Attach wood to aluminum?

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pwood66889

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Feb 10, 2007
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Sopchoppy, Florida, USA
Found some aluminum at the local scrap dealer - good price! At least what I could drop from what was in the wallet.
So my question is how to extend longerons (of an airframe to be covered with cloth) by putting wood on the end? My thinking:
* Aluminum front for crash resistance
* Wood on the back for lightness/cost/whatever
* All materials "Available In Town" to save freight
* And cheaply, so I can "build one to throw away."
The aluminum extrusion has an "C" channel like area that the wood would go into. It would be epoxied in. Questions:
?=Should bolts be used, or would they tear out faster than the strength they add?
?=How much overlap (wood inside the aluminum)?
?=Where to join them? Closer to front? Back?
Thanking you all in advance!
Percy in NM, USA
 

BBerson

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Port Townsend WA
Drilling holes makes the wood weaker obviously.
Consider numerous small nails.
House trusses are joined with steel plates. The plates have "nail like" barbs that press into the wood. This is an efficient way to join metal and wood. I don't trust bonding of aluminum.

Be sure to test any new idea.
BB
 

PTAirco

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Sep 20, 2003
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Corona CA
Do you know for certain what kind of metal you have there? 6063? 6061? Pot metal? Unless you do, I would hesitate to use it any kind of airplane. As far as bonding wood to metal goes; I would forget about that, too - too many variables for the amateur workshop. It can be done, sure, but not worth the uncertainty.

I would not use nails in a structural application in an airplane, contrary to the above post. Airplanes are not roof trusses. Nails damage wood, unlike a properly designed bolt hole.

For your application, bolting the wooden longerons into the C-channel is a perfectly acceptable method of joining the two, with the bolts going through both sides of the channel. It was not unusual for metal and wood parts to be joined by bolting in some old designs - the DeHavilland Leopard Moth for example, had a steel tube front and a wood/plywood rear fuselage.

Drilling holes in longerons does not, strictly speaking weaken them - you have to consider how they are loaded.
The longerons are mostly loaded in tension and compression. Consider a 1" sq. longeron; By itself it could withstand perhaps 10,000lbs in tension and 5000 in compression. So it would be sized for the compression load, since most wood is about twice as strong in tension as in compression. You could theoretically remove up to half the area of the longerons with bolt holes before it's tensile strength drops below the 5000 lbs it can take in compression. In pure compression, the bolt holes make no difference at all.

You will need to assume all the load is taken by the bolts, so the bearing area is important. For shear-out, you need to space the bolts adequately - about 4 diameters minimum., and staggering them very slightly helps too, to avoid putting all the load along one grain line. Looking at old designs you would typically see about 4 or 5 bolts, spaced about an inch apart, with metal fish-plates either side of the longeron. Usually it was done to make production and repair easier.
 
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BBerson

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Thru rivets is another option. This is described in 43.13 (para 4-58) for splicing tubes. It details how to form the shop head without buckling the long rivet by peening the edge of the head.
Soft steel nails can work as rivets if the load is almost pure shear... steel nails are stronger than aircraft aluminum rivets.... a bit heavier. Just peen the edge with a small ball peen hammer rather than striking the rivet shop head in the usual manner.

Always test any new methods first.
 
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