105,99 €
Mehr erfahren.
- Herausgeber: John Wiley & Sons
- Kategorie: Fachliteratur
- Serie: Aerospace Series (PEP)
- Sprache: Englisch
A comprehensive approach to the air vehicle design process using the principles of systems engineering Due to the high cost and the risks associated with development, complex aircraft systems have become a prime candidate for the adoption of systems engineering methodologies. This book presents the entire process of aircraft design based on a systems engineering approach from conceptual design phase, through to preliminary design phase and to detail design phase. Presenting in one volume the methodologies behind aircraft design, this book covers the components and the issues affected by design procedures. The basic topics that are essential to the process, such as aerodynamics, flight stability and control, aero-structure, and aircraft performance are reviewed in various chapters where required. Based on these fundamentals and design requirements, the author explains the design process in a holistic manner to emphasise the integration of the individual components into the overall design. Throughout the book the various design options are considered and weighed against each other, to give readers a practical understanding of the process overall. Readers with knowledge of the fundamental concepts of aerodynamics, propulsion, aero-structure, and flight dynamics will find this book ideal to progress towards the next stage in their understanding of the topic. Furthermore, the broad variety of design techniques covered ensures that readers have the freedom and flexibility to satisfy the design requirements when approaching real-world projects. Key features: * Provides full coverage of the design aspects of an air vehicle including: aeronautical concepts, design techniques and design flowcharts * Features end of chapter problems to reinforce the learning process as well as fully solved design examples at component level * Includes fundamental explanations for aeronautical engineering students and practicing engineers * Features a solutions manual to sample questions on the book's companion website Companion website - href="http://www.wiley.com/go/sadraey">www.wiley.com/go/sadraey
Sie lesen das E-Book in den Legimi-Apps auf:
Seitenzahl: 1278
Veröffentlichungsjahr: 2012
Ähnliche
Table of Contents
Aerospace Series List
Title Page
Copyright
Dedication
Preface
Objectives
Approach
Features
Outline
Unit Systems
Series Preface
Acknowledgments
Symbols and Acronyms
Chapter 1: Aircraft Design Fundamentals
1.1 Introduction to Design
1.2 Engineering Design
1.3 Design Project Planning
1.4 Decision Making
1.5 Feasibility Analysis
1.6 Tort of Negligence
References
Chapter 2: Systems Engineering Approach
2.1 Introduction
2.2 Fundamentals of Systems Engineering
2.3 Conceptual System Design
2.4 Preliminary System Design
2.5 Detail System Design
2.6 Design Requirements
2.7 Design Review, Evaluation, and Feedback
2.8 Systems Engineering Approach in Aircraft Design
References
Chapter 3: Aircraft Conceptual Design
3.1 Introduction
3.2 Primary Functions of Aircraft Components
3.3 Aircraft Configuration Alternatives
3.4 Aircraft Classification and Design Constraints
3.5 Configuration Selection Process and Trade-Off Analysis
3.6 Conceptual Design Optimization
Problems
References
Chapter 4: Preliminary Design
4.1 Introduction
4.2 Maximum Take-Off Weight Estimation
4.3 Wing Area and Engine Sizing
4.4 Design Examples
Problems
References
Chapter 5: Wing Design
5.1 Introduction
5.2 Number of Wings
5.3 Wing Vertical Location
5.4 Airfoil Section
5.5 Wing Incidence
5.6 Aspect Ratio
5.7 Taper Ratio
5.8 The Significance of Lift and Load Distributions
5.9 Sweep Angle
5.10 Twist Angle
5.11 Dihedral Angle
5.12 High-Lift Device
5.13 Aileron
5.14 Lifting-Line Theory
5.15 Accessories
5.16 Wing Design Steps
5.17 Wing Design Example
Problems
References
Chapter 6: Tail Design
6.1 Introduction
6.2 Aircraft Trim Requirements
6.3 A Review on Stability and Control
6.4 Tail Configuration
6.5 Canard or Aft Tail
6.6 Optimum Tail Arm
6.7 Horizontal Tail Parameters
6.8 Vertical Tail Design
6.9 Practical Design Steps
6.10 Tail Design Example
Problems
References
Chapter 7: Fuselage Design
7.1 Introduction
7.2 Functional Analysis and Design Flowchart
7.3 Fuselage Configuration Design and Internal Arrangement
7.4 Ergonomics
7.5 Cockpit Design
7.6 Passenger Cabin Design
7.7 Cargo Section Design
7.8 Optimum Length-to-Diameter Ratio
7.9 Other Fuselage Internal Segments
7.10 Lofting
7.11 Fuselage Design Steps
7.12 Design Example
Problems
References
Chapter 8: Propulsion System Design
8.1 Introduction
8.2 Functional Analysis and Design Requirements
8.3 Engine Type Selection
8.4 Number of Engines
8.5 Engine Location
8.6 Engine Installation
8.7 Propeller Sizing
8.8 Engine Performance
8.9 Engine Selection
8.10 Propulsion System Design Steps
8.11 Design Example
Problems
References
Chapter 9: Landing Gear Design
9.1 Introduction
9.2 Functional Analysis and Design Requirements
9.3 Landing Gear Configuration
9.4 Fixed, Retractable, or Separable Landing Gear
9.5 Landing Gear Geometry
9.6 Landing Gear and Aircraft Center of Gravity
9.7 Landing Gear Mechanical Subsystems/Parameters
9.8 Landing Gear Design Steps
9.9 Landing Gear Design Example
Problems
References
Chapter 10: Weight of Components
10.1 Introduction
10.2 Sensitivity of Weight Calculation
10.3 Aircraft Major Components
10.4 Weight Calculation Technique
10.5 Chapter Examples
Problems
References
Chapter 11: Aircraft Weight Distribution
11.1 Introduction
11.2 Aircraft Center of Gravity Calculation
11.3 Center of Gravity Range
11.4 Longitudinal Center of Gravity Location
11.5 Technique to Determine the Aircraft Forward and Aft Center of Gravity
11.6 Weight Distribution Technique
11.7 Aircraft Mass Moment of Inertia
11.8 Chapter Example
Problems
References
Chapter 12: Design of Control Surfaces
12.1 Introduction
12.2 Configuration Selection of Control Surfaces
12.3 Handling Qualities
12.4 Aileron Design
12.5 Elevator Design
12.6 Rudder Design
12.7 Aerodynamic Balance and Mass Balance
12.8 Chapter Examples
Problems
References
Appendices
Appendix A
Appendix B
Index
Aerospace Series List
Introduction to UAV Systems 4e
Fahlstrom and Gleason
August 2012
Theory of Lift: Introductory Computational Aerodynamics with MATLAB®/Octave
McBain
August 2012
Sense and Avoid in UAS: Research and Applications
Angelov
April 2012
Morphing Aerospace Vehicles and Structures
Valasek
April 2012
Gas Turbine Propulsion Systems
MacIsaac and Langton
July 2011
Basic Helicopter Aerodynamics, 3rd Edition
Seddon and Newman
July 2011
Advanced Control of Aircraft, Spacecraft and Rockets
Tewari
July 2011
Cooperative Path Planning of Unmanned Aerial Vehicles
Tsourdos et al
November 2010
Principles of Flight for Pilots
Swatton
October 2010
Air Travel and Health: A Systems Perspective
Seabridge et al
September 2010
Design and Analysis of Composite Structures: With applications to aerospace Structures
Kassapoglou
September 2010
Unmanned Aircraft Systems: UAVS Design, Development and Deployment
Austin
April 2010
Introduction to Antenna Placement & Installations
Macnamara
April 2010
Principles of Flight Simulation
Allerton
October 2009
Aircraft Fuel Systems
Langton et al
May 2009
The Global Airline Industry
Belobaba
April 2009
Computational Modelling and Simulation of Aircraft and the Environment: Volume 1 – Platform Kinematics and Synthetic Environment
Diston
April 2009
Handbook of Space Technology
Ley, Wittmann Hallmann
April 2009
Aircraft Performance Theory and Practice for Pilots
Swatton
August 2008
Surrogate Modelling in Engineering Design: A Practical Guide
Forrester, Sobester, Keane
August 2008
Aircraft Systems, 3
rd
Edition
Moir & Seabridge
March 2008
Introduction to Aircraft Aeroelasticity And Loads
Wright & Cooper
December 2007
Stability and Control of Aircraft Systems
Langton
September 2006
Military Avionics Systems
Moir & Seabridge
February 2006
Design and Development of Aircraft Systems
Moir & Seabridge
June 2004
Aircraft Loading and Structural Layout
Howe
May 2004
Aircraft Display Systems
Jukes
December 2003
Civil Avionics Systems
Moir & Seabridge
December 2002
This edition first published 2013
© 2013, John Wiley & Sons, Ltd
Registered office
John Wiley & Sons Ltd, The Atrium, Southern Gate, Chichester, West Sussex, PO19 8SQ, United Kingdom
For details of our global editorial offices, for customer services and for information about how to apply for permission to reuse the copyright material in this book please see our website at www.wiley.com.
The right of the author to be identified as the author of this work has been asserted in accordance with the Copyright, Designs and Patents Act 1988.
All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, except as permitted by the UK Copyright, Designs and Patents Act 1988, without the prior permission of the publisher.
Wiley also publishes its books in a variety of electronic formats. Some content that appears in print may not be available in electronic books.
Designations used by companies to distinguish their products are often claimed as trademarks. All brand names and product names used in this book are trade names, service marks, trademarks or registered trademarks of their respective owners. The publisher is not associated with any product or vendor mentioned in this book. This publication is designed to provide accurate and authoritative information in regard to the subject matter covered. It is sold on the understanding that the publisher is not engaged in rendering professional services. If professional advice or other expert assistance is required, the services of a competent professional should be sought.
MATLAB® is a trademark of The MathWorks, Inc. and is used with permission. The MathWorks does not warrant the accuracy of the text or exercises in this book. This book's use or discussion of MATLAB® software or related products does not constitute endorsement or sponsorship by The MathWorks of a particular pedagogical approach or particular use of the MATLAB® software.
Library of Congress Cataloging-in-Publication Data
Sadraey, Mohammad H.
Aircraft design : a systems engineering approach / Mohammad H. Sadraey.
pages cm
Includes bibliographical references and index.
ISBN 978-1-119-95340-1 (hardback)
1. Airplanes–Design and construction. I. Title.
TL671.2.S3136 2012
629.134'1–dc23
2012009907
A catalogue record for this book is available from the British Library.
Print ISBN: 9781119953401
To Fatemeh Zafarani, Ahmad, and Atieh
Preface
Objectives
The objective of this book is to provide a basic text for courses in the design of heavier-than-air vehicles at both the upper division undergraduate and beginning graduate levels. Aircraft design is a special topic in the aeronautical/aerospace engineering discipline. The academic major of aeronautical/aerospace engineering traditionally tends to have four main areas of expertise: aerodynamics, flight dynamics, propulsion, and structure. A qualified aircraft designer employs all these four scientific concepts and principles and integrates them using special design techniques to design a coordinated unique system; an aircraft. Design is a combination of science, art, and techniques. A designer not only must have sufficient level of knowledge in these four areas, but also needs to employ mathematics, skills, experiences, creativity, art, and system design techniques. It is true that aircraft design is not completely teachable in classrooms, but combining class lectures with a semester-long aircraft design project provides the best opportunity for students to learn and experience aircraft design.
Every aeronautical engineering discipline offers at least one course in aircraft design or aerospace system design. The lack of an aircraft design textbook with academic features—such as full coverage of all aspects of an air vehicle, aeronautical concepts, design methods, design flowcharts, design examples, and end-of-chapter problems—combined with the newly developed systems engineering techniques was the main motivation to write this book.
In the past several years, I have talked to various aircraft design instructors and students at conferences and AIAA Design/Build/Fly design competitions. I came to the conclusion that the great design books published by such pioneers as Roskam, Torenbeek, Nicolai, Stinton, and Raymer need more development and expansion. This is to meet the ever-increasing need of universities and colleges for aircraft design education, and of industries for design implementation. The new text should possess significant features such as systems engineering approaches, design procedures, solved examples, and end-of-chapter problems. This book was written with the aim of filling the gap for aeronautical/aerospace engineering students and also for practicing engineers.
Approach
The process of air vehicle design is a complex combination of numerous disciplines which have to be blended together to yield the optimum design to meet a given set of requirements. The systems engineering approach is defined as an interdisciplinary approach encompassing the entire technical effort to evolve and verify an integrated and lifecycle-balanced set of system people, products, and process solutions that satisfy customer needs. Multi-discipline system engineering design involves the application of a systems engineering process and requires engineers with substantive knowledge of design across multiple technical areas and improved tools and methods for doing it. Complex aircraft systems, due to the high cost and the risks associated with their development, become a prime candidate for the adoption of systems engineering methodologies. The systems engineering technique has been applied in the development of many manned airplanes. An aircraft is a system composed of a set of interrelated components working together toward some common objective or purpose. Primary objectives include safe flight achieved at a low cost. Every system is made up of components or subsystems, and any subsystem can be broken down into smaller components. For example, in an air transportation system, the aircraft, terminal, ground support equipment, and controls are all subsystems.
Throughout the text, the systems engineering approach is examined and implemented. The book has been arranged to facilitate the student's gradual understanding of design techniques. Statement proofs are provided whenever they contribute to the understanding of the subject matter presented. Special effort has been made to provide example problems so that the reader will have a clear understanding of the topic discussed. The reader is encouraged to study all such solved problems carefully; this will allow the interested reader to obtain a deeper understanding of the materials and tools.
Features
Some of the unique features of this textbook are as follows. It:
follows a systems engineering approach;
is organized based on components design (e.g., wing design, tail design, and fuselage design);
provides design steps and procedures in each chapter;
derives a number of design equations that are unique to the book;
provides several fully solved design examples at the component level;
has many end-of-chapter problems for readers to practice;
includes a lot of aircraft figures/images to emphasize the application of the concepts;
describes some real design stories that stress the significance of safety in aircraft design;
provides various aircraft configurations, geometries, and weights data to demonstrate real-world applications and examples;
covers a variety of design techniques/processes so that the designer has freedom and flexibility to satisfy the design requirements in several ways;
encourages and promotes the creativity of the reader.
For these reasons, as aeronautical/aerospace engineering students transit to practicing engineers, they will find that this text is indispensable as a reference text. Some materials, such as “design optimization” and “design of control surfaces,” may be taught at the graduate level. The reader is expected to have a basic knowledge of the fundamentals and concepts of aerodynamics, propulsion, aero-structure, aircraft performance, and flight dynamics (stability and control) at aeronautical/aerospace engineering senior level.
The following is a true statement: “design techniques are not understood unless practiced.” Therefore, the reader is strongly encouraged to experience the design techniques and concepts through applied projects. Instructors are also encouraged to define an open-ended semester/year-long aircraft design project to help the students to practice and learn through application and experiencing the iterative nature of the design technique. It is my sincere wish that this book will help aspiring students and design engineers to learn and create more efficient and safer aircraft.
Outline
The text consists of 12 chapters and is organized in a standard fashion according to the systems engineering discipline: conceptual design, preliminary design, and detail design. In summary, Chapter 3 presents the aircraft conceptual design; Chapter 4 introduces the aircraft preliminary design; and Chapters 5–12 cover the aircraft detail design. The outline of this book is as follows.
Chapter 1 is an introduction to design fundamentals and covers such topics as engineering design principles, design project planning, decision-making processes, feasibility analysis, and tort of negligence. Design standards and requirements such as Federal Aviation Regulations (FARs) and Military Standards are reviewed in this chapter, and addressed further throughout the text.
Chapter 2 deals with the systems engineering approach. Major design phases according to systems engineering are introduced: conceptual system design, preliminary system design, and detail system design. In this chapter, several concepts and fundamental definitions such as technical performance measures, functional analysis, system trade-off analysis, design review, and design requirements are reviewed. Implementations of systems engineering into aircraft design via aircraft design phases, aircraft design flowcharts, aircraft design groups, and design evaluation and feedback loops are explained. At the end of the chapter, the overall aircraft design procedure in terms of design steps is outlined.
Chapter 3 covers aircraft conceptual design, and examines the aircraft configuration selection. The primary function of each aircraft component such as wing, fuselage, tail, landing gear, and engine is introduced. Furthermore, various configuration alternatives for each component are reviewed. In addition, the aircraft classification and design constraints are addressed. In this chapter the design optimization and its mathematical tools are briefly reviewed. The chapter ends with a configuration selection process and methodology, and also a trade-off analysis technique.
Chapter 4 discusses the topic of aircraft preliminary design. In this chapter, the technique to determine three aircraft fundamental parameters is presented. These parameters are: maximum take-off weight, wing area, and engine thrust/power. The weight build-up technique is examined for estimation of the aircraft maximum take-off weight. The matching plot technique is utilized in the calculation of wing area, and engine thrust/power. These three parameters are computed based on the aircraft performance requirements such as range, endurance, maximum speed, take-off run, rate-of-climb, and ceiling. Two fully solved examples illustrate the application of the two techniques.
Chapters 5–9 and 12 present detail design of the aircraft components of wing, tail, fuselage, propulsion system, landing gear, and control surfaces respectively. In these chapters, the techniques to calculate all aircraft components parameters such as wing/tail span, chord, airfoil, incidence, sweep angle, tail arm, tail area, landing gear height, wheel base, wheel track, fuselage diameter, fuselage length, cabin design, cockpit design, number of engines, and engine selection are examined. Furthermore, the features of various component configurations and their relationship with the design requirements (e.g., performance, stability, control, and cost) are addressed. Chapter 12 introduces the detail design of the conventional control surfaces of aileron, elevator, and rudder. In each chapter, the design flowchart and design step for each component is also presented. Each chapter is accompanied by several examples, including a fully solved chapter example to demonstrate the applications of design techniques and methods.
Chapter 10 introduces the technique to calculate the weight of the aircraft components, equipment, and subsystems. The technique is derived mainly based on past aircraft weight data and statistics.
Chapter 11 addresses the topic of aircraft weight distribution, and weight and balance. The aircraft center of gravity (cg) calculation, aircraft most aft and most forward cg, and cg range are also covered in this chapter. In addition, the technique to determine the aircraft mass moment of inertia about three axes (i.e., x, y, and z) is examined.
Unit Systems
In this text, the emphasis is on SI units or the metric system, which employs the meter (m) as the unit of length, the kilogram (kg) as the unit of mass, and the second (s) as the unit of time. It is true that metric units are more universal and technically consistent than British units. However, currently, many FARs are published in British units, where the foot (ft) is the unit of length, the slug is the unit of mass, the pound (lb) is the unit of force (weight), and the second (s) is the unit of time. In FARs, the pound is used as the unit for force and weight, the knot for airspeed, and the foot for altitude. Thus, in various locations, the knot is mainly used as the unit of airspeed, the pound for weight and force, and the foot for altitude. Therefore, in this text, a combination of SI and British unit systems is utilized. A common mistake in the literature (even in the Jane's publications) is the application of kg for the unit of aircraft weight. Throughout the text, whenever the unit of kg is used, the term “aircraft mass” is employed. Some texts have created the pound-mass (lbm) as the unit of mass, and the pound-force (lbf) as the unit of weight. This initiative may generate some confusion; so in this text, only one pound (lb) is employed as the unit of weight and force.
Series Preface
The field of aerospace is wide ranging and multi-disciplinary, covering a large variety of products, disciplines and domains, not merely in engineering but in many related supporting activities. These combine to enable the aerospace industry to produce exciting and technologically advanced vehicles. The wealth of knowledge and experience that has been gained by expert practitionersin the various aerospace fields needs to be passed onto others working in the industry, including those just entering from University.
The Aerospace Series aims to be a practical and topical series of books aimed at engineering professionals, operators, users and allied professions such as commercial and legal executives in the aerospace industry. The range of topics is intended to be wide ranging, covering design and development, manufacture, operation and support of aircraft as well as topics such as infrastructure operations and developments in research and technology. The intention is to provide a source of relevant information that will be of interest and benefit to all those people working in aerospace.
Aircraft design brings together the key aeronautical engineering disciplines: aerodynamics, flight dynamics, propulsion and structures, which must be combined to produce designs that meet today's stringent performance, economic and environmental demands. As such, aircraft designis a key component of all undergraduate aerospace engineering courses, and all aerospace students usually tackle some form of aircraft design project.
This book, Aircraft Design: A Systems Engineering Approach, extends the classical aircraft design approaches through the implementation of systems engineering techniques for the conceptual, preliminary and detailed design of heavier-than-air vehicles. As a very readable and informative text reference, with plenty of examples from a wide range of contemporary aircraft designs, and solved examples at the end of each chapter, it is a worthy addition to the Wiley Aerospace Series.
Peter Belobaba, Jonathan Cooper, Roy Langton and Allan Seabridge
Acknowledgments
I am enormously grateful to the Almighty for the opportunity to serve the aerospace community by writing this text. The author would like to acknowledge the many contributors and photographers who have contributed to this text. I am especially grateful to those who provided great aircraft photographs: Anne Deus (Germany); Jenney Coffey (UK); Anthony Osborne (UK); A J Best (UK); Vlamidir Mikitarenko (Germany); Rainer Bexten (Germany); Hideki Nakamura (Japan); Akira Uekawa (Japan); Luis David Sanchez (Puerto Rico); Tom Houquet (Belgium); Toshi Aoki (Japan); Miloslav Storoska (Slovakia); Tom Otley (Panacea Publishing International, UK); Jonas Lövgren (SAAB, Sweden); Jeff Miller (Gulfstream Aerospace Corporation, USA); Michael de Boer (Netherland); Konstantin von Wedelstaedt (Germany); Augusto G. Gomez R. (Mexico); Randy Crew (Singapore); Robert Domandl; Serghei Podlesnii (Moldova); Orlando J. Junior (Brazil); Balázs Farkas (Hungary); and Christopher Huber and www.airliners.net. In addition, the efforts of the author were helped immeasurably by the many insights and constructive suggestions provided by students and instructors over the past 16 years. Unattributed figures are held in the public domain and are from either the US Government Departments and Agencies, or Wikipedia.
Putting a book together requires the talents of many people, and talented people abound at John Wiley & Sons, Inc. My sincere gratitude goes to Paul Petralia, commissioning editor, for coordinating the whole publication process; Clarissa Lim for coordinating the production project; Sarah Lewis for editing the manuscript; Jayashree Saishankar for typesetting; and Sandra Grayson for helping in the copyright process. I am particularly grateful to my editors, Liz Wingett and Sophia Travis, for their comments and guidance. My special thanks go to the outstanding copyeditors and proofreaders who are essential in creating an error-free text. I especially owe a large debt of gratitude to the reviewers of this text. Their ideas, suggestions, and criticisms have helped me to write more clearly and accurately and have influenced the evolution of this book markedly.
Symbols and Acronyms
Symbols
Symbol
Name
Unit
a
Speed of sound
m/s, ft/s
a
Acceleration
m/s
2
, ft/s
2
A
Area
m
2
, ft
2
AR
Aspect ratio
—
b
Lifting surface/control surface span
m, ft
B
Wheel base
m, ft
C
Specific fuel consumption
N/h kW, lb/h hp
Mean aerodynamic chord
m, ft
C
D
,
C
L
,
C
y
Drag, lift, and side-force coefficients
—
C
l
,
C
m
,
C
n
Rolling, pitching, and yawing moment coefficients
—
C
h
Hinge moment coefficient
—
Aircraft side drag coefficient
—
Rate of change of drag coefficient w.r.t. sideslip angle;
1/rad
Wing/fuselage pitching moment coefficient (about the wing/fuselage aerodynamic center)
—
Rate of change of pitching moment w.r.t. angle of attack
1/rad
Rate of change of pitch rate w.r.t. angle of attack
1/rad
1/rad
1/rad
1/rad
1/rad
Rate of change of yawing moment coefficient w.r.t. sideslip angle
1/rad
Rate of change of yawing moment coefficient w.r.t. yaw rate
1/rad
Zero-lift drag coefficient
—
Induced drag coefficient
—
C
f
Skin friction coefficient
—
Wing/tail/aircraft (3D) lift curve slope
1/rad
Airfoil (2D) lift curve slope
1/rad
Maximum lift coefficient
—
D
Drag force, drag
N, lb
D
Diameter
m, ft
d
c
Distance between the aircraft cg and center of the projected side area
m, ft
E
Endurance
h, s
E
Modulus of elasticity
N/m
2
, Pa, lb/in
2
, psi
e
Oswald span efficiency factor
—
F
Force, friction force
N, lb
F
C
Centrifugal force
N, lb
FOM
Figure of merit
—
g
Gravity constant
9.81 m/s
2
, 32.17 ft/s
2
G
Fuel weight fraction
—
GR
Gearbox ratio
—
G
C
Ratio between the linear/angular movement of the stick/wheel to deflection of the control surface
deg/m, deg/ft, deg/deg
H
Altitude
m, ft
h
,
h
o
Non-dimensional distance between cg (
h
) or ac (
h
o
) and a reference line
—
H
Height, wheel height
m, ft
H
Control surface hinge moment
Nm, lb ft
i
h
Tail incidence
deg, rad
i
w
Wing incidence
deg, rad
l
Length, tail arm
m, ft
I
Mass moment of inertia
kg m
2
, slug ft
2
I
Second moment of area
m
4
, ft
4
I
Index (e.g., design, performance)
—
K
Induced drag factor
—
L
,
L
A
Rolling moment
Nm, lb ft
L
Length
m, ft
L
Lift force, lift
N, lb
(
L
/
D
)
max
Maximum lift-to-drag ratio
—
M
Mach number
—
M
,
M
A
Pitching moment
Nm, lb ft
m
mass
kg, slug
Engine air mass flow rate
kg/s, lb/s
MTOW
Maximum take-off weight
N, lb
MAC
Mean aerodynamic chord
m, ft
n
Load factor
—
n
Number of rows in cabin
—
n
Rotational speed
rpm, rad/s
N
Normal force
N, lb
N
Number of an item
—
N
,
N
A
Yawing moment
Nm, lb ft
P
Pressure
N/m
2
, Pa, lb/in
2
, psi
P
Power
W, kW, hp, lb ft/s
P
s
Seat pitch
m, ft
P
req
Required power
W, kW, hp, lb ft/s
P
av
Available power
W, kW, hp, lb ft/s
P
exc
Excess power
W, kW, hp, lb ft/s
P
,
p
Roll rate
rad/s, deg/s
q
,
Dynamic pressure
N/m
2
, Pa, lb/in
2
, psi
Q
,
q
Pitch rate
rad/s, deg/s
Q
Fuel flow rate
kg/s, lb/s
R
Range
m, km, ft, mile, mi, nmi
R
Air gas constant
287.26 J/kg K
R
Radius
m, ft
R
Rank
—
Re
Reynolds number
—
ROC
Rate of climb
m/s, ft/min, fpm
R
,
r
Yaw rate
rad/s, deg/s
s
Semispan (
b
/2)
m, ft
S
Planform area of lifting/control surface
m
2
, ft
2
S
A
Airborne section of the take-off run
m, ft
S
G
Ground roll
m, ft
S
TO
Take-off run
m, ft
SFC
Specific fuel consumption
N/h/kW, lb/h/hp, 1/s, 1/ft
SM
Static margin
—
t
Time
s, min, h
T
Engine thrust
N, lb
T
Temperature
°C, °R, K
T
Wheel track
m, ft
T
,
t
Thickness
m, ft
t
/
c
Airfoil thickness-to-chord ratio
—
T
/
W
Thrust-to-weight ratio
—
U
Forward airspeed
m/s, ft/min, km/h, mi/h, knot
V
Velocity, speed, airspeed
m/s, ft/min, km/h, mi/h, knot
V
Volume
m
3
, ft
3
V
max
Maximum speed
m/s, ft/min, km/h, mi/h, knot
V
mc
Minimum controllable speed
m/s, ft/min, km/h, mi/h, knot
Minimum drag speed
m/s, ft/min, km/h, mi/h, knot
Minimum power speed
m/s, ft/min, km/h, mi/h, knot
V
R
Rotation speed
m/s, ft/min, km/h, mi/h, knot
Maximum rate of climb speed
m/s, ft/min, km/h, mi/h, knot
V
s
Stall speed
m/s, ft/min, km/h, mi/h, knot
V
T
True speed
m/s, ft/min, km/h, mi/h, knot
V
TO
Take-off speed
m/s, ft/min, km/h, mi/h, knot
V
W
Wind speed
m/s, ft/min, km/h, mi/h, knot
,
Horizontal/vertical tail volume coefficient
—
W
Weight
N, lb
W
Width
m, ft
W
f
Fuel weight
N, lb
W
TO
Maximum take-off weight
N, lb
W
/
P
Power loading
N/W, lb/hp
W
/
S
Wing loading
N/m
2
, lb/ft
2
x
,
y
,
z
Displacement in
x
,
y
, and
z
direction
m, ft
Y
Side force
N, lb
y
Beam deflection
m, ft
Greek symbols
Symbol
Name
Unit
α
Angle of attack
deg, rad
β
Sideslip angle
deg, rad
γ
Climb angle
deg, rad
θ
Pitch angle, pitch attitude
deg, rad
λ
Taper ratio
—
ϕ
Bank angle
deg, rad
δ
Pressure ratio
—
δ
Control surface deflection
deg, rad
σ
Air density ratio
—
σ
Sidewash angle
deg, rad
ρ
Air density, materials density
kg/m
3
, slug/ft
3
μ
Dynamic viscosity
kg/m s, lb s/ft
2
μ
Friction coefficient
—
μ
Mach angle
rad, deg
η
Efficiency, dynamic pressure ratio
—
Λ
Sweep angle
deg, rad
ω
Angular velocity
rad/s, deg/s
ω
n
Natural frequency
rad/s, deg/s
ω
Frequency
rad/s, deg/s
ψ
Yaw angle, heading angle
deg, rad
π
3.14
—
Ω
Spin rate
rad/s, deg/s, rpm
τ
Control surface angle of attack effectiveness
—
Γ
Dihedral angle
deg, rad
ϵ
Downwash angle
degr, rad
Downwash slope
—
Sidewash slope
—
Take-off rotation angular acceleration
deg/s
2
, rad/s
2
Δ
Non-dimensional range of center of gravity
—
Subscripts
Note
AR,
S
,
b
,
λ
, Λ, Γ, and
C
without a subscript indicate a wing property
0, o
Zero-lift, sea level, about aerodynamic center
0.25
Quarter chord
1
Steady-state value
a, A
Aileron
aft
The most aft location
A
Aerodynamic
ac
Aerodynamic center
avg
Average
a
Aircraft
b
Baggage
c/4
Relative to the quarter chord
c/2
Relative to the 50% of the chord
cs
Control surface
cross
Cross-section
C
Crew, ceiling, cruise, cabin
d
Design
D
Drag
e, E
Elevator, equivalent, empty, exit
eff
Effective
E
Engine
f
Fuel, fuselage, flap, friction
for
The most forward location
GL
Glide
h
Horizontal tail
i
Item number, inboard, ideal, initial, inlet
ISA
International Standard Atmosphere
L
Lift, left, landing
LG
Landing gear
max
Maximum
min
Minimum
m
Pitching moment
mg
Main gear
mat
Materials
o
Outboard
opt
Optimum
ot
Overturn
p
Propeller
PL
Payload
r, R
Rudder
R
Rotation
r
Root
ref
Reference
s
Stall, stick
ss
Steady-state
SL
Sea level
S
Side
SR
Spin recovery
t
Tip, tab, twist, horizontal tail
T
True
TO
Take-off
tot
Total
ult
Ultimate
v, V
Vertical tail
VT
Vertical tail
w, W
Wing, wind
wet
Wetted
wf
Wing/fuselage
x
,
y
, or
z
In the
x
,
y
, or
z
direction
xx
,
yy
, or
zz
About the
x
-,
y
-, or
z
-axis
Acronyms
ac or AC
Aerodynamic center
ca
Center of area, center of action
cg or CG
Center of gravity
APU
Auxiliary power unit
CAD
Computer-aided design
CAM
Computer-aided manufacturing
CDR
Conceptual design review
CFD
Computational fluid dynamics
cp
Center of pressure
DOF
Degrees of freedom
DOD
Department of Defense
EASA
European Aviation Safety Agency
ETR
Evaluation and test review
FDR
Final (critical) design review
FAA
Federal Aviation Administration
FAR
Federal Aviation Regulations
FBW
Fly-by-wire
GA
General aviation
HALE
High-altitude long-endurance
HLD
High-lift device
IATA
International Air Transport Association
ISA
International Standard Atmosphere
JAR
Joint aviation requirements
KTAS
Knot true air speed
KEAS
Knot equivalent air speed
LG
Landing gear
LE
Leading edge
MAC
Mean aerodynamic chord
MDO
Multidisciplinary design optimization
MIL-STD
Military Standards
NACA
National Advisory Committee for Aeronautics
NASA
National Aeronautics and Space Administration
NTSB
National Transportation Safety Board
np or NP
Neutral point
OEI
One engine inoperative
PDR
Preliminary design review
rpm
Revolutions per minute
rad
Radian
RCS
Radar cross-section
STOL
Short take-off and landing
TE
Trailing edge
Turboprop
Turbopropeller
VTOL
Vertical take off and landing
WWII
World War II
Conversion Factors
Length, Altitude, Range
Area
Volume
Speed, Airspeed, Rate of Climb
Mass
Force, Weight, Thrust
Mass and Weight
Work, Energy
Power
Mass Moment of Inertia
Pressure
Time, Endurance
Angle
Chapter 1
Aircraft Design Fundamentals
1.1 Introduction to Design
Aircraft design is essentially a branch of engineering design. Design is primarily an analytical process which is usually accompanied by drawing/drafting. Design contains its own body of knowledge, independent of the science-based analysis tools usually coupled with it. Design is a more advanced version of a problem-solving technique that many people use routinely. Design is exciting, challenging, satisfying, and rewarding. The general procedure for solving a mathematical problem is straightforward. Design is much more subjective, there is rarely a single “correct” answer. The world of design involves many challenges, uncertainties, ambiguities, and inconsistencies. This chapter is intended to familiarize the reader with the basic fundamentals and overall process of design. This book has been written primarily to provide the basic tools and concepts required to create an optimum/efficient aircraft design that will meet the necessary design requirements.
A very basic and simplified model of a design process is shown schematically in Figure 1.1. In general, a design process includes three major operations: analysis, synthesis, and evaluation. Analysis is the process of predicting the performance or behavior of a design candidate. Evaluation is the process of performance calculation and comparing the predicted performance of each feasible design candidate to determine the deficiencies. The noun synthesis refers to a combination of two or more entities that together form something new. In this text, synthesis is employed interchangeably with design. Hence, synthesis is defined as the creative process of putting known things together into new and more useful combinations. Synthesis is the vehicle of the design, with evaluation being its compass. The candidate designs that fail to satisfy (partially or completely) the requirements are reiterated. That is new values, features, characteristics, or parameters are determined during synthesis operation. The redesigned candidate is reanalyzed again for compliance with the design requirements. This iterative process is continued until the design requirements are met. A design process requires both integration and iteration, invoking a process that coordinates synthesis, analysis, and evaluation. These three operations must be integrated and applied iteratively and continuously throughout the lifecycle of the design.
Figure 1.1 Interrelationship between synthesis, analysis, and evaluation
A design operation often involves two activities: (i) problem solving through mathematical calculations and (ii) choosing a preferred one among alternatives (Figure 1.2). The first activity is performed in Chapters 4–12 in designing various aircraft components. The second design activity is in general a decision-making process. The fundamentals of decision making are reviewed in Section 1.4; and employed entirely in aircraft conceptual design (Chapter 3). In addition, there are various decision-making processes in aircraft components design (e.g., wing design, tail design, and propulsion system design), as will be discussed in several chapters. The major components that comprise a conventional aircraft are wing, fuselage, horizontal tail, vertical tail, engine, landing gear, and equipment. The decision-making process plays a significant role in the configuration design of these primary components.
Figure 1.2 Two main groups of design activities in aircraft design
The traditional engineering education is structured to emphasize mathematics, physical sciences, and engineering sciences. The problem is the lack of sufficient concentration on design and creativity. Creative thinking and its attitudes are essential to design success. Producing a new design requires an ability to be creative and overcome strong barriers. To address this significant issue a new organization, CDIO,1 was established in the late 1990s. The CDIO initiative is defined to be an innovative educational framework for producing the next generation of engineers. The framework provides students with an education stressing engineering fundamentals set within the context of conceiving/designing/implementing/operating real-world systems and products. This textbook has been written with a strong emphasis on creativity, and the freedom of the designer to go beyond current aircraft designs.
Throughout this text, various techniques for generating creative design alternatives are introduced. An effective approach in creative design as a source of new ideas is brainstorming. Brainstorming is a structured group-oriented technique for conceiving design alternatives. It consists of a group of individuals letting their imaginations run wild, but in accordance with central procedural rules. The ultimate goal is that the group members will inspire and support each other. The outcome is that the group will be able to conceptualize design alternatives that are more elegant than those the individuals could have achieved independently. In order to encourage members to describe their ideas, even totally impractical ones, a crucial brainstorming rule is that no criticism of individuals or ideas is permitted. The emphasis is on generating as many ideas and concepts as possible, without worrying about their validity. Rectifying, organizing, and combining the ideas suggested in a brainstorming session is performed out of the group meeting. The brainstorming technique is mainly applicable at the conceptual design phase (see Chapters 2 and 3).
In general, aircraft design requires the participation of six (Figure 1.3) fundamental disciplines: (i) flight dynamics, (ii) aerodynamics, (iii) propulsion, (iv) aero-structure, (v) management skills, and (vi) engineering design. The first four items are primary expertise areas of aeronautical engineering. This text has no particular chapters on any of these four topics; so the reader is expected to be familiar with the fundamentals, concepts, technical terms, and engineering techniques in such areas. Management is defined [1] as coordinating work activities so that they are completed efficiently and effectively with and through other people. An aircraft designer needs to be equipped with managerial skills and act as a manager throughout the design process. This topic is not covered in this text; however, a few aspects of management—such as project planning and decision making—are reviewed in this chapter (Sections 1.3 and 1.4).
Figure 1.3 Aircraft design required tools and expertise
Finally, engineering design [2–4] is at the heart of the design process and is assumed as the sixth discipline necessary for design of an air vehicle. Section 1.2 briefly examines various aspects of engineering design. It must be noted that aircraft engineering design has its own science, concepts, fundamentals, technical terms, and techniques. Chapters 3–12 all address various aspects of designing aircraft components as well as introducing aircraft design procedures.
This chapter will first examine the engineering design profession. Next, design project planning is addressed and tools such as Gantt charts are introduced. Then the principle of decision making, a very significant section of any design process, is presented. Feasibility study is also discussed in Section 1.5. Finally, the tort of negligence will be described to warn aircraft design engineers to take the utmost care in order to prevent liability.
1.2 Engineering Design
Aircraft design is essentially a branch of engineering design. Design is the culmination of all engineering activities, embodying engineering operations and analysis as tools to achieve design objectives. Many engineering professors find it more difficult to teach design than to teach traditional engineering science-based analytical topics. Every undergraduate engineering curriculum has a design component, although the extent and structure of that component may vary widely. Engineering design fundamentals are common to all engineering disciplines – aeronautical, mechanical, electrical, civil, and computer. Engineering design is a methodical approach to dealing with a particular class of large and complex projects. Engineering design provides the design engineer with a realistic design process. Design is the central activity of the engineering profession, and it is concerned with approaches and management as well as design techniques and tools. In this section, the fundamentals of engineering design as well as the definitions of a few technical terms are presented.
There is a clear distinction between classical mathematics and science problem-solving techniques, and design operation. There is inherently a beauty embedded in the design process which is usually felt after the design output is created. The mathematics and science problems have three main features: (i) the problems are well-posed in a compact form, (ii) the solutions to each problem are unique and compact, and (iii) the problems have an identifiable closure. However, a real-world engineering design problem does not share these characteristics. In fact, engineering design problems are usually poorly posed, do not have a unique solution, and are also open-ended. The Accreditation Board of Engineering and Technology (ABET) [5] defines engineering design as follows:
Engineering design is the process of devising a system, component, or process to meet desired needs. It is a decision making process (often iterative), in which the basic sciences and mathematics and engineering sciences are applied to convert resources optimally to meet a stated objectives. Among the fundamental elements of the design process are the establishment of objectives and criteria, synthesis, analysis, construction, testing, and evaluation.
Just as the ABET statement is only one of many definitions of engineering design, there are several approaches to describing how design is done. This text formalizes the ABET description into a simplified step-by-step model of the design process based on a systems engineering approach [6]. A very basic block diagram of the design process is shown in Figure 1.4. It represents the road from customer need to design output, including feedback based on evaluation. The problem formulation is discussed in this section, and project planning is examined in Section 1.4. A large part of this text is on design operations, including Chapters 3–12.
Figure 1.4 Engineering design block diagram
The evaluation not only influences the design operation, but most of the time may affect problem formulation and project planning. A clear current example is the Space Shuttle, which started in 1981 but retired in 2011. After more than 30 years of successful operations (135 space missions), the National Aeronautics and Space Administration (NASA) figured out that the current design concept is not viable. Besides economic factors, two reasons that forced NASA to re-engineer the Space Shuttle (Figure 1.5) are the disasters that happened in 1986 and 2003. On January 28, 1986 Space Shuttle Challenger broke apart, just 73 seconds into its flight, leading to the deaths of its seven crew members. On February 1, 2003, shortly before it was scheduled to conclude its 28th mission, Space Shuttle Columbia disintegrated over Texas during re-entry into the Earth's atmosphere, resulting in the death of all seven crew members. Until another US launch vehicle is ready, crews will travel to and from the International Space Station aboard Russian Soyuz spacecraft or possibly a future American commercial spacecraft.
Figure 1.5 The original Space Shuttle concept and mission profile. Reproduced from permission of NASA
After the need is clearly defined, the designer has to turn his/her attention to describing how he/she envisions meeting the need. This fundamental step requires achieving a delicate balance between establishing the general scope of the design efforts, and avoiding being so specific that opportunities are unnecessarily narrowed for creative design solutions. Problem formulation includes recognizing the need, identifying the customer, market assessment, defining the problem, functional analysis, and establishing design requirements. A problem statement needs to be constructed in such a way that it consists of three components: goal, objectives, and constraints (Figure 1.6).
Figure 1.6 Three elements of a problem statement
A goal statement is a brief, general, and ideal response to the need statement. The need describes the current, unsatisfactory situation, while the goal describes the ideal future condition to which we aspire in order to improve on the situation described by the need. The goal is defined by describing the current situation that is unsatisfactory. Hence the goal is to improve the current situation to a higher level. The goal is generally so ideal that it could never be accomplished. The goal is usually revised through a process called benchmarking. Benchmarking involves explicitly comparing your design to that of the competitor which does the best job in terms of satisfying customer requirements.
The objectives are quantifiable expectations of performance which identify those performance characteristics of a design that are of most interest to the customer. In addition, the objectives must include a description of conditions under which a design must perform. In the lifecycle, the objective is to specify the whats and not the hows; that is, what needs to be accomplished versus how it is to be done. When the operating conditions are specified, the designer is able to evaluate the performance of different design options under comparable conditions. Each of the objectives must be defined using words that convey the desirable aspect of performance. The term “performance specification” is often a synonym for objectives. However, the term “design specification” refers to the detailed description of the completed design, including all dimensions, material properties, weight, and fabrication instructions.
Restrictions of function or form are called constraints; they limit our freedom to design. Constraints define the permissible conditions of design features and the permissible range of the design and performance parameters. They are features that all design must have in order to be eligible for consideration. Most engineering design projects essentially include a variety of realistic constraints, such as economic factors, safety, reliability, aesthetics, ethics, and social impacts. For instance, the height of the new system cannot exceed 1.4 m; or its mass may not exceed 3.6 kg; or it must operate year-round during cold and hot days.
The value-free descriptors associated with each objective are referred to as criteria. For instance, an objective for a design is that it must be “inexpensive.” The criterion associated with this objective is “cost.” The criteria are quantified using the same bases for measurement and the same unit as their corresponding objectives. In other words, the criteria are more compact ways of identifying objectives. Table 1.1 demonstrates a number of typical design objectives and related criteria to design a vehicle.
Table 1.1 Typical design objectives and related criteria for a vehicle design project
Fundamentally, design products are developed and created to satisfy needs and wants and provide utility to the customer. The customer's needs have to be translated into design requirements through goal and objectives. Design requirements mainly include customer requirements plus engineering requirements. The customer requirements refer to objectives as articulated by the customer or client. The engineering requirements refer to the design and performance parameters that can contribute to achieving the customer requirements.
Figure 1.7 illustrates conceptually the status of various design features during the design process. It indicates that there will be a large commitment in terms of configuration, manufacturing technology, and maintenance techniques at the early stages of a design program. In addition, it is at this point that major decisions are made and product-specific knowledge is limited. Moreover, it is estimated that about 70% of the projected lifecycle cost for a given product can be committed based on engineering design and management decisions during the early stages of design. As the design progresses, changes to the design get harder and harder. Therefore, the impact of a decision at the early stages of a design program is more profound than a decision at the later stages. Hence, it is crucial to be highly confident about any decision a designer makes at the conceptual design phase.
Figure 1.7 Status of various design features during the design process
The cost of aircraft design is about 1% of the total lifecycle cost; however, this 1% determines the other 99%. Furthermore, the design cost is about 20% of the production (acquisition) cost. Thus, any necessary investment in design team members is worth it. Most aircraft manufacturers do not make any profit in the first couple of years of production, in the hope that in the future, they will make money. The large aircraft manufacturers get back their money after about 10 years; after that, they will make a profit. In the past, there were a few examples where aircraft manufacturers were bankrupted and only resurrected by government through long-term loans.
Wind-tunnel testing costs from 200 US$/hour for GA (General Aviation) small aircraft to 5000 US$/hour for large transport aircraft. The design and fabrication of some aircraft—such as supersonic transport aircraft Aerospatiale-BAC Concorde (Figures 7.24 and 11.15)—was a great achievement, but when the international market does not purchase it, the production has to be stopped.
1.3 Design Project Planning
In order for a design project schedule to be effective, it is necessary to have some procedure for monitoring progress; and in a broader sense for encouraging personnel to progress. An effective general form of project management control device is the Gantt chart. It presents a project overview which is almost immediately understandable to non-systems personnel; hence it has great value as a means of informing management of project status. A Gantt chart has three main features:
Like many other planning/management tools, Gantt charts provide the manager/chief designer with an early warning if some jobs will not be completed on schedule and/or if others are ahead of schedule. Gantt charts are also helpful in that they present graphically immediate feedback regarding estimates of personnel skill and job complexity. Table 1.2 illustrates a typical Gantt chart for the design of a light single-seat aircraft in the form of a combined bar/milestone chart. Such a chart provides the chief designer with a scheduling method and enables him/her to rapidly track and assess the design activities on a weekly/monthly basis. An aircraft project such as Airbus A-380 (Figure 1.8) will not be successful without design project planning.
Table 1.2 A typical Gantt chart for the design of a light single-seat aircraft
Figure 1.8 Airbus A-380, the newest Airbus production. Reproduced from permission of Anne Deus
A preferred method of scheduling is through the use of program networks [2] such as the program evaluation and review technique (PERT) and the critical path method (CPM). The application of network scheduling is appropriate for both small- and large-scale design projects and is of particular value for a system development where there are several interdependencies. The definitions of new terms in Table 1.2, such as preliminary design and critical design review, and their associated techniques are addressed in Chapter 2.
1.4 Decision Making
First and foremost, it must be emphasized that any engineering selection must be supported by logical and scientific reasoning and analysis. The designer is not expected to select a configuration just because he/she likes it. There must be sufficient evidence and reasons which prove that the current selection is the best.
The main challenge in decision making is that there are usually multiple criteria along with a risk associated with each one. In this section, a few techniques and tools for aiding decision making under complex conditions are introduced. However, in most design projects there are stages where there are several acceptable design alternatives and the designer has to select only one of them. In such cases, there are no straightforward governing equations to be solved mathematically. Thus, the only way to reach the solution is to choose from a list of design options. There are frequently many circumstances in which there are multiple solutions for a design problem but one option does not clearly dominate the others in all areas of comparison.
A simple example is a transportation design problem where a designer is required to design a vehicle to transfer one person from one city to another. It is assumed that the two cities are both seaports and located at a distance of 300 km. The design solution alternatives are bicycle, motorbike, automobile, train, bus, ship, and aircraft. A traveler may select to travel using any of these vehicles. Three common criteria in most engineering design projects are: (i) cost, (ii) performance, and (iii) safety (and reliability). Table 1.3 shows a typical comparison of these design options and the ranking of each alternative. As the ranking illustrates, no one option clearly ranks first with respect to all three criteria to dominate the other six alternatives.
Table 1.3 A typical multi-criteria decision-making problem (1 is the most desirable)
If the designer cares only for the cost of operation and safety, he/she has to select the bicycle, but if the only criterion was travel speed, the aircraft would be chosen as the vehicle. The bicycle is often the slowest vehicle; however it is the cheapest way to travel. In contrast, the aircraft does the best job in terms of speed (fastest to travel), but it is usually the most expensive option. It is evident that, for a typical traveler and designer, all the criteria matter. Thus, the question is how to come up with the best decision and the optimum vehicle. This example (Table 1.3) represents a typical multi-criteria decision-making problem that a design engineer frequently faces in a typical engineering design project. After the type of vehicle is selected, the calculations begin to determine geometry and other engineering characteristics.
A designer must recognize the importance of making the best decision and the adverse consequences of making a poor decision. In the majority of design cases, the best decision is the right decision, and a poor decision is the wrong one. The right decision implies design success, while a wrong decision results in a failure of the design. As the level of design problem complexity and sophistication increases in a particular situation, a more sophisticated approach is needed.
The approach for making the best decision to select/determine the best alternative is to take five steps, as follows.
Step 1.
Specify all the alternatives to be included in the exercise. Try to generate as many design concepts as possible using the brainstorming technique. However, given the resources required to include and consider all alternatives, you need to give considerable thought to reducing the alternatives to a manageable number.
Step 2.
The second step in selecting the best design is to identify and establish the criteria (e.g.,
Table 1.1
). These criteria serve later as the guidelines for developing the options. Some design references employ the term “figures of merit” instead of criteria.
Step 3.
The next step is to define the metrics. The metrics are defined as a shorthand way of referring to the criteria performance measures and their units. Metrics are the tool to overcome a non-comparable complex situation (e.g., comparing apples and oranges) by establishing a common evaluation scale and mapping each criterion's metric onto this scale. A simple evaluation scale is to map each criterion as either excellent, adequate, or poor. So, each design option may be rated with respect to each criterion using this common scale. A better and more quantifiable scale is a numerical scale, as demonstrated in
Table 1.4
. Typical metrics for measuring performance of an aircraft are maximum speed, take-off run, rate-of-climb, range, endurance, turn radius, turn rate, and ceiling.
Table 1.4 Common scale and criteria metrics and three examples
Step 4.
The fourth step is to deal with criteria that have unequal significance. A designer should not frequently treat all criteria as being equally important. The designer must try to ascertain how important each requirement (i.e., criterion) is to the customer. The simplest approach is to assign numerical weights to each criterion (or even at a metrics level) to indicate its importance relative to other criteria. These weights ideally reflect the designer's judgment of relative importance. Judgment as to whether one design alternative is superior to another may be highly dependent on the values and preferences of the evaluator. In some cases, the designer has no way other than relying on personal “feelings” and “judgments” for the basis of the numerical weights. As a starting point, you may pair up each criterion with every other criterion one at a time and judge which of the items in each pair is more important than the other. The weights may later be normalized (i.e., mathematically convert each number to a fraction of 1) in order to make them easier to compare.
Step 5.
Select the alternative which gains the highest numerical value. It is expected that the output of the decision-making process will yield the most desirable result.
1.5 Feasibility Analysis
In the early stages of design and by employing brainstorming, a few promising concepts are suggested which seem consistent with the scheduling and available resources. Prior to committing resources and personnel to the detail design phase, an important design activity—feasibility analysis—must be performed. There are a number of phases through which the system design and development process must invariably pass. Foremost among them is the identification of the customer-related need and, from that, the determination of what the system is to do. This is followed by a feasibility study to discover potential technical solutions, and the determination of system requirements.
It is at this early stage in the lifecycle that major decisions are made relative to adapting a specific design approach and technology application, which have a great impact on the lifecycle cost of a product. At this phase, the designer addresses the fundamental question of whether to proceed with the selected concept. It is evident that there is no benefit or future in spending any more time and resources attempting to achieve an unrealistic objective. Some revolutionary concepts initially seem attractive but when it comes to the reality, they are found to be too imaginary. Feasibility study distinguishes between a creative design concept and an imaginary idea. Feasibility evaluation determines the degree to which each concept alternative satisfies the design criteria.
