In most of the traditional areas of performance studies the central effort is directed towards a refinement of aerodynamic design in order to ensure efficiency of flight. In studying take-off and landing performance attention is directed more to the capacity of the engines to accelerate the aircraft in a condition of high drag coefficient when the available distance is limited, and to the braking capacity during landing for the same reason. There is limited opportunity for refinement of aerodynamics, although in recent years some progress has been made in reducing the drag of an aircraft in the take-off configuration. In the crucial periods close to lift-off and touchdown the behaviour of the aircraft is strongly dependent on piloting technique and there is a need to define standard procedures for these two manoeuvres, based on reference speeds which are used in defining the criteria laid down in airworthiness regulations for safe operations. The lengths of the ground run and the airborne sector in a take-off are both strongly influenced by engine performance and the effective drag polar, whereas in landing there is an airborne sector which is critically dependent on piloting technique to produce a tangential flare to touchdown, followed by a ground run which depends on braking capacity.

The take-off performance cannot be defined so simply when allowance is made for failure of an engine and this chapter considers not only the ideal performance when the manoeuvre proceeds as planned, but also the reduced performance obtained after an engine failure.

The range of an aircraft is the distance that can be flown while burning a specified mass of fuel and is one of the few simple performance parameters by which the commercial value of an aircraft may be judged. Any change in design which leads to an increase in range is always desirable because it gives either

1. (i) a reduction in the fuel needed for a given distance, or

2. (ii) an increased distance for a given fuel load.

The reduced fuel of (i) reduces the cost of fuel for a specified flight and may also allow more payload to be carried for a specified total aircraft weight, provided there is the necessary space in the aircraft. The increased distance of (ii) allows the aircraft to fly over a long distance with fewer stops for refuelling and for a military aircraft it gives a valuable increase in the radius of action.

Any flight involves take-off, climb, cruise, descent and landing but except for short flights the greater part of the fuel is used in the cruise and the emphasis in this chapter is on cruising range, although the fuel used in climb and descent is also considered. The speeds and heights that should be used in the cruise to give maximum range are discussed in some detail, taking account of the constraints that are usually imposed by Air Traffic Control. It is assumed that the aircraft is flying in still air, except in §7.12 where it is shown that a wind affects not only the range relative to the ground but also the optimum speed for achieving maximum ground range.

On an aircraft in supersonic flight shock waves are always present, extending to a great distance from the aircraft. As mentioned in §3.1.2 they are the cause of an additional component of drag, the wave drag, which has been neglected in the preceding chapters but which must be considered in transonic and supersonic flight as an important component of the total drag. The coefficient of wave drag of an aircraft depends on the lift coefficient and on the Mach number M and hence, with this component of drag included, the drag polar relating CD to CL now depends on M. The drag polar may still be represented approximately by the simple parabolic drag law as given by Equation (3.6) but now the coefficients K1 and K2 are functions of Mach number.

In the preceding chapters where wave drag was neglected, a curve relating β to Ve, as in Figure 2.9, could be applied to all heights, for a given aircraft weight, but with wave drag included this is no longer valid, because for any given value of Ve the Mach number varies with height. Thus no simplification can be obtained by the use of Ve, rather than V or M, and consequently the use of Ve* and the speed ratio ν is no longer helpful in the calculation of performance. In contrast, the quantity (ƒ − β) is still of prime importance in determining rate of climb and acceleration, as it is at lower speeds, and in general this quantity must be calculated for each combination of speed and height.

The drag acting on an aircraft is of supreme importance in determining either the performance obtainable with a given thrust or the thrust required to achieve a specified performance, the latter being important because for a given speed the rate of consumption of fuel is approximately proportional to thrust. This chapter gives a brief account of the principal components of drag and the essential flow mechanisms on which they depend. Particular attention is paid to the nomenclature used for the drag components and to the conditions for various minima, because the undisciplined use of some of the terms has led to confusion in the past.

The drag polar for an aircraft has been discussed in Chapter 2 and in this chapter the representation of the polar by a simple mathematical expression is considered. The commonly used simple parabolic drag law has the great advantage of allowing many aspects of performance to be expressed in terms of simple equations, but the limitations of this drag law are emphasised and some alternative laws are introduced. In using either the simple parabolic law or any of the alternatives it is important to ensure that the constants in the equations are chosen to give the best possible agreement with the real drag polar over the range of CL that is important.

The flight conditions for minimum drag and for minimum drag power have been discussed briefly in Chapter 2 and in this chapter they are examined in more detail, making use of the parabolic equation for the drag polar mentioned earlier.

Components of drag

The total drag of an aircraft may be regarded as the sum of several components.

Noise, or unwanted sound, is generated whenever the passage of air over the aircraft structure or through its power-plants causes fluctuating pressure disturbances that propagate to an observer in the aircraft or on the ground below. Since the flight condition cannot be maintained unless these air- and gasflows are controlled efficiently, there are ample opportunities for sound to be produced. Fluctuating pressure disturbances result from inefficiencies in the total system and occur whenever there is a discontinuity in the airflow-handling process, particularly in the engines, where power generation involves large changes in pressure and temperature. This is not to say that the airframe itself is devoid of sound-producing opportunities, for it has a large surface area and, in the configuration that is adopted for take-off and approach, both the landing gear and high-lift devices (slats and flaps) create significant amounts of turbulence.

To the community beneath the aircraft, the self-generated noise from the airframe is normally significant only during the approach phase of operation, where the sources shown in Figure 3.1 can combine to exceed the level of each major noise source in the power-plant. For this reason, airframe noise has been thought of as the ultimate aircraft noise “barrier”. Perhaps we should briefly consider this and other airframe-associated sources before moving into the more complex subject of the aircraft power-plant.

Over the past thirty to forty years, a vast knowledge of aircraft noise-control techniques has been acquired. Some of the findings have contributed to the improved airport-noise climate of today; the less successful and the failures have taught both useful and bitter lessons. This chapter concentrates on success – success in the field of the jet engine, for it was this propulsion system that really started the serious noise problem.

For a broader understanding of the problem, the specific means of controlling aircraft power-plant noise should be discussed in relation to design philosophy, which, in turn, is related to either date of concept or aircraft mission requirements. The following sections deal with the range of power-plants in service in two convenient categories, low- and high-bypass-ratio types.

Suppression of the early jets

As explained in Chapter 3, it is the basic cycle and the maximum thrust level of the engine that determines the level of jet noise produced. The first generation of pure jets and, to a large extent, the low-bypass-ratio engines that succeeded them, all had extremely high exhaust velocities, which caused high levels of jet noise. At full power, supersonic exhaust flows with velocities of up to 600 m/sec were not uncommon and the characteristic crackling and tearing sounds of the mixing noise were augmented by the presence of shock-associated noise.

Without today's turbine cooling technology, there was no opportunity whatsoever to reduce the jet noise in these engines by modifying the engine cycle.

The aircraft noise problem is an environmental “minus” that was born alongside the commercial gas turbine aeroengine more than thirty years ago. The issue came to a head in the 1960s, around the time when the number of jet-powered aircraft in the commercial fleet first exceeded the number of propeller-powered aircraft. During that decade, significant sums of money were expended in research and development exercises directed at quietening the exhaust noise of the subsonic fleet and in researching the noise of even higher-velocity jets that would have been a major problem if the supersonic transport had become a commercial success. Later in the 1960s, with the advent of the bypass-engine cycle, the emphasis was more on the noise generated inside the engine, with the large proportion of the extensive research funds necessary being provided by the taxpayers in those nations with aircraft- and engine-manufacturing capabilities.

Government action was not only limited to supplying the funds for research contracts, but major nations cooperated on a political front to develop noise certification requirements that were demanded of the manufacturers of all new aircraft produced from 1970 onwards. Coincidentally, technology moved forward and produced the high-bypass or turbofan-engine cycle, which, in reaping the benefits of the accelerated noise programmes of the 1960s, was only about one-quarter as noisy as the engines it replaced. Initially, however, the turbofan cycle was limited in its application to the new breed of larger wide-body jets, which had at least twice the passenger-carrying capacity of their forebears.

Aircraft noise prediction involves two types of activities: predicting the noise of an individual aircraft and assessing the cumulative effect of the complex pattern of operations in and out of a specific airport. The latter depends on the former for a wide range of aircraft types.

Aircraft noise

Without available direct measurements, the only method of assessing the impact of a completely new aircraft or power-plant design is to utilise a reliable prediction procedure. Such a procedure may be able to make use of a limited amount of directly relevant data, for example, engine test data where a development programme is under way, or it may have to rely entirely upon empirical component prediction procedures. The latter situation arises at the advanced project stage of any new aircraft design – a current example could be an aircraft with propfans or a second-generation supersonic transport with a novel variable-cycle propulsion system concept.

To be successful, aircraft noise prediction must be based on a reliable definition of aircraft performance and a confident prediction of the noise characteristics of the power-plant (as a function of power setting, altitude and flight speed). Where there are substantial and related measured data to support a new concept, their projection to the new aircraft situation can follow fairly well-defined routes. For example, if the new aircraft incorporates power-plants that are not much different from versions already in service, flight test data can be transposed fairly simply to a new situation. Where the measured information is obtained from static engine tests during the development programme, methods are being established for transposing these data to the flight situation.

Over the years, much has been said about the future. Usually, environmentalists predict that worse is to come, the industry expects a rosy future, whilst governments try to present a “balanced” view.

The future always depends on the starting point; and no two observers, or victims of aircraft noise, have the same viewpoint. Some live near well-developed and possibly operationally saturated airports and therefore experience the benefits of advancing technology in the form of lower individual aircraft noise levels and a generally improving environment; others are adjacent to new or expanding fields, where the growth in the number of operations is the overriding factor; a small percentage of people would complain in any case – if aircraft were silent there would be some who would perceive a “stealth” factor.

The importance of aircraft noise in society in the future will be a function of several factors. Uppermost are whether the manufacturing industry will be successful in advancing noise control technology, whether air transportation is going to expand to any significant degree and how sensibly government and airport operators apply controls. All these factors bear examination in trying to build a picture of the future, as does the question of whether the available noise-forecasting tools are right for the job. This is most important; the weighting applied to individual elements of forecasting methods must be examined, since they can have a tremendous influence on predicted trends in noise “exposure” and, hence, any decisions based on those predictions.

Noise contours, or statements of the noise level heard at various positions on the ground around an airport, are computed rather than measured because of the large areas of ground covered and the length of time over which noise data have to be averaged.

The noise at any point on the ground in the vicinity of an aircraft operation will depend upon a number of factors. Uppermost amongst these are the types of aircraft and their power-plant; the power, flap setting and airspeed conditions throughout the operation; the distances from the points on the ground to the aircraft; and the effect of local topography and weather on propagation of the noise. Most airport operations will include a variety of aircraft and flight procedures and a wide range of operational weights. Because of the large quantity of data (which can be regarded as aircraft or airport specific) required to compute the noise of each individual operation, it is normal to make certain simplifications to reflect average noise exposure over long time periods. These time periods may range from as little as one day to several months.

The normal process of computation embraces three main steps:

1. the calculation of noise level from individual aircraft movements at a matrix of observation points around the airport;

2. the integration of all the individual noise levels over a defined period of time; and

3. the interpolation and plotting of the information in the form of noise contours.

The simplifying assumptions that are most frequently made include the noise levels of “groups” of similar aircraft types, average climatic conditions and the average operational pattern over the time period in question.