Consider an aircraft in steady, level flight, with weight $ W$ , as shown in Figure 13.1. The rate of change of the gross weight of the vehicle is equal to the fuel weight flow:
For steady, level flight, $ T = D$ , $ L = W$ , or The rate of change of aircraft gross weight is thus Suppose [画像:$ L/D$] and $ I_{sp}$ remain constant along the flight path: We can integrate this equation for the change in aircraft weight to yield a relation between the weight change and the time of flight: where $ W_i$ is the initial weight. If $ W_f$ is the final weight of vehicle and $ t_\textrm{initial}=0$ , the relation between vehicle parameters and flight time, $ t_f$ , is The range is the flight time multiplied by the flight speed, or, The above equation is known as the Breguet range equation. It shows the influence of aircraft, propulsion system, and structural design parameters.
The combustion efficiency is near unity unless conditions are far off design. We can therefore regard the two main drivers as the thermal and propulsive13.1 efficiencies. The evolution of the overall efficiency of aircraft engines in terms of these quantities was shown in Figure 11.8.
