INTRODUCTION

The engine for a high-speed aircraft is required to operate over a wide range of conditions and some of these have been discussed in Chapters 13, 14 and 15. Of particular importance is the variation in inlet stagnation temperature, which can vary from around 216 K up to nearly 400 K for a Mach-2 aircraft. As a result the ratio of turbine inlet temperature to engine inlet temperature T04/T02 alters substantially, even when the engine is producing the maximum thrust it is capable of. In contrast, for the subsonic civil aircraft the value of T04/T02 changes comparatively little between take off, climb and cruise, the conditions critical in terms of thrust and fuel consumption, and it is normally only when a civil aircraft is descending to land or is forced to circle an airport that T04/T02 is reduced radically.

In Chapter 8 the dynamic scaling and dimensional analysis of engines was considered. There the engine non-dimensional operating point was held constant, for example T04/T02 is not constant, so the engine remained ‘on-design’. To designate the engine operating condition the value of N/√(c pT0) or any of the pressure ratios or non-dimensional mass flows could also be used, but T04/T02 has the intuitive advantage since engine thrust is altered by varying fuel flow rate to change T04. In Chapter 12 the more challenging issue of a civil engine operating away from its design condition was addressed, i.e. the case when T04/T02 ≠ constant, and the subject of this chapter is the similar case for military engines.