Lost foam wing?

[PVenkman]

I understand how doing a lost-foam fuselage works, and plan to try it in the not too distant future. But thinking about it made me wonder why not do the same thing with wings?

Specifically what I mean is after you vacuum-bag a wing, why not disolve the foam out of the wing when you're done?

Does the foam add enough to the strength of the wing that this is a bad idea? Or is it just not worth the weight savings? I think particularly for an HLG any weight saving is worth while.

[usrobotics]

Depending on the type of wing and construction method used removing the foam could be seriously detrimental to the models health!

On many composite models the foam constitutes considerably to the strength of the overall wing, acting as a partial load bearing structure or shear web. It would take a very heavy lay up to allow the skin to carry all the loads by itself.

One way to do it would be to do the sandwich style of construction, cut out the foam wing, and build a full-depth carbon spar with integrated flange and shear webs.

See https://www.cstsales.com/How_to_Articles/SparBuilding.htm

Glue the spar into the wing. Cover the foam part of the wing with tape but leave the carbon spar exposed. Wax the tape and put a nice spread of epoxy over the spar then lay up the glass/balsa/glass sandwich skin. Once it's cured melt out the foam and work out some way to join the halves.

Now it's up to you to see if it works!!!

- Dan

[PVenkman]

So the foam does actually contribute to the strength of the wing. I guess that's not too surprising.

What I was thinking of is a variation on the technique you described. Cut out the foam wing, and then slice it in half lengthwise right along where the spar would go. Vac-bag the two halves seperately, with some nice carbon fiber where the wing was split to form the 'spar'. Warp the carbon up over the surface of the wing, like a ] shape, to form spar caps. Now join the two halves back together with a final layer of fiberglass over the whole thing to give a nice surface and vac-bag it again, and you have a wing with an embedded cf spar shaped like this ][. Melt the foam out and you're done.

What do you think? I do plan to try this myself some day, but that's a ways off.

[usrobotics]

It would be worth a shot. However, you will need to be careful to get the material orientation correct for the spar and its going to be hard to get adequate stiffness into the fibreglass skins. Even with a 'standard' foam HLG wing getting enough torsional rigidity is difficult. Perhaps a complete carbon 'shell' would be necessary?

What you are trying to make is what engineers term an 'I' beam. The top and bottom horizontal ( - ) bits (caps or flanges) carry all the bending load so the optimum material to use here is uni-directional carbon running from tip to tip. The vertical part ( | ) (the web) is used to keep the caps the correct distance apart by absorbing the load/shear force. The shear force acting on the caps is greatest at an angle of 45 degrees. The material you want to use here is bi-directional carbon cloth turned so that its fibres are running at 45 degrees in relation to the caps.

So if you imagine seeing it from the front what you would see would be something like this (really the uni-directional carbon '-------' would actually be bonded to the 'xxxxxx' 45 degree carbon.

-----------------------------------------
xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
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Hope this makes sense!

- Dan

[m_acree]

If you want a bagged wing with an embedded carbon I beam/spar read this documented build. I will be building a wing like this fairly soon.

http://www.favonius.com/soaring/bagging/bagging.htm

[PVenkman]

If you want a bagged wing with an embedded carbon I beam/spar read this documented build. I will be building a wing like this fairly soon.

http://www.favonius.com/soaring/bagging/bagging.htm


This is the kind of thing I'm thinking about. Have you ever tried melting the foam out of the wing afterward, or would it weaken the wing too much?

[davidfee]

Removing the foam from a foam core wing would weaken it immeasurably because there would no longer be anything to support the skin. You can make hollow wings, but the skins must be self-supporting sandwich panels.

-David

[m_acree]

The only reason for doing a lost foam fuselage is to make room for the radio, pushrods, etc. Just like a fuse you can do a heavy enough layup on the wingskins so that removing the foam would not be detrimental to the strength but at that point you will have a wing that is heavy enough that the added weight of the foam is negligble, which it is anyway. Leave the foam and build a lighter stronger wing with embedded carbon spars. You will be much happier and it will make repairs much easier! I have repaired hollow molded wings and it's much more time consuming (no backing to work against). Don't forget to install you're servo wiring before you bag the wings!