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Questions about making intersection fairings over expanding foam

WB-avl

Active Member
My idea is to use a similar technique that was shown by Dan Horton in this post: http://www.vansairforce.com/community/showpost.php?p=1378151&postcount=23

I came up with a theoretical series of steps to do the gear leg to wheel pant intersection fairings.

Any comments or suggestions would be appreciated!

End goal is to have the fairing bonded to the wheel pant.

Steps:

- Tape off gear leg fairing and all openings around wheel pants (question here is do we want to protect the wheel pant from the foam sticking or not?)

- Build foam dam with tape, paper, etc.

- Pour expandable foam

- Shape cured foam using pvc with stick on sand paper etc.

- Prep foam for fiberglass layup. Best method? Coating of epoxy, drywall compound etc? wax, pva?

- Clean, sand and clean (again) wheel pant where layup will bond

- Layup fiberglass over foam and wheel pant to form intersection fairing

- Cut layup (intersection fairing) along wheel pant split line.

- Remove wheel pant / fairing halves

- Clean up / cut out inside of wheel pant and reinforce.

- Remove foam, tape, etc. from gear leg fairing
 
I bought some expandable foam and used it to create a portion of a mold I needed. In the end, it came out well, but it was much more difficult than I expected. The exterior of the foam once expanded was incredibly hard, much more so than the interior of the foam. So once you get through the outside layer when sanding, you end up dealing with some surface that is still rock hard and soft which means sanding is much more tedious while you try not to eat into the soft(er) foam as you try to get through the exterior.. You will also need to use filler or something since the surface of the expandable foam isn?t smooth enough. I had good luck using duratec surfacing primer but if you have a lot of surface irregularity, you may want to start with some other easy sanding filler. Unless you have your heart set on using the stuff, I would consider trying to work with something else, like maybe a chunk of clay or just cutting and hot gluing pieces of molding foam together to cover the area. Sanding and shaping would be much simpler. I believe you?d save yourself a lot of grief.
 
I Used the expanding foam to build my rear canopy skirt out of fiberglass. Wonderful to work with. I used the carbide sanding blocks to sand it to its final shape then covered it with packaging tape as a release agent. Worked great!
 
When I built the front part of my plenum intake to fit the rings on the cowl, I used a stack of 2" pink foam from HD glued together and shaped with a saw, rasp, then sandpaper. I think it is easier than expanding foam. Everything is the same firmness and it can be sanded very smooth. With the gear legs there, you could easily use them to tape it to for support while fitting. It worked for my airbox too.
 
Layup

Dan is "The Man" as they say. Do what he says.
I do know the better the form, the less work later. I used copious amounts of heavy packing tape on my canopy skirt. I applied two coats of carnuba wax followed by two coats of PVA. Layup popped right off.
Another trick I picked up was Gorilla yellow Delicate Surface tape. It has less tendency to leave residue behind and markings really show up well. The packing tape was layed over it. It was easy to see and trace markings through five layers of 8-harness. Made the cutting really easy.
 
Good info

Thanks for the feedback.

I think I'm going to play around with a test piece on the work bench before I foam up the wheelpant / landing gear fairing!
 
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End goal is to have the fairing bonded to the wheel pant.

Steps:

- Tape off gear leg fairing and all openings around wheel pants (question here is do we want to protect the wheel pant from the foam sticking or not?)

Not. You'll want the new layup firmly bonded to the wheel pant, so do the usual surface prep for bonding (heavy sanding scuff) all around the shaped foam, just before doing the layup.

- Shape cured foam using pvc with stick on sand paper etc.

Try removing all excess foam with a hand-held hacksaw blade prior to sanding blocks. Just cut a rough 3-view.

- Prep foam for fiberglass layup. Best method? Coating of epoxy, drywall compound etc? wax, pva?

None. It's easy to shape the foam to a near exact contour, then do the fabric layups right on it. Tape will result in a worse surface contour. Attempting to coat or otherwise surface finish the foam risks contaminating the bond area on the wheel pant surface. The layups will extend up the gear leg fairing (which masked and coated with mold release, or slick tape, or similar) an extra inch or so, and bond out onto the wheel pant surface.

- Cut layup (intersection fairing) along wheel pant split line. Remove wheel pant / fairing halves. Clean up / cut out inside of wheel pant and reinforce.

Exactly.

You'll split the new fairing layup first; firmly tape a metal straight edge guide to the leg and wheel pant, them score a few times with a box cutter knife or similar until you cut through the glass.

Now remove the wheel pant halves, each with its new section of fairing. Cut out the wheel pant glass inside the fairing and remove all the foam from inside the new layup. Sand the cut edge of the wheel pant to blend with the new layup, and add two plies of glass on the inside, over the joint.
 
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Forgot to mention foam density.

Common 2 lb density works fine when the form doesn't have a lot of detail. The RV-8 can have a very small fairing at the wheel pant, and the shape is not complex, so 2 lb will work fine. I've used a lot of 2 lb for big things like plenum lids and cowl exit panels too.

Shaping foam for more complex shapes, or for more detail, is easier to do using a higher density foam. Here's one source, and there are others: http://www.uscomposites.com/foam.html
 
Thanks for all the great info Dan!

Forgot to mention foam density.

Common 2 lb density works fine when the form doesn't have a lot of detail. The RV-8 can have a very small fairing at the wheel pant, and the shape is not complex, so 2 lb will work fine. I've used a lot of 2 lb for big things like plenum lids and cowl exit panels too.

Shaping foam for more complex shapes, or for more detail, is easier to do using a higher density foam. Here's one source, and there are others: http://www.uscomposites.com/foam.html
 
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