The propeller converts power from the engine into thrust to drive the airplane forward. A propeller may either be fixed pitch or variable pitch.
Constant Speed Propellers
With a variable pitch propeller, the pilot has a control that allows for the adjustment of propeller pitch from inside the cockpit. This allows the pilot to select the propeller blade angle which will result in the most efficient performance for a particular flight condition. In a "forward" position, the propeller has a smaller pitch and takes small bites of air each time it turns. In doing so, each turn of the propeller moves the airplane a smaller distance. This is similar to a low gear on a car. The forward position is used in times such as takeoff, climb out, and go-around.
The propeller may also be positioned at a higher angle so that it takes a bigger bite of air with each turn. At this higher pitch, the propeller moves the airplane a further distance with each turn. This is similar to the high gear on a car, and is typically used during cruise.
Intermediate propeller angle positions are used for climb.
A constant speed propeller is equipped with a propeller governor, which maintains a specific engine speed. As a result, the pilot cannot use engine speed to gauge engine power output. Instead, the pilot refers to the manifold pressure gauge, since manifold pressure indicates engine power output.
An important precaution concerning the operation of an airplane with a constant speed propeller is to avoid high manifold pressure setting at low engine speeds.
To user a constant speed propeller, the pilot uses the propeller control to select the engine's RPM. As engine power and propeller load vary, a device on the propeller, called a governor, changes the propeller's blade angle to maintain the desired engine RPM setting.
Fixed Pitch Propeller
A fixed pitch propeller is usually set to a pitch which is a compromise of climb and cruise performance. A fixed pitch propeller is simpler for the pilot to operate and cheaper. It is often not necessary to place a variable pitch propeller on a lower performance airplane.