412 REPORTS ON THE STATE OF SCIENCE, ETC. 
increase in the angle does not increase, but decreases, the lift. The wing 
is then said to ‘stall’, and the limiting angle is known as ‘stalling 
incidence ’. 
The business of the elevators in an aeroplane is to control the angle 
at which the wings are inclined to the relative wind: when we set the 
elevators ‘up’, the tail is pushed down-by the wind, but the resulting 
change in the attitude of the wings causes a net increase of ‘lift’, and 
the aeroplane ‘climbs’. But if a pilot tries to climb too rapidly, he will 
‘stall’ his wings. Down goes his nose, and he begins to descend, still 
with his wings at a large angle to the relative wind. 
© Autorotation ’. 
33. Now comes in the tendency to ‘ autorotation ’, which I can explain 
with the aid of this apparatus (Fig. 1). Here is a model wing which 
I can expose to a horizontal jet of air; the purpose of the honeycomb is to 
straighten the jet, which otherwise would have a swirl in it, picked up 
from the fan. Suppose that I set the wing at a small angle to the wind- 
stream, and giveitaspin. You will realise that the side which is travelling 
down to meet the wind is meeting it at a greater angle than before; the 
side which is travelling up, at a smaller angle. Hence the lift is greater 
on the downward moving side, and the rotation is quickly damped out. 
But now let me set the wing at an angle close to the stalling incidence. 
Now, the downward-moving side is ‘ stalled ’, and its lift is largely reduced ; 
so the air forces tend to maintain the motion. And this is true whatever be 
the direction of the initial spin. We shall find, when the wind is turned 
on, that the wing exhibits a considerable amount of nervousness and 
indecision ; if, however, I start it spinning, it will settle down contentedly 
to a steady rate of rotation; and it is equally ready to rotate in either 
direction. 
Application to the ‘ Spin ’. 
34. Let us apply this result to the aeroplane which we have imagined 
to be accidentally ‘ stalled’. We have seen that it begins to descend, with 
its wings at a large angle to the relative wind. Any tendency to rotate, 
in either direction, will now be seized upon by the wings, and the aeroplane — 
will descend, steadily turning, in a state outside the pilot’s power of 
control. And this is the important point: his instinct is to try to pull his 
nose wp, to avoid the ground; but we can see that his only chance of 
safety is to put it down, so as to reduce the angle of his wings to the wind. 
Now that this is understood, spinning, except close to the ground, has no 
terrors for the skilled pilot: previously, it was responsible for an endless 
succession of fatal accidents. 
To Professor Bairstow belongs the credit for this elucidation of the 
spin, and I claim it as another triumph for the attitude which seeks to 
understand. : 
III. The Attainment of Low-Speed Control. 
35. But nothing can save the pilot whose machine begins to spin teo 
close to the ground. We have seen that he must put his nose down, and 
this involves a rapid loss of height while his aeroplane is gaining the — 
