Overview

The coursework now starts with this chapter. It follows the mock market study in Chapter 2, which generated customer-specified aircraft requirements. Civil and military aircraft configuration layouts are addressed separately because of the fundamental differences in their approach, especially in the layout of the fuselage. A civil aircraft has “hollow” fuselages to carry passengers. Conversely, a combat-aircraft fuselage is densely packed with fixed equipments and crew members.

Industry uses its considerable experience and imagination to propose several candidate configurations that would satisfy customer (i.e., operator) requirements and be superior to existing designs. Finally, a design is chosen (in consultation with the operators) that would ensure the best sale. In the coursework, after a quick review of possible configurations with the instructor's guidance, it is suggested that only one design be selected for classwork that would be promising in facing market competition. This chapter describes how an aircraft is conceived, first to a preliminary configuration; that is, it presents a methodology for generating a preliminary aircraft shape, size, and weight. Finalizing the preliminary configuration is described in Chapter 11.

The market specification itself demands improvements, primarily in economic gains but also in performance. A 10 to 15% all-around gain over existing designs, delivered when required by the operators, would provide market leadership for the manufacturers. Historically, aircraft designers played a more dominant role in establishing a product line; gradually, however, input by operators began to influence new designs. Major operators have engineers who are aware of the latest trends, and they competently generate realistic requirements for future operations in discussion with manufacturers.

Overview

Chapter 11 completed the aircraft configuration in the conceptual study phase of an aircraft project by finalizing the external dimensions through the formal-sizing and engine-matching procedures. The design now awaits substantiation of aircraft performance to ensure that the requirements are met (see Chapter 13). Substantiation of aircraft performance alone is not sufficient if the aircraft-stability characteristics do not provide satisfactory handling qualities and safety, which are flying qualities that have been codified by NASA. Many good designs required considerable tailoring of the control surfaces, which sometimes affected changes to and/or repositioning of the wing and incorporated additional surfaces (e.g., dorsal fin and ventral fins).

Preliminary stability analyses, using semi-empirical methods (e.g., DATCOM and RAE data sheets [now ESDU]), are conducted during the conceptual study as soon as the three-view aircraft configuration is available. The analyses include the CG location (see Chapter 8) and preliminary stability results from geometric parameters (e.g., surface areas, wing dihedral, sweep, and twist), which are determined from past experience and statistics. Aircraft dynamic-stability analysis requires accurate stability derivatives obtained from extensive wind-tunnel and flight testing. These are cost-intensive exercises and require more budget appropriation after the project go-ahead is obtained in the next phase (i.e., Project Definition, Phase 2). Manufacturing philosophy is firmed up during Phase 2 after aircraft geometry is finalized, when the jig and tool designs can begin. Phase 2 activities are beyond the scope of this book.

This book is about the conceptual phase of a fixed-winged aircraft design project. It is primarily concerned with commercial aircraft design, although it does not ignore military aircraft design considerations. The level of sophistication of the latter is such that were I to discuss advanced military aircraft design, I would quickly deviate from the objective of this book, which is for introductory but extensive course work and which provides a text for those in the industry who wish to broaden their knowledge. The practicing aircraft design engineer also will find the book helpful. However, this book is primarily meant for intensive undergraduate and introductory postgraduate coursework.

A hundred years after the first controlled flight of a manned, heavier-than-air vehicle, we can look back with admiration at the phenomenal progress that has been made in aerospace science and technology. In terms of hardware, it is second to none; furthermore, integration with software has made possible almost anything imaginable. Orville and Wilbur Wright and their contemporaries would certainly be proud of their progenies. Hidden in every mind is the excitement of participating in such feats, whether as operator (pilot) or creator (designer): I have enjoyed both no less than the Wright brothers.

The advancement of aerospace science and technology has contributed most powerfully to the shaping of society, regardless to which part of the world one refers. Sadly, of course, World War II was a catalyst for much of what has been achieved in the past six decades.

Overview

An aircraft must ascend to heights by defying gravity and sustain the tiring task of cruise – naturally, it is weight-sensitive. Anyone who has climbed a hill knows about this experience, especially if one has to carry baggage. An inanimate aircraft is no exception; its performance suffers by carrying unnecessary mass (i.e., weight). At the conceptual design stage, aircraft designers have a daunting task of creating a structure not only at a low weight but also at a low cost, without sacrificing safety. Engineers also must be accurate in weight estimation, well ahead of manufacture. This chapter presents a formal method to predict an aircraft and its component mass (i.e., weight), which results in locating the CG during the conceptual design phase. The aircraft inertia estimation is not within the scope of this book.

In the past, aircraft weight was expressed in FPS units in pound (lb) weight in the United Kingdom and the United States. With the use of kg as mass in SI, the unit for weight is a Newton, which is calculated as the mass multiplied by gravitational acceleration (9.81 m/s2). This book uses both the FPS and SI systems; this chapter addresses mass in SI and weight in FPS, sometimes interchangeably.

Material strength contributes to structural integrity. As stated previously, aircraft conceptual designers must have broad-based knowledge in all aspects of technology; in this case, they must have a sound knowledge in material properties (e.g., strength-to-weight and strength-to-cost ratios).

Overview

Computational fluid dynamics (CFD) is a numerical tool for solving equations of fluid mechanics. CFD is a relatively recent development that has become an indispensable tool in the last two decades. It was developed originally for aeronautical uses but now pervades all disciplines involving flow phenomena, such as medical, natural sciences, and engineering applications. The built-in codes of the CFD software are algorithms of numerical solutions for the fluid-mechanics equations. Flow fields that were previously difficult to solve by analytical means – and, in some situations, impossible – are now accessible by means of CFD.

Today, the aircraft industry uses CFD during the conceptual study phase. There are limitations in obtaining accurate results, but research continues in academic and industrial circles to improve prediction. This chapter aims to familiarize newly initiated readers with the scope of CFD in configuring aircraft geometry (those already exposed to the subject may skip this chapter). This is not a book about CFD; therefore, this chapter does not present a rigorous mathematical approach but rather an overview.

CFD is a subject that requires considerable knowledge in fluid mechanics and mathematics. CFD is introduced late in undergraduate studies, when students have mastered the prerequisites. Commercial CFD tools are menu-driven and it is possible to quickly become proficient, but interpreting the results thus obtained requires considerable experience in the subject.

An accurate 3D model of an aircraft in CAD significantly reduces preprocessing time.

Overview

The engine may be considered the heart of any powered-aircraft system. This book is not concerned with engine design, but it covers the information needed by aircraft designers to find a matched engine, install it on an aircraft, and evaluate its performance. The chapter begins with an introduction to the evolution of an engine followed by the classification of engine types available and their domain of application. This chapter primarily discusses gas turbines (both jet- and propeller-driven) and – to a lesser extent – piston engines, which are used only in smaller general aviation aircraft. Therefore, a discussion of propeller performance is also included in the chapter. The derivation of thrust equations precedes propeller theory.

It is difficult to obtain industry-standard engine-performance data for coursework because the information is proprietary. The performance of some types of engines in nondimensional form is described in Section 10.11. Readers must be careful when applying engine data – an error could degrade or upgrade the aircraft performance and corrupt the design. Verification and substantiation of aircraft design are accomplished through performance flight tests. It is difficult to locate the source of any discrepancy between predicted and tested performance, whether the discrepancy stems from the aircraft, the engine, or both. The author suggests that appropriate engine data may be obtained beyond what is provided in the scope of this book. As mentioned previously, the U.S. contribution to aeronautics is indispensable and its data are generated using the FPS system.

Overview

This chapter assesses whether the aircraft being configured, thus far, meets the FAR and customer requirements given in the form of specifications. Coursework follows linearly from the mock market survey (see Chapter 2). Specification requirements addressed in this chapter include aircraft performance to meet the (1) TOFL, (2) LFL, (3) initial rate of climb, (4) maximum speed at initial cruise (especially for civil aircraft design), and (5) payload range. Chapter 16 computes the aircraft DOC, which should follow the aircraft performance estimation.

Aircraft performance is a subject that aeronautical schools offer as a separate course. Therefore, to substantiate the FAR and customer requirements, this chapter addresses only what is required – that is, the related governing equations and computational examples associated with the five substantiation parameters listed previously. Substantiation of the payload range requires integrated performances of climb and descent that show fuel consumed, distance covered, and time taken for the flight segments. Integrated climb and descent performances are not specification requirements at this stage; therefore, their detailed computational examples are not provided. Instead, the final results in graphical form carry out the payload-range estimation. It is suggested that readers refer to appropriate textbooks for details on this topic. The turboprop example is not worked out but there is sufficient information to compute it similarly.

The remainder of the book after this chapter (except Chapter 16) presents information that aircraft designers should know and apply to their configurations.

Overview

Aircraft structures must withstand the imposed load during operations; the extent depends on what is expected from the intended mission role. The bulkiness of the aircraft depends on its structural integrity to withstand the design load level. The heavier the load, the heavier is the structure; hence, the MTOW affecting aircraft performance. Aircraft designers must comply with mandatory certification regulations to meet the minimum safety standards.

This book does not address load estimation in detail but rather continues with design information on load experienced by aircraft. Although the information provided herein is not directly used in configuring aircraft, the knowledge and data are essential for understanding design considerations that affect aircraft mass (i.e., weight). Only the loads and associated V-n diagram in symmetrical flight are discussed herein. It is assumed that designers are supplied with aircraft V-n diagrams by the aerodynamics and structures groups. Estimation of load is a specialized subject covered in focused courses and textbooks. However, this chapter does outline the key elements of aircraft loads. Aircraft shaping dictates the pattern of pressure distribution over the wetted surface that directly affects load distribution. Therefore, aircraft loads must be known early enough to make a design “right the first time.”

What Is to Be Learned?

This chapter covers the following topics:

• Section 5.2: Introduction to aircraft load, buffet, and flutter

• Section 5.3: Flight maneuvers

• Section 5.4: Aircraft load

• Section 5.5: Theory and definitions (limit and ultimate load)

• Section 5.6: Limits (load limit and speed limit)

• Section 5.7: V-n diagram (the safe flight envelope)

• Section 5.8: Gust envelope

Coursework Content

This chapter provides the basic information required to generate conceptual aircraft configurations.

Overview

Chapter 6 illustrates how to arrive at a preliminary aircraft configuration of a new project starting from scratch, with the expectancy of satisfying the market specification. To progress further, the next task is to lay out the undercarriage (also known as the landing gear) position relative to the aircraft CG, which is accurately established in Chapter 8. This chapter addresses the undercarriage quite extensively but not the detailed design; rather, it focuses on those aspects related to undercarriage layout and sizing during the conceptual study phase. More details on undercarriage design are in the cited references.

This chapter first introduces the undercarriage to serve vehicle ground handling, followed by basic definitions, terminologies, and information used in the design process and integration with an aircraft. Finally, methodologies for layout of the undercarriage and tire sizing are presented to complete the aircraft configuration generated thus far. Considerable attention is required to lay out the undercarriage position and to determine tire size and geometric details to avoid hazards during operation. This book limits the topic to the fundamentals to the extent of the requirements for positioning the undercarriage and sizing the wheels and tires. These fundamentals are shown schematically in the three-view aircraft drawings. Relevant information on wheel tires is also presented in this chapter.

The undercarriage is a complex and heavy item and, therefore, expensive to manufacture. It should be made right the first time.

Overview

This chapter presents important information on aircraft configuration that is required in Chapter 6 coursework. The current design and configuration parameters from aircraft production and operations serve as a template for identifying considerations that could influence new designs with improvements.

During the last century, many aircraft configurations have appeared; today, most of those are not relevant to current practice. Older designs, no matter how good they were, cannot compete with today's designs. This book addresses only those well-established designs as shown in the recent Jane's All the World's Aircraft Manual; however, references are made to interesting and unique older aircraft configurations. The chapter starts by examining growth patterns in the aircraft operational envelope (e.g., speed-altitude capabilities). It continues with a classification of generic aircraft types that show distinct patterns within the class in order to narrow down the wide variety of choices available. Statistics is a powerful tool for establishing design trends, and some pertinent statistical parameters are provided herein.

This chapter compiles the available choices for aircraft-component configurations, including types of wing planform, fuselage shape, intake shapes and positioning, and empennage arrangements. These are the “building blocks” for shaping an aircraft, and as many configurations as possible are described. Artistic aesthetics are considered as long as they do not unduly penalize cost and performance – everyone appreciates the attractive streamline aircraft shapes. The new Boeing 787 Dream liner (see Figure 1.8) shape is a good example of the company's latest subsonic commercial transport aircraft.

Overview

An important task in aircraft design is to make the best possible estimation of all the different types of drag associated with aircraft aerodynamics. Commercial aircraft design is sensitive to the DOC, which is aircraft-drag–dependent. Just one count of drag (i.e., CD = 0.0001) could account for several million U.S. dollars in operating cost over the lifespan of a small fleet of midsized aircraft. This will become increasingly important with the increasing trend in fuel costs. Accurate estimation of the different types of drag remains a central theme. (Equally important are other ways to reduce DOC as described in Section 2.1; these are discussed in Chapter 17.)

For a century, a massive effort has been made to understand and estimate drag, and the work is still continuing. Possibly some of the best work on aircraft drag in English is compiled by NACA/NASA, RAE, AGARD, ESDU, DATCOM, Royal Aeronautical Society (RAeS), AIAA, and others. These publications indicate that the drag phenomena are still not fully understood and that the way to estimate aircraft drag is by using semi-empirical relations. CFD (see Chapter 14) is gaining ground but it is still some way from supplanting the proven semi-empirical relations. In the case of work on excrescence drag, efforts are lagging.

The 2D-surface skin friction drag, elliptically loaded induced drag, and wave drag can be accurately estimated – together, they comprise most of the total aircraft drag.

Appendix

Overview

This chapter is concerned with the aerodynamic information required at the conceptual design stage of a new aircraft design project. It provides details that influence shaping and other design considerations and defines the various parameters integral to configuring aircraft mould lines. Any object moving through air interacts with the medium at each point of the wetted (i.e., exposed) surface, creating a pressure field around the aircraft body. An important part of aircraft design is to exploit this pressure field by shaping its geometry to arrive at the desired performance of the vehicle, including shaping to generate lifting surfaces, to accommodate payload, to house a suitable engine in the nacelle, and to tailor control surfaces. Making an aircraft streamlined also makes it looks elegant.

Aeronautical engineering schools offer a series of aerodynamic courses, starting with the fundamentals and progressing toward the cutting edge. It is assumed that readers of this book have been exposed to aerodynamic fundamentals; if so, then readers may browse through this chapter for review and then move on to the next chapter. Presented herein is a brief compilation of applied aerodynamics without detailed theory beyond what is necessary. Many excellent textbooks are available in the public domain for reference. Because the subject is so mature, some nearly half century-old introductory aerodynamics books still serve the purpose of this course; however, more recent books relate better to current examples.

The Arrangement

In a step-by-step manner, I have developed an approach to aircraft design methodology at the conceptual stage that can be followed in the classroom, from the initial stages of finding a market to the final stages of freezing the aircraft configuration. In the aircraft industry, after the “go-ahead” is obtained, the development program moves to the next phase (i.e., the Project [or Product] Definition Phase), which is not within the scope of this book. The book covers two semesters of work: the first, from Chapters 1 through 13, encompasses the conceptual design; and the second, from Chapters 14 through 17, deals with a more detailed exposition of the first semester's work, advancing the concept through more analysis. Some of the second-semester work on cost and manufacturing considerations may require outside, aeronautical school assistance. The recommended two-semester curriculum is outlined at the end of this road map.

The chapters are arranged linearly; there is not much choice in tailoring a course. I attempt to keep the treatise interesting by citing historical cases. The main driver for readers is the motivation to learn. Except for Chapter 1, the book is written in the third person. (Actual coursework starts in Chapter 6 after a brief mock market survey by the students, as discussed in Chapter 2.)

I omit discussions of vertical takeoff and landing/short takeoff and landing (VTOL/STOL), as well as helicopters in their entirety – these subjects require their own extensive treatment.

Overview

Cost analysis and manufacturing technology are subjects that require specialized instruction in academies, and they are not the main topics of this book. They are included to make readers aware that the classical aeronautical subjects of aerodynamics, structures, and propulsion are not sufficient for a successful aircraft design. Cost analysis and manufacturing technology must be considered during the conceptual design study and integrated with classical aeronautical subjects. The following terms are used extensively in this chapter; some were referred to previously: