Left turning tendencies are a characteristic of the Cessna 172 Skyhawk and other aircrafts. There are actually three major causes of the left turning tendency of aircraft.
In discussing torque, Issac Newton’s third law comes into play which states – for every action, there is an equal and opposite reaction. The right turning of the propeller creates an effect in which the opposite is the aircraft turning left. In the air, the torque tend to “roll” the airplane left.
On the ground, the torque causes the aircraft to yaw to the left as the left side of the aircraft is being forced down by the left rolling motion from the torque. The aircraft cannot physically roll due to the wheels being on the ground, however, this does cause more weight and friction on the left main landing gear. This added friction and force down creates the left yaw tendency and the need to apply more right rudder pressure to offset this force.
Propeller Factor (P-factor) or (asymmetrical thrust)
The P-factor or asymmetrical thrust occurs when one side of the propeller produces more thrust than the other. So greater thrust from one side = yaw in the opposite direction.
The P-factor is an effect of the aircraft flying at a high angle fo attack whereas there is a greater load – more thrust – coming from the propeller. since the propeller is moving in the direction that the left side of the propeller blade gets a greater chunk of air. This difference in thrust creates the tendency to yaw left.
The slipstream effect is also know as spiraling slipstream because the air pulled in from the propellers rotated and sent backwards. This spiraling slipstream moves air in a clockwise manner around the fuselage and puts an added air pressure on the vertical stabilizer which hits the left side of the tail pushing the nose to the left.
This slipstream also caused a slight rolling moment to the right on the longitudinal axis.
Aircraft Aerodynamic Design to Compensate
Since this left leaning effect is well known amongst aircraft manufacturers, airplanes are often designed with aerodynamic compensations to offset this left leaning tendency and the engines are often offset slightly to the right, the trailing edge of the vertical stabilizer may be canted a little to the right and the left wing may be designed to have a greater angle of attack.
These aerodynamic designs are intended to compensate for the aircraft moving at cruise speed so anything other than cruise speed in a straight path needs some compensation by holding the right rudder.
At a lower power setting and higher speeds such as descent with some power, these compensating designs tend to overcorrect in which case left rudder will need to be added.
There is actual another factor which is less noticeable which can cause the aircraft to yaw known as gyroscopic precession.
When the tail is being raised on the takeoff roll, gyroscopic precession is more prominent in tailwheel aircraft.
Procession is the resultant action, deflection or a spinning rotor – when force is applied to change an axis of rotation.
The deflection will occur in 90 degrees in the direction of rotation to where the force was original applied. In this case, the rotating propeller acts as a gyroscope so when the pitch of the aircraft is changes, a force is applied to the axis on which it is spinning.
Pitching the nose down applies forward force to the top of the propeller which in return the resulting force acts at 90 degree rotation to the right and a force on the right side.
On a tailwheel airplane raising the tailwheel on takeoff creates the tendency to yaw the aircraft to the left due to gyroscopic precession.
When pitching up (tilting the propeller up), the force is on the bottom of the propeller’s arc of rotation causing the force to be at the left side and causing the nose to the right.