Left Turning Tendencies
Torque Reaction
For every action, there is an equal and opposite reaction. As applied to the aircraft, this means that as the propeller spins in one direction an equal force is trying to rotate the aircraft in the opposite direction.
When the aircraft’s wheels are on the ground during the takeoff roll, an additional turning moment around the vertical axis is induced by torque reaction. As the left side of the aircraft is being forced down by torque reaction, more weight is being placed on the left main landing gear. This results in more ground friction, or drag, on the left tire than on the right, causing a further turning moment to the left. The magnitude of this moment is dependent on many variables such as the size and horsepower of the engine, the size and RPM of the propeller, the size of the aircraft and the condition of the ground suraface
Corkscrew Effect
The high-speed rotation of an aircraft propeller gives a corkscrew or spiralling rotation to the slipstream. At high propeller speeds and low forward speeds, this spiralling rotation is very compact and exerts a strong sideward force on the aircraft’s vertical tail surface.
When this spiraling slipstream strikes the vertical fin, it causes a yawing moment about the aircraft’s vertical axis. The more compact the spiral, the more prominent this force is. As the forward speed increases, however, the spiral elongates and becomes less effective.
Gyroscopic Effect
Any time a force is applied to deflect the propeller out of its plane of rotation, the resulting force is 90° ahead of and in the direction of rotation and in the direction of application, causing a pitching moment, a yawing moment, or a combination of the two depending upon the point at which the force was applied.
Asymetrical Thrust
During cruise, the propeller’s plane of rotation is perpendicular to the flight path. However, at lower speeds and in a high nose-up attitude, the propeller shaft is inclined up and the rotational plane of the propeller is no longer perpendicular to the flight path. The down-going blade has a greater angle of attack than the up-going blade.
This results from the change in the direction of the relative wind each blade experiences. The downgoing blade rotates into the oncoming airflow and experiences an increasing headwind which alters the relative wind in such a way that the angle of attack increases along with the thrust produced. The opposite is true for the upgoing blade. It rotates away from the oncoming air and, as a result, experiences a change in relative airflow that results in a decreased angle of attack and decreased thrust. The combined effect is that the centre of thrust moves to the right, causing a yawing moment towards the left around the vertical axis.