Wing strut compressive load test.

Discussion in 'Aircraft Design / Aerodynamics / New Technology' started by Dave Hodges, Jul 12, 2019 at 4:11 PM.

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  1. Jul 12, 2019 at 4:11 PM #1

    Dave Hodges

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    This backhoe trailer was used to test several wing truss and tail boom mock-up components. It was also used as an anvil to hammer out wing trailing edges. It's a handy work bench. The wing strut mock-up is attached, at one end, to the top of the A-frame. The other end is attached to the wing truss mock-up. This strut mock-up was plenty strong enough to handle the 6.7-G tensile load when the wing truss mock-up was tested. Tomorrow we plan to see how much of a compressive load (for negative G's) it will withstand. The steel strut mock-up dimensions (our best estimation) are: 1-3/16-inch OD, 0.060-inch wall thickness and 85 inches long. It is pinned at both ends. Anybody want to tell me how much force it will withstand? IMG_20190712_103136480.jpg
     
  2. Jul 12, 2019 at 4:42 PM #2

    Hot Wings

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    Not me, but I'll take the #460 square on the pool sheet......single strut without a jury strut.
     
  3. Jul 12, 2019 at 5:59 PM #3

    wsimpso1

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    There is no 1-3/16 tube in the catalogs, and tubing comes in comes in 0.058 wall and 0.065 wall, so there might be some problems there. I also assumed 4130 annealed for strengths.

    I ran 1-3/16 OD, 0.058 wall, 85 long and with pivoting ends. It calcs out to 1349 pounds in compression from combined Euler and Johnson method as presented in Shigley.

    Then I consulted the Summerill tables for 0.058 wall tubes, interpolated between the 1-1/8 and 1-1/4 which plays out at about 2500 pounds and 55-60 inches, so it does not provide much help.

    Billski
     
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  4. Jul 12, 2019 at 6:43 PM #4

    Hot Wings

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    Some fool left aluminum E in his spread sheet cell.:oops:

    Billski is obviously much better at the 'details'... and only 7 pounds more than my sheet.
     
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  5. Jul 12, 2019 at 7:00 PM #5

    Dave Hodges

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  6. Jul 12, 2019 at 7:09 PM #6

    Dave Hodges

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    Thanks, Bilski. This is EMT, not 4130. I used the equation I found on assakkaf.com (Fcr=0.877Fe) and obtained 1,216 pounds (assuming 36,000 psi yeild stress and 30 million psi Young's modulus). We'll find out tomorrow how strong it is when we test it.
     
  7. Jul 12, 2019 at 9:20 PM #7

    wsimpso1

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    This is a very slender column where the basic Euler formulation is applicable, so difference in yield strength won't matter much. Now the seam weld in EMT, that might have some influence...

    Billski
     
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  8. Jul 13, 2019 at 2:18 AM #8

    BBerson

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    NACA Tech note 306 has charts for 1.25" .058" 4130 annealed, about 1200 pounds loose ends or 2200 pounds fixed ends.
     
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  9. Jul 13, 2019 at 3:03 AM #9

    Dave Hodges

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    Thanks.
     
  10. Jul 14, 2019 at 2:45 AM #10

    proppastie

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    I must be missing something....I see what looks like a TV antenna mast, an A frame one or two struts, a scale on a flat plate.....where is the load?
     
  11. Jul 14, 2019 at 3:05 AM #11

    Dave Hodges

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    The load is the difference in my weight pulling on the lever, which was 50 horizontal inches away from the pivot point of the chain binder. The logging chain parallel with the compressed strut was in a line 1-7/8 inches away from the pivot point. The strut buckled when 42 pounds of force was applied. So the mechanical advantage of (50/1.875) times 42 pounds equals 1,120 pounds of compressive load withstood by the strut. One pin was off center about 1/4 inch. So this test validates the Fcr=0.877Fe formula.
     
  12. Jul 14, 2019 at 3:09 AM #12

    Dave Hodges

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    Now we need to figure out how long a teardrop-shaped pipe, riveted to (outside of) this existing pipe, will increase the strength by a factor of 2.7.
     
  13. Jul 14, 2019 at 3:49 AM #13

    Hot Wings

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    Can you add a jury strut? if you can cut the unsupported length to around 50 inches you might reach your ~3k# load.
     
  14. Jul 14, 2019 at 4:54 AM #14

    Dave Hodges

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    We were thinking about 35 inches of bigger pipe in the middle, with 25 inches of existing pipe on each end. But the jury strut is certainly something to consider.
     
  15. Jul 14, 2019 at 5:14 AM #15

    BBerson

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    Thinner wall 1.5" or 2" diameter tube is lighter in compression. Cessna struts are 2", I think.
     
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  16. Jul 14, 2019 at 11:35 AM #16

    wsimpso1

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    Terrific, the math works.

    I am concerned that this light a tube will sag under airloads and its own weight enough to give you curved column effect. There is a reason the Summerill tables play out where they do. With a strut this slender, the jury strut recommendations are particularly appropriate. Think on that more...

    Putting the rivet holes in can further destabilize the tube. Test is necessary...

    Might be lighter to just go with an appropriate streamline steel tube than adding a streamline shape. They have a bunch of sizes here.
    https://aedmotorsport.com/store/materials/4130-chrome-moly-streamline-tube-1

    Billski
     
    Last edited: Jul 14, 2019 at 11:41 AM
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  17. Jul 14, 2019 at 2:41 PM #17

    pictsidhe

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    I'll third the suggestion to use a larger diameter.
     
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  18. Jul 14, 2019 at 3:42 PM #18

    BBerson

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    The minor axis of those rolled streamlined tubes in post 16 is still too small for that length. The streamline fairing could be a simple .016" 2024 sheet cover over a big tube. The DHC-2 Beaver strut is built up with a deep channel and thick skin wrap with blind rivets.
     
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  19. Jul 14, 2019 at 5:46 PM #19

    Hot Wings

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    Interesting find. Oval tubing might be nice for some things. Bellanca tail feathers, for example are built using oval tubing. The large radius of these streamlined profiles probably reduces the chance of cracking - which was (maybe still is?) a problem with the standard profiles.
     
  20. Jul 15, 2019 at 4:22 PM #20

    proppastie

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    Last edited: Jul 15, 2019 at 4:30 PM
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