It is shown that the trailing vortex wakes generated by large aeroplanes can cause serious upsets (loss of control) to smaller following aircraft that may encounter them and may damage buildings near an airport. The increase in separation behind big aircraft imposed as a safety measure leads to a serious loss of runway capacity.

After discussion of the nature of wake vortices, it is demonstrated that they may be attenuated, so avoiding unnecessary separation between aircraft at an airport and, at the same time, obviating ground damage.

Discussion of alternative devices available for moderating the vortex wake indicates that practical solutions may be confidently expected.

Occasionally, in the midst of satisfying customer requirements, we have to assess whether or not we are really making progress in terms of achieving more effective helicopters. We should identify both the areas which offer the greatest potential for improvement and those in which improvement is the most required. Then, we must decide on the amount of effort we will put into improving these areas.

The major helicopter development activities at Boeing today centre on three programmes: the CH-47 Medium Lift helicopter (MLH), the YUH-61A Utility Tactical Transport Aircraft System (UTTAS), and the XCH-62 Heavy Lift (HLH) helicopter.

The CH-47 Chinook helicopter was introduced into the US Army inventory in 1962 and has had a history of dynamic engineering development. The initial CH-47 A helicopter had a maximum gross weight of 33 000 lb; the CH-47B, introduced in 1967, had a maximum weight of 40 000 lb; the CH-47C (1968) had a maximum weight of 46 000 lb; and the Canadian Forces CH-147 (1974), a maximum weight of 50 000 lb. Our current development efforts are aimed at modernisation of early aircraft to equip the US Army fleet for operation through the 1980s.

A simple rational fraction is derived which gives the Mach number with an error of less than 0·05 per cent for a given value of the Prandtl-Meyer function.