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sheet alu. skin over solid foam core?

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bmcj

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This might be a dumb question but why not use fiberglass instead of aluminum sheet? Isn't it easier to work with and a fraction of the cost ?
Fiberglass is more labor intensive. Aluminum can be cut, drilled and stamped by computer controlled machinery (and sometimes even riveted).
 

HumanPoweredDesigner

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Anybody using sheet metal skin over solid foam core instead of ribs? I know the Cri Cri uses sheet metal bonded to ~1/2" Klegicell ribs, but how about over hotwired or block sanded solid foam core of, say, 2 lb./cubic foot density?

This way, you'd get the smoothness and known strength of alu. skin, and the ease of hotwired or block sanded core material, with the stresses distributed evenly over the whole core, rather than concentrated in rivet holes on narrow ribs. This would allow use of thinner alu. skin, given no stress concentrations at rivets & ribs.

(Warning: I just started learning about aerodynamics and structures 3 months ago.)

Here is my two cents on this idea:

The specific compression strength of foam tends to be less wood, unless you have some really good foam. Then, you have to look at the compression modulus, since it does not matter how much it can take if squishes through a range before failing, which makes the skin fail first. Finally, for something as big as an big wing, there is a lot of volume, and the foam tends to be much heavier than if you had just used ribs, though it may work in the tail or in a propeller.
Finally, since the foam is stuck inside the aluminum on all sides, that actually might increase the compression strength, but I don't know how much. It might just be as much as a balloon of air.


I just realized the mass of the foam might be small compared to the thick skin of a powered airplane. I was thinking for human powered fight. But still, that may mean the foam would be even less strong compared to the skin strength, and not be able to perform its webing job. But I could certainly be wrong. Some foams are very rigid and very strong. So definitely look into that. Google Spyder foam. It might be one of the good ones. Try to find the percent compression. It needs to be small. But on the flip side, I don't know if two rigid means something else won't be able to absorb shock and will break. So definitely ask these guys about that.
 
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M61A!

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I've done a fair amount of testing on the bonding of PVC (Klegicell) foam ribs and stiffiners to aluminum skin. You can find a lot of my posts in the design section of this website. In short, I have determined that Epoxy is brittle and therefore vulnerable to vibration and peel loads, but is very strong when the design is good, the materials are stiff and the preparation is done absolutely correct. Proseal is highly resistant to vibration and less sensitive to peel, but is much weaker than what epoxy is capable of achieving. Proseal has a long working time, and Methacrylates setup fairly quickly. Certain Methacrylates exhibit the best qualities of both in terms of strength and reliability. Methacrylates bond more readily to aluminum than epoxy. Epoxies are very unforgiving of poor surface preparation. The Methacrylates that are more elastomeric exhibit a much higher level of reliability than epoxy.

The foam is weaker than any of these adhesives. Foam strength is directly proportional to its density.

For additional detail, go to the design section of this website...

http://www.homebuiltairplanes.com/forums/design-structures-cutting-edge-technology/5293-conclusions-aluminum-adhesive-bonding-tests.html
What about the more flexible epoxy's like EC2216 scotchweld? We have used this a lot on F111's. It is quite flexible, bonds well to aluminium, has a relatively good pot life, and can be accelerated in curing with heat or done at room temp.
 
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