AIRCRAFT BUILDING

COMPOSITE CONSTRUCTIONConclusion

BY RON ALEXANDER

This article concludes the series on composite construction. The two previous articlesdefined the word "composite," discussed safety issues, listed the stages of building acomposite airplane, and presented the basic composite construction techniques.

Again, the five stages of compositeconstruction are: (1) decision andplanning, (2) basic building and as-sembly, (3) systems installation, (4)filling and finishing, (5) inspection,certification, and final pre-flight. Lastmonth's article concluded with a pre-sentation of most of the building andassembly steps. In this issue I willcomplete the discussion of buildingsteps and then review the steps of sys-tems installation and finishing. Theinspection, certification, and final pre-flight procedures were discussed in theJune issue of Sport Aviation.

The majority of plans-built aircraftrequire the builder to completely formthe entire structure using the hot-wireand foam cutting techniques previouslypresented. Some of the kit aircraft alsouse this building procedure for certainportions of their design. Usually, how-ever, a kit aircraft will only demand asmall amount of shaping and forming.Composite kit airplanes are often soldwith pre-molded parts that need only tobe assembled — not unlike a model air-plane. This, of course, reduces theamount of construction time consider-ably. The kit manufacturer assumes theresponsibility for a properly shapedwing or fuselage. Molds are constructedby the manufacturer and the parts of theairplane actually built within the molds.You then receive the various compo-nents and bond (glue) them together toassemble the airplane. This also allowsthe kit manufacturer to ensure thequality of construction, i.e., theproper mixing of resins, orientationof fiberglass, etc. The number of90 DECEMBER 1997

Sanding prior to bonding.

parts pre-molded and supplied to thebuilder vary from one kit to another.Often, a kit manufacturer will design anairplane to use a combination of bothpre-molded parts and moldless (thebuilder forms the piece) construction.

So, instead of having a foam corewing, as an example, a kit manufac-turer wi l l supply us with a hollowwing. With this type of constructionthe strength of the wing is found insome type of spar system similar to awood or metal wing. A wood or metalwing uses a spar and wing rib combi-nation to support the aluminum skin.

The number and spacing of ribs, thesize of spars, etc., determine thestrength of the wing. The same appliesto composite construction. The advan-tage of a pre-molded composite wingis found in the sandwich type con-struction that is used. Recall the term"sandwich construction" that meanswe are using a foam core, reinforce-ment material and resin together toform our structure. A pre-molded wingwill use the sandwich type construc-tion on the wing skin only. Obviously,the wing skin will only utilize a thinlayer of foam or core material instead

of the entire wing being constructed inthe sandwich manner. The pre-moldedwing is curved which also providesstrength and the wing is held togetherwith ribs just like conventional con-s t ruc t ion . Spars constructed fromcomposite materials and ribs are thenused in a similar manner to a conven-tional airplane. These spars and ribsare often supplied by the manufac-turer. This type of construction allowsthe wing to be assembled using a mini-mum amount of ribs versus a metalairplane that may require considerablymore ribs to acquire the same strength.

With this type of composite con-struction all of these parts must beproperly glued (bonded) together. It isessential tha t the bonding be com-pleted properly.

Bonding

Bonding is not a new process in air-craft building. In fact, bonding has beenused in aircraft construction since thevery beginning. The technique of glu-ing wood structures together has beenused for years. Many of the same glu-ing elements found in wood are alsofound in composites. The term bonding,as applied to composites, is used to de-scribe a common method for joiningcomposite structures. Bonding is theprocess in which previously manufac-tured component parts are attachedtogether during assembly of the air-plane. Bonding composites can also becompared to welding metal. It is de-signed to be a permanent joining

method. Several important points mustbe considered in bonding. We mustknow how much strength is needed inthe joint, the bonding area required,what type of material must be used toprovide the adhesion, and the procedureused to apply the bonding mater ia l .Preparing the surfaces that are to bebonded together is also crucial.

The first method of bonding used inamateur-built aircraft involves a fourstep process. The first step is to cutand trim the component parts to get theproper shape and fit. The second stepis to position the two pieces together.This can be accomplished by usingtemporary jigs or by temporarily glu-ing them together with a non-structuraladhesive. Third, we must fill any gapsthat may exist as a result of butt ingtwo pieces together. The f ina l stepconsists of actually creating the struc-tural joint using wet (resin laden) stripsof reinforcement material (usuallyfiberglass) bonded over the area con-necting the two components together(see Figure 1). The example in Figure1 represents a typical fuselage withtwo pre-molded halves being bondedtogether by the builder. If we are bond-ing toge ther two pieces tha t areperpendicular to each other as in Fig-ure 2, then we must create a fillet onceagain using wet lay-ups of reinforce-ment material. An example of this typeof construction would be in mating awing rib to the wing skin.

The strength of a joint that is joinedby a f i l l e t is derived from the rein-forcement material and not the fillet

itself. The fillet is only needed to pre-vent the reinforcement fibers frommaking a direct 90° bend without anyradius. Composite materials must havea bending radius just like sheet metal.The number of strips of reinforcementmaterial laid down over the fillet de-termines the strength of the bond.

The second method of compositebonding is termed "adhesive bond-ing." Adhesive bonding involvesassembling component parts togetherusing a structural adhesive. Structuraladhesives range from pre-formulated,two part mix tu res tha t are in pasteform to structural laminating resinsthat are mixed with flocked cotton ormiller fiber to provide the necessarystrength. The first method of bondingdiscussed used laminating resins andreinforcement material to create abonding overlap. Adhesive bondingrequires a bonding area to be formedinto the part when it is molded. Thisis usually accomplished by loweringone side of a part and raising a side ofthe second part. This allows the twopieces that will be bonded to slideover each other providing a precisefit. The joint that is formed when thepieces are joined in this manner is re-ferred to as a "joggle" (see Figure 3).With this type of overlap the builderis only required to lay down the struc-t u r a l adhes ive and apply someclamping pressure. Figure 4 showsadhesively bonded joints similar tothe wet lay-up joints in Figure 1.

Some kit manufacturers prefer tocombine both bonding methods to

Figure 2

Figure 4

^Adhesive-

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achieve the greatest possible strength.The key to achieving strength in anyjoint is to properly prepare the surfacesthat will be joined. The laminatingresin or structural adhesive must bondwell to the surfaces. The surfacesshould be cleaned properly and sanded.

The main alternative to bonding ismechanical fastening using rivets,screws, bolts, etc. Metal aircraft typi-cally use mechanical fastenersexclusively. Composite aircraft usemechanical fasteners in areas whereparts will be disassembled for mainte-nance or inspection. These areasinclude cowlings, fairings, inspectionopenings, etc.

Bonding composite pieces togetherhas an added benefit over mechanicalfastening in that a bond is created alongthe entire surface of the joined parts in-stead of only where fasteners areinstalled. A bonded joint will be asstrong as or often stronger than a me-chanically fastened joint as long as thebonding is properly done. As a review,to accomplish a proper bond the surfacesmust be properly prepared, an adequatebonding area presented, and the appro-priate adhesive material applied.

Cleaning prior to bonding.

SYSTEMS INSTALLATIONInstalling the various systems in a

composite airplane will consume ap-proximately 1/3 of the building time.Typically, the time required to build acomposite airplane will consume ap-proximately 1/3 of the building time.Typically, the time required to build a

composite airplane will consist of 1/3spent in basic building, 1/3 in systemsinstallation, and 1/3 in finishing. Thesystems installation varies consider-ably depending upon the actualairplane you are building and howmuch has been completed by the man-ufacturer. Installation of systems iscomposed of control surface tubes or

Vacuum bagging92 DECEMBER 1997

cables, engine installation, engine andpropeller controls, instruments, seatbelts, landing gear and brakes, etc. Asyou build and assemble the airplaneyou wi l l install the various systems.Your plans will specify when to in-stall or fabricate each system. Systeminsta l la t ion is an ongoing process.The bu i lder can also legally h i r esomeone else to assist wi th certainsystems installation. An example ofthis would be in engine installation orinstalling avionics. Advisory Circular20-139 explains in detail what you asthe builder are allowed to contract outcommercially without jeopardizingthe major portion rule.

FILLING AND FINISHINGAs previously mentioned, finishing

a composite airplane consumes fully1/3 of the total building time. Obvi-ously, th i s stage of construction isvery important to the builder becauseit determines the final look of the air-plane. Two choices are ava i lab leregarding when to finish a compositeairplane. You can fill and finish com-ponent parts prior to assembly. Thisrequires even more time because youare assembling the airplane then dis-assembling it to complete thefinishing process. Then the completedparts are joined together. Most peopleprefer to finish the airplane after it hasbeen assembled. In both cases the fi-nal painting is usually accomplishedafter the airplane has been test flown.

Why is finishing necessary? A com-pleted composite part wi l l exhibi t arough look. The weave of the rein-forcement mater ia l w i l l be veryapparent. Filling is usually required asthe first step to a smooth finish. Wehave all seen the extremely smoothsurfaces found on composite aircraft.That finish is the result of a lot of hardwork. There are many rough imperfec-tions that exist before the f i l l ing andfinish process. It is also interesting thatmost composite aircraft are paintedwhite or a light color. This is neces-sary because of the heat build-up whenthe airplane is in the sun that creates ahigh skin temperature. This is detri-mental for two reasons: (1 ) it causesepoxy to shrink more than normal, and(2) it will overheat and damage foamcores. In 90° ambient temperatureswhite paint has a skin temperature of140°F and black painted skin can reach

210°F. You have two choices — eitherfly only at night or paint the airplanewhite or a light color.

F i l l ing and finishing does a lotmore than simply creating an awardw i n n i n g look. Composite a i rcraf thave about a twenty-year track recordwhich can be examined. There havebeen a lot of composite airplanes builtsince the KR series and the Kutan rev-olution that began in the 1970s. Hereare some observations regarding fin-ishing that have surfaced as a result ofthis history.

First of all, many builders haveused too much filler on their airplanes.Too much f i l ler of any sort is badnews in high flex areas or on leadingedges. Fillers are to be used for fillingand not for building. Several of thesefillers have been made from polyesterresins. In previous art icles I havestated that polyester should not beused for aircraft application. The rea-son — it cracks and peels off insheets. That is beginning to occur inseveral composite airplanes that havebeen flying for a number of years.

Secondly, polyester surfaceprimers have been used on a numberof airplanes. Same problem! Mostpaint cracking is caused by heavy ap-plication of these primers that w i l lresult in shr inkage over the years.When it shrinks it takes the topcoatwith it, even high-dollar polyurcthanepaints . There arc a number of air-planes being repainted today becausetoo much polyester primer was used.

Thirdly, th ick coats of high buildautomotive polyurethane wi l l alsocrack. Most two-part polyurethanewill flex very well as topcoat paintsbut t h i ck coats of the product wi l lthen quit. The quest for the perfectfinish should be done with sandpaper,not the spray gun. Professionalpainters realize that surface prepara-tion is 90% of the job.

Lastly, epoxies must be protectedfrom UV radiation. Epoxy resins aresubject to deterioration when exposedto sunlight. One resin manufacturercautions that their highest grade epoxycan totally break down in 15 monthsif not protected from the sun. This is

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Adhesive bonding.

true of all epoxies. The symptom ischalkiness followed by delamination.The best way to protect epoxy is touse a primer that will block sunlight.When paint manufacturers state thattheir products have 100% UV pro-tection, they are talking about thepaint or primer that is being pro-tected from UV radiation and not thesubstrate they are covering. Primersthat totally block the sunlight aresimple insurance policies.

Aircraft composite filling and fin-ishing has taken most of itstechnology from the automotive in-dustry. The reason for this is becauseautomotive technology has beenavailable and people are familiarwith it. The problem is airplanes flexmore than cars. Again, this can resultin a cracking problem if the wrongtype of filler or primer is used. Auto-motive products are usually polyesteror lacquer. Polyester has been dis-cussed. Lacquer products are alsosubject to the same cracking prob-lem. We have all seen lacqueredfurniture that is crazed.

5 Finishing Steps

Step One — FillingComposite structures usually have

two major areas that need rough fill-ing: depressions caused by seams orjoints and the weave pattern of the

reinforcing material (usually fiber-glass) used in the lay-up. Therougher the fabric the more fillingrequired. Molded kit airplane partsare usually fairly smooth when theycome out of the mold. These partsrequire little or no weave filling butwhere these parts are joined (bonded)together there are troughs andcrevices. Rough filling in these areasis inescapable.

The classic method of fillingrough areas or weave patterns is touse a homemade "micro or slurry," amix of epoxy with microballoons.Remember, microballoons are smallbubbles or miniature balls made ofglass or plastic. The idea behind thisis to offset the epoxy resin with alighter material. You add microbal-loons to epoxy until you get aconsistency like peanut butter. Youthen trowel or squeegee the mixtureinto the area you want to fill.

Many people have used Bondo inplace of micro. Recall our earlierdiscussion considering polyesters.Bondo is a polyester and will shrinkwith time. It is also heavier than ourmixture. I do not recommend the useof Bondo on an airplane unless youwant to repaint it after a few years.

Another product that is now avail-able is called SuperFil . Thiscommercially formulated product is apre-mixed epoxy filler. It eliminatesthe guess work necessary in mixing

your own micro. It is made in a high-shear mixer that allows more filler tobe used. When mixing your own mi-cro, if you add too little f i l ler themixture is difficult to sand and if ithas too much filler it will becomeweaker in shear. Many builders arenow using SuperFil instead of mixingtheir own slurry. Of course, weight isimportant when we are filling. Ourown mixture of micro can weigh aslittle as 6 pounds per gallon com-pared to Bondo that weighs about 12pounds per gallon. SuperFil weighs inat 3-1/2 pounds per gallon. The bot-tom line with fillers is use only anepoxy filler or a polyurethane fillersuch as PPG's Rage.

The filler is mixed by weight andthen spread onto the area to be filled.You must be careful not to put toomuch filler on the surface. Too muchfiller of any sort has the potential ofcracking over the years. You startwith very thin coats of filler forcedhard into the surface. Technique be-comes very important in thisapplication and will be discussed in afuture article devoted entirely tocomposite finishing.

After application of the filler ma-terial our soon-to-be favorite activityof sanding begins. Hand sanding ispreferred over machine sanding. Useof high quality sandpaper is also es-sential. There is also a new line oftools available for sanding manufac-tured by the Perma-Grit Company.Be aware that you may have to applyseveral layers of filler to get the de-sired final result.

Step Two — PrimingActually, priming a composite air-

plane usually consists of a smallamount of filling. The filling stepcompletes 90% of the needed surfacepreparation. The remainder is usu-ally accomplished using a fi l ler/primer. Several primer/fillers areavailable on the market. FeatherCoat, Feather Fill, and Smooth Primeare examples. The objective of afiller/primer is to fill small imperfec-tions left from the major filler and tofill all pinholes. Filler/primers areusually sprayed on the surface. Afterabout the second coat those dreadedpinholes (every composite builders'curse) appear. Several coats offiller/primer will be needed to fill

94 DECEMBER 1997

these pinholes. A new product thathas just appeared on the market willactually fill pinholes. The name ofthat product is Smooth Prime. It isalso a water based primer that is partof an entire water based compositefinishing system called Flight Gloss.It is a Poly-Fiber product availablethrough major suppl ie rs . Manyfiller/primers only bridge pinholeswhich means they reappear aftereach sanding.

The actual primer designed for aspecific topcoat paint will do littlefi l l ing. A topcoat primer is definedas a coating that is used to ensure thesubsurface does not deteriorate andto provide a base for the topcoat. Incomposite applications, this typeprimer is usually not necessary if afiller/primer has been used. Remem-ber, most primers wi l l not protectresins from the UV rays of the sunand there are no corrosion issueswith composites. If you are going touse a pr imer use the one recom-mended by the topcoat manufacturer.

Concerning UV protection, theFlight Gloss composite f inishingsystem has a step to block the UV ra-diation. The product name is SilverShield and it contains mica that is aknown blocker of UV rays. It is awater borne product that is sprayedon over the filler/primer and it wil lprotect the epoxy resin.

Step Three — Final TopcoatThe topcoat is one of your choice

as long as it is light in color — usu-al ly white. There are a number ofexcellent topcoats on the market.Most of them are polyurethane paintsand you need to be aware of thehealth hazards involved if you arespraying them. A forced air breath-ing system must be used such as theone manufactured by HobbyAir. Usethe product as directed by the manu-facturer.

This concludes our discussion ofcomposite aircraft building. I hopeyou will consider the pleasures andbenefits to be derived in building acomposite a i rplane. The choicesavailable for composite airplanes arealmost un l imi ted . Composite con-struction is the leading technology inthe aviation industry today. Ama-teur-built aircraft designers have

certainly contributed to the overalldevelopment of composite technol-ogy. Composite airplanes arc sleek,efficient, lightweight and extremelystrong. Bui ld ing a composite air-plane is a very rewarding experience.I would recommend the workshoppresented by the EAA and SportAiron composite construction. This two-day course is available in variouslocations around the country. I want

to acknowledge the fo l lowingSportAir instructors for their input tothis article: Jeff Russell Acrocad,Inc., Greg Kress — Kress PrecisionComposites, and Jon Goldenbaum— PolyFiber, Inc. Other referencesinclude Basic Composites by AndrewMarshall, Composite Construction byJack Lambie, and Understanding Air-craft Composite Const ruct ion byZeke Smith. *

EAA/SportAir Workshop ScheduleDecember 6-7 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Griffin, GAJanuary 16-17, 1998 . . . . . . . . . . . . . . . . . . . . . . . . . Sebring, FLFebruary 7-8 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Griffin, GAFebruary 2 1 - 2 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Chino, CAMarch 21 -22 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Denton, TXApril 4-5 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Minneapolis, MN

Information on these workshops can be obtained by calling 1-800/967-5746 or contacting the website at www.sportair.com. The author can beemailed at ralexander@sportair.com

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