ORIGINAL: ED STEFAN
ORIGINAL: broke_n_bummin
... But adding downthrust is only going to help when the propeller is turning. He said that when he cuts the throttle, it pitches down. Sounds like a stall ... like you're going too slow. The extreme dihedral wing will keep the plane straight, but it sounds like the plane is falling out of the sky. After it dives a bit, does it straighten out for a minute then dive again (porpoising) ?
If there is too much up-thrust, he's having to dial in down elevator for straight flight under power. Reduce the power and the down trim causes the plane to nose downward. To correct this, you shim some down-thrust in the engine to hopefully take out the down trimmed elevator. Once the correct engine thrust is dialed in, the plane should initially continue flying straight when the power is cut. After air speed is reduced, then the plane will lose altitude.
I agree, the pattern planes use this method to trim the model for different power settings to stay in level flight.
Shim the thrust angle down a little more and try the throttle test again.