Mahogany vs birch plywood skin

Discussion in 'Wood Construction' started by 4trade, Dec 15, 2011.

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  1. Dec 15, 2011 #1

    4trade

    4trade

    4trade

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    I am building Cassutt and because long storage time i don´t want to use original 1/8 mahogany ply for skin. That skin ply looks like it have too much humid at some time for storage. I live in Finland, so i can buy aircraft quality birch ply relative easy...just this is much heavier than mahogany.

    How can i know, how much i can reduce thickness for birch skin. I can buy 2 mm or 2,5 mm here, but i prefer use thinner one (lighter plane) if i could. Is there any conversion table for this kind of problem?

    Cassutt have 275-300 mph Vne... it is flutter tested for 400 mph (TAS)
     
  2. Dec 15, 2011 #2

    harrisonaero

    harrisonaero

    harrisonaero

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    Years ago I did some pull and bending tests of baltic birch vs okoume and found that the okoume was slightly stronger and about 70%-80% the weight. It's good stuff and what the sailboat racers use. The only thing you have to watch is moisture and rot... seal it well.

    And of course do your own tests using actual proposed material. Find a college or machine shop with an axial load tester.
     
  3. Dec 15, 2011 #3

    4trade

    4trade

    4trade

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    I just double check plans. Plans request 3/32 (2,38 mm) aircraft quality plywood, but that´s all. Plans don´t say what ply....Cassutt kit that i order have 1/8 ply (3,17 mm) for wings....

    Marcel Jurca Mustang MJ-77 plans say that i can reduce thickness 1/3 if use birch ply at his plane.
     
  4. Dec 15, 2011 #4

    harrisonaero

    harrisonaero

    harrisonaero

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    A test is worth a thousand expert opinions. In a short time you can build up a small, scaled, 2-panel wing section section and do a destructive pull with a come-along winch and load cell (fish scale) to see how the skin reacts. This also tests your manufacturing methods (cohesion vs adhesion, etc.). I guarantee you'll learn more than just the answer to your original question. Never underestimate the time-value of simple tests.
     
  5. Dec 15, 2011 #5

    4trade

    4trade

    4trade

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    I build small "mock up" wing for fiberglass work, but i haven´t glue skin for that yet....maybe i glue that mahogany other side and birch another side of it. How i shold do that stress test? Here is picture of it:
    [​IMG]
     

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