Building Terms Reference Guide

A few of you non-building types have told me that you have no idea what in the world I’m talking about when I refer to certain terms in these build descriptions.  Well, below are a few terms, references and descriptions that we Canardians (those who build and/or fly canard aircraft) use during the construction of our various canard aircraft.  So get educated so we can communicate!  (I pulled a majority of the definitions below right out of the plan’s Chapter 3 – Education).

Fiberglass:

The most basic structural material in building the Long-EZ is glass cloth.  Glass cloth is available commercially in hundreds of different weights, weaves, strengths, and working properties.  The use of glass in aircraft structures, particularly in structural sandwich composites (i.e. the Long-EZ), is a fairly recent (1970’s) development.  Back in the 1970’s, very few of the commercially available glass cloth types were compatible with aircraft requirements for high strength and low weight.  Even fewer were suitable for the hand layup techniques developed by Burt Rutan for the homebuilder.  The glass cloth used in the Long-EZ (and Cozy) has been specifically selected for the optimum combination of workability, strength, and weight.

The glass cloth in the Long-EZ carries primary flight loads and stresses, and the correct application and orientation of the weaves are vitally important.    The two main types of cloth used are a bi-directional (BID) cloth, where the fiber weave goes up and down, left and right, and crosses at a 90 degree angle (just like +).   Undirectional cloth (technically UND in the plans, but I refer to it as UNI), has all major fibers heading in one direction, with small cross fibers just holding it together (think ↔ ).   Both UNI & BID come in 38” wide rolls on thick cardboard tubes in the amount (yards) in which it’s ordered.

BID is normally cut at a 45 degree bias to the sides of the roll—which makes for a lot of leftover triangular scraps—which allows it to be laid up easier and provides a desired torsional and/or shear stiffness.  UNI is used in areas where the primary loads are in one direction, and maximum efficiency (strength vs weight) is required, such as the wing skins.

Finally, multiple layers of glass cloth are laminated together to form the Long-EZ’s structure.  Each layer of fiberglass cloth is called a PLY and that term will be used often, such as “added 1-ply of BID.”  Also, you’ll see me use the term glassing quite a bit just to describe the actual act of laying up fiberglass cloth.

Layup:

Speaking of laying up, ‘layup’ or ‘laid up’ are terms you’ll see in my posts as well: “Made a 3-ply UNI layup.”  A layup in composite construction (unlike basketball, yuk, yuk) is when fiberglass is applied in varying numbers of plies to foam/wood/metal/other fiberglass to then cure and become part of the aircraft/component structure.

Wood:

There are two types of wood used in the construction of a Long-EZ.  Both have amazing physical properties, like strength-to-weight ratio, far beyond “normal” or “standard” woods (like pine or oak) that you may be accustomed to.  The Long-EZ uses Aircraft Grade Spruce and Aircraft Finnish Birch plywood in its construction.

The Spruce is used in approximately 1”x1” pieces that run almost the whole length of the airplane, in all four corners of the fuselage top & bottom, left & right, called “longerons.”  It’s also used in the lower rear fuselage area for the attach hard points for the main gear and the extremely strong center section spar (incidentally is what holds the wings on!).  All of these Spruce pieces tie into the firewall (the protective wall between the engine and passengers) at the rear of the aircraft.

The Finnish Birch plywood used in the Long-EZ is all 1/4” thick and is used as the firewall (with fireproof Fiberfrax covering and then aluminum sheeting on top) and for the myriad of small hard-points used throughout the aircraft.

And that’s it as far as the wood goes.

Foam:

Three types of rigid, closed-cell foam is used in the construction of the Long-EZ, with varying degrees of densities for each type.

A low density (2 lb test) light blue Styrofoam is used as the foam core for the wings, winglets (vertical things at the end of the wings), elevators and canard.  This type of Styrofoam is exceptional for smooth hotwire cutting of airfoil shapes.  The large cell type (“grainier”) used in the Long-EZ provides better protection from delamination than the more common pink, blue or purple insulation-grade stuff found at Home Depot.

PolyVinyl, or Divinycell core foam in low, medium and high densities is used extensively throughout the Long-EZ.  The darker blue color (3 lb test) type R45 is used in the fuselage and fuel tanks.  It has excellent compression strength (75 psi), good laminate peel strength and is completely fuel compatible (fuel will melt/dissolve the the 2 lb light blue Styrofoam above in a matter of seconds, but not so with Divinycell, which is highly fuel resistant).  The light yellow color (6 lb test) type R100 PV-core is used in the center section spar and canard elevator hard-point inserts.  Also light yellow in color, the (16 lb test) type R250 is used in several bulkheads.

Low-density light tan color (2 lb test) urethane foam is used only in non-structural areas, where ease of shaping is important.  Urethane foam is fantastically easy to carve and contour, and has been traditionally used to shape major areas such as the nose and canopy.  In fact, the Vari-Eze used urethane as a primary foam for the majority of the fuselage and other major components.  On significant drawback on urethane foam is that it has a higher propensity for the glass to delaminate (or separate) from the foam.  For this reason, I’ve greatly minimized the use of urethane in my Long-EZ .

Last thoughts on foam:  The densities of these foams are critical since they carry structural and aeronautical loads of the aircraft.  For the most parts, whatever is called for in the plans is what I’ve used.  Also, these foams are sensitive to long term exposure to the UV rays in sunlight so the are kept and stored out of the sun.  Finally, these foams are sensitive to heat over 215° F, so measures are taken to keep the foam out and away from extreme heat.  These foams’ sensitivity to high heat is exactly the reason why you see the majority of composite canard aircraft painted white, or other light colors, to minimize the generation of heat on the skin of the aircraft and thus damage the underlying foam.

Epoxy:

Epoxy is the adhesive matrix that keeps the plies of load-carrying glass cloth together.  Epoxy alone is weak and heavy.   The goal in building a composite airplane is to get the full strength of an epoxy/glass mixture with minimal weight.

An epoxy system is made up of a resin and a hardener.  Mixing resin with its hardener causes a chemical reaction called curing, which changes the two liquids into a solid.  Typically hardeners are identified by the time in which they facilitate the curing of the mixture.  I started with the MGS 335 epoxy system, and then moved to the higher-grade MGS 285 system.  Both of the MGS systems use an epoxy with either a fast hardener (cures in 15 min to an hour) or a slow hardener (cures in 2-6 hours), or a mixture of the two.

A builder uses different hardeners depending on the scale of the layup.  On a small layup it’s fine to use fast hardener because of something called “Pot Life,” or simply how long the epoxy/hardener combo stays a liquid inside your paper mixing cup.  Since the mixture is a chemical reaction in the making, the more hardener you have (thus, the more epoxy you have), then the more likely the curing reaction will kick off sooner.  If the epoxy/hardener combo isn’t getting used quickly enough, and there’s a lot of it in the mixing cup (or “pot”), the chance of it “exotherming” and rapidly transitioning into a cured state is much more likely.  When the epoxy/hardener mixture exotherms, it transitions into a cured stated within a matter of seconds (anywhere from 10-45 seconds on average from my experience, and of course depending on when you become “aware” of the exotherming state).  The epoxy cup will become extremely hot (thus the term “exotherm”) and immediately harden, enjoining with it any brush or item within the epoxy for ETERNITY (i.e. Not good… and sucks!)

One last tidbit on epoxy and hardeners: depending on the system (brand), epoxy and hardeners are always mixed in an exact ratio.  For MGS 335 that ratio is 38:100, or 38 parts hardener (in weight) to 100 parts epoxy.  MGS 285 actually gives just a bit more wiggle room with its ratio 38-40:100, hardener to epoxy.  This ratio is achieved with either the old school Rutan-designed balance scale, or by simply pouring epoxy into a mixing cup (after the cup weight alone has been zero-tared) and multiplying the amount in the cup by 1.4 for a total mixture weight to be achieved by then adding the hardener.

Micro:

Microspheres or microballoons (“Micro” for short) are a very light filler or thickening material used in a mixture with epoxy.  Micro is used to fill voids and low areas in foam, to glue foam blocks together, and as a bond between foams and glass skins.  Microballoons are very light and essentially make any given volume of epoxy when mixed as micro much lighter since much of that volume is nothing more than air.  For this reason, micro is spread on foam before laying fiberglass onto the foam since it fills in the voids of the foam, with, yes, bubbles (See? Aren’t composites fun?! … by golly we’re talking about bubbles and balloons! HA!) . . . and thus air, so it aids in significantly decreasing weight.  Also, because it creates a more uniform base on the surface of the foam to which the fiberglass is being laid up, it creates a stronger bond between the foam and the glass.

Micro is used in three (3) consistencies when building the Long-EZ:

  • Micro slurry: 1:1 ratio of microballoons to epoxy
  • Wet Micro:  ~2-4:1 ratio of microballoons to epoxy
  • Dry Micro: Paste that will not sag or run (~5:1 ratio)

When looking at the pictures on this site, any time you see a bright white material, whether it be paste, liquid, in joints or spread across foam, it will almost always be micro.  Also, throughout the posts you’ll often see the term “micro” used as the verb “micro’d.”

Flox:

Flox is a mixture of cotton fiber (flocked cotton) and epoxy.  Flox is used in structural joints and in areas where a very hard durable buildup is required.  Flox is mixed much like Dry Micro, but only about a 2:1 ratio of flox to epoxy is required.  Usually just enough flox is added to epoxy to make it into a paste, or to make it ‘stand up.’  If I use the term “wet flox,” it means it’s mixed to be a little more saggy and runny.  Flox makes an extremely strong bond.  In fact, flox is so strong that it is also used to bond metal parts to either foam or fiberglass.

As with micro, you’ll see “flox” used as verbs: “flox’d” or “floxed”.

Cabosil:

Cab-O-Sil is a fumed lightweight silica thickener used to reduce the flow of epoxies on vertical surfaces, as well as filling pinholes with its smooth texture.

Bondo:

Bondo is used extensively throughout the construction of a Long-EZ… not structurally in the aircraft itself, but for holding jig blocks, forms and dams in place, and other temporary fastening jobs.  The bondo called for in the plans and what I use is standard automotive bondo that you can buy down at your local auto parts store or Wal-Mart.

Peel Ply:

Peel ply is a layer of light (typical 2.7 oz) Dacron fabric which is laid up over a fiberglass layup while the fiberglass is still wet.  It is later removed after the layup has cured by lifting an edge and “peeling” it off.  Peel ply comes in all different widths, and quite often as “surface tapes” in 1″, 2″ and 3″ wide sizes.  I also use large sheets of peel ply on a few specific layups.

Composite builders use peel ply for two purposes:

1.  Any area that will later be structurally attached to another fiberglass layup will get peel plied, since once the Dacron fabric is peeled off the surface is ready for another layup with no other surface prep (i.e. sanding) required.  If peel ply isn’t applied to the layup, then the surface would have to be sanded completely dull (read: no shiny spots) before another layup is added.  This sanding is hard, itchy work and ruins the strength of the outer ply of fiberglass.

2.  Peel ply is also used to transition the area where the top ply of a layup terminates on the fiberglass surface.  These areas are found at the wing/winglet junction where the fuel tank, cowling lip, and nose all respectively join the fuselage.  If the top ply of a overlapping layup is laid up bare, without peel ply, it will normally result in a rough edge that can delaminate if a little dry.  Plus, the resulting layup without peel ply will have the glass fibers cure in all manner of position, and can leave quite snaggy, tiny glass barbs that tend to cause puncture wounds to unsuspecting hands and fingers…. and although some may cite the resulting blood stains on the build as cool, cuts ‘n slices from rough fiberglass edges just ain’t fun!  Thus, we use peel ply to make those transitions both smooth as silk (almost!) and ready for additional layups.

Pre-Preg:

Pre-preg is a term for “pre-impregnated” composite fibers where epoxy is already present. The actual term is something that we Canardians, or technically composite builders, have bastardized for our own selfish purposes.  Technically pre-pregs “already contain an amount of the matrix material [epoxy & glass] used to bond them together and to other components during manufacture.”  Again in “real” pre-pregs the resin is only partially cured to allow easy handling, this is call B-Stage material and requires cold storage to prevent complete curing. B-Stage pre-preg is always stored in cooled areas since complete polymerization is most commonly done by heat. Hence, composite structures built of pre-pregs will mostly require an oven or autoclave to finish the complete polymerization.

Whew, did you get that?!  (Shamelessly lifted from Wikipedia!)

What do we use that we call pre-preg?  Well, to be precise the full title is “Poor man’s pre-preg.”  Our pre-preg setups consists of making a composite sandwich by laying down a sheet of plastic or aluminum foil, then placing 2-3 plies of glass on it, dumping some epoxy into the middle of the glass area, and then covering it with another sheet of plastic or aluminum foil (you can see better with plastic obviously).

So hopefully we get some of that polymerization going on!  But luckily for us, our pre-preg ain’t got nothing to do with no ovens or autoclaves!

This setup allows us to work the epoxy into the glass fibers without damaging the glass fibers, and at the same time driving out a LOT of excess epoxy! (Remember: epoxy=weight!)

Once all the glass is wetted out, and the excess epoxy squeegeed out, you can then draw lines on it to, say, make four 2″ x 20″ “tapes” to put in the corners of your front seat to fuselage junction.  Once those are cut, they last a bit longer than regular layups because they’re being starved from air.   We can cut a batch and take them out to our work areas without making a huge mess.  Then, when we need to use it, we simply pull off one side of the plastic (just like a Band-Aid), throw away the plastic strip and layup the remaining glass with one side still with plastic on it (plastic facing the builder please!).

The outside plastic with the glass underneath of it, now on the surface to be glassed, is used to hold the fiberglass strip to its shape (fiberglass believes wholeheartedly in entropy and attempts to go into a state of disorder in a microsecond . . .) and to get an EZ-squeegee completed on the layup.  Once it all looks good, we simply, carefully peel that outer piece of plastic off, and voila, finished!  (Most likely we would add some peel ply at this point to overlap everything and get a nice looking finish after it cures).

Vacuum Bagging:

Think of vacuum bagging kind of like a pressure cooker that puts the layup under pressure.  On top of the fiberglass plies that are wetted out with epoxy will be a layer of peel ply.  In vacuum bagging, on top of the peel ply will sit a layer of plastic covered with small holes no more than about an inch apart (kind of like the surface of an air hockey table).  On top of that is a couple of layers of paper towels.  When the layup is put under pressure, it squeezes the excess epoxy up through those little holes where it is soaked up by the paper towels.  When the layup has cured, you pull the plastic and the epoxy soaked paper towels off and throw them away.  Then you peel the hole-riddled plastic and peel ply off, and you’re left with a beautiful layup that has just the right amount of epoxy, since all that extra epoxy, which does nothing more than add weight, is gone!  Vacuum bagging produces some very strong and lightweight parts.  For a more detailed understanding, read on below.

Vacuum bagging is essentially a big giant “clamp” that uses atmospheric pressure to keep epoxy-coated parts of a layup in place until the resin cures.  Modern room-temperature-cure epoxies are one of the main reasons that vacuum bagging techniques are available to us homebuilders since they eliminated the need for high-tech, expensive equipment that was required in the past to get the same results we get today for a whole lot cheaper.

Vacuum bagging uses atmospheric pressure as this clamp to hold the epoxy-saturated glass plies in a layup together.  The entire layup is sealed in an airtight plastic bag, or envelope. The envelope is usually some type of plastic bag or sheet that’s sealed to a solid surface (sometimes fancier dudes actually use molds).  When the plastic sheeting is sealed, pressure on the outside and inside of this plastic envelope is equal to atmospheric pressure: approximately 29″ of mercury (Hg), or 14.7 psi. Now, add a vacuum pump that removes air from the inside of the sealed plastic sheet/bag/ envelope, and the air pressure on the inside of the plastic sheet gets reduced while the air pressure on the outside remains the same at 14.7 psi.  So, atmospheric pressure forces all the sides of the plastic sheet/bag and everything inside it together, putting equal and even pressure over the entire surface of the envelope and its contents: the underlying layup.

Vacuum bagging is all about the difference in pressure between the inside and outside of the plastic sheet/bag.  The amount of difference will determine the amount of clamping force (think of weight) on the layup. In reality, the most pressure that can be applied on the layup, even if it were possible to get a perfect seal/vacuum and get all of the air out from the inside of the plastic, is one “atmosphere,” or 14.7 psi.  A more realistic difference in pressure from the inside to the outside would 6–12.5 psi.

 

 

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