In this chapter are described some of the trends in aerodynamic design which in the latter part of the twentieth century and the beginning of the twenty-first century are making the helicopter a considerably more efficient flying vehicle than it formerly was. In earlier years the low power-to-weight ratio of piston engines necessitated the use of large rotors to provide the all-important vertical lift capability: both profile drag and parasite drag were unavoidably high in consequence and forward speeds were therefore so low as to consign the problems of refining either the lift or drag performance to a low, even zero, priority. With the adoption of gas-turbine engines, and an ever-increasing list of useful and important applications for helicopters, in both military and civil fields of exploitation, forward flight performance has become a more lively issue, even to the point of encouraging comparisons with fixed-wing aircraft in certain specialized contexts (an example is given in Chapter 7). Some improvements in aerodynamics stem essentially and naturally from fixed-wing practice. A stage has now been reached at which these appear to be approaching and, in certain areas, to have arrived at, optimum levels in the helicopter application and therefore a substantial description here is appropriate. Further enhancements, concerned with the fundamental nature of the rotor system, may yet emerge to full development: one such is the use ...