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Wingtip flexing

Electrogunner

Well Known Member
I finished one of my wingtips and was a little concerned about the flexibility of the large surface. I could see a flutter possibility there ie: oil canning type action. Has anyone added support inside via maybe glassing some foam ribs in maybe? Thoughts?
 
I have seen some people's build logs where they install foam ribs to strengthen the FG tips. However, it seems like most were doing something else too, like installing auxiliary fuel tanks or something. I did not install anything and my tips seem fine. Check with Vans, but I don't think a little stiffening would hurt.
 
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I have a -7A but had the same concern, as the tips are pretty flimsy and folks walking by often put a hand on the wing tip, resulting in oil-canning and hairline paint cracks. I removed the wing tips and applied a single ply of 8.8 oz bidirectional fiberglass in the area of most concern. Worked great at improving the stiffness; wish I had done it sooner.

erich
 
By all means, reinforce this area. My wing tip top is cracked from someone leaning on it at an airshows. Its gotten worse once started, by flight loads. Been waiting for warmer weather to fix. My buddy reinforced his on his initial build and his does not suffer the oil canning effect. One hopes some schmuck at an airshow will not lean their fat appendages on it hard enough to crack like mine.
 
You can use 2mm Soric, a core material with a layer of 8 oz cloth over top. After sanding, just wet the inside lay the soric on, and cover with pre wet cloth. It is best to overlap an inch w/o the core, but I tested pieces that indicated little structural benefit. Make a test piece using your process first. Let me know if you need more details.

The gel coat is about half of the thickness of the 7-8 tips, don't know about the 10. quite fragile.

Use the link above, and sign up at fiberglass and you will get more discounts. They need them.

It will reduce deflection by about 8x for the same load.

Edit: Here is the test article with the 2mm core. The test piece without the core touched the bench with the same weight.
9IkNQhItRssuOYXpRxNRL5WiKtaPP_HwNrH9i-wI7RWhUgz8jcN7p5yaD0WGR4dcCxQ0TeTd546gmj0Szy3czlFVyUtyu6d20O8UEkQ-mAaMBiAwLsExoGwyuaXrNWlH8AkmAVI872K9KfQcXb-0tx_xx-0syiRNFfwNx_n0qW2vyEORlSN3MrPUiDTvdRclM3IKyd5S7hXvrkq_U3PmD6yI_8x0VRC21goVFMI2zODn2EOzI-mLEs2RRtBMmXRH7Xpc1cKIXC7IifBZtdCpce3V6aaxdmYuYh8OX1TVAgRlHAd89kA1lj7EofbEpRIGp_P0yNLoJPjSN3Xxpi4gGkJu6T4_borx1G7UN0zKPGx2e8YRXUwJDla2XFsevajQ8CKgnSQ_MhNAaOA0ZN-CSme-8FKgU4FEKIFinx1FWATqYQbsV3uDBk3MGozDEoLcVdEXle6n8dswosqRTFfjEexwaFjhqaxMNbfm_G9gixsOvbytA3WG1iazZ9P-BnTy6k1K9jD5fLfL7MXE7ejB2MJxFerQGkjzmRRAgrXY3ZVqIx6fky4tjf9OJ0vnfZUI1t2by9wLswyeNLNslKIZYc-gfuBT6wgKPGMhHi6N8Cw6q3fNbMja=w500-no
 
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I never thought of doing the wing tips but i did reinforce the inside of my wheel pants and filtered air box with additional layers of epoxy, basically to fill in the weave until it was smooth.
 
Tips for tips

There's a great set of tips in the "Articles" section.
Opening VAF page.
Go to the left column of links.
Click on "Articles"
Download all the "Tips for Tips"
 
I never thought of doing the wing tips but i did reinforce the inside of my wheel pants and filtered air box with additional layers of epoxy, basically to fill in the weave until it was smooth.

Not to flame, but epoxy alone adds no strength, just weight. If you just want to fill in weave, use micro. If you want to reinforce, add glass.

Carl
 
I finished one of my wingtips and was a little concerned about the flexibility of the large surface. I could see a flutter possibility there ie: oil canning type action. Has anyone added support inside via maybe glassing some foam ribs in maybe? Thoughts?

Have you screwed it to the wing? IMO, oil canning isn't a worry with it attached. The convex shape helps.
 
It is attached with hinges. Per the advice I think I will do a layup on the inside across the widest area to stiffen it up a bit. Thanks all for the advice and ideas.

Without some core material, it will not be very satisfying. Painting on resin does nothing. One layer of glass is difficult to measure. 2mm Soric and one layer of glass is dramatic. If the price of Soric is too much, take 1/4" thick pink styrofoam and make a strip with it. That work just fine. Even 2" wide. Also dramatic. Cut some triangles 1/2" on a side, get some depth. There is a reason floor joists are placed on edge and not flat.
 
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Without some core material, it will not be very satisfying.
One layer of glass is difficult to measure.

I can assure you that one ply of FG is both easily measured and adds considerable stiffness to the wing tips. I know because I did it. If even more rigidity is desired, add a 2nd ply, but I didn't find it necessary.

Erich
 
Is this an application where carbon fiber would be useful?

Obviously not where antennas will reside in wingtips, but otherwise?
 
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I can assure you that one ply of FG is both easily measured and adds considerable stiffness to the wing tips. I know because I did it. If even more rigidity is desired, add a 2nd ply, but I didn't find it necessary.

Erich

Good to know, I only did the bench test. It was with 4 layers 8 oz, then 2-core-2 in the photo. I later did a 2-core-1 test that "seemed" as stiff as the 2-core-2, but only tested it for strength. The 2-C-1 piece was cut to expose the core to see if it would fail in shear, it didn't. Also, the pict was an infused piece. I got the Soric for plenum covers.


It seems the tips may only have a couple of glass layers, at least from my measurements the rest is gel coat. So, if a single layer works excellent! If more stiffness is desired a core will give much greater stiffness per oz, but it also has weight, probably equal to another glass layer. All in the expectations, I suppose.

Edit: Lets add some numbers. Deflection is inversely proportional to the moment of inertia. 2 layers of 8 oz is about .020", 3 is .030" So, one additional layer - Proportionally 1.5X^4 ~5X stiffer. With a 2mm core .020 becomes .070 thick and ~150X stiffer. Conclusion: 5 is pretty good (but not for a plenum)
 
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Did you have a problem with the surface of the wingtip deforming at all when you stiffened it with the layer of bid glass? Is one layer enough, or would you do more if you had it to do over? Thanks for your advice as I'm also concerned about the flexing after paint.
 
a guy leaned on my wing tip early on and "pow", a big delamination. I repaired it by removing the delaminated material and adding big pieces of glass to stiffen it. then I added the same to the other side. I added the glass an then immediately attached it to the wing tip to cure. no deformation.
 
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