Landing transition paths which optimize fuel, time or distance for jet-lift VTOL transport aircraft in steep approaches

Abstract

Optimal landing transition manoeuvres are studied for jet-lift VTOL aircraft in which the range of engine-tilt is sufficient to allow approach angles up to 20 degrees. The optimal use of incidence, thrust vector angle and thrust to control the transition is studied using simple physical arguments. All the natural constraints such as incidence, engine tilt, maximum permissible decelerations, minimum permissible lift engine thrust, etc., can thus be taken into account and simple rules formulated for the optimal strategy. It is shown that provided control programs are formulated as functions of aircraft speed, flight path shape and the control program can be dealt with as separate problems. It is known that for minimum distance transitions the optimum path shape involves selecting a let-down speed V* at which the deceleration function for horizontal flight is a maximum. The optimal manoeuvre is then to decelerate horizontally to V*, to lose height at this speed and to conclude the transition with a further horizontal deceleration to the hover point (i.e., the 'stepped manoeuvre'). However, for the minimum fuel or time transition the ideal is to use the same stepped manoeuvre but to select the let-down speed as high as is compatible with considerations of the danger of high rates of descent near the ground and the need to avoid obstacles near the airfield.