The Asymmetric blade effect, also known as ‘P-factor’ or ‘Asymmetric disc effect’, is basically an aerodynamic phenomenon that relates to the motion of air in relation to solid objects such as the front fan blade or wings of an Aeroplane. This factor is mostly seen when an Aeroplane has high angles of attack. An angle of attack is the angle formed by the mass of air and the reference line on the Aeroplane or its wing.
When an aircraft is flying in a straight line and has a rather low speed, the propeller disc (a propeller disc is the front disc or blade of an aircraft) is perpendicular to the relative flow of the air. Each of the blades of the propeller will come in contact with the air at the same angle and speed and thus the reaction force that is produced is eventually centered all across the propeller and its blades. As the angle of attack of the aircraft increases, the propeller disc rotates towards the horizontal side. Here the airflow will meet the propeller disc at an increasing angle because the angle between the airplane wing and the airflow has increased.
The propeller blades that are moving down and forward in a clockwise rotation, will face a greater relative angle of attack and thus they will produce greater reaction force. While the propeller blades that are moving up and back will have a lesser relative wind and angle of attack and therefore, they will face a decrease in the amount of reaction force. This kind of asymmetry in the propeller tends to displace the center of thrust (reaction force) of the propeller disc towards the blade with an increased amount of thrust.
A helicopter has its own mechanism with the help of which it can reduce or increase the angle of attack of its individual blades of the rotor. There it tends to decrease the angle of attack of the front blades, and on the other hand, it increases the angle of attack on the blade of the backside in order to keep the lift of the disc balanced. This mechanism in a helicopter is of great use to it because if the blades of the rotor were unable to change their angle of attack independently then there would be a regular backward roll motion due to the increased lift on the side of the rotor disc.
There are also some aircraft in which the wings are fixed and they cannot adjust themselves independently. In such cases, there is usually no other way to somehow adjust the angle of attack of their individual blades of the propellers. So here the pilot of such an aircraft must struggle with P-Factor and use the rudder (same as the steering wheel in case of a car) to overcome the shift of this thrust.
The aircraft that has single engines and the situations where the aircraft is operating at high power and has a high angle of attack as in the case of a take-off, will cause a slight deviating (also called yawing) motion. The engine with the down-moving blades often produces more deviation and roll than the other engines. The asymmetric blade effect is entirely dependent upon thrust that is the reaction force of the upcoming air and is proportional to the forward velocity of the aeroplane. It is important to take into consideration that if a high angle of attack is used during the rotation, then the effect can also be direct. The effect is not so apparent during the time of landing of the airplane, and also during the flare and rollout of the aeroplane because of the relatively low power setting of the propeller.
IMPACT ON PROPELLER
As already stated, when a propeller is tilted backward means an aircraft is at a high angle of attack, the down-going blade will produce more reaction force than the upper going one. The thrust vector will then not act through the center of the propeller disc now and instead it will be shifted to the right or to the left. This entirely depends on which way the propeller is rotating. Thus, we can say that in the case of a right-hand tractor propeller the thrust vector will act on the right-hand side of the propeller disc and will deviate the aircraft to the left. In the case of a left-hand tractor propeller, the thrust vector will act on the left-hand side of the propeller disc and will divide the aircraft to the right.
The above-stated effect can be due to the two reasons, first the blade that is going down meets the relative airflow at a higher angle of attack than the blade that is going in the upper direction. Thus, there is more thrust on the right side due to a higher angle of attack.
Second, If the axis of the propeller is not in line with the direction of the flight of an aircraft then the path length of the blades passing through the air will vary. If a propeller is tilted towards the backside then the downgoing blade has a longer path to travel when compared to the blade going in the upper direction during the same time period. This means it has to travel faster. Thus, there will be more force on the right-hand side due to more speed.
Thus, we can clearly say that it is all a matter of the different scales of the forces. An aircraft propeller will always experience a force in its pitch as a result of the diverting torque produced by the thrust that acts unequally in different parts of the blade (asymmetric thrust).