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(Also propellor). A propeller is not only used on boats! Propellers were used to, er, propel the earliest airplanes and continue to push and pull craft both heavier and lighter than air today. The propeller allows lower cost, more efficient engines than turbojets or turbofans to be used. Some airplanes and many blimps actually use electric motors to spin their propellers.

An aircraft propeller (also called airscrew in early parlance in homage to the nautical origin of the word) is typically a long, flat-bladed device. It operates in exactly the same manner as a boat propeller, but the differing properties of air vs. water make its shape dramatically different for efficiency. Aircraft propellers come in two basic types; fixed-pitch and variable-pitch. A fixed-pitch propeller, which is essentially a solid piece of material such as composite, metal or wood, is lower inperformance but makes the mechanical workings of the airplane very simple. On the other hand, the speed of the propeller will vary with engine speed, and since the 'most efficient' shape will change depending on the rotational speed and the speed of the propeller through the air, it means that a fixed-pitch propeller is not often operating at its most efficient.

A variable-pitch propeller has the ability to change the angle of attack or pitch of its blades, usually though mechanisms in the hub at the center. Variable-pitch propellers are much more efficient than fixed-pitch, but add yet another dimension of control to deal with while flying the airplane. As an added advantage, however, most variable-pitch airplanes can rotate their blades far enough to produce some measure of reverse thrust, which is useful when taxiing on the ground.

As mentioned earlier, propellers are not always attached to piston engines. Modern aircraft use the more efficient turbo-prop, and some few airplanes (and many lighter-than-air vessels) use electric motors. They have also been fitted both forwards and backwards; aircraft with propellers mounted behind the engine are typically called 'pusher prop' in a triumph of literalness. One reason to do this is that with pusher props, you can mount two engines and two propellers which both on the airplane's midline. Multi-engine airplanes with their props mounted inline like this are much easier to handle if one engine fails than the more widely-seen 'twin engine' with the propellers to either side of the centerline. In that case, loss of an engine means an immediate off-center thrust, and even in regular operations both engines must be carefully adjusted to even power in order to avoid stressing the airframe or destabilizing the airplane.