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sheet alu. skin over solid foam core?

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Bart

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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.
 

lr27

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The European free flight model guys used to do this.
 

bmcj

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When you say solid foam core, I'm assuming you still intend to put wood or metal spars in. As with eny new design, you still need to fully engineer and analyze the structure for the materials used. Aside from that, one of the possible issues that come to mind is repairability should the structure become dented. I do agree, though, that a solid core (with metal spars and skins) provides more shape support than one with just foam ribs.

Bruce :)
 

BBerson

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Since aluminum has a much higher modulus of elasticity, aluminum would be stiffer.
The problem is getting a durable bond without a huge investment in equipment and chemical cleaning.

I have considered using perforated aluminum to get a good bond through the holes. A mechanical bond rather than a surface bond. Also perforated sheet is lighter. But the perforations would require a filler , of course, before painting.
BB
 

bmcj

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As an added note, I would recommend some rivets at the trailing edge as a back-up to prevent delamination from the core (assuming that the sheet is flat wrapped around the leading edge and joined at the trailing edge).

Bruce :)
 

GESchwarz

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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...

https://www.homebuiltairplanes.com/forums/design-structures-cutting-edge-technology/5293-conclusions-aluminum-adhesive-bonding-tests.html
 

GESchwarz

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That's very interesting Shannon. I was thinking that I could do the same thing. I'll look into the T-51. I suppose most of the shear loads are taken by the ribs that are located between the front and rear spar.

Polystyrene is a bit surprising. Are you sure it's not PVC?
 

MadRocketScientist

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I was talking to the owner of a company that assembles them here in NZ, I actually asked him again when he mentioned it at first as I was also surprised at the use of polystyrene, I am guessing that the load is low and well spread out in the area that it was in. It also seemed to be a thick wing. The rest of the wing behind the spar is conventional aluminium riveted construction.

Shannon .
 

dviglierchio

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After all this engineering effort what is the advantage over typical composite construction??
 
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MadRocketScientist

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Possibly only about USD$60 worth of polystyrene per wing, that stuff is really cost effective. It doesn't weigh much. The main thing is to be sure of the loading that it is subjected to. It has a nasty failure mode if not designed within it limits. A lot of radio controlled planes use polystyrene with balsa sheeting for the wings.

Shannon.
 

GESchwarz

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By contrast, the PVC foam is very pricy. So my plan is to use aluminum for my nose rib webs, and the caps will be made of PVC foam, as described in my other posts in the design forum. With this configuration, the aluminum is there to do what it does best, which is to take the shear loads. And the PVC is there to join the rib web to the skin, doing it without any stress risers.
 

Starman

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That sounds like a good idea. I also had the idea of not using brittle glues but rather something very flexible, like silicone sealer, like it, but much more liquid?
 
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