Counter-rotating Propellers

On twin-engine aircraft, the props of each engine may rotate in opposite directions with the top blade rotating in towards the fuselage. These are known as counter-rotating propellers.

The main advantage of counter-rotating propellers is during take-off and climb-out after an engine failure. On a conventional twin-engine aircraft with both propellers turning clock-wise, asymmetric thrust causes the greatest yaw when the left-hand engine is shutdown. This is due to thrust generated on the

down-going side of the propeller disc, remember the P-factor or asymmetric disc loading! On right-handed propellers the center of thrust to displaced to the right of the propeller axis. On the right-hand engine it is further away from aircraft’s normal axis, and the centre of thrust on the left-hand engine will be closer to the aircraft’s normal axis. If the left-hand engine fails, the right-hand engine will produce the greatest yawing moment due to the centre of thrust being displaced further outboard. The drag of the windmilling left-hand prop will contribute to the yawing force. In this instance, the left – hand engine is said to be the critical engine, due to the greater yaw force caused by the thrust from the right-hand engine.

On a twin-engine aircraft with counter-rotating props, both props will have the centre of thrust an equal distance from the aircraft’s normal axis. Therefore, the failure of either engine will produce an equal yaw force. The critical engine is

Four sets of contra-props power the Avro Shackleton
AEW.2 maritime patrol aircraft. This aircraft resides in the
Museum of Science & Industry, Manchester, England.

eliminated and single-engine performance will be the same with either engine failed.

Airplanes with propellers rotating anti-clockwise, or ‘left­handed’ propellers, will have their right-hand engine as their critical engine. The ‘critical’ engine is so named due to the control problems being more critical when the critical engine is shut down. The Fokker F.27 Friendship is one aircraft that comes to mind with a right-hand (Number 2) critical engine, due to the left-hand rotation of its propellers powered by their Rolls Royce Dart fixed-shaft turboprop engines.

The location of the wing-mounted engines on twin-engine aircraft is also important. Placing the engines too near to the fuselage will not only increase noise in the passenger cabin, it can also affect the amount of thrust produced by the propeller. The closeness of the fuselage affects the free air flow between the prop and fuselage. This has an affect on the prop by slightly reducing the prop thrust of the prop on one side of the aircraft, while the prop on the other side remains unaffected. Although this imbalance of thrust is not as great as the ‘P’ factor or asymmetric disc loading, it is still present to a certain degree .