Through testing in the 1920s, researchers discovered that fixed landing gear was responsible for approximately 40 percent of fuselage drag and realized that in-flight performance would be greatly improved if this drag could be reduced.
Engineers have always had to wrestle with five requirements of aircraft design that are often at odds with one another: weight, performance, reliability, cost and maintenance. In the case of retractable landing gear, the idea was perfect for improving performance but it negatively affected the other factors.
The faster an aircraft travels, the more important it becomes to reduce drag, since this also increases with speed. As airplane speeds increased throughout the 1930s, the weight of retractable landing gear became less important; at speeds greater than 200 miles per hour, the benefit of retractable landing gear outweighs the cost.
Today's lower-speed private aircraft continue to have fixed landing gear because of reduced cost and ease of maintenance. However, some models also have what are called "pants" that cover the landing gear and reduce drag.
Airplanes were originally built with their landing gear fixed to the bottom of the plane. While airplane flight was still in its infancy, aircraft designers sought ways to decrease drag so they could improve speed, fuel efficiency, maneuverability and control.