ESDU 90012:1990

ESDU 90012 discusses and illustrates the use of the energy height method to minimise time or fuel used in transferring from one (velocity, height) point to another. Energy height is an "equivalent" height that yields an "equivalent" potential energy at any point equal to the sum of the kinetic and true potential energies of the aircraft. The method allows additional constraints to be applied, such as structural or aerodynamic limits (for example, maximum Mach number or lift coefficient) or operational limits (for example, horizontal distance covered in the climb). It involves the simplifications that net thrust is assumed always to be aligned with the flight path and that drag is that in level flight at the given weight irrespective of aircraft attitude or manoeuvres. In addition, transfers between trajectories are assumed instantaneous. The effect of these assumptions is studied and shown to be generally small for a range of typical aircraft configurations. Four worked examples illustrate the use of the method: for a combat aircraft minimum time to height and speed, and to height, speed and distance, and for a subsonic transport aircraft minimum fuel to height and speed, and to height, speed and distance.