The airplane strut is part of the undercarriage, more commonly known as the landing gear. There are two main types – straight leg and trailing link – but their function is the same: to absorb the impact of the landing as the aircraft touches the ground. Each strut contains a shock absorber (a collection of springs), hydraulic oil and gasses which work together to reduce the impact felt by the passengers.
On some aircraft, such as those used by student pilots, the airplane strut is made entirely out of spring steel. This type of steel is treated in such a way that it can absorb the shock of landings repeatedly, bending automatically back into shape
Aircraft currently operating in the world’s civil use airports have been designed with various configurations of their landing gear. Most aircraft are designed with one of three basic landing gear configurations; the single-wheel configuration, defined as a main gear of having a total of two wheels, one on each strut, the dual-wheel configuration, defined as a main gear of having a total of four wheels, two on each strut, and the dual-tandem configuration, defined as two sets of wheels on each strut.
Planning and Design of Airports/ Fifth Edition