STABILITY OF AEROPLANES—WRIGHT. 215 
height of less than 200 or 300 feet, the machine is likely to strike 
the ground before the speed necessary to recover control is acquired. 
The danger from ‘‘stalling’’ comes in the operator attempting to 
check the machine’s downward plunge by turning the main bearing 
surfaces to still larger angles of incidence, instead of pointing the 
machine downward, at a smaller angle of incidence, so that the 
speed can be recovered more quickly. It is safe to say that fully 
90 per cent of the fatal accidents in flyimg are due to this cause. 
Most of the serious ones occur when, after long glides from con- 
siderable heights, with the power of the motor reduced, an attempt 
is made to bring the machine to a more level course several hundred 
feet in the air. The machine quickly loses its speed and becomes 
“stalled.” All of us who have seen the novice make a ‘‘pancake”’ 
landing have seen the beginning of a case of ‘‘stalling’”’ which might 
have been fatal had it taken place at a height of 100 or 200 feet. 
The greatest danger in flying comes from misjudging the angle 
of incidence. If a uniform angle of incidence were maintained, there 
would be no difficulty in fore-and-aft equilibrium. As has already 
been stated, for any given surface and any given angle of incidence 
the position of the center of pressure is fixed. Under these conditions, 
if the center of gravity were located to coincide with the center of 
pressure and a uniform angle of incidence maintained, the machine 
would always be in equilibrium. 
It is in accordance with this principle that experiments the past 
year have brought about a considerable advance in the development 
of automatic stability. A small horizontal wind vane is so mounted 
on the machine as to ride edgewise to the wind when the machine 
is flying at the desired angle of incidence. In case the machine 
varies from the desired angle, the air will strike the vane on either 
its upper or lower side. The slightest movement of the vane in either 
direction brings into action a powerful mechanism for operating the 
controlling surfaces. 
If the wind strikes the vane on the underside, as would be the 
case when the machine takes a larger angle of incidence, the elevator 
is turned to cause the machine to point downward in front till the 
normal angle is restored. If the air strikes the vane from above, a 
smaller angle of incidence is indicated, and an opposite action on the 
elevator is produced. In this system no particular angle of the 
machine with the horizontal is maintained. It is the angle at which 
the air strikes the aeroplane surface that is important. If the vane. 
is set at an angle of 5° with the main supporting surfaces, and the 
machine is traveling on a level course, increasing the power of the 
motor will cause it to begin taking on more speed. But as the lifting 
effect of an aeroplane surface is the product of two factors—its speed 
and its angle of incidence—any increase in speed will produce a greater 
