The emphasis of Part 1 of the book has been overwhelmingly towards the aerodynamic and thermodynamic aspects of a jet engine. These are important, but must not be allowed to obscure the obvious importance of a wide range of mechanical and materials related issues. In terms of time, cost and number of people, mechanical aspects of design consume more than those which are aerodynamic or thermodynamic. Nevertheless this book is concerned with the aerodynamic and thermodynamic aspects and it is these which play a large part in determining what are the desired features and layout of the engine. Clearly, an aerodynamic specification which called for rotational speed beyond what was possible, or temperatures beyond those that materials could cope with, would be of no practical use.

An aircraft engine simultaneously calls for high speeds and temperatures, light weight and phenomenal reliability; each of these factors is pulling in a different direction and compromises have to be made. Ultimately an operator of jet engines, or a passenger, cares less about the efficiency of an engine than that it should not fall apart. Engines are now operating for times in excess of 20,000 hours between major overhauls (at which point they must be removed from the wing), and this may entail upward of 10,000 take-off and landing cycles. In-flight engine shut-downs are now rare and the rate for the fleet of modern civil aircraft is one shut-down in about 250,000 flying hours. As a result most pilots will never experience a compulsory engine shut-down during their whole careers.

12.0 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.

15.0 Introduction

Figure 15.1 shows a cross-section through a modern engine for a fighter aircraft and the large differences between it and the modern engines used to propel subsonic transport aircraft, Figure 5.4, are immediately apparent. Above all the large fan which dominates the civil engine, needed to provide a high bypass ratio, is missing. Engines used for combat aircraft typically have bypass ratios between zero (when the engine is known as a turbojet) and about unity; most are now in the range from 0.3 up to about 0.7 at the design point, though the bypass ratio does change substantially at off-design conditions.

This chapter seeks to explain why fighter engines are the way they are. It begins with some discussion of specific thrust, equal to net thrust per unit mass flow, since this is a better way of categorising engines than bypass ratio; fighter engines have higher specific thrust than civil transport engines. Then the components of the combat engine are described, pointing out features common to the civil engine and drawing attention to their differences. Features peculiar to the combat engine, such as the mixing of the core and bypass stream, the high-speed intake, the afterburner and the variable area propulsive nozzle are also considered. A brief treatment of the thermodynamic aspects of high-speed propulsion leads into the constraints on the performance of engines for combat aircraft and the rating of engines.

19.0 Introduction

When the engine for a new civil transport, the New Large Aircraft, was considered in Chapters 1 to 10 many assumptions were introduced to make the treatment as simple as possible. In the treatment of the engine for a New Fighter Aircraft in Chapters 13–18 the level of complexity was increased. The properties of the gas were allowed to be different before and after burning of the fuel and the effect of the mass flow of fuel added to the gas passing through the turbine was included. The effect of the cooling air supplied to the turbines was allowed for and the effect of the pressure loss in the combustor was accounted. It is appropriate to recalculate the performance of an engine for the civil aircraft with some of these effects included and that level of fidelity will apply tomost of this chapter.

Another difference between the treatment for the civil engine in Chapters 1–10 and the treatment for the combat aircraft was the mixing of the core and bypass streams upstream of the final propulsive nozzle in the combat engine. Some large civil engines are mixed and this chapter therefore opens with a brief consideration of this option. Following this the consequences of different levels of fidelity in modelling will be addressed. A significant part of the chapter uses the most accurate model to look at the impact of cooling air, pressure drop in the combustor and component efficiency on the thrust and sfc of engines; this is done first for the engine on-design and then off-design. The chapter concludes with a brief consideration of propulsion for high-speed civil aircraft.

9.0 Introduction

The compressor, which raises the pressure of the air before combustion, and the turbine, which extracts work from the hot high-pressure combustion products, are at the very heart of the engine. Up to nowwe have assumed that it is possible to construct a suitable compressor and turbine without giving any attention to how this might be done. In this chapter an elementary treatment is given with the emphasis being to find the overall features of the compressors and turbines, including the number of stages, the suitable rotational speeds, the diameters and some indication of the flowpath. The details of blade shape will not be addressed. Further information is obtainable in a recent book by Dixon and Hall (2013) and at a more specialised level for compressors in Cumpsty (2004).

The description of turbomachinery is based on the fan, compressor and turbine for the engines of the NEA. To avoid unnecessary duplication, the design is restricted to the case with a fan pressure ratio of 1.5 at cruise. At this condition the stagnation pressure and temperature entering the engine are p02 = 35.6kPa and T02 = 245.4K. From Exercise 7.4 the mass flow through the engine to give the required thrust is 392 kg/s and of this 32.1 kg/s goes through the core (bpr = 11.2). The turbine inlet temperature is 1500 K, the overall pressure ratio is 45 and the HP compressor pressure ratio is 18. From Exercise 7.6 the fan diameter is 2.66 m.

8.0 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.

8.1 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.

13.0 Introduction

This part of the book begins the consideration of the engine requirements of a new combat aircraft. In parallel with the treatment in earlier chapters for the engines of the New Efficient Aircraft, the approach chosen is to address the design of engines for a possible new aircraft so that the text and exercises can be numerically based with realistic values. The specifications for the New Fighter Aircraft used here have a marked similarity to those available for the new Eurofighter or Typhoon.

The topic of the present chapter is the nature of the combat missions and the type of aircraft involved. Figure 13.1 shows the different regions in which aircraft operate in terms of altitude versus Mach number, with the lines of constant inlet stagnation temperature overlaid. We are concerned here with what are referred to in the figure as fighters, a major class of combat aircraft. Figure 13.1 shows the various boundaries for normal operation. Even high-speed planes do not normally fly at more than M = 1.2 at sea level because in the high density air the structural loads on the aircraft and the physiological effects on the crew become too large. At high altitude high-speed aircraft do not normally exceed M ≈ 2.3, largely because the very high stagnation temperatures preclude the use of aluminium alloys without cooling. The boundary to the left in Figure 13.1 corresponds to the aircraft having insufficient speed to create the necessary lift; this is the stalling boundary and can be expressed in terms of the equivalent airspeed, that is the airspeed at sea level having the same value of 1/2ρV2.

11.0 Introduction

Up to this point consideration has been given only to the design point of the engine. This is clearly not adequate for a variety of reasons. Engines sometimes have to give less than their maximum thrust to make the aircraft controllable and to maintain an adequate life for the components. Furthermore all engines have to be started, and this requires the engine to accelerate from very low speeds achieved by the starter motor. The inlet temperature and pressure vary with altitude, climate, weather and forward speed and these need to be allowed for.

To be able to predict the off-design performance it is necessary to have some understanding of the way the various components behave and this forms the topic of the present chapter. It is fortunate that to understand off-design operation and to make reasonably accurate predictions of trends it is possible to approximate some aspects of component performance. The most useful of these approximations is that the turbines and the final propulsive nozzle are perceived by the flow upstream of them as choked. Another useful approximation is that turbine blades operate well over a wide range of incidence so that it is possible to assume a constant value of turbine efficiency independent of operating point. These approximations make it possible to consider the matching of a gas turbine jet engine and to see how the various components operate together at the conditions for which they are designed (the design point) and at off-design conditions. Off-design performancewill form the topic of Chapter 12.

5.0 Introduction

This chapter looks at the layout of some jet engines, using cross-sectional drawings. This begins with relatively simple engines and leads to engines for a recent large aircraft, the Boeing 787 and an engine for the smaller Bombardier C-series. Two concepts are introduced in the chapter. One is the multi-shaft engine with separate low-pressure and high-pressure spools. The other is the bypass engine in which some, very often most, of the air compressed by the fan bypasses the combustor and turbines.

Any consideration of practical engines must address the temperature limitations on the turbine. The chapter ends with some discussion of cooling technology and of the concept of cooling effectiveness.

5.1 The turbojet and the turbofan

Figure 5.1 shows a cut-away drawing of a Rolls-Royce Viper engine. This is typical of the simplest form of turbojet engine, which was the norm in the 1950s when it entered service, with an axial compressor coupled to an axial turbine, all on the same shaft. (The shaft, the compressor on one end and turbine on the other are sometimes referred to together as a spool.) Even for this very simple engine, which was originally designed to be expendable as a power source for target drones, the drawing is complicated. For more advanced engines such drawings become unhelpful at this small scale and simplified cross-sections are therefore more satisfactory and will be shown. A simplified cross-section is also shown for the Viper in Figure 5.1, as well as a cartoon showing the major components.

Preface to the third edition

The civil aircraft, the Airbus A380, which resembles the New Large Aircraft of Part 1 of earlier editions, has been in service for several years. Attention in the aircraft industry has now shifted to two-engine aircraft with a greater emphasis on reduction of fuel burn, so the model created for Part 1 is the New Efficient Aircraft, a twin aimed at high efficiency. There is another change to highlight, which is the switch to using fan pressure ratio as the independent design variable rather than bypass ratio. In the time since the first edition, the typical fan pressure ratios have been reduced, and this has necessarily led to a considerable increase in complexity. The changes relating to military combat engines are relatively small.

Another major change is the inclusion of a co-author. Andy Heyes had used the second edition in teaching a course in Imperial College and was well placed in terms of knowledge and experience to work on the third edition.

For the third edition, we would like to acknowledge additional help from friends and colleagues. From Rolls-Royce, we would mention Conrad Banks, John Bolger, Simon Gallimore, Peter Hopkins, Glen Knight, Paul Madden, Alan Newby, Ian Rainbow and Joe Walsh; from Pratt and Whitney, Yuan Dong, Alan Epstein and Jayant Sabnis; from Stanford University, Juan Alonso and Anil Variyar; from the University of Cambridge, Chez Hall and John Young; from Imperial College, Aaron Costall, Ricardo Martinez-Botas and Peter Newton; from Ohio State, Mike Dunn, retired from NASA, Tony Strazisar; retired from General Electric, Meyer Benzakein; and retired from Airbus, Jeff Jupp.

2.0 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.

2.1 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.