Advanced Aircraft Design

Conceptual Design, Analysis and Optimization of Subsonic Civil Airplanes

Egbert Torenbeek, Peter Belobaba, Jonathan Cooper, Roy Langton, Allan Seabridge

• Publisher: Wiley
• Year: 2013
• ISBN:
  9781118568095
• Edition: 1
• Topic: Technology & Engineering
• Subtopic: Aeronautic & Astronautic Engineering
• Index: Technology & Engineering

Although the overall appearance of modern airliners has not changed a lot since the introduction of jetliners in the 1950s, their safety, efficiency and environmental friendliness have improved considerably. Main contributors to this have been gas turbine engine technology, advanced materials, computational aerodynamics, advanced structural analysis and on-board systems. Since aircraft design became a highly multidisciplinary activity, the development of multidisciplinary optimization (MDO) has become a popular new discipline. Despite this, the application of MDO during the conceptual design phase is not yet widespread.

Advanced Aircraft Design: Conceptual Design, Analysis and Optimization of Subsonic Civil Airplanes presents a quasi-analytical optimization approach based on a concise set of sizing equations. Objectives are aerodynamic efficiency, mission fuel, empty weight and maximum takeoff weight. Independent design variables studied include design cruise altitude, wing area and span and thrust or power loading. Principal features of integrated concepts such as the blended wing and body and highly non-planar wings are also covered. The quasi-analytical approach enables designers to compare the results of high-fidelity MDO optimization with lower-fidelity methods which need far less computational effort. Another advantage to this approach is that it can provide answers to "what if" questions rapidly and with little computational cost.

Key features:

• Presents a new fundamental vision on conceptual airplane design optimization
• Provides an overview of advanced technologies for propulsion and reducing aerodynamic drag
• Offers insight into the derivation of design sensitivity information
• Emphasizes design based on first principles
• Considers pros and cons of innovative configurations
• Reconsiders optimum cruise performance at transonic Mach numbers

Advanced Aircraft Design: Conceptual Design, Analysis and Optimization of Subsonic Civil Airplanes advances understanding of the initial optimization of civil airplanes and is a must-have reference for aerospace engineering students, applied researchers, aircraft design engineers and analysts.

1

Design of the Well-Tempered Aircraft

Let no new improvement in flying and flying equipment pass us by.

—Bill Boeing (1928)

As our industry has matured … we have become increasingly enslaved to our data bases of past successful achievements. Increased competitive pressures and emphasis on control of rapidly escalating costs have combined to preclude the level of bold risks taking in exploring possible new configuration options that might offer some further increase in performance, etc., but for which no adequate data exist to aid development.

—J.H. McMasters [57] (2005)

1.1 How Aircraft Design Developed

1.1.1 Evolution of Jetliners and Executive Aircraft

The second half of the twentieth century has been truly revolutionary. In particular, the period 1945–1960 produced some highly innovative projects which demonstrated that propulsion of transport aircraft by means of jet engines had become feasible. In combination with the appearance of the sweptback wing, this resulted in a jump in maximum cruising speeds from about 550 to more than 850 km/h (Figure 2.1). Having pioneered the B-47 swept-wing bomber, Boeing introduced its basic jet concept to the 367-80 tanker transport and later to the 707 passenger transport; see Figure 1.1(a). This concept proved successful and has been adopted for jetliners almost universally since the 1960s. When one realizes that in the early 1950s designers did not yet avail themselves of the advantage of electronic computers, it will be appreciated that this revolution in design technology was a monumental achievement.

Modern jetliners are mostly low-wing designs with two or four engines installed in nacelles mounted underneath and to the fore of the wing leading edge. It should not be concluded, however, that since the Boeing 707 little progress has been made in configuration design. An early example of an unusual mutation was the Sud-Est Caravelle, see Figure 1.1(b), the airliner that pioneered jet engines attached to the rear fuselage. Even though this was a patented concept, several short-haul designs soon emerged with a similar layout and some of these were very successful. The introduction of bypass engines (~1960) and large turbofans (~1970) further improved the productivity and economy of jetliners. In combination with the strong worldwide economic expansion, this resulted in an unprecedented growth of air traffic and the almost complete extinction of competing modes of transportation over long distances, including the long-haul piston-powered and even the brand-new turboprop-powered propeller airliners.

Short-range jets initially suffered from poor low speed performances and high fuel expenditure. This market niche was filled by the four-engine Vickers Viscount and other turboprops designed in the 1950s. The twin-engine Fokker Friendship – see Figure 1.1(c) – had its Rolls-Royce Dart turboprop engines mounted to the high-set wing. This configuration was difficult to improve on and became the standard for similar propeller aircraft appearing later. Short-range turboprops have survived the twentieth century thanks to their excellent fuel economy and low operating costs. The idea of producing economy-size jets for large companies and wealthy individuals came around 1960. A prime example of a successful business jet was the Learjet depicted in Figure 1.1(d). Seating six in a slim fuselage (‘no-one walks about in a Cadillac’), it outperformed jetliners of its time in maximum speed. Learjet’s general arrangement, a low-wing design with engines attached to the rear fuselage and a high-set horizontal tail, has been adopted on most executive jets.

Figure 1.1 Prime examples of early post-WW II passenger aircraft. (a) Boeing 707 (1954): the first jet-powered airliner of US design. (b) Sud-Est Caravelle (1959): the first airliner with rear fuselage-mounted jet engines. (c) Fokker F 27 (1955): turboprop designed as a regional aircraft; still operational in 2012. (d) Gates Learjet 24B (1963): business jet designed in the early 1960s

Since the introduction of the first jetliners, subsonic civil airplane technology development has advanced in an evolutionary way. During the time span between 1950 and 2000, considerable improvement has been accomplished in all technical areas, but none could be regarded as revolutionary. The basic properties of traditional designs – such as lift, drag, weight and flight performances – have become well understood. Computational methods supporting advanced design (AD) have steadily developed over a long period of time and a wealth of empirical evidence confirms their accuracy. Consequently, aircraft with a conventional layout can be developed with a high degree of confidence in the analysis. Though designing an innovative configuration will always be challenging from an engineering viewpoint, its application in an industrial project entails many challenges. This may lead to the situation that, after several years of costly configuration development, the project has to be terminated by a show stopper. It is also observed that airline management tends to avoid the uncertainties of an unusual general arrangement and prefers the purchase of a traditional configuration.

The conformity between modern airliners is not caused by the lack of conceptual creativity of designers; arguments supporting this statement can be found in publications such as [12] and [14]. In fact, several innovative designs proposed during the last decennia of the twentieth century have not been developed into a for-sale aircraft because airlines were reluctant to order them for non-technical reasons. The following projects serve as examples.

• The Boeing 7J7 project of the 1980s – Figure 1.2 (a) – was a 150-seat airliner in which new technologies were integrated: a fly-by-wire control system, unducted fan (UDF) engine technology, advanced system and flight deck technologies, and advanced aluminium alloys. The 7J7 did not find favour with the airlines mainly because the anticipated spike in fuel prices did not occur.
• Boeing’s Sonic Cruiser – Figure 1.2 (b) – was designed to connect typical long-range city pairs at Mach 0.95 or above. In a business class layout for 100 seats it would attract passengers who would be willing to pay a fare premium to save several hours on long distance flights with increased comfort. The 300-seat version would be used for continental flights circumventing the large hubs. The Sonic Cruiser became the victim of the aftermath of the events following September 2001, when airlines began to re-evaluate their business models resulting in a preference for a more economical (slower) design which became the 787 [41]. The Sonic Cruiser was not developed into a for-sale product because potential customers would rather see its advanced technology developed for integration into an airplane optimized for lower Mach numbers.

Figure 1.2 Boeing design projects which were not put into production. (a) 7J7 open rotor-powered narrow body airliner of the 1980s. (b) Sonic Cruiser long-range wide body Mach 0.95 airliner 1999–2002

1.1.2 A Framework for Advanced Design

The non-recurring costs of a commercial aircraft development programme are so enormous that even a relatively minor technical hiccup may be magnified into an unacceptable commercial risk. Consequently, a certain amount of conservatism is inherent in the development of civil aircraft design. In spite of this, conservatism in design is risky because it can lead to missed opportunities when maturing aerodynamic, structural and propulsive technologies are becoming available which find their best application in concepts different from the current dominant configuration.

In civil aircraft development programmes, far-reaching decisions concerning top level specifications, general arrangement, propulsion and enabling technologies are made before and during the concept finding and the conceptual design phases. The preliminary design phase is then entered during which the aircraft’s characteristics are defined in more detail, initial assumptions are verified and the feasibility and risk level of the project are investigated. A year or more may elapse before management will decide to give the green light or withdraw from further development. The next phase consists of design verification (testing) and detail design during which major modifications of the basic configuration can be very labour-intensive and costly. Clearly, ESDU’s trademark phrase, ‘get it right the first time' is highly relevant for the initial aircraft system design process.

The observation has frequently been made that no more than a few percent of the pre-production costs are attributed by a few designers committing to a large fraction of total aircraft programme cost. In some cases this observation was made in favour of strengthening the advanced design capability of the aeronautical industry and/or the effort in academia to offer excellent aircraft design teaching. Although these arguments are fully justified, it is not always acknowledged that a large portion of aircraft programme costs is committed by merely specifying the need for the particular vehicle rather than by defining its technical and operational characteristics. If a new airplane has been developed for which no market exists, the project will be doomed to fail. The project design team cannot be blamed for a wrong go-ahead/exit decision and devoting more manpower to advanced design is not necessarily a panacea for avoiding misjudgement of the market. Although concept finding is not, in general, considered a part of the design project, it is at least as crucial to the success of a programme as the actual concept development phase.

1.1.3 Analytical Design Optimization

Since advanced design is highly relevant to the company’s viability, one would expect that the discipline of design optimization has traditionally received a great deal of attention from the aeronautical community – in fact, this is not the case. Until the time of large-scale computer applications, only a few systematic efforts were made to develop a fundamental framework for non-intuitive decision-making. Most of these were small-scale programs initiated by individuals in research institutes and academia and their impact on the actual practice in design offices has not become entirely clear. Nevertheless, from the educational point of view, several approaches and trends from the past still deserve to be mentioned even though not all of them have received widespread recognition.

Early parametric surveys were made on a limited scale in the industry by experienced designers. Until the 1960s, efforts to include optimization in conceptual design were based on relatively simple methods with minimum take-off gross weight (TOGW) considered as the criterion for the figure of merit. The analytical approach to sizing and improving a design in the conceptual stage was discussed in 1948 by Cherry and Croshere Jr [19]. Though their methodology was based on experience with propeller airplanes, its systematic character appeared useful for jet aircraft as well. In 1958, G. Backhaus proposed a comprehensive (quasi-)analytical optimization of jet transports [20]. His article did not get the recognition it deserved, probably because it was published in German. Another pioneer of the analytical approach to concept optimization was D. Küchemann. During the 1960s, he and his co-workers at the Royal Aircraft Establishment in the UK developed analytical design methods of aircraft intended to fly over widely different ranges at different (subsonic, supersonic and hypersonic) speeds [21]. Part of this work was based on research in connection with the conception of Concorde and was compiled in a unique book [1]. The elegance and lucidity of Küchemann’s analysis inspired the present author to initiate a systematic study of fundamental design considerations [27]; some of its results are included in the present book in a modified form. After the advent of computational design analysis and optimization technology in the 1970s, the (quasi-)analytical approach has appealed to only a few researchers; see, for example, W.H. Mason and B. Malone in [34, 35].

1.1.4 Computational Design Environment

During the first decennia after WWII, aircraft design was performed manually with the use of hand calculators and drawing boards. Despite the commercial success of several excellent airliners and business airplanes developed during this period, the ‘paper method’ is nowadays consi...

Table of contents

• Cover
• Aerospace Series List
• Title Page
• Copyright
• Foreword
• Series Preface
• Preface
• Acknowledgements
• Chapter 1: Design of the Well-Tempered Aircraft
• Chapter 2: Early Conceptual Design
• Chapter 3: Propulsion and Engine Technology
• Chapter 4: Aerodynamic Drag and Its Reduction
• Chapter 5: From Tube and Wing to Flying Wing
• Chapter 6: Clean Sheet Design
• Chapter 7: Aircraft Design Optimization
• Chapter 8: Theory of Optimum Weight
• Chapter 9: Matching Engines and Airframe
• Chapter 10: Elements of Aerodynamic Wing Design
• Chapter 11: The Wing Structure and Its Weight
• Chapter 12: Unified Cruise Performance
• Appendix A: Volumes, Surface and Wetted Areas
• Appendix B: International Standard Atmosphere
• Appendix C: Abbreviations
• Index