Propellers (021.10.09)

The purpose of a propeller is to convert engine power into thrust, by accelerating a large mass of air to low speeds.
In order to produce this thrust, a propeller blade is cambered and can be considered as a small rotating wing producing lift (total reaction force).

The total reaction force on the thrust face (cambered side of the the blade) can be split into two components:

1. Thrust: acting at 90° to the plane of rotation;
2. Torque:acting parallel to the plane of rotation;

If the propellor torque (drag) is equal to the engine torque, then the propellor speed is constant. (If less, its speed would increase, if higher the engine rotation speed would decrease).

Angle of attack (α):	the angle between the chord of the blade and the relative airflow.

When considering a fixed pitch propeller (so not adjustable), the angle of attack (α) is dependent on both the forward speed (TAS) and the engine RPM (blade’s rotation speed).

With a constant rotation speed the angle of attack (α) will be large at a low TAS, and subsequently small at a high TAS.

The same applies considering the blade’s rotation speed. As the rotational velocity on any part of the blade is a function of the radius of the propeller, the rotational velocity is larger at the tip of the blade than it is at its root.

So, if the blade would be equally shaped from root to tip, the angle of attack would be much smaller at the tip, than it would be at the root. This would obviously result in the production of unequal thrust along the radius of the propeller.

In order to equalize the thrust from root to tip, the blade of a propeller is twisted.

At a low forward speed the blade / pitch angle will be large (coarse).

At a high forward speed the blade / pitch angle will be small (fine).

Blade / pitch angle:	the angle between the chord of the blade and the plane of rotation.
Coarse pitch:	large angle between the chord and the plane of rotation.
Fine pitch:	small angle between the chord and the plane of rotation.