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A Further Question About Aluminum Tubes, if I may.

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Tom Kay

Well-Known Member
Joined
Aug 10, 2007
Messages
409
Location
Canada
Hello;

I have been trying to learn a bit about strength of materials related to aluminum tubes, in hopes of building an airframe one day.

The Murphy Renegade looks like a good aircraft, and is my goal at the moment. I find that the materials are brutally expensive from that company, and I have begun the search for alternatives, yet I still want to stick with 6061-T6 tubing.

In many cases, this will mean adding weight, because I can't find an exact substitute for their proprietary materials. Here's an example: for their wing spars they use an extrusion of 1.5w by 3.5h by .078" wall. They own the dies. This is a near-impossible size to find on the open market, but I can get close with a size of 1.5 by 3.0 by .125" and this seems to be as strong when I do the math. I'll also demo how I did the math, since I am severely stress analysis challenged.

So I'll show some pics, and provide a link to the website which had the stress formulas. It was easy to work with, and when I did the math, I used a force of 200 pounds at 100 inches, acting at the tip of a cantilever beam. Deflection was 5+ inches, almost identical.

Here are a couple questions:

1. Are square corners in aluminum tubes a concern with airframe design? Murphy's extrusions are well rounded at the corners.

2. There's a weight penalty of about 13.5 pounds for the entire set of 4 spars for my biplane. I know that adding weight here and there is bad, but the cost of the off-the-shelf tubing is $5.00 per foot at a local store, (correct alloy and temper). The Murphy proprietary tubing is $20.50 per foot and I'd need 40 feet shipped 4000 miles to me. Is it worth it to go for the correct stuff from the aircraft company? I don't feel that it is, unless I'm missing some safety concern.

3. Does anyone see any flaws in this plan that I am missing? Is this strength calculation a good way to verify that a substitute piece of tubing is strong enough, as long as the weight penalty is acceptable? If the answer is yes, I'd use this approach to choose other substitutes for the fuselage tubes.

It might help to remember that this is a biplane, originally an ultralight, with lots of flying wires to brace the spars at the I-strut, about 2/3rds of the way toward the wing tips. So each spar has support at two locations, not just at the root.

Here's the website for beam calc's. I found it very handy:

Deflection formula for comparing different tubing sizes

Thanks all, Tom.
 

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