Originally posted by FLS
Traveler P-factor is modeled but the turning effect is minor.
Gyroscopic Precession is modeled but it's only noticeable with pitch changes so it's a factor on the takeoff roll momentarily when the tail comes up and again on rotation when you pull the nose up.
What liqermixr is describing is caused by helical propwash.
an oversight on my part, you are 100% correct. It would be nice if the propeller would just take the air and throw it straight backwards, but it doesn’t. The propeller airfoil necessarily has some drag, so it drags the air in the direction of rotation to some extent. Therefore the slipstream follows a helical (corkscrew-like) trajectory, rotating as it flows back over the aircraft.
The next thing to notice is that on practically all aircraft, the vertical fin and rudder stick up, not down, projecting well above the centerline of the slipstream. That means the helical propwash will strike the left side of the tail, knocking it to the right, which makes the nose go to the left, which means you need right rudder to compensate.
You don’t notice the effect of the helical propwash in cruise, because the aircraft designers have anticipated the situation. The vertical fin and rudder have been installed at a slight angle, so they are aligned with the actual airflow, not with the axis of the aircraft.
In a high-airspeed, low-power situation (such as a power-off descent) the built-in compensation is more than you need, so you need to apply explicit left rudder (or dial in left-rudder trim) to undo the compensation and get the tail lined up with the actual airflow.
Conversely, in a high-power, low-airspeed situation (such as initial takeoff roll, or slow flight) the helix is extra-tightly wound, so you have to apply explicit right rudder.
Helical propwash sometimes contributes to left/right asymmetry in multi-engine aircraft
One think I noticed flying an AT6 and even my J3 Cub was that the helical prop wash was damped some what, while the tail wheel remaines on the ground. May be that the propwash can't actually corkscrew around until the tail is raised. But at that point without the proper application of right rudder the aircraft will make a hard left if left unchecked.
