Undercarriage
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The undercarriage or landing gear is the structure (usually wheels) that supports an aircraft when it is taxiing or stationary. The assembly usually has wheels and some sort of shock absorber apparatus, but sometimes skis for snow or floats for water, and skids or pontoons (helicopters). To decrease drag in flight the undercarriages on many aircraft, particularly large modern ones, retract behind doors which close flush with the fuselage.
A design for retractable landing gear was first seen in 1876 in plans for an amphibious monoplane designed by Frenchmen Alphonse Pénaud and Paul Gauchot . Aircraft with at least partially retractable landing gear did not appear until 1917, and it was not until the late 1920s and early 1930s when such aircraft became common. By then, aircraft performance was improved to the point where the aerodynamic advantage of a retractable undercarriage justified the added complexity and weight.
Wheeled undercarriages come in two types: either taildragger, where there are two main wheels towards the front of the aircraft and a single, much smaller, wheel or skid at the rear; or tricycle undercarriage where there are two main wheels (or wheel assemblies) under the wings and a third smaller wheel in the nose. Most modern aircraft have tricycle undercarriages or variants thereof. Taildraggers are considered harder to land and take off, and usually require special training. Sometimes a small tail wheel or skid is added to aircraft with tricycle undercarriage, in case the tail strikes the ground during take-off. The Concorde, for instance, had a retractable tail "bumper" wheel.
As aircraft grow larger, they employ more wheels to cope with the increasing weights. The Airbus A340-500/-600 has an additional four-wheel undercarriage bogie on the fuselage centreline. The Boeing 747 has five sets of wheels, a nose-wheel assembly and four sets of four-wheel bogies. A set is located under each wing, and two inner sets are located in the fuselage, a little rearward of the outer bogies. Tricycle undercarriage aircraft are usually steered by the leading wheel(s) when taxiing. On the Boeing 747 the two inner bogies, and on the Boeing 777 the last two wheels on each leg, are also steerable with the nose wheels in order to reduce the lateral stresses on the undercarriage.
Some planes use wheels only for take off and drop them afterwards to gain the improved streamlining without the complexity, weight and space requirements of a retraction mechanism. In this case, landing is achieved on skids or similar simple devices. Historical examples include the Messerschmitt Me 163 and the Messerschmitt Me 321.
Other examples of unusual undercarriage configuration include the Hawker-Siddeley Harrier, which has two mainwheels in line astern under the fuselage (called a bicycle or tandem layout) and a smaller wheel near the tip of each wing (on second generation Harriers, the wing is extended past the outrigger wheels to allow greater warloads to be carried). A multiple tandem layout was used on some military jet aircraft during the 1950s such as the Lockheed U-2, Myasishchev M-4, Yakovlev Yak-25, Yak-28 and the Boeing B-47 because it allows room for a large internal bay between the main wheels. A variation of the multi tandem layout is also used on the B-52 Stratofortress which has four main wheel bogies underneath the fuselage and a small outrigger wheel supporting each wing-tip. The B-52's landing gear is also unique in that all four pairs of main wheels can be steered. This allows the landing gear to line up with the runway and thus makes crosswind landings easier (using a technique called crab landing).
For light airplanes, a landing gear which is economical to produce is a simple wooden arch laminated from ash, as used on some homebuilt aircraft. A recent addition to this type of gear is the fixed-gear RJ.03 IBIS canard homebuilt aircraft.
