STABILITY OF AEROPLANES—WRIGHT. 211 
always feasible. Flying machines must be built to land safely as well 
as to fly. A high center of gravity tends to cause a machine to roll 
over in landing. A compromise is therefore adopted. The center of 
gravity is kept high enough to be but a slight disturbing factor in flight 
and at the same time not so high as to interfere in making safe 
landings. 
The three forces acting on an aeroplane in the direction of its line 
of motion are the thrust of the propellers, the momentum or inertia of 
its weight, and the resistance of the machine to forward travel. If 
traveling in any other than a horizontal course, a component of 
gravity in the line of motion will have to be reckoned with. When 
these forces are exerted in the same line, with the centers of thrust 
and momentum acting in the opposite direction to that of the center 
of resistance, a variation in the quantity of any one, or of all, of these 
forces will not in itself have a disturbing effect on the equilibrium 
about the lateral horizontal axis. But these forces in the ordinary 
flying machine do not act in the same line. Usually the center of 
thrust is high, in order to give proper clearance between the propellers 
and the ground; the center of gravity is low, to enable the machine to 
land without danger of being overturned; and the center of resistance 
is usually between the centers of thrust and gravity. When a flying 
machine is traveling at uniform speed the propelling forces exactly 
equal the resisting forces. In case the thrust of the propellers is 
diminished by throttling the motor, the momentum of the machine 
acting below the center of resistance carries the lower part of the 
machine along faster than the upper part, and the surfaces thus will 
be turned upward, producing a greater angle and a greater resistance. 
The same effect is produced if the machine be suddenly struck by a 
gust of wind of higher velocity from in front. The thrust of its pro- 
pellers will be temporarily slightly decreased, the resistance due to the 
greater wind pressure will be increased, and the momentum of the 
machine (the center of gravity being low) will in this case also turn the 
surfaces upward to a larger angle. While these variations in the 
forces acting in the horizontal line have of themselves a certain amount 
of disturbing effect, yet it is from the changes of incidence which they 
introduce that. one encounters the greatest difficulty in maintaining 
equilibrium. 
The two principal methods used in preserving fore-and-aft equilib- 
rium have been, first, the shifting of weight so as to keep the center of 
gravity in line with the changing center of lift; and, second, the utili- 
zation of auxiliary surfaces, known as elevators, to preserve the posi- 
tion of the center of pressure in line with a fixed center of gravity. 
The first method has been found impracticable on account of the im- 
possibilty of shifting large weights quickly enough. The second 
method is that used in most of the flying machines of to-day. 
