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engine/landing gear mount crack


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During the Condition Inspection on a new to me 1996 -4 with 1049TT, I discovered a crack in the engine/landing gear mount. It would have been very easy to overlook since that part of the mount is not easily visible on the plane. Thankfully I have a good habit of wiping dirt off parts and looking at everything. (Comes from years of experience working on race cars). I think it had been cracked a while, slowly growing, and then not far from failing. I removed the mount, cleaned it, welded the crack, fabricated and welded on some gussets, beadblast and repaint. Back in service and enjoying flying my RV-4.
Thanks, nick


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Now that you have strengthened that area you'll need to inspect the structure on the other side of the firewall on a regular basis. Sort of kidding but not really. Other side is mainly 3/4" alum angles that (hopefully) reaches the main wing spar, held in place with 0.032 alum (which may also be riveted to bottom skins?).

Point is that when you strengthen one area you can expect connected areas to be affected.

Oh, and check for main gear (wheel) fore/aft shimmy, taxiing, take-off and landing roll. (Mount camera at tie-down ring looking at wheel pants.)

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A thing of beauty..

Nick and share the same airspace, and he lives close by. His metal work is pure art, and his race car chassis build shop/machine shop is a dream. His repair is robust, but very aircrafty..not overkill. His 4 is one of the nicest legacy builds I've seen, and had no signs of frontal abuse. I can't officially sign him up for weld repairs, but..he's really good!
Thanks for the kind words gentlemen. My reason for the post was to make aware to other rv'ers a potential place for a crack, where it is difficult to see up close. I saw something after wiping away all the gunk, using a USB camera confirmed the extent of the crack.
I did look at the load path beyond the firewall. It is backed up with the aluminum angles to the main spar and is boxed in.
To me, the connection between the cross tube in between the tubes that the landing gear legs slide into and the sleeves that the firewall bolts go into, needed a little something more. Did I overdo it, probably. The area circled labeled "A" is a sharp corner with a HAZ(heat affected zone). Easy place for a crack to start. The direction of the load on the top end of the strut is forward. It is trying to twist that cross tube and pull it off the firewall. And, I could be wrong.
I am no professional engineer, I hated math in school, but I know enough just enough to kill me:) Use my "wisdom" at your own risk.
Half the fun of owning an experimental aircraft for me is working on it, legally.
This is my part on giving back to the RV community, I certainly have learned a whole lot from Vansairforce. I look forward to learn more and give more back.

Thanks, nick


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Yes, beautiful work! I wish I could weld like that.

I guess I didn't make my point clear. Why did it crack? Hard landing(s)? Wheel shimmy (if so, is it still present)?

If the cause of the crack is still present, strengthening one area just transfers the stress to the next, now weaker point.

No argument that it's a high stress point. Matter of fact, originally the RV-3 did not have those two firewall contact points which were added later and carried over to the RV-4.

On similar subject I assume you're aware of the weldments inside the fuselage where longerons attach to the firewall and engine mount bolts? They were also prone to cracking and were strengthened. I'm sorry I don't have the details handy.

Good catch. I would have thought by now this issue would be widely known.

Very common on mounts without gussets. Vans has gussets and used to send them on request. I used the 7 motor mount as “newer”, some 20 years ago, mounts have gussets.

Although the repair is very well done, I would encourage folks to use Vans gussets or follow their repair procedure. They have been down this road and to my knowledge, no cracks have ever been found in the newer motor mounts or any repaired per Vans instructions.

I am surprised there isn’t an SB on this issue.