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Variable pitch propellers allow a pilot to adjust the position of the propellers and engine speed for any situation while flying, maintaining the most efficient engine thrust. The basic design of variable pitch propellers serves different purposes. The constant speed propeller lets the pilot choose the revolutions per minute (RPM) desired. RPM is maintained by the angle, or pitch, of the propeller blades during flight. A full-feathering propeller permits one propeller to achieve a very high pitch when an engine fails on a two-engine aircraft.
Most propeller airplanes employ variable pitch propellers. A pilot can “feather” the propellers while on the ground to prepare for take-off without creating thrust. Once in the air, and as the pilot increases power, the angle of the blade adjusts to the RPM setting. At cruising altitude, the pitch of the propeller can be decreased while maintaining the desired RPM. Both features improve the aircraft’s performance and efficiency.
Controllable propellers also enable ships at sea to conserve fuel. Variable pitch propellers increase maneuverability of the vessel, whether it is moving forward or in reverse. The position of the blades can be adjusted for the weight of the load carried on the ship, unlike a fixed pitch propeller.
The first idea for a variable pitch propeller arose in France before the Wright brothers ever took flight in 1903. Several airplane designers recognized the advantage of a propeller that could change engine thrust without changing the speed of the propeller itself. Early variable pitch propeller systems improved aircraft performance, but the benefit was not significant enough to generate widespread interest in the invention.
It wasn’t until 1910 that the first variable pitch propeller was used on airships to permit rapid decrease in speed and reversal of motion. Although the design was suitable for ships, the technology was not considered safe enough for airplanes. At the time, fixed propellers served adequately because airplanes flew at relatively low speeds. As improved airplane design increased air speed and altitude, the demand for variable pitch propellers grew.
World War II accelerated the demand. Engineers from several countries began perfecting propeller design for use on military and commercial aircraft. In the early 1930s, variable pitch propellers were installed on the first Boeing 247 commercial passenger plane. Improvements to the craft’s takeoff, cruising, and climbing rate were so impressive that variable pitch propellers were added to airplanes internationally. The angle of the propellers is changed hydraulically to maintain desired RPM during flight.