Sir George Cayley (1773–1857), a remarkable 19th century scientist, was the first to formulate the principle of powered flight when in 1809 he stated in .his paper ‘On Aerial Navigation’: ‘To make a surface support a given weight by the application of power to the resistance of the air’. More specifically, Cayley must be considered the founder of the science of aerodynamics, while also being the first to design and build and successfully fly gliders, thereby inaugurating the concept of the modern fixed-wing aircraft controlled by elevator and rudder.

The 757–200, which started its development in 1974, will fly for the first time early in 1982. When this aircraft enters airline service in January 1983, it will set new standards for quiet, fuel-efficient operation. The roots of the current 757 configuration go back to the 727–300 study programme. The design evolution proceeded through the 7N7 study programme which lasted from early 1976 through 1977, and developed into the 757–200 now in production (Fig. 1).

There is currently some interest in supersonic sea-skimming as a possible terminal phase for anti-ship guided weapons. The flight of a supersonic vehicle, close to a density interface, will produce an additional series of shock waves from the reflection of the shock waves generated by the vehicle from the interface. The angle of reflection of these shock waves will depend, interalia,on the slope of the surface upon which they are incident. Given a surface with a periodic positional displacement, such as would be produced by a train of surface waves, the variation in position of the reflected shocks, in relation to the generating vehicle, will also be periodic in nature. This coherent motion could produce periodically fluctuating forces and moments on the vehicle generating the incident shocks and these, in turn, could cause stability or control problems for the missile autopilot.

The ‘universal’ Strouhal number, SB, which was derived by Bearman using the Kronauer vortex stability criterion is presented for two-dimensional rectangular-section prisms of depth to width ratio, d/h, from 1 to 5, for a flat plate with and without splitter plates, for a circular cylinder with splitter plates and for a circular cylinder with two blowing slots to control its wake width. A value of SB close to Bearman's value of 0·181 is obtained for the prisms of small d/h, for the plate without a splitter plate, for the cylinder with small splitter plates and for the cylinder for low blowing rates. For the other cases, however, SB is found to vary within the range 0·137 to 0·208 which is an indication that the general application of this concept to two-dimensional bluff bodies is superficial.