AIRCRAFT BUILDING

WORKING WITHCOMPOSITES

BY RON ALEXANDER

During the fall of 1997, I wrote aseries of three articles on compositeaircraft construction. These articlesprovide an overview of compositesas they relate to aircraft building.The articles began with the October1997 issue of Sport Aviation. I amgoing to again focus attention on thispopular method of aircraft construc-tion by discussing in more detaileach step involved in building a com-posite aircraft. A certain amount ofreview will be necessary to achievethe goal of explaining the steps in-volved in this type of building.

Once you have made the decisionto build a composite aircraft, either akit aircraft or a plans built, the firststep is to set up your workshopspace, purchase the necessary toolsand organize materials and parts.

WORKSHOP SPACE

To begin this discussion it is im-portant to note that you do not need apristine laboratory to build a compos-ite airplane. Like most aircraftbuilding projects, if you have a two-car garage you have what is needed.It has been my experience that havingyour workshop in or near your homesolves two problems. First of all, youwill be much more likely to spendtime on the project after getting homefrom work versus having to drive 30minutes to another location. Thisequates to more hours on the actualproject. Secondly, your f ami ly ismore likely to become involved. Thisis very important if you are to suc-cessfully complete the project.

If you had an ideal compositeshop you would have a "clean room"

for doing layups, cutting cloth, etc.and a "dirty room" for sanding oper-ations. Most of us do not have apartition in our garage so we must becareful during our sanding opera-tions not to contaminate our work.Sanding should be accomplished af-ter completed parts are cured andcovered — not jus t after doing a

fresh layup.You will need a table on which to

cut your reinforcement fabrics (usu-ally fiberglass). Since most of yourfabric wil l be cut on a 45-degreebias, it may be handy to have a tableset up just for that. You can shape thetable by cutting one end at a 45-de-gree angle to facilitate cutting on a

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bias. The table should be wideenough to handle the fabric you willbe using (60 inches should beenough). You should be able to un-roll about 4-5 feet of fabric on thetable. You will want to place a hardplastic cutting surface on the top ofthe table to allow you to cut the fab-ric with a cutting blade. (More aboutcutting fiberglass later.) This mate-r ia l can be 1/8-inch thickhigh-density polyethylene or some-thing similar.

Another table can be constructedto do your resin mix ing and basiclayups. This table should be roughly3 feet by 8 feet depending upon theamount of space avai lable. Thelength of the table needed will alsovary with the aircraft you are build-ing. The table should be placed in anarea that will allow you to walk com-pletely around it. In addition, somebui lders prefer to have anothersmaller table dedicated to mixing106 MAY 1999

resins. After completing a part youshould remove it from the area if atall possible or hang it from the ceil-ing.

A large thermometer should beplaced where you can view it alongwith a humidity indicator. As youwill learn, temperature and humiditycontrol is very important when mix-ing and working with resins. Ideally,you should be able to control thetemperature of your workshop. This,of course, is not always practical.Place a large clock with a sweep sec-ond hand on the wall where you cansee it while working. The clock is al-ways running on your resins afterthey have been mixed. You will haveonly a certain amount of time withwhich to apply the resin before it be-gins to gel. Of course, you need afirst aid kit and an eye wash station.The eye wash station must be easilyaccessible.

Proper venti lat ion of the work

area is necessary. When workingwith resins or when sanding you willwant to move the air through theworkshop space. A fan can be set upto move the air outside the work-shop. If you really want to do it right,mount an exhaust hood over yourlayup table. This is not that difficultto do and is very effective in remov-ing fumes created from the resinswhen you are working with them.

Storage of materials, parts, etc.must be addressed. If you are build-ing a composite kit a i rcraf t thepre-molded parts must be carefullystored. Wing panels, as an example,can bend and adapt to any shape towhich they are subjected. Warpingcan result from improper storage.The best way to store parts is to sim-ply leave them in the shipping cratein which they arrived. You may alsowant to save the shipping materialsfrom the crate to use as padding, etc.for completed parts.

Resins should be stored in a warmarea if at all possible. When the tem-perature is less than about 65 degreesresins become thick. The colder thetemperature the thicker the resin.That means you will have difficultypouring the resins from their con-tainers. Several bui lders havedesigned heated areas wi thin theirshops to store resins if the shop itselfis not maintained at a normal tem-perature. If resins are stored inextremely cold temperatures they aresusceptible to crystallizing. This isnot a major problem and can be cor-rected by placing the resin containerin a pan of water and heating the wa-ter to about 160 degrees F or so untilthe crystals dissolve. Resins may bestored for several years prior to be-ing used. This is termed the i r"shelf-life." However, with epoxyresins the accompanying hardenerusually has a shelf life of less thanone year. Vinylester resins oftenhave even less time for shelf life es-pecially if they have been promotedprior to shipment.

COMPOSITE TOOLS

Most of the tools you will need tobuild a composite airplane are read-ily a v a i l a b l e and somewhatinexpensive. The following is a par-tial list of tools you will need:

• Scales, mixing pump or balancescales to mix resin

• Sanding blocks• Saws — hacksaw, coping saw,

and pad saw• Carpenter's level• Carpenter's square• Clamps• Electric hand drill• Fabric scissors• Rotary cutter• Grooved laminate rollers• Knives — including utility knife

and large serrated knife• Respirator• Rubber squeegees• Straight edge• Vacuum cleaner• Hair dryerOther tools that are nice to have

consist of a Dremel tool with bits forshaping and cutting, a die grinder,dr i l l press, band saw, rotary or or-bital sander and the list can go on.The tools I have mentioned are spe-cific to composite construction. You

will also need basic tools and usuallysome sheet metal tools for a smallamount of riveting, etc. The best wayto determine the exact tools you willneed is to read the kit manufacturer'sassembly manual or the designer'splans. They wil l almost always pro-vide you with a list of basic toolsneeded to construct their airplane.

Now that we have established whatkind of workshop space you will needalong with several of the tools thatare required let's get down to the ba-sics of construction. I will talk abouteach type of material used in com-posite construction and how to workwith each separate one. After we haveestablished a foundation, in upcom-ing issues I wil l discuss the propermethods of doing a composite layup,methods of bonding and tape glass-ing, forming hardpoints, post curing,and most of the activities you will be-come involved in if you decide tobuild a composite airplane.

If you want a complete review ofbasic composites I invite you to read

the previous articles I mentioned inthe b e g i n n i n g of t h i s a r t ic le . 1 dowant to briefly review some of thematerials used in composite con-struction with an emphasis on howto work with each one.

CORE MATERIALS

Let's begin with the core materialsthat usually consist of some type offoam. Polystyrene is the first corematerial that wi l l be discussed. Poly-styrene comes in large blocks and isnormally used to form large struc-tures such as wings, control surfaces,etc. If you are bui lding a plans builtairplane you wi l l bui ld a large por-t ion of the a i rp l ane out of th i smaterial. Polystyrene can be cut witha knife , saw or it can be hot-wiredinto the shape of an airfoil. Usuallythe la t ter w i l l be called for in theplans. You can find plans for a hot-wire device in the R u t a n bookletcalled Moldless Composite SandwichHomebuilt Aircraft Construction

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available from supply companies.This device is easily constructedfrom common materials. Templatesare made from the aircraft plans youreceive and are used as a guide incut t ing the foam to proper shape.One thing in particular when work-ing wi th all foams and especiallywith polystyrene foam, the cells orvoids in the foam must be filled priorto applying the reinforcement mate-rial. This is accomplished by mixinga slurry compound or using a com-merc ia l f i l l e r manufac tured byPoly-Fiber called SuperFil. This isthe first step in the layup processthat will be discussed in detail later.It should also be noted that vinylesterresins wi l l dissolve polystyrenefoams, therefore, they are not usedwith this type of core material.

Most of the kit aircraft use eitherpolyurethane (urethane) or polyvinylchloride (PVC) foam. These foamscome in different densities and thick-ness. Usually the thickness will befrom about one-quarter inch to twoinches or so. With most kit aircraftthe large airfoils wil l be partial lycompleted and you wi l l simply berequired to construct ribs, bu lk-heads, etc. and glue them in place.These foams are easily cut wi th aknife or saw. DO NOT HOT-WIREURETHANE FOAMS. They wi l lemit poisonous gases if hot-wired.They are also f lammable. Do notburn the scraps of material left overas the same gases are emitted. Sand-ing blocks are used to shape foams.Band saws and routers may also beused to cut and shape.

Honeycomb cores are used in sev-eral kit aircraft. You will usually notbe required to work with this mate-rial, as the kit manufacturer wi l lsupply the completed parts that use ahoneycomb core.

REINFORCEMENTMATERIALS

This is a term used for the fabricmaterials found in composite con-struction. We will find three differenttypes of materials used in most com-posite aircraft. They are fiberglass,Kevlar, and carbon fiber (graphite).Fiberglass is the most commonlyused material. It has the best physi-cal characteristics at the lowest price.108 MAY 1999

Sportair instructor Terry Schubert mixes resin.

Without going into great detail,there are a few basic things you re-al ly should know about fabrics.Fiberglass is made up of filaments ofglass that are twisted together toform a yarn. This yarn, or fiber as itis often called, is then woven intocertain styles of fiberglass. When theweaver looms fiberglass they useterms such as "warp", "fill", and"selvage edge" (see Figure 1). Warpdefines the fibers that run the lengthof the fabric as it comes off the roll.The warp direction is designated asO degrees. Fill fibers run perpendi-cular to the warp fibers. They aredesignated as 90 degrees. The f i l lfibers or threads interweave with thewarp fibers. Selvage edge is the wo-ven edge produced by the weaver toprevent the edges from fraying.Some of the new fabrics today ap-pear to not have a selvage edge. Theedges have been stitched with a light-weight thread.

Unidirect ional Fiberglass —

With unidirectional fiberglass, all ofthe major fibers run in one direction.All of the strength of the fabric isfound in that one direction. The fi l loften consists of threads designed tohold together the glass fibers. Acommon term for this glass is "uni."It is manufactured in both glass clothand in tapes. A common style num-ber used by many composite air-planes is designated as 7715. Thiscloth is typically used where the pri-mary loads are in one direction suchas a spar cap.

Bi-directional Fiberglass — Inth i s glass, the major fibers run intwo directions, both the warp and thefill. In other words, instead of usingthreads as a f i l l , glass fibers areused. Thus we have glass fibers inboth O degrees and 90 degrees. Inother words, the cloth has half of thefibers in one direction and half in theother direction at right angles. Thismeans that the cloth has the samestrength in both directions. This type

90°

WARPFIBERS RUNALONG THEFABRIC

CUT AT 45'ACROSSWIDTH

SELVAGE EDGE

Figure 1

FILL FIBERS RUNACROSS FABRIC

of cloth is commonly called "bid."Of course, there are many differentstyles and weaves that are available.7725 and 7781 are two very com-mon cloths used in amateur-builtaircraft. In your plans they will oftenbe referred to as bid cloth.

Bid cloth can be stitched togetherin more than one layer to form whatis known as biax cloth or triax clothdepending upon the number of layersinvolved. The most important thingfor you to unders tand is tha t youmust use the type and style of clothcalled for in your plans. Do not ex-periment with cloths. The designerhas specified the cloth to use basedupon structural analysis. Use whatthey tell you to use.

Keeping it simple. I am not goingto discuss all of the different weavesof cloth, etc. that are available. Youcan read Andrew Marshall's book,Composite Basics, for a good discus-sion of this. I want to concentrate onthe basics you need to know to safelybuild your airplane.

Handling and Cutting Fiber-glass — First of all , you must becareful when handl ing fiberglass.Remember to cut the glass in a cleanarea. Do not drop fiberglass on thefloor. It wi l l be contaminated withdirt and debris. If your fiberglassgets wet do not use it in the struc-ture. Be careful when hand l ingfiberglass as its shape can be easilydistorted. Mark the cloth using aSharpie marker. These marks wi l lnot show through the final f inish.Your plans will usually require youto cut your cloth at a 45-degree an-

gle. This is done to achieve maxi-mum strength in the final structure.So we w i l l u sua l ly be cu t t ing theglass on what is referred to as a 45-degree bias. You need a Sharpiemarker, a straight edge, a measuringdevice, and a good pair of scissors or

a rotary cutter. When you make acut, allowance for small deviations isusually built into the dimensions. Ifyou are wi th in one-half inch or sothat should be good. As you make acut the cloth may slightly distort. Ifso, it can be carefully pulled backinto its proper shape by pull ing onan edge. Cutting can be done using agood pair of scissors or a rotary cut-ter or they are sometimes referred toas a roller blade. Many people callthis a pizza cutter — which is a termfor the rotary cutter — it is not a realpizza cutter. Get a rotary cutter fromone of the supply companies.

After you have cut the cloth to theproper dimensions, carefully roll itinto a fair ly large roll. In otherwords, do not roll it tight. This is thebest way to transport the fabric toyour structure. We wil l see how toapply it later. If you pick it up by theends it wi l l distort and not fit thearea of the part correctly. It is alsoimportant to note that the selvageedge must be removed prior to ap-

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Unrolling fiberglass onto surface.

plying it to the structure. (Note: thiswill not apply when using the typeof fiberglass wi thout a selvageedge.) Cutting on a 45-degree biaswill cause a certain amount of waste.However, it is necessary that you cutth i s way to achieve max imumstrength. By the way, the angle is notcritical. You do not have to measureit accurately. Eying it will work fine.Let me emphasize that you must cutthe fabric in the orientation calledfor by your plans.

RESINS

To emphasize the importance ofthe resin matrix I would like to quoteAndrew Marshal l from his bookComposite Basics. "Basically, theresin matrix is the key to the wholeoperation of producing compositestructures. It was noted earlier that theresin matrix is the mass in which thefibers exist, but the resin does muchmore than just contain the fibers. Itsprimary job is to carry the load fromone fiber to the next, and from thebundles of fibers or groups of rein-forcements into an adjacent structurewhich may either be embedded in the

composite during manufacture, or ad-hesively bonded to it at a later stage.The resin material thus distributes andtransfers the load within the structureso that each reinforcing fiber carries aproportional share of the load."

There are two types of resins thatare most commonly used on compos-ite aircraft. They are vinylester resinsand epoxy resins. I am not going todiscuss polyester resins, as they shouldnot be applied on aircraft except forvery limited non-structural use.

Vinylester Resin — This type ofresin is used by several of the kitmanufacturers. Vinylesters are lowin viscosity making them easy touse. The cure time can also be easilyaffected s imply by adding morehardener thus speeding up the curetime. Despite the cure time, hard-ened vinylester u sua l ly exh ib i t sconsistent properties of strength andflexibility. Working time with thisresin is dependent upon the ambienttemperature and the amount of cata-lyst that is added. Vinylester resin isless expensive than epoxy and it willwithstand high temperatures withoutpost curing.

The negative side of it results fromthe mixing process. Vinylester resinmust be "promoted" prior to mixingthe catalyst. It is promoted using achemical called cobalt napthenate(CONAP). This chemical must beadded into the resin before catalyzing.Vinylester resin is catalyzed using achemical called methyl ethyl ketone

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peroxide (MEKP). CONAP andMEKP mixed together prior to beingplaced in the resin can cause a fire orexplosion. You will not encounter thishazard as long as you remember toplace the CONAP into the vinylesterresin prior to adding MEKP. Extremecare must also be taken when usingMEKP. This chemical is very danger-ous to the eye.

Overall, vinylester resins providean easy to use, strong, high tempera-ture, and inexpensive resin. Skinirritation problems are also less likelyto occur than with epoxy resin. Justremember to take proper precautionswhen you are mixing vinylester resins.Be sure not to mix CONAP wi thMEKP and always wear a face shieldwhen using MEKP.

Epoxy Resin — Epoxy resin hascome to dominate the aerospace in-dustry and it is widely used oncustom-built aircraft. Epoxy resinsdiffer from vinylester resins in thatthey harden through a process knownas "crosslinking." Epoxies are pack-aged in two parts: a resin and a

hardener. Unlike vinylester resin, themixing ratio of resin to hardener iscritical. Adding more hardener wil lnot accelerate the cure time. In fact, itmay seriously impede the curing ofthe resin resulting in less strength ofthe final cured part. Different types ofepoxy resins are available. Again, usethe type of epoxy called for by the de-signer. Working time may be variedusing different types of epoxies. A 5-minute epoxy is commonly used tosimply hold two pieces together forfurther bonding. These epoxies set upwithin five minutes and should not beused for structural purposes. Struc-tural epoxies will have a working timeof approximately 45 minutes depend-ing upon the type of epoxy and theambient temperature.

Proper skin protection is a mustwith epoxies due to skin dermatitisthat can be caused by the chemical. Inthe next issue I wi l l discuss how toproperly protect your skin from thisproblem. How to mix fillers and theactual process of completing a com-posite layup will also be presented. 4

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