Right thrust is usually needed in propeller-driven aircraft (even full-size) because the rotating prop not only imposes a rearward impetus to the airstream, but a sideways component as well. This sideways component is in the direction of the propeller rotation. Some of the airstream is thus moving aft and towards the right-hand side of the model and will strike the tail. The rightward force on the vertical surface will push the nose left, so you need to counteract it. That's why you need right steering on takeoff.

Once in the air, the effects are much less, because the forward motion of the model reduces the effect (a vector diagram will show this), but some of the left-turning force is still present. To reduce the amount of changes, especially with throttle-up and throttle-down, a thrust angle is built into the engine mounting system.

An examination of a full-size Aircoupe (Ercoupe, and other variant spellings) will show significant thrustline angles. You can also see this on a DC-9 airliner. The engine pods are angled downward towards the rear, giving the same effect as downthrust on an airplane with a propeller in the nose.

Regardless of the actual direction and magnitude of the thrust angles, they are built into an aircraft to help deliver the handling characteristics desired by the designer.