Jet Propulsion A Simple Guide to the Aerodynamics and Thermodynamic Design and Performance of Jet Engines

Introduction

The engine requirements for an aircraft depend upon the size, range and speed selected, but they also depend on the aerodynamic behaviour of the aircraft and the way in which it is operated. In this chapter some very elementary aspects of civil aircraft aerodynamic performance are described (if further explanation is needed the reader is referred to Anderson (2011)). These lead to a brief description of the conditions which are most critical for the engine: take-off, climb and cruise. It is possible to see why cruising fast and high is desirable, and to calculate the range. We can also develop the New Efficient Aircraft, the NEA, to form the basis of the exercises in Part 1 of the book. Knowing the ratio of lift to drag it is possible from this stipulation of the NEA to estimate the total thrust requirement.

Payload versus range

It is common to show aircraft performance by plotting maximum allowable payload against range capability, and an example corresponding to the Airbus A330-300 is shown in Figure 2.1. The presumption is that only enough fuel is carried to enable the mission, though reserves are also carried to enable diversions or other contingencies to be handled safely – it is the intention that these reserves will not be used. As the range is increased the weight of fuel at take-off must increase. The maximum payload allowed by the structure is marked by line OA and, as the range is increased along the line OA, the payload can remain constant at its maximum value, despite the increased weight of fuel, until the point A is reached. At point A the total weight of the aircraft has reached the maximum take-off weight and the range for this is denoted by R1. For the range to be increased beyond R1 requires additional fuel but, since the maximum take-off weight has been reached, for extra fuel to be carried the payload must be reduced. Payload may be substituted by fuel in this way until the tanks are full, denoted by point B in Figure 2.1. To increase range beyond that for B implies the drag, and consequently aircraft weight, has to be reduced in order to reduce fuel consumption, and this leads to a steep drop in payload for a small increase in range.