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2076 results in Aerospace engineering

Page 38 of 104

  • 5 - What's Under the Skin?: Structure and Propulsion

  • John P. Fielding, Cranfield University, UK
  • Book:
    Introduction to Aircraft Design
    Published online:
    28 May 2018
    Print publication:
    03 April 2017, pp 65-82

  • Summary

    GENERAL

    Having looked at the reasons for designing a new aircraft, its requirements and the reasons for its external shape, it is now the time to look at the interior. One might use the analogy of a human body to see that for successful operation it requires a skeleton, muscles, digestive system, sensors and a control system. These all work together efficiently even when they have different functions. The aircraft designer must produce a similar harmony within the aircraft interior. Each system is important, but each may have conflicting requirements. A good designer must weigh up the conflicts, use relevant analyses and synthesize the aircraft into an efficient whole. It is a truism that the outside of the aircraft has to be bigger than the space required inside the aircraft. It is often necessary to modify the external shape to accommodate the interior, with consequent aerodynamic changes.

    To return to the body analogy, this chapter will describe the aircraft's skeleton, muscles, sensors, etc. in terms of structure and propulsion. Fuel, flying controls, avionics, furnishing and weapon systems will be described in Chapters 6 and 7.

    THE STRUCTURE

    An aircraft structure should be designed to meet a number of conflicting requirements which include:

  • (i) Low weight.

  • (ii) Acceptable material and manufacturing costs.

  • (iii) Adequate strength to meet the maximum expected loads, with a suitable safety factor.

  • (iv) Adequate stiffness so that distortions are kept within acceptable limits.

  • (v) Good in-service properties, such as fatigue and corrosion resistance, together with tolerance of expected temperatures and other atmospheric conditions.

    • Materials

    Aircraft designers have a wide range of structural materials to choose from, including those described in the following subsections.

    Light Alloys

    Light alloys are essentially based on aluminium and are used on the majority of aircraft. Their advantages are the high strength and stiffness-to-weight ratios, relatively low cost, general ease of handling, except in special cases, availability, choice of many fabrication processes and, above-all, familiarity. Against this is the dependence upon relatively low-temperature heat-treatment processes to obtain the desired material properties, which implies poor strength and stiffness at elevated temperatures and relatively poor fatigue properties. The higher-strength alloys are poorer in fatigue and their use is restricted to predominantly compression-sensitive areas. Tensile loads produce more fatigue problems.

  • 8 - Why do Aircraft Cost so Much?

  • John P. Fielding, Cranfield University, UK
  • Book:
    Introduction to Aircraft Design
    Published online:
    28 May 2018
    Print publication:
    03 April 2017, pp 122-143

  • Summary

    GENERAL

    This chapter discusses the costs of buying and operating civil and military aircraft. The former costs are termed acquisition costs for both classes of aircraft, whilst the latter are termed operating costs for civil aircraft, and life-cycle costs (LCCs) for military aircraft. The acquisition costs are included as important elements of both operating costs and LCCs. The costs associated with aircraft reliability and maintenance are significant contributors to operating and LCCs, and design to reduce these costs is outlined at the end of this chapter.

    ACQUISITION COSTS (THE COSTS OF BUYING OR ACQUIRING THE AIRCRAFT)

    The Reasons for High Aircraft Acquisition Costs

    The main reasons are:

  • (i) High performance requirements – Civil and military aircraft operations are competitive, hence each new type is required to show improvements relative to existing aircraft. It is therefore necessary to undertake extensive research programmes and also to incorporate customer/operator refinements, which imply complexity.

  • (ii) Safety considerations – Civil and military aircraft have, rightly, very stringent safety requirements that must be proved before the aircraft enters service. Extensive proving and testing is required and there is a need for added complexity so that failure of individual components or systems can be tolerated. An aircraft has to continue functioning safely even after a failure has occurred. This applies to both hardware and software.

  • (iii) Quality control – The complexity and safety demands make it essential to have elaborate systems that record the complete history of not only each individual aircraft, but of each piece of material or part used in it. Inspection has to be complete, not sampled, and continuous throughout the life of the aircraft.

  • (iv) Tooling requirements – Complexity requires extensive and expensive tooling for production purposes, and the launch cost of a new type of aircraft is considerably influenced by this. Numerically controlled machine tools have considerable flexibility, and are very expensive. They can more than pay for themselves, however, on reasonable production runs, and their use is increasing. Automatic rivetting and bonding facilities also reduce labour content (see below). Another source of expense is that of assembly jigs.

  • (v) Labour-intensive production – In spite of the elaborate tooling, the complexity of an aircraft means that labour costs are by far the greatest part of the direct production costs (Fig. 8.1). Two approaches to reducing such costs are increasing automation or use of lower-cost labour.

  • 11 - What Can Go Wrong?: Some Lessons from Past Aircraft Projects, and a Glimpse into the Future

  • John P. Fielding, Cranfield University, UK
  • Book:
    Introduction to Aircraft Design
    Published online:
    28 May 2018
    Print publication:
    03 April 2017, pp 180-201

  • Summary

    INTRODUCTION

    The design, manufacture and operation of aircraft are very complex and potentially risky activities. There is a natural tendency towards trusting tried and tested configurations, and adopting an evolutionary approach. There are many cases, however, of revolutionary design changes, such as the advent of the jet engine, or the delta wing. Revolution involves more risk, but can be very rewarding. The history of aircraft design has had many examples of excitingly innovative, not to say weird designs. A minority of these are successful, some marginally successful, but many have been failures, some of them disastrously expensive failures! This chapter will describe a number of different projects and attempt to classify the reasons that led to their difficulties.

    On a more positive note, the chapter will conclude by describing a number of innovative designs that might have a great influence on the future of aeronautics – positively or negatively!

    AIRCRAFT THAT SUFFERED FROM REQUIREMENTS THAT WERE TOO RESTRICTIVE, TOO AMBITIOUS OR WERE CHANGED DURING DEVELOPMENT

    Ward [34] contains many examples of such projects from the depressing record of British aircraft developments since 1945, and several examples will be repeated here, both civil and military. Other sources were used for later aircraft.

    The Hawker Siddeley Trident

    This aircraft (Fig. 11.1) had many years of successful service in the UK and in China, and was a pioneer in the field of automatic landing, but sales were disappointing. British European Airways (BEA) were initially offered a short-haul jet transport, (the DH121) which would have carried 110 passengers over stage lengths of up to 1800 nmiles. The manufacturers’ projections in 1958 predicted sales of 550 aircraft. According to Ward [34] BEA had been examining market trends that showed a marked slip in 1958–9 and thus considered that the DH121 was too big. They required that the aircraft only have 97 seats over an 800 nmile range. The Trident was designed round this specification, as was the Spey turbo-fan engine. The aircraft satisfied the requirement, but it and the engine had limited development potential. The Boeing 727 design team developed a competing tri-jet to almost the same requirements as for the original DH121.

  • 2 - Why Should We Design a New Aircraft?

  • John P. Fielding, Cranfield University, UK
  • Book:
    Introduction to Aircraft Design
    Published online:
    28 May 2018
    Print publication:
    03 April 2017, pp 6-19

  • Summary

    The world has accepted that flying is an extremely efficient means of quickly transporting people, cargo or equipment, and performing a wide range of other activities. All operators need to increase efficiency, cost-effectiveness, environmental compatibility and safety. It is often possible to do this by modifying either the design or operation of existing aircraft. This is limited, however, by the inherent capabilities of the original design and the cost-effectiveness of modifications. Under these circumstances, it is necessary to consider the initiation of the design of a new aircraft. Aircraft manufacturers are usually in the business of making profit out of building aircraft. They may do this by means of building their own existing designs, modifying their designs or licence-building the designs of other companies. Another reason for the initiation of new designs is to retain or enhance their design capabilities.

    Safety is the most important aspect of aircraft design. This is the moral and legal responsibility of designers, manufacturers and operators of both civil and military aircraft. The regulations that have emerged aim to maximize safety, and must be complied with before an aircraft may receive a Type Certificate to allow it to be sold. A brief summary of such requirements is contained in Chapter 9.

    In recent years there has been a considerable increase in the importance of requirements associated with aircraft-related effects on the environment. These particularly concern noise, airport local air quality and global warming. Current environmental requirements are summarized in Chapter 9.

    Operator requirements and aircraft specifications come from a number of different sources, but they must all consider the needs of the aircraft operators, whether they are airlines or air forces. A certain path to disaster is to produce an aircraft that no one will buy!

    Descriptions will be given of the two main means of deriving a requirement specification, namely the results of market surveys and individual aircraft operators’ specifications.

    MARKET SURVEYS

    The major aircraft manufacturers employ marketing departments that produce annual reports [2]. Historical data are analyzed and extrapolated in such areas as world economic indicators. Figure 2.1 shows gross domestic product (GDP) and highlights the recent recessions, and shows the close correlation between GDP and the world air travel growth in revenue passenger miles (RPM).

  • 7 - What's under the skin?: Avionics, Flight Control and Weapon Systems

  • John P. Fielding, Cranfield University, UK
  • Book:
    Introduction to Aircraft Design
    Published online:
    28 May 2018
    Print publication:
    03 April 2017, pp 101-121

  • Summary

    AVIONIC SYSTEMS

    Avionics is one of the most rapidly developing fields of aircraft design. Its importance and range has increased over recent years and as much as 40% of the cost of a new aircraft can be attributed to avionics. There is a bewildering range of avionic systems, each of which usually requires the use of many acronyms.

    Figure 7.1 shows an avionics fit for a medium-range airliner of the future. This aircraft is intended for flight with a crew of one or two pilots. If this aircraft had been in operation during the early post-war years, three additional crew-members would have been required, namely navigator, wireless operator and flight engineer. Modern efficient, reliable avionics have dramatically simplified these operations and have eliminated to need for these crew members.

    The growth in the capabilities of military avionics has been even more dramatic and make it possible for pilots of single-seat aircraft to navigate, communicate, detect and attack targets at heights of 100 ft and speeds approaching the speed of sound. Figure 7.2 shows a schematic of such an avionic system whilst Fig. 7.3 shows the installation of components on the instrument panel.

    Having looked at typical aircraft installations, the next stage is to examine the functions of various types of avionic systems in the following major groups: communications, navigation systems, radar systems and others.

    Communications

    Airborne communications systems vary considerably in size, weight, range, power requirements, quality of operation and cost, depending upon the desired operation.

    The most common communications system in use today is the very high frequency (VHF) system. In addition to VHF equipment, large aircraft are usually equipped with high frequency (HF) communications systems and some are fitted with ultra high frequency (UHF) systems.

    VHF airborne communication sets operate in the frequency range from 100 to 150 MHz. VHF receivers are manufactured that cover only the communications frequencies, or both communications and navigation frequencies. In general, the VHF radio waves follow approximately straight lines. Theoretically, the range of contact is the distance to the horizon and this distance is determined by the heights of the transmitting and receiving antennas. However, communication is sometimes possible many hundreds of miles beyond the assumed horizon range. Typical ranges are 200 miles at 20 000 ft. UHF systems are similar to VHF but operate in the 200–400 MHz band.

  • 6 - What's Under the Skin?: Airframe Systems

  • John P. Fielding, Cranfield University, UK
  • Book:
    Introduction to Aircraft Design
    Published online:
    28 May 2018
    Print publication:
    03 April 2017, pp 83-100

  • Summary

    SECONDARY POWER SYSTEMS

    We have looked at an aircraft's primary power system, its propulsion, and will now examine its secondary power systems (SPS). Secondary power is usually provided by an aircraft's main engines when they are operating, but can be supplied by other sources such as an auxiliary power unit (APU) in the aircraft or an external source on the ground.

    Almost all present aircraft SPS are of three types, shown in Fig. 6.1.

    Power is extracted from turbo-fan powerplants in two forms. Bleed air is tapped from one or more points along the engine compressor to provide pneumatic power. Drive shafts, from the engine's high-pressure shaft, drive an accessory gear box on which are mounted generators and hydraulic pumps, as well as the engine's own accessories. A typical large turbo-fan is shown in Fig. 5.13.

    Turbo-jet powerplants can also supply bleed air and shaft power for systems, but turboprop powerplants in general have difficulty in supplying sufficient bleed air for airframe requirements. In such a case, extraction of shaft power to drive separate compressors may be considered.

    SPSs are often linked with each other, but it is convenient to separate them into the types described below.

    Air-Conditioning and Pressurization

    On many civil transports the largest amounts of secondary power are extracted in the form of bleed air and the largest continual user of this power is the aircraft's air-conditioning and pressurization system.

    It is usual to maintain an aircraft cabin at a pressure equivalent to that at 8000 ft or less, and this requires a constant supply of pressurized air. In addition to this, oxygen used by the occupants must be replaced and the temperature and contaminant levels controlled, requiring a significant throughput of air, although some of it is recycled through the cabin, after filtering.

    It is the control of temperature that usually determines the rate at which air must be supplied and, with the type of air-conditioning system installed on most aircraft, this also requires the air be supplied at pressures significantly above those in the cabin.

    Figure 6.2 shows the major elements of an air-conditioning system. There are extremely remote events where the pressurization system fails. In this case, on-board emergency oxygen systems are used to preserve life. The combination of air-conditioning, pressurization and oxygen systems are usually termed the environmental control system (ECS).

  • 1 - Introduction

  • John P. Fielding, Cranfield University, UK
  • Book:
    Introduction to Aircraft Design
    Published online:
    28 May 2018
    Print publication:
    03 April 2017, pp 1-5

  • Summary

    WHY ANOTHER AIRCRAFT DESIGN BOOK?

    Aircraft design is a complex and fascinating business and many books have been written about it. The very complexity and dynamic nature of the subject means that no one book can do it justice.

    This book, therefore, will primarily act as an introduction to the whole field of aircraft design leading towards the subjects summarized in Fig. 1.1. It will not attempt to duplicate material found in existing design books, but will give information about the whole aircraft design environment, together with descriptions of aircraft and component design. It also presents otherwise unpublished data and design methods that are suitable for aircraft conceptual, preliminary and detail design activities.

    TOPICS

    The following chapters are arranged as a series of questions about aircraft design, the answers to which give largely descriptive overviews of all aspects of aircraft design. This will provide an introduction to the conflicting requirements of aircraft design specialists in a design team, with a view to improving understanding and the integration of a sound overall design.

    The book is divided into chapters that answer a number of significant design questions.

    The question, ‘Why design a new aircraft?’ is answered in Chapter 2, which shows the derivation of aircraft requirements for civil and military aircraft from market surveys, and gives examples of operator-derived specifications.

    Chapters 3 and 4 answer the question, ‘Why is it that shape?’ with an initial discussion of aircraft wing and tail shapes, followed by descriptions of the configurations of a wide range of civil and military aircraft types.

    The question, ‘What's under the skin?’ is answered in Chapters 5, 6 and 7, which deal with structures and propulsion, airframe systems, avionics, flight controls and weapons, respectively. These chapters describe the interiors of aircraft, ranging from structures to weapons systems via airframe systems, avionic systems and landing gears.

    In Chapter 8 the crucial areas of acquisition and operating costs are discussed, some prediction methods are described and the importance of good reliability and maintainability are stressed in order to answer the question, ‘Why do aircraft cost so much?’

    The answer to the question, ‘What help can I get?’ is provided in Chapter 9, which contains a bibliography of the most important current aircraft design books.

  • 10 - The Shape of Things to Come: Should the Project Continue?

  • John P. Fielding, Cranfield University, UK
  • Book:
    Introduction to Aircraft Design
    Published online:
    28 May 2018
    Print publication:
    03 April 2017, pp 165-179

  • Summary

    INTRODUCTION

    This book has attempted to give an introduction to most aspects of aircraft design, with emphasis on giving the reasons for the shapes of aircraft, and descriptions of their constituent parts. The preface clearly stated that it was not intended to produce a book about aircraft conceptual design, as many good texts are currently available. Some of these are referred to in the previous chapter, and they should be used, with help, to produce aircraft conceptual designs. Conceptual designs, however, are not usually end-products, but merely important steps in the whole design–manufacture–operation cycle.

    Conceptual designs must be objectively assessed to see if they warrant the significantly increased expenditure that would be required to perform preliminary and detailed design. A vital precursor to this decision-making process is a clear definition of the characteristics of the conceptual design, and this is discussed in Section 10.2. Section 10.3 shows the options that are available following a conceptual design process and Section 10.4 describes simple decision-making techniques. The final section of this chapter gives an example of a conceptual-design definition, and describes the decisions that were made about its future.

    CONCEPTUAL DESIGN DEFINITION

    It is important to summarize clearly the results of the conceptual design process by providing the following types of information:

  • (i) Configuration description – This will be defined by means of conventional engineering drawings, tabular data, computer models, physical models or a combination of these methods, together with a brief description of the rationale of the design.

  • (ii) Mass estimates – These will include gross and empty masses, together with component mass estimates to be used as targets for subsequent work.

  • (iii) Performance estimates – Drag and lift predictions will be produced, together with powerplant data to give payload-range curves, field-performance, and direct operating cost targets for commercial aircraft. Military aircraft will also require predictions to be made of sortie performance, and point performance criteria such as attained turn rate, sustained turn rate and specific excess power.

  • (iv) Reliability and maintainability targets – The direct operating cost targets, mentioned above, imply estimates of acquisition costs as well as reliability and maintainability (R and M) characteristics. These may be used to set R and M targets for subsequent design stages (Appendix C).

Foundations of Gas Dynamics
  • Ruey-Hung Chen
  • Published online:
    31 March 2017
    Print publication:
    09 March 2017
  • Foundations of Gas Dynamics covers supersonic and subsonic flow phenomena where compressibility of the fluid cannot be ignored. It finds application in jet and rocket propulsion systems as well as handling industrial gas flow at high speeds. Students and engineers in the mechanical, aerospace, and chemical disciplines will find it useful. It begins with basic concepts such as isentropic flows, shock, and supersonic expansion waves in one dimension. These are followed by one-dimensional flows with friction and heat exchange. Two-dimensional theory with small perturbations is presented, with its applications illustrated by supersonic airfoils. Method of characteristics is used for flows with two independent variables, either with two spatial coordinates or with time variations in one dimension. In later chapters, acoustic wave propagation, supersonic flow combustion, and unsteady shock formation are treated thoroughly. The book ends with a chapter on basic hypersonic flow, with a discussion of similarity rules.

  • Preface

  • Iain D. Boyd, University of Michigan, Ann Arbor, Thomas E. Schwartzentruber, University of Minnesota
  • Book:
    Nonequilibrium Gas Dynamics and Molecular Simulation
    Published online:
    13 April 2017
    Print publication:
    23 March 2017, pp xvii-xx

  • Summary

    There are two top-level goals that we aim to address in this book: (1) to provide a description of a gas by considering its most basic constituents, i.e., atoms and molecules, and (2) to introduce readers to computer simulation techniques that are available to analyze a gas at this fundamental level.

    The first question we must ask ourselves is, Why should we consider a gas at the molecular level? After all, there are well-established equations and ideas that provide accurate descriptions of gas flow at the macroscopic level that employ variables such as density, flow velocity, temperature, and pressure, and these represent properties that take into account the molecules in an aggregate sense. Certainly, the molecular approach will provide us with a deeper understanding of all gas flows. However, more than that, under certain conditions, the aggregate, or sometimes called fluid, approach is not able to provide a physically accurate picture of the gas.We will find that these conditions arise when there is either not enough time or physical space for a sufficient number of intermolecular collisions to occur to maintain the gas in the well-understood equilibrium state. We refer to such conditions as nonequilibrium. To describe nonequilibium flows accurately, we need to study the molecular nature of the gas.

    There are a number of important application areas in aerospace engineering where nonequilibrium gas flows arise. In general, we will find that nonequilibrium occurs when the gas flow is at low density and/or involves very small length scales. One important application area for nonequilibrium is the flight of high-speed vehicles at very high altitude in the Earth's atmosphere. Examples include spacecraft returning from orbit, such as the space shuttle, or hypersonic cruise vehicles. These vehicles have a length scale of several meters, and move at very high speed so that the flow field surrounding them involves very high temperatures. However, it is their operation in the low-density environment of near-space that leads to nonequilibrium gas flow phenomena. We focus on low-density, high-temperature air in many of the examples and analyses presented throughout the book.

    A second important technology area involving nonequilibrium gas flow is micro- and nanoelectromechanical systems (MEMS/NEMS) that involve fabrication and operation of very small machines based on microfabrication technology.

Page 38 of 104