Introduction

This chapter sets out the background to the new airliner which is to form the basis of the first part of this book. The aircraft, to be called the New Efficient Aircraft (NEA), will be a large wide-body aircraft designed to give low fuel burn, in anticipation of the likely rise of fuel price and pressure to reduce CO2 emissions. The aircraft will have two engines.

The costs and risks of a new aircraft or engine project are huge, but the profits might be large too. Some background is first discussed concerning the history and business of jet propelled aircraft and the impact of concerns for the environment. In explaining the requirements some of the units of measurement used are discussed. Design calculations in a company are likely to assume that the aircraft flies in the International Standard Atmosphere (or something very similar) and this assumption will be adopted throughout this book. The standard atmosphere is introduced and discussed towards the end of the chapter.

1.1 Some background

The age of jet travel really got started when the Boeing 707 entered service in 1958. By the time this aircraft was initiated, Boeing had already acquired considerable experience of large multi-engine jet aircraft, bombers and tankers, so it was in a strong position to make good design choices. The 707 was conceived as a long-range aircraft, which in those days meant it was capable of flying across the Atlantic non-stop with a full load of passengers, typically 110 in a two-class cabin. The range with maximum payload was only 2800 nautical miles (nm), but the shortest distance between London and New York is 2991 nm and going west there are normally headwinds that increase the effective distance. Such flights would therefore operate with less than the maximum payload, which would mean less than maximum freight on board, if all seats were taken.

6.0 Introduction

In treating the gas turbine it is essential to make proper acknowledgement of the compressible nature of the air and combustion products. Compressible fluid mechanics is a large and highly developed subject, but here only that which is essential to appreciate the treatment and carry out the designs is given. There are also special approaches for handling the compressible, high-speed flow inside ducts which need to be introduced, and that is the purpose of this chapter. The most important book dealing with this topic is Shapiro (1953), but a more accessible account is given, for example, by Munson et al. (2009).

6.1 Incompressible and compressible flow

For liquids the changes in density are normally negligible and it is possible to treat the flow as incompressible. Thus the equation for steady frictionless flow along a streamline,

VdV + dp/ρ = 0,

can be integrated directly, assuming the density is constant, to give Bernoulli's equation

1/2V2 + p/ρ = p0/ρ, a constant.

p0 is the stagnation or total pressure and corresponds to that pressure obtained when the flow is brought to rest in a frictionless or loss-free manner. The term 1/2ρV2 is known as the dynamic pressure or dynamic head. A pitot tube records the stagnation pressure whereas a pressure tapping in a wall parallel to the flow records static pressure. The use of stagnation pressure is something like a book-keeping exercise – it indicates the pressure which would be achieved if the flow were decelerated to rest is a loss-free manner. The stagnation pressure also represents the pressure in a reservoir from which the fluid could be accelerated to velocity Vj and this is illustrated in Figure 6.1. The difference between stagnation pressure and static pressure is the dynamic pressure 1/2ρV2. An analogy which is sometimes helpful can be drawn between the hydraulic system and a mechanical system: static pressure is analogous to potential energy and dynamic pressure is analogous to kinetic energy.

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.