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

Introduction

In Chapter 11 the performance of the main aerodynamic and thermodynamic components of the engine was considered. In earlier chapters the design condition of a high bypass ratio engine had been specified and a design arrived at for this condition. At the design point all the component performances would ideally fit together and only the specification of their performance at this particular condition would be required. Unfortunately engine components never exactly meet their aerodynamic design specification and we need to be able to assess what effect these discrepancies have. Furthermore engines do not only operate at one non-dimensional condition, but over a range of power settings (for take-off, climb, cruise and descent) and there is great concern that the performance of the engine should be satisfactory and safe at all off-design conditions. For the engines intended for subsonic civil transport the range of critical operating conditions is relatively small, but for engines intended for high-speed propulsion and for combat aircraft, performance may be critical at several widely separated operating points. Although the chapter is predominantly aimed at the engine for the New Efficient Aircraft, the chapter also lays the ground for the off-design behaviour and treatment of combat aircraft.

The treatment in this chapter is deliberately approximate and lends itself to very simple estimates of performance without the need for large computers or even for much detail about the component performance. The ideas which underpin the approach adopted are physically sound and the approximations are sufficiently good that the correct trends can be predicted; if greater precision is required the method for obtaining this, and the information needed about component performance, should be clear. The programme GasTurb is widely available and allows for more precise modelling of engine behaviour.

Assumptions and simplifications

Engine performance is primarily determined by the inlet air stagnation pressure and temperature and by the fuel flow. It may also be affected by the ambient static pressure, as discussed in Chapter 8. Of these inputs only the fuel flow may be treated as the control or independent variable. Depending on the fuel flow are the thrust of the engine, the mass flow of air, the rotational speeds of the shafts and the temperatures and pressures inside the machine. If the engine operates at a constant condition the appropriate non-dimensional values of these quantities must be constant.