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Thread: Laminated wood landing gear

  1. #46
    Registered User Mac790's Avatar
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    Re: Laminated wood landing gear

    Quote Originally Posted by davidb View Post
    I still have to figure out how to edit and post the video.
    Do you have youtube account? YouTube - Broadcast Yourself.
    If you dont have, just create one it's very simple, free, etc. Check out Jake's thread about posting videos http://www.homebuiltairplanes.com/fo...eos-posts.html

    Seb

    btw It's sad to hear about your landing gears.
    Amor Patriae Nostra Lex

    "Time, training, training, training and more training is the key to any success."
    Francis "Gabby" Gabreski

  2. #47
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    Re: Laminated wood landing gear

    Quote Originally Posted by Mac790 View Post
    Do you have youtube account? ...
    Actually, I just opened one about an hour ago and have been trying to upload the un-edited video. Problem is it is a huge file and I don't have editing software. My buddy also filmed it and he has editing software but it might be a day or two before we can get together and post it on YouTube.

  3. #48
    Registered User wsimpso1's Avatar
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    Re: Laminated wood landing gear

    Davidb,

    So, what drop heights did you use? How many drops at each height did you run. What was the height that caused the cracks? How much did the center section move? C'mon man, how close did you get before you broke the gear? For you and everyone else to learn from this, you need to look at it from a perspective of how strong was it?

    Billski

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    Re: Laminated wood landing gear

    Quote Originally Posted by wsimpso1 View Post
    Davidb,

    So, what drop heights did you use? How many drops at each height did you run. What was the height that caused the cracks? How much did the center section move? C'mon man, how close did you get before you broke the gear? For you and everyone else to learn from this, you need to look at it from a perspective of how strong was it?

    Billski
    Billski, I'm afraid I might have disappointed you. With all the logistics involved with setting up the test and the time constraints we were working with, I decided to go with one drop that would either validate that the gear was strong enough or not. We dropped it from 9.75 inches. The test did verify that the weaknesss of the gear was the concentrated stress at the bends and failure mostly manifested in the glue lines in that area.

    So I learned that the shape of a laminated gear is critical. Also, getting a good glue bond is no small feat. More to follow...

  5. #50
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    Re: Laminated wood landing gear

    Here's the video.



  6. #51
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    Re: Laminated wood landing gear

    I will try to post the individual frames of the video that shows the flexing of the gear in stop action. That YouTube video doesn't make it easy to see how the gear reacted. I'm still puzzled by the fact that the center laminations failed and not the outer laminations. Wouldn't the outer laminations be subjected to the highest tension and compression and therefore be the most likely to fail? Is it because the shearing force would be greatest in the center, i.e. where tension meets compression?

    If I decide to make a new gear design, I'll still be constrained by the already built fuselage meaning the center section would still have to be flat. I know that the optimal shape would be an arc. I'm wondering if curved legs meeting a straight center section would still cause a concentrated stress.

  7. #52
    Banned George Sychrovsky's Avatar
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    Re: Laminated wood landing gear

    Putting the sharp bend in it was a big mistake. Its not the sheering force that does this, as the load stretches the inner fibers the high curve causes it to split itís the pull apart force that is caused by the inner fibers to straighten up, the very high thickness off wood required makes it worse but a fiberglass gear with a sharp bend will fail exactly the same way also, thatís why they are always shallow arch, only metallic materials can have sharp bends, you had the first gear the right shape.
    In the post of Ibis wooden gear construction you posted yourself here
    http://www.homebuiltairplanes.com/forums/design-structures-cutting-edge-technology/4712-landing-gear-drop-test-2.html#post35418
    It specifically warns about maintaining the shallow radius in order to avoid delamination, the choice of birch wood instead of ash might have something to do with it also.
    As to your question , the meeting of the arch and straight part doesnít create any more stress then the arch has itself
    Last edited by George Sychrovsky; February 19th, 2009 at 02:05 AM. Reason: is always the same

  8. #53
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    Re: Laminated wood landing gear

    Quote Originally Posted by George Sychrovsky View Post
    ...Its not the sheering force that does this, as the load stretches the inner fibers the high curve causes it to split itís the pull apart force that is caused by the inner fibers to straighten up, the very high thickness off wood required makes it worse but a fiberglass gear with a sharp bend will fail exactly the same way also, thatís why they are always shallow arch, only metallic materials can have sharp bends, you had the first gear the right shape.
    Thanks for that explaination George--that makes perfect sense. That pull apart force is visually evident in the slo-mo--there was a 1/2 inch gap between laminations at the peak bending. I really wish I had wrapped it in fiberglass but even that probably wouldn't have compensated for the poor shape.

    At this point I don't know if I'll continue to toy with wood or just cut my losses and go with aluminum. If I toy with wood, I might try hickory or ipe as that would allow for thinner gear.

  9. #54
    Rom
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    Re: Laminated wood landing gear

    The failure is due to the cross grain tensile strength of the wood which is about 500 psi. Exopy has a tensile strength of ~9000 psi, so the failure at the joint is probably in the wood. Composite gear with the same geometry will most likely be of sufficient strength in your test. The bond strength of epoxy to wood is also something to factor in.
    Does the adhesive where the crack propagated have wood attached?

  10. #55
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    Re: Laminated wood landing gear

    Quote Originally Posted by Rom View Post
    The failure is due to the cross grain tensile strength of the wood which is about 500 psi. Exopy has a tensile strength of ~9000 psi, so the failure at the joint is probably in the wood. Composite gear with the same geometry will most likely be of sufficient strength in your test. The bond strength of epoxy to wood is also something to factor in.
    Does the adhesive where the crack propagated have wood attached?
    I haven't tore apart the gear yet to see if the failure was due to glue line or wood fiber. I suspect uneven clamping pressure and glue starvation played a factor. I tried my best in that regard but that was a pretty tough glue-up to get perfect. Some of the cracks purely follow the wood fiber pattern so even with a perfect glue-up I think it would fail.

  11. #56
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    Re: Laminated wood landing gear

    David:
    Im using T-88 epoxy in my assembly.
    It is important in gluing with epoxy not to starve the joint.
    Im sure your gear is twice as strong as it needs to be, however you suffered glue starvation. Epoxy likes minimal clamping pressure. It is possible with epoxy to apply an adequate amount of glue, yet squeeze too much out due to clamping. This is the only explanation that explains the failure that you had.
    If you would have used a standart glue like, gorilla, or even urea-formaldehyde glues, they love clamping pressure .
    I would try injecting epoxy into the joint and test it again. the wood is definitely more than strong enough.
    I would do another test were you statically load it to :

    Weight of test= (3+ .133s) Gross weight ( s= wing loading)
    this way you can watch the landing gear as you load it. If it holds then your are in business.
    My .02 worth
    jacq

  12. #57
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    Re: Laminated wood landing gear

    I'm just now getting around to re-working the gear. I've just injected the cracks with epoxy. Next, I will wrap the bends with fiberglass tape. Then I will put some steel plates/bolts (compression clamps) in the bend area to counter the "pull apart" force in that area.

    Oh, then I'll spend $1200 on Grove aluminum gear.

  13. #58
    Registered User K-Rigg's Avatar
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    Re: Laminated wood landing gear

    any updates?

  14. #59
    Registered User Dan Thomas's Avatar
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    Re: Laminated wood landing gear

    Quote Originally Posted by Rom View Post
    The bond strength of epoxy to wood is also something to factor in.
    Does the adhesive where the crack propagated have wood attached?
    Epoxy has low peel strength, good shear strength. Sharp bends are what this glue would not like. Take a look at the curved composite gear on the VariEze, for example. No sharp, short bends.

    My uncle tried to make a wooden tailspring years ago. It broke easily, too.

    Dan

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    Re: Laminated wood landing gear

    Quote Originally Posted by K-Rigg View Post
    any updates?
    Update: I did re-glue the cracks and wrapped fiberglass tape around the separation area. Also, I did the compression plates where the separation originated. The "frankenstein" gear has been sitting in my buddy's hanger for a couple of months now waiting for our schedules to match-up so we can try another drop test--maybe this weekend. Frankly, even if this gear survives several drop tests I'm not sure I want it on my project.

    I still like the idea of wood gear and I still might make another set in the proper shape without the tight bends. I'm also thinking about FlyBaby style gear again but don't really want rigid gear. I just don't know how much more effort I'm willing to put into the wood gear dream. I've learned a lot and could probably make it work but I may have spent all the energy I care to--Grove gear is easy.

    I'll post pictures and results when I drop test it again.

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