Energy Approach to the General Aircraft Performance Problem

The general aircraft performance problem is considered from the point of view of the balance that must exist between the potential and kinetic energy change of the aircraft, the energy dissipated against the drag, and the energy derived from the fuel. This approach yields as one result a basic equation for the rate of change of the sum of the potential and kinetic energies of an aircraft. The form of the equation points to the use of aircraft total energy rather than altitude as the significant independent variable in the climb performance of high-speed aircraft. Using this equation, a method is outlined which permits finding either the path of minimum time or minimum fuel to change from one combination of speed and altitude to another. The energy balance approach yields as another result a form of the aircraft range equation which incorporates a correction for the total energy change during the cruising portion of the flight. This form of the range equation shows that the dimension and the order of magnitude of aircraft range are given by the heat content of the fuel. Finally, the parameters of the range equation are examined to obtain an indication of the range capabilities of aircraft at supersonic speeds relative to those at subsonic speeds.