Category AIRCRAF DESIGN

A crude development cost up to certification (in year 2000 U. S. Dollars) is shown in Table 2.1. Typical unit aircraft costs by class are also given (there is variation among companies). A substantial part of the budget is committed to Phase 1.

2.3.1 Typical Time Frame

Typical time frames for the phases of different types of projects are shown in Chart 2.4. All figures are the approximate number of months. Exploratory work continues year-round to examine the viability of incorporating new technologies and to push the boundaries of company capabilities – which is implied rather than explicit in Chart 2.4.

2.3 Typical Task Breakdown in Each Phase

Typical task obligations in each phase of civil aircraft design are defined in this sec­tion. Military aircraft designs follow the same pattern but more rigorously. Military aircraft must deal with new technologies, which could still require operational prov­ing; therefore, there is uncertainty involved in military aircraft projects.

All phases do not work under uniform manpower-loading; naturally, Phase 1 starts with light manpower during the conceptual study and reaches peak manpower (100%) at Phase 3; it decreases again when flight testing starts, by which stage the design work is virtually done and support work continues. Figure 2.2a is a typical distribution of cost and manpower loading (an average percentage is shown); the manpower-loading forecast must be finalized during the Phase 1 study. Figure 2.2b shows the cumulative deployment. At the end of a project, it is expected that the actual figure should be close to the projected figure. Project costs consist primarily of salaries (most of the cost), bought-out items, and relatively smaller miscellaneous amounts (e. g., advertising, travel, and logistics). Chain lines in Figure 2.2 illustrate the cost-frame outlay.

Aircraft manufacturers conduct year-round exploratory work on research, design, and technology development as well as market analysis to search for a product. A new project is formally initiated in the four phases shown in Chart 2.2, which is applicable for both civil and military projects. (A new employee should be able to sense the pulse of organizational strategies as soon joining a company.)

Among organizations, the terminology of the phases varies. Chart 2.2 offers a typical, generic pattern prevailing in the industry. The differences among ter­minologies are trivial because the task breakdown covered in various phases is

approximately the same. For example, some may call the market study and specifi­cations and requirements Phase 1, with the conceptual study as Phase 2; others may define the project definition phase (Phase 2) and detailed design phase (Phase 3) as the preliminary design and full-scale development phases, respectively. Some pre­fer to invest early in the risk analysis in Phase 1; however, it could be accomplished in Phase 2 when the design is better defined, thereby saving the Phase 1 budgetary provisions in case the project fails to obtain the go-ahead. A military program may require early risk analysis because it would be incorporating technologies not yet proven in operation. Some may define disposal of aircraft at the end as a design phase of a project. Some companies may delay the go-ahead until more informa­tion is available, and some Phase 2 tasks (e. g., risk analysis) may be carried out as a Phase 1 task to obtain the go-ahead.

Company management establishes a DBT to meet at regular intervals to con­duct design reviews and make decisions on the best compromises through multidis­ciplinary analysis (MDA) and MDO, as shown in Chart 2.3; this is what is meant by an IPPD (i. e., concurrent engineering) environment.

Specialist areas may optimize design goals, but in an IPPD environment, com­promise must be sought. It is emphasized frequently that optimization of individual goals through separate design considerations may prove counterproductive and usu­ally prevents the overall (i. e., global) optimization of ownership cost. MDO offers

Manpower

Cost Frame

100% deployment

Customer

Support

a. Phase-wise deployment

Figure 2.2. Resource deployments (manpower and finance) good potential but it is not easy to obtain global optimization; it is still evolving. In a way, global MDO involving many variables is still an academic pursuit. Indus­tries are in a position to use sophisticated algorithms in some proven areas. An example is reducing manufacturing costs by reshaping component geometry as a compromise – such as minimizing complex component curvature. The compromises are evident in offering a family of variant aircraft because none of the individuals in the family is optimized, whereas together, they offer the best value.

When an aircraft has been delivered to the operators (i. e., customers), a manu­facturer is not free from obligation. Manufacturers continue to provide support with maintenance, design improvements, and attention to operational queries until the end of an aircraft’s life. Modern designs are expected to last for three to four decades of operation. Manufacturers may even face litigation if customers find cause to sue. Compensation payments have crippled some well-known general aviation compa­nies. Fortunately, the 1990s saw a relaxation of litigation laws in general aviation – for a certain period after a design is established, a manufacturer’s liabilities are reduced – which resulted in a revitalization of the general aviation market. Military programs involve support from “cradle to grave” (see Section 1.7.)

This emphasizes that the product must be done right the first time. Midcourse changes add unnecessary costs that could be detrimental to a project – a major change may not prove sustainable. Procedural methodologies such as the Six Sigma approach have been devised to ensure that changes are minimized.

The typical aircraft design process follows the classical systems approach pattern. The official definition of system, adopted by the International Council of Systems Engineering (INCOSE) [5] is: “A system is an interacting combination of elements, viewed in relation to function.” The design system has an input (i. e., a specification or requirement) that undergoes a process (i. e., phases of design) to obtain an out­put (i. e., certified design through substantiated aircraft performance), as shown in Figure 2.1.

As subsystems, the components of an aircraft are interdependent in a multi­disciplinary environment, even if they have the ability to function on their own (e. g., wing-flap deployment on the ground is inert whereas in flight, it affects vehi­cle motion). Individual components such as the wings, nacelle, undercarriage, fuel

system, and air-conditioning also can be viewed as subsystems. Components are sup­plied for structural and system testing in conformance with airworthiness require­ments in practice. Close contact is maintained with the planning engineering depart­ment to ensure that production costs are minimized, the schedule is maintained, and build tolerances are consistent with design requirements.

Chart 2.1 suggests a generalized functional envelope of aircraft design architec­ture, which is in line with the Aircraft Transport Association (ATA) index [6] for commercial transport aircraft. Further descriptions of subsystems are provided in subsequent chapters.

Extensive wind-tunnel, structure, and systems testing is required early in the design cycle to ensure that safe flight tests result in airworthiness certification approval. The multidisciplinary systems approach to aircraft design is carried out within the context of IPPD. Four phases comprise the generic methodology (dis­cussed in the next section) for a new aircraft to be conceived, designed, built, and certified.

Civil aircraft projects usually proceed to preproduction aircraft that will be flight-tested and sold, whereas military aircraft projects proceed with technical demonstrations of prototypes before the go-ahead is given. The prototypes are typ­ically scaled-down aircraft meant to substantiate cutting-edge technologies and are not sold for operational use.

Chart 2.1. Aircraft system

With guidance from the instructor, students conduct a mock market survey. Stu­dents generate a bar chart (i. e., Gantt) to monitor progress during the semester. The remainder of the chapter is recommended easy reading. The coursework activ­ity begins in Section 2.6 with a mock market survey to generate aircraft specifica­tions and requirements and helps students understand its importance in the success or failure of a product.

1.2 Introduction

Existing aircraft indicate how the market is served and should indicate what is needed for the future. Various aircraft have been designed, and new designs should perform better than any existing designs. Designers are obligated to search for proven advanced technologies that emerge. There could be more than one option so the design team must conduct trade-off studies to arrive at a “satisfying” design that will satisfy the customer. Economy and safety are possibly the strongest drivers

in commercial transport. Aircraft design drivers for combat are performance capa­bility and survivability (i. e., safety).

Despite organizational differences that exist among countries, one thing is com­mon to all: namely, the constraint that the product must be “fit for the purpose.” It is interesting to observe that organizational structures in the East and the West are beginning to converge in their approach to aircraft design. The West is replacing its vertically integrated setup with a major investor master company in the integrat­ing role along with risk-sharing partners. Since the fall of communism in Eastern Europe, the socialist bloc is also moving away from specialist activities to an inte­grated environment with risk-sharing partners. Stringent accountability has led the West to move away from vertical integration – in which the design and manufacture of every component were done under one roof – to outsourcing design packages to specialist companies. The change was inevitable – and it has resulted in better products and profitability, despite increased logistical activities.

The aircraft design process is now set in rigorous methodology, and there is considerable caution in the approach due to the high level of investment required. The process is substantially front-loaded, even before the project go-ahead is given. In this chapter, generic and typical aircraft design phases are described as prac­ticed in the industry, which includes market surveys and airworthiness require­ments. A product must comply with regulatory requirements, whether in civil or military applications. New designers must realize from the beginning the importance of meeting mandatory design requirements imposed by the certifying authorities.

Exceeding budgetary provisions is not uncommon. Military aircraft projects undergo significant technical challenges to meet time and cost frames; in addition, there could be other constraints. (The “gestation” period of the Eurofighter project has taken nearly two decades. An even more extreme example is the Indian Light Combat Aircraft, which spanned nearly three decades and is yet to be operational; the original specifications already may be obsolete.) Some fighter aircraft projects have been canceled after the prototype aircraft was built (e. g., the Northrop F20 Tigershark and the BAC TSR2). A good design organization must have the courage to abandon concepts that are outdated and mediocre. The design of combat aircraft cannot be compromised because of national pride; rather, a nation can learn from mistakes and then progress step-by-step to a better future.

This chapter covers the following topics:

Chapter introduction

Management concept of aircraft design process in the industry; describes project phases and systems approach to design, includ­ing management in phases, a typical work schedule, resource deployment, and the time frame involved Task breakdown in each phase and functional activities, high­lighting the conceptual study phase

Aircraft familiarization (civil and military); indispensable infor­mation about various aircraft components Market survey (civil and military); coursework begins with a mock market survey to generate customer specifications (i. e., requirements)

Typical civil aircraft design specifications Typical military aircraft design specifications Comparison between civil and military designs Airworthiness requirements, mandatory requirements for air­craft design and configuration Coursework procedures

2.1 Overview

This chapter is concerned with how aircraft design projects are managed in a com­pany. It is recommended that newly initiated readers read through this chapter because it tackles an important part of the work – that is, to generate customer specifications so that an aircraft configuration has the potential to succeed. A small part of the coursework starts in this chapter. The road to success has a formal step- by-step approach through phases of activities and must be managed.

The go-ahead for a program comes after careful assessment of the design with a finalized aircraft configuration having evolved during the conceptual study (i. e., Phase 1). The prediction accuracy at the end of Phase 1 must be within at least ±5%. In Phase 2 of the project, when more financing is available after obtaining the go-ahead, the aircraft design is fine-tuned through testing and more refined analysis. This is a time – and cost-consuming effort, with prediction accuracy now at less than ±2 to ±3%, offering guarantees to potential buyers. This book does not address project-definition activities (i. e., Phase 2); these are in-depth studies conducted by specialists and offered in specialized courses such as CFD, FEM, Simulink, and CAM.

This book is concerned with the task involved in the conceptual design phase but without rigorous optimization. Civil aircraft design lies within a verified design space; that is, it is a study within an achievable level of proven but leading-edge tech­nology involving routine development efforts. Conversely, military aircraft design lies within an aspirational design space; that is, it is a study of unproven advanced technology requiring extensive development efforts. Obviously, the latter is tech­nologically more complex, challenging, and difficult. Generally, the go-ahead for a project is preceded by a demonstration of the technology to prove the concept.

Jane’s All the World’s Aircraft Manual [1] is an indispensable source of aircraft statistics vital for any aircraft-design work. The following three magazines are also highly recommended resources:

Flight International [2]. A weekly publication from the United Kingdom. It is a newsletter-type journal, providing the latest brief coverage of aerospace activi­ties around the world.

• Aviation Week and Space Technology [3]. A weekly publication from the United States that provides more in-depth analysis of aerospace developments and thoroughly covers the U. S. scenario as well as worldwide coverage.

• Interavia [4]. A bimonthly publication that covers aerospace news, specializing in topics of interest in an essay format. The commercial airline business is well covered.

Aircraft design strategy is constantly changing. Initially driven by the classical sub­jects of aerodynamics, structures, and propulsion, the industry is now customer – driven and design strategies consider the problems for manufacture and assembly that lead the way in reducing manufacturing costs. Chapter 16 addressed cost con­siderations in detail. In summary, an aircraft designer must be cost-conscious now and even more so in future projects.

It is therefore important that a basic exercise on cost estimation (i. e., second- semester classwork) be included in the curriculum. A word of caution: Academic pursuit on cost analysis to find newer tools is still not amenable to industrial use – manufacturers must rely on their own costing methodologies, which are not likely to appear in the public domain. How industry determines cost is sensitive information used to stay ahead in free-market competition.

I emphasize here that there is a significant difference between civil and military programs in predicting costs related to aircraft unit-price costing. The civil aircraft design has an international market with cash flowing back from revenues earned from fare-paying customers (i. e., passengers and freight) – a regenerative process that returns funds for growth and sustainability to enhance the national economy. Conversely, military aircraft design originates from a single customer demand for national defense and cannot depend on export potential – it does not have cash flowing back and it strains the national economy out of necessity. Civil aircraft designs share common support equipment and facilities, which appear as indirect operational costs (IOCs) and do not significantly load aircraft pricing. The driv­ing cost parameter for civil aircraft design is the DOC, omitting the IOC compo­nent. Therefore, using a generic term of life cycle cost (LCC) = (DOC + IOC) in civil applications, it may be appropriate in context but would prove to be off the track for aircraft design engineers. Military design and operations incorporating discreet advances in technology necessarily have exclusive special support systems, equipment, and facilities. The vehicles must be maintained for operation-readiness around the clock. Part of the supply costs and support costs for aircraft maintenance must be borne by manufacturers that know best and are in a position to keep con­fidential the high-tech defense equipment. The role of a manufacturer is defined in the contractual agreement to support its product “from cradle to grave” – that is, the entire life cycle of the aircraft. Here, LCC is meaningful for aircraft designers in minimizing costs for the support system integral to the specific aircraft design. Com­mercial transports would have nearly five times more operating hours than military vehicles in peacetime (i. e., hope for the life of the aircraft). Military aircraft have relatively high operating costs even when they sit idle on the ground. Academic lit­erature has not been able to address clearly the LCC issues in order to arrive at an applicable standardized costing methodology.

Aircraft design and manufacture are not driven by cost estimators and accoun­tants; they are still driven by engineers. Unlike classical engineering sciences, cost­ing is not based on natural laws; it is derived to some extent from manmade policies, which are rather volatile, being influenced by both national and international ori­gins. The academic pursuit to arrest costing in knowledge-based algorithms may not prove readily amenable to industrial applications. However, the industry could benefit from the academic research to improve in-house tools based on actual data. I am pleased to present in this book a relevant, basic cost-modeling methodology [11] from an engineer’s perspective reflecting the industrial perspective so engineers may be aware of the labor content to minimize cost without sacrificing design integrity. The sooner that engineers include costing as an integral part of design, the better will be the competitive edge.

The postwar dominance of British and American aeronautics has kept the use of the foot-pound-second (FPS) system current, despite the use of nondecimal frac­tions and the ambiguity of the word pound in referring to both mass and weight. The benefits of the system international (SI) are undeniable: a decimal system and a distinction between mass and weight. However, there being “nowt so queer as folk,” I am presented with an interesting situation in which both FPS and SI systems are used. Operational users prefer FPS (i. e., altitudes are “measured” in feet); how­ever, scientists and engineers find SI more convenient. This is not a problem if one can become accustomed to the conversion factors. Appendix A provides an exhaus­tive conversion table that adequately covers the information in this book. However, readers will be relieved to know that in most cases, the text follows current interna­tional standards in notation units and the atmospheric table.

Aircraft performance is conducted at the International Standard Atmosphere (ISA) (see Section 3.3). References are given when design considerations must cater to performance degradation in a nonstandard day.

This extended section of the book can be found on the Web at www. cambridge .org/Kundu and gives a brief overview of near-future military-aircraft design trends, covering typical, new, and emerging operational roles (e. g., UAVs and design chal­lenges). Figures 1.18 and 1.19 are associated with the section.

Figure 1.18. JUCAS prototypes (X47B)

Figure 1.19. Future design type

1.3

Learning Process

To meet the objectives of offering close-to-industrial practice in this book, it is appropriate to reiterate and expand on remarks made in the preface about the rec­ognized gap between academia and the industry. It is impertinent to explain the aircraft-design process before outlining the intended classroom learning process. The methodology suggested herein is the same as what I experienced in industry.

It is clear that unless an engineer has sufficient analytical ability, it will be impossible for him or her to convert creative ideas to a profitable product. Today’s innovators who have no analytical and practical skills must depend on engineers to accomplish routine tasks under professional investigation and analysis and to make necessary decisions to develop a marketable product.

Traditionally, universities develop analytical abilities by offering the fundamen­tals of engineering science. Courses are structured with all the material available in textbooks or notes; problem assignments are straightforward with unique answers. This may be termed a “closed-form” education. Closed-form problems are easy to grade and a teacher’s knowledge is not challenged (relatively). Conversely, industry requires the tackling of “open-form” problems for which there is no single answer. The best solution is the result of interdisciplinary interaction of concurrent engi­neering within design built teams (DBTs), in which Total Quality Management (TQM) is needed to introduce “customer-driven” products at the best value. Offer­ing open-ended courses in design education that cover industrial requirements is more difficult and will challenge a teacher, especially when industrial experience is lacking. The associative features of closed – and open-form education are shown in Figure 1.20 ([9] and [10]).

To meet industry’s needs, newly graduated engineers need a brief transition before they can become productive, in line with the specialized tasks assigned to them. They must have a good grasp of the mathematics and engineering sciences necessary for analysis and sufficient experience for decision making. They must be capable of working under minimal supervision with the creative synthesis that comes from experience that academia cannot offer. The industrial environment will require new recruits to work in a team, with an appreciation of time, cost, and quality under TQM – which is quite different from classroom experience.

The purpose of my book is to provide in the coursework close-to-industry stan­dard computations and engineering approaches sciences necessary for analysis and

Figure 1.21. Typical CAD drawing of Airbus A400

enough experience to work on a team. The level of mathematics in this book is not advanced but contains much technological information.

Here, I compare what can be achieved in about 36 hours of classroom lectures plus 60 hours by each of about 30 inexperienced students to what is accomplished by 20 experienced engineers each contributing 800 hours («6 months). Once the task was clearly defined shadowing industrial procedures, leaving out multi­ple iterations, I found that a reduced workload is possible in a classroom environ­ment. It cuts down manhour content, especially when iterations are minimized to an acceptable level. My goal is to offer inexperienced students a powerful analyt­ical capability without underestimating the importance of innovation and decision making.

For this reason, I emphasize that introductory classwork projects should be familiar to students so that they can relate to the examples and subsequently sub­stantiate their work with an existing type. Working on an unfamiliar nonexistent design does not enhance the learning process at the introductory level.

Although it is not essential for the classwork, I highly recommend that modern conceptual aircraft designers be conversant with 3D modeling in CAD (Figure 1.21 is a CAD drawing example) (most recent graduates are). The 3D mod­eling provides fuller, more accurate shapes that are easy to modify, and it facilitates maintenance of sequential configurations – benefits that become evident as one starts to configure.

There are considerably more benefits from CAD (3D) solid modeling: It can be uploaded directly into CFD analysis to continue with aerodynamic estimations, as one of the first tasks is to estimate loading (CFD) for structural analysis using the FEM. The solid model offers accurate surface constraints for generating internal structural parts. CAD drawings can be uploaded directly to computer-aided man­ufacture (CAM) operations, ultimately leading to paperless design and manufac­ture offices (see Chapter 17). Today’s conceptual aircraft designers must master many trades and specialize in at least one, not ignoring the state-of-the-art “rules of thumb” gained from past experience; there is no substitute. They need to be good “number-crunchers” with relatively good analytical ability. They also need assistance from an equally good support team to encompass wider areas. Vastly
increased computer power has reached the desktop with parallel processing. CAE (e. g., CAD, CAM, CFD, FEM, and systems analyses) is the accepted practice in the industry. Those who can afford supercomputers will have the capability to conduct research in areas hitherto not explored or facing limitations (e. g., high-end CFD, FEM, and multidisciplinary optimization [MDO]). This book is not about CAE; rather, it provides readers with the basics of aircraft design that are in practice in the industry and that would prepare them to use CAD/CAE.

Finally, I recommend that aircraft designers have some flying experience, which is most helpful in understanding the flying qualities of aircraft they are trying to design. Obtaining a license requires effort and financial resources, but even a few hours of planned flight experience would be instructive. One may plan and discuss with the flight instructor what needs to be demonstrated – that is, aircraft character­istics in response to control input, stalling, “g” force in steep maneuvers, stick forces, and so forth. Some universities offer a few hours of flight tests as an integral part of aeronautical engineering courses; however, I suggest even more: hands-on experi­ence under the supervision of a flight instructor. A driver with a good knowledge of the design features has more appreciation for the automobile.