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Broken engine/gear leg mount

I’m not sure if there is anyone out there who can help but it’s worth a try. I have an RV-6 that I built and have been flying for 19 years. I have a little over 1100 hours on it and almost 2300 landings logged, at least half of them on grass. On Memorial Day I landed at my home airport in North Arkansas and noticed when taxiing to the hangar that the right wing was significantly lower than the left. I first thought I might be getting a flat so I got to the hangar as soon as I could before it went completely down. Unfortunately it turned out to be the engine mount which surprised me since I have not experienced any landings lately that I would consider rough. I am thinking now that it may have been cracked for a while and the last landing was the straw that broke the camel’s back.

At any rate, here is my problem. The technical advisors at Van’s looked at the pictures and have recommended not attempting to have it welded, but to replace it with a new mount and new gear legs which are match drilled by the manufacturer. I have ordered it but received an email yesterday stating that the parts will not be available until September 13. I’ m wondering if anyone near me would have the mount in their kit but might not be ready to use it in the near future, and might be willing to swap out with me. The mount that I have coming is the 7 DYNA-1 O-360 for the 6,7, or 9. If not too far away I would be willing to come after it and pay a little extra to make it worth your trouble. I could have Van’s ship my new one to you. Oshkosh is coming up and I sure would like to take my RV for the 18th time. Sorry for the long post.
 
photos please

Sorry to hear about your -6 being grounded but glad you were able to return to the hangar under power.

I can't help with the mount but would love to see some photos of your cracked mount. Am interested if the root of the failure appears to be from the same area where small cracks are often found.
 
I have never posted a picture so will have to figure out how to do so. If you like I would be happy to text or email pictures to you directly.u
 
rv6cardinal check you PM's, I sent you my email address so you can send me pics and I will post them for you. Like Sam, I am interested in seeing them. Thanks,
 
Pics of Mount

Received these pics of the mount from the OP.

IMG_0133.jpg

IMG_0135.jpg

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I'll be the first one to jump in and say, holy ****. My first assumption was cold weld that separated, but the weld didn't fail, it pulled the entire tube apart and cracked another one. If your based on a grass strip, I would go walk it looking for gopher holes, cuz you hit something hard at some point. Sorry I can't help with the new mount, but that one needs to go on the wall, cuz thats an impressive level of damage. Super happy that it didn't take anything else out with it.
 
The new mount that you will receive has additional gussets welded in areas that the older 6 mounts did not have. If you do find one to get before yours arrives, I'd make sure it is the latest generation.
 
From a picture, it is hard to tell what caused the failure.

BUT, what seems apparent is that painting the engine mount black could have made it much harder to identify a crack.

Think about that if you're re-painting an engine mount or are painting one of the older ones that wasn't factory powder coated.
 
Mount

Failure from stresses in excess of design limits should have occurred on the diagonal tube from the top of the firewall to the top of the gear socket. The diagonal tube should be lesser wall thickness than the socket tube.
This is indicative to me of air hardened weld of the diagonal to the gear socket. Any failure should be well away from the weld.
The tube to the engine mount portion is a secondary failure due to excessive movement from initial weld failure.
This raises the question of whether the socket doubler would be better on the outside of the main socket tube as Steve Wittman originally designed it. While the inner socket makes it much easier to fit and install the gear leg I decided on the outer socket for my Wittman Tailwind. I have a ittermittent shimmy on the left side only that can sometimes get quite violent. I have tried all the standard fixes including balance weights in the wheel pant with limited results. Low tire pressure has helped. Rough paved runway is a problem. Most of my local flying has been with the aft portion of the left wheel pant removed. No wheel pant has not made much difference.
An interesting test would be to file the failed weld with a very fine tooth file. If it doesn't file easily but instead seems like glass, it is an air hardened weld.
Similar failures have occurred from time to time on Pitts aircraft and on the Christen Eagle. On those failures cold weld shop temperatures could be an issue. Too much heat and moving too fast with the TIG torch is also a possible cause. I can produce near identical TIG welds where one is ok and the other is air cooled and will fail.
 
Sorry to hear about your -6 being grounded but glad you were able to return to the hangar under power.

I can't help with the mount but would love to see some photos of your cracked mount. Am interested if the root of the failure appears to be from the same area where small cracks are often found.

Wow....that is a catastrophic failure. However, it is not at the same location as the small compression fatigue cracks we sometimes see in RV-6 mounts and have been addressed with gussets in later mounts.

This mount appears to have been severely over-stressed at some point....or cracks with unknown origins went undetected for a very long time. My guess (speculation) is that the gear suffered a huge hit (or repeated punishment?) which created cracks that propagated over a period of time until failure occurred. I really doubt this is due to design flaws or poor workmanship.

Thank you for posting the photos, they are very helpful.
 
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.
 
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.
 
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.
 
Flat spotting?

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.
 
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!!!
 
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:
 

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    Upper left gear leg crack with paint.jpg
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  • Upper left gear leg crack with paint removed.jpg
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  • WD-603 left mount cracking.jpg
    WD-603 left mount cracking.jpg
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