. 214 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1914. 
Instruments have been produced which indicate closely the angle of 
incidence at which the machine is flying, but they are not in general 
use. Nor does the average flier realize how exceedingly dangerous 
it is to be ignorant of this angle. Most of the fliers are aware that 
‘‘stalling’’ is dangerous, but do not know when they really are 
“stalling.” 
A flying machine is in great danger when it is flying at its angle 
of maximum lift. A change either to a smaller or a larger angle 
results in a lesser lift. There is this important difference, however, 
whether the angle be increased or decreased. While a smaller angle 
gives less lift, it also has less drift resistance, so that the machine is 
permitted to gain speed. On the other hand, the larger angle gives 
not only less lift but encounters a greater resistance, which causes 
the speed of the machine to be rapidly checked, so that there is a 
double loss of lift—that due to angle and that due to a lesser speed. 
The maximum lift is obtained in most flying machines at some 
angle between 15° and 20°. If the machine be gliding from a height 
with the power of the motor throttled or entirely turned off, and 
the operator attempts to turn it to a level course, the speed of the 
machine will soon be reduced to the lowest at which it can support 
its load. If now this level course be held for even only a second 
or two, the speed and the lift will be so diminished that the machine 
will begin to fall rapidly. 
The center of pressure on a cambered aeroplane surface at angles 
greater than 12° to 15° travels backward with increase of angle of 
incidence, so that when a machine approaches the ‘“‘stalled”’ angles 
the main bearing surfaces are generally carrying practically all of 
the weight and the elevator practically none at all. Under these 
conditions the main surfaces fall more rapidly than does the rear 
elevator. The machine noses downward and plunges at an exceed- 
ingly steep angle toward the earth. This plunge would tend to 
bring the machine back to normal speed quickly were the machine 
flying at its usual angle of incidence. But at the large angles of 
incidence the drift is a large part of the total pressure on the sur- 
faces, so that, although plunging steeply downward, speed is recov- 
ered but slowly. The more the operator tries to check the downward 
plunge by turning the elevator, the greater becomes the angle of 
incidence, and the greater the forward resistance. At ordinary 
stalled angles the machine must descend at an angle of about 25° 
with reference to the horizontal in order to maintain its speed. If 
the speed be already below that necessary for support, a steeper 
angle of descent will be required, and considerable time may be con- 
sumed before supporting speed can be recovered. During all this 
time the machine is plunging downward. If the plunge begins at a 
