In the past, due to the low speed of the airplane, not very strict with the aerodynamic shape of the plane, so the plane's landing gear consists of a fixed frame and wheels, for manufacturing, so do not need to have a high level of technology. When the aircraft is flying, landing gear still exposed outside the fuselage. With the improvement of aircraft flight velocity, the plane soon crossed the sound barrier, due to resistance to flying as flight speed increases dramatically increases, then, exposed gear would seriously affect the aircraft's aerodynamic performance, hindering the further increase in speed.
Therefore, people designed retractable landing gear as the plane flew in the air as soon as it receives the landing gear within the wings or fuselage, for good aerodynamic performance, then put down the landing gear when the plane landed. However, there is gain and loss, the deficiencies of this is due to the landing gear to increase the complexity of the operating system, makes the aircraft's total weight. But on the whole is greater than the loss, so modern aircraft, whether military or civil aircraft, which most of the landing gear can be retracted, only a small proportion of ultra light aircraft with fixed landing gear (such as bee series ultralight aircraft).