Energy–maneuverability theory
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 Air Force, and is useful in describing an aircraft's performance as the total of kinetic and potential energies or aircraft specific energy. It relates the thrust, weight, drag, wing area, and other flight characteristics of an aircraft into a quantitative model. This allows combat capabilities of various aircraft or prospective design trade-offs to be predicted and compared.
All of these aspects of airplane performance are compressed into a single value by the following formula:
In words, the specific excess energy is proportional to the ratio of net motive forces compared to the weight of the plane and proportional to velocity. (Note that dimensionally, has units of "velocity," not "specific energy" (energy per unit mass).)
The net motive force is found by calculating the engine's ability to move the plane after accounting for friction and other aerodynamic issues that slow down the plane. The ratio (T-D)/W is similar to T/W, the Thrust-to-weight ratio, which is also used as a figure of merit for airplanes and rockets. By normalizing the motive forces to the weight of the plane, it is clear how efficient the plane is. A very large engine may be able to generate a huge thrust but would be so heavy that it could not even lift itself. The ratio is unity (T-D)/W = 1 when the engine is only powerful enough to keep the plane at constant speed in a 90 degree ascending trajectory. Fighter jets, such as the F-16 have a T/W ratio close to 1, depending on fuel weight and armament.
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