Effect of weight on gliding

It is commonly thought that heavy aeroplanes should glide more steeply that light aeroplanes, but a moment’s reflection will make one realise that this is not so, since the gliding angle depends on the ratio of lift to drag, which is quite independent of the weight. Neither in principle nor in fact does weight have an appreciable influ­ence on the gliding angle, but what it does affect is the air speed during the glide.

Look back for a moment at Fig. 6.1. Imagine an increase in the line repre­senting the weight; there will need to be a corresponding increase in the total aerodynamic force, and a greater lift and a greater drag. But the proportions will all remain exactly the same, the same lift/drag ratio, the same gliding angle. But the greater lift, and greater drag, can only be got by greater speed. If we now think back to flying for range, it will be remembered that the con­dition was the same: greater weight meant greater speed. But there is an interesting and important difference in this case. In flying for range, greater speed meant greater drag, greater thrust, and so less range. In gliding without engine power, greater speed means greater drag, but now the ‘thrust’ is pro­vided by the component of the weight which acts along the gliding path and this, of course, is automatically greater because the weight is greater. So greater weight does not affect the gliding angle and does not affect the range, on a pure glide – but it does affect the speed.