When the effects of parasite drag and induced drag are plotted against airspeed, the resulting line is called the drag curve. Parasite drag increases as airspeed increases because airflow resistance over the aircraft surfaces becomes greater. Induced drag behaves in the opposite manner; it increases as airspeed decreases because a higher angle of attack is needed to support the aircraft’s weight.
Aircraft performance charts are presented in two common ways. Jet aircraft usually use thrust versus drag graphs, while propeller-driven aircraft use power versus drag graphs. Since this article focuses on propeller aircraft, the discussion refers to power required rather than thrust required.
In steady, level flight, the power produced by the engine must exactly match the aerodynamic drag acting on the aircraft. For this reason, the drag curve can also be interpreted as a power required curve, which represents the amount of engine power necessary to maintain a constant airspeed at a constant altitude.
Figure 1 shows the relationship between airspeed and power required for level flight. The lowest point on the curve represents the speed where total drag is at its minimum. At this airspeed, induced drag and parasite drag are equal, and the aircraft operates at its greatest aerodynamic efficiency, achieving the maximum lift-to-drag ratio.
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| Figure 1. Thrust and power required curves |
As airspeed increases beyond this point, parasite drag rises quickly and additional power is required. As airspeed decreases below this point, induced drag increases sharply because the wing must operate at a higher angle of attack to maintain lift. In both cases, more power is needed than at the minimum drag speed.
For propeller-driven aircraft, the available power changes with airspeed and does not remain constant. Propellers typically operate at a peak efficiency of approximately 80 to 88 percent. Only a portion of engine horsepower is converted into useful thrust, while the remainder is lost as aerodynamic and mechanical inefficiencies. As airspeed increases, propeller efficiency improves until reaching a maximum value and then decreases at higher speeds. Because of this, the power available curve changes with airspeed rather than remaining perfectly horizontal.
The interaction between power required and power available explains many aircraft performance characteristics. At lower airspeeds, the required power increases rapidly. To maintain altitude, the pilot must add power while simultaneously increasing pitch attitude.
Figure 2 illustrates this region where decreasing airspeed requires increasing power. In this condition, pitch primarily controls airspeed while power primarily controls altitude, which is different from the normal relationship during cruise flight.
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| Figure 2. Regions of command |
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 be 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 1) and is primarily due to induced drag. Figure 2 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.
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 3. 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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| Figure 3. Region of speed stability |
An aircraft flying in steady, level flight at point C is in equilibrium. [Figure 3] 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 does not produce any significant excess or deficiency in power. It has the characteristic of neutral stability (i.e., 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. 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 AOA in slowspeed flight.