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Engine basics => Engine set up tips => Topic started by: Dennis Toth on July 19, 2012, 08:13:54 AM
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Guys,
I am getting close to completing the motor crutch for my El Diablo with the Fox 35. At this point I would normally have mount pads on the mounts but with the Fox there is a concern with the bottom of the mount lug not being flat. If you look close there is a very small mold release angle to them. When we used to just bolt them to the wood mount it wasn't a problem cause the wood would crush down and all would be fine. Now that we like to spread the motor load over more of the mount for a smoother run we add the mount AL pads. With the mount pad when you bolt down the motor it stresses the case. Some have said its only a few thousants but it does cause problems. The Fox 35 case is very thin and pulling down on the tabs causes things to go out of alignment and the motor run becomes unpredictable. One option is to just bolt it to the wood mount, using the mount pad does seem for most installation of other motors to spread the load and help the consistency of the run. Filing the pad to match the relief angle is another option as is using JB Weld under the pad to fill the gap then pull the hold thing together and it should be aligned. I don't know if the JB Weld is the right stuff for the vibration load. I would like to see how others have addressed this issue.
Best, DennisT
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Get a machinist friend to mill the mounts flat. I have carved a wedge out of 1/32 plywood on wood mounts in the past.
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Dennis,
On my Fox racers with flat alum motor mounts: I use 1/32 plywood pads under the engine lugs.
I find the small plywood mount pads work well to prevent the c/case from warping when mounted ( screwed down) onto a flat surface. Milling the engine lugs to me is too much like work, when easier methods like this are available...
Good luck!
Paul G.
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It's easy to scrape a bit from maple mounts to get the crankcase flush. If you start construction by bolting the engine on the bearers, then adding ply firewall, etc. and 1/2" balsa between bearers, then you can remove the engine and build the fuselage around the completed mount assembly. The mounts will be slightly skewed, but the engine will always fit when you are finished.
Floyd
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Get a machinist friend to mill the mounts flat. I have carved a wedge out of 1/32 plywood on wood mounts in the past.
Sounds like a new product for Jim Lee to add to his line of products: machined pads to level the mounts of the Fox .35. y1
BIG Bear
RNMM/AMM
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Hmmmmm..
Well, I would mention here that I think this is a lot of "ado" over something not worth worrying about.
The aluminum pads are mounted on Maple, in most cases, and that still provides some elastic deformation (The compressive strength of maple is low enough to permit some deformation) when the engine is bolted in place. "Viola", not much different than bolting the engine directly to the maple mounts. There is also a small amount of torsional "give" in the fuselage mounts. Add all this together and it simply overcomes the very small amount of draft in the Fox mounts.
If one must...it's a simple matter to slightly bevel the maple mounts {with a file) prior to putting the aluminum plates in place, but entirely unnecessary in my opinion.
Between the natural flexibility of the engine mounts and the case, and the maple mounts, any distortion in the engine case will be completely negligible.
Randy (sensible engineering) Cuberly
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Paul's plywood shims worked great, give a solid mount and has no problem with the draft angle on the Fox case.
Best, DennisT