Transport aviation is expanding rapidly in America both because of improved business conditions and the excellent record of the transport companies. The new aeroplanes being laid down, both land transports and boats are, on the average, twice the size of their immediate predecessors measured in terms of gross weight. Never before in the history of our industry has there been such a large increase in size in such a short period (Fig. 1). Coupled with the demand for larger aeroplanes is the requirement for decidedly higher cruising speeds. To meet these demands the total thrust horse-power per aeroplane has been materially increased. Operating costs are becoming more and more important due to the competition among air lines, as well as between them and other forms of transportation. The engineer is consequently forced to give serious consideration to utilising the power available in the most efficient manner, as well as to reducing maintenance costs.