Why do my single-engine propeller planes keep rolling uncontrollably?

[MAFman]

When I make a single engine prop plane, it always rolls uncontrollably opposite the direction of the motor. I know this is a reaction torque to the torque of the motor, but why don't real planes have as much of a challenge with roll stability as my planes, and how do I fix it?

[CFYL]

Real planes are affected. Although real planes mostly use turboprop engines (or piston engines), the torque is still applied to the plane.

IRL The elevons on the two sides are not set to the same angle. i.e. the prop spins clockwise, looking to the prograde direction, then the elevon on the right wing goes up and the left one goes down (also for pitch/AoA stability) to counter the torque.

In KSP, without autopilot mods, you can set your main wing elevons to "deployed" and adjust the deployed angle in PAW in editor or in flight. Also rember to click all three axis to "pitch inactive, yaw inactive, roll inactive". The deployed angle could require some trial-and-error to be correct. Changes in speed and output torque of the propeller can also change the "correct" angle. You can (mostly) rely on SAS to tackle small torques. For flight control, use your tail fins or canards.

This is my example craft.

Spoiler

I don't have breaking ground. The NFA propellers provide only thrust, no torque. So this plane flies well.

So I added a pair of small rotated small wings to simulate the torque provided by propellers. The "torque" elevons are placed with symmetry off, angle snap on. Then I used offset to put the "torque" elevons in a symmetric place, without auto symmetry. 

First try---setting "torque" elevons to deployed at 13 degrees. The PAWs are arranged in sequence:

1st row is main wing elevons. left → right is outer → inner. The outer elevon (elevon 2) controlls roll. The middle elevon is the "torque" elevon to counter the propeller torque. The inner elevon are flaps; when deployed they give the aircraft more drag and more lift, useful for landing.

2nd row is tail wing elevons. left → right is outer → inner. The outer elevon controlls roll and pitch. Thin inner elevon controlls roll. (I think I wanted to have it control pitch. So I made a mistake.)

The vertical tail fin controls yaw. I didn't show its PAW.

The plane rolls to the left after takeoff. This means the 13° deployed angle of "torque" elevons is an overkill. After some trial and error...

Its flight is stable! All flight screenshots are taken with SAS on and no manual control. One problem I found is that, when manually pitching the plane, it will also rotate on the roll axis. SAS can't handle it, but manual correction can make up for it.

This is the final status. The first row is main wings, left → right is left → right on the main wings. The second row is tain fins, left → right is left → right on the tain fins.

Hope this could help you

Edited May 19, 2022 by CFYL

[ColdJ]

On 5/19/2022 at 7:43 PM, MAFman said:

When I make a single engine prop plane, it always rolls uncontrollably opposite the direction of the motor. I know this is a reaction torque to the torque of the motor, but why don't real planes have as much of a challenge with roll stability as my planes, and how do I fix it?

In KSP if you don't mind the length you can add a second rotor that turns in the opposite direction with reversed blades in a coaxial set up. You power the outer most and have the closest to the body unmotorised. The game will then automatically power both and the opposing torque will cancel each other out. Or you can make a 2 engine plane with opposite spinning motors, 1 on each wing.

You said single so the coaxial is closest to what you want.