The Critical EngineIn conventional twin engine aircraft where the propellers rotate clockwise as viewed from the cockpit, the left engine is considered the critical engine. The Seneca has counter-rotating propellers. Some say that this eliminates the critical engine. Taking the pessimistic view may help you respect the dangers of an engine failure: if either engine fails, it is a critical situation. Assuming clockwise rotating propellers, the following conditions make the left engine “critical”. 1. P-Factor – Yawing moment This is the greatest reason why the left engine is critical. At high angles of attack (such as what would be encountered while pitching for Vyse), the descending blade takes a bigger “chunk” out of the air than the ascending blade. The descending blade of the right propeller is a greater distance from the longitudinal axis of the aircraft than that of the left propeller, this giving it a greater arm and moment. 2. Spiraling slipstream – Yawing moment Most of the spiraling slipstream from the propellers misses the vertical stabilizer in coordinated two engine flight. In single engine flight, the slipstream from the left engine will hit the tail (because the aircraft is yawing to the right) and help bring the aircraft nose back to the left. In single engine flight with the left engine inoperative, this slipstream does not aid in directional control, and because of the position and direction of rotation of the right propeller, the slipstream from the right side does not aid either. 3. Torque – Rolling moment This is the same as in a single engine aircraft – engines turn right, aircraft rolls left. Think of the center of rotation (rolling) in this case to be the engines. If the right engine is inoperative, the rolling moment from the left engine has to “pick up” everything to the right of the engine, including the fuselage, right engine, and right wing. The weight of the plane will help resist this rolling force. If the left engine is inoperative and the right engine is producing the torque effect, it only has to “pick up” the right wingtip – all of the weight is acting in the same way as the rolling moment from the torque. 4. Accelerated Slipstream – Rolling moment When the propellers are turning, they create wind behind them. This wind passes over the wings, increasing the lift produced by that portion of the wing. For a similar reason to P-factor, there is more air “blowing” behind the descending blade than the ascending blade. Again, the descending blade has a greater arm from the longitudinal axis on the right engine than the left engine, thus increasing the rolling effect from this added lift. |
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