My purpose is to try to place before you three broad main problems which cover numbers of subsidiary matters of great significance. Taking safety as read (and safety itself is not a problem but a necessity, begetting various problems in varying circumstances), my divisions of the subject are: —

1. 1. Reliability including regularity.

2. 2. Safety in the context of economy.

3. 3. Economy of operation.

In the past few years, two problems in applied aerodynamics have received much attention. The first is the problem of low drag in which it is desired to reduce considerably the drag of aerofoils over a certain range of lift coefficient by suitable design and construction. The second is the problem of increasing the lift in which it is desired principally to obtain a higher maximum lift coefficient.

Each time a new undercarriage design is laid down by an aeroplane or undercarriage designer, the geometry of the undercarriage retraction system nas to be investigated. This often involves a review of known solutions and since the designer cannot be expected to remember all these known solutions, the following notes have been prepared to put on record most of, and possibly all, the known solutions.

The basic linkage from which nearly all solutions are derived is the well-known fourbar linkage, as illustrated in Fig. 1. Links A and C rotate, link B is “floating,” while the airframe forms the fourth link.