An oleo strut is a pneumatic air–oil hydraulic shock absorber used on the landing gear of most large aircraft and many smaller ones. This design cushions the impacts of landing and damps out vertical oscillations. It is undesirable for an airplane to bounce on landing as it could lead to a loss of control, and the landing gear should not add to this tendency. A steel coil spring stores impact energy from landing and then releases it, while an oleo strut instead absorbs this energy, reducing bounce. As the strut compresses, the spring rate increases dramatically because the air is being compressed. The viscosity of the oil dampens the rebound movement. History and applications The original design for the oleo-pneumatic shock-absorbing strut was patented by British manufacturing conglomerate Vickers Armstrong during 1915. It had been derived from the recuperative gear design of the Vickers gun, controlling recoil by forcing oil through precisely sized orifices. Vickers' oleo strut was first applied to an aeroplane by the French aircraft company Breguet Aviation. The design proved to be viable and was extensively adopted across the aviation industry for fixed undercarriages, becoming simply referred to as an "Oleo unit" or undercarriage leg. However, Vickers' initial design had placed air above the oil, an arrangement that did not pose any problem until the introduction of retractable landing gear during the mid-1930s. The engineer Peter Thornhill devised a novel undercarriage strut that used a free-floating piston, not only being a lighter arrangement but enabling the whole strut to be inverted and to work while at an angle, eliminating the weakness of using an oil and air mixture. Oleo-pneumatic technology was subsequently reused by the manufacturer to produce several other products, including hydraulic railway buffers and industrial shock absorbers. During 1926, the Cleveland Pneumatic Tool Company designed and introduced its own oleo strut, one of the first to be purpose-designed for use on airplanes. The company subsequently marketed the product as an Aerol strut, which had entered widespread use within the United States within the space of a decade. By 1931, innovations in the field were being made in the United Kingdom, France, and North America. During subsequent decades, the oleo strut became commonly used for aviation purposes around the world. By the twenty-first century, a wide range of different shock-absorbing struts were in use, but typically employ common principles, despite considerable variations in size, weight, and