Theorie Basiskurs

Some aircraft without landing gear actually exist, they have slide rails at the fuselage, but the vast majority of airplanes have landing gear with wheels.  

The landing gear allows gentle acceleration, because nothing slides, grinds or glides over the ground.  In addition, brakes are installed in the chassis which function like in a car and thus help shorten the landing distance.  The landing gear also supports the aircraft on the ground the necessary distance so the propeller does not touch the ground.  

Landing gear can be rigid or retractable.  A rigid chassis is less complicated, more robust and less expensive to manufacture and maintain.  However, it also means a significantly higher air resistance, which has a negative effect on speed, fuel consumption and thus on the range.  While rigid landing gear was used in the First World War, retractable landing gear were introduced in the Second World War.  

The vast majority of landing gears are so-called three-point undercarriages, where one wheel each is mounted to the right and left in the middle of the aircraft and one wheel either at the very back or at the very front in the centre.  Just as a three-legged stool cannot tilt, a three-legged gear ensures that the aircraft is stable and evenly loaded during landing.