To withstand the gravity of the aircraft on the ground, taxiing, take-off and landing; to withstand, consume and absorb the impact and bump energy of the aircraft during landing and ground movement; braking during taxiing and taxiing; maneuvering the aircraft during taxiing and taxiing . In the past, due to the low flight speed of the aircraft, the aerodynamic shape of the aircraft was not very strict, so the landing gear of the aircraft was fixed, so that no high technology was required for manufacturing. When the aircraft is flying in the air, the landing gear is still exposed outside the fuselage. As the flight speed of the aircraft continues to increase, the aircraft quickly crosses the obstacle of the speed of sound. As the resistance of the flight increases sharply with the increase of the flight speed, the exposed landing gear seriously affects the aerodynamic performance of the aircraft. It hinders the further increase of flight speed. Therefore, people have designed a retractable landing gear. When the aircraft is flying in the air, the landing gear is received inside the wing or fuselage to obtain good aerodynamic performance. The landing gear is placed when the aircraft is landing. . However, there are certain losses. The downside of this is that the total weight of the aircraft is increased due to the addition of a complicated retraction system to the landing gear. But in general it is more than lost, so modern aircraft, whether military or civil aircraft, most of their landing gear can be retracted, only a small number of ultralight aircraft still use a fixed form of landing gear ( Such as the bee series of ultralight aircraft).