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:

Overview

This book begins with a brief historical introduction in which our aeronautical legacy is surveyed. The historical background illustrates the human quest to conquer the sky and is manifested in a system shaping society as it stands today: in commerce, travel, and defense. Its academic outcome is to prepare the next generation for the advancement of this cause.

Some of the discussion in this chapter is based on personal experience and is shared by many of my colleagues in several countries; I do not contest any differences of opinion. Aerospace is not only multidisciplinary but also multidimensional – it may look different from varying points of view. Only this chapter is written in the first person to retain personal comments as well as for easy reading.

Current trends indicate maturing technology of the classical aeronautical sciences with diminishing returns on investment, making the industry cost-conscious. To sustain the industry, newer avenues are being searched through better manufacturing philosophies. Future trends indicate “globalization,” with multinational efforts to advance technology to be better, faster, and less expensive beyond existing limits.

What Is to Be Learned?

This chapter covers the following topics:

• Section 1.2: A brief historical background

• Section 1.3: Current design trends for civil and military aircraft

• Section 1.4: Future design trends for civil and military aircraft

• Section 1.5: The classroom learning process

• Section 1.6: Units and dimensions

• Section 1.7: The importance of cost for aircraft designers

Coursework Content

There is no classroom work in this chapter, but I recommend reading it to motivate readers to learn about our inheritance.

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 course work 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.

Overview

Chapter 6 proposes a methodology with worked-out examples to conceive a “firstcut” (i.e., preliminary) aircraft configuration, derived primarily from statistical information except for the fuselage, which is deterministic. A designer's past experience is vital in making the preliminary configuration. Weight estimation is conducted in Chapter 8 for the proposed first-cut aircraft configuration, revising the MTOM taken from statistics. Chapter 9 establishes the aircraft drag (i.e., drag polar), and Chapter 10 develops engine performance. From these building blocks, finally, the aircraft size can be fine-tuned to a “satisfactory” (see Section 4.1) configuration offering a family of variant designs. None may be the optimum but together they offer the best fit to satisfy many customers (i.e., operators) and to encompass a wide range of payload-range requirements, resulting in increased sales and profitability.

The two classic important sizing parameters – wing-loading (W/S) and thrust loading (TSLS/W) are instrumental in the methodology for aircraft sizing and engine matching. This chapter presents a formal methodology to obtain the sized W/S and TSLS/W for a baseline aircraft. These two loadings alone provide sufficient information to conceive of aircraft configuration in a preferred size. Empennage size is governed by wing size and location on the fuselage. This study is possibly the most important aspect in the development of an aircraft, finalizing the external geometry for management review in order to obtain a go-ahead decision for the project.

Appendix

Introduction

In Chapter 1, it is noted that the dynamic behavior of many rotors can be divided into three different classes: lateral, axial, and torsional. This chapter addresses both the axial and torsional behaviors of rotors. Generally, these two categories of behavior do not interact with one another, except in worm drives and bevel gears. Treating axial and torsional behavior together is justifiable because, mathematically, they are close analogies of one another. For cyclically symmetrical rotors, the analysis of both axial and torsional behavior is relatively simple. Thus, it is possible to provide an overview treatment of both in the space of a single chapter.

The degree to which a single rotor can be analyzed in isolation is different for the three classes of vibration. Lateral vibrations of a rotor are usually strongly coupled to the vibrations of the supporting stator and structure. The only significant exception to this is where very flexible bearings are in use. Passive magnetic bearings often provide this condition. By contrast, there is usually little coupling between torsional or axial vibrations of a rotor and any motion of the stator, except in the case of a geared system. Although this fact is helpful in analyzing the rotor, it often has the undesirable effect that even very severe torsional or axial vibrations in a rotor easily may go undetected by vibration probes on the stator. The analysis of torsional behavior is often carried out for a complete shaft train, whereas it is sometimes possible to analyze separately the axial and lateral behavior of the individual rotors in a shaft train.