AIRCRAFT BUILDING

AIRCRAFT WOODConclusion

BY RON ALEXANDER

This article concludes our serieson wood that is used in the construc-tion and restoration of aircraft. Uponexamining the problems associatedwith selecting wood for use in yourairplane, it is apparent that you mustbecome familiar with how to properlyinspect wood prior to installation.The previous articles provide an in-depth look at the types of wood youcan use, the common defects found inthese types of wood, and how to prop-erly inspect the wood. Mil i tarySpecification 6073 is presented anddiscussed as it relates to Sitka spruce.The articles suggest that you should

purchase wood from a reputable kitmanufacturer or aircraft supplier —one that further inspects lumber priorto shipment. Most of these compa-nies complete a final inspection forobvious defects before shipping theorder to you — the end user. Severalof them will even do testing on sam-ples of wood. The procedure ofaccomplishing a simple test on woodsamples used by TEAM, Inc. wasoutlined in the March 1999 issue.

Prior to beginning our discussion, Iwould like to correct an error made inthe December 1998 article. I statedthat the famous Spruce Goose aircraft

was constructed largely of spruce.That statement is incorrect. Mr. DickWood of the Evergreen Aviation Edu-cation Center sent an ar t iclereferencing the building of the SpruceGoose. This aircraft is on display atthe above-mentioned center located inMcMinnville, Oregon.

The article states that after thor-ough testing of many types of wood,birch was selected as the primary woodto be used in this unique airplane. Ac-cording to the article found in HowardHughes & His Flying Boat by CharlesBarton, "Birch was selected as themain structural material, not only be-

Plywood plate on a spar88 APRIL 1999

cause of the structural properties, andits good strength-weight ratio, but alsobecause spruce of the quality desiredwas difficult to obtain. However,spruce, poplar, maple, and some balsafor fairings were also used. The mainstructural material for the flying boatwas built up by bonding several pliesof birch veneer with glue. The bondswere formed under both heat and pres-sure, but some cold setting was used incertain cases."

As you can see from the referencedarticle, spruce was becoming difficultto obtain during the construction ofthe Spruce Goose in the early 1940s.As I have repeatedly mentioned, thescarcity of spruce has become muchmore pronounced through the years. Itis very difficult for the aircraft supplycompanies and kit manufacturers toobtain quality spruce.

I will conclude our series on air-craft wood this month by discussingplywood, glues used in aircraft woodconstruction, and how to properly fin-ish wood.

AIRCRAFT PLYWOOD

Confusion often results from theterms veneer and plywood. The termveneer is used to describe the relativelythin sheets of wood cut with specialmachinery from the surface of a log.The log typically revolves in a massivelathe and thin sheets are sliced or sawedfrom the log to form veneer. Plywood,on the other hand, refers to the combi-nation of several sheets of veneer thatis glued together. These sheets aretermed plies. You will usually hear ply-wood referenced according to a certainnumber of plies, each being one layerof veneer.

Plywood does have certain advan-tages over solid wood when used inaircraft construction. As compared withsolid wood, one of the major advan-tages of plywood is the presence ofmore equal strength properties alongthe length and width of a specific panel.Plywood is also more resistant to check-ing and splitting and it has less changein dimension with correspondingchanges in moisture content. These ad-vantages are obtained by alternating thedirection of grain in the plies of veneer.

Plywood used in aircraft construc-tion must be manufactured according tocertain specifications. The regular ply-wood you find at your local lumberyard

should not be used in any structuralcomponent of your airplane. Why?Most common grade plywood is com-prised of plies of veneer that probablyhave several defects. These defects re-sult in voids or gaps between the sheetsof core veneer. These voids cause aweak spot in the plywood itself. If mois-ture creeps into a void it can lead torotting or delamination of the plywood.In addition, the glue used in commonplywood may not have adequatestrength or be waterproof.

Aircraft grade plywood, on the otherhand, must be free of all voids and theveneers used must be free of most wooddefects. The glue used must also meetcertain specifications. Most manufactur-ers use phenol-phenolic glue applied ina hot press. The glue is both waterproofand fireproof. A military specificationpertaining to the manufacture of aircraftplywood was developed by the govern-ment years ago. This specification isreferred to as Mil Spec 6070B. It out-lines in detail the types of wood that canbe used, adhesives to be used, thickness

of veneer, defects not allowed in veneer,thickness tolerances, sample testing re-quirements, etc. To my knowledge onlyone company manufactures plywood tothis Mil Spec. That company is AircraftPlywood Manufacturing, Inc. owned byJerome Hediger. Jerome is also the man-ager of Wicks Aircraft in Highland,Illinois. Jerome provided informationfor this article.

"All plywood manufactured by hiscompany is done so in accordancewith Mil Spec 6070B. This plywood ismeasured in fractions of an inch andcan legally be used in production air-craft. The veneer used is eithermahogany or birch with a core mate-rial between the sheets of veneer. Thiscore material is usually poplar. Birch,basswood, or maple is also acceptableas a core material. Each ply must be 90degrees to the adjacent ply. The out-side plies of material are called facesand the inner plies are termed core andcross bands." The basic construction ofa 5-ply panel consists of a center plymade of core material with 2 inner

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Two-part Resorcinol glue

plies whose grain will be oriented at a90 degree angle to the 2 face plies.

Birch plywood is stronger than ma-hogany but is also heavier. Somebuilders prefer mahogany because itweighs about 10-15% less than birch.The appearance of mahogany is some-times preferred over birch.

Plywood manufactured under MilSpec 6070B must pass a boil test for20 minutes. After the boil, the ply-wood is subjected to a peel test and thewood must fail before the glue linefails. This plywood is often referred toas fractional plywood and it is avail-able in 4 ft. x 8 ft. panels. It is alsosold both as 90 degree plywood and 45degree plywood. The degree referencepertains to the outside or face veneerorientation. Forty-five degree plywoodis more expensive and is mainly usedwhere torsional stiffness is required.

METRIC PLYWOOD

Metric plywood is manufacturedaccording to a European standard. It istermed GL-1 or GL-2 — the GL mean-ing German Lloyd. GL-1 is a morestrict specification than GL-2 that al-lows more defects. Metric plywoodobviously is measured using the metricsystem with 1.5mm (approximately1/16 inch) being a common thickness.It is only available in 4 ft. x 4 ft. pan-els. Metric plywood is a suitablealternative to fractional plywood forexperimental aircraft. It is my under-go APRIL 1999

standing that most FAA inspectors willrequire fractional plywood to be usedin production aircraft. This is becausefractional plywood is manufacturedaccording to a military specification.You can inquire further concerningthis with your local FAAoffice. You will findGL-2 grade metric ply-wood readily availablefor purchase. It is ac-ceptable to use GL-2rather than GL-1 for ex-perimental aircraft.Also, the glue used inmanufacturing metricplywood is similar tothat used in fractional.

MARINEPLYWOOD

Several aircraft buildersuse marine grade plywoodin aircraft construction.It is available from air-craft supply companies.It is sold in two differentgrades, A-A and A-B.The A-B grade meansthat the A side does notallow too many defectswhere the B side allowsfor more defects. Marineplywood is usually madeof Douglas fir. It is inex-pensive but of poorquality. Care should be

taken if you decide to use this type ofplywood. I would not recommend us-ing it for anything structural.

USE OF PLYWOODPlywood is used for a number of

purposes in aircraft construction. Ex-amples are gussets for wing ribs,reinforcement plates for wing spars,instrument panel construction, floor-boards, skins for entire wing panels,leading edges of wings, etc.

REPAIRING AND STORAGEOF PLYWOOD

Repairs on plywood surfaces arediscussed in detail in FAA AdvisoryCircular 43-13. This publication dis-cusses repairing ribs, patching ply-wood skin, bending plywood, etc.AC 43-13 has several pages devotedto this subject and I would recom-mend referring to it prior to doingany wood repairs.

Concerning the storage of plywoodpanels, you will want to ensure thatyou store this and any type of wood ina dry area. Remove it from the ship-

Epoxy Adhesive

ping crate and inspect it for moistureor damage. Pieces of plywood may bestacked on top of one another. Do notstore plywood on concrete floors or inany area where moisture may present aproblem. Store in a dry, well-venti-lated area with all edges exposed topermit adequate ventilation.

WOOD GLUE

Certainly, the type of glue used inwood construction is of the utmost im-portance. Glue is the material usedalmost exclusively as a means of join-ing wood in aircraft construction. Acomponent part is considered joinedsatisfactorily if the strength of the gluejoint is approximately equal to thestrength of the wood itself. The AircraftWoodwork Technical Manual states, "Astrong glue joint is characterized bycomplete contact of glue and wood sur-faces over the entire joint area, with acontinuous film of glue between thewood layers, unbroken by air bubblesor foreign particles. The details of thegluing operation control the result."

Wood surfaces must be clean (freeof oil, grease, varnish, paint, etc.) priorto gluing. It is best not to sand thepieces prior to assembly. Sanding dustwill often fill the pores of the wood re-sulting in a weak bond. Apply glue toboth pieces that are to be joined andthen place them together. After joiningthe wood pieces together you will wantto apply pressure. The amount ofclamping pressure does vary depend-ing upon the type of glue to be usedand the type of wood. Pressure is ap-plied in order to distribute the loadfrom the point of contact to other partsnot directly under the load. Pressurecan be applied using clamps or brasscoated nails. As a rule of thumb, useabout four nails per square inch ofwood. Remember that the purpose ofthe nails is simply to hold the piecestogether until the glue dries. As a gen-eral rule of thumb, a lighter pressurewill be used with thin glue and a cor-responding higher pressure used withthicker glue. It should be noted thatusing epoxy adhesives requires mini-mal clamping pressure. Application oftoo much pressure will starve the gluejoint through excessive squeezing outof the glue itself.

If you are unsure about how muchpressure to apply you should make uptest samples. After allowing the glue to

properly dry in the sample pieces,place the piece in a vise and try tobreak the wood. The wood itself shouldfail prior to the glue joint. Once youhave made this determination you canthen use the same gluing techniqueson your structural pieces. It is also ad-visable to glue together two or threesample pieces. You can present onesample to the FAA inspector and keepanother sample for several years (10-20) to test again at that time. That willhelp you determine how the glue is

holding up inside your airplane. Again,AC 43-13 has detailed information onthe proper application of pressure dur-ing wood gluing.

TYPES OF GLUE

Now we enter the world of contro-versy. There are as many opinionsconcerning what type of wood glue touse as there are builders. Basically, thetypes of glue can be broken down intothe following categories: Casein glues,

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45° birch plywood — note Mil Spec stamp

plastic resin glues, Resorcinol glues,epoxy glues, and polyurethane glues.One very impor tant note regardingyour choice of adhesives, use the typeof glue recommended by the kit manu-facturer if you arc bui ld ing a kit

aircraft. Be sure to use any type ofglue strictly in accordance with the in-structions of the glue manufacturer.(An in-depth discussion of aircraft ad-hesives can be found in the July 1996edition of the EAA Experimenter.)

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CASEIN GLUES

Casein glue was used in aircraftduring the early 1900s. It remained inuse until about 1940. It was difficult tomix and slow to dry. It was not water-proof and could not withstand highertemperatures. Do not use casein glueon your aircraft. The new AC43-13states, "Casein adhesives should beconsidered obsolete for all repairs."

PLASTIC RESIN GLUES

Plastic resin glue has been used inaircraft for many years. It is powderedglue that is mixed with water prior touse. It uses urea-formaldehyde to pro-mote a chemical reaction. This type ofglue is not waterproof but does havemuch better water resistance than ca-sein glue. However, plastic resin gluehas been shown to deteriorate in hot,moist environments. It is a fact thaton a hot day an airplane parked on anasphalt ramp can experience tempera-tures in the inside of a wing that exceed200 degrees F. It has been determinedthat at temperatures of 120 degrees F,

urca-formaldchydc glues beginto deteriorate. Prolonged expo-sure to heat has a cumulativeeffect on the glue. AC 43-13now contains a warning con-cerning the use of plasticresin glues. It basically statesthat you should consider thistype of glue obsolete for air-craft wood repairs and anyproposed use should be dis-cussed with the appropriateFAA office prior to use on acertificated aircraft.

RESORCINPLGLUES

At approximately the sametime plastic resin glues wereintroduced Resorcinol gluewas manufactured. These gluesconsist of a two-part mixturethat is very thick and also is adark purple color. The glueconsists of a resin and a hard-ener. Because of its color it iseasily recognized after beingapplied on aircraft structures.Proper mixing and recom-mended clamping pressure areboth critical to achieving

92 APRIL 1999

proper bonding strength. Aircraftbuilders and restorers have used Resor-cinol glues with success for a numberof years. Some find the dark color ofthe glue objectionable. It does retainthis dark color after it has dried on thewood. This type of glue is readily avail-able from your aircraft supply companyand can be used with success.

EPOXY GLUES

There are many epoxy glues on themarket today. A popular glue used bymany amateur builders is called T-88and it is manufactured by SystemThree, Inc. Epoxy adhesives consist ofa resin and a hardener that are mixedtogether just prior to use. The pot life(working time) of cpoxies varies withthe type of adhesive and with the tem-perature where you are working.Generally, you will have 30 minutes ormore to complete the gluing processbefore the epoxy begins to gel. Curingtime to maximum strength also varies.It is very important that you mix epoxyadhesives according to the instruc-t ions. Do not vary the amount ofhardener to resin ratio. To do so willadversely affect the strength of thebond.

Fast curing epoxies, often referredto as 5-minute epoxies, are occasion-ally used to hold pieces together forfur ther bonding . Do not use theseepoxies for anything structural. Theydo not develop the necessary strength.

There exists some controversy con-cerning the use of epoxy adhesives onaircraft wood structures. There is apossible problem with high tempera-tures. I consulted Kern Hendricks ofSystem Three, Inc. and he stated thathe is not aware of any documentedfailures due to heat in any aircraft builtwith T-88 epoxy adhesive. He did saythat if you arc bonding wood at tem-peratures below 60 degrees you shouldallow the bond to cure at temperaturesabove 70 degrees for at least 24 hoursprior to use. He also said, "Like allepoxy adhesives minimal clampingpressure should be used to avoid starv-ing the glue joint through excessivesqueeze out."

POLYRRETHANE GLUESWayne Ison of TEAM, Inc. men-

tioned two new polyurethane gluesavailable. One is Excel, made in Bel-gium. It requires no mixing. Strength

and water resistance appears to be good.The second is called Timber Tix andwas developed for boat builders. It toorequires no mixing. I am not aware ofother polyurethane glues and their re-spective properties. I know they existbut I have not investigated them com-pletely enough for a recommendation.

So, what is the bottom line con-cerning glues? A number of buildersare using either Resorcinol glue orone of the many epoxy glues avail-able. T-88 and FPL-16A are twoexamples. Remember to protect yourskin if you are using epoxies. Many ofthe polyurethane glues are probablyvery good. Mr. Vick with the ForestProducts Laboratory has written a re-cent article in Sport Aviation regardingglues. No matter which type of glueyou decide to use, test several samples.Try to destroy the test pieces after theglue has cured. You should destroy thewood prior to the glue joint failing. Thedecision is yours unless you arc build-ing a kit aircraft. Then you should usethe type of glue supplied with the kit.The same applies with a plans-built air-

plane. If a certain type glue is recom-mended by the designer — use it.

FINISHING WOOD

Proper f in ishing of the woodenparts you have constructed is very im-portant. The wood must be sealed andprotected from moisture. It must alsobe protected from any chemicals thatmight permeate fabric during the cov-ering process. The best types ofvarnish to use are epoxy andpolyurethane varnishes. After propercuring, either of these will seal thewood and protect it from most chemi-cals. There are other varnishesavailable but they usually do not offerprotection from chemicals. The addedcost of an epoxy varnish or apolyurethane is insignificant com-pared to the protection they will offer.Both of these varnishes are two-partmixtures. Hardware store polyurethanevarnishes are usua l ly not t ruepolyurethanes. The varnish must becatalyzed to be considered a truepolyurethane. Polyurethane varnish is

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recommended if you are exposing thewood to direct sunlight. Otherwise,epoxy varnish is the choice.

If you are applying varnish to anold surface, remove as much of the oldvarnish as possible by sanding. Youneed not remove all of the varnish un-less you are using a polyurethanevarnish. If so, the polyurethane willoften act as a paint stripper on the oldvarnish. Usually, you can simply re-move the flaking pieces, thoroughly

clean the surface, and apply a coat ofepoxy varnish.

On new wood, you must ensure thesurfaces are free of grease, oil, and allcontaminants. Remove any excess glue.Fill all holes using a wood filler such asSuperFil. Clean all enclosed spaces ofsawdust, wood chips, etc. If you havemarked the wood using any marker otherthan a pencil, remove the marking.

On interior surfaces that fabric willnot contact, one heavy coat of epoxy

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varnish should be sufficient. It can bereduced and sprayed or reduced andbrushed on the wood. The resultingfilm will, of course, be smoother if thevarnish is sprayed. Proper thinning isimportant. Follow the directions.Spraying an assembled wing will bemuch easier than brushing. Large,broad surfaces will be easy to brush.The penetration into the wood is es-sentially the same if proper thinning isaccomplished. The solvents cause thevarnish to soak into the wood.

Pay particular attention to the endgrain surfaces. They must be sealed andprotected and are often overlooked.Also, if you are covering large woodensurfaces — such as a wing — with fab-ric, be sure to properly prepare thevarnished surface according to the fab-ric covering procedure. This is of theutmost importance. Follow the manualfor the covering system you select.

This concludes our series on air-craft wood. Wood is used in manyaircraft and it is a user-friendly ma-terial in most instances. If you canmaintain the items in your home youprobably have the tools and knowl-edge required to bui ld a woodenairplane. Obviously, the level ofknowledge needed will vary fromone design to another. If you arebuilding a kit aircraft supplied by areputable manufacturer, the wood inyour kit should have been carefullyinspected and tested. If you are re-pairing or restoring an airplane,review the inspection tips foundwithin these articles and accomplisha thorough inspection on all woodyou intend to use. +

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94 APRIL 1999

The EAA/SportAir workshopschedule is as follows:

March 20-21, 1999 Denton, TXApril 24-25,1999 Salt Lake City, UTMay 1-2,1999 Greensboro, NCJune 5,1999 Columbus, OHJune 26-27,1999 Frederick, MD

Information on these work-shops can be obtained by calling800-967-5746 or by contactingthe website at www.sportair.com.

The author may be emailed atralexander@sportair.com