Propellers can be thought of as a rotating wing. The propeller blades are shaped similar to an aerofoil and the rotation of the propeller creates airflow over the propeller blade. Just like a wing, the airflow over the propeller creates a pressure differential, with relatively higher pressure behind the propeller blade compared to the front. The result is a force similar to lift, but the force from the propeller acts forwards and is called thrust.
The propeller blade also experiences a force similar to the drag on a wing, but in the case of the propeller it is called torque. The torque has to be overcome by the engine’s power to keep the propeller rotating and to continue to create thrust.
In flight, the propeller is both rotating (because of the engine power) and traveling forwards (because of the aircraft’s forward airspeed). This results in the motion of the propeller being a combination of the two.
We can look at this motion as two vector components (remember those from the lesson on Forces?) One component is the velocity due to rotation and the other component is the forward velocity of the aircraft’s direction of flight. We can combine these two components into one resultant force – the propeller’s direction of movement in flight.
Just like the wing, the propeller’s relative airflow is opposite its direction of motion. The angle of attack (AoA) is the angle between the propeller blade’s chord line & this relative airflow.
The entire propeller rotates together at a certain rate (given in revolutions per minute or RPM), but the outer parts of the propeller move through a bigger distance, causing the tip to move at a faster speed. This means the propeller tip has a higher velocity component in the direction of rotation.
This change in velocity would normally lead to an increase in angle of attack. Since the propeller has an optimum angle of attack (just like the wing’s optimum AoA which gives the best lift/drag ratio), we want to maintain a constant AoA across the whole propeller blade. In order to achieve this, the propeller blade is twisted. At the propeller hub, the blade angle is much larger than at the tip. This is known as blade twist.