A Critical Study of Aircraft Landing Gears

Abstract

This report contains some of the results of a continuing study of the behavior of aircraft landing gears. This study was prompted by the general awareness tha t landing gears are a principal source of accidents and troubles throughout the aviation industry and by the desire to determine, if possible, technological advancement for future aircraft. Some of the findings to date, indeed, appear of sufficient importance to warrant presentation at this time. Factual data are presented showing that , exclusive of accidents due to crew error, weather, or unknown causes, landinggear failures have been consistently responsible for more commercial and military accidents than all the rest of the structure combined, that landing-gear structural failures are predominantly from drag loads, that this is in spite of steady increases in design loads, and tha t a large number of these failures have been in landings. Some statistical data are available to show that design conditions are amply severe and, in fact, are seldom realized perhaps even in hard landings. Strength diagrams show that landing-gear strengths on comparable aircraft are about equivalent and are all very high. Likewise, behavior characteristics are shown to be similar. In a critical search for an underlying cause of excessive loads in apparently normal landings with aircraft of all types, Lockheed Constellation experience is utilized. Various possible explanations, such as the general strength level, fatigue, shock strut characteristics, extreme ground friction, and structural deflections, are considered and found to aggravate and contribute to the problem but fail to explain service failures. The fundamental cause is deduced to be dynamic overloading of landing gears resulting from critical rates of application of wheel spin-up drag forces, which critical rates have a random probability of occurrence on any aircraft unless the motion is suitably damped. Also, at least one source of self-excited vibration arising from skids on wet runways is found to exist. Some possible solutions are discussed, and hydraulic damping of horizontal wheel motion is advocated. Typical test results show that such damping does indeed offer a most effective solution.