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Jul 19, 2019, 09:42 AM
Everything's A Composite
Knoll53's Avatar
Of course it takes a well trained eye, but if you look closely, you can see a slight bow in the wing.

For durability, I too like a spar as opposed to just foam core and stressed skin. Once you've got a good spar, the skin need only span to the TE.
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Jul 20, 2019, 06:52 AM
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EdSoars's Avatar
Kent: Not sure what you mean: "Skin need only span to the TE."

I was never able to break that hemp twine spar. It just kept bending, which with better fiber-to-resin mix would probably make a near-indestructible spar, IF the flex was acceptable, and that depends on what and where and how fast you want to fly. Hemp fiber still has great promise for spars, but I haven't yet found a source for long fibers in an easily-usable form like carbon fiber or fiberglass tow.

Planks provide me the most enjoyable flying in the conditions near my home, which are usually boisterous. Among the guys I usually encounter at the flying sites, the frustrations they have experience with planks seems to be due to play in the control surface linkages, and inaccurate measurements of the planform to calculate CG for the maiden flight.

I am still inspired by the videos and designs of Uwe Heuer and of course Peter Wick. Jaw-dropping performance. Of course, those are 3 - to 6 servo wings, and I'm not going there, but the 80-20 rule is good enough. With 20% of their advanced building methods and complexity, 80% of their performance should be achievable.

So back to the drawing board. Or I should say, "Onward to the drawing board." A lightweight, E-powered thermal plank is in the future.
Jul 23, 2019, 10:42 PM
Everything's A Composite
Knoll53's Avatar
Once the spar is strong enough to carry all loads all the way out to the wing tip, then there is nothing for the skin to do except span from the spar to the LE and the TE.

Without a spar, the skin carries all the bending loads and the entire core is the shear web. This stressed skin approach is probably more efficient, but requires a good bond between the skin and foam. Once you get a wrinkle in the skin or any delamination, you've lost all the strength. Typically a compression failure. I have broken many such wings with what I thought was a good landing. Hard landings probably far exceed the flying loads. That momentary 10G impact will crack anything brittle. That is why your EPP protected spar is such a good idea for your (assumed) high speed landings on rock. The EPP absorbs the shock and the durable spar stays intact.
Jul 25, 2019, 07:50 AM
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EdSoars's Avatar
Gotcha. Yes, your analysis is exactly right. On the current Max, I pecked holes in the foam to increase the surface area of the skin-foam bond., but this time i pushed them deeper into the foam. The elevons were difficult to cut out due to the epoxy-filled holes. They were also quite rigid in torsion, a good thing with the tapered design, which puts the control horn in a small-chord portion of the elevon.

The rigidity was due to the top and bottom skins being connected to each other through the thin part of the foam cores by the epoxy-filled holes. The next wing will have holes through the foam everywhere, and I'm thinking about some kind of chopped fiber to increase the epoxy's tension strength.

If I can make holes that stay open in EPP, this might make a very strong wing. It would also be easy to repair, another good thing, given my lack of restraint in choosing landing zones.


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