Landing an aircraft is a difficult and stressful procedure for parts, as they deal with the weight of the aircraft and the friction of landing. Therefore, landing gear struts that support the aircraft must be resilient and durable. There are four main types of landing gear struts, all of them designed to take the shock out of landing.
Rigid struts were the first type of landing gear struts invented. Their execution was simple at first: simply weld the wheels to the airframe. The problem with this approach, however, is that a hard landing transfers an immense amount of shock into the airframe, which can damage parts of the aircraft and is extremely uncomfortable for pilots and passengers. Inflatable tires helped soften the impact load, but other forms of wheel struts soon became more popular. They are still frequently used on helicopters, however.
Spring wheel struts are common on smaller aircraft like Cessnas. Using strong, flexible materials like steel, aluminum, and composites, they help absorb the impact of landing by flexing and bending, transferring the impact load into the airframe at an easier rate.
Bungie cords are often used on tailwheel and backcountry aircraft like the Piper Cub. Bungie cords are just as their name implies, a series of elastic cords wrapped around the airframe and flexible gear system, which allows the gear to transfer impact load to the aircraft at a rate that doesn’t hurt the fuselage. Some use a donut-type rubber cushion, while others use lots of individual strands of elastic material to dissipate the shock.
The last type is the only type that is a true shock absorber. Shock struts, or oleo or air/oil struts, use a combination of nitrogen (or sometimes compressed air) and hydraulic fluid to absorb and dissipate shock loads on landing. They are most often used on larger aircraft, like commercial airliners and business jets.
Shock struts use two telescoping cylinders, both closed at the external ends. The top cylinder is attached to the aircraft, and the bottom is attached to the adapter landing gear, called the piston. The piston can freely slide in and out of the upper cylinder. The bottom cylinder is filled with hydraulic fluid, and the top is filled with nitrogen, with a small hole connecting the two. As the aircraft lands, pressure from the wheels touching the ground, forces hydraulic fluid up through the orifice and into the nitrogen-filled top chamber. The kinetic energy is transferred into thermal energy, and the shock of landing is absorbed.
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