STABILITY OF AEROPLANES—WRIGHT. 213 
producing more or less inherently stable aeroplanes. An inherently 
stable aeroplane may be described as one in which equilibrium is 
maintained by an arrangement of surfaces, so that when a current 
of air strikes one part of the machine, creating a pressure that would 
tend to disturb the equilibrium, the same current striking another 
part creates a balancing pressure in the opposite direction. This 
compensating or correcting pressure is secured without the mechanical 
movement of any part of the machine. 
The first to propose the use of this system for the fore-and-aft 
control of aeroplanes was Penaud, a young French student, who 
did much experimenting with model aeroplanes in the seventies of 
the last century. His system is used only to a slight extent in the 
motor-driven aeroplanes of to-day, on account of its wastefulness 
of power and on account of its restriction of the maneuvering quali- 
ties of the machine. 
Penaud’s system consists of a main bearing surface and a hori- 
zontal auxiliary surface in the rear fixed at a negative angle in 
relation to the main surface. The center of gravity is placed in 
front of the center of the main surface. This produces a tendency 
to incline the machine downward in front, and to cause it to descend. 
In descending the aeroplane gains speed. The fixed surface in the 
rear, set at a negative angle, receives an increased pressure on its 
upper side as the speed increases. This downward pressure causes 
the rear of the machine to be depressed till the machine takes an 
upward course. ‘The speed is lost in the upward course, the down- 
ward pressure on the tail is relieved, and the forward center of 
gravity turns the course again downward. While the inherently 
stable system will control a machine to some extent, it depends 
so much on variation in course and speed as to render it inadequate 
to meet fully the demands of a practical flying machine. 
In order to secure greater dynamic efficiency and greater maneu- 
vering ability, auxiliary surfaces mechanically operable are used in 
present flying machines instead of the practically fixed surfaces of 
the inherently stable type. These machines possess the means of 
quickly recovering balance without changing the direction of travel 
and of maneuvering with greater dexterity when required. On the 
other hand, they depend to a greater extent upon the skill of the 
operator in keeping the equilibrium. It may be taken as a rule 
that the greater the dynamic efficiency of the machine and the 
greater its possibilities in maneuvering, the greater the knowledge 
and skill required of the operator. 
If the operator of a flying machine were able to ‘‘feel” exactly 
the angle at which his aeroplane meets the air, 90 per cent at least 
of all aeroplane accidents would be eliminated. It has been the 
lack of this ability that has resulted in so large a toll of human lives. 
