Flying, in common with all means of transport, is affected by adverse weather conditions, but the necessity of aeroplanes to maintain flying speed introduces a major difficulty of its own. The older forms of transport are able, in the last resort, to evade their difficulties by coming to a dead stop. An aeroplane must, literally, fly in the face of its difficulties. It must fly blind in clouds and perhaps land in fog. Over and above this, flight under certain meteorological conditions introduces a danger unique to aircraft. Ice may deposit at all leading edges and grow to windward, at critical regions of the relative airflow, in shapes which increase drag and seriously decrease lift. The accumulated ice adds to the weight. Unsymmetrical ice deposits on the airscrew blades cause dangerous engine vibrations which can only be kept in check, if at all, by throttling back at the expense of thrust. Venturis and pressure head orifices become blocked with ice, rendering the instruments they serve useless. External controls may become jammed. In short, many adverse factors to prevent flight may be brought into play simultaneously by the mere fact that particular meteorological conditions have been encountered.

The recently issued Report of the Aeronautical Research Committee draws attention to the fact that much work, apparently of academic interest only, is now of direct practical utility. In particular, the theories of the boundary layer and of turbulence, though admittedly very incomplete and uncertain and lacking a secure scientific foundation, have been developed to the stage in which they can be applied to explain and predict the behaviour of aircraft, and to show how aircraft may be improved. The physical structure of the boundary layer will be described, and the methods by which the problems have been attacked mathematically to get useful results and to check the theories.

In conclusion, the theory of the boundary layer will be applied to the calculation of the drag of an airship or aeroplane fuselage under full-scale conditions, and to an explanation of the behaviour and performance of different types of aerofoils.