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Tip: Fiberglass Forms

DanH

Legacy Member
Mentor
Built a custom cowl part this weekend and thought it might be a good time for a tip. I've had several recent PM's and emails asking how to form shapes, about mold release, etc. As usual the former canard builders among us should probably click out now or risk being bored to death;).

Generate the desired male shape using any material that works; clay, block foam, balsa, sheet metal, gypsum....whatever. This one is pour in place two-part urethane. Slap up some cardboard dams, pour the foam, wait 15 minutes and carve away everything that doesn't look like the part.

Intake%20Form.jpg


The Prime Directive with male form "moldless" parts is to spend as much time as necessary to make the form perfect. It won't be perfect of course, but getting real close pays a huge time and weight dividend later when you get into surface filling and finishing.

Chipping out or dissolving foam afterward, then sanding, is a PITA. Cover your form with any thin, slick tape. I use ordinary clear box packing tape for most parts. However, note the red tape on the forward end of this form; there's a reason. The forward end has some severe curves and a tight angle down by the flange where the finished part will attach to the cowl. Curvature, in particular 90 degree bends, tends to result in air bubbles between the plies as you add layers. No big deal, just stipple or use a roller to remove them after all the plies are in place. However, you must be able to see them to find them. Using a colored tape makes them very, very obvious; great color contrast.

This is sort of a big part and doesn't have much draft angle at the forward end to make release easy. So, with the tape in place, fire up your cheap HF minigun and spray a coat of PVA mold release. Later when the part is cured, pry up an edge and blast under it with a squirt of air. It will pop right off the slick tape. The backside of the part will be smooth and slick, but it will have some tape lines. Leave 'em or sand them out as you please. Don't forget to rinse off the PVA.

The aluminum ring at the inlet was set in place with flox. The blue clay just formed a transition between the foam and the location where the flox started. The flox was placed and then the glass fabric was immediately laid over it while it was wet.

Speaking of cloth, in the past I've mentioned my favorite general use "keep in the shop" cloth is 8.9 oz 8-harness satin. Builds thickness fast, but more important, the 8-harness weave lets it drape around compound curves without fighting wrinkles and bubbles.

Intake%20Draped.jpg


Full layup, with peel ply.

Intake%20Form%20Layup.jpg


When ripped off after cure, the peel ply will leave a "fractured" surface, ready for subsequent bonding or filling. The alternative is heavy sanding with a coarse grit before a subsequent bond. Only light sanding is needed here, just to eliminate any ridges and bumps.

Raw%20Scoop.JPG


Trimmed part, ready for surface filling with micro.

Intake%20Trimmed.JPG


Same basic technique works for all kinds of parts.

Plenum%20Form.JPG


Exit%20%233.JPG
 
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Dan,

How did you get the ring aligned with the horizontal intake?

I am in the process of finishing up a mod to use a ram air intake with an internal "y" for filtered alternate air. The greatest challenge is getting the ring to within .25" of the intake and aligned with its circle.

dd
 
David,
<<How did you get the ring aligned with the horizontal intake?>>

Foamed it into place with the cowl and intake plenum on the airplane. Wrap the Y-duct in tape to protect it and hang it in place on the engine. Mine would stay there without clamps; you might need to fixate loosely. Also tape up the cowl around your opening. Mount the lower cowl. Use some tape or cardboard to make a dam inside or outside as necessary. I just taped a cardboard box on the outside of the cowl and poured into it.

Pour some foam and let it cure to fix the intake to the cowl. Pull the assembly off the airplane, carve away unnecessary foam, and do another pour so you can start shaping the outside around the fixated opening.

On mine I carved the foam back enough to slip on the silicone tube (1" long)which connects the intake plenum to the aluminum ring. It too was taped, with the tape wrapping around the end and tucked back inside. Then I shoved the aluminum ring into it and faired to shape with clay, leaving just the ring area exposed where I planned to place flox.

Later you chip out the fixating foam from the back, remove the plenum, and peel the tape off the cowl and plenum to leave clean parts:

Vlad,
<<Do you have specs for the cloth handy? What Spruce # is that? >>

Wicks #7781, and sorry, had a brain fart earlier, it is 8.9 oz.

BTW, I make it a point to spread business between Wicks and Spruce, with a preference for Wicks, the little guy. If we all put our money with Spruce without thinking, pretty soon you'll have one source, and prices will rise.
 
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Another great piece of education

Thanks, Dan.

You have no idea how much your mini-seminars in glass help those of us that are composite-impaired.

Michael Wynn
RV 8 FWF
San Ramon, CA
 
Dan,

Do you have specs for the cloth handy? What Spruce # is that?

Vlad,

FWIW, the original Burt Rutan invention, BID FIBERGLASS RA5277, works very well also. It is 8.8 ounces per yard, very workable around odd shapes, and is available from Wicks and ACS.

Dan, I imagined what you did to get it aligned with the intake. I did something similar with a piece of dense .25 foam across the intake opening and placed the ring up against it and then temporarily glued it to the cowl. I figured a quarter inch was close enough considering how much the engine can move about during start and shut down.

Thanks for posting the pictures, it helps a lot to see how others do these things.


dd
 
Dave just to clarify - do you mean RA5177?
I am not a big expert in composites and I often double check... Sometimes I can not find an item when it is in plain sight:)

Vlad,


FWIW, the original Burt Rutan invention, BID FIBERGLASS RA5277, works very well also. It is 8.8 ounces per yard, very workable around odd shapes, and is available from Wicks and ACS.


dd
 
Dave just to clarify - do you mean RA5177?
I am not a big expert in composites and I often double check... Sometimes I can not find an item when it is in plain sight:)

Vlad,

Burt Rutan designed 2 types of glass for his airplanes, one was called UNI, the other BID. UNI (RA5177) has a single row of fibers in one direction, weighs 7.7 oz, and is .009 thick with each ply. BID (RA5277) has two rows of fibers crossing each other at 90 degrees, weighs 8.8 oz, and is .013 thick with each ply.

UNI does not follow contours very well, it is used primarily on straight structures like a wing or spar each ply running at a modest angle to each other.

BID is the stuff you want to use for a cowl if you decide to go with this product.
 
Good lesson David. Thanks. I am not that bold to do a mod like Dan did. What I am looking for is a suitable cloth for engine plenum ( sorry Dan for thread creep:)). I have read multiple accounts on how to do it and a bit confused - builders use any type of cloth. Could you guide me in right direction? Looks like BID is the way to go?
 
There's a little more to it than just BID vs UNI.

Bidirectional (BID) describes any cloth with roughly the same fiber count in both the X and Y axis.

Unidirectional (UNI) puts most of the fiber in one axis. The other axis has a very low fiber count, just enough to hold the fabric together. We have no practical use for UNI on an RV.

There are many different weave patterns available in BID fabrics.

Plain weave is one over, one under, one over, one under.

Twill weave is usually two over, two under, two over, etc. It can also be one over, two under.

Crowfoot is usually considered to be one over, three under, one over, etc. It is also known as 4-harness.

8-harness is seven over, one under, seven over, etc.

Plain weave has a lot of empty space between fiber strands. Impossible to be otherwise because the over/under/over/under pattern doesn't allow adjacent fiber bundles to slide up tight against each other. Harness weaves (twill, crowfoot, 8-harness) are tighter because they skip some of the over/under. They also produce a slightly stiffer finished laminate because a greater percentage of each fiber strand is straight instead of bending over and under each cross fiber in a constant series of S-curves. The weave pattern is also less apparent in the finished laminate surface, and less likely to show through finish coats after sitting in the sun a while.

Which harness weave to choose? In general, higher harness numbers mean greater formability. You can smooth high harness fabrics around compound curves because the individual fiber strands are less locked into position in relation to their neighbors. They can slip and slide in relation to each other as you smooth out the fabric with your hands.

When should you use plain weave vs a harness weave? Well, plain weave is very fast to wet out and bubbles are easily removed, if they get trapped at all. High harness weaves are more dense, harder to wet, and easily trap air. With that in mind, select whatever works the best for the application. I'd go with plain weave for large, mostly flat parts, twill or crowfoot for moderate compound curves and a nice properties balance, and high harness counts for serious compound curves.

BTW, both Spruce and Wicks are terrible about identifying weaves.
 
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simplified glass layups

the real advantage of BID (bidirectional) is the strength in both X and Y directions. whereas the UNI (unidirectional) has all of the glass fibers running in one direction the strength component is 1.0 in the X direction and ~0.0 in the Y direction. with the BID fibers running at 45 degrees to both the X and Y directions the strength component is 1.414 (sqrt 2) in both directions. the minimum strength component in any direction is 1.0. maximum advantage is obtained laying the fibers at 45 deg angle to your anticipated stress. (i know the math is greatly simplified and assumes equil wt glas; for illustrative purposes only)

glass is really much easier to work with than many builder comments would lead you to believe.

here's my technique:
get yourself a piece of 6 mil clear visqueen (polyethylene vapor barrier) from home depot. lay it out on the item you want to cover with glass. draw your shape with a sharpie on the visqueen. cut to approximate shape. lay the pattern on a piece of glass fabric and use your sharpie to draw pattern shape on the glass with 1" excess all the way around. cut glass fabric to sharpie line.

cut 2 more pieces of visqueen (a rectangle is fine) 4-5 inches larger in each direction than your pattern. center the glass on one piece of the visqueen. you are going to make a plastic-glass-plastic sandwich. mix your epoxy very well (3 min constant stirring) and pour over the glass. lay plastic sandwich top over the glass and use stiff plastic kitchen spatula or wooden tongue depressor or even the sharpie to move epoxy around. continue moving epoxy around till all the glass is transparent. any dry glass will be readily apparent. you may need to mix another batch of epoxy.

use this approximation;
glass = 9oz/yard at a ratio of ~1.5
so make 16 oz (2 cups) of epoxy/yard^2 of glass.
some excess is ok and will likely be blotted up. penalty is the plane is 2 oz overweight.

after all glass has been wetted. lift off the visqueen sandwich top and lay down 1 layer of paper towels over the glass. put sandwich plastic top back down over the paper towels and apply some pressure all over to blot excess epoxy. you just got your 2 extra oz back.

take your original pattern and lay it over the sandwich. trace pattern outline with sharpie. use scissors and cut to pattern line. lift off top visqueen and towels, invert glass onto your mold in proper position. slowly peel back the 2nd piece of visqueen.

you can do 2 or even 3 layers of BID simultaneously with same process. use proportionally more epoxy for more layers. three layer glass layup like Dan has shown can be done start to finish in 20 minutes. not a drop of epoxy on you or on the shop floor. well, i sometimes wear a pair of latex gloves and tap the glass into the corners with my fingers. however, a 69 cent 1" fiber paint brush dipped in epoxy can be used for same purpose. after glass is in place on your mold use a paper towel to lightly blot up excess epoxy pools.

cutting and moving around dry glass is difficult to maintain in the desired shape. lift a BID corner and a circle becomes a football. this process prevents that problem as well as distortion from applying epoxy to the glass.
]
the quantity of frayed edges goes to near zero doing layups this way. notice the frayed edges in Dan's 2nd foto. endemic problem with glass BID. cut glass 1" larger than the pattern and don't mess with the stragglers, just cut 'em off. all those frayed edges are discarded.

for a superior surface buy some 100% nylon from fabric store and lay over the glass/epoxy while it cures. nylon wicks epoxy to the surface leaving a smooth finish. oversize the nylon (roll ends at fabric store often sell for $1/yd) so no edges are in the layup. if the layup is so big you need an edge then use the factory hem. any cut fabric edges in the epoxy will leave fibers embedded in the epoxy. fibers aren't structurally weakening but they'll drive you crazy wishing you had done differently. after 24 hrs grab a corner of the nylon and pull it off. more days might mean more pulling!!

you'll fill the garbage can with lots of expendibles. polyethylene vapor barrier is ~$57/10x100 ft roll. that's a nickel/ft2. but, you're gonna buy a roll anyway to build your paint booth. a dozen paper towels and $5 scissors/month of layups. seems like a lot of stuff.. but doesn't really amount to much.
 
And for more...

.......
There are many different weave patterns available in BID fabrics.

Plain weave is one over, one under, one over, one under.

Twill weave is usually two over, two under, two over, etc. It can also be one over, two under.

Crowfoot is usually considered to be one over, three under, one over, etc. It is also known as 4-harness.

8-harness is seven over, one under, seven over, etc.

.......

BTW, both Spruce and Wicks are terrible about identifying weaves.

.....purchasing confusion, Aircraft Spruce calls their 7781 cloth "crowfoot" on their web site.....:)

http://www.aircraftspruce.com/catalog/cmpages/7781.php

But the picture does look like 7 over, one under.

Dan - thanks for the weave explanation.

It's nice to know I've been using the best one for curved surfaces....:)

I want the East coast folks to keep using Wicks, so I can use the original Aircraft Spruce location - 2 days shipping to AZ with no expedited charges, unfortuneately Wicks is nearer a week for shipping here.
 
Great Source

Thayercraft ( www.thayercraft.com ), on the other hand, is a goldmine of glass.

For roughly 8 years I produced hundreds of R/C Pattern fuselages using glass from this supplier.

Great folks. You'll have to purchase material in bolts, usually mill ends making it more likely for group purchases. The prices were (seven years ago) incredibly reasonable. If you can put together an order with a few buddies, give them a try.


Carl Raichle
RV-9A N194CR
Lutz, FL
 
All, can some one walk through the process of finishing fiberglass? After sanding as smooth as you can feel what primers do you use and what do you use to fill pin holes, imperfections, ect?

I realize this is a fiberglass "form" tip, but I didn't know where else to ask.
 
Finishing

Larry,
Dan will probably give you some good advice but here are a couple places I've gone for help from composite award winners and had good luck:

http://curedcomposites.netfirms.com/finish.html
(PS: I'd avoid the polyester fillers....)

Cory Bird's masterpeice Symmetry used a resin wipe to fill pinholes and provide a durable finish that eventually became almost optically perfect:

http://www.sportsmanpilot.com/AL/articles/spr2003/Spring2003.htm
(PS: he describes it later in the article a couple paragraphs above his picture)

Here is a good explanation of the process with pictures:
http://www.maddyhome.com/canardpages/pages/waynehicks/chapter_25_skimcoating.htm

Good luck,
Dave
 
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