Aircraft Propellers

Without a method of converting the power produced by the engine into useful thrust an aircraft would just be sitting still and creating a lot of noise and not get anywhere.

In the past, all kinds of different propeller design were used: sometimes flat wooden blades, from one bladed to multiple blade models rotated at various speeds.

Until it was realised that a propeller is just like a wing but instead of carrying the aircraft it displaces a volume of air backwards. Albeit slowly, compared to a pure JET engine, which displaces a small volume of air at high speeds.

The same aerodynamic laws that apply to wings are valid for propellers too only not horizontal but in a vertical motion and so thrust (lift) is created and the aircraft is able to move forward, after reaching liftoff speed it will get airborne.

These pages shine a small light on propeller design, some aerodynamics and operation of controllable propellers during preflight, cruise and the influence on fuel consumption.

Basic Design

Propellers are usually constructed of wood, aluminum alloy or a combination of composites and wood. The leading edge is sometimes reinforced with nickel for protection against rain, sand and stones. They come in two, three or more blades depending of the application. Some high end turbo prop aircraft even have six blades to absorb the power. But, more blades means more weight and more propeller torque, the engine must be able to handle this.

The function of the propeller is to convert brake horse power from the engine into thrust. To do this there are two types of propellers in use: fixed pitch (or ground adjustable, which remains a fixed pitch during flight) and the constant speed propeller with some variations possible.

Blade angle

Looking at a propeller, its blade angle varies from the root to the tip. This is because angular speed of the blade is at it highest at the tip (reaching the speed of sound) and lowest at the root. If the blade angle would be constant then the angle of attack (and thrust) of the relative airflow (result of forward speed and RPM) would vary across the propeller disc, and the blade would probably be stalled. To make sure that the generated thrust is equal from root to tip, the blade angle is high at the root (low angular speed) and low at the tip (high angular speed).

Propeller thrust

During propeller rotation the airflow past the propeller blade produces an aerodynamic reaction with can be resolved into thrust and propeller torque (called lift with wings). To rotate the propeller the engine needs to create a torque. The resistance to this rotation is called propeller torque and when these two forces are in balance or stabilized, engine/propeller RPM is constant. Thus power created by the engine is absorbed by the propeller and as a result thrust is generated.

Fixed pitch

With this propeller the blades are fixed and cannot move (except rotate), thus propeller blade angle is fixed (but the blade angle changes from the root to the tip as explained above). It can be seen that there is also one RPM / airspeed combination where the propeller is operating at its optimum angle of attack and produces its maximum thrust. On all other airspeeds (or conditions of flight) there is less thrust available from the propeller. In other words, its effective angle of attack depends on RPM and airspeed.

To compensate, manufacturers sometimes have a climb and a cruise propeller available for the same model or type. Although convenient and simple to operate, a fixed pitch propeller will always be a compromise between a number of factors as RPM, airspeed, relative airflow, angle of attack, two or three blades, blade chord and length, emitted sound level etc...

Constant speed

To overcome all these disadvantages the constant speed (or variable pitch) propeller is able to adjust its blade angle. Thereby operating closer or at its optimum angle of attack over a wide range of RPM and airspeed combinations. The result is that the maximum amount of thrust is obtained from standstill to maximum speed (V_NE) or from minimum to maximum propeller pitch angle. A shorter takeoff roll and much better climb performance may be expected and a lower specific fuel consumption is also one of the advantages.

Propeller control

With a fixed pitch propeller the pilot has only one control, throttle, to control power and RPM. With a constant speed propeller there are two controls: power (throttle, black knob) and RPM (propeller, blue knob). In addition there should be a MAP indicator (manifold pressure) which relates to engine power. Moving the propeller control (pitch) changes the RPM at which the engine and propeller will rotate and moving the throttle changes the amount of power delivered to the propeller at the preset RPM held constant by the propeller governor.

An aircraft with a constant speed propeller is bit more complex and is not usually used for basic (ab initio) flight training. Those training for higher licences (CPL, ATPL) will need to understand and properly operate these more complex aircraft.