Design for endurance

The purpose of an aircraft is not always to transport people or cargoes between two locations, sometimes the aircraft is used as a radar or visual observation platform, and in this case the main design consideration will be the length of time it can remain airborne, or its endurance.

In this case we require, not the minimum fuel flow over a given distance, but the minimum fuel flow in unit time. Here we will adopt the same approach as before and look at the airframe from an idealised point of view to get an initial idea of the way things behave. Following this we will look at the real engine behaviour to get a more accurate picture of the operational requirements of the complete aircraft.

If we take an initial guess, we would suppose that the best way to operate the airframe for maximum endurance would be to fly at the condition at which the smallest amount of work needs to be expended in unit time in order to overcome the drag force. The rate at which work is done is equal to power, so this operating point is equivalent to the flying speed and cor­responding aircraft attitude which results in minimum power, rather than minimum drag.

Because we are now concerned with power, rather than drag, we will con­sider the power required by the airframe and powerplant and plot them in a similar manner to the drag curves of Fig. 7.4. The power required curve is very easily derived from the drag curve. All we have to do is multiply each value of the drag by the speed at which it occurs and replot as in Fig. 7.10. Then we superimpose the power, rather than the thrust, curve for the particular power – plant we are using.

We find that the power reaches a minimum value at a speed slightly lower than the minimum drag speed. In constructing the power curves we must again remember that we are talking about an aircraft flying straight and level at a constant weight.

Now we have decided what the airframe is doing, we will take a simpli­fied look at the compromise which must be reached for the different power plant types, as we did when considering how to operate for best economy and range.