A. The landing gear (or undercarriage) is intended to support the airplane in proper location for take-off and landing and to provide the shock absorption. The shock is usually absorbed by a sort of pneumatic tyres and shock-absorbing struts. The landing gear usually consists of a pair of wheels carried either from the fuselage or from the wings by a framework of hollow tubes called struts. In addition to these main wheels a support is needed at the rear of a machine. This is a tail wheel (or skid) carried on a swivelling mounting.
B. Two different arrangements of landing wheels are in use today. They are conventional tricycle gears and the landing gear with a skid.
C. The first, the tricycle type, has the main wheels mounted slightly aft of the centre of gravity and the third wheel (the nose wheel) in front. The second type comprises two main wheels located slightly forward of the airplane's centre of gravity and a tail skid at the rear.
D. The tricycle landing gear of the aircraft consists of one nose leg and two main legs. The nose leg is mounted under the nose section of the fuselage. The main legs are installed under the wing or the fuselage symmetrically with respect to its centre line. Tricycle gear has many advantages. It simplifies landing, eliminates the danger of nosing over and carries the airplane in normal take-off position. It permits an airplane to land and come to rest within a shorter distance.
E. Consequently, it is the rule today to employ retractable landing gear which can be drawn up (or retracted) in flight into the wing or fuselage structure. Most high-speed airplanes have retractable landing gears. The retracting mechanism may be either mechanical, powered by electric motors, or hydraulic. Various linkages are employed to perform the retraction of wheels and struts into the fuselage, wing or nacelles.
F. After take-off the nose leg is retracted into the well provided in the fuselage and the main legs are retracted into the well of special nacelles. The landing gear legs have oleo-pneumatic shock absorbers. The shock absorber comprises an outer steel tube with a welded top head which attaches a plunger.
G. The landing wheels are fitted with large diameter low-pressure tyres which allow the airplane to taxi over rough ground and also assist in absorbing the shock of landing. The landing gear is designed to withstand the loads imposed by rough landings and fast taxing. It must also carry the braking loads in a fully braked landing.
H. The design of the tail wheel is similar to that of the main legs and usually consists of a single oleo unit. The tail wheel may be of the conductor type. When it is resting on the ground it provides an electrical earth contact and so prevents the aircraft and crew from damage through static electrical charges.
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