Landing gear

(aerospace engineering) Those components of an aircraft or spacecraft that support and provide mobility for the craft on land, water, or other surface.
(mechanical engineering) A pair of small wheels at the forward end of a semitrailer to support the vehicle when it is detached from the tractor.

That portion of an aircraft consisting of the wheels, tires, brakes, energy absorption mechanism, and drag brace. The landing gear is also referred to as the aircraft undercarriage. Additional components attached to and functioning with the landing gear may include retracting mechanisms, steering devices, shimmy dampers, and door panels.

The landing gear supports the aircraft on the ground and provides a means of moving it. It also serves as the primary means of absorbing the large amounts of energy developed in the transition from flight to ground roll during a landing approach. The brakes, normally located in the main wheels, are used to retard the forward motion of the aircraft on the ground and may provide some control in the steering of the aircraft. In most modern aircraft the landing gear is designed to retract into the aircraft so that it is out of the airstream and drag is thus reduced.

Main-wheel bogie for the XB-70A aircraft. (After S. Pace, North American Valkyrie XB-70A, Aero Series vol. 30, Tab Books, 1984)

Early aircraft and many small aircraft use a tail-wheel (or skid) in a conventional, or tail-dragger arrangement, in which the main landing gear is located ahead or forward of the center of gravity of the aircraft. The popular arrangement on modern aircraft is a tricycle landing gear, with the main gear located behind or aft of the center of gravity, and a nose gear located forward which carries about 20% of the static weight of the aircraft. Large aircraft such as the wide-body commercial aircraft and military aircraft like the C-5A employ multiple-wheeled bogies to support their huge weight and, in the case of the C-5A, to provide soft terrain landing and takeoff capability. See also Military aircraft.

The most accepted method of absorbing the energy due to landing is an air-oil strut called an oleo. The basic components are an outer cylinder which contains the air-oil mixture and an inner piston that compresses the oil through an orifice. The flow of oil through the orifice is metered by a variable-diameter pin that passes through the orifice as the gear strokes. The flow of oil in effect varies the stiffness of the compression of the gear.