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curiosity question on motor mount

danny

Well Known Member
I'm getting my 9 inspected by the DAR soon but now I'm worried about my firewall/motor mount install that I actually did a long time ago. The mount is actually quite flat at all 6 lugs. I ran into problems when it mounted so close to the firewall at the landing gear that I had to trim the corners of the fuselage skin to the firewall. When that wasn't enough I ended up putting 3 an970 washers under each lug to get the little clearance I have (approx .125"). I do not want to start grinding away at the firewall and making holes there.
Anyway, this is kind of freaking me out a little. Van's says use spacers as required but wondering how others overcame this problem.
Anybody have some words of wisdom?
thanks
danny
 
The intent for installing a tail dragger mount is that the bottom fwd fuselage skin and firewall be trimmed as needed to clear the engine mount gear leg sockets.
Did you add washers at all 6 bolt locations?
If not, then you adjusted the thrust line of the engine to be more nose up than is desired. This could potentially be destabilizing for trim in the pitch axis.

If you did put washers at all bolt locations then the thrust line will be fine but you will have a different shear loading on the bolts (it is no longer just a single point). I don't know whether that will cause a problem in the future or not.
 
Yes, washers are at all 6 places. I used the an970's to distribute the load on a wider area. I truly am not comfortable trimming into the firewall.
thanks for responses
danny
 
A standard tail dragger build is with the skin and firewall flange trimmed away in the lower corners to clear the leg sockets.
All tail dragger RV's except for the RV-8 are done this way.

There should not be any need to trim the firewall it self.... only the flange, so I am not sure if that is what you meant by not cutting the firewall.

As mentioned already, with washers added, it is no longer a single shear point on the bolts. Because the washers are not a close tolerance fit on the bolts (a pretty loose fit actually) the bolts can now have a bending load induced on them in the area between the engine mount and the face of the firewall.

This is the thing that causes me concern at this point, and it seems like it has you as well since you said "I'm worried about my firewall/motor mount install that I actually did a long time ago."

There is not much you can do about it at this point (not easily anyway) so it would probably be a good idea to keep a close watch on the area where these fasteners are. Particularly after any very hard landing events.
 
I trimmed the skin and the firewall flange right up to the firewall. Any more and it would be trimming the firewall itself. I plan on just keeping an eye on it and checking bolt torque at the first condition inspection or if something looks or feels unusual.
Thank you guys for the feedback
danny
RV9, Superior IO-360, WW ground adjustable
almost a real airplane
N109DK
Woo Hoo!
 
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