The landing gear, also known as the undercarriage, are the wheels upon which the aircraft maneuvers whilst it is on the ground. These were originally fixed in position and a practical retractable undercarriage did not appear until 1929 when one was fitted to a Dayton-Wright racer. Tricycle undercarriages (fitted with a nose wheel rather than a tail wheel) were widely used during the 1940's.
Tricycle undercarriages provide the pilot with better visibility during take-offs and landings, given a level fuselage which helps in loading and keeps hot exhaust gases away from the runway surface which avoids damaging it.
Bicycle undercarriages have the main wheels arranged in tandem and are fitted with outrigger stabilisers to give them strength. This design is utilised by aircraft with thin cross-sectional wings which are not capable of accommodating retracted wheels.
Today's high landing speeds and subsequently heavier planes require hydraulic systems that incorporate a long stroke, particularly with high angles of attack used by most airliners on landing.
Larger airplanes such as the Boeing 747 employ bicycle assemblies which contain up to sixteen main wheels to spread the weight of the plane on take-off and landing.
