– Effect on the Aircraft’s Stability

The previous section covered propeller drag and introduced prop torque. Propeller torque is now covered in greater detail, along with its associated propwash force, precession, asymmetric disc loading, ‘P’ factor and the effect these have on the aircraft’s stability. These factors are mostly de-stabilising, however in some cases they can enhance the stability of the aircraft

Prop Torque Force

The engine torque will produce an equal and opposite torque reaction at the propeller creating a turning moment, which will tend to rotate the aircraft around its longitudinal axis in the opposite direction to the prop’s rotation. With a ‘right-handed’ prop, this will cause the aircraft to rotate or roll to the left, in accordance with Newton’s Third Law of equal and opposite reaction. This can present as a problem during take-off due to asymmetric loading on the undercarriage, where the left-hand wheel is pressed down on the runway more so than the right – hand wheel. This excess pressure results in wheel drag and in turn, causes the plane to yaw to the left. For most modern aircraft types, the effect of torque and the accompanying roll and yaw in flight can be considered negligible and is easily corrected by use of the controls. Pilots of tail-wheel aircraft, especially World War II fighters with their greater power/ weight ratios, have considerably higher torque forces to contend with. A pilot who is not ‘ahead’ of his/her aircraft with a high power/weight ratio, could experience a torque roll on take-off, or during a go-around that could end with catastrophic results. The earlier versions of the Supermarine spitfire were equipped with the Rolls Royce Merlin engine that rotated the prop clockwise, while the later versions of the

Spitfire from the Mark 12 onwards were equipped with Rolls Royce Griffon engines, which rotated the prop anti-clockwise. Pilots converting from the earlier ‘mark’ of Spitfire to the later models had to be ready to counteract opposite torque forces with the rudder pedals. The torque forces are at a maximum during full power operations such as during take-off and climb out, but the force can be considered as zero during a descent with the engine throttled back to idle setting.