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

Introduction

It would be possible to calculate the performance of an engine in the manner of Exercises 7.2 to 7.7 for every conceivable operating condition, e.g. for each altitude, forward speed and rotational speed of the components. This is not an attractive way of considering variations and it does not bring out the trends as clearly as it might. An alternative is to predict the variations by using the appropriate dynamic scaling – apart from its usefulness in the context of engine prediction, the application of what is conventionally called dimensional analysis is illuminating. The creation of groups which are actually non-dimensional is less important than obtaining groups with the correct quantities in them. The reasoning behind these ideas is discussed in Chapter 1 of Cumpsty (2004). For compatibility with the usual terminology, however, the phrases ‘dimensional analysis’ and ‘non-dimensional operating conditions’ will be retained here.

Using the ideas developed on the basis of dynamic scaling it is possible to estimate the engine performance at different altitudes and flight Mach numbers when the engine is operating at the same non-dimensional condition. From this it is possible to assess the consequences of losing thrust from an engine at cruise and, in some cases, to estimate how the engine will behave on a static test bed.

Engine variables and dependence

Figure 8.1 shows a schematic engine installed under a wing. For simplicity this is a mixed-flow engine, where the core and bypass streams are mixed before entering a single propulsive nozzle. Later the treatment can be generalized by considering the two streams separately. Alternatively, with bypass ratios to 10 or more, the errors may not be large if the analysis concentrates on the bypass stream and assumes the ratio of core jet velocity to bypass jet velocity stays approximately constant.