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Old Apr 27, 2015, 09:37 PM
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Eliminating flex on hotwire cut wings?

I have a pylon racer style plane with a very thin airfoil (don't remember the exact one), it has a 3/16" fiberglass spar from a dollar store kite but the wings still flex like crazy, it's flapping around all over the place. Are there any strategies for reinforcing wings? It seems like it should be stronger than it is.
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Old Apr 27, 2015, 10:02 PM
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Carbon fiber is much stiffer than glass. I had some of those kite fiberglass rods and they are pretty much useless.
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Old Apr 27, 2015, 11:30 PM
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As an experiment you can try stretch pack taping over the wing in a cross hatch pattern at a 45 degree angle. I think you glass a wing this way, they call it laying out the cloth on bias. I've done it both ways and the bias or cross hatch is by far superior in stiffness.
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Old Apr 28, 2015, 01:46 AM
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Rods - regardless of whether it's carbon or fiberglass are going to flex. They are tolerable with depron wings because the wing loadings are generally light enough and wings are short enough in span so the degree of bending is generally minimal.

A tube rather than a rod will have better resistance to bending, however depending on the weight and forces encountered a more traditional type of spar might be better.

Alternatively, a relatively thin piece of 3/16 carbon strip embedded vertically would probably offer the best balance of weight vs bending resistance overall and would be relatively easy to install, slice through the wing and epoxy it in then fill in any low spots.
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Old Apr 28, 2015, 07:00 AM
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A few of my planes are made rigid by slitting both upper and lower surfaces along the thickest part of the wing about 1/8 in. deep. Insert 1/16 in. or 1.5 carbon rod in the slit. It should be deep enough to almost disappear. If it's EPP foam flood with thin CA along the slit so it soaks in to lock the wire. Thin foamsafe CA if it's regular foam. Makes for a very rigid, light wing.

Gord.
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Old Apr 28, 2015, 08:27 AM
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Made the speedy, a small pylon type foamy from a member here on RCGroups. The wing is dollar tree foam with carbon tube spar covered in packing tape. Has lots of flex with a not to fast power setup. Making a new one now, got the wing almost done. 6mm depron carbon tube to within one inch from tips. Fiberglass weave tape for drywall 6inch width, applied to both sides trimmed around the edges then sprayed with a heavy duty spray adhesive. Smooth out the adhesive with a drywall knife. Much better but still need to tape to make smooth, which will add a bit more stiffness.
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Old Apr 28, 2015, 09:19 AM
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Hey Steve! Monocote iron on low heat will make the glue on the tape stick better and help shrink it a bit too.

Gord.
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Old Apr 28, 2015, 01:39 PM
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Quote:
Originally Posted by rhokita View Post
As an experiment you can try stretch pack taping over the wing in a cross hatch pattern at a 45 degree angle. I think you glass a wing this way, they call it laying out the cloth on bias. I've done it both ways and the bias or cross hatch is by far superior in stiffness.
Depends on the sort of stiffness you want.

If you're talking about bending, then a strip of fiber packing tape underneath and a rod or strip of just about anthing that doesn't compress on top works really well. I used 1/8"x3/8" basswood as the top cap on the foamystang. The plastic packing straps also work well and are near free- carbon flat works, too.

If you're talking about twist or flutter, then the "on the bias" crosshatch (or "disser") pattern works best. Fiber reinforced tape is better than regular tape.
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Old Apr 28, 2015, 04:05 PM
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Originally Posted by DaveK View Post
A tube rather than a rod will have better resistance to bending...
That is not entirely true. Since the rod has more material it is more resistant to bending. But the effectiveness in the material decreases the closer to the core it is. The tube is of course lighter so it is a better choice though.
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Old Apr 28, 2015, 04:26 PM
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A hollow shaft is always stronger than a solid shaft. Large heavy duty drive shafts are all hollow. This may only apply to twisting resistance and not bending, not sure.

Thanks for the tip flypaper, will try.
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Old Apr 28, 2015, 05:34 PM
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It applies to both actually and you are wrong Sir.

Think of a tube Outer diameter of 6 mm and inner 4 mm.
Then place another tube in that O diam. 4 mm and I diam. 2 mm, and in the center a rod diam. 2 mm. If it is that a hollow shaft always is stronger as you say, then the inner parts would apply force in the same direction as the force that is twisting or bending the whole structure! This is against everything.
And if we think that the inner parts have no effect at all, then if you remove the inner rod for example it would bend like an overcooked spaghetti since it doesn't have any resistance at all against any force! And that is not the case.

There is other reasons for drive shafts to be hollow like inertia and weight.
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Old Apr 28, 2015, 11:41 PM
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A thin spar top and bottom of the wing will give you a much stiffer wing than a single rod or tube, they are called cap spars this is how most full size plane wings are put together.
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Old Apr 29, 2015, 02:12 AM
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Cap spars work very well for bending, but are they good for torsion too? Seems to me that a hollow tube would do both jobs for bending and twist.
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Old Apr 29, 2015, 05:08 AM
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Yes, I agree with "Loco1", a spar on top and on the bottom are best for a wing. They don't handle torsion well though.
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Old Apr 29, 2015, 06:09 AM
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I think this one comes down to weigh. The thin spars are much lighter than a tube spar. Foam itself is relatively rigid as compares to built up structure.

Gord.
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