how do i get a single propeller plane to not roll once in air?

[KyZeNickX]

how do i get a single propeller plane to not roll once in air? i am trying to recreate the f4u corsair but it keeps rolling when i get in air, and it just turns to one direction on the ground. do i really need a second propeller?

[Vanamonde]

Yes, that's physics properly modeled by the game, so you'll have to correct for it one way or another. Real aircraft apply trim, but in KSP I prefer to mount props in counter-rotating pairs. 

Oh, and your thread has been moved to the Gameplay Questions sub since you are seeking help rather than offering it. 

Edited December 31, 2020 by Vanamonde

[lemon cup]

Just like real airplanes, flight control rigging! You have an aileron on the end of the both wings limited to Roll only, yes? If so, right click one of them and select Remove From Symmetry. Then use the rotate gizzo to give it a small amount of incidence compared to the other, so that the wing that wants to roll “down” will end up getting more lift.

[Echo__3]

For each action there is an equal and opposite reaction. The force rotating the propeller one way is rotating the plane the other way. Typically I will set the trim on my airlerons to counter the roll.

[Zeiss Ikon]

Actually, it's not torque reaction that makes a  real Corsair (or R/C model) want to roll over -- it's a yaw force known as P-factor, cause by angled airflow through the propeller when you rotate for takeoff (at least in the real world; I don't know if FAR models to that level of detail and I'm pretty sure the stock game doesn't), which then brings dihedral into play to cause a roll to the left.  This is exacerbated by slipstream rotation acting on the tail surfaces, turning the airplane to the left as well (and again, I don't know if FAR models this, but stock surely doesn't).

In the stock game, it's almost certainly just torque reaction, similar to a reaction wheel.

The Corsair was often called "The Widow Maker" because, due to the very high engine power, it had far more of this P-factor than whatever aircraft new pilots were transitioning from, and required a large right rudder pedal force to counteract this, applied as soon as back stick was brought in to lift off.  Letting the aircraft build up more speed on the ground helped (rudder was more effective, and the angle of attack to lift off was less), but in theater, short fields were the rule and the aircraft was also usually flying heavy (with fuel and munitions) when launching for a mission, which increased the necessary angle of attack.

In game, aileron trim is most of the answer -- but you'll need to add a bunch before starting your takeoff roll, and take some out once you're in the air (the faster you fly, the more effect you get from a given amount of trim).  If you have FAR, you may also find you want some rudder trim -- or simply some right-yaw taps during takeoff.