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 United States 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, aerodynamic 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. Formula. All of these aspects of airplane performance  are compressed into a single value by the following formula: Introduction. 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 speed. Dimensionally, formula_2 has units of speed 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 formula_3 is similar to formula_4, 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 could be so heavy that it would not even lift itself. The ratio is unity formula_5 when the engine is 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, armament, and altitude (complicated, because lessening drag trades off with lessening  thrust due to decreased air intake). The difference between formula_6 and formula_3 is that formula_6 does not include the effects of friction and other aerodynamic losses. When a plane is moving very slowly, these losses are small and can be ignored. However, formula_6 does not accurately describe the performance of the plane at its normal operating conditions. By including drag in the formula, the aerodynamics of the plane are also summarized in the formula_2 value. The specific excess energy model is proportional to the speed of the plane. This means that the faster the plane is capable of flying, the  better its score. The other parts of this model (thrust, drag, and weight) may say that a plane is excellent but a good fighter must also go fast. History. John Boyd, a U.S. jet fighter pilot in the Korean War, began developing the theory in the early 1960s. He teamed with mathematician Thomas Christie at Eglin Air Force Base to use the base's high-speed computer to compare the performance envelopes of U.S. and Soviet aircraft from the Korean and Vietnam Wars. They completed a two-volume report on their studies in 1964. Energy Maneuverability came to be accepted  within the U.S. Air Force and brought about improvements in the requirements for the F-15 Eagle and later the F-16 Fighting Falcon fighters.