Endurance on the glide

The conclusion of the previous paragraph might perhaps lead one to ask whether, in that case, there is any need for a sailplane to be built of light con­struction. The answer is definitely – Yes. A sailplane (Fig. 6A, overleaf) must have a flat gliding angle if it is to get any distance, any range from its starting point; but, even more important, it must have a low rate of vertical descent or sinking speed; it must be able to stay a long time in the air and be able to take advantage of every breath of rising air, however slight. Sailplane pilots do sometimes add ballast so as to increase the flight speed as this can be useful under certain circumstances. However, a description of such advanced sailplane techniques is best left to books devoted specifically to that subject. It is easy to see that the rate of vertical descent depends both on the angle of glide and on the air speed during the glide. Therefore to get a low rate of descent we need a good lift/drag ratio, i. e. good aerodynamic design, and a low air speed, i. e. low weight.

Actually we shall get a lower rate of descent by reducing speed below that which gives the flattest glide; this is because we gain more by the lower air speed than we lose by the steeper glide. Thus there is an ‘endurance’ speed for gliding just as for level flight and, as before, it is lower than the range speed, and corresponds to the speed for minimum power requirement.