Thrust Lines !

The reason for right thrust is to counteract the spiral effect that the prop wash places on the fuselage & fin. The air hits the fin on one side only and the fin finding itself in a cross wind tries to turn (Yaw) towards the direction of the air, it’s the same effect as a weather cock.

The following comes from Phil Williams of Probuild (probably the UK’s premier supplier of aerobatic models and equipment) and an old flying buddy:-

Torque actually causes the model to roll! If you fly past on a calm day when the model is trimmed to fly at full throttle, then cut the throttle, the model will roll slightly. This is where the model is trimmed at full throttle (full torque) & when you cut the throttle the torque is reduced, therefore the trim is then wrong. I always have about 3 per cent engine to aileron mix running.

Here is a copy of a recent RCM&E article on the subject.