Static Stability Static Stability

   Stability of an airplane in flight is slightly more complex than just explained, because the airplane is free to move in any direction and must be controllable in pitch, roll, and direction. When designing the airplane, engineers must compromise between stability, maneuverability, and controllability; and the problem is compounded because of the airplane's three axis freedom.

   By comparing this three axis freedom with the limited flexibility of an automobile, the problem may be better understood. Pitch control of a car is limited to a shock absorbing and leveling problem while roll control is a matter of the lateral spacing of the wheels and the banking of highways around curves. Essentially, then, the car has only one degree of real freedom - directional; in many cases, though, the lack of pitch and bank control have caused it to lose directional control. Automobile designers provide "centering" in the steering mechanism or "toe in" on the wheels to provide directional stability so the car will maintain a straight path when the steering wheel is released. Too much centering or "toe in" is, of course, objectionable because of the excessive effort required of the driver to turn the car. When power steering was introduced to the driving public, many considered it objectionable because of insufficient centering or "feel." Even now, most automobile models are offered either with or without power steering to satisfy the public.

   The conclusions, then, are that too much stability is detrimental to maneuverability, and similarly, not enough stability is detrimental to controllability. In the design of airplanes, compromise between the two is the key word.