Profile motor mounts

LAST EDITED ON Aug-26-01 AT 03:55 PM (CDT)

Do any of you radius them? By this I mean instead of having a square cut at the rear of the notch in the fuse plank for each engine bearer (and on the bearer itself), you have a half-circle, and the end of the mount beam is sanded on a belt sander to match that. I usually push the mounts as far back as I can, and still have the ply doublers cover them, also cut away the doublers where the engine mount lugs go. I use blind nuts (under the inboard doubler if I remember!) to help reduce compression of the doublers. Another trick is of course to epoxy aluminum sheet on both sides of the nose to get the same effect.
I got the tip on the radius bit years ago, where I can no longer recall. It helps eliminate a stress point at the sharp rear corners (especially in a crash) of the mount notches, and is supposed to strengthen the joint (and the nose) thereby.
Does anyone else out there bother to do this?

JAK
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I've recently started to round the rear of my maple motor mount beams and round the balsa fuselage slot to match.

I've also been using the Sig or Brodak nylon profile mount pads under the engine. These are "C" shaped. Instead of merely slapping it on top of the ply doublers (I hate moving the C/L of the engine outboard). I made a template to rout out the ply doublers to recess the 1/8" nylon mount pad. This sits directly on the maple motor mounts and gives the motor a very solid mounting.

I've also installed the blind nuts directly into the maple mounts and relieved the inboard ply doubler to fit over them (or just put a 1/4" hole in the ply). I like about a 1/4" length of bolt to protrude from the blind nuts so I can use a nylon lock nut to jam against the blind nut.

How do you cut the motor mount slots in the fuselage? If you use a router you would have a nice round end to the slot. In that case I would round the end of the maple motor motor mount to fit nice and snug in the radius.

I just cut mine on my old trusty scroll saw. I use a drafting circle template to mark the back ends of the slots with a pencil. A router would likely work very well too.