Until recently an aeroplane undercarriage has generally been called upon to serve two purposes only :—

To absorb the energy due to the vertical velocity of the aircraft when landing;

To enable the aircraft to be flown off from, and manoeuvred over, ground which may be very rough or very soft.

To these requirements are now added a third :—

To absorb the energy due to the horizontal velocity of the aircraft by means of wheel brakes.

These three duties must be carried out with the minimum possible weight and head resistance, for the undercarriage must be as self-effacing in flight as it is all-important on the ground.

The problems connected with wheel brakes are very specialised in themselves and have a particular interest at the present time. For this reason it was decided to deal with them first, and later to turn to the more general aspects of undercarriage development.