3.0 Introduction

This chapter looks at how a jet engine produces thrust, which is a simple consequence of Newton's laws of motion applied to a steady flow. It requires the momentum to be higher for the jet leaving the engine than the flow entering it, and this inevitably results in higher kinetic energy for the jet. The higher energy of the jet requires an energy input, which comes from burning the fuel. This gives rise to the definition of propulsive efficiency (considering only the mechanical aspects) and overall efficiency (considering the energy available from the combustion process).

With few exceptions this book will be concerned with bypass engines. These are engines where some or most of the incoming air passes around and outside the core of the engine: this is the bypass stream. A fraction of the air enters the core and passes through the combustor. The bypass ratio is defined by mass flows of air as

The total mass flow rate is given by

Early bypass engines had more air going through the core than through the bypass, that is a low bypass ratio, but modern high bypass engines have around ten times as much air in the bypass stream. The jet velocity from the core and bypass need not be equal but they are normally designed to be similar and for the present purposes may be taken to be equal.

History and regulation

As jet air transport increased in the 1960s 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 Figure A1.

The noise at the lateral position is the highest noise measured along a line parallel to the runway whilst the aircraft is departing at full power and the maximum usually occurs when the aircraft has climbed to about 1000 feet. Flyover noise is measured directly under the flight path after take-off and at an altitude where it is normal to cut-back the power to reduce the noise whilst still maintaining a safe rate of climb. The approach noise is also measured directly under the flight path as the aircraft prepares to land, with the glide slope carefully controlled. The flights are for the maximum allowed weight of the aircraft and correspond to standard day temperatures (which will generally require corrections to be made to the measurements since tests are rarely carried out at precisely the standard conditions). Needless to say, aircraft do not always operate as specified for the tests, but the tests do at least provide a standard way of comparing aircraft and thereby regulating airport operations.

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.