Laminated wood landing gear

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wsimpso1

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Good plan. Anything to spread out the load at the mount.

Good luck with the drop testing. I hope that we can all learn a lot.

Billski
 

davidb

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:depressedFAILURE!

I did the drop test tonight and the gear failed. As many had pointed out, the peak stress would be concentrated at the bend and that is indeed where it failed. No signs of damage at the fuselage mounting points or the axle/mounting plates. But at the bend I got some delamination and some splitting diagonally through the wood. Interestingly, the splitting was in the center laminations. There must have been some strong torsional forces as the splitting did not match up front to rear. The gear did not collapse so I could have walked away from the crash.:)

I got video and pictures. I'll try to post those tommorow. For now I'll just cry myself to sleep.
 

davidb

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Here's the pictures of the damage. I still have to figure out how to edit and post the video.

Its hard to tell in the pictures, but not not all the splitting was along the glue lines. One area had splitting through several layers of wood with no glue-line failure.

Things I noticed in the video:

--The legs flexed quite a bit but the center section flexing was barely noticeable.

--One of those splits can be seen opening almost 1/2 inch at the peak flex.

--The greased plates/ramp moved forward--we had blocked it so it wouldn't move but missed resetting the blocks after an adjustment. The tires did move outward freely 'though (about 5 inches each way).

--The tires only compressed about 3-4 inches--lower pressure would have been better.

As Orion pointed out, those bends might have benefitted from a wrapping of fiberglass--oh well:depressed. Somewhere in the rush to testing, I discounted the glass and tire pressure issues. At this point part of me wants to inject the cracks with epoxy and wrap the bends with glass for another test but the rational part of me says just abandon this bad design and move on to saving up for aluminum gear.

BTW, I ended up using a seat belt buckle for the release mechanism--worked great!

One more big thanks to all on the forum that offered advice and ideas (Orion, Billski,...). I just wish I had followed more of that advice:emb:.
 

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addaon

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If you know the tire pressure, you can calculate the actual peak Gs from tire deflection, no? At least it will let you validate the drop test for next time.
 

davidb

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If you know the tire pressure, you can calculate the actual peak Gs from tire deflection, no? At least it will let you validate the drop test for next time.
I didn't get an accurate measurement of the actual tire deflection but in viewing the video and comparing it to expected results from the landing gear spread sheet program, all flexing and compression was in the ballpark of predicted results. The tire pressure was 20 psi but had I lowered the pressure I could have gotten a couple more inches of deflection from the tires.

But realistically, the lamination didn't fail by a little bit--it would have most likely failed even with softer tires.
 

Mac790

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davidb

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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.
 

wsimpso1

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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
 

davidb

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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...
 

davidb

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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.
 
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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 it’s 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, that’s 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
https://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 doesn’t create any more stress then the arch has itself
 
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davidb

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...Its not the sheering force that does this, as the load stretches the inner fibers the high curve causes it to split it’s 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, that’s 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.
 

Rom

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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?
 

davidb

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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.
 

NorthwestJack

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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
 

davidb

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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.;)
 

Dan Thomas

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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
 

davidb

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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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