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.
  => For this reason, a failure of the right engine would be most favourable.

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A right engine failure requires a lot of left rudder. A condition where left wind is present would reduce the required left rudder => remember that left wind results in left yaw.