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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 .
Airflow operating at higher thrust will cause the axial induction factor to rise above the optimum value. Higher thrust causes more air to be deflected away from the turbine. When the axial induction factor falls below the optimum value, the wind turbine is not extracting all the energy it can.
If a powered aircraft is generating thrust T and experiencing drag D, the difference between the two, T − D, is termed the excess thrust. The instantaneous performance of the aircraft is mostly dependent on the excess thrust. Excess thrust is a vector and is determined as the vector difference between the thrust vector and the drag vector.
In flight a powered aircraft can be considered as being acted on by four forces: lift, weight, thrust, and drag. [1] Thrust is the force generated by the engine (whether that engine be a jet engine, a propeller, or -- in exotic cases such as the X-15-- a rocket) and acts in a forward direction for the purpose of overcoming drag. [2]
The type of jet engine used to explain the conversion of fuel into thrust is the ramjet.It is simpler than the turbojet which is, in turn, simpler than the turbofan.It is valid to use the ramjet example because the ramjet, turbojet and turbofan core all use the same principle to produce thrust which is to accelerate the air passing through them.
Since the power equals thrust times speed, the efficiency is given by = / where V is speed and h is the energy content per unit mass of fuel (the higher heating value is used here, and at higher speeds the kinetic energy of the fuel or propellant becomes substantial and must be included).
The particular take-off distance required may be shorter than the available runway length. In this case a lower thrust may be used. Lower thrust settings increase engine life and reduce maintenance costs. The take-off thrust available from a civil engine is a constant value up to a particular ambient temperature.
A corollary of this is that, particularly in air breathing engines, it is more energy efficient to accelerate a large amount of air by a small amount, than it is to accelerate a small amount of air by a large amount, even though the thrust is the same. This is why turbofan engines are more efficient than simple jet engines at subsonic speeds.