The problem of landing an aeroplane under conditions of low visibility has been a subject for speculation for the last twenty years, but it is only within the last ten years that it has been given any serious practical attention. This practical attention has been forced by the necessity for aircraft to maintain schedule regardless of weather conditions in competition with other forms of transport.

Although certain schemes involving radio were amongst the earliest proposals for a solution of the problem, the first practical work appears to have been concerned with methods of a more mechanical or optical nature, including a proposal to deposit the fog by high tension discharge.

The object of the present paper is to draw attention to some of the recent researches which have had as object the improvement of the aerodynamic characteristics of aircraft. Such characteristics can be classified into two groups; those concerned with performance and those concerned with stability and control. In the former group come all those researches which aim at the reduction of drag, while in the latter group come those directed to the attainment of satisfactory stability and control in normal flight, as well as those concerned with the behaviour of auxiliary devices such as flaps and slots which are used under special conditions and in particular during landing and taking-off. The problems associated with flutter also belong to this group. The two groups may be broadly characterised by the words economy and safety.

Theoretical critical stresses have been obtained for four cases in each of which a curved panel has been subjected to a compressive stress, parallel in direction to a generator and uniformly distributed around its top and bottom circumferential edges.