a device for converting the energy of a natural source or of a mechanical motor to useful work, causing a vehicle to move.
The wheel is the basic form of propelling device in land transportation. The wheel interacts with a road surface (as in automobiles and motorcycles) or with a rail track (as in streetcars and locomotives). The caterpillar drive is used in moving a vehicle across soft ground and in absence of roads (as in tractors, tanks, and snow-going vehicles). The movement of large masses across small distances can be accomplished by step drives (as in an excavator).
The sail is a propelling device for converting wind energy. The energy of ascending air currents is utilized by the soaring planes of a glider or by the flexible canopy of a parachute. Propulsion through air or water and on land is achieved by the use of a propeller (as in airplanes, helicopters, airships, aerosleighs, planing boats, and hovercrafts). Ships are propelled by oars, propellers, and paddle wheels, by rotating-blade propellers, and by water-jet propellers. The reaction jet is used widely in air travel and in space travel.
A. A. PARKHOMENKO