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Bush gear failures


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Posted (edited)

I have had several landing gear failures since going to the spring "bush gear". I have been researching the subject and have noted quite a few similar failures of that type gear, all in about the same location and of the same nature, Inward bending of the forward strut leg. I have been reluctant to re enforce that leg for fear of promoting a more serious failure elsewhere. The gear is a Chinese copy of the gear used by Piper from the 1930s onward but the Piper gear does not seem to suffer the same failure. Today I made a cardboard model of the gear geometry and I believe I have located the problem. The bottom anchor bolt for the spring leg on the "bush gear" is fastened to a metal plate welded between the front leg and the axle. When the spring strut is fully extended and bottoms out, or the spring becomes coil bound, that point then becomes a rigid fulcrum with the axle as a lever. The upward motion or the axle is then translated into an inward rotary motion on the front gear leg. The failure always seems to occur at the point halfway between the axle and the fuselage, which makes sense. I have never bent a rear strut, nor the spring strut, always the front one.

Looking at photographs and real life examples of Piper, Hatz, and Pietenpol gear shows that the anchor point on those gear is at the inboard end of the axle. My model shows a significant difference in the movement of the middle of the forward gear leg when the anchor point is moved to the end of the axle. As a matter of fact, the farther inboard the pivot is, the lesser the rotary movement at the front gear leg center.

I have yet to figure a force analysis on the gear, but I think i am onto something. I'll post more as I find more.

On a positive note, my welding is getting better.

Bob McCaa

Edited by whizzers
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Posted

That's exactly how mine failed.

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Posted

How about some photos or drawings?

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Posted

Isnt this exactly why Akflyer got different springs with more spacing between coils and lengthened the slots to give more extension while keeping the springs from being coil-bound?  See his posts.  EDMO

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Posted

Also why Joey went to the Monster Shocks, to give a progressive cushion rather than a linear rate to a hard stop. 

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Eventually when I get some time, I may play a bit more with my landing gear design that mimics the Aeronca oleo strut setup.  I think that design could have plenty of travel, and a built in shock absorber as well.   To busy now trying to get the Kitfox 4 fuselage painted before it gets to cold.  JImChuk

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Posted

I have had several landing gear failures since going to the spring "bush gear". I have been researching the subject and have noted quite a few similar failures of that type gear, all in about the same location and of the same nature, Inward bending of the forward strut leg. I have been reluctant to re enforce that leg for fear of promoting a more serious failure elsewhere. The gear is a Chinese copy of the gear used by Piper from the 1930s onward but the Piper gear does not seem to suffer the same failure. Today I made a cardboard model of the gear geometry and I believe I have located the problem. The bottom anchor bolt for the spring leg on the "bush gear" is fastened to a metal plate welded between the front leg and the axle. When the spring strut is fully extended and bottoms out, or the spring becomes coil bound, that point then becomes a rigid fulcrum with the axle as a lever. The upward motion or the axle is then translated into an inward rotary motion on the front gear leg. The failure always seems to occur at the point halfway between the axle and the fuselage, which makes sense. I have never bent a rear strut, nor the spring strut, always the front one.

Looking at photographs and real life examples of Piper, Hatz, and Pietenpol gear shows that the anchor point on those gear is at the inboard end of the axle. My model shows a significant difference in the movement of the middle of the forward gear leg when the anchor point is moved to the end of the axle. As a matter of fact, the farther inboard the pivot is, the lesser the rotary movement at the front gear leg center.

I have yet to figure a force analysis on the gear, but I think i am onto something. I'll post more as I find more.

On a positive note, my welding is getting better.

Bob McCaa

 

All of the "bush gear" I have seen has a straight pull from the axle point through the vee to the upper mount.  By lowering the intersection of the Vee like you see on the piper gear you will better distribute the forces.  This along with changing the springs to ones with more travel before they bind is the key to an easy fix.  The gear I now have on my plane had been bent more than once and was bent when I got it.  I straightened the tube and lengthened the slots as well as changing out the springs.  Since that time I have flown the plane a a weight much greater than the published numbers and have dropped it in pretty hard with no ill effects.

The gear legs that I have were custom built, but if one would supply me with measurements on the gear they have I can mock it up and make replacement vee and struts that would take care of the issues without having to completely swap the gear legs.  Would be a quick matter of lifting the plane, pull a few bolts, pop the new struts and vee in and lower it back down. 

:BC:

 

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Posted (edited)

I finally got some time to play with the gear issue. What I think I found is surprising, and pretty much confirmed my suspicions. I did document it, but the document is quite long, so I will post it here as a .doc file. I figure if you want to read it, go ahead, if not, ignore it. I also put it here because I figure if there are any flaws in my logic, you guys will find them. Landing Gear Geometry.doc

Landing Gear Geometry.doc

Edited by whizzers
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Posted

I like it!  Nice write up on it.  I think if you move it to the end of the axle AND lower the center point of the cabane vee more like a cub, the failures will be a thing of the past.  I have the cad files done and all the cut lengths and templates for a gear exactly as your describing, I have just not yet found the time to build it.  Maybe I need to get off my ass and get it built while I am in the production mode making the engine mount for Randy.  Being a single dad with a 4 yr old copilot that loves to help dad work in the shop kind of slows me down a bit on production though :lol:

:BC:

 

 

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Posted

Yea, I think ideally scaling the gear using the Angles of the cub gear would be the best overall solution, but would involve building new gear all around. I'm thinking about milling a nut out of square stock with a fork on it to thread on the inboard end of the axle shaft. That would be relatively easy and inexpensive, and just might do the trick.

Good to hear from an expert, thank you. Please stay in touch.

 

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Posted

you could probably do the same with a couple of 1/8" straps with washers welded to them and run a new bolt through the end of the axle.  One end of the strap goes to the axle the other to the cabane strut.

:BC:

 

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I dont have the photos anymore but I bought a Ridge RUnner 3 from a guy in Montana. He had the engine quit and did and off field landing. He picked a spot and headed for it and when he got close, at the last moment he saw a fence so he pulled the flaps and ballooned over the fence and drope the plane in from 15 feet or so. It hit so hard, the 3/8 shock cord stretched and snapped. The plane settled onto its belly. The ONLY damage was the vee cabane got ripped out, and the two shock struts for shredded. The main gear legs, fuselage wing struts, even the muffle which is on the bottom came thru ok. The prop was GSC wood and got shatterd.. So the bungees basically saved the plane from worse damage I think. Each shock strut has 3 wraps on one side and 3 wraps on the other side of the same shock strut. The shock cord is 3/8" aircraft quality and 48" long with loops at each end to ffit over the shock strut ears.

Just as a layman, it  looks like the load is applied to the axle in an upward direction. The shock strut extends until it bottoms out. Now you have a lever and a fulcrum. The pivot point is the lower shock strut attach point. IT applys a force trying to bend the forward gear leg inward until it kinks. IF you in crease the wall thickness, it probably willl fix the problem.

However if you apply enough load next time with thicker wall gear, the axle will likely bend.

In alaska, on operator of Helio couriers had some  much trouble with axles snapping. The main axle is solid 1" 4340  Chrome Moly steel heat treated to rockwell C50. It is so hard  it is brittle. So the company mechanics made their own axles out of the same material 4340 and sent them out to be heat treated to C-40. Now occasionally they bend and axle but can fly home.

A mechanical engineer could determine how much applied force is  needed to make that front tube on the bush gear bend inward and kink...

 

There is one other failure mode I have seen in Alaska with PA-18s with beefed up HD bush gear. On skis, the get to skiding side ways and hit someting solid, the front tube bows outward!

 

 

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