Isn't side thrust related to prop rotation ? What other factors ? Is there a deference from gear drive or direct though rotation stays the same ?? Thanks , KEITH

Imagine an airplane with the motor running, but the prop held such that it can't move. The plane will be spinning in the opposite direction that the prop was originally turning. Normally the airplane has more mass than the prop, so it wins the battle and the prop turns, but that prop is always trying to force the airplane in the opposite direction.

When viewed from the rear, propellers normally turn clockwise, to the right and down. This makes the plane want to roll to the left and up, opposite the rotation. This is the main reason why you will see right thrust and down thrust on tractor (motor in the front) airplanes. On a pusher (motor in the back) airplane, the effect is somewhat opposite: The plane still wants to roll left, but the nose wants to go down, so you need to counter with up thrust.

Also, in a tractor situation, the thrust rolls over the planes surfaces in a biased fashion and the plane reacts as if control surfaces are being actuated. Some muanfacturers have you offset the vert stab to compensate, some have you install the firewall offset and some just have you tweek the motor mount with shims.

theres also a contributor called "p-factor" which is the dissymetry of angle of attack of the prop blades. Imagine a plane climbing at an angle of attack of maybe plus 5 degrees (not 5 degrees nose up in relation to the ground, but 5 degrees positive angle of attack). Assuming a "normal" rotation prop, the descending blade of the prop on the right side of the model has a higher angle of attack compared to the ascending blade on the left side of the model, and hence it creates more drag,..thus rolling the model even more to the left. This is more pronounced in constant pitching maneuvers such as a loop, where the plane is accelerated in pitch to a high angle of attack and that high angle of attack is sustained throughout the maneuver.

When Gary says "5 degrees angle of attack," imagine the airplane moving along horizontally, but with the nose pointed up 5 degrees. The airplane is actually flying in one direction, but is pointing in another direction.

Because of the twist in the propeller, the blade moving down will "dig" into the air more than the blade moving up, when the airplane is flying along in this situation. On a normal propeller, that downward-moving blade is on the right side of the airplane.

The downward-moving blade produces more thrust, and more drag as Gary mentioned. Extra thrust on the right side makes the airplane want to turn left; this is why taildraggers tend to turn left when taxiing and taking off.