INTRODUCTION

The gas turbine has many important applications but it is most widely used as the jet engine. In the last few years, since the regulations changed to permit natural gas to be burned for electricity generation, gas turbines have become important prime movers for this too. Many of the gas turbines used in land-based and ship-based applications are derived directly from aircraft engines; other gas turbines are designed specifically for land or marine use but based on technology derived for aircraft propulsion.

The attraction of the gas turbine for aircraft propulsion is the large power output in relation to the engine weight and size – it was this which led the pre-Second World War pioneers to work on the gas turbine. Most of the pioneers then had in mind a gas turbine driving a propeller, but Whittle and later von Ohain realised that the exhaust from the turbine could be accelerated to form the propulsive jet.

This chapter looks at the operation of simple gas turbines and outlines the method of calculating the power output and efficiency. The treatment is simplified by treating the working fluid as a perfect gas with the properties of air, but later some examples are discussed to assess the effect of adopting more realistic assumptions. It is assumed throughout that there is a working familiarity with thermodynamics – this is not the place to give a thorough treatment of the first and second laws (something covered very fully in many excellent text books, for example Van Wylen and Sonntag, 1985).

INTRODUCTION

This part of the book begins the consideration of the engine requirements of a new fighter aircraft. In parallel with the treatment in earlier chapters for the engines of the new large civil 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.

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 book has been well received and Cambridge University Press approached me with the invitation to bring out a second edition. This was attractive because of the big events in aerospace, most significantly the decision by Airbus Industrie at the end of 2000 to launch their new large aircraft, the A380. This meant that some changes in the first ten chapters were needed. Another major development is the decision to develop an American Joint Strike Fighter, the F-35.

Another more personal change took place when I left academia to become Chief Technologist of Rolls-Royce from the beginning of 2000. It should be noted, however, that the character and ideas of this second edition remain those of the university professor who wrote the first edition and do not reflect my change of role.

The aim and style of the book is unchanged. The primary goal of creating understanding and the emphasis remains on simplicity, so far as this is possible, with the extensive use of relevant numerical exercises. In a second edition I have taken the opportunity to update a number of sections and to include some explanatory background on noise; noise has become a far more pressing issue over the last four or five years. The book remains, however, very similar to the first edition and, in particular, numerical values have been kept the same and the exercises have not been changed.

INTRODUCTION

This chapter looks at the layout of some jet engines, using cross-sectional drawings, beginning with relatively simple ones and leading up to the large engines for one of the most recent aircraft, the Boeing 777. Two concepts are introduced. 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.

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 were the norm about 40 years ago, 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 and for more advanced engines such drawings become unhelpful at this small scale. Simplified cross-sections are therefore more satisfactory and these will be shown for more advanced engines. A simplified cross-section is also shown for the Viper in Fig. 5.1, as well as a cartoon showing the major components.

INTRODUCTION

In this chapter we will consider three separate engine designs corresponding to distinct operating conditions. For convenience here the three design points are at the tropopause (altitude 11 km; standard atmosphere temperature 216.65 K and pressure 22.7 kPa) for Mach numbers of 0.9, 1.5 and 2.0. The thrusts required for these conditions were determined in Exercise 14.4. At each condition a separate engine is designed – this is quite different from designing the engine for one condition and then considering its operation at different conditions, which is the topic of Chapter 17.

For this exercise all design points will correspond to the engine being required to produce maximum thrust, even though the ultimate suitability of an engine for its mission may depend on performance, particularly fuel consumption, at conditions for which the thrust is very much less than maximum. The designs will first be for engines without an afterburner (operation ‘dry’) and then with an afterburner; the afterburner will be assumed to raise the temperature of the exhaust without altering the operating condition of the remainder of the engine so the stagnation pressure entering the nozzle is unchanged.

The engines considered will all be of the mixed turbofan type – such an engine was shown in Fig. 15.1 with a sketch showing the station numbering system adopted. Note that the numbering shows station 13 downstream of the fan in the bypass and station 23 downstream of the fan for the core flow; in the present simplified treatment it will be assumed that p023 = p013 and that T023 = T013.

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. This increase in complexity included allowing the properties of the gas to be different for burned and unburned air; the effect of the mass flow of fuel added to the burned air 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 losses in the combustor, the bypass duct and the jet pipe were demonstrated. It is appropriate to recalculate the performance of an engine for the civil aircraft with some of these effects included.

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 propelling nozzle in the combat engine. Some engines on subsonic transport aircraft also have mixed streams; Fig. 19.1 shows photographs of an unmixed and a mixed engine on the wing of two contemporary aircraft, the Airbus-330 and Airbus-340-300. By a simple treatment it is possible to demonstrate the advantages which the mixed configuration brings.

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, Fig. 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, 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 engine are described, pointing out special features of components common to the civil engine, and giving a general treatment of the special features: the mixing of the core and bypass stream, the high-speed intake, the afterburner and the variable nozzle. 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.

In earlier chapters the cooling air supplied to the turbine was neglected in calculating the cycles, so too was the mass flow rate of fuel in the gas through the turbine.

INTRODUCTION

This chapter will look only briefly at the design of the compressors and turbines for combat engines, following on from Chapter 9 for the civil transport engine. The flow Mach numbers inside the compressors tend to be higher than for the subsonic transport and this makes the treatment of each blade row rather special; the design rules must take account of the presence of strong shock waves. A very important design consideration is how the compressor will behave at off-design conditions since combat engines are off-design for so much of their operation. The problem arises because of the large density ratio between inlet and outlet of the compressor, and the reduction in this ratio when N/√(c pT0) is decreased. The turbine stages do not suffer from this off-design problem. The turbines are required to produce large work output in relation to the blade speed, that is Δh0/U2 must be high, but at off-design conditions for the engine the turbine condition is essentially unaltered from the condition at design. This, it may be recalled from Chapters 12 and 17, is because the turbines and the propelling nozzle are effectively choked, so the turbines are forced to operate at the same non-dimensional condition.

In this chapter the consideration will be based on the case 1 engine of Chapter 17, with fan pressure ratio 4.5 and HP compressor ratio 6.66 at design point, sea-level static.

History and regulation

As jet air transport increased in the 1960's the annoyance to people living and working around major airports was becoming intense. Regulations affecting international air transport are governed by the International Civil Aviation Organisation (icao), but this body was moving so slowly that in 1969 the US Federal Aviation Agency (faa) made proposals for maximum permitted noise levels. After extensive discussions in the USA these were formally approved as Federal Aviation Regulation (FAR) Part 36 in 1971, retroactive with effect from 1969, but only for new aircraft. Shortly afterwards the icao Committee on Aircraft Noise published similar recommendations, to be known as Annex 16, a formal addendum to the 1944 Chicago Convention on Civil Aviation; each member state had then to accept the rules in Annex 16 and write them into their legal framework. The underlying principle for the noise certification of aircraft under FAR Part 36 and Annex 16 are similar and has remained unchanged ever since, with the levels under the US and icao rules subsequently becoming virtually identical.

The certification for noise relies on measurements at three positions, two for take off (referred to as lateral and flyover) and one for landing (referred to as approach). The levels are expressed in decibels (EPNdB) using effective perceived noise level (EPNL), described in outline below. The layout for testing is shown in Fig. A1.

INTRODUCTION

This chapter looks at some of the commercial requirements and background to the proposals to build a new civil airliner capable of carrying about 600 people. The costs and risks of such a project are huge, but the profits might be large too. 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. The chapter ends with brief reference to recent concerns about environmental issues.

SOME COMMERCIAL BACKGROUND

In December 2000 Airbus formally announced the plans to go ahead with a new large aircraft, dubbed the A380, intended in its initial version to carry a full payload (with 555 passengers) for a range of up to 8150 nautical miles. First flight is intended to be in 2004 and entry into service in 2006. There are already plans afoot for heavier versions, carrying more that 555 passengers and for all-freight versions with a larger payload. In December 2000 Airbus Industrie had received enough orders to justify the expected cost of over $10 billion, with an expected breakeven point with a sale of 250 aircraft. They forecast delivery of the 250th aircraft in 2011.

INTRODUCTION

A fighter aircraft is required to be agile, which requires it to turn sharply, to accelerate rapidly and usually to travel fast. It is no surprise that accelerating rapidly or travelling fast require large amounts of thrust from the engine. What may be more of a surprise is that rapid changes in direction require high levels of engine thrust. The reason is that the drag of the aircraft rises approximately with the square of the lift coefficient and making rapid turns demands high lift from the wings. An aircraft normally banks in order to turn so that the resultant of the gravitation acceleration and the centripetal acceleration is normal to the plane of the wings, Fig. 14.1, and the force they produce is exactly balanced by the wing lift. It is normal to express the increase in acceleration in terms of the load factor, denoted by n: a load factor of unity corresponds to an acceleration g perpendicular to the wing, when the lift is the normal weight of the aircraft, whereas a load factor of 5 corresponds to an acceleration of 5g and the lift is five times the weight. For a modern fighter aircraft structures are designed to withstand the approximate limit on acceleration set by the human pilot and load factors can be as high as 9.

For the civil airliner the turns are normally so gentle that the lift on the wings is little more than the weight of the aircraft, and the size of the engine is normally fixed by requirements at the top of the climb.

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.