ORIGINAL: Villa
I have always felt that if you cut the POLY firewall with a shoulder (rabit?) that not enough material is left for the screws holding the firewall to the PVC. If you put in a plain POLY firewall and then cutout four POLY right angle triangles with two equal sides about 1 inch, they can be located at the corners of the firewall so they overlap the PVC and thus serve as a shoulder at the corners. One screw in each should do it. Might work fine with just two triangles in diagonally opposite corners. Comments?
i always worry about the same thing - especially with the poly - on 1/2" ply its not as bad, but still bad enough that i don't bother with them anymore. The other disadvantage as far as i'm concerned is that when you use the rabbit, if you crash nose in - you transfer all of the force into the engine - instead of the screws breaking out of the pvc, and allowing some give. I guess it depends on how hard you hit. but personally, i'd rather replace a fuse than an engine.
But, i think there might be a good compromise - why not cut a piece of pvc to about 2 5/8"x 2 1/2" - and either goop or screw it to the firewall (between the motor mount, and the firewall) - that'll provide the locating qualities of the rabbit, give you plenty of meat to screw into, but still flex free in an unplanned nose first landing - and be considerably lighter than gluing a whole second firewall to the nose. I don't see why you couldn't use light ply to do the same thing.
Personally, i think the whole rabbited fit is overrated. Really, i think the whole firewall/ motor mount is overrated. Last several planes that i've built with pvc just use a hdpe motor mount - Picture a Spa3dt motor mount slid into a pvc fuse, and screwed to the bottom of the fuse - thats what i mean.
Anyway, here's one more that i used to use alot -
Basicly 2 hdpe bars with the motor mounted to it, and then screwed into the fuselage - they all work fine, its just a matter of personal preference.