Curtiss JN-4D. Built by Andy Anderson, this plane has asquare tube fuselage but many new Jenny parts. Poweredby OX-5.

Original S.P.A.D. VII. Plane was owned for years by aschool in Oregon until purchased by "Swede" Ralston whosold it to Wings and Wheels. Dummy twin machine gunssince replaced by single, real Vickers.

ANTIQUERS . . .(Continued from preceding page)

The "Wheels" part of the collection includes suchfamous names as Buick (1911), Hudson (1912), Ford(Model T and A), Chevrolet (1927), Cadillac (1939) —and even a 1923 Fordson tractor! The stars will be a1922 Anderson — the only car ever produced in SouthCarolina — and a 1931 Duesenberg, never before on pub-lic display.

The collection continues to grow — Al Williams'Curtiss "Gulfhawk" is soon to be delivered, the nation'sbest-known Staggerwing Beech will be in on loan, andby early next year, a full-size, working reproduction ofAmerica's first scheduled train, the "Best Friend ofCharleston" will be completed and running on a trackaround the grounds!

Wings and Wheels is solely owned by Dolph Over-ton of Smithfield, N. C., a jet ace in the Korean War,who, in addition to collecting antique airplanes, is involv-ed in several industrial pursuits. He is well along theroad toward realizing every antiquer's dream, "to ownone of everything!"

Wings and Wheels is open daily and on a year-roundbasis. If you fly in, the airport is on the 290 degreeradial about 3A of a mile from Vance VOR. The facili-ty has its own restaurant, gas, oil, and repairs are avail-able. Free transportation is provided to nearby motelsand golf courses. If you are planning a trip to Floridathis winter to escape those Yankee winter blasts, makeWings and Wheels a "must" stop. It's lots warmer inSouth Carolina and if you love old airplanes — it willwarm your heart. ®PAGE 40 NOVEMBER 1969

Comparison ofLoading ParametersFor Various SingleEngine Light Airplanes

By Howard L. ChevalierEAA 31823

Texas A & M UniversityAerospace Engineering Dept.

College Station, Texas

I N GENERAL, most homebuildersare building existing airplane de-

signs which have been proven to havecertain stability and performancecharacteristics. However, for the in-dividuals who are modifying existingdesigns or designing their own air-plane, the questions that arise are:how will it handle, how will it per-form, and how can one improve onthe current class of homebuilt air-planes? For some of us who have thetechnical background, the problem be-comes one of many hours of calcula-tions for any one design configuration.Those without the technical back-ground are forced to make guesses(same as the engineer at the end ofall the elaborate calculations).

The writer, during the design ofhis airplane, which is currently underconstruction, found a wide variety ofopinions from fellow homebuilders onthe subject of engine size and air-plane geometry. Also, the writer wasadmittedly confused as to why manyhomebuilt airplanes were termed asbeing "hot" with high landing speedswhen the wing loading on these air-planes is relatively low as comparedwith current commercially manufac-tured airplanes. For example, generalaviation airplanes have wing loadingsapproaching 20 lbs./ft.2 and commer-cial transports exceed 100 lbs./ft.2.This article presents a comparison ofexisting airplane data used by thewriter to obtain an empirical approx-imation of the relationship betweenairplane weight, wing area, wing span,and engine size for desirable stabilityand performance. In view of the typeof comparisons made, the results alsoinclude an evaluation of the aerody-namic efficiency of homebuilt air-planes as a representative class ofaircraft.

Values of airplane weight, wingarea, wing span and engine size frompublished material were used to de-fine three of the most frequent andimportant aerodynamic parametersfound in stability and performanceanalysis of aircraft: wing loading,span loading, and power loading. Asan example of the importance of theseparameters, the following two equa-tions of statics related to wing loadingand power loading yield the majorcharacteristics of airplane perform-ance in unaccelerated flight:

Note:AA(—1 is functionally

equivalent to (——)V hp/

1) V = 2 Mr

2)R/C =

'L

CL

Where:V = aircraft velocityP = air densityCL = aircraft lift coefficientCD = aircraft drag coefficientR/C = aircraft rate of climbW— = aircraft wing loadingSW— = aircraft thrust loading (func-T tionally equivalent to power

loading, W/hp)

—Npte that increased wing loadingwill not result in increased rate ofclimb as implied by this equation dueto the relation between required liftcoefficient and resulting drag coef-ficient for a given airplane weight.

Wing loading, span loading, andpower loading can also be related inequation form to airplane stability.However, to show all the functionalrelationships of these parameters tostability and performance would bean involved subject and beyond thescope of this article. However, a fewwords of caution are inserted here topoint out some of the critical changesthat can occur as a result of changesin these loading parameters.

Increased wing loading can resultin the following aerodynamic changes:

(1) Increased stall speed(2) Decreased rate of climb(3) Increased rate of descent (pow-

er-off condition)(4) Decreased maneuverability

and/or increased stick forces(5) Increased inertia loads(6) Decreased range (for constant

fuel weight)

Increases in span loading can resultin high roll rates and directional andlateral instabilities such as a spiralinstability. Generally, wings with largespan loadings also have low aspectratios resulting in high drag. Decreas-ing power loading can result in triminstabilities due to the location of theengine thrust line. The writer also isconvinced that some homebuilt air-planes have speed instabilities (oper-ating on the so-called "backside ofthe power curve") during the ap-proach to landing, but cannot provethis point because of the lack ofdata.

Figure 1 shows the variation ofspan loading with wing loading forcurrent single and twin-engine light

airplanes. It should be noted that thedata for special purpose airplanes,biplanes, and STOL aircraft were notused. The aircraft data used are iden-tified by symbols to show the wing,engine, and landing gear configura-tion. Only U.S. design homebuilt air-planes are shown for comparison.Most data were obtained from JanesAll the World Aircraft, 1965-66. Sincethe objective was to compare a classof aircraft as a whole, manufacturersand designers are not identified onthe figures.

Since any straight line drawnthrough the origin in Fig. 1 is a lineof constant aspect ratio, one obviousfact shown is that most aircraft are

(Continued on next page)

SYMBOLS AND UNITS

b = wing span (-Ft.)

5= wino, arta (s<^.-F-fc.)u 2

AR = aspec-t ra-fcio o-P wing = ^

W= aircraft gross weigh-t (Ibs.)

hp = engine, horsepower

Vc = aircra.-P-t cruise Speed (mph)

W—^ =• span loadinq (ibs./-Ft1)b J

W—— = wing loading (lbs./s<V-ft-)

—— = power loading (lbs./hp)

AIRCRAFT TYPE SYMBOLSPt-oa'tic.-t'ion Aircrtx-T-b

Single Engine ;

O : low wing y -Fi^cd gear

_Q_ : low wing , retractable gear

f_j ' high winj, -Fixed gear

C )̂ : high wing , re-trac-table gear

Tw in E ng i n e, :

9& ' wing < gear as aWwe.

-pixed aea

1̂ : hi^h w.n^ J -fixed g«a*-

Note ; Al l speci-Fica-tions based,

on original^ design gross

w«igk-ts , wing plan-forms ,

WING DESIGN

SPORT AVIATION 41

Comparison of Loading . . .(Continued from preceding page]

designed with an aspect ratio between5.5 and 7.5. In general, the homebuiltairplane has low aspect ratio, probablydue to structural reasons, which, alongwith other factors, has resulted in poorperformance.

The areas bounded by the dashedlines are arbitrary and yet are justi-fied by the foregoing logic. For wingloading above 20 lbs./ft.2, single en-gine configurations are unsafe in theevent of engine failure on take-off orlanding. Disadvantages of high stallspeeds and low rate of climb forhomebuilt airplanes exist in the area

between 16 and 20 lbs./ft.2. However,these disadvantages can be offset byefficient wings, wing flaps, and re-tracting landing gears. Commerciallymanufactured aircraft shown in thisarea have been developed with veryefficient wings and wing flaps. Thishas been accomplished through bothengineering and long periods of flighttesting. History shows that more air-planes have failed to perform in thisarea than the ones that have succeed-ed. Although it is doubtful that thehomebuilt airplane could approach thewing efficiency needed to operate inthis area, it should serve as a goal tobe reached. Therefore, the maximumwing loading selected by the writer

POWER REQUIREMENT

wTV

Power Loading

(Ibs./hp)

•ft

~<*of Climb Inet-me

High Drag and/o

10 <i 14 16 18 20 ZZ M

•j- Wing Loading (lb»./so,.-fi.)

FIG. 2

CRUISE: SPEED COMPARISON

j_ Cruise Speed corresponds -to 75 %

power wri«r«. olot-ta. o-vjuJoblt-

Low Droiq Aircrgf-t

in IZO ISO 140 150 I f rO 170 180 190

FIG. 3

was 16 lbs./ft.2 A wing loading of16 lbs./ft.2 corresponds to a sea-levelstall speed in the neighborhood of 63mph without flaps.

The interesting fact shown in Fig. 1is that homebuilt aircraft, by compari-son, are not high wing loading air-planes. The high stall speeds associat-ed with some homebuilt airplanes aresimply a result of poor wing design.

The absence of single-engine air-craft span-loading data above 3.25lbs./ft.2 resulted in the upper boun-dary on span loading. In general, ithas been difficult to achieve satisfac-tory lateral stability for aircraft withspan loadings above 2.5 lbs./ft2. Sim-ilar to the area of wing loading be-tween 16 and 20 lbs./ft.2, theoreticaldesign precision is unsatisfactory inthe region above 2.5 lbs./ft2. In somecases the addition of a mere foot ortwo of wing span resulted in chang-ing the airplane handling qualitiesfrom poor to very good. It should benoted that aircraft scale is an im-portant factor to be considered in spanloading. A 20 ft. span airplane will bemore difficult to trim hands-off thana 30 ft. span airplane of the samespan loading at low speeds.

Now that the weight and corres-ponding size of the wing is estab-lished by using the limitation of as-pect ratio between 5.5 and 7.5, wing-loading maximum of 16 lbs./ft2, andspan-loading maximum of 2.5 lbs./ft2,the problem becomes one of deter-mining engine size. Fig. 2 shows thevariation of power loading with wingloading for the same airplanes asshown in Fig. 1. Although the pointsappear random in nature, most of theaircraft, except homebuilt airplanes,fall in the region defined by the twocurves. Some of the commercially pro-duced airplanes in the excess powerregion are used for crop dusting,glider towing, etc., even though theoriginal design was for a general-pur-pose airplane. The two equations inFig. 2 can be used to determine ap-proximate engine size. As an example,assume a wing loading of 14 lbs./ft2

and gross weight of 1,800 Ibs. Thisresults in a required power loadingbetween 13 and 15 Ibs./horsepower oran engine horsepower between 139 hpand 120 hp.

It is the opinion of the writer thatengine size for most homebuilt air-planes is adequate, and apparent de-ficiencies cannot be corrected by in-creasing engine horsepower. As seenin Fig. 2, most homebuilts could notbe rated as efficient low-drag air-planes. The apparent high drag ofhomebuilts is also indicated in Fig. 3which shows the variation of power

PAGE 42 NOVEMBER 1969

loading with cruise speed. In view ofthe wide range of cruise speed for agiven power loading, Fig. 3 is for com-parison only and could not be usedfor design purposes. Also, increasingengine horsepower does not result ina linear increase in speed. This is dueto factors such as propeller design,and the fact that drag increases bythe square of the velocity. An import-an aspect of Fig. 3 is that it indi-cates a general need for aircraft dragimprovements for both homebuilt andcommercially manufactured airplanes.

One obvious conclusion from allfigures is that almost all of the U.S.homebuilt airplane designers haveoverlooked the importance of the wingdesign. Higher lift can be obtained byproper selection of airfoil and flapgeometry. Lower drag can be obtainedfrom the selection of airfoil, plan-form, and tip design. This conclusiondoes not imply that high aspect ratiowings with elliptical planforms arethe only answer to efficient wing de-sign. Recent commercially manufac-tured airplanes are proving efficientwith wings having aspect ratios be-tween 5.5 and 6.0, 15 percent thickchord, and constant chord lengths.Therefore, it is felt that improvementsin wing design can be made withoutadding to the structural weight anddifficulty of fabrication. An extensiveamount of experimental wind tunneland flight data on wing design areavailable from the NACA library ofreports, Summary of Airfoil Data,NACA Report No. 824. Credit shouldbe given to the several homebuilt de-signs shown in Fig. 1, 2, and 3 (withdesigner's recommended engine size)which compare favorably perform-ance-wise to commercially manufac-tured airplanes.

Other probable sources for improve-ments are fuselage, trim, and land-ing-gear drag. Two typical examplesof how cruise speed can be increasedby retracting the landing gear areshown in Fig. 3, one for a high-wingconfiguration and the other a low-wing configuration.

One of the objectives of this articlewas to point out the possibility that amore important aspect of airplanestability and performance than enginesize is physical shape. Another wasthat parametric studies can be madeusing current aircraft information forhomebuilt designs. This method canbe expanded to obtain approximationsof needed tail volume, tail size, fuse-lage length, etc.

The writer wishes to acknowledgethe work of Dennis Riddle in helpingwith the collection of data and prep-aration of the figures used. ®

"POP"Rivets InAircraft

By John W. ThorpEAA 1212

909 E. MagnoliaBurbank, Calif.

RIVETS are the pro-duct of the United

Shoe Machineiy Corporation of Shel-ton, Conn. "Pop" is their registeredtrademark. They are a hollow rivet,available in several styles and ofseveral material combinations. Thereare also a number of imitations of"pop" rivets, both good and bad,that pass under the general headingof "pop" rivets. This discussioncovers cnly the product of UnitedShoe Machinery Corp., and data istaken from their publications.

Few products available to the am-ateur builder are more controversialthan are "pop" rivets. You likethem or you don't, whether you area builder or the FAA. United ShoeMachinery Corp. had done little tosell the use of "pop" rivets in air-craft, their principal market beingin the commercial metal trades in-dustry. Actually, there are legiti-mate uses for practically all "pop"rivets in aircraft, even the hardwarestore variety, but because these aremuch weaker than solid "AN" alumi-num rivets or the higher strength"pop" rivets from which they arealmost indistinguishable, it is thoughtprudent to use only the zinc-platedmonel "pop" rivet with the zinc-plat-ed 1050 steel mandrel in aircraftwork. This will prevent a low-strength rivet from inadvertently be-ing used where a high-strength rivetis required. In Vs-in. diameter, themonel "pop" rivet has a publishedsingle-shear strength of 420 Ibsi.against 388 Ibs. for an A-17ST solidaluminum rivet.

"Pop" rivets in aircraft are usuallyset from one side of the work withan inexpensive hand-operated tool,eliminating the need for a compres-sor and rivet gun. This is an im-portant factor in homebuilding ametal airplane. The one man opera-tion and reduction of noise are alsoimportant considerations. The onedisadvantage seems to be that therivets do not pull the parts together

as tightly with "pop" rivets as withsolid aluminum rivets. In a fewcases, "pop" rivets have worked looseunder severe vibrating conditionswhere solid rivets were more dur-able.

Costwise, "pop" rivets are moreexpensive than solid "AN" aluminumrivets, but far less expensive thanthe normally accepted blind "cherry"rivet that they slightly resemble.

Because they do not swell as muchin driving, any "blind" rivet is morecritical to hole size than solid rivets.However, the "pop" rivet does bet-ter at filling an oversize hole thanother "blind" rivets.

A number of T-18s have now beenbuilt using "pop" rivets exclusively.By now some rivets have needed re-placing. This can also be said ofT-18s using solid rivets.

"Pop" rivets can in many instancesbe used as a replacement for "P-K"screws in installations requiring occa-sional dismantling. In this case, softaluminum "pop" rivets with the alu-minum mandrel are used. They areso easy to drill out that it can bedone as quickly as taking out the"P-K" screw. Wing and tail tipshave been installed in this man-ner. Similar use has been made inattaching plexiglas to tubular canopyand windshield frames.

A hole is left in the head of a"pop" rivet when the mandrel breaksafter setting the rivet. These canbe filled with a two-part epoxy auto-mobile body putty before painting.Where extreme smoothness is requir-ed, countersunk-head "pop" rivets areavailable. These may also be filledwith body putty. In using counter-sunk "pop" rivets, it should be notedthat the included angle of the headis 120 degrees instead of 100 degreesas is the case for AN 426 rivets.Where the sheets are dimpled thereis no problem as the "pop" rivet willpull the overformed dimple in to con-form to its head shape. Where ma-chine countersinking is used, it isbetter to use a twist drill than the100 degree countersink. This op-eration requires care in any case.

In summation, the "pop" rivetoffers no panacea. It is a good wayto build a metal structure. If itslimitations are kept in mind, safemetal airplanes can be built using"pop" rivets. In the writer's view,"Pop" rivets have made a worth-while contribution to the techniquesemployed in turning out homebuiltaircraft. There is no justification foranyone to arbitrarily ban their use.

SPORT AVIATION 43