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Figure 1 is a plot of speed stability, which is the manner in which static longitudinal stability is demonstrated in flight. It measures the relationship between airspeed and longitudinal control force. Simply stated, speed stability is a measure of the control force required to hold the airplane at an airspeed other than the trimmed airspeed, with the throttles fixed at the trimmed thrust setting. Airplanes with positive static longitudinal stability require a pull force to maintain a speed below the trimmed speed, and a push force to maintain a speed above the trimmed speed. For conventional airplanes (those without stability augmentation), this is a design requirement of the U.S. Federal Aviation Administration Regulations (FAR) Part 25. However, in an unstable airplane--one with negative static longitudinal stability--if the airplane is in trim and the flight crew applies a pull force, the airplane will initially pitch up and slow down, but the pitch rate will quickly become large enough to require a relaxing of the pull, and eventually a push force, to maintain a constant speed below the initial trim speed. The converse is true when the flight crew attempts to accelerate the airplane from the trimmed state. This difficult task can be accomplished without increasing the crew's workload by using electronic flight control computers for stability augmentation.

SPEED STABILITY
In practice, flight test for certification of static longitudinal stability by the U.S. Federal Aviation Administration measures speed stability, a parameter equivalent to Cm-alpha. Simply stated, speed stability is a measure of the control force required to hold the airplane at an airspeed other than the trimmed airspeed. The throttles are fixed at the trimmed thrust setting to eliminate pitching moment changes due to thrust. Speed stability is measured by trimming the airplane in level flight, with throttles fixed at the trimmed condition, then slowly varying airspeed with control column input.

STATIC LONGITUDINAL STABILITY
Static longitudinal stability is a measure of the tendency of an airplane to maintain its trimmed angle of attack in 1g flight. More strictly speaking, it is a measure of the initial pitch response of an airplane to a disturbance in angle of attack. Following a disturbance, a statically stable airplane tends to return to the angle of attack for which it is trimmed. Conversely, a statically unstable airplane tends to move away from the trimmed angle of attack following a disturbance. The term "static longitudinal stability" is the name of the stability coefficient (Cm-alpha) for the pitching moment due to a change in angle of attack. In a stable, conventional airplane, the CG is forward of the neutral point of the airplane (wing plus tail). An increase in angle of attack from trim increases the amount of lift generated by the wing and results in an increasing pitch-down moment. This drives the airplane back toward its original angle of attack. If the CG is aft of the neutral point, increasing the angle of attack causes the airplane to pitch up, away from its original trimmed condition.