A controllable pitch propeller is able to vary the pitch of airplane propeller blades from inside the cockpit. By adjusting a switch on the instrument panel, a pilot is able to change the pitch of the propeller blades to best suit any given situation. At start-up, the blades of the controllable pitch propeller are feathered, or set so they are neither pulling nor pushing the aircraft. This allows the engine to warm-up without placing a strain on either the engine or the braking system. Also, when encountering a strong head wind, the controllable pitch propeller can be changed to a more power-producing pitch to combat the added force that is fighting against the aircraft.
As the pitch or angle of the propeller is altered, the power-producing effects of the propeller are altered. The design of the propeller enable it to attack the air and grab it, much like the threads of a screw. This is why a ship's propeller is often referred to as a screw. Similar to how the threads of a screw grab the wood and pull the screw into the wood, the propeller blades grab the air and pull or push the aircraft forward. Changing the pitch of the propeller blades is similar to changing the threads of a screw from extremely coarse threads to very fine threads.
Like a finely threaded screw, changing the controllable pitch propeller to a certain pitch allows the propeller to spin many more revolutions to cover the same amount of air space. This gives the aircraft more power, but less speed. Conversely, changing the controllable pitch propeller to a pitch more like a coarsely threaded screw allows the engine to turn fewer revolutions while covering the same amount of air space. This gives the aircraft more speed, but less power. By manipulating the controllable pitch propeller, the pilot is able to adjust the pitch to best suit the aircraft and the weather conditions.
In multiple engine aircraft, the controllable pitch propeller allows the pilot to feather the propeller blades if an engine fails. This reduces drag from the non-running engine as well as keeps the disabled engine from being turned by the wind hitting the propeller blades. This also allows a pilot of a cargo plane to change the controllable pitch propeller to aid in takeoff when the plane is heavily loaded. As the plane reaches a cruising altitude, the propellers are adjusted to aid in cruising and to save fuel.