Aircraft Performance

deriuqer rewoP The variations of speed and power required must be specific fuel consumption for values of brake horsepower monitored by the pilot as part of the cruise control procedure below the maximum cruise power rating of the engine that to maintain the L/D . When the aircraft’s fuel weight is a is the lean range of engine operation. Thus, an increase in MAX small part of the gross weight and the aircraft’s range is small, altitude produces a decrease in specific range only when the the cruise control procedure can be simplified to essentially increased power requirement exceeds the maximum cruise maintaining a constant speed and power setting throughout power rating of the engine. One advantage of supercharging the time of cruise flight. However, a long-range aircraft has a is that the cruise power may be maintained at high altitude, fuel weight that is a considerable part of the gross weight, and and the aircraft may achieve the range at high altitude with cruise control procedures must employ scheduled airspeed the corresponding increase in TAS. The principal differences and power changes to maintain optimum range conditions. in the high altitude cruise and low altitude cruise are the TAS and climb fuel requirements. The effect of altitude on the range of a propeller-driven aircraft is illustrated in Figure 11-13. A flight conducted at Region of Reversed Command high altitude has a greater true airspeed (TAS), and the power The aerodynamic properties of an aircraft generally determine required is proportionately greater than when conducted at the power requirements at various conditions of flight, while sea level. The drag of the aircraft at altitude is the same as the the powerplant capabilities generally determine the power drag at sea level, but the higher TAS causes a proportionately available at various conditions of flight. When an aircraft greater power required. is in steady, level flight, a condition of equilibrium must prevail. An unaccelerated condition of flight is achieved NOTE: The straight line that is tangent to the sea level power when lift equals weight, and the powerplant is set for thrust curve is also tangent to the altitude power curve. equal to drag. The power required to achieve equilibrium in constant-altitude flight at various airspeeds is depicted on a The effect of altitude on specific range can also be appreciated power required curve. The power required curve illustrates from the previous relationships. If a change in altitude causes the fact that at low airspeeds near the stall or minimum identical changes in speed and power required, the proportion controllable airspeed, the power setting required for steady, of speed to power required would be unchanged. The fact level flight is quite high. implies that the specific range of a propeller-driven aircraft would be unaffected by altitude. Actually, this is true to the Flight in the region of normal command means that while extent that specific fuel consumption and propeller efficiency holding a constant altitude, a higher airspeed requires a higher are the principal factors that could cause a variation of power setting and a lower airspeed requires a lower power specific range with altitude. If compressibility effects are setting. The majority of aircraft flying (climb, cruise, and negligible, any variation of specific range with altitude is maneuvers) is conducted in the region of normal command. strictly a function of engine/propeller performance. Flight in the region of reversed command means flight in An aircraft equipped with a reciprocating engine experiences which a higher airspeed requires a lower power setting very little, if any, variation of specific range up to its and a lower airspeed requires a higher power setting to absolute altitude. There is negligible variation of brake hold altitude. It does not imply that a decrease in power produces lower airspeed. The region of reversed command is encountered in the low speed phases of flight. Flight speeds below the speed for maximum endurance (lowest point e u d on the power curve) require higher power settings with a a l e v el At altit decrease in airspeed. Since the need to increase the required e power setting with decreased speed is contrary to the normal S command of flight, the regime of flight speeds between the speed for minimum required power setting and the stall speed (or minimum control speed) is termed the region of reversed command. In the region of reversed command, a decrease in airspeed must be accompanied by an increased power setting Constant weight in order to maintain steady flight. L/D MAX Figure 11-14 shows the maximum power available as a Speed curved line. Lower power settings, such as cruise power, would also appear in a similar curve. The lowest point on Figure 11-13. Effect of altitude on range.