This is based on Newton’s Law of action and reaction. Applying this law to an airplane with a propeller rotating in a clockwise direction, as seen from the rear, a force is produced which tends to roll the entire airplane about its longitudinal axis in a counterclockwise direction. To better understand this concept, consider the air through which the propeller rotates as a restraining force. This restraining force acts opposite to the direction the propeller rotates, creating a tendency for the airplane to roll to the left. [Figure 1-18]

Spiraling Slipstream

 This theory is based on the reaction of the air to a rotating propeller blade. As the airplane propeller rotates through the air in a clockwise direction, as viewed from the rear, the propeller blade forces the air rearward in a spiraling clockwise direction of flow around the fuselage. A portion of this spiraling slipstream strikes the left side of the vertical stabilizer forcing the airplane’s tail to the right and the nose to the left, causing the airplane to rotate around the vertical axis. The portion of the spiraling slipstream traveling under the fuselage is not obstructed, therefore, creating a different resistance between the obstructed and the unobstructed flow which causes the left-turning tendency. [Figure 1-18]

Gyroscopic Precession

 This theory is based on one of the gyroscopic properties which apply to any object spinning in space, even a rotating airplane propeller. As the nose of the airplane is raised or lowered, or moved left or right, a deflective force is applied to the spinning propeller which results in a reactive force known as precession. Precession is the resultant action or deflection of a spinning wheel (propeller in this case) when a force is applied to its rim. This resultant force occurs 90° ahead in the direction of rotation, and in the direction of the applied force. [Figure 1-18]

Asymmetric Propeller Loading (“P” Factor)

 The effects of “P” factor or asymmetric propeller loading usually occur when the airplane is flown at a high angle of attack.
 The downward moving blade, which is on the right side of the propeller arc, as seen from the rear, has a higher angle of attack, greater action and reaction, and therefore higher thrust than the upward moving blade on the left. This results in a tendency for the airplane to yaw around the vertical axis to the left. Again this is most pronounced when the engine is operating at a high power setting and the airplane is flown at a high angle of attack. [Figure 1-18]