Almost every modeller is aware of the torque reactions caused by rotating propellers. It is fairly easily understood by all, that with the rotation of a propeller, there is a twisting (roll) force in the opposite direction to that of the propeller. For most models it is a rolling force to the left.

Not so well understood is the so called ‘P’ factor. Under normal flight conditions the propeller meets the airflow ‘head on’. Under some conditions, especially take off and landing, the model (and hence the propeller) is flying at a higher angle of attack, compared to the airflow.....

This angle of the blade to the airflow creates uneven forces on one side of the prop to the other, and so the ‘P’ force will be seen as an effect in yaw. This ‘P’ effect on the normal model engine rotation is to the left in a nose up attitude, and to the right in a nose down attitude.

Gyroscopic forces, in particular gyroscopic precession are best demonstrated by a spinning top or a bicycle wheel. If you hold a spinning object in a plane simulating a propeller, and move the object up, down or sideways (whilst spinning), there will be a pitching or yaw effect on the spinning object.

To summarize the forces, assuming the normal model engine rotation of anti clockwise (from the front)

Under normal flight conditions the above forces caused by the rotation of the propeller are usually not noticed, but are always in effect, and can be a problem for models that are inherently unstable. These forces are most noticeable at take off, especially taildragger models such as a scale Focke Wolf 190, which has a long undercarriage, a short tail moment and small tailplane. Under these circumstances all 3 forces will combine to create a potential crash on take off. If left unchecked (mainly with right rudder) the model will take off with a violent yaw and roll to the left.

Full size propeller driven fighter aircraft have similar problems, (some legendary) to models.

For F2B (aerobatics), control line models, gyroscopic precession is a major factor, especially when using large diameter propellers. Upon the application of up elevator, the model will yaw into the circle, loosing line tension. The opposite is true for down elevator where the line tension usually increases.

F3A (aerobatics) models all use a degree of side thrust, and very long tail moment arms. These 2 factors reduce the effect of these forces, and make for rock steady flying. the effects generally only show up in rolls and snaps, where the model will roll quicker one way than the other.