Laminated wood landing gear

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davidb

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Well, Jack's gear design certainly will have much more resistance from deflecting aft under a strain(assuming the mounting bolts and fuselage mounting points are up to the task), but David's gear width at the fuse is a fairly common size for one piece aluminum gears sets. If the aluminum gear has proven to be successful in the past on the VP2, I would suspect that the same mounting system would work out OK. Depending on how it's mounted, crush plates might be a good idea for the mounting area so the wood does not crush under load.

George
Yes, I have incorporated crush plates in the design and the yellow birch is quite hard but obviously not as hard as aluminum. Hopefully I'll be able to work thru that bearing strength issue (with Billski's help) as it pertains to the flimsy axle mounting. At this point I'm less than optimistic. BTW, I do have most of the logistics worked out for the drop test rig--could be ready to start testing in two weeks.
 

Hugh Lorimer

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I made up a block and cast layers of glass, filled centre with 1/8" x 1" straps which were pre bent and fibre glassed the top side, lifted moulding and trimmed the edges then wrapped the whole thing. Made up axle stress plates, filled with matrix ( resin and chopped strand ) Drilled axle holes and mounted with thru bolt. Both passed the drop test.
 

Attachments

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I hope you will have a video camera rolling when you drop test it.
With all this gear talk I’m inspired to video tape my gear drop test but it has to get warmer first, maybe end of April.
 

addaon

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If you have any techie friends locally, ask around if anyone has a Casio EX-F1 digital camera. This is a relatively cheap digital "still" camera that is capable of doing 300 frame per second video at decent (television quality) resolution. Having the ability to see what's going on at 10x to 20x slow down would be amazingly cool; probably not cool enough to justify getting the camera, but cool enough to see if you can borrow one.

(Check out a video from the same camera at 600 fps; 300 fps gives much better image quality. You'd definitely be able to measure amount of flex, or find out where breakage started.)
 

davidb

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(Check out a video from the same camera at 600 fps; 300 fps gives much better image quality. You'd definitely be able to measure amount of flex, or find out where breakage started.)
Cool video demo! For a case of beer, I'm sure I can get a few guys to have cameras rolling--I won't drop it 'til I have at least one high speed video camera on scene. BTW, I'm still soliciting ideas to avoid that breakage thing.:)
 

Hugh Lorimer

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Hugh,

What did the gear only weigh? Also, what gross is it going to support?

GusMan
Hi GusMan, I don`t remember actually weighing the U/C seperately but the dry empty weight of the Quaiche tipped the scales at 141Kg. so with a pilot at 90Kg and 25Ltrs. fuel (18Kg), auw comes to 249Kg. The vertical drop test was not very high, around 30cm? as laid down by BCAR Section S. The laminated straps in the core are straight grained Phillopina mahogany and I used glass and epoxy as opposed to carbon since it is tougher.
Hughie.
 

davidb

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... I’m inspired to video tape my gear drop test but it has to get warmer first, maybe end of April.

George, did you have some expensive high tech release mechanism for the drop test or did you fabricate some kind of inexpensive simple release mechanism? I was thinking eye bolts with a pullable clevis pin but I suspect that would be hard to pull when loaded to 700 lbs.
 

Mad Man Mike

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Go to a sailboat shop and get a "snap shackle" -they come in various sizes and will open under load VERY easily---but they aint cheap -expect to spend 30-40 bucks!
 

davidb

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Go to a sailboat shop and get a "snap shackle" -they come in various sizes and will open under load VERY easily---but they aint cheap -expect to spend 30-40 bucks!
That was a big help!:) Having "snap shackle" to Google I was able to find one on-line for 10 bucks--its in the mail now. Just what I need to do the job.
 

davidb

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I didn’t do any drop test yet, I did a 4G load test. I think cutting the rope will do when it comes to it .
Interesting. You have me wondering. I predict that the peak G force of a successful drop test will be 3.2 G's so if I statically load the gear to say 3-4 G's would that replace the need for a drop test? I'm thinking a drop test would probably be the prudent course to take in the case of my gear as that 3.2 G's is a guess-timate at best given all the unverifiable factors.
 

PTAirco

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I noticed you drilled though the landing gear - it's a little after the fact, but might it not have been better to use a clamping plate, like the typical metal spring gear does? Are the holes a tight fit or or are they slightly oversize to allow flexing of the middle portion of the gear?
 

davidb

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I noticed you drilled though the landing gear - it's a little after the fact, but might it not have been better to use a clamping plate, like the typical metal spring gear does? Are the holes a tight fit or or are they slightly oversize to allow flexing of the middle portion of the gear?
Well, that might be a good question. The VP gear per the plans is aluminum and it is mounted with holes drilled through it like I did but obviously the aluminum is thinner than my wood gear. The gear per plans is rigid in that it is cable braced but another option is thicker aluminum without cable bracing--presumably it would flex and the manufacturer of that gear suggests using radiused plates to relieve the stresses that flexing would impose on the fuselage. I did radius those green plates you see in the pics. The actual mounting area in the fuselage is only 1 1/2 inches thick vertically. I didn't see the slight outward force on those bolts as a problem but maybe it is. In the drop rig I used longer bolts but I think the 2 x 4s will yield to that force. Also, I believe this gear will not flex as much as predicted so the mounting method might be the least of the problems. If I enlarge the holes it might make the gear less able to handle the drag forces. At this point I'm not sure what to do. I will do the drop test gradually to see if I can identify where the problems develop. More thoughts welcomed.
 

wsimpso1

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Geez, I hate to see that... Some primary info here:

Stresses in one direction always influence how much stress a material can take in another direction. It is usually a strongly negative influence too. And in wood, cross fiber strength is pretty darn low.

Holes in an area of high stress practically guarantees it will try to fail at the hole, even if no additional load is added around the hole;

The bolts must have preload to keep the structure together, and that adds cross grain compression to the material around the hole, which reduces the bending and shear load at which failure will commence;

Hard surfaces and sharp corners at the bearing points will produce high cross fiber compressive stresses and trip early failures in the leg at the saddle;

Drag loads and/or landing loads that are not perfectly aligned will require the joint to carry torsion and put really big cross grain compression on the wood at the bolts and at the saddles in addition to the bending stresses;

With spring beam (Wittman type) gear legs, (even in hardened steel) you have to do everything you can to spread out the mounting loads and keep bending as the primary stressor. That usually means a couple of gently cambered saddles for the leg to be clamped in, with a rubber pad (1/8" rubber gasket stock or similar) between the saddles and the leg. Then, significant clamp bolts are used to get the saddles tight. Another way is to build a saddle around the leg out of fiberglass and pick up the saddle with a big bolt serving as an axle. This lets the leg flex and imposes only small cross compression on the part.

Now, we still do not know which issue will trip a failure first, but I would hate to see the clamp method cause it to fold up really early... With the bolt hole not tight on the bolt, a nice wooden saddlefor the leg to sit on and another nice big pad to spread out the bolt and torsion loads, you could still learn a lot...

Billski
 

davidb

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Billski, I think I understand what you are saying, but...

One area I haven't talked much about is the mounting structure in the actual VP-2 fuselage. I built that to plans to include the recommended strength modifications. I can post pics and diagrams of that structure if anyone is interested but suffice it to say that if you viewed the fuselage structure it would be obvious that any failure mode will most certainly occur there. Yes, that structure was designed for rigid aluminum gear but my gear is proving to be much more rigid than predicted.

Last night with my "not so free" fall I was getting a lot of tire compression and very little flexing of the gear. I really don't think I'll get a failure mode in the gear around those bolt holes before something else fails.

BTW, official drop testing is planned for Monday night if I can figure out a way to get a clean release.
 

davidb

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Geez, I hate to see that... Some primary info here:

...Hard surfaces and sharp corners at the bearing points will produce high cross fiber compressive stresses and trip early failures in the leg at the saddle;

Billski
I epoxied steel bearing plates to the wood around the mounting area/bolts to help minimize wood fiber damage from the pressure against the aluminum radius blocks. I concidered putting some thin rubber between those metals but didn't think it would do much as it would always be in pretty high compression anyway.
 
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