Aircraters’ Dave Saylor (let), with author Bob Fritz, discuss the diferences in basic composite materials.
In Part 1 of our newest hands-on series, we’ll open your eyes to the wonders of modern composite materials. BY BOB FRITZ
W
hat do Burt Rutan, the police and a trickedout oyota have in common? An of-ais question no doubt, and the answer is not the one Benjamin Braddock received in Te Graduate, (“I want to say one word to you. Just one word. word. Plastics.”). Plastics.”). No, the linkage here is composite materials, which are everywhere you look. Burt makes magic airplanes with them—has done so since the 1960s and continues to even today—the police wear Kevlar vests, and the guy that customized his oyota oyota loves the look o a carbon-ber ca rbon-ber panel or two on his car. Sure, you’re amiliar with composite’s use in skateboards, bicycles and bulletproo vests, and you’ve seen it in the body o Corvettes since 1953. But did you know that it was in use 5000 years ago?
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Peel-ply is used to help absorb excess resin in berglass or carbon-ber cloth and helps create a clean (but comparatively rough) surace or secondary bonding.
Bricks and Mortar OK, I’ll I’l l back up a bit. What’s a composite? Answer: It is two t wo or more materials that are bonded into a single structure where the origina originating ting materia materials ls retain their identity and contribute to the creation o new material. Simply put? Combine the materials such that neither dissolves and the sum is greater than the parts. By that denition bellybutton lint mied into Elmer’s Glue qualies, though I doubt it enjoys wide usage. More realistically, composites didn’t www.kitplanes.com www .kitplanes.com
Let’s start with the materials: What are they, why use them, and how do you select which to use? Actually, this decision is made or you by the kit manuacturer, but let’s take a look at some o the elements that went into their thinking. For aircra use, we start with what is popularly known as berglass. Tere are two basic types: E-glass and S-glass. Te most common is E-glass. It’s strong, water resistant, easily ormed and easy to repair. It’s cheap to manuacture and was initially optimized or its electrical properties, hence the E. It’s now ound its way into surboards and boats. Te materials science guys wanted to keep their jobs, so they tweaked the Fiberglass cloth comes in a variety o weights and weaves. The variations accommodate not only difering strength considerations but the ways in which the nished composite structure will be strong—it’s possible to ne-tune composites to an amazing degree.
change much rom the mud and straw used to make bricks 5000 years ago in Sumeria until the Romans came up with concrete (cement and gravel). Te net leap, although one that has yet to be adopted, was Pykrete. Tat was a dandy material composed o water and sawdust rozen into blocks. It came within a year or two o being the material rom which torpedo-proo ships would be built or WW-II in the North Atlantic. You’ve probably come across terms such as E-glass, S-glass, Kevlar, carbon ber, ceramic and a host o options that boggle the beginner and make the entrepreneur salivate. Tese materials have come to common use in homebuilt aircra because they give the designer signicant reedom in choosing shapes—both or aesthetics and aerodynamics—but also because they are com paratively easier to use than traditional materials. We say comparatiely because while composite construction is most oen used in large chunks—reducing the total number o pieces in any given airrame design—the medium requires its own set o skills and specialized understanding. Photos: Marc Cook and Bob Frit z
Looking Ahead
Carbon ber, mysteriously black, is dramatically stronger than berglass but considerably more expensive.
In this series we’ll look at those options as they relate to amateur-built aircra and take some o the mystery out o them. We’ll dene the terms, eplain the options, and ofer some hints, help recipe a bit. What they got was S-glass. and advice. Moreover, or this rst It’s a tiny bit lighter, but has 20% more installment, we have worked through a tensile strength, hence the S or strength. small composites project to give you a Great stuf! However, as with all things general lay o the land in photographs. scientic, there’s always a better version As we move orward in this series, we’ll just around the corner. connect the dots a bit more thoroughly, In this case, the wonder material was and ofer you the benet o eperience carbon ber. Te bers themselves are and crasmanship rom several sourc- stifer than steel and are about 25% the es, including Dave Saylor, who owns weight o the grey stuf. But the matri Aircraers in Watsonville, Caliornia, material in which the bers are embeda thriving builder-assist shop that also ded lowers the modulus (engineer speak does inspections and repairs on Eperi- or stifness) to being equal to or lower mentals. than that o steel while not losing the KITPLANES
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BYS Composites, Part 1 continued weight advantage. Te result is a nished part that weighs a quarter as much as steel while being in the same strength range as steel. Compare CF to aluminum, and it still has the advantage. Tat CF part is almost twice as stif as aluminum at about hal the weight. Tis stuf has “airplane” written all over it…until you see the cost. Te net ratchet on that wheel is Kevlar. Manuactured by DuPont, Kevlar has ve times the strength o steel on a per-weight basis, 60% o the weight o berglass, is re resistant, electrically insulating, sound damping and, like Superman, can even stop a speeding bullet. But, alas, each o these products is more epensive than the previous, with Kevlar being realistic only i you want a bulletproo airplane to carry you to your Swiss bank.
Saylor begins our sample project at a table dedicated to resin/epoxy preparation.
Underpinning It All: Technique Tere are trade-ofs to these wonder materials. All o them have to be careully handled in the construction o the nished product to make sure that air is not trapped between layers, and none o them is especially abrasion resistant. Tey will all cause your cutting tools to dull quickly, carbon ber is not as good in impact resistance as the others, none is nearly as heat-resistant as metal, and even the wonder material Kevlar—also called Aramid—alls behind the others when in compression. A quick eplanation is in order here: ension means pulling on the individual ber; compression is as i we had one end o a ber being pushed directly towards the opposite end; shear is what one does to a ber with scissors. Keep in mind that a cloth made o bers has to be seen as individual bers, each loaded individually in tension, compression or shear. Tat said, the strength-to-weight ratio, ease o abrication and the ability to mold these materials into compound curves make them the rst choice or the airrames o the latest go-ast aircra. So, what goes into these wonder materials? In modern terms, the two compo38
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Using a specialized pump, Saylor is able to dispense the precise proportions o resin and hardener or the job.
nents are the binder, also known as the matri, and the reinorcement. It’s the reinorcement (the bers) that gives the strength, while the binder holds the bers in position and transmits the load among them. Let’s simpliy that a bit. As you can see rom the photos, the reinorcement isn’t very impressive; wet noodle comes to mind. But tensile strength is where composites shine. Generally speaking, resistance to breaking when
Mixing takes place in a wax-ree cup using wood tongue depressors to emulsiy the resin and hardener. The purpose is to have both components mixed thoroughly in a way that introduces as ew air bubbles as possible.
being pulled is known as tensile strength and it is key to these materials. “But we’re not pulling on them in an airplane, we’re bending them,” you say? Ah, but we are pulling. ry this: ake our similar boards, each a halinch thick, stack them loosely on a pair o sawhorses and then stand on them. www.kitplanes.com
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Thickening agents, such as ox, cabosil or microballoons can provide a strong bond that doesn’t run out between layers.
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Tey bend quite a bit. Now drive a ew nails through them and stand on them. Tey’re a lot stifer, right? Tat’s because they cannot slide against one another anymore; the bottom two boards are being stretched (tension loading) and the top two boards are being squeezed (compression loaded). Te nails combine the strength o the boards just as using a solid, 2-inch-thick board is stronger than a group o our hal-inch boards. Now substitute a stack o bers and epoy or the boards and nails and you see the mechanism o tension.
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Aligned with the Load o take advantage o the tensile strength o the material we have to align the bers with the load. An airplane sees stresses in many directions, so we have to lay the bers in many directions, too. Tere are a couple o ways o doing that. You can build the uselage like they do at the super-high-tech shops with the entire uselage on a rotisserie and a machine that lays down a tape that winds around the circumerence on its way to the tail, and then continues the same lament back to the nose. It’s done that way on cylindrical shapes because it KITPLANES
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BYS Composites, Part 1 continued gives a antastic strength-to-weight ratio on pressurized containers. It’s a couple o million bucks or the equipment, so i your airplane uselage isn’t a cylinder or pressurized, it’s not an option or the do-it-yourseler—or even an etremely well-unded kit manuacturer, were there such a thing. Te practical way or homebuilders is hand layup. It can, i done well, be just as strong as a lament wound structure. It takes a bit longer, but it requires only a ew basic tools and the right ormat or the bers, and that ormat is usually a cloth. Beore we get started on this discussion, let’s dene some terms. • Weight : Tat’s in ounces per square yard o dr y cloth. • Tickness: Tis is primarily a unction o weight and ber type, but also depends on the weave. • Filament : A single ber sometimes known as a monolament. • Strand or End : A group o monolaments. • Roving or ow: A group o monol-
Our practice project is a hardpoint—in this case a #10 screw embedded in a oam sandwich—but the techniques will carry through many types o composite construction. Here, the oam is being wetted with plain epoxy.
The next step is to slather on a thin layer o microballoon mixture.
The two pieces o oam can be placed together. The critical concern here is to watch or sucient “push out” so you know that the suraces have su cient micro/epoxy or a good bond. 40
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Now the screw stud is placed in the hole and surrounded with a thick ox/epoxy mixture.
The ox can be smoothed with a brush dipped in the plain epoxy.
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Next we create a micro/epoxy llet where the two oam pieces join. Fiberglass cloth doesn’t like to go around sharp corners, so this transition is important.
Ater smoothing the llet with a brush dipped in epoxy, it’s time to lay down the rst layer o cloth.
aments all parallel to one another. Tese are usually designated by the number o laments, e.g. 3K or 12K. • Yarn: A group o monolaments twisted to hold the bers together. wisting lowers the strength o the bers a bit. • Denier : A weight in grams o 9000 meters o yarn. Te lower the number, the ner the yarn. • Warp or Ends: Tese are the yarns that run in the direction o the roll o material. • Fill or Picks: Tese yarns are shorter because they go across the roll. • Count : Tis reers to the number o warp yarns (ends) and ll yarns (picks) per inch. For eample, a 24 x 22 abric has 24 ends in every inch o ll direction and 22 picks in every inch o warp direction. • Drape-ability: Tis is a bit subjective, but it describes how easily the cloth conorms to a non-at shape. Aluminum oil, or instance, has poor drapeability in that it has to be olded to wrap around a sphere. • Weae: Tis describes how the warp and ll yarns are interlaced to make what we see as cloth. It determines the drapeability and the strongest direction o the cloth i the cloth is woven with a bias to the warp or ll direction. o complicate the issue urther, there are three classications o reinorcement: particulates, discontinuous ber and continuous ber. An eample o a particulate composite would be concrete, with the gravel being the particulate. I the key to the strength o composites is the tensile strength o the bers, then longer strands are better because there are more places or the epoy to grab on, loading the individual ber to its maimum. Tat’s why they put long rather than short steel bars in concrete. Discontinuous-type construction uses short bers and is, thereore, stronAter the cloth is positioned, it can be wetted 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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BYS Composites, Part 1 continued ger than particulate. But because the bers are short, their strength is not ully employed. Instead, the load goes the length o a ber and is then transmitted through the epoy to the net ber. However, it can be sprayed to make the layup, as is done in boat hulls. Spraying also means the bers are randomly oriented; thereore, the strength is not directionally dependent. Te last group, continuous ber, is what we want or maimum strength. It’s our very long tug-o-war rope where enough guys can get their hands on it to really use its strength.
A clipped brush is efective or stippling out unwanted air bubbles.
Fabric, Meet Your Binder Tat analogy o guys grabbing a rope can be etended to the other hal o this material, the binder. Tere’s no point in having a strong rope held by weak peo ple, and the more hands on the rope the better. I we compare two teams, one called Vinylester Resin and the other Epoy Resin, and look at how they grab the rope, it becomes obvious that the ER team is stronger. Te VR molecules are long ellows that wrap around the ber, Peel-ply can be used to absorb excess resin. Apply it like another layer o berglass. while the ER molecules connect not only to the ber in many places, but also One trick to doing good to each other. Te advantage is that the berglass work (true with ER molecules put more “hands” on the carbon and Kevlar, too) is to ber. It’s as i one team had etra playknow when to stop ussing. ers pulling on the belts o the g uys hold This project’s peel-ply is ing the rope. just about perectl y wetted Te ER hands are a lot stronger as out. Stop now, have a cup o cofee and let it cure. well. In act, they’re about our times as strong as the VR guys. Not only that, ER will bond dissimilar or already cured materials where VR sometimes has diculty. Te real icing on the cake is that cost is about hal that o ER? weight and, because the cloth is, well, ER also has better resistance to microConsequently, the kit makers use ER cloth, it can be easily ormed into comcracking. where it’s needed or strength. VR is the pound curves that make love to the air What’s the downside to ER? It’s a bit binder o choice or Glasair, Glastar and rather than just shoving it out o the tougher to work with in that to achieve the Van’s series, which uses it wrapped way. as many 3D bonds as possible, you have around a metal rame. Lancair, with Right now you’re probably eeling to mi the constituent components very no metal, opted or ER. Tat’s what like someone just dumped a 1000-piece accurately. I not, the ecess material is makes so many great airplanes possible: jigsaw puzzle on the table. Not to worry. just mied in without nding a partner options. In uture installments we’ll show you and weakens the nished product. VR, Te bottom line is that i we lay down the bo and point out all the edge pieces. on the other hand, is a long chain that really long bers o glass, and bind them You’ll nd that handling the materials can be made longer, so miing is a bit with a resin, we get an afordable air- is actually much easier than you think. more orgiving. Did I mention that the plane with phenomenal strength, light In act, this will be un. 42
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