AIRCRAFT BUILDING

BUILDING A COMPOSITEAIRCRAFT

BY RON ALEXANDER

Within the sport aviation world,the term "composite aircraft" is syn-onymous with sleekness of designand speed. These airplanes, com-posed largely of fiberglass, arebecoming more and more popular.Certainly when we attend a large fly-in we see rows and rows of compositeaircraft. To many of us these air-planes are somewhat mysterious.How are they built? What does theword "composite" actually mean?Are they safe? How difficult are theyto build?

Actually, composite aircraft con-struction is not a new idea. Glidershave been constructed using fiberglassfor many years. Throughout aviationhistory, advances in design have beenmade. Beginning with wooden struc-tures that were covered with fabric,technology then advanced to weldedsteel framework and on to aluminum.As each type of construction was in-troduced, design improvements weremade in strength and aircraft perfor-mance. Composite construction is yetanother advancement for the aircraftindustry. Fiberglass construction hasbeen and continues to be used in man-ufacturing a number of parts found onmost airplanes. Of course, we nowsee many airplanes that are con-structed almost exclusively out ofcomposite material. Composite tech-nology has certainly changed theentire aviation industry and in particu-lar sport aviation.

Amateur built composite airplaneswere actually introduced during the1970s when Ken Rand introducedthe KR-1. Burt Rutan also intro-duced the VariViggen that featuredsome composite construction, andthe VariEze in 1976. This airplane92 OCTOBER 1997

RESIN MATRIX REINFORCEMENT FIBERS

FOAM CORE

STRENGTH 1.0WEIGHT 1.0

design included a more comprehen-sive type of composite constructionusing moldless techniques. The termmoldless will be defined later. TheVariEze was very successful inspir-ing Rutan to develop the Long-EZ.During the 1980s, several other de-signs were introduced to sportaviation enthusiasts as popularity ofthis type of construction heightened.It was during this period of time thataircraft "kits" were first introduced.Supply companies began offeringmaterial kits to builders to simplify

the building process. Plans for com-posite airplanes could be purchasedand then materials for each phase ofconstruction could be obtained on anas needed basis. The amount of timeneeded for completion is a factor inbuilding an airplane from a set ofplans. With this in mind, severalcompanies began introducing theirown airplane designs in kit form.The objective was to allow thebuilder to spend less time actuallyconstructing the airplane. A largenumber of parts and pieces were

manufactured by the company andsold to individuals. This concept in-troduced the pre-fabricated kitairplane that is popular today in alltypes of construction.

From the late 1980s through todaywe have seen many composite air-craft kits offered to prospectiveairplane builders. This decade(1990s) has seen a tremendousgrowth in the popularity of amateurbuilt composite airplanes. Higherperformance airplanes with manyvarying appearances are being of-fered by a large number of kitmanufacturers and also by designerswho offer plans. This is truly an ex-citing time for our industry.

Before beginning our discussion ofcomposite construction, let's definethe word "composite." The dictionarydefines a composite as "a complexmaterial such as wood or fiberglass,in which two or more distinct, struc-turally complementary substancescombine to produce structural or func-tional properties not present in anyindividual component." In simpleterms, a composite structure has morestrength than the individual compo-nents that make up the structure itself.For our purposes, the component partscomprising a composite structureconsist of a core material, a reinforc-ing material and a resin binder. Eachof these substances alone has very lit-tle strength but combined properlythey become a composite structurethat is very strong.

To further explain the structure,the core material keeps the rein-forcement fibers separated so theycan be kept in maximum tensile (ten-sion or stretching) strength. Thereinforcement fibers carry the load.They must be properly oriented toachieve their maximum potential.The resin keeps the fibers in place sothey can maintain straightness anddeliver their maximum strength. Theresin also binds the fibers to the core.Therefore, a composite structure isreally a mixture of critical compo-nents. When loads are applied to awing, as an example, the majority ofthe stress occurs at the outer sur-faces. To take advantage of thisprinciple, a sandwich panel is de-signed with two working skins onthe outside that are separated by alightweight core. This type of designconcentrates the strength in the area

of high stress (outer surfaces) whilereducing the weight in the area oflow stress (inside the wing). I willfurther expand on the specific typesof materials used later in the article.

To further complicate the issue,you will hear the words moldlessand molded used in composite con-struction. To define these words asthey apply to us is relatively simple.Moldless construction, as the nameinfers, does not use a mold. Thistechnique allows the builder to con-struct a part by forming a corematerial to a desired shape and thenlaminating the reinforcement mater-ial to the shaped piece to make upthe final part. The core structure,usually a foam like material, allowsthe builder to employ virtually anyshape desired. Original designs suchas the VariEze used moldless typeconstruction. Many airplane designscontinue to use this type of fabrica-tion. Moldless techniques allow thebuilder to produce a safe, superiorairplane without the requirement ofexpensive equipment or extensiveexperience.

In contrast, molded fabricationuses a mold to build the part. A mas-ter mold or "plug" must first be builtin the same manner as you wouldbuild a moldless part. You then con-struct a working mold from themaster and then finally make the ac-tual part from the working mold.Within our industry, molded com-posite construction is very popular.A large majority of kit manufactur-ers use this type of fabr icat ion.Molds are made by the kit manufac-turer who then fabricates the partsfrom the mold. The manufacturerthen supplies you, the builder, withthe parts. As an example, a wing kitmight consist of two wing halves,built from a mold, along with thenecessary ribs. You would then as-semble the wing by bonding the ribsto the wing halves and, of course,bond the halves themselves together.Compare this with moldless in whichyou actually form the wing, comply-ing with a set of plans, out of a foammaterial. You then place several lay-ers of fiberglass on the foam usingresin to bind the two. The end resultwould be very similar. One type ofconstruction (moldless) has a corematerial you have shaped that issolid whereas molded usually has

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thin cores that are sandwiched be-tween skins and you actually assemblethe supplied parts. Building a moldedtype composite kit is very similar toassembling a plastic model airplane.The building of most amateur builtcomposite airplanes will require useof both types of construction.

To summarize our general discus-sion, composite structures that com-bine the best qualities of diverse mate-rials have opened a new world to theairplane builder. Modern compositeconstruction offers several advantagesover conventional techniques. Whilesafety tolerances for metal structuresare often designed at 1.5 to 1, light-weight reinforced composites allow"overdesign" by factors of severaltimes, increasing both safety and per-formance. These designs also achievebetter aerodynamics by eliminatingjoints and rivets in addition to reduc-ing problems of corrosion. Compositedesign allows an easy way to achieve alow drag airfoil. Composite airplanesare usually faster for a given horse-power than their counterparts becauseof airfoil shape and smoothness. Onecommon misconception that docs existis that composite airplanes alwaysweigh less than metal airplanes. Thisis often not the case. Fiberglass isheavy. If we were to construct an air-plane wing out of solid fiberglass wewould have a very heavy airplane. Re-member though, instead of doing thiswe insert a piece of core material be-tween layers of fiberglass to reduce theweight. Kit airplanes use ribs and morecontemporary types of construction toachieve the high strength with a lowerweight. t , ; •-. v.

J STEPS IN BUILDING A! COMPOSITE AIRPLANEii1 Building a composite airplane en-tails five stages of construction.These five stages are (1) decisionand planning, (2) basic building andassembly, (3) systems installation,(4) filling and finishing, and (5) in-spection, certification, and finalpreflight.

i Decision and PlanningAs we have previously discussed,

this phase of construction is critical toour successful completion of an ama-teur built airplane. You cannot spend94 OCTOBER 1997

Composite workshops attendees

too much time planning. A large partof the planning process is technicalknowledge. Composite construction,like all types of construction, requiresa certain amount of basic knowledge.EAA and SportAir offers a 2-dayworkshop explaining the techniquesof composite construction with timespent actually building airfoil sectionsutilizing this method of construction.More information on these workshopsis presented at the end of the article.

In our discussion on decision andplanning we will look at the types ofmaterials used, tools required andworkshop requirements.

Materials Used inComposite Construction

Core Materials

A word of caution. The specifica-tions for the materials to be used foryour airplane should be stated withinyour plans or provided with your kit.It is important that you conform tothe plans of the designer.

Choosing the proper core materialis critical to the overall composite'sperformance. Note the illustration inFigure 1. The first item is one piece ofmaterial with its respective weightand strength being shown as 1.0.When we insert a core material dou-bling the thickness of the compositenotice that the strength increases to3.5, the stiffness to 7.0 but the weightonly increases by 3%. Further strength

is noted by increasing the thicknessfour times. Observe even in this casethe weight only increases by 6%.

Lightweight core materials in-clude wood, foam and honeycomb.Wood has obviously been around fora long time. It serves as a good corematerial for many composite de-signs. It is stiff, strong and has highshear properties. However, its varia-tions in density and physicalproperties along with the difficultyin fabricating limits its application.

Foam is usually the choice of ma-terial for the custom aircraft builder.Foams are easy to shape and reason-able in cost. Three types of foam aregenerally used within our industry.Polystyrene foam is the first. It isblue in color and is supplied in largebillets. Polystyrene foam is oftenused to construct boat docks. Thistype of foam can be easily shapedusing a "hot-wire" technique de-scribed later in this article.Polystyrene foam is the type used inseveral popular composite airplanesin the wings and control surfaces. Itdoes have the disadvantage of beingsoftened by exposure to gasoline andseveral other solvents. This type offoam cannot be used with polyesteror vinyl ester resins, both of whichwill be discussed later.

Polyurethane foam is basically alow-density insulating type foam alsoused for the construction of surfboards. Polyurethane foams are oftenused within a fuselage structure or for

parts requiring detailed shaping. Thistype of foam is impervious to mostsolvents. Its color is usually tan orgreen. Polyurethane foam has certainhazards. It emits a poisonous gaswhen burned. DO NOT USE A HOTWIRE DEVICE TO CUT POLY-URETHANE FOAMS. You also donot want to burn any scraps of thistype foam. Carving and cuttingshould be accomplished using a knife,saw or other cutting tools.

Polyvinyl chloride foams (PVC)are based on the same chemistryused in common PVC water pipematerial. Divinycell™ and Klege-cell™ are trade names for this typeof foam. Both of these are suited forstructural cores. This material is re-sistant to most solvents and it canwithstand a high temperature.

The last type of core material ishoneycomb. This material has an ap-pearance much as the honeycombfound in a bee hive. The sheet mater-ial used to form honeycomb can bewoven fabric, metal or paper. Honey-comb cores are used very extensively

in the aerospace industry. Varyingthicknesses are available along with awide variety of materials. Honey-comb is usually supplied in four feetby eight feet sheets. Honeycomb ma-terials offer exceptional strength toweight ratios but reliable bonding toouter skins is more difficult toachieve.

Reinforcement Materials

Many types of reinforcement ma-terials are available for aircraft use.Three types are used most often tobuild custom aircraf t . These arefiberglass, carbon fiber and Kevlar®.

Glass fiber or fiberglass is themost widely used reinforcing mater-ial. Fiberglass is manufactured withvarying physical characteristics andcost. One of the most widely used istermed E-glass. This type of glassfiber has the best physical character-istics at the lowest price. One othertype with limited use in our area isS-glass that is about 30% strongerthan E-glass but the cost is often 2-3

times higher. Fiberglass is also of-fered in various weaves. The termsunidirectional and bidirectional areused. Unidirectional simply meansall of the glass fibers are running inone direction lengthwise. They areheld together with threads runningparallel to the glass fibers. Bidirec-tional fabric means the same numberof fibers go across the material asfound lengthwise. The type of weaveis then defined. Several weaves areavailable such as plain, basket, satin,twill, etc. Fiberglass also is availablein varying weights from less thanone ounce per square yard to over 10ounces per square yard.

Carbon fiber or graphite is a verystrong reinforcement material. It isused on sail boat masts, golf clubs,etc. Carbon fibers combine lowweight, high strength and high stiff-ness. In the custom aircraft area,carbon is used in critical areas suchas spars, etc. Working with carbonfiber is somewhat difficult and whenit fails it will snap like a carrot. Ofcourse, the failure point where this

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occurs is extremely high.Kevlar® is a product of the DuPont

Corporation. It is a very tough mater-ial with a high strength and is used inmaking bulletproof vests. Kevlar® isvery effective in applications requir-ing resistance to abrasion andpuncture. However, its use in primarystructures is often limited by the rela-tively low compression strength anddifficulty in handling.

Resin Matrix

The resin component in a compositeserves to maintain fiber orientation,transfer loads and to protect the struc-ture against the environment. While acomposite's stiffness, flexibility andtensile strength are more affected bythe reinforcement material, its heat re-sistance, shear and compressivestrength are more dependent on theresin system. Three types of resin sys-tems are available: polyesters, vinylesters and epoxies. All three requirethe user to mix a specific amount ofhardener with a base chemical. Thechemicals involved are shipped sepa-rately and combined only when thebuilder is ready to use the resin.

Polyesters are most widely used forindustrial applications and within theboat industry. They are cheap and setup fast. A typical polyester is Bondo.Polyesters are easy to mix with theamount of hardener added only affect-ing the time needed to develop fullstrength. Polyesters are not suitable forapplications requiring high strength.They also will shrink over a period oftime. You may have noticed an auto-mobile fender repair where the paintcracked over a period of time. Chancesare Bondo was used as a filler, andsince it is a polyester, it cracked underthe paint. In a few words, polyestersare the least capable resin for struc-tural aircraft use.

Vinyl esters are used extensivelythroughout our industry. Vinyl estersare a crossbreed between polyestersand epoxies. They are much more ca-pable than polyesters in strength andbonding. Vinyl esters are low in vis-cosity making them easy to use. Thecure time can also be easily affectedby adding more hardener thus speed-ing up the cure time. Despite the curetime, hardened vinyl ester usually ex-hibits consistent properties of strengthand flexibility. Vinyl esters are not96 OCTOBER 1997

Sanding and shaping tools

Filler Material

subject to moisture problems duringapplication and are also lower in pricethan epoxies. One of the disadvan-tages found in using vinyl esters is inthe mixing of the chemicals. Vinylester resin is usually "awakened"from its dormant state with cobaltnapthenate (CONAP) prior to use.Just before using the system dimethylaniline (DMA) is added as an acceler-ator that determines how quickly themix will cure and, in addition, methylethyl keytone peroxide (MEKP) isadded as the hardener that actuallystarts the curing process. Mixing ofthese chemicals can be somewhatcomplicated in addition to being haz-ardous. MEKP mixed directly withDMA or CONAP, apart from the baseresin, can be explosive. Overall, vinyl

esters provide an easy to use, inex-pensive resin system. Proper carecertainly must be taken during themixing process.

Epoxies have come to dominate theaerospace industry and are the basicresins used in most amateur built air-craft. Epoxies differ from polyestersand vinyl esters in that they hardenthrough a process termed "crosslink-ing." Epoxies are essentially longchains of molecules that intertwinewhen hardened to form a strong matrixof crosslinked chains. This provides aninner structural strength to the resin.When combined with the proper rein-forcement material, compositestructures using epoxies are unmatchedin strength and lightness. Epoxies arepackaged in two parts: a resin and a

hardener. Unlike polyesters and vinylesters, the resin to hardener mixturemust be strictly followed. Addingmore hardener will not accelerate thecure time, in fact, it may seriously im-pede the drying and strength of thecured resin. Epoxies are offered withdifferent characteristics includingstrength, curing time, etc. Care mustbe taken to follow the manufacturer'srecommendation regarding the type touse. Most epoxy cures at room temper-ature. Once this is complete additionalstrength is obtainable by raising thetemperature of the epoxy through aprocess called "post curing." Usuallythis involves raising the temperatureabove 140 degrees Fahrenheit for a pe-riod of time. If this has not beenproperly accomplished the heat from aramp on a hot day can "post cure" theepoxy on an airplane. Working timewith epoxies can be much longer thanpolyester and vinyl ester because youcan use specific hardeners that havecustom working times, some as shortas four minutes, others over 24 hoursat 70°. This makes removing excessresin that may accumulate much lessof a problem. Proper skin protection isa must with epoxies due to skin der-matitis which can be caused by thechemicals.

Tools Required ForComposite ConstructionThe tools needed to build a compos-

ite airplane are inexpensive and readilyavailable. The most expensive tool re-quired will be the scales or mixingpump necessary to measure the resinmaterial. A set of postal scales can bepurchased for about $70-$80. This is avery efficient and precise method ofmeasuring epoxies. Special shears tocut fiberglass and other reinforcementmaterials is necessary. Some peoplelike to have a Dremel tool to do shap-ing and cutting. A hot wire device canbe constructed with little cost. Othercutting and sanding tools can be pur-chased at your option. A list of toolsneeded for most composite projects in-cludes:

• Scales or mixing pump• Fabric shears• Band saw (optional)• Utility knife• Rotary pizza cutter• Rubber squeegees• Grooved laminate rollers

• Disposable paint brushes• Sanding blocks• Portable electric sander (op-

tional)• Belt sander (optional)• Charcoal filtered respiratorIn addition, you will need mixing

cups, tongue depressors for stirringand a large supply of latex gloves.

Workshop RequirementsLike most airplane building pro-

jects, if you have a space the size of atwo-car garage, you can begin. Ideallyyou should have a room to do your ac-tual "layup work" and another area orroom in which to sand. You do notwant the sanding particles to floataround your fresh resin on your layersof fiberglass. Good ventilation is nec-essary along with a way to somewhatcontrol the temperature. Resins do notlike cold temperatures. Remember,you will need a workbench in additionto a work table. The work table shouldbe large enough to cut your fiberglassand to assemble component parts. A

table three feet wide by up to 15-20feet long is sometimes recommended.Remember to lay out your tools andyour shop very neatly. This will saveyou a tremendous amount of time dur-ing the building process.

Building a composite airplane canbe a very rewarding experience. Thebasics of composites have been pre-sented in this article. Next month I willexpand on the actual building tech-niques used with this type ofconstruction. I will discuss safety is-sues, cutting and shaping foam, mixingresins, applying layers of fiberglasscloth, post curing, vacuum bagging,bonding and many other compositebuilding procedures.

For additional information on theEAA/SportAir workshops, includingtype of workshops available dates andlocations call 1-800/967-5746 or visitthe website www.sportair.com

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