Sleeved tube vs single tube

Discussion in 'Aircraft Design / Aerodynamics / New Technology' started by kirbylee, May 24, 2019.

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  1. May 24, 2019 #1

    kirbylee

    kirbylee

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    For example, the plans call for the landing gear legs to be made of 1"x.058 aluminum tubing, with a 7/8" x.058 tube of equal length inside.
    Why not simply use just one single tube with equal wall thickness ? Maybe a 1" x .125.
    Without guessing, does anyone know the reason for two tubes instead of one?
     
  2. May 24, 2019 #2

    TFF

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    A tube has an inside and outside wall so there is actually two “skins” that the mechanical properties can work through. A solid bar only has one. A tube inside a tube has four. One tube only has two. There is going to be a gain in strength, whatever it is, using one inside the other. Maybe some flexibility advantages too.
     
  3. May 24, 2019 #3

    pictsidhe

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    Usually finding suitable tube. With sleeved tubes, you can alter the weight and strength of tube along its length easily by only sleeving the part that needs more strength than a single 058
     
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  4. May 24, 2019 #4

    Pops

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    Diameter means a lot more than wall thickness in column strength. Adding an inside tube does little to increase the column strength, mostly just adding weight. Weight is the enemy. Work out the column strength on the Euler scale for the dia and wall thickness needed for the LG leg. Then the bearing strength of the aluminum tube and sleeve wall thickness needed and then the rivets needed to attach the tube and sleeve.
    IF your landing gear is the Cub type, make sure the column strength of the shock strut is not under strength for side loads as in a ground loop with a tail dragger. Nothing worse than a collapsed shock strut with the wheel folding under the fuselage with a wing, stab and prop strike.
     
  5. May 24, 2019 #5

    wsimpso1

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    You got the guy that thinks guessing is to be avoided...

    I ran the numbers, a 1 x 0.125 is about 6% stiffer in bending than a 1 x 0.058 plus a 7/8 x 0.058. So the thicker tube is a little stiffer and a little lower stress.

    Is the landing gear leg a Wittman style leg (one piece spring leg) or is it part of a suspension system like a Cub? How it is loaded matters to the designer. If it is a Wittman leg and their drop test came in showing enough strength but they needed a few percent more travel, they could get it by going with the sleeved set up. Or maybe the 0.125 was not available when they built the prototype.

    I am also the guy who says stick to the plans, so, stick to the plans. Law of unintended consequences usually comes in big time once you start changing things.

    Billski
     
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  6. May 24, 2019 #6

    Pops

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    The 6% is about right. Like you, I say stick to the plans. But the designer could have saved a little weight by going to a larger diameter tube with less wall thickness for the strength needed , but so many other things to consider for a very small weight saving so maybe he thought that this was a better way.
     
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