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 designs were tried: sometimes flat wooden blades, from one bladed counterweighted to multiple blade models rotating at various speeds.
Until it was realized that a propeller is just like a wing creating lift. But instead of carrying the aircraft it displaces a large volume of air backwards. Albeit slowly, compared to a pure JET engine, which displaces a small volume of air at very high speeds.
The aerodynamic laws that apply to wings are valid for propellers too, only not in a horizontal plane but in a vertical motion and so thrust (horizontal lift) is created and the aircraft is able to move forward, and after reaching lift-off speed it will get airborne.
These pages shine a small light on propeller design, explains some of the aerodynamics and discusses operation of controllable propellers during preflight, takeoff and cruise and the influence on fuel consumption.
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 small stones. They come in two, three or more blades depending on the application and engine power. Some high-end turboprop aircraft even have six blades to absorb the power. But, more blades means more weight and more propeller torque (drag) and the engine must be able to handle this.
The function of the propeller is to convert brake horse power from the engine into useful thrust. To do this there are two types of propellers in use: fixed pitch (or ground adjustable, which remains at a preset fixed pitch during flight) and the constant speed propeller with some variations possible in the way they are controlled.
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 of the relative airflow (result of forward speed and RPM) would vary across the propeller disc (and the thrust too), and the blade would probably be stalled. To make sure that the generated thrust is somewhat equal from root to tip, the blade angle varies from high at the root (low angular speed) and low at the tip (high angular speed).
During propeller rotation the airflow past the propeller blade produces an aerodynamic reaction with can be resolved into thrust (with wings we call this lift) and propeller torque (drag). To rotate the propeller the engine needs to create a force, torque. The resistance to this rotation is called propeller torque and when these two forces are in balance or stabilized, engine/propeller RPM is also constant. Thus power created by the engine is absorbed by the propeller and as a result thrust is generated.
With this type the blades are fixed and cannot move (except rotate), thus propeller blade angle is fixed (but the blade angle still 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 the most efficient and 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 most effective angle of attack depends on RPM and airspeed.
To compensate, manufacturers sometimes offer a climb and a cruise propeller available for the same model or type so that operators may choose the best product for their day to day flying.
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... You get the picture.
To overcome all these disadvantages the constant speed (or variable pitch) propeller is able to adjust its blade angle. Thereby operating closer to 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 aircraft standstill to its maximum speed (VNE) or from minimum to maximum propeller pitch/blade 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.
These propellers are, as expected, more expensive in terms of acquisition cost and maintenance. But should you require maximum performance from your aircraft in most circumstances, then this is the way to go.
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 (MAP) to the propeller at the preset RPM held constant by the propeller governor.
An aircraft with a constant speed propeller is usually a bit more complex and is not normally used for basic (ab initio) flight training. Those training for higher licenses (CPL, ATPL) will need to understand and properly operate these more complex aircraft with higher powered engines.