The Specific Fuel Consumption of Aircraft Engines (TSFC versus PSFC)

From a fundamental consideration of the efficiency (eta = P_out / P_in) it already follows that the power-specific fuel consumption, PSFC or c_P of an aircraft engine should be approximately constant, while c = c_P * V applies to the thrust-specific fuel consumption, TSFC or c in a first approach. Obviously, fuel is consumed already at static thrust (V=0). For this reason the thrust-specific fuel consumption needs an extended approach c = c_a + c_b * V. Breguet's range equation can certainly be described with a constant thrust-specific fuel consumption c, if c is determined for the cruise speed in question. However, this leads to an error if you want to use it to calculate an optimal flight speed in a flight performance calculation. It is recommended (for a first simple consideration) to write Breguet's range equation for jets with a constant power-specific fuel consumption c_P. This then leads to an optimal cruising speed for maximum range at minimum drag (md) V_md instead of 1.316 * V_md as it is determined with the "classic" derivation. For more detailed considerations, the "Herrmann model" should replace the simple equation c = c_a + c_b * V.