Evolvability and design reuse in civil jet transport aircraft

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Abstract

A comprehensive investigation of evolvability and design reuse in new and historical civil jet transport aircraft was undertaken. The main purpose was to characterise the techniques and strategies used by aircraft manufacturers to evolve their designs. Such knowledge is essential to devise improved design methods for promoting the evolvability of new aircraft. To perform the study, jet aircraft from three large western manufacturers (Boeing, Airbus, and McDonnell Douglas) were investigated in depth. The academic and industrial literature was combed to find descriptions of design reuse and change across each major model of all three manufacturers. The causes and effects of the changes are explored, and the amenability of the different airframes to change are discussed. The evolution of the payload and range capabilities of the different aircraft was also investigated. From these studies, it was found that the initial approach to derivative designs appears somewhat ad hoc and that substantial modifications were devised in quick succession to increase both range and capacity. From the 1970s, two distinguishable patterns started to appear – a ‘leap and branch’ and a ‘Z’ pattern. The leaps correspond to major changes in both propulsion and airframe, whereas the branches are simple ‘stretches’ or ‘shrinks’. The Z pattern, also documented by other authors, is a progressive increase in range, followed by a simple stretch, and then another increase in range. Design changes were investigated further by grouping them according to the assumed payload-range objectives set for the derivatives. Finally, the maximum changes found for salient geometrical design parameters amongst all the aircraft surveyed were documented. Developing methods to support the creation of leaps (especially across configurations) appears to be one of the most promising avenues for future research.

Introduction

The development of a brand-new aircraft is often an expensive and risky undertaking. In fact, when decision-makers at the large aeroplane manufacturers elect to pursue a ‘clean-sheet’ design, they are often considered to be ‘betting the company’. Although sometimes necessary to remain competitive, this type of clean-sheet development programme is rare, precisely because of the risks involved. Manufacturers are far more likely to devise only incremental advancements to existing designs. For such a strategy to be successful, however, the original (‘baseline’) design must be ‘evolvable’. The evolvability (of an engineering system) is the extent to which the design of the system can be “inherited and changed across generations (over time)” [1].

An evolvable baseline design may help the manufacturer to reduce development and certification cost and time for descendants of this design. This is because it affords them the opportunity to reuse many design elements (such as segments of the airframe, components, systems, as well as development, manufacturing, and assembly processes), while upgrading only those that are necessary for subsequent generations to be competitive.

Several methods to design for and explore the evolvability of engineering systems have been devised. For an overview of these, the reader is directed to Cardin [2], which covers methods that promote ‘flexibility’, many of which are directly applicable to evolvability. A number of aircraft family-specific design strategies have also been proposed (see for example Willcox and Wakayama [3]). However, there seems to be a paucity in the literature of collated practical information related to the evolution of passenger transport aircraft. Such information was required by the authors to conduct related research involving the development of computational techniques to support designing aircraft to be more evolvable (see van Heerden [4]). In essence, to perform that work, it was required to obtain a detailed understanding of the design changes used in practice to evolve the designs of airliners.

Therefore, an investigation was conducted into evolvability practices in actual commercial transport aeroplanes. The purpose was to obtain i) qualitative descriptions of how components, design features, and airframe segments were reused or changed across generations of airliner designs; ii) to determine whether there are any distinguishable patterns in payload-range capability evolution, along with associated airframe changes; and iii) to determine the maximum changes in airframe geometrical design parameters achieved across baseline and derivative pairs.

Several sources were employed to conduct this study, including the manufacturers’ websites, books devoted to the technical history of the aircraft, industry periodicals, three-view drawings, and airport planning manuals. Only western airliners from the main current and historical OEMs, namely Boeing, McDonnell Douglas, and Airbus, were investigated because there was a reasonable amount of accessible information available for these. The two major regional jet manufacturers, Bombardier and Embraer, usually face similar challenges regarding evolution than the companies studied here, such as retaining commonality while not penalising performance too much, and incorporating new technologies into existing airframe architectures. They therefore often respond in similar ways (i.e. stretching tubular fuselages, re-engining existing designs, and many others). However, there are also some unique challenges that regional jet manufacturers face, such as a larger number of competitors in a smaller market segment [5] and providing comfort approaching that of larger airliners, while meeting the unique operating cost demands of regional operators [6]. Therefore, studying evolvability and design reuse in their products deserves separate attention and is reserved for future work.

Also, only aspects that would normally be considered at conceptual design, i.e. the airframe major components, and the airframe-propulsion integration, were investigated. Freighters and special variants were generally excluded but do receive some mention in the text.

The rest of this paper is organised as follows: first, in the next section, some background on the concept of evolvability is provided, along with a discussion on enablers for evolvability. Section 3 covers the case studies for Boeing aircraft, Section 4 covers McDonnell-Douglas aircraft, and Section 5 is devoted to Airbus aircraft. The results of the studies are collated in Sections 6, 7, and 8, which respectively cover investigations into i) the evolution of payload and range for the different aircraft, ii) modifications by payload-range modification objectives for the derivative, and iii) the maximum changes achieved in important design parameters. Finally, conclusions are drawn in Section 9.

Section snippets

The concept of evolvability

Evolvability is a type of change ‘ility’. Ilities are desired life-cycle properties of an engineering system that are not intended to fulfil primary functional requirements [1] but which, nonetheless, often constitute much of the value of the system. It is therefore important for designers to recognise what ilities are required and to what extent these must be present in their products. Other change-related ilities include ‘flexibility’, ‘adaptability’, ‘versatility’, ‘robustness’, and the

Case studies in evolvability: Boeing aircraft

The world's largest aerospace company [30], Boeing, has a long and illustrious history of producing successful airliners. As will be discussed below, some of their designs have evolved significantly over several decades.

Case studies in evolvability: McDonnell Douglas aircraft

McDonnell Douglas (and especially the preceding Douglas Aircraft Company) produced many famous and celebrated transport aircraft. The company experienced increasing financial problems and merged with Boeing in the 1990s.

Case studies in evolvability: Airbus aircraft

The formation of Airbus reflected a rising level of European collaboration and the merging of many aerospace companies during the 1960s and 1970s. Airbus has since grown into an enormously successful enterprise, producing some of the most competitive and innovative airliners in the world.

Payload-range capability evolution strategies

In this section, the evolution strategies (or lack thereof) of the three manufacturers considered above are investigated. For this purpose, the design payload and range capabilities of the different ancestor aircraft and their descendants (or derivative) were plotted for each manufacturer in single combined payload-range diagrams.

Most of the information employed for the diagrams originates from the ‘aircraft characteristics for airport planning’ documents provided for by the manufacturers for

Design changes by payload-range modification objectives

In this section, the major design changes are summarised and classified according to the design objectives that the manufacturers set for derivatives. For this purpose, the data collected for the payload-range evolution plots, discussed in the previous sub-section, along with all the information provided in the previous discussions on the individual aircraft families. The raw data employed in this section, along with references used, can be viewed on line at 1//doi.org/10.17862/cranfield.rd.7123178.v1

Maximum changes

In this section, a list of the most substantial airframe modifications identified in this paper is provided together with the maximum increase/decrease in the geometric parameter values associated with each change. As no published information could be found that state when a specific type of design change is impractical (as compared with a new design) the maximum values identified here could be considered to be the maximum ‘reasonable changes’. The changes are summarised in Table 23. Note that

Discussion and Conclusions

The purpose of this research was to characterise the techniques and strategies employed by the major civil jet transport aircraft manufacturers to evolve their existing designs. To do this, the main products of three western manufacturers, namely Boeing, McDonnell Douglas, and Airbus were investigated in depth.

In the early jet-age (1950s and 60s), it seemed that the approach to devise derivative designs was somewhat ad hoc and substantial design changes were frequent. Later, two distinguishable

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