LONGITUDINAL STABILITY

LONGITUDINAL STABILITY

 

Longitudinal stability is pitch stability, or stability around the lateral axis of the airplane.
To obtain longitudinal stability, airplanes are designed to be nose heavy when correctly loaded. The center of gravity is ahead of the center of pressure. This design feature is incorporated so that, in the event of engine failure, the airplane will assume a normal glide. It is because of this nose heavy characteristic that the airplane requires a tailplane. Its function is to resist this diving tendency. The tailplane is set at an angle of incidence that produces a negative lift and thereby, in effect, holds the tail down. In level, trimmed flight, the nose heavy tendency and the negative lift of the tailplane exactly balance each other.
Two principal factors influence longitudinal stability: (1) size and position of the horizontal stabilizer, and (2) position of the center of gravity.

The Horizontal Stabilizer

The tail plane, or stabilizer, is placed on the tail end of a lever arm (the fuselage) to provide longitudinal stability. It may be quite small. However, being situated at the end of the lever arm, it has great leverage. When the angle of attack on the wings is increased by a disturbance, the center of pressure moves forward, tending to turn the nose of the airplane up and the tail down. The tailplane, moving down, meets the air at a greater angle of attack, obtains more lift and tends to restore the balance.
On most airplanes, the stabilizer appears to be set at an angle of incidence that would produce an upward lift. It must, however, be remembered that the tailplane is in a position to be in the downwash from the wings. The air that strikes the stabilizer has already passed over the wings and been deflected slightly downward. The angle of the downwash is about half the angle of attack of the main airfoils. The proper angle of incidence of the stabilizer therefore is very important in order for it to be effective in its function.

Center of Gravity

The center of gravity is very important in achieving longitudinal stability. If the airplane is loaded with the center of gravity too far aft, the airplane may assume a nose up rather than a nose down attitude. The inherent stability will be lacking and, even though down elevator may correct the situation, control of the airplane in the longitudinal plane will be difficult and perhaps, in extreme cases, impossible.

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