Designing

MODEL liRe FT

Designing

MO EL I C

Designing

© Peter Miller 1995

All rights reserved . All trademarks and registe red nam es ack nowledged . No part of this book may be co pie d ,reproduce d or transmitted in an y form without the written conse nt of the Publi shers .

The information in this book is true to the best of our knowled ge at the time of compilation. Recommendation sare mad e witho ut any guarantee , implied or otherwise, on the part of the author or pu blisher, who also disclaim an)

liability incurred in co nnec tion with the use of data or specific information co ntained within this publicat ion .

I , Peter Miller , hereb y asse rt and give no tice of my right unde r section 77 of the Copyright, Designs andPatents Act 1988 to be ide ntified as the author of this book .

First published by Trap let Publi cation s Limited in 1995Rep rinted 2002Trap let House ,Severn Drive,

Upton-upon-Severn ,Worceste rsh ire . \'(1R8 O]L

United Kingdo m.

ISBN 0 9510589 6 7

Front CoverQuark is a C02 powered modelfor tbe CETO su b miniature radio, total, ready to fly weigbt is 2 ounces .

Inset is Pee Tee Spo rtsterfor .40 to .45 fo ur stroke engines.

Ba ck CoverThe author witb bts unp ublished "Belgarion 'tfo r Sa ito 90 twin .

Technical Dratoings by Lee Wiseda leCm 100ns by Simo n Bates

TRAPLET~'~

P U8 L I C A TI U N S

Printed and bo und by Stephens & George Limited ,Merthyr Industrial Estate, Dowla is, Merthyr Tydfil, Mid Glamo rgan CF48 3TD

About the AuthorP

ete r Miller started modellin g in 1952 w ith rubberpow ered kits before moving on to pow er mod elsin 1954. After several attempts to learn to fly co n­

trol line mod els with out ac hieving one co mplete lap hedesigned his own train er with whi ch he finally taughthimself to fly.

Being tota lly obsessed with aircraft, Pet er spent 12years as as airfra me fitter in the R.A.F. whi ch removedsome of h is illusions abou t aircra ft. As Peter puts it ,"Most aircraft are de sign ed by sadists and you need tobe a cross be tween an oc to pus and a co ntor tionist towork on them ."

Dur ing this time he co ntinued to build models fromkits, plans and his own designs.

In 1974 Peters first plan was published , a control lineaeroba tic bipl ane . Over the next 8 yea rs ano the r 9 co n­trol line plans found their way into British and Amer icanmagazines togeth er with several articles.

1982 saw th e pu b lica tion of the author's first R/ Cdesign wh ich has been followed by a who le host of va r­ied des igns publis hed in most o f th e magazines inBritain and the USA.

Peter is not only known as a designer, he wrote the"Engine Bay" co lumn in RC Mode l World for 7:;' yearsand cu rre n tly has tw o co lum ns running w ith o thermagazines. As well as the co lum ns, Pe ter has writte narticles on a wide ran ge of mod ellin g subjects and pro­du ced numerou s kit and product rev iews , as well aswriting "Four Stroke Model Engines", the first book onthe subject of the modern four stroke eng ines wh ich wa spubli shed by Traplet Publication s.

ContentsPage

Chapter 1

Chapter 2

Chapter 3

Chapter 4

Chapter 5

Chapter 6

Chapter 7

Chapter 8

Chapter 9

Chapt er 10

Introduction 9

Why and What With? 11

Essential Aerodynamics 15

The First Design Layout 22

Lightness Equals Strength 31

Fuselages 39

Wings and Tails 46

Undercarriages and Other Bits .54

Scaling Up Drawings 64

Flight Testing 69

Selling Your Plan 73

Usefu l Addresses 78

Notes 79

IntroductionO ne of the first questions any specta tor asks a

modeller is 'Did you make it yourself?' and therecan be few modellers who do no t fee l a touch

of pride when they say 'Yes, I did.' Imagine then thepr ide you would fee l it yo u could say, 'It is my owndesign.' A response which brings a look of respect to thespectators eye .

Quite apart from the ego factor , designing your ownmo dels adds a tremendous amount of interest to thehobby and, not on ly can it save you money it can actu­ally be profi tab le.

There are other books on designing mod els but thisbook brings the subject down to the simples t possibleterms and methods. No maths tha t cannot be hand led ona basic calcu lator, the minimum of th e o ry which isreduced to the barest facts in understandable terms.

Even if you do no t want to design models, knowing alittle about design will he lp you to improve on kits, spotpoten tial weak spots, modify other designs without m in­ing them and it will he lp you to understand why thingsare done in a particular way.

I have not tried to simplify the su bject for the book,what I have done is lay out the methods tha t I use , nomore and no less, to produce my own designs.

Judge for yourself how successfu l my methods are, 48published plans in 10 magazines in three countries over20 yea rs at the time of wri ting with another 8 awaitingpublication, and I don't know how many designs tha thave never been sent in to magazines.

Designing Model Aircraft 9

Chapter 1

Why and What With?W

hy bother to design your own mod els? Afterall, look at all the kits arou nd. Isn't it enoughthat yo ur fellow mod ellers wi ll admire yo ur

ar ti sti c pl a cing o f th e s t ickers o n th e seve n tee n th"Maniac" in the pits.

Wh at is wrong w ith paying £5 0 for £1 2 worth o fmateri als in a fancy box, after all, it is all cut o ut for yo uand yo u only have to ge t the parts to fit. Of co urse , ifyou are fussy yo u might have to repl ace some of thepa rts like the one ult ra so ft fuselage side (or the rockhard one if you prefer).

Want so mething a little more adventuro us? One of theplans published in the magazines, fine , lots of them . Ilike peop le who build from plan s, it me an s that th emagazines will bu y my designs. Actually I wonde r why Iam writing this. I'm cutting my ow n throat doin g this atth e pri ce . Ju st ca ll me "Cut Me Ow n Throat " Mille r.(With apologies to Terry Pratchet t.)

Of co urse you are limited to building wha t othe r peo­ple like and there is a good choice but do n't you everwant to do so me thing origina l? Don 't you have an urge

Tbe a uthor 's firs t RIC design was tbis Luton Minor.

-

- - ..... ---:-. :._- ~\:..

Designing Model Aircra ft

to create a truly beauti ful model. Isn 't there a sca le sub­ject that yo u lon g to build but there are no plan s for it?J ust once wouldn't yo u like to have a mod el that no oneelse has? If the answer to ALL these qu estion s is "No" allI ca n say is, "\V'atch ou t, the tran spl ant surgeons arelooking for spa res ."

Perhaps you think it is hardThe reason that people don't design their own mod­

els is that they believe that it is difficu lt. Bull . .. Er . . .Codswallop.

I know 16 year old youths who have designed excel­lent projects for their G.C.S.E., I know of so me that havethen gone on to se ll the plans to magazines. I imaginethat most modell ers have as mu ch int elligen ce as ab right 16 year old .

All you need to kn ow to design a mod el are a fewsimple facts. If yo u have built a few kits or the odd planyo u alrea dy kn ow most of wha t you need , you just don 'trealise it.

11

Tim Rose designed 'R b u ba rb' as a scboolproject. Plans tben published ill RCMlV.

16y ear old Gm). Gooderbam designed tbis modelfor bis GCSE project, bis first destgn. fteto ve l)' welL

12

Do you understand the co-efficie nt o f lift or the co ­efficient of drag? Do you understand neutral points andpolar mom ent s of inert ia? Do you study the gra phs sup­plied with the co-ord inates of wing sections? You do?Well , you don 't need to read this becau se I don't andnor will the readers of this bo ok .

You will learn all that you need from this book, truerule of thumb designing and the first rule of thumb is,don 't hit it with a hamme r becau se you can 't hold apen cil afterwa rds.

O ut of the goodness of my heart and be ca use I amge tting paid for it, I am goi ng to tell you how to designmod els the easy way witho ut any of the mys te ry or for­mulae.

What you needYou can desig n a mod el wi th a pencil, a ru ler, a tab le

and a sheet of brown paper an d an erase r (do n't forgetthe eraser). It isn 't ve ry easy but it ca n be done .

At the othe r end of the sca le yo u ca n have an AOdrawi ng board with parallel moti on , a full se t of drawinginstruments, French cu rves, ass orted templates , se ve ralpencils and pen s and Mylar film at £.50 a ro ll. All ve rynice but it won't mak e your design s any better, easierperhap s but not bette r.

Most people will sta rt out with a bit more than themost basic and slowly build up as they progress. Let'slook at a simple and chea p kit of equipme nt.

You need a drawing board , it must be big enoug h todraw a co mplete fuselage o f the s ize yo u prefer and atleast one wing panel. You don 't have to do eve rythingon o ne sheet so it doesn't need to be that big. Let 's sayab out 48" by 36". You might eve n be able to use theback of your building board . You can buy a nice piece

Desig lli llg Model Aircraft

o f y," plywood or bla ckboard for abo ut £.4. Make surethat the edges are squa re , sa nd pa pe r it smooth and youhave an excellent board , I used one like that for man y,man y yea rs.

You need a Tee Squ are , a chea p one will be fine , ifyou ca n ge t one as lon g as the board , good , but a sho rt­er o ne is OK.

You need a couple of rulers, a Metre rule which mustbe straight. You ca n find them in Woolwo rths or DIYstores at very se nsible prices (under a fiver) bu t checkfor ,stra ightness or yo u will have funn y sha pe d wingsand strange inciden ce angles . You also need a 12" or 24"plastic ruler, bot h is even better.

A good large se t square is vital, ge t a 12" one as thiswill enable you to extend lines from a side view right upto a plan view as we sha ll see in a future chapter. I alsouse a square 5" prot ractor , th is is a wonderful tool andcan be obtaine d from pilot 's suppliers as it is used fornavigation. It will cost about £.4 , it isn 't essential but it isex treme ly useful and you do need a protractor .

Penci ls a re as essen tia l as a mod ell ing kn ife is tobuilding. Use a 2H as th is will give a dark enough linebut will ho ld its point , buy a good sharpe ne r to go wi thit. You can buy Pentel prop elling pencils with differentsize leads, 0.50mm is ab out right , they are not expen ­sive.

You will need a good pen cil e raser, ge t a nice bigsoft one , a quality on e , the nasty little things so ld forschool use will soon ge t dirty and leave dirty smears onyou r draw ing, an untidy drawi ng is not as nice to workon and can lead to mistakes.

A French curve is usefu l, you can buy se ts but I findthat I o nly use o ne (see fig 1.1), this is quite expens ivebut cheape r ones which are sup posedly the same shapedo no t have quite the same flow from curve to curve.You will need this for airfoil sections .

A pair of co mpasses is needed for drawing wheels

YOIl,. ruler must be straight

First model design by retiredfull s ize designer ,ouer strong but not too beauy, needed more pourerbut was 11ice flier.

an d the front of sp inner rings. O ne that can be conve rt­ed to dividers is useful. Ge t a reason able pair, not oneof those things that you stick a co mp lete pen cil in.

You now have a se t of equipment that wi ll p roducealmost any design tha t you want. There are one or twoothe r item s but they w ill be men tioned late r and eithe rdon't cos t mu ch or alterna tive meth od s can be used.

You sho uld be ab le to get all the above eq uipmentfor far less than a cheap kit and you will save that withyour first design.

It is worth goi ng to car boot sa les and auc tions w hereyo u can so me times find items . My AO draw ing boardco mple te with parallel mot ion and stand ca me from aco lleague via a ca r boot sa le , it cost me 3 ounces oftob acco .

Designing Model Aircraft 13

Fig 1.1 -Tbe most useful French curve

My favourite sbape ofFrench curve. Figure is toillustrate uibat you call do tuitb French curves . . .maybe 1 sbould re-phrase tbat!

14

PaperDrawing paper and tracing paper can be ex pensive

whe n bou ght on a roll or in sheets, but yo u are going tobe drawing, correc ting and altering, and qu ite often youwill tear a fusela ge side vie w off the board and sta rtaga in, then , wh en you have don e the drawing yo u aregoing to be building on yo ur plan so you wa nt so me­th ing chea p and tou gh .

On e so urce of pap er is the back of oth er plan s, theones that yo u bought becau se you th ou ght that youmight build them onl y to find that they were design edby a sado -masochist wh o would not use one piece ofwood whe n he co uld use ten .

For a reall y good so urce of drawing paper go to aDIY sto re and find their lin ing pap er , tou gh off whitepap er and the price of a roll will be che ap . .. I p ickmin e up fro m a local sho p at about 40p a roll.

Another typ e of paper which is harder to find is whiteshe lf lining paper, this is thinner and smo other, I like itbut ca n rarel y find it these days.

On ce yo u a re in to d es igning yo u ca n g o on to"Detail" pap er , this is a thin wh ite pap er wh ich will alsoact like an opaque tracing pap er, it does cos t nearly £20a roll though .

In futur e you may also wa nt tracing paper so that youcan have prints run out from your plan , it also allowsyou to keep a copy whil e yo u build on the scruffy origi­nal , thi s is useful it yo u want to se ll th e plan to amagazine after the test flights .

Tbe basic needs ill tbe way ofdratoing instruments,

Tbe writer's bargain drauiiug board, sbeer IIlXIIIJ'

after )'ears toitb pieces ofplywood.

Designing M odel A ircraf ]

Chapter 2

Essential AerodynamicsA erodynamics is a vas t and co mplicated subjec t

but the bas ic theory of flight is much simpler,. and what you need to kn ow to design a model is

eve n simpler than that.I knew a University gradua te who design ed a ser ies

of bea utiful co ntrol line stunt models, all the form ulae ,the works and they flew ve ry well indeed . One day heneeded a stunte r in a hurry and design ed a simple bo xand plank design for quick buildi ng ... it flew far betterthan the fancy ones.

Don't think that all the co mputers and wind tunnelsand expertise of the full size wo rld are infallible , theyget it horribly wrong as well at times.

I am goi ng to tell you the minimum that yo u need tokn ow to design a model , if yo u wa nt to understandaerod ynam ics be tter I ca n recommend "The Mechanicsof Flight" or "Flight with out Formulae" by Kermode , oldbooks bu t st ill two of the easiest to understand . Theysho uld be ava ilab le from your library or aviation bookshops.

Allyou uiant to hnoui about flightAn ae ro p la ne fl ie s b e ca use th e w ing is mo ve d

through the air and ge nera tes lift, the lift is ge ne ratedfrom one or bot h of two action s.

Reason one is that because the shape o f the cross ­section (called the airfoil from now on) mak es the airtravelling ove r the top sur face move faster than the airmoving under the lower surface, this redu ces the pres­sure of the ai r ab ove the wing.

The second reaso n is that the wing meets the air atan angle fo rcin g air down and the refo re the w ing ispu shed up. This angle is called the ang le of attack.

I cOllkk~_ into details as to why th is is and ex plain ita ll but yo u do n 't need to know that and it will tak e upva luable space.

The wing needs to be stab ilised in flight and for thiswe use a tailplane , we co uld use a forep lane and build aca nard or even a flying wing but that wou ld be runningbe fore we ca n wa lk .

Tips)' Nipper is bigbly aerobatic, note umtsual toing sect io n and larg e incidence angle compared uiitbtailplane and engine tbrustline.

Designing Model A ircraft 15

\Ve also need a powe r source to pro pel the w ingthrough the air. On a glider this is simply the weight ofthe aircraft, it is literally sliding downhill all the time. YesI know gliders can gain height , look at it this way , if youare in a lift and launch a paper glider which takes fiveseconds to reach the floor from six fee t and the lift isgoi ng up at ten feet pe r second the glide r will have go neup 44 feet in the five seconds it took to reach the floor.

There are four forces acting on an aircraft; weight , lift,dr ag arid th rust. When the aircraft is in stable flight , Le.travelli ng at a co ns tant speed in a co ns tant di rection ,these forces will ba lance out. Tru st me , they do and youdon 't really need to worry about tha t at the mom ent andI w ill ex p lain as and when yo u do need to know.

Fig 2.1 -Airfoil Types

The Airfoil SectionI have said tha t the shape of the airfo il produces lift

and it does but there are hundreds of airfoil sections ,wh ich do we use?

For our very simple minds we can divid e them intofour types. Flat bottomed , se mi-symme trica l (o r Bi-con­vex as th e purists ca ll th em), symmet rical and undercambe red . Flat plate can be considered sym me trical an dcurved plate is under cambered .

These are rath er ine fficient sections for wings and willonl y be mention ed in passing, although flat plate sec­tion s are very co mmon for tailplanes and the flat plat e isa lso used for very simple model s w ith a solid sheetwing.

cFlat bottomed

Bi-convex

ESymmetrical

'- -Under cambered

16

---Desig ning Model Ai rcraft

Higb iotng cabin models ca ll be sleek and beautiful,tbe author's Lady Boioyn design tohicb is ill RClIflVplans '·allge.

Fig 2.2 - Terms Explaiued

Flat bottom ed sect ions are used for some trainers andare quite common on certain sca le types, Clark Y is thegrea t classic fiat bott omed section . They are easy tobuild because they can be placed flat on the buildingboa rd.

Semi-sym metrical sec tions are very co mmo n, they areused o n many full size d and mod el aircra ft and are goodfor the faster tra ine rs and sports aeroba tic models.

Full sym metrical sec tions are for true aerobatic mod­els as they will fly in exactly the same way upside downas the right way up if they are se t at the correct angle.

Under cambered sec tions are used on vintage mod elsand so me floate r type glide rs, they. are for slow flyingmod els only as they do produ ce a lo t of drag bu t theyalso provide a lot of lift.

For o ur purposes we will o nly use the first thr eetypes of sec tion.

Angles ofD ... Different SortsIn describing how a wing produ ces lift (and inciden ­

tally, giving all aero dynamics expe rts apop lexy ) I men­tion ed 'Angle o f Attack ', forget abou t it. Th e angle ofattack is constantly variable and the o nly time you needto worry abo ut it is wh en you exceed the angle when thewing stalls and eve n that varies with spee d, but we arenot talking about /lying so , as I sa id, forge t it.

The angle that we are co nce rned with is the ang le ofincide nce. Th is is the angle of the wing to a purely arbi ­trary line called the datum. The datum is normally takento be a centre line drawn through the fuselage to whichall other ang les are related . You cou ld have a datum lineat 45° to the fuse lage but it would make drawing difficult.

Cbord lille

\Angle ofattack

Direction ofatrfloto(variable)

A ng les

C!.Jord Une

Desig n ing Model A ircra ft

17Jis lin e parallel to d~tum/ Angle of in cidence

17

This model is based loosely 011 control line stuuterlayout,jlew well. Four strokes are best in ligbtmodels.

The an gle of the wing is measured from the chordline and the cho rd line is a line drawn from the centre ofcurv ature of the leadin g edge to the ce ntre of the trailin gedge. Sometimes th e chord lin e is ta ken as runn ingalong the bott om of a flat bottom ed wing.

Other items which are se t at some angle in relation­sh ip to the d a tum lin e a re th e th ru st lin e a nd th etailplane chord line.

The only othe r angle that we are co ncerned wi th isthe d ihedral ang le which affects stab ility and ailero nresp on se, bu t this will be d iscussed when we get downto putt ing pen cil to pap er.

WillgAreasThe wing area o f mo de ls va ries depending on the

type o f model. Powered glide rs wil l have large wi ngareas for ve ry small eng ines.

My Falco n design has 620 sq . in . and is mild ly ae ro­ba tic o n a .12, w h ile Te q u ila Su nr ise has o n ly gotaround 230 sq . in. for similar power and is fully aeroba t­ic but naturally do es not have the same low speed per­formance o r glide .

The mod el with th e smalle r wing w ill have bett erpenetr ation in a wind becau se the drag is less in propor­tion to the powe r. Fit a .40 to a Falc on and it wouldhandle even stronger winds, but it would then have tobe struc tura lly stronger to tak e the power and sudd enlyit is not a powered glide r any more .

I have g ive n a ta ble o f suggeste d w ing a reas formost sizes of eng ine . Thi s ta ble is fo r average sports

Full size Skybolt and model Vollesplane obey tbe same rules forfligbt.

18 Desigllillg Model A ircraft

aeroba tics models which is the most popular typ e ofmodel and is only intende d as a guide.

Biplanes will have more area, so me ligh t types o fsca le mod els will 'have much more area and pow eredglide rs eve n more .

Going be low the smaller areas would not be a goodidea for any type of mod el as it wo uld be very difficultto keep the wing loading within the bo unds of reason.

.09 - .15

.20 - .25

.29 - .40

.40 - .45

.45 - .60

200 to 350 sq . in.350 to 450450 to 550550 to 650650 to 800

Competition FilII Fly models take low aspect ratio and low icing loading toextremes for incredible performance.

Areas and LoadingsOne of the mos t importan t factors in any design is the

wing area and the win g loading. Too high a wing load­ing and you will have a model that is a pig to fly, if itwill even get of the gro und .

This is where we have to do a little maths, no thingco mplicated, you don 't really need a calculator unl essyou are a pro duct of the mod ern education system.

Wing area is quite simply the chord or width of thewing multiplied by the span or length of the wing. Egad!How basic can I ge t!

Wing lo ading is thefigure tha t yo u ge tw hen yo u d iv ide th ees timated we ight of themod el in ounces by thew ing area in squarefeet.

Let 's take an exam­pl e . Th e wingspan is50", the cho rd is 9" so :50 x 9 = 450 sq ua rein ches . Divide by 144wh ich is the numb er ofsq ua re in ch es in asqua re foot and that is3.125.

Assume that we aredesign ing a mod el fo rsp orts ae robatics with awing o f th e size givenabove, if we build fairlylight it should come outat about 4 Ibs in weight.4 Ibs is 16 ounces x 4 =

64 ounces.If we di vide 64

ounces by 3.125 squarefee t we ge t 20 ouncesp er sq uare foo t wingloading.

After you have builtth e mo d el an d foundout wh at it does weighyou can see how closeyou go t.

The same system willallow you to decide onho w bi g a wing yo uneed . Assum e that you

Desig ning Model Aircraft

Small, ultra simple design call be built quickly andstill provides fun and satisfaction:

are designing a mod el for a .40 engine, a realistic weightfor this wo uld be up to 6 lbs maximum. A sens ible wingload ing would be up to 24 ounces per square foot.

6 Ibs = 96 o unces . 96 di vided by 24 eq ua ls th edesired nu mber of square feet = 4. Multiply by 144 = 576square inches. That is the size of the wing bu t we canbring it up to 600 for safety.

I had planned to include a tab le of suggested wing

19

loadings for diffe rent sizes of model , but when I beganto lay it out I reali sed just what a wid e range there are,even for one size of eng ine.

For exa mple, a Competition Fun Fly mod el might bepow ered with a .25 to .40 and have 700 square inches ofw ing area , a wi ng loading of ar ound 10 ounces persq ua re foo t, but a sports model mi ght go up to 25ounces pe r square foot which would defin itely be thetop limit for the .25 size of eng ine .

Lets look at so me sca le models. The Sig Pip er Cub is72" span and ca n be built to weigh aro und 4lbs , it flieslike a dream on a 20 four stroke . The Grea t Planes Pip erCub is the same size, bu t they claim that the ir kit willweigh 61, to 71, lbs and need s a .40 to .60 two stroke .

Th e only co mme nt that I will mak e is that the realCub is light and flies very slowly.

Runn ing th ro ugh so me o f the figures for my ownde sig ns, I suggest that , for norm al spo rts aerobat ic typesyou do not exceed 20 ou nces per square fo ot for modelsup to .15 eng ines and 22 ou nces per squa re foot fo r .25size models , up to .40 you can go up to 26 ounces andfor .60 you might go as high as 30 ounces per squarefoo t.

Goi ng a little over these figures will not be disastrous,but keepi ng below them will help improve performanceand han dlin g of the model.

Fig 2.3 -Aspect Ratio

AIR 20:1

AIR 6:1

AIR 5:1

AIR 4:1

2 0

Sailplanes use bigb aspect ratios, tbis Condor MSbas all A.R. ofabout 20.

Powered gliders, lightweight sca le models like PiperCubs ete. sho uld have much low er wing loaclings.

Desigu inp.Model A ircraft

Full size Kitfo ... is microlig b t, velJ! like a model ill construction.

Wing Volume Loading

Giant sca le mod els use mu ch higher wing loadingsan d if you start working out the loadings for full size air­craft they becom e astron om ical in co mpa rison with theirmod el counter parts. This is because of the claimed extraefficiency with larger sizes .

Many yea rs ago Dave Platt did so me comparisons andfound that if one took the volume of a wing as a unit ofloading instead of the area and co mpared that at differen tsca les of a part icular mod el of a subject the figures veryclose ly matched .

Dave 's figures sho we d that increasing the size of asc a le w ing from 500 s q u a re in ches to 700 sq . in .increased the area by 40%, but increased volume by 98%.

'Dave went on to show that a loading of 1 ounce per 5cubic inches was about right for a hea vy sca le mod el andthat this co incided nicely with the heavier load ings possi­ble with larger mod els.

It would seem likely that if this wa s taken to it's logi­cal co nclus ion, the volume loading on a model would bevery close to that of the full size prototype .

I don't suggest you try work ing the volume out, but i tdoes ex plain wh y larger mod els can use high er loadings.

Presumably the computer "Whizz Kids " could write apro gramm e that would calculate the desired figures butfor our simple purposes it would be very complica tedbearing in mind that eac h aerofoil sec tion wo uld give adifferent volume .

Design ing M odel Aircraf t

Aspect Ratio

Yet anothe r of the secret term s of the designer. All itmean s is the sh ap e of the wing, for a parall e l cho rdwing it is simply the wing span divided by the chord .Example , from the ab ove , 50" spa n divid ed by 9" chord= 5.55 , a nice figure .

Nor mal range of aspect ratios for paralle l cho rd win gsis abo ut 4.5 to 6. Glid ers use A.R.s as high as 20:1.

For a tap ered wing we add the tip chord and rootcho rd , divide by 2 whi ch gives the average chord andthen divide that into the spa n.

Balance Point or Centre ofGravityOne of the mo st vital thin gs about a model is whe re

it balances. This balance po int or ce ntre of gravity (CG)will affec t the stability to such an ex tent that if it is just alittle too far ba ck it will be impossibl e to fly the mod el.

The CG has to be in front of the point through whichthe lift acts. Now wor king out th is latter point is not onlyco mplica ted but rath er hit or miss as it dep ends on soman y factors so we adopt a rule of thumb for the timebeing.

On co nventiona l mod els the CG sho uld be be tween Y,and Y, of the cho rd back from the leading edge of thewing , 25% to 30% if yo u prefer and yes, I kno w that Y,rdis more than 30%. The nearer the forw ard limit the saferthe mod el .

21

Chapter 3

The First Design LayoutI n Chapter two we looked at the basic requirem ents

o f an aircraft , we talked about win gs and angles andbalan ce points and so on , the aerodynamic side of

th ings.Th e typ e of model that we want will decide the aero­

dy namic layout and the aerodynamic layout will haveso me co ns ide rable effect on the shape o f the model,thus all basic trainers are high wing and all pattern shipsare mid or low wing. Now you kn ow why eve ry airlinerloo ks the sa me as all the res t and why mod ern owne rsof full size sa ilp lanes ca n't tell one from the other.

The rea son tha t military ai rcraft all look different isbecause they are d rawn to specifica tions decid ed byco mmittees who don't kn ow wh at they want anyway.

Wh at sha ll we design? Well , I imagi ne that you arepast th e trainer stage a nd th e most popular typ e ofmod el is the sports aerobatic type so that is the way wewill go . It can be for .25 size eng ines or .40s, I w ill givefigures for both but I suggest the sma ller size as it ischeaper to build .

Fig 3.1 - Tbe Th ree Most Usef ul Sections.

Ultr a boxy camera plane is tbe easiest type ofmodel to des ign. Tbis model w ould us e flatbottomed wing or semi-symmetrical section.

~KY

22 Designi ng Mode! Aircraft

Experimental design by Geo rge Strobm does not baue a single curve ill it, evelJ Iuiing s ection is a d ia mond. Flew a t b lg b speed but was d iabolical a t loto speed.

SpecificationBefore we ca n s ta rt

drawing w e ne ed tod ecide o n th e s ize o fth e mod el, th e wi ngsect ion that we want touse and the sha pe andpo sition of the wing .

For a .25 size modela w ing o f a b o u t 4 50sq .in . is a bo u t r igh t.No w w e p lay a bou twith s pa n and cho rduntil we ge t an as pectra tio that is sensibl e , Itook 50" s pa n and 9"chord for an as pect ratioof 5 .5 but yo u c o u ldvary this from a span of45" a n d c ho rd of 10"(AIR 4.5) to a span of to52" and 8.6" chord (AI R6) . The s ho rte r wingspan will roll faster.

You ca n round fig ­ures up and down, a lit­tle more area or less isnothing to worry about. For a .40 s ize model the areawou ld be 550 or more sq . in.

The wing can be located in the sho ulde r, mid or lowpositions. The mid-wing pres ents structural problem sand takes up valu ab le space , the shoulder win g is nicebut can make for extra work later, it is ide al wh en themodel has to be hand launched and also mak es wo rkingon the model eas y as it will sit on its undercarriage .

The low wing model is more common and has man yad vant ages but may present problems when it comes tolocating the undercarria ge in the ideal position. We willcover both shoulder wing and low wing in the exe rcise ,after all, this is your first design and you are going tobuild it. We also ne ed to decide on the wing section .Here the choice is bewildering. You co uld use a semi­symm etric al wing or fully symmetrical, you could eve nuse Clad, Y se t at true 0 incidence as this works like athin semi-symme trical wing inverted but it is a bit thin .

I favour a th ick symme trical sec tio n with a blunt lead ­ing ed ge, this will slow the model slightly and gives verygentle stall characteristics.

In this chapter you will find three airfoil sec tions , youcan have these enl arged on a photocopier to the sizetha t yo u need , take th e measurement of yo ur w ingchord and divide that by the measurem ent of the sec tionprinted here , that is the enlarge me nt factor to set on thecop ier whi ch mu st hav e a full zoom capab ility. e .g. Yourmode l chord 9.5", in th e bo ok , 5.375, yo u ne ed toenlarge by 1.76.

You could des ign you own sec tio n, I have shown thema in points that you need to watch . If you go this routeyou will probably not no tice the differen ce . You couldeve n use the MILLER T.L.A.R. (tha t looks about right)section .

Paper and PencilTak e your piece o f pap er and attach it to your draw­

ing board, I sug gest masking tap e at the co rners butdrawing pins or even proper drawing board clip s can beused .

Now draw a line about 10" inches from the bottom of

Fig 3.2 - Ouin Desig n Section

Max thicknes s at 3 0% cb ord

st«lit L.E.

Designing Model Aircraft

Miller T.L.A.R.

Fla t fo r easy build tng

23

the paper right along . Th is is your datum line and thewhole mod el will be drawn round this.

Th e datum line is arbitra ry , it ca n be on th e thrustline or it ca n be the to p of the fuselage side, it co uldeve n be outside the model but this is not wise . \'\fe willmake it the top of the fusela ge side althoug h you won 'tknow it at th is stage.

About 12" in from the left side of th e paper drop aline down from the da tum, this will be the L.E. of thewing.

Now we draw in the chord lin e . If this was a trai ne rw e would p ut it well above the datum line , fo r a shoul­der w ing mod el we ca n leave it on the datum and or alow w in g model it must , naturall y, be belo w . Put itab out 3)1," be low an d paralle l w ith the datum.

Now, on the datum, measure back fro m the L.E. thedis tance th at yo u have decid ed on fo r th e chord anddrop ano ther line , th is is the T.E. No te that yo u alwayswork on the datum and drop or raise lines .

\'\f e n ow h av e two mo re d e ci si ons to make , th elength of the nose and the tail mom ent arm or the dis ­tance from th e T .E . o f the wing to th e L.E. o f th etailplan e.

Th e length of the nose de pen ds on the weight of theengine, the weight of the rear end and the length of thetail moment and yo u can't work it out so , for most typ esof model , we ca n work to a distan ce from the pro p dri­ve r to the L.E. of about one chord . Becau se four strokesare he avier you co uld reduce it a bit if yo u are go ing touse one.

Th at chor d dimen sion is a very handy one fo r givingproportion s for any size of mod el.

Fig 33 - Basic Layouts

----------

Higb wing

Th e tail mom ent arm and the size of the tai lp lane arelin ked w ith sta b ili ty . A long moment arm and sm alltailplan e give the sa me stability as a sho rt ar m and bigta ilplane, howeve r, contro l is smoother with the lon gerarm.

A good co mpromise moment arm is about one and ahalf chords with a tailpl an e about 18% to 20% the areaof the wing . Patt ern ships have longer arms , so me of thewilder fun fly models use less .

Having decid ed on these d imensions we dro p a linedown 9" in fro nt o f th e L.E., th is is th e front o f th emode l an d we drop one down about 13" behind the T.E.this is the L.E. of the tailplan e .

Th e tailpl ane s iz es h ave to b e ca lc u la te d no w .Percentage of the wing area is easy , divide the w ing areaby 100 and mu ltip ly by what ever pe rcent age yo u wa nt .E.g., we have a w ing area of 450 sq .in . so 450 divid edby 100 equa ls 4.5. If we go for 18% we multiply by 18an d ge t a tailplan e area of 81 and so on , most calcula­tors have a pe rcentage button which mak es life eveneasier. I prefer a little mor e area so we w ill try 20% andge t 90 sq.i n .

\\Te now want to know the chord of the ta ilp lane , notquite as simple , w e may have a parallel chord w ing butthe tailplan e is much nicer if it is tapered. Play w ith yourca lculator aga in . Hit 90 and div ide by 5, that gives us aspan of 18" and an average chord of 5". Doodle aroundon some scrap pa per an d see what looks nic e , a taper oftwo inc hes looks good , right, root chord of 6" and a tipchord of 4". You can vary th is and you can put moretaper on the L.E. o r on the T.E., just keep the area aboutthe sa me.

'" Datum line

pSbOO'de=1II

1.,- __r--..... ----

Low wing

24 Design ing Model A ircraf t

Fig 3.4 -s Aerodynamic Layout

Prop driver I.E. T.E. TIP I.E,1 I

-------l...I+------cHORD (C)-----.'<llI...f-----C X 1.5-------._ _1 _ _ _ _ _ _ _ _ _ L

Datum

Put the tailp lane's 6" root chord on your datum line .You can decide wh ere the tailplane will sit now, I likeputting it on top of the fuselage s ide .

Variations 011 the Theme\X/hat you have drawn above is the ae rodynamic lay­

out for a sp ort s ac robatic mod el. With a mod e l of thistyp e the wings, tailpl an e and engine will norm ally be se tat 0° to the datum, this wi ll give neutral stability or, inothe r word s, the model will stay point ing in any direc­tion that you have placed it in. Some oth er typ es need tobe different.

Tra iners need som e positive inciden ce on the wing tomak e them pull out of a dive if the learner releases thes tick. Th ere ar e so ca lle d trainers that ha ve neutral

stab ility and so me that are ac robatic . . . they are notbasic train ers whatever the box top says .

We talk ed briefl y abo ut the balance o f forces anddra g in Part 11 , now we need to look a little furth er.

Take a high win g mod el, the wing produces lift butalso d rag and it is high . The engine is low er down andproduces thru st, as you ca n see from the drawin g thistend s to pull the nose up so we tilt the engine downand this co unterac ts the imbal an ce of forces.

Likewi se , it we mount an engine on a pylon , on aseaplane o r a powered glider for example , the thru st iswell above the dra g and so the model will tend to nosedive and in this case we would use up thrust.

Qu ite hon estly, it is impossible for the average mod­eller to calculate the exact angles need ed and so provisionmust be made for adju stment during the flight test period .

Higb 1II00m ted pusber engine on tbe Polisb bomebuilt needs some 'up' thrust but, because it is pushing tbeengine is pointed downwards s ligbtty.

Designing Model Aircraft 25

Fig 3.5 - Thrust Line Adjustment

Drag

Nose up fo rce counter urith doum thrust

D r ag

Nose doum force

Fitting the Gear

It may so und strange to talk of fitting radios and tankswhen you don't have more than a few lines on the pap erbut now is the time to plan it, before you are restr ictedby sha pes.

Start off by placing your win g section on the plan , ifyo u have co pied a section from the chapter you can stickit d own o n the drawing, just ex tend the chord line andthe line dropped for the L.E. and use these to line thesection up with your draw ing.

Now yo u need to d raw in th e a mou n t that youraileron servo will stick up above the wing. I find that cut­ting ou t card shapes the same size as my servos, receiveran d batt ery pack is useful, you can jugg le them aro undand be sure that all the co ntro l runs will clea r each other.

Don't forge t that the ailero n servo will be beh ind themain spar which is normally about 30% of the chordback from the L.E.

You also have to locate the engine at th is point. Is itgoi ng to be up right or side mounted ? You ca n also posi­tion the fue l tank, this sho uld have its ce ntre level withor just be low the ce ntre line of the carboYou must allowroom for the eng ine mount and a former between thetank and the eng ine .

You do not have to draw these part s in detail , just anou tline is fine.

Once you have worked out where all the internals willgo you ca n, at last, draw the side view of your model.Here you can let your imag ination have free rein , racer ,jet style, wh atever you like .

I would suggest that you keep it simple at this stage,you still have to draw all the othe r parts and then build it.

Start with a Bo x90% of all model aircraft are based on a plain box

fuselage . Some , like the Stik family don 't go any further,othe rs have a simple turtle deck and othe rs have the boxhidden unde r stringers and fairings .

Your box will look something like the drawing , theslo pe up to the ta il may vary but at thi s stage in thelearning curve little e lse will. What you do with it is up toyou , a simp le turtle deck will be easy to design and canbe varied in height , position of co ckp it and so on.

Draw it in lightl y, th is is wh en the era ser co mes intoits own as you cha nge things until you get a shape thatyou like. Of course you should have had a fair idea ofwha t you wanted from those ske tches on the back ofenve lopes, but ge tting it to look right at full size can takea little time .

A very simple jet style mod el is sho wn in Fig 7 thisuses th e box and a few pieces o f sheet to create anattractive looking model.

26 Designing Model Aircraft

Fig 3.6 -s Bquipmeut Layout and Outline

--~~fE-=tfR8f~+---

You can draw in the fin, this should have about halfthe area of the tailplane, more if you have a very bulkyfront end of the fuse lage . Fin shape is not critical, you canhave any shape you like, just keep the areas correct andrem ember that part of it has to be hinged for a rudder.

The rudder area is no t critica l, highly aerobatic mod­els will benefit from large rud ders to kick them over ingood stall turn s and produce crisp flick man oeu vres an dspins , sports mod el do not need as much . A good com­prom ise is about Y3rd the tot al area of the fin and rudder.The rud der is norm ally ex tende d down to the bottom ofthe fuse lage .

With th e side vie w dr awn we ca n d raw th e p la nview. Draw a line ab ove or below your side view , th isline mu st be pa ra lle l w ith th e da tum line . Now d roplines from the various critical locat ion s like the front for­mer , the L.E. and T.E. of the wing and the tailpl an e L.E.and the end of the sides .

You now have to decide on the width of the fuse­lage , this will dep end on the size of the se rvos and tank.Always leave so me roo m to play with and don't forge t toallow for the th ick ness of the sides and any doublers .

The fuselage can be pa ra lle l fro m the nose to th ewing T.E. and tap ered from there to the end . You ca n

Planning installation during designing will save a lot of'problems later.

Design ing Model Aircraft 27

Fig 3.7 -Styles 011 same Basic Box

o

o

o

ouse a s moot h , natu ral curve o r a tighte r curve andstraig ht taper.

To draw a smooth sweeping curve you need a spline,this is not ex pens ive becau se it is just a str ip of Y.."sq uare balsa or spruce . Using mask ing tape hold o neend down to the stra ight po rtion of the fusel age andthen pull the free end to line up with the ce ntre line atthe rear of the fuselage.

A light pen cil line alon g each s ide of the spline and

28

you have the plan view of a curve d fuselage side drawn .The alterna tive is to use the French curve to draw a

ge ntle be nd behind the wing and then add the stra ighttaper back to the en d .

The only othe r task at th is stage is to measure downfrom the thru st line a distance , that will a llow plenty ofclearance for the intended prop eller and draw a line par­alle l with the datum, this is the ground line and willdefine how lon g the und ercarriage is .

Designing Model Aircraft

That takes ca re of thefuselage outline . Make su re tbe ra dio toillfit

The WingT he wi ng ca n b e

draw n as a re c tang lebe ca use we a re n o tgoi ng to g o in fortapere d w in gs at th isstage . Straight wings areeas ier be cau se all thewing ribs are the sa mewith jus t m inor va ria­tion s.

At th is stage we ca ndesign the sha pe of thet ip s a n d we c a n al sop lot the ailerons .

Th e tips can be an yshape that yo u want butyou really want to keepthem simple and light atthi s s tage . The s hapeshown o n the drawingis as good as any for a first model.

For ease of co ns truc tio n strip ailerons are hard tobeat. I have foun d that 14% of the wing area is a pre ttygood rule of thumb, so take your wing area , le t's say 450sq. in. and punch that into the ca lculator and we ge t 63sq .in. , if we have a spa n of 50" divide that into 63 and

we ge t 1.26" so a I X" wide aileron is what we ge t. Drawa line I X" forwards of the T.E., that will be our aileronhinge line .

That is the shape of the wing, everything else on thedrawin g will be struc ture which we sha ll co me to in dueco urse .

T1J'different outlines, something sligbtly unusual call produce an eye catching model.

Design ing Model Ai rc raft 29

Midwest Kits use a basic box and uitng for a ,.a llge of "SOI·t of' scale models.

area , so it is a matter ofjugglin g the tap er into asha pe that looks nice .

Slig htly more sweepo n th e L.E. lo o ks bestand gives a se nsible sorto f sha pe to th e e leva­tors.

Don 't w orry toomu ch about keeping toexact areas and percen t­ages , mo d el s wi ll flywit h wid e variations .You sho uld now have adr awing that looks likeFig 3 althou gh the wingsand tail may be on sepa­rate p ieces of paper.

TailplaneWe worked out the size of the tailp lane and its shape

when we were drawing the fuselage side in Part Ill , sonow it is a simple ma tter to lay it out.

Draw a line whic h will be the hin ge line and drop averti cal throu gh it which will be the ce ntre line.

The eleva tors should be between 20% and 25% of thetail plane area, I pr efer the larger figure becau se it ismore e fficie nt to move a large r ar ea over a sma lle rang le.

We kn ow the roo t chord and the tip chord and the

What have we got?The result of our work so far sho uld be a drawing of

the shapes of all major compone nts of the model andwe know the relationsh ip between them. We know thatthe radi o and tank will fit in the mod el and we knowwh ere the eng ine will fit.

The next stage is to de sign and draw in the structurebut before we do th a t you will need to th ink abo utweight and struc tures.

Fig 3.8 -s Aerodynam ics and Shape Outline

I

I

II

Ii\I------_-+---_----jl/

Straight lilies tangent to cu rue

RIC omittedfor clarity

30 Designing Model Aircraft

Chapter 4

Lightness Equals Strengthmod el, there is les s ine rtia and and so the struc ture ca ncope with the load s.

Another effec t of inert ia is to mak e a model more dif­ficult to fly accurately. Take a wing and hold it in themiddle and roc k it as though it wa s being put into abank. Now add some lumps of modelling clay to each

t,.~

dIl'----ti ff • .

d \".,,~ \~ . t' ......~""o.l ....

.~h ;\1 \\.~'"

~ \\ I ""'#I~

~\ ..

~....~

.•"~_~, .,I .-1'

..": . ........... .. ,

/1./. . r: ~ "~t · ..u~_

( ,.. ...,H.""" ....,Hft. . . .",

~ -a

-

When one bit stops, tbe rest try to ca,.,y on

Sir Isaac Newton sa idthat a body had a ten­d ency to rema in in aco ns ta n t s ta te unl essacted on by an outsideforce , Le . the apple keptfalling until his he ad gotin the way and sto ppe dit o r, a mo re m od erne xa m p le , s it ting in acha ir until the wife tellsyou to go and mow thelawn . .. this is inertia .

In the sa me w ay amodel ha s inertia andeach pa rt of that mod elha s iner tia and wh en itis m o ving none of itw ants to s to p . Whenone part sto ps the restwa n ts to ca rry o n andthe heavier eac h part is,the more iner tia it has.

No w imagine yo u rni ce s tro ng , he a vymodel. The sp inne rsto ps because it meet sth e g ro u nd but every­thing else wants to carryo n until it too , meetsthe ground and so yourheavy wingtips hit th eground and the wing isin two (o r a lot more)bits and the tail tries toge t up front with th ee ngine and your solid'looking mod el is a heapof splinters.

Now take a lighte r

Inertia

Y OU have all seen those models that are built likea bri ck built sh . . . chi ck en coo p . You haveseen them cras h and dem olish themselves and

perh ap s you have wondered wh y such a strong modelbroke in a relatively minor crash.

I will tell you wh y, because the idea that large andhe avy lumps o f w oodar e s tro nge r is wro nga n d in fac t the y co n­tribute to the damage .

Designing Model A ircraft 31

Front view ofa Piper Cub fin, about -%" thick,bracing wires are II0t mucb thinner. Note also waJIfabric is used to fair fin to fuselage, something tbatis often missed Oil scale models.

tip and repeat the ac tio n. Did yo u not ice how muchharder it is to sto p the rota tion once it is moving? Wellthat happen s in flight and the same th ing happen s in thep itch mode when the eleva to rs are used .

I once had a ve ry spectacular dem on strat ion of ine r­tia . I was flying a .60 powered , se mi-sca le Fairy Fireflyco ntro l line stun t mod el. This aircraft had wing mountedund ercarr iages with big, heavy wheels.

I was flying inverted and came down a bit too lowand mad e a hard , heavy but flat inve rted land ing in thegrass.

The model sto p ped in a co u p le of fee t wi th o n lymi no r dam age , but both unde rca rriage legs depart edfrom the wings and ended up se veral yards fur the r on,co mplete with sections of ribs and bits of spa r and L.E.sheeting .

Weight ill FlightWe have seen that , for level flight, lift has to equa l

weight. If a mod el is heavier it has to fly faster or at ahigh er angle of attack. Increasing power is not a goodop tion becau se you need four times the power to dou­ble the speed an d it will not help the lan din g speed.

In creasin g th e ang le o f att ack increa ses d rag andslows the mod el until yo u reach a stage whe re the wingis flying just be low the stalling po int. From th is it can beseen that the more weigh t the lower the ac tua l flyingspeed.

At the sa me time the stalling speed will rise with theincrease in weight. It is quite possible to ge t a sta lling

32

Plying a uintage model is tbe best illastration ofbow nice ligbt models are.

speed high er th an th e maximu m flyi ng speed of amod el , th is is a safe mod el becau se it can 't fly but that isnot the ob ject of the exercise.

Conve rse ly, of co urse , a light model will fly fas ter andslower and this w ide speed envelope is mu ch sa fer .

A ligh t model will accelera te and decelerate faster,th is mean s q uic ke r take- o ffs because th e mod el willreach flying speed in less time . It a lso mean s grea tersafe ty in landi ngs.

Conside r a heavy mod el co ming in to land in a 15mph wind, the model has a stalling speed of 30 mphand is approaching the landing at 35 rnph. Now thewind is not constant, a nominal 15 mp h wind is drop­ping to 5 to 7 mph and rising to 20 mph.

If the wind drops to 5 mph the mod els airs peed issudden ly re d uced to 25 mph , 5 mph below sta ll ingspeed , the heavy mod el canno t accelerate fast eno ug h tomake this speed up . . . instant scrap balsa .

A light e r model with a stalling speed of 20 mph canmake its approach at 30 mph or less and will acceleratequickly enough to prevent a disastrou s stall.

Light er mod els are safer and fly bette r.

Structures Full Size Classification\V'e can learn a lo t abo ut mod el struc tures fro m the

full size. Most mod els, kits or plans , are poorly designedfrom the struc tura l point of view. This co uld be becauseit is easier to man ufacture the kit that wa y or sim ply du eto ignorance.

Fu ll s ize aircra ft struc tu res are d ivided into threeclasses. Primary, secondary and tertiary.

Primary structure takes all the main load s, the flyingload s, the landing loads, everything . It is very stro ng .Spars, lon geron s, eng ine mou nt ings and underca rriagemountings wo uld all be primary struc ture. In so me casesthe ski ns of the aircraft may be p rima ry struc ture. Anydam age to the primary struct ure will ground tha t aircra ft.

Seco ndary struc ture carries some load s an d may havesome of the loads on prim ary struc ture transferr ed to it.Giving exa mples of secondary structure is mu ch hard er ,on a mod el we migh t co ns ider lead ing edge sheeting assecond ary but as in all cases w here some thi ng lie sbetween two ex tremes it is harder to de fine.

Tertiary structure is non load bearing, wing tips andfairings are obvious exa mples , very often wing ribs aretertiary str ucture on fu ll size aircraft , lead ing edges andtrail ing edges are also terti ary. Damage to te rtiary struc­ture wo uld no t preven t an ai rcraft from flying apa rt fro m

Design ing Model Aircraft

Uniden tified eleva to r banging 011 a w all, 1I0te bow ligbt all tb e m embers are andIigbtening boles ill ribs.

aerodynamic co nsidera­tion s and the possibilitythat the damage mightex te nd to second arystructures .

Keep th e a bove inmind when designingyour mode l. Afte r a ll ,wh y mak e yo ur w in gtips out of rock hard X"shee t whe n some so ft~" with a little X/ th cando th e same job. Yo uwi ll s a ve w e ight a ndmake your mod el muchn ice r to fly w ith lig httips.

Even ligh te r wi ngtips ca n b e made bylaminat ing strips o f X,,"sheet, these are remark­ably strong but still verylig h t. No , they won 'ttak e a full power win gtip lan d ing but if theydid the wing spar mightgo instead , ask yourse lfwhich is easier to repair.

Tai ls ca n be mademuch light er than man y wou ld believe . The Piper Tri­Pacer fin looks so substantial wh en it is covered but itco nsists of a stro ng tub e which acts as hinge post andthe rest is ~" tube and channe l sec tion , it loo ks positivelyflimsy.

To take a mod el exa mple, the wild and woolly co m-

petition fun fly models have tails built up from X" x Mr,"and X" x ~" strip, very ligh t an d tlex ible, they surviveso me very violent aerobatics but they are pron e to nutterbadly.

On our simple q uick built model we may prefer soli dsheet for ease of co nstructio n but we can use soft sheet

Fig 4.1 - Tapering COIISt'"IICtiOIl

E ==Tapered sp a r

Centre s ection s beet

-.-,

II

- /rII

{ o-' --'--+------'-_ _ ---1

Good/

-=--. -- --- - -

Dihedral braces

~--= -Radius 011 gussets

Balsa sidePly doubier

-=. -_""'::: -~--

Balsa trebler

Design ing Model Aircraft 33

T he p a rt s rh at . done e d to be s tro ng arethe e ng ine mo unting ,th e u n d e rcarriagemoun t, the wing attac h­ment areasand thespa~

but eve n th ese can bemad e a bit lighter withso me thou gh t.

The RottenCarrotSyndrome

T here a re ve ry fewmo d el s that aredes ign ed to spread theload s prope rly. We seedoublers th a t e nd in as tra ight lin e just a t th erear of the wing , ce ntresec tio n s h eeti ng th atjo ins the L.E. sheet andT.E. at a right ang le an dso on.

Th is sudden cha ngeof section always resultsin a high stress area an d

tha t is where the mod el will break in a crash. The cor­rect way to design any stru cture is to tap er the change ofsection and to taper the streng th to suit the loads.

and pu nch holes in the surfaces. A strip of harder woodfo r the hinges will also act as a spa r to stiffen th ings upa little.

This 15powered aerobatic model used tbe tapered spar toitb webs mid velJIligbtureigbt structures, note centre section sheet blended to spars and tips. Flewlike a dream.

This Tiger Motb kit fuselage could be improved at tbe [unction bettoeen sbeet and built up section

34 Desig ning Model Ai rcraft

Ultra light structure Oil lVaco PG -2jilselage show­ing curved ends to doubler,

Look at a main spar. The no rma l model uses Yl"sq ua re top and bottom which may be webb ed and the nis sheered over the L.E.

The stre ng th is the same from roo t to tip but the loadis highest at the roo t an d lowest at the tip.

The ideal spar would be laminated from Xl" x Vi", threelamination s over the first th ird , two over th e secondthird and one from there to the tip and the cha nge oflamin ation sho uld be tapered . I have on ly found a sparlike tha t in one kit. Now th is will no t save weight but itputs the streng th and weight near the roo t and keeps thetips ligh t.

Fig 4.1 shows this along with othe r ideas for taperingstrength.

The we bs may be X." near the roo t and }:"nd over thenext th ird w ith no webs at the tip.

The L.E. sheeting can be om itted an d a sma ll sub sparused to help preselve the section or the sheeting can bevery so ft X,," or even reduced to X,nd sheet.

Looking at the fuse lage, any doublers mu st end in along curve or tap er and if there are cut-outs in the fuse­lage such as a cockp it ope ning and wing openings thedoubler mu st ex tend beyond th em. Doublers ca n beth inn er than you may imagine , X,nd ply bonded to Xl"ba lsa is qui te strong enough for any .40 size d enginean d if trebl ers are used right at the fro nt it would beade qua te for .60s.

If yo u ins ist on Vii," doublers yo u ca n cut holes inth e m because the ce ntre of any sheet co mponent isund er far less load tha n the edges .

One of my favo uri te fuse lage sides consis ts of Vi,,"balsa , YJ," pl y doublers and Yl" x Xi' longero ns anduprights a t fo rm er locati ons . I have flo wn .46 fourstrokes in a fuse lage with this type of construc tion, Le.my Turbulen t in RCMW plan s service among others .

Believe it or no t a slightly flexible structure will with­stand crashes better than a roc k so lid one. Even mod ernfull size jets have flexible wings , just look ou t the win­dow and watch them go ing up and down in fligh t andtwist ing wh en reverse thru st is applied .

I have a came ra plane design, the front of the fuse­lage is a very so lid box to pro tec t the camera and radio,the rea r is bu ilt up from strip . In crashes it was found 'that th e rigid box would bre ak but the rear portionwo uld be undamaged .

We don 't wa nt a floppy struc ture but so me "give" isqui te acce p ta b le. Now read thi s aga in a nd d igestbecause now we are goi ng to draw the co ns truc tion onour plan .

Design ing Model Aircraft

.049 powered aerobatic model, velJ' light,flew well,guess where the toing broke ill the end.

MaterialsTo build struc tures you need materia ls. Yes , I know,

pretty obvious but the choice of materials is important.You co uld double the weight of a mod el qu ite eas ilyw ith the wrong choices.

There are th ree main types of material that we use ,balsa wood, plywood and hardwoo d and, by carefulchoices, we ca n bu ild the lightes t and strongest poss ib lestruc ture.

Th is is where the mod el built from a plan scoresover the one bu ilt fro m a kit , the q ua lity contro l is inyour hands and the designer w ho understands his mat e­rials will do the best job of all.

We will look at each type of wood in turn and seehow they can be used to best adva ntage.

BalsaOf a ll woods , bal sa is th e most variable , th is is

because it grows ve ry fast and so a very dr y or part icu ­larly wet season has a mu ch more dram at ic effec t onthe woo d .

You can be prett y ce rtai n that any plank of oak willbe mu ch like any o ther, but a sheet of balsa ca n berock hard on one side and, in the space of two inches,go to very soft.

More often you will find less var iat ion than that inany given sheet but any stack of sheets in the mo de lshop w ill show th at degree of va ria tion be tw een thehardest and softes t sheet, and for hard and so ft you canalso read heavy and light.

This varia tion in hardness is very usefu l, we ca n usehard sheet for he avily load ed areas, medium fo r lessstresses parts and so ft for those that do not need thestreng th. .

Not onl y do sheets vary in hardness, they also varyin stiffness , this is controlled in part by the hard ness butalso by the way it has been cut. Quarter grain sheet isreluc tant to bend across it's w idt h an d so is used forth ings like fuselage sides and w ing ribs .

The softe r, easi ly curled sheet is best for L.E. sheet­ing and turt le decks where it will curve wit hout spli t­ting.

Str ip wood shou ld always be hard , it is used forspa rs, lon gerons and stringers , the first two have to takethe main loads of the mod el while str ingers take han­dlingIoads and soft stringe rs w ill break very eas ilywhe n you p ick the mod el up .

Solid Tail surfaces sho uld always be mad e out of so ft

35

sheet, you ca n't afford the weight of anything heavier atthe tail end . If it is very so ft yo u ca n glue hard stripsround the edge or a hard spa r to tak e the hin ges and itca n pay to let in a stiffener to prevent the sur face warp­ing .

As you design yo ur mod el yo u sho uld look at eachpart and decid e then what gra de of wood yo u will usefor each part , co ns ide r if it will do the job in the light estway.

Think abo ut the tailplan e , this is a large componentfa r ba ck fro m th e C.G . No w assuming th at yo u aregoing to mak e it o ut of sheet, whi ch is lig hter, a %."sheet with hard er edges and stiffene r o r a piece of Yo"

sheet, or wo uld it be better to use hard strip wood ? Itwill depend on various factors such as sha pe and sizeeven the mom ent arm. Never forget , a surp lus ounce atthe ta il needs up to four ounces in the nose to co mpe n­sate .

Ano ther factor that need s to be taken into account isthe grain, we all know that wood splits a long the grainand ye t I have seen part s in kits cut-o ut with the gra inrunning across the nar row dim en sio n.

It may be th e o nly wa y to get the part in and sosaves the ma nufacturer so me mon ey but it w ill cos t youthe mon ey to re pla ce . At least with yo ur own des ignyou ca n ge t it right.

Inside view ofTaylor Monoplane fuselage sbototng W' sq wood and ply co veri ng:Gtrssets are formed by solid blocks glued illto corners ofjoints.

Unidentified bomebuilt fuselage under construction. Square cut-out is for toing,1I0te large gussets spreading load ill bigl» stress area. Tbe frame will be coveredtuttb ply late,'.

36

PlywoodPlyw o o d is mu ch

stronger than balsa butit is a lso mu ch heaviera n d s o mu st b e us edwith care .

T he re a re m a nytypes o f p lywood a ndse ve ral usefu l thickness­es , most have thei r useso n ou r models.

Th e most co mmo nlyused s izes are Yt. ,", y.,",X,,", X" and Y.". The th in­ner s izes are often usedfo r d oublers as th e yten d to be rather flexi­bl e a nd bonding th emto ba lsa mak es a verystron g, stiff laminate .

Doublers a re o fte nmad e out o f X,." ply butth is is thicke r than yourea lly need . y'," p ly lam­inated to hard W' ba lsais q ui te stro ng e no ug hto ho ld a powerfu l .40e n g ine , w hi le 10 ." p lya nd .y,," b a ls a is ade­quate for the nose struc­tu re o f m od el s wi the ngin e s u p to .15 ,increase the balsa to X"for eng ines up to .20.

I o nce sa w a mode lwhi ch used o b ech ive nee r ins tea d of pl yfo r th e doublers . Th egra in was runn in g inthe sa me direct ion asth e b a ls a g ra in a n do bec h i is n ot mu chstro nger than balsa any­way so I formed ara the r lo w o p in io n o fthe design.

Th e thicke r s ize s o fpl y a re used for form­e rs . T he form er o nw hi c h the e ng ine ismou nted must be birch

Designi ng Model Aircraft

Metal bomebuitt, 1I0te oarying size of' tubes, biggest is 0111)' W'. Turtle deck isfibreglass, rest is fabric cooering.

pl y and , if 1'. " pl y isused for th e lar g erengines , this sho uld bemulti pl y with a t lea stfive layers.

Th ere are s e ve raltyp es of plywood , so meo f this is wh at used tob e ca lle d "Do u b leThi ckn e ss Ce n treLamination ", it is ofte nmad e from mah ogan y,it is easy to recogn isebe ca use th e ce n tre isthi ck with th e outsi delayers being very thin . Ifind that it cu ts badl y ,tends to se parate and isn ot reall y strongen ough.

Lit epl y is a mu chbett er material and ca nbe used for some form­e rs . Man y kit s u selite ply for mu ch of theco ns tructio n , includi ngall th e fu selage parts .This is hea vy and hard­er to cut-out and so wewould onl y use it for afew parts.

Formers ca n be cutaway lea v ing ju st a no u tli ne a nd th e a reaswhere o the r part s area ttached, suc h as th eplates for the nuts thathold the win gs on.

l1H" ply is a ve ry use­ful material , it feel s soweak and floppy but inthe righ t place it ca n bebett er than balsa.

My Velie Mon ocoupehas a fusel age built ofWt sq ua re spruce w ithan inn er skin of )(,," plyin the ce ntre area, thiswas pow ered by a CoxQu e en Be e .074 a ndwas full y aeroba tic . Inth e fin al cras h whichdemolished the top por­tion of th e ca bi n, th ewing , buried the eng ineand snappe d the crank­shaft, the basic fusela gewas not broke n.

l-f, ." pl y can also beused for turtl e de cks asit will roll so easily andI have used it to sheet ce ntre sections on wings. I a lsoknow of so me mode ls where th is ply is use to sheet theL.E. of the win g.

Use d carefully, y", " pl y ca n ad d valuab le s tre ng thwith very little we ight pen alty.

Designing Model Aircraft

HardwoodsThere are two main hardw ood s so ld for mod el co n­

struc tion . Beech for engi ne bea rers and undercarriagemountings , and sp ruce w hich is used for spa rs a ndstringers.

37

One offour bolts bolding doum a Cessna fln; pound coin gives scale. Note rivetsholding bracket to fill. Tbis is primal)' structure.

Few designs use hard wood bearers these days, thenylon engine mount is so much easier to use .

The hardwood bearer had one big advantage , it tiedthe front of the mod el together as it ran through at leasttwo formers. Th e disad vantages were that they mad epositioning a large fuel tan k difficult and they we re verydifficult to replace' if the y go t broken in a cras h.

Both these disadvantages are overcome if the bearersare glue d to the fuselage sides and the eng ine is mount­ed on a Paxolin plate , wh ich will snap in a crash pro­tectin g the bearers and the engine . Being sprea d apa rt,fitting a tank between the bea rers is easy.

Spruce is often used for spars and stringers and canbe used fo r longerons. Th e we ig h t penalty is lowbecause it is used to repl ace hard balsa and one can usea sma ller sec tion.

Spruce is idea l for sca le mod els whe re the frameworkis visible and the members wo uld be over sca le if ba lsawas used , this applies especially to stringe rs.

Referring again to my Velie Monocoupe , the rear por­tion of the fusela ge was mad e from W' sq uare sprucelon geron s with ~" square ba lsa cross members, the crashdid not damage that part of the mod el either.

An incredi bly strong fuselage can be built with spruceif yo u gusset ea ch joint wi th y,;," ply . I ac tually built afuselage fram e with hard 1(6" balsa and gusseted eac hjoint. I decided to scrap the fuselage and I used it like aclub against a metal cha ir and the bench, holding it bythe tail end an d smashing it down . I had to hit it aga ins tthe ben ch an d the cha ir as hard as I could several timesbefore it even started to break up .

Strength and Lightness in Full SizeAircraft

I have menti on ed using full size class ification of struc­tu re but let 's look at so me ex amples of full size con­struc tio n. You will be surp rised as jus t how lightly builtmost aircraft are .

38

Take the ave rage allwood home-built. Herewe ha ve a n a irc raftw h ic h is built of aframe wo rk of ro" squa reand covered with ~" ply.The re a re o ne o r tw oth at use eve n thinn erplywood.

Lo ok a t th e s ize o fthe tube on a metal air­frame with fabric cover­ing, 'X" tube is la rgestand most are eve n thin­ne r. Even e ngi nemounts use quite sma lld iame ter tub ing carefullyarranged in triangles.

Wh en yo u lo o k a tco ntrol surfaces you cansee th at th e y a re builtup fr om a fe w thinpi eces o f wood a nd aspar.

Where parts are bolt-e d toge th er th e b olts

a re often s ma lle r th an o ne mi gh t expect. To runthrough so me examples . Th e Taylor Monoplan e tail­pane is held on by four W' bolts th rough a co uple ofsma ll metal brackets. Th e fin o f a Cess na 152 is helddown with four X" bolts th rou gh four small brackets.

Pe rhaps an aero batic aircra ft w ill have mor e sub­stantia l bolts. No. The To p win g of the Steen Skybo lt, ave ry aerobat ic machi ne , is held o n by two );,." bolts butof co urse the flying wi res tak e so me of the loads.

Stru t b raced wings o n a ircra ft lik e Cessnas a ndAusters are held to th e fusel age by two sma ll boltsth rou gh brackets , the stru ts tak e a lot of the lift but notall.

The Application ofModelsModel s do suffer stresses th at full s ized a irc raft

co uld not survive. We pull "G"s th at would have apilot squas he d fla t on the cockpit floor and of co urseman y, if not mo st , of o ur landings should be calledco ntro lled crashes, but we ca n still mak e ou r models abit lighter.

Even allowing for these high er load s we co uld usesma ller s izes of mat eri als that we normally do .

My Monocoupe 110 Specia l has the wings held onwith four 4mm nylon bolt s , I co uld have used 8 BAstee l and still have been co mplete ly safe. I had a .40powered co ntro l line Pitts specia l once, and that usedtwo 6 BA bo lts to hold the top wi ng on.

Tak e th e Tay lor Monoplane , a favourit e sub jec tbecau se I spe nt a lot of time doin g the sca le draw ings ofthe full size aircraft. A Y. sca le version wo uld only be 63"spa n and the fuselage fram e co uld be %, sq ua re hardbalsa an d W,," ply or X6" hard balsa skin for a velY stro ngscale structure, such a model wo uld be quit e happy on.40.

By th inn ing abo ut weight and stre ng th, usin g mate­rials to the best adva ntage it is possible to buil d a lightmod el that will withstand appa lling punishme nt andonly need min or repairs.

Design ing Model Aircraft

Chapter .S

FuselagesM

any people draw their plans with single linesto represe nt pa rts, for exa mple just the front ofthe spa r and the rear of the T.E. They also use

single lines for the for mers and fuselage sides.This is bad policy in my view for two reason s. The

most immediate reason is that you have to draw all theparts using previously drawn items. You need the fuse­lage side and plan views to draw the formers. It is alltoo easy to ge t mixed up wh en taking these measure­ments if you only have one line .

Imagine that you are only using a single line for eachformer. The fron t former would have to be represen tedby the front face because that is where the engine sits,but the next one back wo uld have to be shown by therear face which is wh ere the wing L.E. locates and theformer at the T.E. would aga in have to be shown by thefron t face . . . See how easy it would be to ge t confused ?

Tbe majority ofmodels u se tbe basic boxfuselage.. . this is Miss 5 7/"0111 tbe R CMlV Pians seruice . . .

Fig 5.1 -Collstrllctioll Layout

I ' I

r\\ ~ uI 1 "

' I I , I , ,

"ift l

11

11,I I

10 r I, I I

"" ~

,~ L _ I

,.' "

I .L- - - - ---.- -" 11 I I erI

11

I ~ 11 'I I

i/It :1

.-/1 .' 1 Il: .. : ,

\- I ...J.

! I

optiollal sub spar

I'11,.

r l - ---=,.......,'='-~­I.

- "f!!I"- --,"-,-- ---;.;.

"

!!

I' , / II I

11

" DOllbler',--I ~.... ....

Trebler,

,'1;;' <,

r' - ~ I Ii-------==-~ .

---,--"

I li' ;1

- i- -'l-- i(=

RIC Omitted/or clarity

.-~-----n=-=,..-=--==~""-=-=~:::::::::----l

Unsbeeted L.E,

-i --ttr>~~~~~=n==~Note:Cockpit opening over tuing cu t-o u t is the weak point, exte nd dOll,blers beyond area.

Design ing Model A ircraf t39

... some just leave it like that . . . Blue Movie 2 from tbe RCMlV Plans Seruice, tbere is not mucb else youcall do witb Correx . ..

The second rea son is that yo u just might want tooffer yo ur plan to a magazine and they will need morethan a few thin lines. A typ ical plan is shown in thedrawing Fig 5.

FuselageIn th is part I sha ll tell yo u how to lay out yo ur

drawing but I won 't cover everyth ing in great detailbe cau se anyone designing a model sho uld ha ve builtone o r two kits and so will have a little ex pe rience .For exa mple, you sho uldn't need me to tell you thatth e e ng ine mount ing former mu st be pl ywo od andmu st be firml y attache d to the side s. Design er s use

. . . some dress it up II bit . . . Triuial Pursuit sbotosthat angular turtle deck call look smart . . .

40

many standard techniqu es so , if so me thing is not cov­ered in detail , use a method that you have seen else ­wh ere .

The sid e view will sho w any doubler s and the form­ers. Ind icate the dou bler , not forgetting to make it endin a curved line aw ay from any othe r changes in sec­tion (Yo u did read the last chapte r, didn 't you).

Form ers come at logical points , the engine mount­ing bul khead, at the L.E. and T.E. of the wing and atthe L.E. of the tail plan e . You will need at least onebetween the wing and the tail and the turtle deck mayneed a co uple of ex tra formers.

Drop lines to the plan view and draw the doublerand formers in to this . If you are feelin g lazy yo u ca nindi cate the th ickness of the sides and doublers in theplan view at the former locat ions only.

Now to plot the formers. Draw a lon g datum lineand drop verticals from th is, also ex tend the ve rtica lsupwards . It is at this po int that sq ua re protract o rco mes into its own.

Now, usin g a pai r of divid ers tak e the distan ce fromth e datum on th e s ide v ie w to th e bottom at th eeng ine fo rme r and tran sfer thi s to th e first former,repeat with the top d imen sion and then using the planview tak e the width fro m the ce ntre line to the sideand tran sfer this to each side of the former drawing.

Using yo ur square protractor plot o ut th e sq ua resha pe of th e ba sic former. Th e top portion will beplott ed later. Repeat for all the othe r formers.

The re are var iou s styles o f turtl e de ck , yo u canleave it flat (Ug h l), yo u ca n mak e it ro u nded or asharp ang led deck .

The sha rp angled top is easier to plot as you canuse fixed points o n each former. The rounded turtl e

Designing Model Aircraft

WItr1---- JLi-__tJ2 _lIT 1 1 l1i72

1111

~I

Advanced FuselagesThe design of your average sports mod el is pretty

basic and there are limits to what one ca n do with aplank wing and modified box fuse lage with dural U/C.

Add round fuse lages and tapered wings to the per­mut ations, th row in a variety of undercarri age typesand yo u can pro duce some rea lly differen t look ingmod els.

You may feel that such co mplications are not worththe effo rt and for sports mod els this could be true , butwhe n it co mes to sca le mod els some of the nicest sub­jects are more co mplex.

The aerodyna mics rem ain the sa me , the probl em sco me in design ing the structure so that it is still lightand relatively easy to build.

I 1Vl

~il'

- HT2

,,., , -

I11

I11

HS2

I r I ,

lIT

- ---+= -

Fig 5.2 - Former P lotting

I

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d e ck lo o ks b e st a n dca n be pl otted usingth e he ight and widtha nd drawing it withcom passes . A p icturebe ing wo rt h a th ou ­sa n d words yo u w il lfind two d raw in gss howi ng th e methodelsewhere in this cha p­ter. Fig 2.

The meth od sho w nfor pl otting roundto pped fo rmers workswe ll , but so me timesyou may want a differ­ent sha pe to the turtledeck and yo u will findthat interm edi ate form ­e rs ca nno t be plottedeasily. The simpl e wa yis to lea ve them over­si ze and s a n d downwith th e aid o f astraig ht edge until theyblend in.

Curve d turtle deck sw ill ne ed to b eplanked a nd thi smethod of co nstruc tionallows you to use for­me rs th at vary qu ited ramaticall y. Th is ca nma ke for very attractivesha pes w hic h are noto ften seen these days.

When drawin g form­ers out don 't forget toallow for the th ickn esso f th e s ides and top ,and remember that thedoublers do not ex tendto the tail , all these aregood reason s for draw­ing the model in full.

Oth e r it ems thathav e to b e added toth e fu se lage d rawingwould be wing boltpl at e s , a ny rei nfo rce-ment for attach ing theundercarriage , perhaps a sheet inse rt for gluing downthe tailplan e and a ply mount for the tail wheel andsk id .

Draw all th ese parts in on bo th drawings. Veryoften you will spo t possibl e problem s w hile doing thedrawin g which is a lot better tha n finding them whe nco ns truction is well under way.

For o ur in itial design we ca n use a very simpleundercarriage , the be nt dural U/C bo lted to the bott omof the fuse lage so incorporate a ply plate jus t in frontof the wing an d use triangular stoc k to reinfor ce thejoin ts bet w een th e pl at e a nd th e s ides a nd a lsobetween the eng ine mounting form er and side s.

This wh ole area is primary structure as defined inCha pter 4.

Design ing Model Airc raft 41

Internal view ofrearfuselage ofall Auster undergoing restoration. Tbln outlineply formers support stringers, nearestformer is coaming tobicb takes cabingtaxing: Mai"framework is steel tube.

Rounded FuselagesIf you study 3 views and photos of full size aircraft

you will see that a ll fuselages are variations on a fewmethods.

The slab sid ed fuselage has been dealt with but afew , like the Piper Cub have a stringer or two down theside wh ich just gives a little more shap e.

More rotund fabr ic covered fuselages will be the sim­ple box with sub form ers and stringers. For this typ e offuselage it is well worth building the box up from striplike a vintage mod el, as the stringers will add strengthand yet the whole thin g will be no heavier than a plainbox. Fig 3.

The next type is the metal cove red fuselage and herecareful analysis can pay off. l et 's take two examples , theB-17 Flying Fortress and the Ryan STA or PT se ries.

The first C/ l 3-17 that I built was plunked . a verylong and ted ious task but then I found better drawingsand studied mor e ph oto s and I realised that the fuselag eis a cylinde r and a cone, the o nly double curvature wasat the no se . You ca n do the whole thing with rolledsheet. This is wh at I did with my second 3-1 7 and thenose sheeting was slit and curve d and pulled in just likeshort planks.

l ook at the Ryan STA and you will see that it is thesame , straight lines whi ch are very easy with balsa.

It is possible to get so me double curvature into balsasheet. The method is quite simple . You need a fram ework of formers with substantial stringe rs about 3" apa rt.

Take soft balsa sheet and soak it in water for a whil e,if you add a little ammonia the soa king time is reducedto about 15 minut es.

Now take the sheet and carefully bind it to the frame­work with bandage and lea ve it to dry. Two oppos itepan els can be don e at a time . On ce the panels havedried the y can be trimmed to fit and glued into place .

\X'h en building a fuselage like the B-17 it would be awaste of weight to have much internal struc ture , as theskin and Y; sq. longerons would be qu ite strong eno ugh,

but some sort of jig isne ed ed to bu ild th efuselage.

The simple answer isto buy a pie ce of woodabout X" (I umm) by 3"or 4" a nd cu t s lo ts totak e the form ers whi chca n b e lined up andsp o t g lue d in pla ce .O nce most of the fuse­lage is built it ca n becrac ke d away from thejig. Fig 4.

Thi s typ e o f jig ca nbe used on oth er mod­e ls, th e Hirth Acrostarhas a fuselage that goesfrom slab side d to roundo ver a ve ry sho rt di s­tance and the only wa yto build it is with the jigdescribed .

On e o f th e mostbeautiful se ries o f a ir­craft were the LockheeclVeg a, Sirius , Altai r, e tc.

Stringers

IStrip frame

_0 ~ ,

Rounded box

'- ­- ....- ---

Fig 5.3 -i Rounded so«

Sbeeted

42 Designing Model Aircraft

. . . toitb stringers Oil tbe s ides oftbe box . . . like Tequila Sunrise . . .

Th is range o f a irc ra ftused. o ne monocoquepl y ". fu se lage s he llformed in a co nc re temou ld a nd a s tandardwi ng ;with slight varia­tions on dihedra l.

Fro m th ese co mpo­nents Lockhee d b u iltthe p a raso l w in g AirExp re ss , the sho ulderw ing .Vega , th e lo wwi ng Altai r, Sirius andO rio n, so me w ith andsome without retractingundercarriages.

These aircra ft we rev ery fa mo us , Wi nnieMae for exa mple hel dthe record for a roundthe wo rld flight.

I have o fte n w on­dered w hy so me kitmanufactu rer has notprod uced a series ofmod e ls of these a ir­c ra ft. Even th ou gh Id isli ke fibregl ass a ndfo am I ca n see th eadv antages he re . O nemain GRP moulding and a few small ones , one basicfoam wing and a varie ty of sheet tail surfaces and hewo uld have five different kits.

From our point of v iew the Lockheed se ries wo uldneed to have the fuselages planked , whic h is the rea ­son that I have never built a mod el of one of these air­craft.

Meamobile, Back at the DrauringBoard

The average sports mod el has a hard life (and a shortone sometimes) , and so we may not want to go to suchlengths as describ ed above but we can use the tech­niqu es.

. . . tobile others bide tbe fact comp lete ly . . . Pee Tee Sportster is built as sboum onfigure 53

Designing Model Aircraf t 43

This Auster has velJ' box)' fuselage but three stringers add a little shape.

. . . but a feto do not use tbe box at all: B-17 built asshow" in figure 5.4 .. .

44

. .. a feto use a box at tbe front . . . like tbe HirtbAcrostar.

Designing Model Aircraft

Typical former~

You call make some really attractive s h apes

The slab side d sports mod el can be transform ed byadding stringe rs dow n each side, instant round fuselage .

A nice dou ble curvature turtle deck can be planked(NO! The man who said hollow block, it's heavy an dve ry exp ensive).

Fig 5.4 - Simple Fuselage fig

Side view ofjig

B-17Jig, only two Iongerons sboum

Designi ng Model Aircraft

Combinations of all the methods can be used to pro­du ce some really beautiful fuselage shapes . The ph otosand drawings will give you some ideas. Just remember therules, kee p it light, no sudden change of sec tion, spreadany load and tert iary structure does not carry any loads.

fotn skill Oil longerons

I

I : II

J ig sboum dotted '-_!

45

Chapter 6 '

Wings and TailsWings

T he w ings are cons idera b ly easie r to d raw ou t, thecross-sec tion is already deci de d by the airfoil section soit is just a matter of decid ing on the size and position ofspars etc.

On a mo de l the norm al locat ion for the main spar isa t the th ickest point in the section , aro und Y.rd backfro m the L.E. O n large r wings yo u may wa nt to putano ther spar at Y.rds chord .

We have talked abo ut tap erin g the spars and whil ethis is the ideal I predi ct that few peopl e will both er ,preferrin g the standard type of spar, Le. X" square topand bottom possibly with we bs.

Leading edge sheet does add strength and provides asmooth surface over the most important section of thewing but it is not essential on smaller and lighter mod elssuch as powered glide rs. Nose ribs between the full ribsor smaller sub spa rs can give equa lly good result s andare cheaper and lighter.

For the more highly loaded and stresse d mod els the'0' box is much stro nger and will prevent wa rps an d

The simplest uiing ofall, parallel chord, 110 aileronsand 110 sheet, suitable for velJI small light models.This is American Dragon forB diesels but has beenscaled up to 60" span for .40follr stroke.

Fig 6.1 - lVillg Sketch Page

Front view

::

POORSplits here

c.Common with or rottbout sheet

Good with or tottbout sheet

c =======Bestfor sbeeted L.E.s

c:Light models, small aerobatic, gliders

46 Design ing Model Aircraf t

Ellip tical tips O il p a rallel cbord look nice, inset ailerons are velJ' effective Oil

tbis Mo nocoupe 110.

12°Washout

11:===4

Standard tapered tuing

)

Dih edral is used to p rovide lateral sta b ility , o n amodel with no aileron s it is esse ntial but not on others.

A mod el with no aile ron s needs abo ut 5° or 6° oneac h panel, this will give reason able rudder control, lessd ihedra l will reduce the efficiency of the rudder.

tap er the braces or the win g will snap wh ere they end.Strip ailerons are normally solid, quit e often ready

sha ped T.E. stock can be used but do not compromisethe airfoil section by using the wrong size of wood , bet­ter by t~I1' to shap e your own stoc k from thicker wood .

Tips must be kept light , rem ember inertia? A co upleo f suggested meth od s are shown in the drawings.

Dihedral

---

-

lSpar" --

Fig 6.2 - lVillg Shapes

distortion of the wing .Be warned th at it isa lmos t imposs ib le torem ove a built in warpfrom this type of wing.

Rib spa cing willdepend on the typ e ofmode l , th e size a ndloads. Closer ribs at thece n tre espe c ia lly if awing mounted und ercar­riage is used and widerspacing out beyond thecentre sec tion shee ting.

All sheeting shou ldfoll ow the rul es abouttaper ing off the strength,large radii at all cornersor ce ntre section sheettap ered into the L.E. andT.E . over two rib bayswill prevent that sudde nchange o f section th atsnaps so easily.

Trailing edges can be made with a strip of W' sq . andtwo pieces of XI," shee t or of more substa ntial strip with­out the sheet. The T.E. does have to take the loads ofthe aileron hinges and so must be strong enough for thetask.

Leading edges do not need to be very strong . Onevery common type of L.E. is Y, sq se t on one corne r, thisis not good because a slight tap will split the ribs. A bet­ter L.E. is shee t glued flat to the front of the ribs.

My favourite L.E. is a variation of the above whe nused with L.E. sheeting. The L.E. is ~" sheet, it can bequit e soft, this is glued to the ribs and sande d to blendin perfec tly. The sheet is glued on and trimmed backflush with the front of the L.E. and then a caps trip of ~"

sheet is glu ed o n the front and sanded to the co rrec tsectio n . Thi s sys te m isvery strong and ve ryea sy to make andalwa ys looks ve ry neat.

J oining tw o w ingscan be done in variousways . Som e Ame rica nkits ha ve used g lasstape and resin just likejoi ning foam wings, itworks if you don 't mindthe mess and e ffo rt ofge tting the joint smooth.

Small models can justhav e th e w ings buttjoin ed and rely on goodglue joints and the cov­ering materi al, but thisis o nly for mod els withengines up to .09.

Th e normal methodis to use dihedral braceso n th e spars a nd L.E.and T.E . Th is syst emhas worked for as longas models have beenbuilt , just rem ember to

Designing Model Aircraft 47

Tapered all sheet couered uiing does not needspars 0" smaller sizes, tbts is Sbrike Commander:

Even more dihedral is needed for a low wing modelwi tho ut ailero ns , up to 10° which looks pret ty ridiculousw hich is why there are so few low wing mode ls witho utailero ns .

With ailerons di he dra l is not essentia l but a slightamo unt will make the model slightly more stable and soeasier to fly. As the dih edral is increased the ailero nsbecome less effective , for normal use about 1° or 2° isabout right.

A low wing mod el with no dihedral will fly very nice­ly and need not be very unstable, altho ugh it canno t beleft to itse lf for very lon g, however this type of mod el isnormally used for aerobatics and so this is not a prob­lem

Wings with no dihedra l do look as though they aredroo ping and so it may be desirable to use a tou ch ofdihed ral for cos metic reasons. On a tapered wing it isenough to make the top of the wing flat with all thedihed ral on the bo ttom of the wing .

Tapered WingsOne of the easiest mod el struc tures to design and

bu ild is the parallel chord wing with strip ailero ns, how­ever, the tapered Wing is a bit more co mp lex becauseeve ry rib is different and the dreaded tip stall is morelikely to occ ur.

Of course if you really wa nt to make life hard youcan go for a fully elliptical wing and if you are a certifi­ab le sado- masochist you could design a wing like CarlGo ldberg's famo us (I'd say infamous) Valkyrie , whichhas each rib buil t up from st rip on that vas t elliptica lwing. My only comment is , if yo u are read y for that ,what the heck are you do ing wasting your time readingthis?

The Straight Tapered WingThe only ex tra work in designing a straigh t tapered

wing is that all the ribs are differen t.The spars can remain in the same relative location on

the rib and this will mean that the L.E. sheeting will betap ered wh ich, as we have seen, is a good th ing.

The actual taper can be on the L.E., the T.E. or both,it is much a matter of taste but it is a good idea to keepthe main spar straight at least on yo ur ea rly designs asthis makes joining the spar easier and also provides ada tum to work to .

Ta pered wings are more prone to the dreaded tipstall and the grea ter the tap er ratio the worse this ca nge t, so for your spo rts mode l it would pay not to go toowild, a tip of Y. the roo t chord looks right an d is notexcessive .

Tapered wings will ben efit fro m wa sh out, a slig httwis t so that the tip is at a lower ang le of incidence thanthe roo t, one or two degrees is fine.

A use ful tip, X,," at about 3W' eq ua ls 1 degree , so if wetake a roo t chord of 10" and a tip cho rd of 7", and wewa nt two degrees of was hou t we divide 7 by 3.5 = 2 so

Homebuilt biplane top using structure, note ve,,' ligbt construction ofribs and beauier compressionmember betuieen spars. Ribs are tertiary structure.

48 Designing Model Aircraft

Laminated strips make strong, Iigbt tips, 1I0te sbeetL.E. blended ill.

Sbeet of tapered uiing ribs from tbe AuiationBookshop, one 0.1a series uiblcb are velJ' useful totbe designer.

o ne degree would equa l Yx and two would be Y; so theT.E. would be rai sed by ~ " a t th e tip to give yo u 2degrees wash out.

Th ese are 'Rules of Thumb' and ca n be broken, forexa mple, the Great Plan es Patr iot kit design has a rootchord of 15" and a tip of 7" and the whole w ing is swe pt,it only uses Y, a de gre e of was ho ut and , so long as theCG is in the specified rang e , tip stalling is not a p roblem.

Ribs for Tapered WingsObviou sly every rib is different on a tapered wi ng

a nd so each rib ha s to be drawn separ at el y a nd beex actly the right size and shape so that the w ing tap er isperfect. Som e w ings on scale models have the adde dco mplex ity that the sec tion cha nges from root to tip .

Th ere are a number o f methods fo r producing ad rawing of a set or win g ribs, for the most part they allhave d isadvantages , they may be ex pe ns ive , time co n­suming or limited by the ran ge of sections avail abl e .

Th e qui ckest and best way is to use a co mputer pro­gramme, this will draw the ribs with all the spa rs a ndshe et, it will ch an ge the section fro m root to tip and it is .unlimited in range because yo u ca n feed in the coord i­nates o f an y sectio n . It will cost yo u quite a lot o fmoney for the programme and even more if you have tobuy a co mputer , but it is worth it if you already ha ve a

Designing Model A i rc ra f t

co mpute r. Th ere are peopl e who can do the job for yo uat rea sonabl e cost.

Th e next method is to use a full zoom ph otocopi er ,just take yo ur o rigina l drawing and enlarge or reduce itby the co rrect factor for each rib , thi s method w ill notallow yo u to change sectio ns a long the w ing . It will alsocost lOp per rib and yo u will have to add the spa rs etcafterwards.

The above method can be used to good advantage ifyo u ca n ge t hold of Martyn Pressnell 's bo ok , Aerofoil sFor Aeromodellers w hich wa s publish ed by Pitm an . Thisbook has large nu mb ers o f aerofoils drawn in differents izes a n d so me are al so drawn w ith th e )'I,, " s ki nremoved. Most are intended for gliders and F/F powerbut th ere ar e several useful sectio ns fo r R/ C powe r.Th ese do reduce the number o f pho tocopies that yo uwill have to ha ve done . Thi s boo k is o ut o f p rint butyo u may be able 'to find co pies aro und .

Anothe r source of tapered ribs is a range of sheetsava ilab le from the Aviation Bookshop. These are the o ldAeromodeller sheets and cover quite a wide range ofsectio ns in useful sizes for the sma ller model but again ,these are mo re suited to glide rs and vintage type mod elsa ltho ugh NACA 0018 is a useful sectio n for aerobaticmodels.

One method of drawing ribs that I have read about ina 1939 book was to take a ply outline of a rib and along piece of wood, you fit a torch bulb at the top and

Taylor Monoplane aileron is top hinged and basmassive differential.

Inset ailerons are more complicated to build butare essential on scale models. Bxtra gusseting atcorners would be all improvement.

49

Fig 6.3 - Inset Ailerons

~;/\\\,/ :-\ '. I

. If\ \i ,.\ '., '

Spar mayco ver aileron

only or

Ply bellcranh mount

Pusbrod

H inge

~ Without~ gussets

Full span

Top hinged

slide the outline up and down until you get the correctsize of shadow and then draw round it. I haven 't tried itbut it might work.

There are ways of drawing the req uired ribs, one is toplot the ordinates for each rib and then draw it out . ..This way lies insanity . . . then there is ano the r meth odwhereby yo u draw the tip rib and root rib and thenabout seven thousand lines (we ll, an awful lot anyway)

Aileron connection 011 all Isaacs Fury set-up forlots ofdifferential; just like a servo arm:

50

and co nnect them up an d draw each rib out. It doeswork, it takes a very long time an d you wo uld be betteroff buying a comp uter or having a foam wing cut.

Elliptical WingsElliptica l w ings can be treated in the same way as

tapered wings bu t the shee ting is much harder becauseit is dou ble curve d .

A straight wing with elliptical tips is one so lution an dthere are Spitfire models with a straight tapered wingwith ellip tica l tips and curved strip ailerons, they arequite convincing in the air. .

Nice, light elliptical tips can be made by laminatingstrips of wood over a pattern, this is ideal for scale andse mi sca le mod els. See the picture of my Monocoupewing.

Elliptical wings do tend to increase the likelihood of

Tiger Moth aileron pusbrod sbotoing connectionfor differential aileron ... just like a seruo disc.

Designing Model Aircraft

tip stalling and so washout is essential, eve n the full sizeSpitfire had washout.

AileronsThe strip aileron is the mos t co mmon type on models

because it is very easy to make , ex treme ly simple to

Fig 6.4 - Linkages for Differen tial Ailerons

connec t to the servo and ge ne rally make s life easy forthe designer. It also makes life easy for the manufacturerof foam wings but it is not the most efficient typ e ofaileron.

Inset ailerons are working out at the tip wh ere theycan apply most leverage , the portion close to the fuse­lage is doing very little work.

Note: Top d rauiing ill each pair iss ta nda rd, lower dratoing shows differen tial

x

Strip Ailerons

Seruo disc

Note: all high uiing models connection ismovedto X

A B

Ins et Aile rons

Bellcrank 90°

Bellcrank 45°

1

j/~I~+7c::============================~JU1 1 1

11_1""""'-----.111 1.....------...1

Designing Model Aircraft 5 1

Tail SurfaceFor in it ia l desi gn

exe rcises the shee t tail isthe best ')Tay to go butyou can build them up

,.',' ~ ... .~ .:, .... \ .:.

they do add some stressareas in the wing, how­ever, they are essentia lfor scale models. I knowthat there is a so calledscale Piper Cub kit thatuses s trip ailero ns butyou are not going to sinkas low as that when youdesign a scale model areyou?

Th e main things torem ember whe n design­ing inset ailerons is thatit s ho u ld be 1\1," to 2times the width of a stripa ile ro n a nd th at yo us ho u ld use gusse ts tosp re ad th e lo ads andrem ove the stress are asin the corners.

Fig 6.3 sho ws a typi­ca l in set a ile ro n . Th eaileron bellcrank can bereversed a nd th e horn

can be fitted on top of the aileron if desired . Note that thebe llcranks point in oppos ite direction s in eac h wing sothat one is pulling as the other pushes.

It is desirabl e but not essential to have differentialaileron movement , this means that the up going aileronshould move up farther than the down going one . This isbeca use the down go ing ailero n crea tes more lift, morelift equals more drag, more drag tries to pull the aircrafttowards the down aileron which is the exac t opposite ofwhat we want.

By mak in g th e up going a ileron mo ve fu rthe rwe increase th e d rag so th at it is equa l to or mo re

tha n the do wn aileron.Th e Ta ylor mono­plan e hom ebuilt , speci­fies aileron travel as "250

up and almos t no downmovem ent. "

Thi s differ ential ha sanother adv ant age . Thedown go ing aileron ineffect increases the angleof attack of the wing inthat area , at slow speedsthis can initiate a tip stallo n th at side, reducingthe throw with differen­tial helps to delay the tipstall. Fig 6.4 shows howto set up the aileron bellc ra nk for d ifferential ,Chart Micro Maid makean adjusta ble bellcrankwhich is useful for this.

Another snag with strip ailerons is that they can twist,this will reduce the angle out at the tip where you needthe deflection most. You can use much harder woodwhich will resist this twisting but that is needless we ightjust where we do n't want it.

One way to overcome this is to connect the controls tothe centre of the aileron by means of a system of bell­cranks and pushrods or, alternatively by using a servo outin each wing. This allows the aileron to be used as flapsvia a mixer, probably the only aerodynamically valid rea­son for using strip ailerons.

Inset ailerons are much more complicated to make and

Tail assembly uslng built up and core methods. Core systems is vel)' goodforscale control surfaces.

Tail toith laminated outline is vel)' strong and light, tbis one is Triuial PursuitforLBcc engines, spars could be addedforlargermodels.

52 Designing Model Aircraft

Fig 6,5 - Tailplanes

with something like X" x )1," outlines and ~" x X" strip ribs.Used medium shee t for the solid tails and hard stock

for built up tailplanes.Of course the draw ing for solid shee t tails is done as

soo n as the outlines have been drawn and I hop e that Idon 't have to describe how to draw the bu ilt up system.

It is rare for a model to finish up nose heavy and soanything that can be done to keep the tail light will help,don 't forget that an ounce of surplus weight at the backend will need three to four ounces up the front, this isuseless weight which we need like a hole in the fuel tank .

Tail surfaces can be built up , there are three commonmeth od s. Ordinary straight strips of wood with ribs asshown in drawing 6.4 of this chapter.

More curvaceous shapes can use the same method butthe outline can be done with laminations as described forwing tips.

Another method which is very goo d and popul ar is thecore method , a thin shee t core is cut to the shape of thetail and then ribs and spars etc are glued on each side ,when covered it looks just like a fabric covered tail but isve ry s tro ng and alsolight. Co mb in ing thi smethod with the lami ­nated outline mak es awonderfully strong lightstructure.

Weight can be savedover a solid tail bybuild ing th e structurefrom strip and coveri ngit with Yl,nd balsa, thismakes what is, in effect,a bo x struc tu re withwebs which is incredibl ystiff for its weight, butyou must use soft woodthroughout.

One method that wasvery popul ar some yearsago was to build an out­line with diagonal brac­ing (\'(farren Girder) andthen put a spar top andbottom and fill in th eribs with small triangulars ha ped pi eces . Thismethod was used on theaerobatic models of theday and is light andstrong. The tail needs tobe stiff. enough to pre­ve n t flu tte r. Th e topCo mpetitio n Fu n Flymodels use inc re di b lylight tail sur faces w ithvery large co ntro l sur­faces bu t these mod elsare not designed to beflow n fast and mostwa rn th e builde r th atflying straight and levelat full throttle will causethe model to flutter andself destruct. I have see nit happen seve ral times.

Designing Model Aircraft

Auster tail assembly. Note bowfabric on fin is usedto make a fatring to fuselage, typical ofmost fabriccovered aircraft.

Ltgbttoetgbt, could be covered tuitb'l$z sheet

Sheet core

c --W®F

Vintage aerobatic e.g. Astro Hog

53

Chapter 7

Undercarriages andOther BitsT he main parts of the model are the fuselage and

the wing but the design of the undercarriage andits mounting is of major importance, this area is

prim ary struc ture.The cowling is al so important be cause a badl y

desi gn ed co wling can ca use the engine to ove rhe atwhich will dama ge the engine and may cause a crash.

UndercarriageTh e undercarriage is only used for a short time at

take-off or landing, however , it is a vital piece of equip­ment. It sto ps the propeller being worn down and thebelly of your model being scra pe d along the ground soit mu st work properly and not co llapse.

To work we ll the wheels must be in the proper placeor tak e-offs will be diff icult or downright impossibl e.The undercarriage must also be of the right tJpe.

Trying to use a tricycle unde rcarriage on anythingothe r than sho rt grass or tarm ac is a dead loss, the lon ggrass will hold the no se down and even fold the noseleg back eno ugh to allow the prop to ge t into the grass,slowing the engine . If the main wheels are too far backnot eve n the most powerful elevators will get the noseup . . . No take-off.

The main wheels on a trike should be just behind theCG, so close that if the mod el is rocked back ont o its tailit will stay there . In this wa y the elevator will be able tolift the nose and allow the mod el to take-off.

The nose leg takes an awful pounding so it must be

Simple dural vie can be held 0" with elastic bands 01' •••

54 Designing Model Aircraft

Fig 7.1 -VIC location

Tricycle VIC

\\\\

\

CG behind wheels uib en model is tipped back

"Tail dragger" VICJ

IN Effect of'foruiard VIC uiben model stoing

CR = Centre ofdrag ofwheels

IN = Inertia acting through CG

Note: Largermoment ofleverage 011

foruiard VICincreasing suiing

Design ing Model Aircraft 55

... bolted 011 toitb 11)'1011 bolts. (Numberplate bolts from Halfords are good).

Extra cosmetic struts dress up a normal single leg wire Ur'C:

strong eno ugh and it must be mounted firmly, no t on lytha t but the form er must be solidly attached to the fuse­lage.

There is a ce rtain kit around whi ch has a trike U/C, italso has a tank hatch on top of the nose . I have seensevera l o f these kits and in quite ge ntle landings theleve rage of the nose leg has pulled the front former out,becau se the former was only held at the sides and notacross the top.

The owner of o ne of these mo dels drill ed two W'holes in the top of the front forme r and 1'-2 and epoxiedhardwood dowels between the form ers to try and tie thefront former in place. It worked.

Tail dragger U/Cs need to be position ed with care .The idea l locatio n is just under the L.E. of the wing .Slightly furth er forward is better on grass but do not betempted to put it too farforwa rd for th a t is thewa y to gro und loops.

M o u n tin g th e U/Cca n be o n the fuselageo r the wing. For sportsmod els / prefer fuselagemo u nt ing beca use it isligh te r, it can be mount­ed o n the ply form er o rbo lted to a sma ll p latein t he botto m of th efuse lage .

The w ing mou nt edU/C needs ha rd w o o dblocks and ply doublerso n the ribs w hich a reheavier but it can lookbe tter a nd d o es all owfor torsi on s pri ng ing .On low Wing trikes it isthe o nly o ption in mostcases.

T he u nd ercarria getakes trem endous loads,just try twisting a Dura lo r 10 SWG u ndercar-

56

riage leg back into shape and see the effort that has tobe used . A lot of the landin g loads have been absorbe dby that tw isting but much of the load has been transmit­ted into th e fuse lage o r wi ng . That load has to bespread.

Look at the drawing of the wing mounted un dercar­riage, note how the doublers on the ribs spread the loadan d transmit it to the main spars. Note that the torsionspringing will absorb so me of the loads but not all.

O n the fuselage mounting not e that the mount isbraced with triangu lar stock to spread the load in thejoint and that the stresses are then transmitted to the plydoublers. One of the best methods of mounting a duralV/C on a tra iner is to attach it with elastic bands, if thelanding is too hard the U/C will flick backwards and themod el will se ttle on its belly with minimum da mage .

Design ing Model A ircraf t

Fig 7.2 - Undercarriage Sketch Page

Typical Li/C mount ill using

Wire u/c

Sheet omlttedfor clarity

,~~"'-- Ply doublers

Triangle stockbrace . ..

iiJl>.,rIZ--- 'APl)'

~...----- 'A6 P l)'

Liteply rib or':-7"--J~~~~ply doublersboth sidesof rib

/~7J.,----,,~,, ~~

Mount for dural

Cowlings

Cowlings may not be so important from the struc tura lpoint of view as they can be classed as terti ary struc ture,but they can have a very se rious effec t on the engi necoo ling if not designed properly and it is velY frustra tingif you canno t ge t in eas ily on the field to sor t out so meminor pro blem.

Cowling the engine neatly is often a problem, man ymo d el s jus t use an exposed engi ne w it h just thecrankcase cove red by a cowl that is part of the fuselage.

This engine is normally cotoled, note bow bafflestake airfrom top ofcylinders dounuoards, forcingit through tbe fills.

This system has many adva ntages as almos t every vitalrequirem en t is covere d.

The vital requirements are, good cooling, good accessto the engine and controls and sec urity of attachment.

Good cooling needs a flow of air directed ove r thecylinder fins and good outlet for the hot air, this sho uldbe bigger than the inlet. Many people think that a nicebig radia l cowl is perfect, but in fact the vas t intake andlimited outlet makes for poor cooling.

The answer is to fit a plate with dummy engi ne in thecowl and this forces air round the cylinder. The plate hasno se rio us effec t o n pe rfo rm an ce as p ro ved by myMonocoupe 110, this mod el was 44" span with a cowlthat hid the 0.5. 15 FP engine, over 4" in diameter. Onemight ex pec t the plate blanking the cowl off to act as anairbrake, not a bit of it, that model was very, very fast infligh t just like the full size .

Contro l line team racers and speed models are operat­ing at the very limits of engine perform ance , good cool­ing is essential and yet, if you look at the cowls on thesemodels they have tiny air inlets. Of cou rse they are flyingvery fast so there is a lot of ram air bu t the main reasonis that that air is forced through the fins and right roundthe cylinder which provides the most efficient cooli ng.

Engines in large cowlings would ben efit from so mebaffles to guide the air flow ro und th e cy linder andthrough the fins . Full size engines use these baffles, theba ffling on a twin row rad ial like the Bristo l Hercules isso mething to be ho ld.

Designing Mo del Aircraf t 57

Tbe inside ofa well designed cowl, note shape ofduct round cylinder and crankcase cooling scoopill otber applecbeek. Note also doubled tuire forVIe, just about tbe best uiire VIC going. Tbls isMoolldallce,..

How radial cowls are made.

O ne ar e a th at most people forget about is th ecrankcase of an engine, this needs cooling just as muchas the cylinde r, it may not show up until you ge t a reallyhot day.

Two exa mples : My biplane design , "Yuppy Love" hada PA\XI 1.49 diesel , this engine overhe ated badly eve nthou gh the cylinde r was out in the open, I fitted a pieceof tube to ac t as an airscoop to direct ai r over th ecrankcase, end of probl em .

A friend has an anc ient Junior 60 powered by an oldO .S. 30 , it has been flying regularl y fo r 14 yea rs .Sudde nly it started playing up. The day was roas ting hot ,the engine was well carbo ned up on the outside but it

Fig 7.3 - Couiling Sketcb Page

7

~ Balsa

Sheet baffle dummycylinders call be

aUacbed

Two laminattons of~z balsa

.:P_'_l),_-----,~----___;r

-----

Cooling airfloios through gap

Fit small scoop

F,.ollt

Ideal cotolfor enclosed cylinder

Crankcase needs cooling alrfloto

58 Designing Model Aircraf t

Tbis uncotoled VlVe"gi"e ill a turbulent needsbaffles to ensure tbat rear cylinder is properlycooled.

had been flown in this state and these conditions before.What was different? My friend had fitted a spinne r whichcovered the slot betw een the crankshaft and the cowl,cutting off the flow of air to the crankcase. Instant ove r­heating.

The reason is that the incoming charge is p ie-heatedin the crankcase, this reduces efficiency but also sendshot mixture into the cylinder, that mixture is part of the

Higb drag but good cooling 0" America" Dragon.

cooling o f th e e ngi ne, th e e ng ine ge ts hotter , theun co ol ed cra nkcase ge ts even hotter and soon th eviciou s circle build s up to the point wh ere the enginewill not run .

One of the hard est thin gs to design is a cowl that canbe rem oved easily and yet will not come loose in flight.Hardw ood blocks and se lf tapping screws can be onesolution whil e cycle spo kes can also be used to goodad vant age.

One thin g to rem ember , cowls are go ing to hit thegro und first and the y will break regardless and they aretertiar y struc ture anyway a nd so do not need to betremendou sly strong .

Having sa id that , the radial cowl shown in the draw­ing is remarkably tou gh , I had one on my first TequilaSunrise design , when that mod el crashed it smashed thecarb back whi ch broke the crankcase and bent the carbspigot. The cowl wa s und ama ged .

Cowlings can also be moulded in plastic in the sameway as cano p ies are .

Canopies and CockpitsYour spo rts model will look a lot better with a smart

coc kpit, this can be an ope n coc kpit or a bubble canopyor so me othe r sor t of enclos ure. This is a styling matterand will probabl y hav e littl e e ffec t o n th e flightperformance .

Designing Model Aircraft 59

Tbe simplestform ofcouii firmly attached, good access and good cooling. Tbisline is from Gerard Peeney "Callisto" design.

I di d sa y 'Pro bab ly'but my Turb u le n td e s ign (RClVIW pl ans)has a big winds creen , ifthis go t kn ocked off orbadly b roken I wouldhave to wind in massesof trim on the elevato rc le v is to make th emod el fly properly andth en take it o u t aga inwhen the w indsc ree nwas replaced.

In th e case o f amoulded ca nopy th eimportant thing is todesign it to suit wha t isa va ila b le , as th ere isnothing more frustratingthan trying to find o nethat fits wh at you havedrawn , indeed yo umi ght even ha ve tomould your own.

Th e use o f vacuumforming pl astic allo wsyou to design any shape of canopy, it also allow s you toproduce involved and tight fitting cowlings , spa ts andfairings .

Most peopl e avo id this tech niqu e because of the needto make pa tte rns . The other probl em is finding some-

where that will do the formin g for you but this is not asdifficult as you might imagine.

The plug or patt ern can be made out of hard balsa ifit is only to be used for one or two parts , ha rder mater ialwould be in orde r for production runs .

Monocoupe bad vel)' small outlet area so front ofcotoling is btanked off, model was still vel)' fast 011 allO.S. FP.15.

60 Designing Model A ircraft

A similar arrangement uiitb Toumend ring 011 Triuial Pursuit, No problem tottboutlet bere.

The finish is imp or­ta n t , es peci a lly fo rcano pies as these needto. be crystal clear. Mostnorm al finishes react toheat and so the woodgrain sl{ows .through o nthe very first draw , thisdoes..not mat ter on athick cowl but a cano pywill be ruined .

I .bave found th atM.D. 'Products Filler willgive a ~ perfect finish andthe heat does no t raisethe grain of the wood,

. resu lt, exce lle nt finishfor at least two formingsand then another appli­cation of the filler willrestor e the fin ish. Th epattern must be slightlysmaller than the finishedpart as it is inside afterthe formi ng process. Toge t it out there must beno undercuts bu t rathera s lig h t taper to ai dremova l of the pattern.

Radial cowlings can be rem oved by drilling a hole inthe centre and applying so me air pressure to this whichwill blow the pattern out of the cowl.

There are home made vacuum formers which use thesuction from a vacuum clea ner , quite frankly these arenot much use for a large, thick cowl or cano py; (A) thesuction is not strong eno ugh and (B) it is d ifficult to ge tthe whole of a large shee t of plastic up to the right tem­perature. So where do we go from here?

Back to school. Most seconda ry schools or upperschools and eve n some middle schools will have a verygood vacuum forming machine in their Craft Design andTechn ology Departme nt. This w ill be capable of formingthick plastic about 10" x 18" wh ich is big eno ug h formos t normal model uses.

You might wo nde r how to ge t the use of one of thesemachines. Of co urse if you have children there is noproblem at all, if you don 't, we ll, there are ways .

Tequila Sunrise cowl su rviv ed toben all eng ine was turitten off inslde it.

Design ing Model Aircraf t 61

Isaacs FlIIJ' Lycoming engine bas baffles like tbeVlVellgille.

All COT departments are sho rt of mon ey, they arealways grateful for materials which may be scrap or off­cuts, timbe r, metal, plastics, goo d cardboa rd, electroniccompo ne nts, anything to help the pupil s make their pro­jects. If you can supply anything in those lines you will bewelcome. Even the offer of goo d old fashioned money.

Tiger Motb air inlet is not ve/J' large. Internalbafflillg directs air tbrougb fills and bettoeencylinders.

To approac h the head of the dep artment you shouldtelephon e just after the lunch o r final bell has go newhen he is still there but has got rid of the pests , I meanpupil s. You will NOT be popular if you phon e duringclasses, the school recepti onist will tell you the bes t timeto phon e.

Tiger Motb air outlets are large, and there is one 011 eacb side and another underneath. To allow for tbeheated and expanded uolume ofair.

62 Designing Model A ircraf t

Open cockpits ca ll look really smart 011 tbe right model.

You may have to wa it for your moul ding to be done,the de partment may not have a full time tech nician whois the person who will be lum bered wi th the job. Bepatient, remember they are do ing yo u a favour andteac hers are very busy people. I know, I use d to be aschool tech nician . You may also need to sup ply yourown ma terials altho ug h sc hools norma lly stock poly­styrene in l mm and 2mm thicknesses. They rare ly stock.clear plastic for moulding .

Tbe Auster's Gips)' engine is hung 011 tbese bearers.I baue seen beauier bearers 011 models!

Design ing Model A ircraf t

Failing the school, you co uld try the Yellow Pages,bu t these co mpa nies may no t have a small e noughmachine and may not want to do a sma ll one -off job .Some of the model manufacturers such as Chart may bewilling to help out.

All th is may seem a lot of hassle but a well mouldedcowl or canopy can make a mod el stand out from all theothers.

Canopies baue to be moulded uibicb call presentproblems.

63

Chapter 8

Scaling Up DrawingsT here are two reasons for sca ling up d rawings.

One is to enlarge 3 view drawings to a suitablesize and then design the struc ture for a flying

model, the second reason which is not rea lly classed asdesigning is to enlarge a plan to suit a different size ofengine , this is most co mmon with vintage models and itwill be discussed here because it is not a straightforward'Blow it up two times and let 's build it' process.

Scaling Up for Larger EnginesYou may co me across a plan that you just have to

build , for exa mple a full size plan in a magazine that isdesign ed for sma ll engines and micro RIC , and wh ichyou would like to buil d for your .40 engine .

Ge nerally speaking this is a pretty simple job wi thmod ern ph otocopi ers because the enlargeme nt factor isonly go ing to be abo ut x2 at the most. Many of my ownsma ll mod els have been enlarged by modellers in thisway and all of them have been highly delight ed . Onemodeller eve n took my Trivial Pursu it up to 10 foot span

and built it in cardboard, you can no w buy a kit of thedevelopment of this model . . . I can 't co mment on thekit because they never had the co urtesy to let me haveone .

Most full zoom ph otocopi ers will enlarge over a widerange and definitely up to x2. All that you need to do isdecide on the en largeme nt factor.

Now, assumi ng tha t you wa nt to go twice size youjust cut your plan up into AS size p ieces and ge t it takenup to A3, Bingo , you are there. Most pages are A4 and ifyou take A4 up to A3 that will be 1.41 times, which ispretty close to I ll, times enlargeme nt.

Anoth er way to look at it is this, decide on the spa ndesired for the larger mod el, let 's say 55". Now look atthe actua l spa n of the original, let 's say 33". Divide 55 by33 = 1.66 times. A good pho tocopier will co pe with that.

Now it is not just a matter of sticking the bits of theplan togeth er and build ing it because your material sizesare go ing to be ridiculous , take a %," sq . spa r enlarge dtwice, it will be %" sq when a X" sq . spa r is the correctsize .

Trioial Pursuit bas bee" scaled up for .25 sized engines and eve" enlarged to 10ft tuingspau and kiltedtoith modified construction and sligbtly altered tail surfaces.

64 Design ing Model Aircraf t

American Dragon is a 30" span for .Bcc engines but bas been scaled up to 60" spa" andfitted tuitb aileronModellel' was delighted toitb performance and handling.

New Zealand modellers scaled Tequila Sunrise up.40 and .60 engines, .40 version was modeller's first lowwing plane.

Designing M odel Aircraft 65

The best way to deal with spar sizes is to co nside r thecross-sec tion, a X,," sq. spar is 3 x 3 = 9 sq X,,", a X" sq.spa r is 4 x 4 = 16 sq X,," and a X" sq spar is 6 x 6 = 36 sq.X"ths, four times the cross-section of the orig ina l spar butthe X" sq. spar is just und er twice the cross -sec tiona l area .

Sheet sizes are the same, X,," sheet on the L.E. canremai n the same size , the sides sho uld be increased tothe size normally used for that size of mod el, so X." sheetsides would go up to l{;" but y.!" side wou ld still only goup to l{;th.

Th e resu lt of all this is a mod el that sho uld we ighabo ut two to three times the orig ina l weight for fourtimes the wing area, but if you double all material sizesit will weigh 8 times the original's weight for only fourtimes the win g area. Which will fly bette r? Which will bestronger? Go back and read chapter 4 again.

As a pra ctical example, I took my O-Fourn ier which isa 56" span powered glide r with a Cox Texaco .049 andwith three channe l micro R/C and which weighed about25 o unces , I scaled it up on a photocopi er 1.41 times, Ired uced the result ing 79" span to 73 to make the bes tuse o f stand ard wood sizes and fin ished up with a

Fig 8.1 -Tbe Effect ofDoubling Matel'ial Sizes

model that weighed 52 ounces, tw ice the we ight, twicethe area. I fitted a Supe r Custom 12 for power and fourchannel radio using small but not micro se rvos .

Enlarging 3-View DrawingsThe ave rage 3 view drawi ng is e ither ~!nd or X"th ,

there are othe r sca les and the same pr inci p les apply.Most R/C sca le models are from ~th sca le up ward s, thismeans that to enlarge a 3 view to mo de l size we arelook ing at factors of 7 times upward s.

There are co mpa n ies who ca n do enlargements tothese factors but let me tell you, it is a very expensiveprocess. You co uld do multiple ph otocopi es but aga in, itwo uld be ex pe nsive and all photocopiers distort so atthe end you wo uld have some rath er non scale shapes.You wo uld also have so me very thick lines.

Believe it or not, the most accurate way is to scale thedraw ing up by hand . You can enlarge the 3 view slightlyso that you have an exact en largement factor because itis eas ier to enlarge 5 times than it is to enlarge 5.76times.

-U6Sq.

-'h Sq.A ll dimensions doubledCross -section area = 4 timesVolume = 8 times

'ASq.Lengtb doubleCross-section x 1.3Cross-section area = 1. 77 timesvolu m e = 3.55 times

O-Folll'llier powered glider bas been enla rged 1.41times untb twice tbe toing area aud exactly twicetbe uieig b t.

66

Proportional dividers are expensioe but savehours oftedious work, tbls pair is 30 y ears old.

Desig ni ng Model A ircraft

Fig 8.2 -i Scattng up a 3 View

Diuider 0,. multiplication method

Grid metbod

+----_.

a

IHorixoutal datum

~Ve,.tiCaldatum

You decide on the spa n of the finished model, divideit by the span of the drawing which gives you 5.76. Nogood. Divide the mod el spa n by 5 and divide the resultby the span of the 3 view and that gives the enlargeme ntfactor for the 3 view .

Tools for the TaskEnlarging with a ruler and calculator is not on, there

is an easier way. You use dividers.Set your dividers on one dim ension, say the spinn er

ring to L.E. on the 3 view . Now using your dividers, stepthat measurement off on the datum line of your drawing.

Now take the measurem ent from the da tum to thecho rd line at the L.E. and step that off on your drawing.All main points are p lotted in th is way and then con­nected by lines until you have completed the outline ofall parts of the aircraft.

I can hear screams of, "It will take a yea r to do that."No it doesn 't, you will be surprised how much you cange t done in a couple of hours once you ge t the hang ofit.

You can save a lot of time and increase accuracy ifyou bu y a pair of Prop ort ion al Dividers, these have asliding joint and a scale and can be se t to any enlarge­ment factor from 1 to 10. They are very , very ex pensive,up to £30 last time I looked, bu t they are a tool thatsaves hours of work and will last for yea rs, mine are stillgoing strong after some 30 yea rs.

You can buy chea pe r on es mad e from two pieces ofwire . . . you ge t exactly what you pay for in terms ofqu ality, accurac y and ease of use .

Enlarging

We have talked abo ut the basic method of enlargingwith dividers and this is pre tty standa rd for the co mpletemod el but there are a few places whe re different tech­niques are need ed .

Plotting the shape of a elliptical w ing tip or fin andrudd er ca n be done by p lottin g seve ral points and join­ing them up with a French Curve, but you ca n also drawgrids over the sha pe and a s imilar grid o n yo ur ownplan and use this to plo t the sha pe.. Man y America nscale drawings actua lly had these grids printed on theplan, Nye and \V'ylam drawings for exa mple.

Possibly tbe ultimate design ch a lle ng e, drato YOIl,.01011 3 uieurs from full size plans and pbotos andtben design tbe model Tbls is tbe author's MongSport control line model.

Designing M odel A i rc ra f t 67

done this for a co uple of models you will know why Ibuild flat sided models.

You can have some of the smaller parts en larged ona photocopier but do remember that the lines w ill beve ry th ick and the re may be d isto rtion.

One metho d which I have not tried is to take a 35mmslide of the 3 view and project it onto a sheet of paper.You w ill need a superb camera to ge t the qu ality andyou will st ill have very th ick lines .

I would suggest that you choose a nice simple sub­ject with lots of straight lines for your first atte mpt atsca ling up a 3 view draw ing, there w ill still be plenty todo but the shape w ill appear qu ite qu ick ly w hich isrewarding and there is less chance of errors and eventhese w ill be seen more easily.

Taylor MOlloplalle, tbe autbor prepared scale 3uieuis from full siz e construction dratoings andthen designed model Witb sca le homebullts youcan follow tbe original construction uiblcb is oftenjust like a model

Most plans provide fuselage cross-sections, in fact itisn't mu ch use if these are no t shown. Unfortu nately thesesections are rarely jus t where we want them and evenworse there are not enough of them, so we have to plotthe intermediate sections, you are going to have fun now.

The method I use works well. Plot and draw thelargest cross-section and then the smallest on top of it.You w ill have four common lines connecting these sec­tions , the da tum lines.

Using a pro tractor pl ot more lines from the centreoutwards. Now divide the lines into the required nu m­ber of sections between the two exis ting ones and co n­nect th ese po ints and you w ill have the in termed iatesections w hich will be yo ur fo rmers . Cut them a littleoversize just to be on the safe side. When yo u have

Scale ConstructionTh ere is no di fference between a sca le model and a

sports model as fa r as constru c tio n goes , th e sca lemodel may have a more complex shape which will ca llfor some of the fancier tec hniques but it is still jus t amodel aircraft.

The average sca le aircraft has a larger fuselage inrelat ion to the wing area than the equivalent size sportsmodel , this can add weight whic h is bad news. There isa natural ten dency to wa nt to add d et ail to a sca lemodel , th is will also add weight an d is even more ba dnews . Fina lly, the co lour scheme on a sca le model maytempt you to do a fancy pa int scheme an d thi s w ill addye t more weight which is even worse news.

Bear all this in mind and mak e a co nscious effort tosave weight and you might keep the model somewhe rew ith in reasonabl e limits but it is diffic ul t. Console yo ur­se lf wi th the thou ght that if there were no weight limitsat all for competition sca le mod els, the builders wouldstill b e moaning th at they co u ldn 't keep wi thin thelimit.

1

200 11

~I

F8F7

~----'''"--t.I4--=---...I.''--='-------''''1Equal diuisions

Fig 8.3 - Plotting Formers from Two Knoum Sections

F6

F6

F6 & F9 k'IOWIl F7 & F8 plottedPlot points 011 radii and connect

uittb French curue

68 Designing Model Aircraft

Chapter 9

Flight TestingY O U stand in the workshop admiring you r first

ever own design model , you picture it takin g offand flying round, you imag ine the aerobatics

and then the approach and greased on landing.This probably takes place at about ten o 'clock on a

Saturday night and you are going to fly her in the morn­ing. You are? Well you have quite a bit of work to dobefore that.

Final CheckoutTh e first th ing to check o n is the CG, don 't tak e

"about right " as go od enough, make sure that it is whereyou wanted it and preferably a bit furth er forward .

With a small tailplane the CG wants to be well for­ward , I favour starting with a CG at 25% cho rd for suchmodels and go ing back as far as 30% for ae roba tic mod­els with large tailplanes, any further back is askin g fortrouble on a first flight.

Chec k that the incidences are as you designed themyou should have chec ked this during build ing but checkagain. I always remember the old RAF safet y posterwhi ch said "Don't Assum e , CHECK".

You will have set up the control th rows, chec k themagain and check for correc t sense . I like to se t full ratesat a little more than I think the model needs and lowrates at a little less, I take-off on low rates. If you haveex po nential rates these are even better.

Check the undercarriage for correc t track ing and ifyou have nosewheel stee ring, check that, I know that itis OK, I had a model and I knew that it wa s OK, fortu­natel y I checked because the steerin g was reversed onthe nos e whee l.

Now charge the radio and hope for decent weath er.

DOli Kicbot, lA scale Polish bomebuilt cruises past,note IIp trim neededfor slowflight. A lovely modelto fly.

Designing Model Aircraft

Flight Test Stage OneOn e is always a little nervous before a first flight and

this really is a first flight. You have a completely newmod el, not onl y has it never flown but no on e has everflown this particular design.

The worst thing you can do is to worry about things .Rem ember , it is only a mod el and if it is a conve ntiona lshape and it follow s the rules set out in this series andhas been built according to plan there is no reason whyit will not fly.

Having said that you mu st give th e model ev erychance because it ca n be badly ou t of tr im, the CGcou ld be a fraction too far back and the engine mightdecide to play up at the wrong mom ent so be careful.

Don't attempt to fly if the wind is too strong for yourlikin g, I have a wind meter and I w ill not test fly inwinds of over 12 mph even if the model is a p rovendesign , it is better to wait for two or three weeks thanwrite a new model off.

Do a range check on the field and get someone todouble check that the co ntrols are working in the cor­rect dire ction. Run the engine up and hold the nose up ,be very, very sure that the engine is go ing to keep goingand that it will respond to throttl e properly.

Some less ex perience d mod ellers get another pilot tofly their mod els for the first time. That is up to you butbe sure that the pilot that you choose is expe rienced .Expe rience is not just a matter of being a good pilot, fortest flying it also means bei ng able to recogni se a prob­lem instantl y and take the correc t action, for example, ifthe model goes into a spin and will not recover do esyour choice of pilo t know wh at to try in an attempt tosave the model?

Taylor MOlloplalle needed a dab of'full doum forspill recouery. It takes expensively earnedexperience to knoto what to do atfiftyfeet.

69

Fantasy (lievel' sent for publication] flew like a dream. First model that I everflew inuerted at 2foot altitude several times,

The First FlightIt is often a good idea to fly ano the r model first , just

to ge t re laxe d and feel the condi tions, you will havebro ug ht a second model just in case anyway.

It is a good idea to ask the other modelle rs for a slotto yo urse lf for the first flight or at least ask them to flywell clea r of the take-off area. If someon e has a videoca me ra ask the m to film the flight for you , this can beuseful later.

Now, go throu gh all your fue lling and start up proce­dures calmly and carefully, have the nose of the modelraised to che ck aga in for correct running, throttl e backand have the mod el placed on the strip . Check that thecontrols move .

\\then you are satis fied that all is well, look aro un d,ope n the throttle and take-off. Don 't try and haul themod el off the ground too soon, hold it down and ge t asmu ch speed as possible before easi ng in up eleva tor. Begent le, at this stage you don 't know how the mod el willreact. Climb to a good height mak ing any turns slow ly.

You will already have so me ide a as to how sens itivethe mod el is to the controls, if it is ve ry se nsitive to ele­vato r it would pay to throttl e back and land and eithe rmove the CG forward or reduce the eleva tor throw or

George Strobm's e....-perimental model uras fine atbigl» speed but deadly as SOOIl as it was throttledback,'

70

both . If the ailerons areto o sensi tive yo u ca nland a n d a d jus t th ethrows or carry on tes t­in g , sensi tive ail e ronsare not as da ngerous ase leva to rs wh ich ca nca use s ta ll s, s pi ns o rflicks.

If th e co nt ro ls aresluggish yo u ca n go tohigh rates and see howthe model pe rforms inthat state . Contro l sensi­tivity is a matter of per­sona l tast e . I lik e ve rypowerful ailerons wh icho the r cl u b membe rsha ve trouble co pi n gwith .

On ce you are happythat the mod el is in trimand flies reasonably yous ho u ld gain some

height , yes, more height , and throttle back , slowly feedin up elevator and watch for any tenden cy to drop awing. Feel out the low speed handling because anyvicious habits will catch you out on landin g if you arenot awa re of them.

While you still have plenty of height (regain it if nee­essa ry) , and are flying slow ly with lots of up elevator , hitfull rudder, the mod el should drop into a spin, allow itto turn three times and then centralise the co ntro ls, themod el should stop sp inning , if it do es no t, give a dab offull down elevator which should stop the spin, oppositerudder may also be need ed. You ca n also open thethrottle , if none of these stop the sp in you are in deeptrouble, but I have yet to find a model that did not stopspinning with down elevator .

On ce the sp in has stoppe d be careful an d pull out asge ntly as the rem aining height will allow because if themodel was reluctant to recover from the spi n there is arisk that it could go back into the spin as you pull out.

The cures for a model that do es not want to stopspinning are, CG forw ard , add washout o r, to be mor edrastic, a bigger fin and rudd er.

If your nerv es will stan d it, you can now try loopsand rolls and then landings.

First Tequila Sunrise flew well but bad poor spillrecovelJ' due to POOl' cboice oftotng section, sbapeand uiing loading.

Designing Model A ircraft

AIlalyse what toent wrollg

Second uersion OfTequila Sunrise was excellent and has been publisbed hereand ill the USA and enlarged with success ill New Zealand.

....oJlI,.. .

\LI, ·

"tK)to . ...... ·

.......~.. .

. P' - . ... ..sIf.,liJI• .• .

~..........

...•u.

The TestProgramme

The elation of havingmade the first fligh t withyour firs t d e s ign wil lha ve yo u ' wa lkingaro und about six inchesa bo ve the ro und a ndyou will want to fly themod el again and again .

The first thing to dois examine the model ,jus t as yo u would anynew model b ut witheven mo re care as yo uwill wa nt to be certainth a t th e s tructure th atyo u design ed is up tothe stresses an d strainsimposed on it.

In subse que nt flightsyo u ca n fly th rou g hae roba tics bu t while youare en joying yo urse lfyou sho uld note if themo de l rolls with equa lspeed in bo th d ire c­tions , if loops and bun tsare equa l, ho w mu chdown trim is need ed ininverted fligh t.

Now you can eva lu­ate your mod el , Is it asae robatic as yo u hadhop ed , is it unstable , doa ll th e contro ls ..,h avero ugh ly the sa mepower? In sho rt, a reyo u hap py wit h themodel,

Ge t other peopl e tofly it and see wha t theysay. You may not agreewi th them but you aregetting feed back whichcan be help ful for yournext design . You maywant to try some modi­fications , bigger co ntro lsurfaces or perhaps cut­ting them down , maybeyou will try fitting a big­ger engine . The mostlikely thing that you willdo is just canyon flyingthe mo de l witho ut any

. major changes bu t withthe knowledge of thingsth at yo u wi ll changeon yo ur next d es ign ,because the best way tole a rn to d es ign is bypractical experience .

The mo re th at youfly your new design the

Designing Model Aircraft 71

One scale lip tb a t didn't work, Aztec Two-step scaled up to 45" span for two .12s, came out grossly over­uieigbt at 600111lCeS, tip stalled andfliclsed as SOO1l as it was clear oftbe ground.

more you will learn about it from the aerodynamic andstru ctural aspects.

On e word of warning, never get complace nt. No mat­ter how man y mod els you design each mod el is differ­ent and you can still have failure s.

\':Ihen you do have a failure, analyse what went wrongbecause you can learn a great deal from your failures.

Failures can be divided into three main gro ups, aero­dynami c, struc tura l or pilot e rror, we will look at each ofthese in turn .

Pilot ErrorThi s is the most co ntentious group because if the

model is marginal the pilot will usually blame the mod eland drawing the line between a situation that a pilotcould have save d and one he could not can cause badfeelin g between peop le . This is the reas on tha t I alwaystest fly my own mod els and hate being asked to test flyothe r people 's mod els.

It must be said that a we ll designed mod el, properlyse t-u p and flown by an ex pe rienced pilot sho uld besa fe, so one of the oth er failure groups will almost ce r­tainl y be a contributing factor in any failure .

Aerodynamic FailureMost e rro rs in aerod ynam ic design w ill show up

immed iatel y after take-off. Ask yours el f exac tly w ha t

72

your model did . Did it stall, did it flick roll or did it justrefus e to lift off?

Stalling or flick rolling on tak e-off ind ica tes a CGthat is too far ba ck or that the model had not rea ch edflying speed before being hau led off the ground andthis , in turn ca n be caused by too high a w ing load ­ing .

Aerod yn ami c failure ca n b e ca us e d b y w a rps,incorrect an gles , wrong CG, excessive wing loading,po or cho ice of wing section or combina tions of these .

If this book ha s been fo llowed a nd th e mode lchecke d ca re fu lly th en any e rro rs sho u ld b e of aminor nature and failures sho uld not be a catastro phe .

Structural FailureTh ese can happen and whe n they happen at height

there is nothing that yo u can do ex ce pt stand backand enj oy the cras h .

Struc tural failures are rarely caused by major designfaults , it is normally the sma lle r things like a wingattach me nt that has not been reinfo rced sufficie ntly orthe use of incorrec t grades of wo od .

Failures after a heavy landing o r minor crash willoften show up wea k po ints in the co ns truc tion , ver yoften ca used by that sudden change in section .

Th e impo rtant thing is to learn from these thingsand go ba ck to the dra wing board and design a newmod el or modify the last on e .

Designing Model Aircraft

Chapter

Selling Your Plan10

There are three monthly R/ C magazines and aboutfive bi-monthly magazines in Britain which pub­lish at least one plan in each issue, most of them

will publi sh two plans.This adds up to a lot of plans every year. Some of the

magazines will want only du cted fan or electric poweredmodels, or perhaps glide rs but they still want plans.

Where do they ge t the se plans? From mod ellers likeyou, people who have designed a mod el and sent it in.

Do they bu y every plan submitte d? No, but if yousend the right design , prop erly prepared and pre sent edyou have a ver y go od cha nce that your plan will bepubli shed.

.What Sort ofDesignjust abo ut any type, but it sho uld have so mething to

attract the editor and potential build ers. It can look beau­tiful, it can be ultra cheap to build or velY quick to build.

Scale mod els are always popular bu t do call for care­ful se lec tion of a subject , p ro to types that have beendon e to death are less likely to be accepted but equallythe very obscure type for whi ch there is no docum ent a­tion would need some th ing specia l. Hom e builts arealways popular.

The one important factor is that it must fly well , if theperformance of your mod el is poor or it has very nastyhab its , don 't se nd it in becau se it will do nothing foryour reputation and could affect the sale of future plans.

My first Tequila Sunrise was pretty good but it wouldflick into a spin at the slightest provocation and it need-

Blue Movie 2 (RCMlV Plans Seruice], is a best sellerbecause it is velJI cbeap and quick to build andalmost indestructible.

Designing Model A ircraf t

ed a lot of height to ge t it out, pret ty soon it came togrief because of that. I design ed a new model whi chwas fine and has been publi shed here and in the USA.

Sometimes a design may have a characteristic that isdan ge rou s but un avoidabl e . My Waco PG-2 (RCMWPlan) powered glide r has two engines and no throttles, ifone engine cuts you have to be very careful not to let itlose speed. In this case it is acceptable becau se it is aco mmo n fault with twins and you put that warn ing inthe text. The Waco is a good subject, easy to build , goodflying perform ance and can be flown as a glide r or co n­verted to electric power. Blue Movie was very quick tobuild , chea p , alm ost indestruc tible and flew superbly,the plan is a best seller.

First ApproachYou have a nice mod el whi ch you think might be of

interest to an editor . Choose one ed itor and se nd him aco up le of ph otos and a description of your model andan SAE.

Do not se nd the same thing to mor e than one ed itorat a time because if they all wa nt it . . . you have mad eyourse lf unpopular with most of them and that is notgo ing to help you. If the editor is interested you have tobe read y to send him the plans etc.

The PlanThe plan need s to be neat and clear, it must show all

the information regarding wood sizes and any instruc­tions, all parts must be shown and identified .

Ryan B-5 Brougbam plans sold ill tbe USA, aircraftis commercial uersiou ofRyan NYP.

73

FILL THE FRAME. Note that all tbe static picturesill this book bave tbe f rame fllled bJI tbe model, II0tlike tbis o ne.

You must reme mber that many people will bu y theplan without eve r read ing the article in the magazine soall the inform ation need ed to build the mod el must beon the plan .

Unless you are a skilled draughtsman your plan willhave to be retraced to suit the standard and format usedby the magazine. It will have to be traced onto Mylarfilm anyway for printing, the pe rson who traces it cannotguess at inform ation that is not includ ed .

The plan that you send in can be don e in penci l but Iprefe r to trace the plan and send in a print , this way youhave not lost much if it ge ts lost in the pos t.

Plying sbots will prove that YOllr design flies, tbeydo II0t bave to be perfect.

A roll of tracing pa per is not too expe nsive and draw­ing pen s can be very chea p for the disp osabl e types,better ones are expe nsive but well worth it once you ge tinto designin g.

Some of my ea rly plans were traced using the oldspring bow pe ns , which co uld be se t to su it th e linebe ing dra wn . Now I use 4 pens, .25mm for fine detailand grain marking, .5mm for most of the draw ing, .7mmfor main outlines and l mm for the border of the plan .This is purely for my own satisfaction, as the draw ingwill be retr aced an yway I sugge st that yo u se ttle for.5mm which is an ideal size of pen .

Till)' Shrike Commander is a bail)' little beast bu t great filii and cheap, text ioarns of pitfalls.

74 D esigning M odel A i rc raf t

Tbis is better, m)' neighbour's drive gets usedfor most ofmy static sbots.

Lettering can be don e free hand so long as it is legi­ble, you can use a stencil but that is vely slow .

Just remember, the better your plan is, the eas ier it isfor the tracer and the less likely he is to mak e mistakes.

The pictures must be properly ex posed and in focus,this is not hard to achieve with the average co mpactcame ra.

What is important is to ge t a good ba ckground, itshould be as free of any clutter as possible and sho uldco ntrast with the subject. Construction photos sho uld betak en agai nst a dark background and th e co mp lete dmod el taken aga inst one that co ntras ts with the TONE ofthe mod el. The photo may be printed in black and whiteso you should think in term s of tone, not colour.

I use a neighbour's tarm ac drive for most of my ph o­tos whil e cons truc tion photos are taken w ith the mod elon a dark coloured bedspread.

The oth er rule for taking p ictures is: "Fill the frame",there is nothing more useless than a tiny figure holdingso mething in a vast expanse of countryside or a minutemod el with a caption telling you to look at so me finedetail.

Vel/e MOllocollpe in flight.for really good in fligbt photos you need a 300m",telephoto lens, a good pilot m' cameramall, and luck.

Photos

Text

Yo u will n e ed agood se t of ph otos ,ordinary gl oss colo urprints 6" x 4" are quit eacceptable, alw ays se ndplent y to allow the edi ­to r to se lect th e oneshe wants.

Your text must be clear and cover all vital points . Itsho uld star t with a short int roduction and cover thecon stru ction in a logical way. It sho uld then cover theflying side of the mod el.

Get someo ne to re ad th rough yo u r text to se e ifthey can spot anything that need s cha nging. You knowwh at you mean but will other peopl e .

The text should be typ ed with double spa cing onon e side of A4 pap er , it should be printed quite dark ifpossible so that a co mputer sca nne r ca n read it.

If you can't typ e it up you can often find a tam etypi st at work who willdo it for a b o x o fcho colates or there aresecretarial se rvices whow ill d o it for a s ma llfee .

Designing Model Aircraft 75

\\\i

\

How lIIall)' th ing s are wrollg with this picture?Noui you k ,IOW what to avoid.

Flying shots are helpful b ut very difficult to take , ifyo u d o not ha ve a go od , powerful telephoto lens,maybe a friend ca n help out. Edi tor's do like to knowthat a mo de l does fly.

\X/hen I sent my first plan to the USA I se nt an 8mmcine film of the mod el in night (long before the da ys ofvideo) . . . it wor ked .

Sending the Material 111Pack everything up in a good , stout, ca rd ba cked

en velop e. The print can be sent rolled but I usually justput th em in th e e nve lo pe . Se nd it by regis tered o rrecorded delivery and then sit back and wait.

Very often an editor will let you kno w that he hasreceived it but ed itors are busy peop le so you may nothear for some time . If you have heard nothing afterabo ut a month you co uld telephon e to make sure that ithas arrived .

Once your plan has been accepted for publicati onforget about it.

You may be lucky and see it publi shed in a matte r ofmonths but you co uld have to wait yea rs. Do not keepringing up and asking because it will not endear you tothe editor and you can't afford to offend them , at least,not if you want to se ll them more plans. You can remindthem after a yea r has passed but do not overdo thin gs,just se nd in more plans.

\'(Th en yo ur pl an is publish ed yo u w ill hav e th eexcitement of seeing your wor k in print and sho rtly afterthat you will ge t a nice at cheque in the post.

76

This is not a good background as it is d is tracting.

Where Next?

With o ne p lan awaitrng publ ication the urge todesign mor e models will grip you, and wh y not , thereis a big market out there .

Once you have had two o r three plans publish edyou can cons ide r the American market. If yo u lo okthrough American plan s catalogues you will find se ver­al plans from British designers that ha ve appeared inmagazines over here .

The reason for this is that there is very little trad e inmagazines across the Atlanti c and even less in the wayof plans, so it is worthwhile for the Amer icans to p ub­lish these p lans.

You ca n offer a plan to an Ame rican magazine thathas been published here but you should talk to the edi­tor of the British magazine first. R/ C Mod el World buysthe copyright but will normally give yo u permission tose nd the plan to the USA, oth er maga zin es onl y buyFirst British Ser ial Rights so yo u still own the co pyright,but it is only polite to tell the edito r an yw ay.

You must also tell the American editor if the planhas been published over here , so me Amer ican maga­zines may onl y want a plan o n an ex clus ive basis .

Any p lan that yo u offer to an Ameri can magazinehas to have some thing a little special ab out it, after all,the y have a lot of designers ove r there . Slightl y unusualscale subjects go down well, out of the rut sports mod ­els or even unorthodox, providing that you can p rovethat it flies we ll.

Get hold of so me American magazin es and studytheir style and presentation. Make your ap proac h to theed itor and see how you get on.

Looking Back

Th is book has tried to show you the simple, un com­plicated wa y of designing mod els, a subject which can,and does, fill bo oks.

You now know the basic mechani cs of design andeno ug h of the very Simple aerody namics and struc turesto design successful models.

The re are bo oks wh ich will teach you more aboutaerodynam ics if you feel the need , there are bo oks onde signing scale mod els as well as sports models . Addthese to your co llection as they will always be usefulreferen ce sources but the most impor tant thin g that yo une ed now is ex pe rience and you can only ge t that bydesign ing mod els.

Designing Model A ircraf t

You need a good pilot and guts to get this close, eoen tuitb a 300mm lens.

Moolldmlcer trick sbot. Tbe model is in fligbt butbow did I get it? Simple, take tbe moon at nighttuitb a 500mm mirrorlens, remind tbe film andreload Oil exactly tbe same mark and take )'ourpictures as normal; toitb tbe moon ill a clear areaofshy.

Tbis is m)' design for a scale Rnssiau poweredglider, tbe MAK 15. It is C02 potoeredfree fltgbtaud pubtisbed b)' SAMS, would be a tuinner as allRIC design.

Filling the frame and pill sbmp,f/)'illg pictures like this are bard to get. Top and bottom pictures are Geminimodel kits.

Designing Model Aircraft 77

Useful AddressesAviation Bookshop656 Holloway RoadLondonN193POTel: (0171) 272 3630Ae rofoil sbeets, modelling books and magaz ines, full sizeaviation books and magazines. Also second band booksand magazines. Get tbeir catalogue.

Transair Pilot Shop50a Camb ridge StreetLon donSW1V 4QQOr :Fairoaks AirportSurreyTel:(01276) 858533 for orders and catalogues.Squa re protractors, books, Videos, av iation novelties a ndfo r tbe full size pilot, all navigation aids, equipment,simulators etc.

Balsacraft (International) LtdNorwich Road Industrial Estate\VattonNorfolkIP25 60 RTop qua lity toood selected to yo ur specification at tbe bestprices, also large range 0/accessories.Real ualue f or money , one stop mail order, 3 fi rst classstamps/or catalogue.

78 Designing Model Aircraft

Notes

Notes

DESIGNING MODEL AIRCRAFTThis book covers everything that you need to know about designing models,starting with what drawing instruments you need and where to buy your drawingpaper in Chapter One.Everything that you need to know about aerodynamics and structural design iscovered in simple terms without any unnecessary complications.Each stage of preparing a design is clearly laid out with suggestions for differenttypes of construction. Methods for developing the more complex shapes of scalemodels are shown.There are chapters on scaling plans up, flight testing your new design and how togo about getting your plan accepted by a magazine.There are no theories in this book. Every technique and principle has been and isused by the author on a regular basis when designing his own models.

r\ Truplct Puhlicat ion