I generally try to avoid crossposting, but since I haven't had any taker in the aerodynamics forum, I figure I will try my luck here.

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If I mount the engine such that the thrust line deviates from that which is drawn on the plan, what corrections, if any, should I make to compensate? Obviously I would need to adjust the fuel tank position to center with the carb. But would I have to adjust the downthrust also?

Specifically, the 4*60 I'm building calls for 2 degrees down thrust. If I move the thrust line farther up (mounting the engine higher up on the firewall), would I need to decrease or increase the down thrust angle?

How about incidence angles, any changes necessary there?

Thanks.

Other things being equal, if the engine goes higher up on the firewall, a down-pitching moment will result, calling for less down-thrust angle (or more up-thrust). The theory is that the engine's vertical thrust line (axis of the propshaft in the vertical-longitudinal plane) should go through the center of drag of the model as a whole for stable flight behavior. The only practical solution is to fly the model straight & level at high throttle, then chop the throttle and watch for change of pitch up or down. Ideally, there should be no change, just slowing down into glide angle. Confirm by flying same pattern at low throttle, then punch up throttle. Only change should be in the model speed, no change in its line of flight other than lifting instead of gliding. Then for maximum enjoyment of the trimming experience, do the same thing inverted. If change of throttle results in change of pitch attitude, your vertical thrust offset needs adjusting. If you're moving your engine up the firewall by only maybe 1/4" the difference in flight behavior could be hard to notice, therefore negligible.

Thanks, majortom. You explained it very well. I was thinking along the same lines and am glad to have it confirmed.

Yes, I am moving it up exactly 1/4", so I don't think I will adjust the downthrust. This was necessary because I'm mounting the engine inverted and 1/4" will let me center the carb precisely with the lowest position I can set the fuel tank.

I will do the flight tests as you described. My only concern is that I won't be able to tell if the effect was due to thrust line offset or nose heaviness, as they produce similar effects.