RE: Phoenix Models Sukhoi SU-31 CG and engine Q's
drbenz,
Where you mount it on the firewall, up down left right, won't do a thing to the thrust directions. The firewall for this model is built with right and down thrust and you're going to get that no matter where on the firewall you screw on the motor mount. And some people have reported that their models don't have what others have reported for thrust directions of their firewalls. And the manual shows thrusts that don't match up to the models people seem to be getting nowadays. So it's a big confusing discussion.
If you put the wing on the airplane and measure it's incidence, it'll probably be parallel to the centerline for your fuselage from back to front. If you then look at the angle the firewall makes to the aircraft centerline, you'll see the up/down thrust if there is any. If there is none, the firewall will be absolutely perpendicular to the CL. My model LOOKED like it had downthrust and measured to have 3-4degrees.
If you stand over the fuselage and look straight down, the right/left thrust will show up about the same way. My model LOOKED like it had right thrust and definitely did. About the same as the downthrust.
Where you center the motor mount on the firewall to screw it on the firewall only really determines where the nose of the engine winds up within the cowl opening. When you've got a right pointing firewall that also points down, the center of the motor mount would usually not be in the center of the firewall. It would be above and to the right of the center if you were looking from the front.
Truth is, with my model, Phoenix whacked out so much of the cowl for the opening that whether or not the prop is centered is almost impossible to tell.
I wouldn't worry at all about centering the prop compared to looking inside the fuselage just behind the firewall for that lead slug Phoenix chose to hide in there. Pull that lead and use it if you wind up needing it.