Refer to figure.

Asymmetric blade effect (also known as ‘P’ Factor)

Propeller blades are not flat, they are in fact shaped as small wings. As a consequence, as the angle of attack of the airplane increases, the air passing by hits the blades differently:

The down-going blade (viewed from the cockpit) will have a larger angle of attack compared to the up-going blade, generating more thrust. This then means that the down-swinging blade exerts a greater force than the up-going blade. For this reason, the thrust line will be displaced to the right of the engine centre line.

• Propellers rotating in the same direction (clockwise): if both engines rotate clockwise, the right engine will have a longer thrust arm than the left engine. This difference in thrust will give a yawing moment to the left with a clockwise rotating propeller in a nose-up attitude. The failure of the left-hand engine will result in a larger yaw effect via the operating right-hand engine, rather than vice-versa. The left engine is therefore the critical engine.
• Counter-rotating propellers: they are called counter-rotating propellers because the propeller on one wing turn in the opposite direction to the one on the other wing. The principle advantage of counter-rotation is to balance propeller torque effects thus eliminating any problems associated with a Critical engine.