STABILITY OF AEROPLANES. 
By OrvitteE Wricut, B.S8., LL. D. 
The subject of “stability of aeroplanes” is too broad to permit of 
a discussion of all of its phases in one evening. I shall, therefore, 
confine myself more particularly to a few phases of the fore-and-aft 
or longitudinal equilibrium. Although in learning to fly the beginner 
finds most difficulty in mastering the lateral control, it is his lack of 
knowledge of certain features of the fore-and-aft equilibrium that 
leads to most of the serious accidents. These accidents are the more 
difficult to avoid because they are due to subtle causes which the 
flyer does not at the time perceive. 
A flying machine must be balanced in three directions—about an 
axis fore and aft in its line of motion, about an axis extending in a 
lateral direction from tip to tip of the wings, and about a vertical 
axis. The balance about the lateral axis is referred to as fore-and- 
aft or longitudinal equilibrium; that about the fore-and-aft axis as 
lateral equilibrium; and that about the vertical axis is generally re- 
ferred to as steering, although its most important function is that of 
lateral equilibrium. 
If the center of support of an aeroplane surface would remain 
fixed at one point, as is practically the case in marine vessels and 
in balloons and airships, equilibrium would be a simple matter. 
But the location of the center of pressure on an aeroplane surface 
changes with every change in the angle at which the air strikes the 
surface. At an angle of 90° it is located approximately at the center 
of the surface. As the angle becomes less, the center of pressure 
moves forward. On plane surfaces it continues to move forward as 
the angle decreases until it finally reaches the front edge. But on 
cambered surfaces the movement is not continuous. After a certain 
critical angle of incidence is reached, which angle depends upon the 
particular form of the surface, the center of pressure moves back- 
ward with further decrease in angle until it arrives very close to the 
rear edge. At angles ordinarily used in flying, angles of 3° to 12°, 
the travel of the center of pressure is in this retrograde movement 
and is located, according to the angle of incidence, at points between 
1 Presented at the stated meeting cf the Franklin Institute held Wednesday, May 20, 1914, when Dr. 
Wright received the Franklin Institute’s Elliott Cresson Medal in recognition of the epoch-making work 
accomplished by him in establishing on a practical basis the science and art of aviation. Reprinted, by 
permission, from the Journal of the Franklin Institute, Philadelphia, September, 1914. 
73176°—sMm 1914——14 209 
