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What do Burt Rutan, the police and a tricked-

out oyota have in common? An of-aisquestion no doubt, and the answer is notthe one Benjamin Braddock received in TeGraduate, (I want to say one word to you.

Just one word. Plastics.). No, the linkage here is composite mate-rials, which are everywhere you look. Burt makes magic airplanes

with themhas done so since the 1960s and continues to eventodaythe police wear Kevlar vests, and the guy that customizedhis oyota loves the look o a carbon-ber panel or two on his car.

Sure, youre amiliar with composites use in skateboards,bicycles and bulletproo vests, and youve seen it in the bodyo Corvettes since 1953. But did you know that it was in use

5000 years ago?

Bricks and Mortar

OK, Il l back up a bit. Whats a compos-ite? Answer: It is two or more materialsthat are bonded into a single structurewhere the originating materials retaintheir identity and contribute to thecreation o new material. Simply put?Combine the materials such that nei-ther dissolves and the sum is greaterthan the parts.

By that denition bellybutton lintmied into Elmers Glue qualies,though I doubt it enjoys wide usage.

More realistically, composites didnt

www.kitplanes.com

In Part 1 of our newesthands-on series,

well open your eyesto the wonders of moderncomposite materials.

BY BOB FRITZ

36 KITPLANES May 2007

Aircraters Dave Saylor (let), with authorBob Fritz, discuss the diferences in basic

composite materials.

Peel-ply is used to help absorb excess resin

in berglass or carbon-ber cloth andhelps create a clean (but comparatively

rough) surace or secondary bonding.

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2/7Photos: Marc Cook and Bob Fritz

change much rom the mud and strawused to make bricks 5000 years ago inSumeria until the Romans came up withconcrete (cement and gravel). Te netleap, although one that has yet to beadopted, was Pykrete. Tat was a dandymaterial composed o water and sawdustrozen into blocks. It came within a yearor two o being the material rom whichtorpedo-proo ships would be built orWW-II in the North Atlantic.

Youve probably come across termssuch as E-glass, S-glass, Kevlar, car-bon ber, ceramic and a host o optionsthat boggle the beginner and make theentrepreneur salivate. Tese materials

have come to common use in homebuiltaircra because they give the design-er signicant reedom in choosingshapesboth or aesthetics and aerody-namicsbut also because they are com-paratively easier to use than traditionalmaterials. We saycomparatiely becausewhile composite construction is mostoen used in large chunksreducingthe total number o pieces in any givenairrame designthe medium requiresits own set o skills and specialized

understanding.

Looking AheadIn this series well look at those optionsas they relate to amateur-built aircraand take some o the mystery out othem. Well dene the terms, eplainthe options, and ofer some hints, help

and advice. Moreover, or this rstinstallment, we have worked through asmall composites project to give you ageneral lay o the land in photographs.As we move orward in this series, wellconnect the dots a bit more thoroughly,and ofer you the benet o eperienceand crasmanship rom several sourc-es, including Dave Saylor, who ownsAircraers in Watsonville, Caliornia,a thriving builder-assist shop that alsodoes inspections and repairs on Eperi-

mentals.

Lets start with the materials: Whatare they, why use them, and how do youselect which to use? Actually, this deci-sion is made or you by the kit manu-acturer, but lets take a look at some othe elements that went into their think-ing. For aircra use, we start with what

is popularly known as berglass. Tereare two basic types: E-glass and S-glass.Te most common is E-glass. Its strong,water resistant, easily ormed and easyto repair. Its cheap to manuacture andwas initially optimized or its electricalproperties, hence the E. Its now oundits way into surboards and boats.

Te materials science guys wantedto keep their jobs, so they tweaked the

recipe a bit. What they got was S-glass.

Its a tiny bit lighter, but has 20% moretensile strength, hence the S or strength.Great stuf! However, as with all thingsscientic, theres always a better versionjust around the corner.

In this case, the wonder material wascarbon ber. Te bers themselves arestifer than steel and are about 25% theweight o the g rey stuf. But the matrimaterial in which the bers are embed-ded lowers the modulus (engineer speakor stifness) to being equal to or lower

than that o steel while not losing the

KITPLANES May 2007 37

Carbon ber, mysteriously black, is

dramatically stronger than berglass butconsiderably more expensive.

Fiberglass cloth comes in a variety oweights and weaves. The variations ac-

commodate not only difering strengthconsiderations but the ways in which

the nished composite structure will bestrongits possible to ne-tune compos-

ites to an amazing degree.

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weight advantage. Te result is a n-ished part that weighs a quarter as muchas steel while being in the same strengthrange as steel.

Compare CF to aluminum, and it

still has the advantage. Tat CF partis almost twice as stif as aluminum atabout hal the weight. Tis stuf hasairplane written all over ituntil yousee the cost.

Te net ratchet on that wheel is Kev-lar. Manuactured by DuPont, Kevlarhas ve times the strength o steel on aper-weight basis, 60% o the weight oberglass, is re resistant, electricallyinsulating, sound damping and, likeSuperman, can even stop a speeding

bullet. But, alas, each o these productsis more epensive than the previous,with Kevlar being realistic only i youwant a bulletproo airplane to carr y youto your Swiss bank.

Underpinning It All: TechniqueTere are trade-ofs to these wondermaterials. All o them have to be care-ully handled in the construction o thenished product to make sure that airis not trapped between layers, and noneo them is especially abrasion resistant.Tey will all cause your cutting tools todull quickly, carbon ber is not as goodin impact resistance as the others, noneis nearly as heat-resistant as metal, andeven the wonder material Kevlaralsocalled Aramidalls behind the otherswhen in compression.

A quick eplanation is in order here:ension means pulling on the individ-ual ber; compression is as i we had

one end o a ber being pushed directlytowards the opposite end; shear is whatone does to a ber with scissors. Keep inmind that a cloth made o bers has tobe seen as individual bers, each loadedindividually in tension, compression orshear. Tat said, the strength-to-weightratio, ease o abrication and the abilityto mold these materials into compoundcurves make them the rst choice or theairrames o the latest go-ast aircra.

So, what goes into these wonder mate-

rials? In modern terms, the two compo-

nents are the binder, also known as thematri, and the reinorcement. Its thereinorcement (the bers) that gives thestrength, while the binder holds the bersin position and transmits the load amongthem. Lets simpliy that a bit. As youcan see rom the photos, the reinorce-ment isnt very impressive; wet noodlecomes to mind. But tensile strengthis where composites shine. Generally

speaking, resistance to breaking when

being pulled is known as tensile strengthand it is key to these materials.

But were not pulling on them inan airplane, were bending them, yousay? Ah, but we are pulling. ry this:ake our similar boards, each a hal-inch thick, stack them loosely on a pair

o sawhorses and then stand on them.

38 KITPLANES May 2007 www.kitplanes.com

BYS Composites, Part 1continued

Using a specialized pump, Saylor is able to

dispense the precise proportions o resinand hardener or the job.

Mixing takes place in a wax-ree cup using

wood tongue depressors to emulsiy theresin and hardener. The purpose is to have

both components mixed thoroughly in away that introduces as ew air bubbles as

possible.

Saylor begins our sample project at a table dedicated to resin/epoxy preparation.

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Tey bend quite a bit. Now drive a ewnails through them and stand on them.Teyre a lot stifer, right? Tats becausethey cannot slide against one anotheranymore; the bottom two boards arebeing stretched (tension loading) andthe top two boards are being squeezed(compression loaded). Te nails com-bine the strength o the boards justas using a solid, 2-inch-thick board isstronger than a group o our hal-inchboards. Now substitute a stack o bersand epoy or the boards and nails andyou see the mechanism o tension.

Aligned with the Loado take advantage o the tensile strength

o the material we have to align thebers with the load. An airplane seesstresses in many directions, so we haveto lay the bers in many directions, too.Tere are a couple o ways o doing that.You can build the uselage like theydo at the super-high-tech shops withthe entire uselage on a rotisserie anda machine that lays down a tape thatwinds around the circumerence on itsway to the tail , and then continues thesame lament back to the nose. Its done

that way on cylindrical shapes because it


Thickening agents, such as ox, cabosil or

microballoons can provide a strong bond

that doesnt run out between layers.

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The two pieces o oam can be placedtogether. The critical concern here is to

watch or su cient push out so you

know that the suraces have su cient

micro/epoxy or a good bond.

Now the screw stud is placed in the holeand surrounded with a thick ox/epoxy

mixture.

The ox can be smoothed with a brushdipped in the plain epoxy.

gives a antastic strength-to-weight ratioon pressurized containers. Its a coupleo million bucks or the equipment, soi your airplane uselage isnt a cylinderor pressurized, its not an option or the

do-it-yourseleror even an etreme-ly well-unded kit manuacturer, werethere such a thing.

Te practical way or homebuilders ishand layup. It can, i done well, be justas strong as a lament wound structure.It takes a bit longer, but it requires onlya ew basic tools and the right ormat orthe bers, and that ormat is usually acloth.

Beore we get started on this discus-sion, lets dene some terms.

Weight: Tats in ounces per squareyard o dr y cloth.

Tickness: Tis is primarily a unc-tion o weight and ber type, but alsodepends on the weave.

Filament: A single ber sometimesknown as a monolament.

Strand or End: A group o monol-aments.

Roving or ow: A group o monol-



Our practice project is a hardpointin

this case a #10 screw embedded in a oamsandwichbut the techniques will carry

through many types o composite con-

struction. Here, the oam is being wetted

with plain epoxy.

The next step is to slather on a thin layer o microballoon mixture.

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Next we create a micro/epoxy llet where the two oam pieces join. Fiberglass cloth

doesnt like to go around sharp corners, so this transition is important.

Ater smoothing the llet with a brush dipped in epoxy, its time to lay down the rstlayer o cloth.

aments all parallel to one another. Teseare usually designated by the number olaments, e.g. 3K or 12K.

Yarn: A group o monolamentstwisted to hold the bers together.wisting lowers the strength o thebers a bit.

Denier: A weight in grams o 9000meters o yarn. Te lower the number,the ner the yarn.

Warp or Ends: Tese are the yarnsthat run in the direction o the roll omaterial.

Fill or Picks: Tese yarns are shorterbecause they go across the roll.

Count: Tis reers to the number owarp yarns (ends) and ll yarns (picks)per inch. For eample , a 24 x 22 abrichas 24 ends in every inch o ll direc-

tion and 22 picks in every inch o warpdirection.

Drape-ability: Tis is a bit subjec-tive, but it describes how easily the clothconorms to a non-at shape. Alumi-num oil, or instance, has poor drape-ability in that it has to be olded to wraparound a sphere.

Weae: Tis describes how the warpand ll yarns are interlaced to make whatwe see as cloth. It determines the drape-ability and the strongest direction o thecloth i the cloth is woven with a bias tothe warp or ll direction.

o complicate the issue urther, thereare three classications o reinorce-ment: particulates, discontinuous berand continuous ber. An eample oa particulate composite would be con-crete, with the gravel being the particu-late.

I the key to the strength o compos-ites is the tensile strength o the bers,

then longer strands are better becausethere are more places or the epoy tograb on, loading the individual ber toits maimum. Tats why they put longrather than short steel bars in concrete.

Discontinuous-type constructionuses short bers and is, thereore, stron-


Ater the cloth is positioned, it can be wet-ted out with a brush. Work rom the center

out, and try to avoid using too much

epoxy. Give the cloth time to wet out.

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ger than particulate. But because thebers are short, their strength is notully employed. Instead, the load goesthe length o a ber and is then trans-mitted through the epoy to the net

ber. However, it can be sprayed tomake the layup, as is done in boat hulls.Spraying also means the bers are ran-domly oriented; thereore, the strengthis not directionally dependent. Te lastgroup, continuous ber, is what we wantor maimum strength. Its our verylong tug-o-war rope where enough guyscan get their hands on it to really use itsstrength.

Fabric, Meet Your BinderTat analogy o guys grabbing a ropecan be etended to the other hal o thismaterial, the binder. Teres no point inhaving a strong rope held by weak peo-ple, and the more hands on the rope thebetter. I we compare two teams, onecalled Vinylester Resin and the otherEpoy Resin, and look at how they grabthe rope, it becomes obvious that the ERteam is stronger. Te VR molecules arelong ellows that wrap around the ber,while the ER molecules connect notonly to the ber in many places, but alsoto each other. Te advantage is that theER molecules put more hands on theber. Its as i one team had etra play-ers pulling on the belts o the g uys hold-ing the rope.

Te ER hands are a lot stronger aswell. In act, theyre about our timesas strong as the VR guys. Not only that,ER will bond dissimilar or already curedmaterials where VR sometimes has di-

culty. Te real icing on the cake is thatER also has better resistance to micro-cracking.

Whats the downside to ER? Its a bittougher to work with in that to achieveas many 3D bonds as possible, you haveto mi the constituent components veryaccurately. I not, the ecess material isjust mied in without nding a partnerand weakens the nished product. VR,on the other hand, is a long chain thatcan be made longer, so miing is a bit

more orgiving. Did I mention that the

cost is about hal that o ER?Consequently, the kit makers use ERwhere its needed or strength. VR is thebinder o choice or Glasair, Glastar andthe Vans series, which uses it wrappedaround a metal rame. Lancair, withno metal, opted or ER. Tats whatmakes so many great airplanes possible:options.

Te bottom line is that i we lay downreally long bers o glass, and bind themwith a resin, we get an afordable air-

plane with phenomenal strength, light

weight and, because the cloth is, well,cloth, it can be easily ormed into com-pound curves that make love to the airrather than just shoving it out o theway.

Right now youre probably eelinglike someone just dumped a 1000-piecejigsaw puzzle on the table. Not to worry.In uture installments well show youthe bo and point out all the edge pieces.Youll nd that handling the materialsis actually much easier than you think.

In act, this will be un.



A clipped brush is efective or stippling out unwanted air bubbles.

Peel-ply can be used to absorb excess resin. Apply it like another layer o berglass.

One trick to doing good

berglass work (true withcarbon and Kevlar, too) is to

know when to stop ussing.

This projects peel-ply isjust about perectly wetted

out. Stop now, have a cupo cofee and let it cure.

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