## Instrument Flying Handbook Aerodynamic Factors Regions of Command and Control Characteristics

Instrument Flying
Handbook

Preface

 Figure 2-8. Thrust and Power Required Curves.
 Regions of Command The drag curve also illustrates the two regions of command: the region of normal command, and the region of reversed command. The term "region of command" refers to the relationship between speed and the power required to maintain or change that speed. "Command" refers to the input the pilot must give in terms of power or thrust to maintain a new speed once reached. The "region of normal command" occurs where power must he added to increase speed. This region exists at speeds higher than the minimum drag point primarily as a result of parasite drag, the "region of reversed command" occurs where additional power is needed to maintain a slower airspeed. This region exists at speeds slower than the minimum drag point (L/Dmax) on the thrust required curve, Figure 2-8 and is primarily due to induced drag, Figure 2-9 shows how one power setting can yield two speeds, points 1 and 2. This is because at point 1 there is high induced drag and low parasite drag, while at point 2 there is high parasite drag and low induced drag. Figure 2-9. Regions of Command. Control Characteristics Most flying is conducted in the region of normal command: for example, cruise, climb, and maneuvers. The region of reversed command may be encountered in the slow-speed phases of flight during takeoff and landing; however, for most general aviation aircraft, this region is very small and is below normal approach speeds. Flight in the region of normal command is characterized by a relatively strong tendency of the aircraft to maintain the trim speed. Flight in the region of reversed command is characterized by a relatively weak tendency of the aircraft to maintain the trim speed. In fact, it is likely the aircraft exhibits no inherent tendency to maintain the trim speed in this area. For this reason, the pilot must give particular attention to precise control of airspeed when operating in the slow-speed phases of the region of reversed command. Operation in the region of reversed command does not imply that great control difficulty and dangerous conditions exist. However; it does amplify errors of basic flying technique making proper flying technique and precise control of the aircraft very important. Speed Stability Normal Command The characteristics of flight in the region of normal command are illustrated at point A on the curve in Figure 2-10. 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 has a 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 has a 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.

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