Gliding flight

Gliding flight is very similar to climbing flight – the main difference being that now we are going down rather than up! The operational requirements are sim­ilar too. We may wish to remain in the air for the maximum possible time, in which case we require the minimum possible rate of descent, or we may wish to travel as far as possible during the glide, in this case it is the minimum angle of glide which is required. Once more we shall find that these two requirements are distinct from each other and the pilot will obtain the minimum sink rate at a different flying speed to the speed at which the minimum sink angle is obtained.

Consideration of the above cases is also very similar to the counterpart in climb. This time, however, we do not have the complication of the engine performance to worry about.

If it is the minimum sink rate that concerns us we merely need to operate at the speed for minimum required power because in gliding flight this must be supplied by loss in the potential energy of the aircraft which is, of course, proportional to the rate of descent (Fig. 7.19).

The argument for minimum glide angle is very similar to that for maximum climb angle. In this case the weight component acting in the direction of flight must exactly balance the drag (Fig. 7.20). This component will be at its small­est when the drag is at a minimum and this condition will correspond to the minimum drag speed.

Weight component in flight direction (balanced by drag)

Fig. 7.20 Minimum glide angle

Minimum glide angle occurs at minimum drag since weight component in direction of motion will then be minimum