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Energy–maneuverability theory is a model of aircraft performance. It was developed by Col. John Boyd, a fighter pilot, and Thomas P. Christie, a mathematician with the United States Air Force, [1] and is useful in describing an aircraft's performance as the total of kinetic and potential energies or aircraft specific energy.
For most unpowered aircraft, the maximum flight time is variable, limited by available daylight hours, aircraft design (performance), weather conditions, aircraft potential energy, and pilot endurance. Therefore, the range equation can only be calculated exactly for powered aircraft. It will be derived for both propeller and jet aircraft.
As noted earlier, , =,. The total drag coefficient can be estimated as: = [()], where is the propulsive efficiency, P is engine power in horsepower, sea-level air density in slugs/cubic foot, is the atmospheric density ratio for an altitude other than sea level, S is the aircraft's wing area in square feet, and V is the aircraft's speed in miles per hour.
Aircraft manufacturers will publish performance data in an aircraft flight manual, concerning the behaviour of the aircraft under various circumstances, such as different speeds, weights, and air temperatures, pressures, & densities. [5] [6] Performance data is information pertaining to takeoff, climb, range, endurance, descent, and landing. [1]
SFC varies with throttle setting, altitude, climate. For jet engines, air flight speed is an important factor too. Air flight speed counteracts the jet's exhaust speed. (In an artificial and extreme case with the aircraft flying exactly at the exhaust speed, one can easily imagine why the jet's net thrust should be near zero.)
The thrust-to-weight ratio and lift-to-drag ratio are the two most important parameters in determining the performance of an aircraft. The thrust-to-weight ratio varies continually during a flight. Thrust varies with throttle setting, airspeed , altitude , air temperature, etc. Weight varies with fuel burn and payload changes.
Extra power, or specific excess power, [3] is a very basic method of determining an aircraft's flight envelope. It is easily calculated but as a downside does not tell very much about the actual performance of the aircraft at different altitudes.
For conventional fixed-wing aircraft with moderate aspect ratio and sweep, Oswald efficiency number with wing flaps retracted is typically between 0.7 and 0.85. At supersonic speeds, Oswald efficiency number decreases substantially. For example, at Mach 1.2 Oswald efficiency number is likely to be between 0.3 and 0.5. [1]