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One of the most exciting types of training, in my opinion, is multi-engine training. I did my multi-engine training in a Piper Seminole. This aircraft is considered to be counter-rotating—AKA a non-conventional aircraft. There are other aircraft that are considered conventional. A convention aircraft means that both propellers move clockwise. Due to the propellers rotating in the same direction, it creates a critical engine. A critical engine is when the left engine failure most adversely affects the directional control and performance of the airplane. Let's explore the four factors that make the left engine critical.
P-Factor
Similar to a single-engine airplane, both propellers turn clockwise viewed from the cockpit. During high angles of attack, such as on takeoff and climb-out, the descending blade of the propeller produces greater thrust than the ascending blade. The center of thrust is an imaginary line from the centerline of the fuselage. The right propeller is further away from the center of thrust creating a greater moment arm (greater leverage) between these two points. The left engine is made critical due to the resulting yaw produced by the failure.
Spiraling Slipstream
Due to the propeller rotating clockwise, it creates a spiraling slipstream off both propellers moving to the right. As it moves off the left engine, it hits the vertical stabilizer from the left, counteracting most of the yaw created by the failure of the right engine. Yet, if the left engine fails, the slipstream from the right engine does not hit the vertical stabilizer at all, because it drifts rightward. Therefore, the spiraling slipstream cannot counteract the yaw toward the left side.
Accelerated Slipstream
Because of P-Factor, the centerline of lift is also farther out on the wing. Because the engines are located on the wings they also induce an increase in high and low pressure above and below the wing. This improves roll effectiveness as well as acceleration through the air. If one engine functions properly and the other fails this results in a greater-rolling tendency. With the failure of the left engine due to that P-Factor the roll tendency is greater on the left than the right.
Torque
Just like a single-engine airplane torque effect works the same way with Newton's third law. As the propellers rotate clockwise, the airplane tends to roll counterclockwise. If the left engine fails, the airplane will have an equal yet opposite reaction causing it to roll left and yaw left, into the inoperative engine.
These are the four left-turning tendencies of a multi-engine aircraft. The concern with these left-turning tendencies is not an issue with a counter-rotating prop.