How are propellers used in aircraft?
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Propellers on airplanes generally have from two to six blades. These are usually made of wood, aluminum alloy, steel, or composite materials. At first, all were of fixed pitch, i.e., the angle of the blades was not variable. Later, advantages in speed and power brought variable-pitch propellers into general use; their blades are set into sockets in the hub with gear arrangements capable of altering the pitch in flight. The development of automatic equipment to alter the pitch as needed for maintaining a predetermined speed produced the constant-speed propeller. Variable-pitch propellers generally take the name of the pitch-controlling device; the principal types are hydraulic, mechanical, automatic, and electric. With modifications they can also act as air brakes. When the number of blades was increased from two to three, then from three to six, to achieve greater thrust or propulsion or to keep the blade size down, new stress problems arose. These were met by the development of contrarotating propellers, in which the blades were arranged as two separate three-bladed units rotating in opposite directions.
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