The Factors Affecting Aircraft Performance: Speed Stability

Normal Command
The characteristics of flight in the region of normal command are illustrated at point A on the curve in figure 2-7. If the aircraft is established in steady, level flight at point A, lift is equal to weight, and the power available is set equal to the power required. If the airspeed is increased with no changes to the power setting, a power deficiency exists. The aircraft will have the natural tendency to return to the initial speed to balance power and drag. If the airspeed is reduced with no changes to the power setting, an excess of power exists. The aircraft will have the natural tendency to speed up to regain the balance between power and drag. Keeping the aircraft in proper trim enhances this natural tendency. The static longitudinal stability of the aircraft tends to return the aircraft to the original trimmed condition.
 
An aircraft flying in steady, level flight at point C is in equilibrium. [Figure 2-7: Regions of speed stability] If the speed were increased or decreased slightly, the aircraft would tend to remain at that speed. This is because the curve is relatively flat and a slight change in speed will not produce any significant excess or deficiency in power. It has the characteristic of neutral stability; the aircraft's tendency is to remain at the new speed.
 
Reversed Command
The characteristics of flight in the region of reversed command are illustrated at point B on the curve in figure 2-7. If the aircraft is established in steady, level flight at point B, lift is equal to weight, and the power available is set equal to the power required. When the airspeed is increased greater than point B, an excess of power exists. This causes the aircraft to accelerate to an even higher speed. When the aircraft is slowed to some airspeed lower than point B, a deficiency of power exists. The natural tendency of the aircraft is to continue to slow to an even lower airspeed.
 
This tendency toward instability happens because the variation of excess power to either side of point B magnifies the original change in speed. Although the static longitudinal stability of the aircraft tries to maintain the original trimmed condition, this instability is more of an influence because of the increased induced drag due to the higher angles of attack in slow-speed flight.
 
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Static longitudinal stability: The aerodynamic pitching moments required to return the aircraft to the equilibrium angle of attack.
 
Slow Airspeed Safety Hint
Be sure to add power before pitching up while at slow airspeeds to prevent losing airspeed.