How many layers of fibreglass

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Battson

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Jan 30, 2012
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Hi team,

How many layers of fibreglass will I need to get enough strength in my custom-made wingtips?
They are a standard hoerner shape for the Bearhawk Alpha wing, NACA 4412, depth about 10" and span about 15". I estimated they will see about 90N of lift each at 1.0g.

Obviously I want to use the least material necessary, to save weight. On advice from the aircraft's designer, I have started with 4 layers of 6oz cloth and used the stronger epoxy resin, including a heavy resin layer with mircoballoons instead of gelcoat. They have sanded up really nice and smooth. There's an extra layer of 2" glass tape around the edge to add strength where the screws will go through.
They are just slightly more flexible than I expected, so I wondered about strength.

If I need to add strength I can add chop strand mat, cloth, or foam plus cloth to create some structure on the inside. But I can't decide whether or not that's necessary. Advice appreciated.
 

Voidhawk9

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Mar 26, 2012
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Timaru, NZ
Are you laying up in a mold? I would have carved the shape from a block of styrofoam, laid up 2 or 3 layers of glass outside, then carved away most of the foam inside, leaving about 6mm as a core, then a layer or two of glass inside. Taper the foam to allow the glass to bond to glass at the edge, with maybe some extra layers just there for the fasteners. You would have to fill and sand the outside in this method, of course.

Using glass alone they'll flex easily - add a foam core and it will be very rigid. You might use coremat instead of foam, or add some foam 'ribs' inside with glass over them for stiffness. Avoid chopped mat, lots of weight, not much strength.
 

BoKu

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It's hard to know how much is enough without some analysis. I suspect that the critical loadings are probably from handling while parking and positioning the airplane, and while installing and removing the wings.

I would probably have started with five layers of 9oz cloth for the first set, and added or subtracted from there.

--Bob K.
 

TFF

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Apr 28, 2010
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Memphis, TN
I would put some small ribs in it if there is some span to it. Is it too late to cure at an elevated temp?
 

wsimpso1

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Battson,

Hmm, are you building to the plans? If you have plans and molds, do what the plans say and be happy, unless the build community has found these parts to be be inadequate. Then do what the build community has found to be a sound solution. If there is no standard, we have to get serious...

Here is my thinking on tips. They are fairings. That means they are not really intended to be structural parts... What do they really have to do on your airplane? They have to survive being built, painted, and installed. then they have to stand people walking around the wingtips, manhandling the bird, etc. Then stay put with flying. Now with 15" of span, they might have enough bending moment being reacted into the wing to matter.

Two plies of BID cloth at 8.8 oz/yd^2 is 17.6 oz/yd^2, and that is about the minimum anyone uses for fiberglass parts. They feel flimsy if any lighter. The outside skin of most fast glass airplanes is generally not less than 3 UNI, 7 oz/yd^2 so 21 oz/yd, and it is seriously structural at that level. That sets your minimums. You also have to attach it with screws or rivets on the edge, so some beefup along the edge makes sense.

So, you are using about 24 oz per yard of glass cloth? Sounds about right.

I have rebuilt wingtips and stabilator tips and other fairings for storebought airplanes (code for "made replacements") using the originals in the process to make molds. 3 Plies BID and epoxy, faired with "dry" micro, sealed with epoxy, and painted with primer that is 100% UV block. They have been flying for a bunch of years with nary a split anywhere. Why did I chose 3 BID? I have handled a lot of STC'd and factory wing tips and fairings and the like - 3 BID is what they felt like stiffness-wise and thickness-wise. Also, there are a bunch of RV's flying with less thickness and I do not hear about any of them getting torn up.

I suspect that you are in the right neighborhood...

You could fly without them if you had to, your stall speeds will be a little higher and your cruise will be reduced, but it will fly and be controllable without the tips. Now if the wingtips are cracked, could they lock an aileron or otherwise mess with being able to fly safely? If yes, they need to be sturdy. In an ideal world, the tips could break free at a couple attachment holes and interfere not a bit, but maybe yours are not ideal. If a screw hole failure or two would cause them to lock the ailerons, you MUST prevent that from happening. Inspect the tips carefully before every flight. Remove the tips if you find any cracks...

What would I do? I would fly the thing and look them over carefully every flight. You will have to include a mirror and light in your flight kit so you can look over both top and bottom on every flight. If they grow cracks, take them off and build a new set with another ply added to the layup schedule in the crack locations, and add some more fasteners to what is holding them.

Billski
 
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TFF

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I think Battson has had his Bearhawk flying for a couple of years. Sounds like he is doing what you are suppose to do with Experimental aircraft, experiment.
 

autoreply

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If you have access to molds; using something like Soric makes you get away with significantly thinner skins since the core is fairly rigid. I'd not be surprised if you could get away with two light layers (100 g/m2) of CFRP. Using traditional foam cores that's way too weak. Check for lift loads though; you might need a spar or local reinforcements.
 

Arthur Brown

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May 1, 2016
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London
IMO the wing tips almost attract damage. They need to be strong enough to hold together in normal service, BUT need to be weak enough to fail before the wing itself. I'd also think that the fastenings should fail at the screw line on the replaceable wing tip rather than the rather more critical end of the wing.
 
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