Engine thrust angles
 

What type of aircraft require down thrust and or right thrust ? I am working on a low wing p-51 and am wonder if offsets are needed.

There is no specific "type" that requires thrust angles to the engine. It's a combination of things including but not limited to wing, horizontal and vertical stab incidences. Speed envelope and wing loading. Engine torque and any number of other things. Even the airfoil of the wing can play into it. The full size Piper Cherokee has some noticeable down and right thrust to it. Some planes will offset the vert stab a few degrees which can be seen when looking down the center line.

I know the full size P-51 has some down thrust to it but not much. I don't believe it has any right thrust though. Some of the models I've seen of this plane have the down and some don't. Again, it's all in the rest of the stuff I mentioned.

If building a kit, I'll trust that the designer has built in any engine thrust he may want. If scratch building I tend to zero everything unless the plans say otherwise. If after flying it I feel a need to add engine thrust, I'll user washers behind the engine mount to "adjust" it. But for me, learning the different flying characteristics of each plane is part of the fun. If every plane flew and landed the same, I would soon get bored with the hobby.

The need or not is related to the location of the thrust line to the wing, which is a major portion of the drag on any airplane. It is also related to the amount of pitch stability trimmed into the model by the selection of where the model is balanced and the resulting decalage angle between wing and stabilizer. The more "stable" the plane is the more it tries to climb from any excess speed or adding power. And thus the more desire for some downthrust to reduce that pitching up.

Which is why a lot of seriously aerobatic models that are trimmed for pretty much neutral stability can get away with a 0-0-0 thrust, wing and tail setup and why high wing trainers might have something more like -2, +4, +2 or -4, +2, 0 setups or something along that line.

Your P51 is a low wing so that helps avoid the need for excess downthrust provided you set the CG location and wing to stabilizer angles fairly flat so it has some but limited pitch stability. That being the case you could likely get by with something like 0, +1.5, 0 and it would fly well yet still have some positive self recovery from fast dives. And you would trim the CG location to arrive at where it flies level at mid throttle at that setup.

Right thrust is not used to compensate for engine torque. The torque of the engine is aligned with the roll axis. Right thrust imparts a moment to the yaw axis. The reason a single engine, single propeller aircraft turns left (when the prop spins CW as viewed from the cockpit) is because we have been building airplanes for over 100 years with half the vertical stabilizer missing. This is done to make the airplane easier to get into and out of and rotate for take off and landing. As a consequence, the airplane flies in a circle. As the air leaves the propeller it curls down the fuselage (spiral propwash). When the air gets to the tail, it beats against the left side of the vertical stabilizer. However, there's no corresponding lower vertical stabilizer for air to beat against the right side of. This results in the plane turning left. Since the problem is power related, using right thrust to compensate is the right thing to do. Angling the vertical stabilizer is not. It's a dynamic problem and requires a dynamic solution. Modern pattern planes have counter rotating propellers and require no right thrust. In spite of the fact that they still have just one motor spinning in one direction producing lots of torque.