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  #11  
Old 06-10-2021, 06:28 PM
jrs14855 jrs14855 is offline
 
Join Date: Apr 2008
Location: Lake Havasu City AZ
Posts: 2,518
Default Mount

Paul Dye wrote a recent article on gear mount failure on RV6 in Kitplane's. In that case I have issues with the positioning of the bushings at the lower firewall as well as the positioning of the lower truss crossmember and the vertical tubes from the lower gear socket to the upper firewall mount. i feel that these all have less than optimal positioning to properly transfer landing loads.
The round spring gear was designed and patented in the early 50's by Steve Wittman. The RV gear mount is a derivative of his mount design of that era. I am now leaning toward the idea that tire manufacturing process that creates out of round tires is a likely cause of shimmy. Also the shimmy that I am currently fighting is much more violent that what I experienced starting in the mid 60's. I am doing a visual inspection of the gear mount at intervals between 5 and 10 hours.
Some time ago I flew a Baron that had a violent nose wheel shimmy. Installed the most expensive Goodyear tire and the shimmy went away. No tire balance, just changed the tire.
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  #12  
Old 06-10-2021, 07:26 PM
rv6cardinal rv6cardinal is offline
 
Join Date: Jun 2016
Location: calico rock ar
Posts: 16
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For what it’s worth, I have had my share of bouncy landings on grass and pavement just like most honest pilots. However I do not remember ever having a “violent” landing that made me wonder if I had damaged anything. I never installed the wooden shimmy dampeners and have never had a real shimmy problem.

I had planned a 3rd trip to Johnson Creek for the weekend that the mount broke. My niece, who just got her private license and just bought a -9a, was going to go with me. He dad is the campground host. I am very thankful that we postponed the trip due to some enroute weather and this did not happen there or anywhere in between, assuming it was already cracked. Also thankful that the gear did not collapse on landing. Fortunately, the air was calm and the last landing was as close to a greaser as I get.
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  #13  
Old 06-10-2021, 09:27 PM
jrs14855 jrs14855 is offline
 
Join Date: Apr 2008
Location: Lake Havasu City AZ
Posts: 2,518
Default Mount

There appears to be a pinhole in the weld on the socket. Note that much of the failure was along the outer edge of the weld. It is very likely that a crack started next to the pinhole and then expanded from there. the lower portion of the socket/ diagonal intersection was last to fail, tearing out a piece of the socket.
I will stand by my previous comment that there is likely a portion of the weld that is air hardened. That is likely adjacent to the pinhole.
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  #14  
Old 06-11-2021, 05:44 AM
BillL BillL is offline
 
Join Date: Sep 2007
Location: Central IL
Posts: 6,045
Default Flat spotting?

Quote:
Originally Posted by jrs14855 View Post
Paul Dye wrote a recent article on gear mount failure on RV6 in Kitplane's. In that case I have issues with the positioning of the bushings at the lower firewall as well as the positioning of the lower truss crossmember and the vertical tubes from the lower gear socket to the upper firewall mount. i feel that these all have less than optimal positioning to properly transfer landing loads.
The round spring gear was designed and patented in the early 50's by Steve Wittman. The RV gear mount is a derivative of his mount design of that era. I am now leaning toward the idea that tire manufacturing process that creates out of round tires is a likely cause of shimmy. Also the shimmy that I am currently fighting is much more violent that what I experienced starting in the mid 60's. I am doing a visual inspection of the gear mount at intervals between 5 and 10 hours.
Some time ago I flew a Baron that had a violent nose wheel shimmy. Installed the most expensive Goodyear tire and the shimmy went away. No tire balance, just changed the tire.
My 7 got a rare shimmy after sitting for a cold period. And recently got an odd vibration from my Michelin auto tires. Although I am thoroughly familiar with the phenomenon with cars, it was never a consideration for my 7 until this recent event.

Has this been considered as a contributor/cause of shimmy? I know it happens just from rolling force from the hangar.

It would seem tire belt cord and "rubber formulations should be selected specifically to avoid this due to the operating cycles, physical and thermal.
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RV-7
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  #15  
Old 06-11-2021, 08:29 AM
DennisRhodes DennisRhodes is offline
 
Join Date: Jul 2007
Location: Taylorsville, Ga
Posts: 862
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Back in another world this one with pressure parts that had separated, we could look at the failure where it had been ripped out and it would be shinny and sharp in the cross section of the material failure. The section that had been cracked for a while and in our case leaking H20 would be dull and discolored due to the crack being opened up.. A microscope analysis could also be handy in evaluating that failure mechanism . In most cases as it turned out, it didn 't really matter because our job was to put it back together and make sure it doesn't happen again!! Steam power plant experience!!!
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RV9 N908DR
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  #16  
Old 06-11-2021, 10:23 AM
Ralph Inkster Ralph Inkster is offline
 
Join Date: Feb 2008
Location: Calgary, Alberta
Posts: 1,388
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I may have what you need. PM sent
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built a few RVs, rebuilt a few more, hot rodded more, & maintained/updated a big bunch more
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  #17  
Old 06-11-2021, 03:41 PM
ernestn ernestn is offline
 
Join Date: Feb 2012
Location: Bowen Island, BC, Canada
Posts: 24
Default Another case of RV-6 mount cracks

I have an RV-6 finished in 1997 with an O-360 and currently has 1661 hours on it. I am the third Owner and have flown it for about 1000 hrs. I was in the process of doing the annual and noticed a crack in the engine mount on the back of the top of the Port gear leg tube. Previously (in 2015) there were cracks at the base of the same gear leg tube. At that time we removed the engine mount and welded up the cracks with some added gussets from Vans Aircraft. Vans Aircraft suggested welding up the current cracks and if it cracked again to replace the engine mount. The current cracks were welded up as suggested. However my problems didn't stop there. On reinstalling the mount we found that the lower left engine mount attach bracket (WD-603) was also cracked. We are currently in the process of replacing this attach bracket with new bracket and backing plates. Below are some pictures:
Attached Thumbnails
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Name:	WD-603 left mount cracking.jpg
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ID:	12495  
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