Bush gear failures


27 posts in this topic

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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Posted

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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Update on the gear. One possible fix for existing gear. I had two yokes made similar to the yoke on a universal joint. They were made out of 1 1/4 cold rolled square stock. I threaded them 3/4-16 inside, and turned that end round to give 3/8 inch wall thickness. The fork end I milled out 7/8 inch wide to provide +1/8  wall thickness, and drilled that 1/4 inch.

I then made new longer axles, but threaded the inboard end to give a tight fit on the thread. The fork was threaded on, then drilled and pinned. I had to make new lower spring strut rods since they are shorter than original. The only issue I had was that the axle end of the spring rod has to be canted a bit to fit to the existing cabane. That and the 3/4 inch tubing AirSpruce provided won't accept a 3/4 inch wheel bearing without polishing it first.

The top (spring) end of the inner rod should be doubled internally to avoid the hole tearing out. I saw that happen on a Super Cub clone.

I put the gear back on the plane, but have not tried it yet.

The other possible fix is to weld tabs to the existing axle tubes. I will try that next.

MVC-011S.JPG

MVC-006S.JPG

MVC-008S.JPG

MVC-009S.JPG

MVC-010S.JPG

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Posted

I am revisiting this as I am working on the cad drawings for the new gear. 

 

Bob, I love the idea you had for a retro fit.  Elegant and should last forever!  I like your safety cables too, that's a great idea the way you have them.

If ya'll had your choice would you want to stick with 3/4" axles or start updating things to the 1.25" Axles?

Also, Does anyone have a measurement on the forward rake of the gear?  I want to get a little weight off the tail wheel but not go animal on it.  Right now I have the gear drawn up with the axle 2" in front of the upper mount just going off memory.

Do you want to stick with springs and make them work or go with the ridge runner style with dual bunggies on each leg?

:BC:

 

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Posted

IMO, 1.25" axles are the way to go.  You can mount real wheels and brakes and get away from the toys currently being put on these planes.  There are plenty of wheel options available instead of the "chicken teeth" Douglass wheels and their associated shit brakes. 

Moving to 1.25" axles and their associated parts was the single best modification I made to my plane. 

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Posted (edited)

This is a great conversation! To add fuel to the fire, here is a sketch I found that supposedly matches the J3 gear, it is also the EAA Biplane cabane gear.

Please note that the dimensions are quite close to the gear I am installing, any differences are due to the Avid's much wider side by side fuselage.Eaa_Biplane.thumb.jpg.594510c14890476e0e

 

Edited by nlappos

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Posted

I you were to build it you should move. point C up a bit. Gives guys the option to run skis that attach there.

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Posted

I you were to build it you should move. point C up a bit. Gives guys the option to run skis that attach there.

Read the MS Word doc that Bob wrote up and I linked to here.  Moving point C is one of the critical errors.

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Posted

IMO, 1.25" axles are the way to go.  You can mount real wheels and brakes and get away from the toys currently being put on these planes.  There are plenty of wheel options available instead of the "chicken teeth" Douglass wheels and their associated shit brakes. 

Moving to 1.25" axles and their associated parts was the single best modification I made to my plane. 

Spring gear with big soft tires being the other.....:)

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Posted

I you were to build it you should move. point C up a bit. Gives guys the option to run skis that attach there.

Read the MS Word doc that Bob wrote up and I linked to here.  Moving point C is one of the critical errors.

That was interesting thanks for posting it.

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Posted

Here is a full analysis of the Lowell Fitt gear. It shows that the spring is not too stiff, it is sized just right for the design conditions (8 feet per second drop at gross weight) and it does not go solid until you exceed that drop and get about 4 G's on impact. Note too that he has addad a cross bar across the top of the Cabane strut so the two aircraft sides now have additional bracing from the side forces the gear puts on them as it shoves the longeron inward.

Fitt Gear.jpg

Gear 338S.jpg

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Posted

As I said before.. I would much rather trust the computer models than the real life pireps, pictures and heart aches that multiple pilots have endured.  I would also not trust the bent gear I had to straighten or the testing I have done myself.  I would not trust the other guys that have actually modeled it and pulled and prodded.  To each their own.  Please make sure that you grease every landing and please don't try any off airport work unless your insurance covers helicoptering out the bent up fuse from a remote strip.

:BC:

 

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For what it is worth, I offered to sacrifice my plane and to beat the shit out of this gear and my plane PRIOR to them labeling it "bush gear"  I was declined and as a result many sets of gear were sold with many of the exact same results.  Bent planes and pissed off pilots.  I could start cranking our gear tomorrow and sell them as "bush gear" but I don't fly off manicured grass strips and I refuse to sell something I don't have 100% confidence in to put on my plane and really beat the crap out of it.  Bush gear on a hangar queen tested on grass and improved strips is an oxymoron to the utmost degree.  I truly hope you are not one of the guys that is soon posting pics of a gear failure and a bent up fuse.

I know that I like to learn from others mistakes.  I damn sure don't have the time nor money to make them all myself.

 

Your calcs show the total load.  Do they take into account the bending moments and how those total forces are applied and to where?  Did you actually look at the pics that Bob posted in his write up?  Did you actually look at the bending of the tubes with MINIMAL force applied? 

:BC:

 

 

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Posted

Lowell told me directly, he quit making this gear due to too many bent airplanes.

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Posted (edited)

I have  around 400 hrs of mainly off airport landings ,at and above gross weight on a Highwing gear,many of my landings have been suboptimal:(.Any gear will break under the right conditions.Lowells gear isn't a true bush gear and could be refined in a few places ,but overall is up to the task intended.I believe at times we are asking well beyond the original design of our aircraft.Any improvements to make our versatile aircraft better can only be a good thing,but there is a finite limit !

Edited by Dusty
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Posted

Lowell told me directly, he quit making this gear due to too many bent airplanes.

I just bought the gear from Lowell a few days ago. We exchanged emails about it and he knows it works, and that it is strong, but how can he control folks who land off site?  As I said above the gear is quite strong enough for landings at 8 to 10 feet per second. As an indicator of how hard that is, the max landing at 8 feet per second is a 4 G landing while not damaging the gear. In a UH-1 helicopter, 8 feet per second (480 feet per minute) will spread the skids and make the belly touch the ground. 

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