Aircraft Design Projects

For Engineering Students

Lloyd R. Jenkinson, Jim Marchman

• Publisher: Butterworth-Heinemann
• Year: 2003
• ISBN:
  9780080498959
• Topic: Tecnologia e ingegneria
• Subtopic: Ingegneria aerospaziale

Written with students of aerospace or aeronautical engineering firmly in mind, this is a practical and wide-ranging book that draws together the various theoretical elements of aircraft design - structures, aerodynamics, propulsion, control and others - and guides the reader in applying them in practice. Based on a range of detailed real-life aircraft design projects, including military training, commercial and concept aircraft, the experienced UK and US based authors present engineering students with an essential toolkit and reference to support their own project work.All aircraft projects are unique and it is impossible to provide a template for the work involved in the design process. However, with the knowledge of the steps in the initial design process and of previous experience from similar projects, students will be freer to concentrate on the innovative and analytical aspects of their course project. The authors bring a unique combination of perspectives and experience to this text. It reflects both British and American academic practices in teaching aircraft design. Lloyd Jenkinson has taught aircraft design at both Loughborough and Southampton universities in the UK and Jim Marchman has taught both aircraft and spacecraft design at Virginia Tech in the US.* Demonstrates how basic aircraft design processes can be successfully applied in reality* Case studies allow both student and instructor to examine particular design challenges * Covers commercial and successful student design projects, and includes over 200 high quality illustrations

1

Design methodology

The start of the design process requires the recognition of a ‘need’. This normally comes from a ‘project brief’ or a ‘request for proposals (RFP)’. Such documents may come from various sources:

• Established or potential customers.

• Government defence agencies.

• Analysis of the market and the corresponding trends from aircraft demand.

• Development of an existing product (e.g. aircraft stretch or engine change).

• Exploitation of new technologies and other innovations from research and development.

It is essential to understand at the start of the study where the project originated and to recognise what external factors are influential to the design before the design process is started.

At the end of the design process, the design team will have fully specified their design configuration and released all the drawings to the manufacturers. In reality, the design process never ends as the designers have responsibility for the aircraft throughout its operational life. This entails the issue of modifications that are found essential during service and any repairs and maintenance instructions that are necessary to keep the aircraft in an airworthy condition.

The design method to be followed from the start of the project to the nominal end can be considered to fall into three main phases. These phases are illustrated in Figure 1.1.

Fig. 1.1 The design process

The preliminary phase (sometimes called the conceptual design stage) starts with the project brief and ends when the designers have found and refined a feasible baseline design layout. In some industrial organisations, this phase is referred to as the ‘feasibility study’. At the end of the preliminary design phase, a document is produced which contains a summary of the technical and geometric details known about the baseline design. This forms the initial draft of a document that will be subsequently revised to contain a thorough description of the aircraft. This is known as the aircraft ‘Type Specification’.

The next phase (project design) takes the aircraft configuration defined towards the end of the preliminary design phase and involves conducting detailed analysis to improve the technical confidence in the design. Wind tunnel tests and computational fluid dynamic analysis are used to refine the aerodynamic shape of the aircraft. Finite element analysis is used to understand the structural integrity. Stability and control analysis and simulations will be used to appreciate the flying characteristics. Mass and balance estimations will be performed in increasingly fine detail. Operational factors (cost, maintenance and marketing) and manufacturing processes will be investigated to determine what effects these may have on the final design layout. All these investigations will be done so that the company will be able to take a decision to ‘proceed to manufacture’. To do this requires knowledge that the aircraft and its novel features will perform as expected and will be capable of being manufactured in the timescales envisaged. The project design phase ends when either this decision has been taken or when the project is cancelled.

The third phase of the design process (detail design) starts when a decision to build the aircraft has been taken. In this phase, all the details of the aircraft are translated into drawings, manufacturing instructions and supply requests (subcontractor agreements and purchase orders). Progressively, throughout this phase, these instructions are released to the manufacturers.

Clearly, as the design progresses from the early stages of preliminary design to the detail and manufacturing phases the number of people working on the project increases rapidly. In a large company only a handful of people (perhaps as few as 20) will be involved at the start of the project but towards the end of the manufacturing phase several thousand people may be employed. With this build-up of effort, the expenditure on the project also escalates as indicated by the curved arrow on Figure 1.1.

Some researchers¹ have demonstrated graphically the interaction between the cost expended on the project, the knowledge acquired about the design and the resulting reduction in the design freedom as the project matures. Figure 1.2 shows a typical distribution. These researchers have argued for a more analytical understanding of the requirement definition phase. They argue that this results in an increased understanding of the effects of design requirements on the overall design process. This is shown on Figure 1.2 as process II, compared to the conventional methods, process I. Understanding these issues will increase design flexibility, albeit at a slight increase in initial expenditure. Such analytical processes are particularly significant in military, multi-role, and international projects. In such case, fixing requirements too firmly and too early, when little is known about the effects of such constraints, may have considerable cost implications.

Fig. 1.2 Design flexibility

Much of the early work on the project is involved with the guarantee of technical competence and efficiency of the design. This ensures that late changes to the design layout are avoided or, at best, reduced. Such changes are expensive and may delay the completion of the project. Managers are eager to validate the design to a high degree of confidence during the preliminary and project phases. A natural consequence of this policy is the progressive ‘freezing’ of the design configuration as the project matures. In the early preliminary design stages any changes can (and are encouraged to) be considered, yet towards the end of the project design phase only minor geometrical and system modifications will be allowed. If the aircraft is not ‘good’ (well engineered) by this stage then the project and possibly the whole company will be in difficulty. Within the context described above, the preliminary design phase presents a significant undertaking in the success of the project and ultimately of the company.

Design project work, as taught at most universities, concentrates on the preliminary phase of the design process. The project brief, or request for proposal, is often used to define the design problem. Alternatively, the problem may originate as a design topic in a student competition sponsored by industry, a government agency, or a technical society. Or the design project may be proposed locally by a professor or a team of students. Such design project assignments range from highly detailed lists of design objectives and performance requirements to rather vague calls for a ‘new and better’ replacement for existing aircraft. In some cases student teams may even be asked to develop their own design objectives under the guidance of their design professor.

To better reflect the design atmosphere in an industry environment, design classes at most universities involve teams of students rather than individuals. The use of multidisciplinary design teams employing students from different engineering disciplines is being encouraged by industry and accreditation agencies.

The preliminary design process presented in this text is appropriate to both the individual and the team design approach although most of the cases presented in later chapters involved teams of design students. While, at first thought, it may appear that the team approach to design will reduce the individual workload, this may not be so. The interpersonal dynamics of working in a team requires extra effort. However, this greatly enhances the design experience and adds team communications, management and interpersonnel interaction to the technical knowledge gained from the project work.

It is normal in team design projects to have all students conduct individual initial assessments of the design requirements, study comparable aircraft, make initial estimates for the size of their aircraft and produce an initial concept sketch. The full team will then begin its task by examining these individual concepts and assessing their merits as part of their team concept selection process. This will parallel the development of a team management plan and project timeline. At this time, the group will allocate various portions of the conceptual design process to individuals or small groups on the team.

At this point in this chapter, a word needs to be said about the role of the computer in the design process. It is natural that students, whose everyday lives are filled with computer usage for everything from interpersonal communication to the solution of complex engineering problems, should believe that the aircraft design process is one in which they need only to enter the operational requirements into some supercomputer and wait for the final design report to come out of the printer (Figure 1.3).

Fig. 1.3 Student view of design

Indeed, there are many computer software packages available that claim to be ‘aircraft design programs’ of one sort or another. It is not surprising that students, who have read about new aircraft being ‘designed entirely on the computer’ in industry, believe that they will be doing the same. They object to wasting time conducting all of the basic analyses and studies recommended in this text, and feel that their time would be much better spent searching for a student version of an all-encompassing aircraft design code. They believe that this must be available from Airbus or Boeing if only they can find the right person or web address.

While both simple aircraft ‘design’ codes and massive aerospace industry CAD programs do exist and do play important roles, they have not yet replaced the basic processes outlined in this text. Simple software packages which are often available freely at various locations on the Internet, or with many modern aeronautical engineering texts, can be useful in the specialist design tasks if one understands the assumptions and limitations implicit in their analysis. Many of these are simple computer codes based on the elementary relationships used for aircraft performance, aerodynamics, and stability and control calculations. These have often been coupled to many simplifying assumptions for certain categories of aircraft (often home-built general aviation vehicles). The solutions which can be obtained from many such codes can be obtained more quickly, and certainly with a much better understanding of the underlying assumptions, by using directly the well-known relationships on which they are based. In our experience, if students spent half the time they waste searching for a design code (which they expect will provide an instant answer) on thinking and working through the fundamental relationships with which they are already supposedly familiar, they would find themselves much further along in the design process.

The vast and complex design computer programs used in the aerospace industry h...