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Composite wing spar web design - composite webs, foam webs, etc.

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UAVGuy

Active Member
Joined
May 1, 2020
Messages
29
I need to design a wing spar for a large RC UAV.

I understand how conventional (ie steel) beams work, can calculate stresses, etc.

I'm mystified at how to design a fully composite spar because I've heard that composites don't like compression loads, leading to buckling on the compressed flange (cap) and web compression between the caps.

If you give me the strength (compression and tensile) of a set of caps and a distance between them, I can calculate ultimate loads, etc, *assuming* the web doesn't fail. But how do I design a composite web ? And how do I prevent the spar cap in compression from buckling ?

I'm mystified at how some designs use foam as the web ! For example, in a LongEze wing there are upper and lower "spar caps" of considerable size, which I understand. But the web between them is insulating foam ! I'm guessing in that design that the spar caps are tied into the wing skins which spread the compressive/shear load between the spars over the entire wing area, thus making the entire wing surface the web. Am I right ?

If so... how far can this be taken ? How narrow of a piece of foam can I use to support the spar caps in a design ? How do I calculate this ?

What if I want to make a more complicated web... say a piece of foam with a composite on each side. How do I calculate the strength of that ?

Any and all hints and advice on composite spar design will be appreciated.

Thanks
 
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