AIRCRAFT BUILDING

COMPOSITE CONSTRUCTIONPartll

BY RON ALEXANDER

L ast month I began a series of ar-t icles int roducing compositeconstruction. As a review, I dis-

cussed the history of composite aircraftwithin the sport aviation field, definedthe term "composite," and listed thestages of building a composite air-plane. The stages of construction arc(1) decision and planning, (2) basic-building and assembly, (3) systems in-stallation, (4) filling and finishing, and(5) inspection, certification, and finalpre-flight. Our discussion this monthwill begin with the basic building andassembly phase.

Basic building starts with safety. Safetyconsiderations are of the utmost impor-tance as you begin the actual constructionof your composite airplane. Working withcomposites can be hazardous if properprecautions are not taken.

SAFETY ISSUES

All resins, hardeners, catalysts, sol-vents . . . in short, all chemicals usedin composite construction should beconsidered hazardous. Some of theseare more hazardous than others but allpose a potential health problem. Ab-sorption of these chemicals throughthe skin is a major hazard. Epoxies canbe absorbed through skin contact andthe effects are cumulative with ex-tended use. You may use a certainepoxy for years with no adverse skinreaction and then you suddenly be-come sensitized and develop a painfulrash or other problem. A wide varianceof opinion exists among professionalsconcerning the best way to protectyour skin (hands in particular). It isimpossible to make an emphatic state-ment concerning how to protect yourhands. It's impossible because thereare individual physiological differ-ences. The bottom line is some people96 NOVEMBER 1997

1T3> -i» i & a

Template for hot wiring.

Hot wire device and polystyrene foam.

are much more sensitive than others. Ifyou are just beginning to work withresins and your chance of contactingthe chemicals is minimal, you can useInvisible Gloves, a skin barrier cream.The key to using Invisible Gloves is torecoat at least every hour. Barriercreams provide adequate protectionwhen you have limited exposure. La-tex gloves also offer protection and arewidely used. Some people will useboth Invisible Gloves followed by la-

tex gloves. Sweating of the hands of-ten contributes to an allergic reaction.To preclude this many people will usecotton glove liners followed by vinylor butyl gloves. Overall, butyl glovesoffer the best possible protection butthey are expensive. You will need todecide which method works best foryou. Avoid skin contact with epox-ies. There are no safe epoxies.

Wear long sleeve shirts to protectyour arms. Never wash your hands with

solvents after you have been workingwith resins. Use only soap and water. Agood cleaner for composite tools is or-dinary apple cider vinegar. Denaturedalcohol also works well. There is reallyno reason to use solvents with compos-ite construction. Do not breathe thevapors emitted when using resins. En-sure that you are in a very wellventilated area and use a charcoal fil-tered respirator as an added precaution.

An additional hazard involved withusing resins is the exothermic reactionthat results from the curing process. Arapid increase in temperature resultswhen the curing process of the resinsystem begins. Mixing large quantitiesof resins should be avoided. Often alarge quantity of resins will exotherm tothe point that the heat can potentiallyreach a temperature that wi l l ignite afire. To avoid this problem mix smallquantities, no more than one quart.

Vinyl ester resins pose another typeof problem. Skin sensitivity is oftennot as pronounced as with epoxies.However, vinyl esters must be cat-alyzed using M E K P (methyl e thylketone peroxide). This chemical isvery hazardous if it contacts your eye.Be sure to wear eye protection if youare using a v iny l ester. Addi t iona lproblems can be encountered if youare promoting vinyl esters. Usually avinyl ester has been promoted whenyou receive it.

As I discussed last month, cuttingthe core materials can pose a safetyproblem. The only core material that

we cut using a hot-wire device is poly-styrene. All other foams emit apoisonous gas when burning. Theymust be cut using a saw or knife. Re-member, do not burn the excess scrapsof urethane foam. The gas emitted iscyanide. When cutting using a saw besure to wear a dust mask to preventbreathing of the particles.

Sanding of reinforcement materialswill release small airborne fibers intothe air. To protect your lungs fromthese particles you should wear a dustmask or a respirator. Also, protectyour skin from these small particles ofglass. Mixing microballoons (smallglass spheres) emits the spheres intothe air. Do not breathe these glassspheres. Milled glass, Cab-O-Sil, andcotton flox also present the same prob-lem. Do not breathe these particles orallow them onto your skin. Eye protec-tion should also be used to prevent theparticles from reaching your eyes.

Composite construction does havecertain hazards. However, with everytype of construction we are confrontedwith different types of safety problems.Proper knowledge and adequate prepara-tion wi l l protect you from the risksinvolved in building a composite aircraft.

BASIC BUILDINGTECHNIQUES

A brief outline of each step involvedin composite construction follows.This discussion is introductory in na-ture providing an overview. The actual

Hot wiring foam.

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steps involved require a more detailedanalysis than space permits.

Cutting Foam Cores

If you are building an airplane froma set of plans you will be cutting thefoam cores into the shape of an airfoil.Many kit airplanes come with premadeparts precluding the necessity of learn-ing how to shape a section of theairplane. Assuming you will need tocut the foam core, I will briefly outlinethe procedure.

You will need a large work table onwhich to lay your foam pieces for shap-ing. If we are using polystyrene foamwe will make a template the shape ofour airfoil from our plans using ma-sonite or aluminum as a backing.Duplex nails are used to secure the tem-plate to the foam. Notice that the

Postal scale to weigh resins for mixing.

template has numbers on one side.These numbers are used to ensure uni-form cutting by the two peoplenecessary to hot wire the foam. Oneperson calls the number where the ac-tual wire is located and the otherensures that the hot wire on their side ison the corresponding number. Our hotwire device is nothing more than an in-conel wire mounted between two postswith a source of electricity providingcurrent through the wire. The wire be-comes hot and actually melts its waythrough the foam forming a verysmooth, even surface. Hot wire devicescan be up to about 60" wide. Anythinglonger than that is difficult to handle.

As you may ascertain, severalpieces of foam will need to be cut andshaped then glued together to form acomplete airfoil such as a wing. Finalshaping of the piece is usually done by

sanding. Once each piecehas been properly shaped,all pieces are then gluedtogether using a resinmixture. This completesthe airfoil section. Usu-ally additional shaping isnecessary after the partsare glued. The entire foamstructure is then preparedto accept the reinforce-ment material.Polystyrene foam haslarge cells that must bef i l led. If these cells arenot filled the resin matrixwill be absorbed into thefoam through these cells.This will result in excessresin being used which

Marking fiberglass for cutting.

adds to the overall weight. In addition,a poor bond with the reinforcementmaterial may result due to voids thatmay be present. These cells are filledusing a filler material. This can be amixture of resin and microballoonsmixed to the consistency of a thickgravy. Another filler often used forthis process is SuperFil that is a light-weight, premixed material manu-factured by Poly-fiber. A thin layer ofthe filler is then placed on the core ma-terial using a rubber squeegee.Urethane and PVC foams usually re-quire a different viscosity of mic-roslurry because their cells are verysmall.

Application ofReinforcement Material

Recalling our composite structure,we have basically three materials. Oneis the core (usually foam), the secondis the reinforcement material (usuallyfiberglass), and the third is the resinmatrix (usually epoxy) which bindsthe materials. The three together forma very strong part.

After the foam has been properlysealed, we now are ready to "lay-up" thelayers of reinforcement material. Thetype of material and the number of lay-ers are determined by the aircraftdesigner. Be sure to follow the manu-facturer or designer's plans. Thefiberglass is usually placed on the foamin layers with the strength required de-termining the number of layers.

The work area should be clean withthe ideal temperature being 70° to 80°F.Cut your pieces of fiberglass usingshears designed for cutting this type ofmaterial. Keep the pieces clean. As agoal to minimize the overall weight ofthe airplane, the weight of the resinshould equal or be slightly less than theweight of the fiberglass you are layingup. If you strive for 50-50 weight dis-tributed between the resin and the glassyou will usually achieve your objective.It is essential that you wet out the fabricthoroughly while being careful not touse too much resin. Excess resin iswasted and simply adds additionalweight. So, weigh the fiberglass or ma-terial you are bonding and mix thatamount of resin material. The most ac-curate way to mix resins is with asimple postal scale. These scales arefairly inexpensive and they provideboth ounces and grams as units of mea-

98 NOVEMBER 1997

BAGGING FILM

SEALING TAPE

CONNECTORVALVE

BLEEDER PLY

PART BEING"BAGGED"

PERFORATEDRELEASE SHEET

PEEL PLYFABRIC FIGURE 1

surement. Prepare yourself for mixingresins by protecting your skin. Using ameasuring cup weigh the properamounts of resin and hardener as notedon the container. Mix the two togetherby stirring with a mixing stick for a pe-riod of at least two minutes to ensureadequate blending. At a temperature of70° you will usually have a workingtime of about 45 minutes, depending onthe resin system used. Place the fiber-glass on the foam surface orienting thefibers according to the design and thenpour a small amount of resin on thefiberglass. Use the rubber squeegee tospread the resin onto the glass. Brushesand grooved laminate rollers are oftenused in the laminate process Be sure tocover the glass uniformly with the resinmixture. Clean up your tools using ap-ple cider vinegar. Points to remember— proper mixing of the resin is essen-tial to ensure adequate bondingstrength, mix small amounts to avoidthe exotherm problem, thoroughly wetthe fabric without using excess resin,and don't forget to protect your skin.

Use of Peel PlyPeel ply is a nylon or polyester fab-

ric (similar to the fabric used onairplanes) which is used after a layuphas been completed to remove excessresin and to ensure an adequate bondbetween layers of glass. This material isplaced on the resin before it has cured.It is squeegeed into place actually wick-ing up resin from underneath the peelply itself. The resin is then allowed tocure and then the peel ply is removedfrom the laminate. The result is a verysmooth surface, derived without sand-

ing, which will result in greater adhe-sion of subsequent layers of material.The use of peel ply on laminates (lay-ers) of material has the followingadvantages: (1) peel ply causes thefibers to lay flat, (2) it reduces theamount of sanding necessary, (3) peelply increases the adhesion in subse-quent bonding and the adhesion of

primers, and (4) it reduces the amountof resin used on the structure.

Vacuum Bagging

The term is fami l iar to manybui lders but often not understood.Vacuum bagging, very simply, is amore sophisticated method used to re-move excess resin and to improvelaminate quality. Vacuum bagging is aprocess us ing a vacuum pump to"draw" a vacuum on several parts of alaminate. This draws the parts veryt ight ly together forcing out all voidsand excess resin. The process alsoserves to hold reinforcements, resins,and core materials in close conformityto complex shapes. Without a doubt,vacuum bagging increases the timeand materials cost of a laminate. How-ever, it offers significant advantageswhen optimum strength to weight isessential. While specific materials mayvary depending on the particular appli-cation, the basic components of avacuum bag assembly include lami-nate (layer of glass), peel ply, bleeder

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Basic lay-up.

Clean up items.

ply, bagging film, sealant, connectorand vacuum pump (see Figure 1).

As noted, peel ply is also used withthis application. The vacuum pump isattached through the connector valveinto the bagging film. The bagging filmcontains the vacuum and applies pres-sure to the laminate. It must be able tostretch and conform without rupturing.Bleeder ply absorbs the excess resinand communicates the vacuum evenlyover the entire surface. A perforated re-lease sheet allows excess resin totransfer from the part being bagged tothe bleeder ply. Peel ply separates thecured laminate from the bag assemblyallowing removal after curing. The re-moval of the peel ply is usually notdone until the surface is ready for paint-ing or secondary bonding. Keeping it inplace will protect the laminate surfacefrom dirt and oil. A tremendous amountof pressure can be applied using thisprocess. As an example, a vacuum of100 NOVEMBER 1997

15 inches of mercury will produce aforce exceeding 1000 pounds per squareinch. As you can see, this is a very effi-cient means of removing excess resinand eliminating voids.

Post Curing

Post curing is a process used to ob-tain maximum strength from a resin. Tounderstand post curing it is necessary todefine the term Glass Transition Tem-perature or Tg. The transitiontemperature of a resin from a hard glassystate to a soft rubbery state is called itsTg. At the Tg the tensile strength, chem-ical resistance, and hardness aresignificantly reduced while the flexibil-ity is increased. Post curing is performedby raising the temperature of the lami-nate above standard room curetemperature. Most resin systems willnot reach their full strength unless theyare cured at a temperature considerably

above room temperature. Usually thistemperature is about 40°F below the Tgspecified for the resin. The post curetemperature should never surpass anymaximum temperature of another mate-rial in the laminate such as the foam.Without post curing the Tg will only beapproximately 40°F above the tempera-ture at which the resin was cured. On ahot day the temperature of a structurecan exceed the Tg which could causethe entire matrix to soften. This soften-ing can result in the matrix of the heatedportion being softened and pulling away.The once smooth surface now exposesthe weave of the fabric. Structural in-tegrity can also be affected by hightemperatures in structures that have notbeen post cured.

With this in mind, it is important thatyou follow a post curing procedure. Youcan do this yourself by introducing theproper amount of heat into a fireprooftent-like structure containing your partor the entire airplane. Introduce the heatgradually to the temperature specifiedby the resin manufacturer. Usually thiswill be between 140° to 180° F. Again,care must be taken to not exceed thebreakdown temperature of other compo-nents such as the foam.

The above discussion will provideyou with a basic understanding of com-posite construction. Most composite kitaircraft do not require shaping the airfoilsection from foam. Instead, you are pro-vided sections of the airplane that haveto be bonded together. Next month Iwill conclude the discussion of compos-ite construction by presentinginformation concerning bonding tech-niques and finishing composite surfaces.Hopefully, at the conclusion of these ar-ticles you will have a basicunderstanding of composite airplanesand how they are assembled. At thatpoint you will be prepared to decidewhich airplane you want to build.

The author may be reached at ralexan-der@sportair.com. Diagram furnishedby Richard Kunc.

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