Sadly, recent events continue to keep this subject to the fore — and memories of Dawsons Field, Entebbe, Mogadishu, Stansted, Beirut and Malta, to mention but a few of the most dramatic hijack incidents, are recalled, as are other events such as those of the early morning of 23rd June 1985 when one Boeing 747 of Air India was blown up in mid-air off the West Coast of Ireland, killing all 329 on board, with chance preventing the simultaneous destruction of another at Tokyo.

The seriousness of such incidents has, in recent months, grown from the mere appalling and tragic but occasional incident to one of major world importance having substantial effect on world peace and economic survival. The fact that this is no exaggeration is shown by the bombing raid carried out by the USA on Libya in April 1986, in retaliation for that country’s alleged involvement in or support of international terrorism. The subsequent reported mass cancellation of travel to Europe by Americans with consequential serious commercial implications not only for the airlines operating on the North Atlantic, where, for instance, British Airways reported a 25% drop in passenger traffic, is but one example of the effect of terrorism on the stability of world trade.

The longitudinal stability and control of a large receiver aircraft was considered during air-to-air refuelling. A simple horseshoe vortex was used to model the tanker wake and approximate expressions were derived for the additional aerodynamic derivatives due to the position and attitude of the receiver aircraft within the downwash field. These derivatives were shown to depend on the mean variation of downwash with vertical displacement at the receiver wing and tailplane. The mean downwash gradients, in turn, depend mainly on the vertical separation between the tanker and receiver aircraft and the ratio of the tanker-to-receiver aircraft wing spans. Solutions of the linearised equations of motion were obtained for a range of values of the downwash gradients. The large receiver aircraft, considered in the paper, typically exhibits two divergent modes which appear to be controlled in flight by frequent alternate movement of the elevators and engine throttle.

An axiomatic aerodynamic model has been developed for the general motion of a two dimensional aerofoil as it passes in and out of stall, which gives realistic unsteady loads as compared to experimental values. A non-linear set of aerodynamic derivatives with time delays have been derived from the axiomatic aerodynamics. ‘Actual’ and ‘predicted’ dynamic responses of an aerofoil, spring restrained in torsion, following an impulsive input show similar trends, including limit cycle oscillations, although there is a slight difference in frequency and a difference in the magnitude of the initial impulse required to trigger the limit cycle.