ORIGINAL: tryan02
Is the whole in the firewall of an arf the thrust line? I just converted my seagull 180 to gas and centered the mounting plate side to side on the fuse and then aligned the hole in the backplate (gas mount) with the hole in the firewall and the cowl slipped on centered. Was I wrong to use that hole in the firewall as a reference?
Lucky? yeah, a bit.
Lots of motor mounts have an open center so the firewalls often have a hole there. That'd be where they expect you to route your hoses. However, it isn't a given that the mounts they expect you to use are symmetrical.
If the design calls for right thrust, that hole should be offset side to side. If it calls for down thrust, it'd be offset up/down.
Everything you assemble in an ARF needs to be checked closely. A Sukhoi ARF I assembled a couple of years back was completely hosed. The manual said there was no thrust offsets. The firewall actually had right and down thrust. The hole was positioned for up thrust and right thrust. It's always a good idea to check everything you can think of, because they don't always get everything right.