Fiberglass help... Underside Layup

I would like to replace an aluminum panel on the bottom of my cabin with either fiberglass or CF. My reason for doing so...

The AL panel is originally a flat piece of .025. To install it requires shaping and bending to match up the screw holes. This creates multiple compound curves. Naturally the AL wants to spring back to it's original shape (flat) which makes it a real PITA to install and remove. This is an area that requires frequent panel removal for access to fuel lines/filters, boost pump and gear leg oleo struts.

In order to match the shape of the panel I would have to cast a layup in place and upside down. There is no way to remove the panel to perform the layup as it will not maintain the desired shape once removed.

Does anyone have any suggestions for casting a layup from the bottom of an original part?

My thought is...

• Cover the in place bottom panel with silicone (release) film
• Brush on epoxy resin
• Let tack up
• Layer on 8 harness fiberglass strips
• Let set
• Wet in the fabric and repeat.

This seems like a gravity fighting sloppy mess...

I am open to anything. I don't know or have the tools to shape AL, so that would be out of the question.

Thank you for any thoughts or insights.

Rob Caldwell

Attachment 9178

I'm no expert, but here's what I'd try. Cut and lay out the fiberglass cloth on an oversized piece of Saran wrap. Pour on the epoxy, cover with another layer of Saran wrap. Roll out the epoxy so the cloth is completely wetted and excess epoxy is squeezed out. Now peel off the top layer of wrap, slap it onto the area to be covered, and wrap over it with more plastic wrap held in place with some tension by some wide painter's tape to hold it in place while the epoxy cures. You don't have to get the cloth cut to the exact size, you could make it a little oversized and trim to fit.

I've done the Saran wrap thing a couple of times, and it works great. Van's recommends it for doing layups on canopy fairings, and I've used it for wheel pant repairs. The only complication in your case is doing it on the bottom side, and some stretch wrap and tape should do the trick there.

I presume the area you're wanting to work on is the unpainted area in the photo. With the exhaust pipes running parallel to it, what about heat? Will the fiberglass tolerate it? It's one thing to run a hot pipe through a generously-cut hole, but in this case the heat gets radiated over several inches.

I had a similar problem when I bought my Fly Baby...there was a panel across the belly that was supposed to be 0.025" aluminum, that the builder apparently made out of battleship armor. Needed to be flexed to be installed, and that was a bear and a half to do.

I got around it by making it a two-piece panel. Use another piece of aluminum for overlap, drill attachment holes BEFORE cutting the panel. Then cut the panel between the two sets of holes. Rivet the overlap strip to one side, and add anchor nuts to the other. This has worked very well for the past ~25 years.

Attachment 9179
The metal strip that joins the two is on the inside of the panel, so from the outside all you see is the rivet heads on one side and the screw heads on the other.

The key factor is to drill all the holes (through the metal strip into the panel) BEFORE you cut the panel in half. That way, everything is solidly aligned.

Ron Wanttaja

Thank you, @DaleB ! Good ideas.

@rwanttaja

No, actually the piece I am referring to is directly behind the tunnel (unpainted area). The piece with the gear leg struts protruding through the back. You can see that I have attempted to try what you have described in your renderings (thank you for that effort!). I divided the panel in two and joined them similar as you described. Unfortunately this did not resolve the issue I am still having, which is fighting with the aluminum panels to get them into the compound shape they assume when fully screwed in. They just want to spring back. I have tried pre-bending, or shaping them to match the compound shape. So I am considering fiberglass or CF to match the shape in place.

What I'm going to suggest has nothing to do with fiberglass...

Seems the trouble lies with the aluminum...

When shaping aluminum...it gets hard and a little brittle...
To keep shaping it without damaging it... Heat is applied to neutralize the tension that built up.
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To know how much heat to apply , mark a "Sharpie" line on it...
When the line DISAPPEARS... you have neutralized the tension and can continue shaping it.
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In your case.... If you can use a propane torch "IN-PLACE" , it might help you fasten the piece you already have in place.
Bend a piece of scrap aluminum and test it out... Sharpie and heat
I like the push button propane burners...the larger ones ( $50 ) not the little flame 15 dollar ones...

Make sure anything is COLD when you unscrew anything you heated to stress relieve.
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Gotta Fly...
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Quote:

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Originally Posted by planecrazzzy

... Heat is applied to neutralize the tension that built up.

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I hear you... Unfortunately major fuel components (gascolator, boost pump & inline filter) are inches away from this panel. The gear struts are also oil filled and have rubber and neoprene surrounding them. Of least concern is that the panel has been painted...

Well there you go... It can't be done.
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Gotta Fly...

I've done upside down wet layups with fiberglass and carbon fiber. It's not easy or fun as gravity is always there and this stuff wants to fall off. If you have little composite experience, it's going to be a very frustrating, messy experience. About the only way to accomplish a wet layup while fighting gravity is using the plastic wrap method. I've had some moderate success with this method, but isn't something I'm anxious to do again. It would be easier if you made a jig to hold the piece in the shape you want, then use it as a mold to fabricate a new piece working right side up on a bench. Then it's easy!

One thing I didn't see in your post was what grade and hardness aluminum was used to construct the existing part. Study your aluminums and hardness for forming to what you need. More than likely the existing panel is 2024-T3. Look into 6061 aluminum and the various hardnesses available and their forming qualities. There are a lot of different Aluminum alloys and most are available in various hardnesses. Each one has different working qualities for stiffness, formability, and resistance to cracking. That doesn't look like a difficult piece to form. You could form it from dead soft material, then use that as a form to do a composite layup on the bench.

Quote:

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Originally Posted by cub builder

I've done upside down wet layups with fiberglass and carbon fiber. It's not easy or fun as gravity is always there and this stuff wants to fall off. If you have little composite experience, it's going to be a very frustrating, messy experience. About the only way to accomplish a wet layup while fighting gravity is using the plastic wrap method. I've had some moderate success with this method, but isn't something I'm anxious to do again. It would be easier if you made a jig to hold the piece in the shape you want, then use it as a mold to fabricate a new piece working right side up on a bench. Then it's easy!

One thing I didn't see in your post was what grade and hardness aluminum was used to construct the existing part. Study your aluminums and hardness for forming to what you need. More than likely the existing panel is 2024-T3. Look into 6061 aluminum and the various hardnesses available and their forming qualities. There are a lot of different Aluminum alloys and most are available in various hardnesses. Each one has different working qualities for stiffness, formability, and resistance to cracking. That doesn't look like a difficult piece to form. You could form it from dead soft material, then use that as a form to do a composite layup on the bench.

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All great thoughts! Thank you very much!

If you used foam to hold your Fiberglass in place... Just sand the foam away after it dries ...?
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Gotta Fly...

If installing this panel is a big hassle as you say, it seems to me it would worth your time to build yourself a mold. First question…are you satisfied with the fit and look of your panels once in place? If yes, I would take poster board, not foam board, and create templates at a spacing close enough to capture the compound curve, say at 1 inch spacing. Then take some sandable foam and slice it to match your template spacing. Assemble the templates and foam together and sand your foam till your templates are just exposed. Now you’ve got your shape and you can do your layup without fighting gravity.

Sounds like your problem is that the panel is 2024 which is very stiff. I would first try making the panels out of either 5052, 6061-0, or 3003-H14. They bend and hold shapes much more easily than 2024.

If you really want to make a composite panel, how about this to take the shape away from the airplane. Aluminum 6061-0 is soft and bendable. Get some 0.040 and bend it into place. Now carefully remove it and you have your mold to lay composite onto. Alternately you might try making a panel of 3003-H14 that is stiffer but bends to shape pretty easily. 5052 is also an option for this but you might decide after putting one of these into place that you do not need to go composite.

Best of luck,

Wes

Quote:

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Originally Posted by WLIU

Sounds like your problem is that the panel is 2024 which is very stiff. I would first try making the panels out of either 5052, 6061-0, or 3003-H14. They bend and hold shapes much more easily than 2024.

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All Aluminum has essentially the same "stiffness" (Young's modulus) of about 10,000,000 psi. What you mean here is that the 2024 is stronger than the other types listed, so that it can take more stress before plastically (permanently) deforming. That's not stiffness - it's strength.

Quote:

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Originally Posted by WLIU

Aluminum 6061-0 is soft and bendable. Get some 0.040 and bend it into place. Now carefully remove it and you have your mold to lay composite onto.

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There's no way that a thin piece of AL will hold its shape anywhere near well enough to act as a mold when taken off the airplane.

There's no reason not to just do the layup in place on the bottom of the airplane, using copious mold release (PVA) on the surface. Use 4 plies of 7725 BID cloth, two at 0/90 and 2 at 45. Around the edges, where screws hold it on, add 2 more 1" wide strips to bring the total thickness to 6 plies. After cure, pop loose, take off the metal panel, trim the composite panel to shape and voila'.

Quote:

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Originally Posted by WLIU

Alternately you might try making a panel of 3003-H14 that is stiffer but bends to shape pretty easily...

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See above - not "stiffer", but stronger.

Perhaps you have an FBO on your airport that has an English wheel that can help you create the compound curve you need. If not on the airfield,maybe a body shop, or sheet metal shop in your area. Youll wonder why you never discovered this wonderful tool before now !!

Quote:

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Originally Posted by JETROCK

Perhaps you have an FBO on your airport that has an English wheel that can help you create the compound curve you need. If not on the airfield,maybe a body shop, or sheet metal shop in your area. Youll wonder why you never discovered this wonderful tool before now !!

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I'm a beginner on the English Wheel.
It helps to have a Rubber Wheel on top sometimes.
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I made a few Mods to a cheap one from Harbor Freight
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https://www.wingsforum.com/viewtopic.php?f=325&t=23279&sid=0a08824e1396c7b415 1d90c089a2d3c3
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Gotta Fly...
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