The propeller engine is generally powered by gas turbines and is known as the turboprop engine. Smaller craft and light planes are powered by conventional internal combustion engines.
The propeller or airscrew as it is also known is a screw like device used to propel the airplane. It obtains thrust by the large mass of air moving over it at low velocity. A typical propeller has two or more blades arranged around a central hub. They are positioned to form part of a spiral surface like a screw thread. They have an airfoil cross-section which means their leading edges are rounded and their trailing edges are sharp. Twisted along their length the propeller blades strike the air at a certain angle and as they rotate they develop thrust creating reduced pressure in front and increased pressure behind.
The angle of the propeller blades known as the pitch determines the speed of the plane through the air. A low pitch setting makes best use of the engines power on take-off and low speeds but a higher pitch setting is required for faster in-flight speed. Most propeller-driven planes have variable angle pitch blades, as a fixed pitch blade would only be efficient at one speed. Adjustment may be either manual or automatic as is more often the case. By altering the pitch the airflow can be reduced acting as an airbrake.
