IFH 4-7

Jet Aircraft Airspeed Thrust Required 4,000 3,000 2,000 1,000 0 0 100 200 300 400 500 Induced drag Parasite drag Total drag Minimum drag or L/DMAX Propeller-Driven Aircraft Airspeed 0 100 200 300 400 500 27 25 23 18 0 Power Required Inches Hg A B Parasite power required Total power required Minimum power required L/DMAX Induced drag Region of reversed command Region of normal command Airspeed Power Required 1 2 Figure 4-9. Thrust and power required curves. Figure 4-10. Regions of command. of Figure 4-9 is based on power versus drag, and it is used for propeller-driven aircraft. This chapter focuses on power versus drag charts for propeller-driven aircraft. Understanding the drag curve can provide valuable insight into the various performance parameters and limitations of the aircraft. Because power must equal drag to maintain a steady airspeed, the curve can be either a drag curve or a power required curve. The power required curve represents the amount of power needed to overcome drag in order to maintain a steady speed in level flight. The propellers used on most reciprocating engines achieve peak propeller efficiencies in the range of 80 to 88 percent. As airspeed increases, the propeller efficiency increases until it reaches its maximum. Any airspeed above this maximum point causes a reduction in propeller efficiency. An engine that produces 160 horsepower will have only about 80 percent of that power converted into available horsepower, approximately 128 horsepower. The remainder is lost energy. This is the reason the thrust and power available curves change with speed. 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 4-9) and is primarily due to induced drag. Figure 4-10 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