Understanding Aircraft Structures

224 Pages·2005·3.3 MB·English

by  Cutler

Understanding
Aircraft Structures
John Cutler
Fourth Edition
revised by
Jeremy Liber
© John Cutler 1981,1992,1999
© John Cutler and Jeremy Liber 2005
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First published in Great Britain by Granada Publishing 1981
Second edition published by Blackwell Scientific Publications 1992
Third edition published by Blackwell Science Ltd 1999
Fourth edition published by Blackwell Publishing Ltd 2005
ISBN-10:1-4051-2032-0
ISBN-13:978-1-4051-2032-6
Library of Congress Cataloging-in-Publication Data
Cutler,John.
Understanding aircraft structures / John Cutler.–4th ed./rev.by Jeremy Liber.
p. cm.
Includes bibliographical references.
ISBN-13:978-1-4051-2032-6 (alk.paper)
ISBN-10:1-4051-2032-0 (alk.paper)
1. Airframes. I. Liber,Jeremy. II. Title.
TL671.6.C88 2005
629.134¢31–dc22
2005048077
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Preface
The fourth edition of Understanding Aircraft Structures builds,naturally
enough,on the firm foundations of the earlier editions in aiming to keep
up to date with an evolving industry.Whilst the fundamentals of how air-
craft are constructed change only relatively slowly, the aerospace world
as a whole tends to change at a more noticeable pace and many of these
changes impinge upon the realm of aircraft structures.The major change
since publication of the last edition has been in the regulatory environ-
ment:all of the national European aviation regulators have been subor-
dinated beneath a new European Union body, the European Aviation
Safety Agency (EASA).This change is reflected in the chapter describing
how to apply for approval of an aircraft modification and, to a lesser
extent,in the section on quality and airworthiness.
The broad range of subjects which support,and interact with,the work
of the aircraft structures designer is covered in greater detail than before
and the opportunity has been taken to provide a ‘round up’ of the key
points at the end of each chapter.In their own studies the authors have
found that this kind of review at the end of a chapter greatly helps the
learning process.
Finally,in recent years we have seen the European Airbus organisation
surpass the great Boeing Company in terms of numbers of aircraft sold.
However,the American aircraft industry remains the dominant force in
the world of aviation.Its influence is such that,probably,the majority of
the industry continues to work in imperial rather than SI units.For this
reason,and after much soul-searching,it has been decided to retain both
imperial and SI units in this book.
Contents
Preface ix
Chapter 1 Introduction 1
Chapter 2 History 3
2.1 Outline 3
2.2 Wire-braced structures 4
2.3 Semi-monocoque structures 9
2.4 Sandwich structures 14
2.5 Review of the key points 15
Chapter 3 Parts of the Aircraft 29
3.1 Terms connected with flight 29
3.2 Terms connected with control 31
3.3 Terms connected with high-lift devices 32
3.4 Terms associated with the shape and
dimensions of the aircraft 32
3.5 Review of the key points 37
Chapter 4 Loads on the Aircraft 38
4.1 General flight forces 38
4.2 Acceleration loads 44
4.3 Further aerodynamic loads 48
4.4 Other loads 50
4.5 Further load factors 51
4.6 Loads acting on the whole aircraft 53
4.7 Review of the key points 55
4.8 References 56
Chapter 5 The Form of Structures 57
5.1 Structure relative to aircraft design 57
5.2 Historic form of structure 57
5.3 General form of structure 59
5.4 The basic load systems in structures 59
5.5 The forms of stress in materials 63
5.6 Bending and torsion 72
5.7 Compression 85
vi Contents
5.8 The whole structure 87
5.9 Review of the key points 87
5.10 References 88
Chapter 6 Materials 89
6.1 Choice of materials 89
6.2 Material properties 92
6.3 Smart structures (and materials) 101
6.4 Cost as a property of a material 103
6.5 Heat treatment 105
6.6 Reference number for materials 106
6.7 Review of the key points 109
6.8 References 110
Chapter 7 Processes 112
7.1 Introduction 112
7.2 Manufacturing 112
7.3 Jointing 119
7.4 Review of the key points 123
7.5 References 123
Chapter 8 Corrosion and Protective Treatments 124
8.1 Nature of corrosion 124
8.2 Causes of corrosion 126
8.3 Protection against corrosion 128
8.4 Review of the key points 132
8.5 Reference 132
Chapter 9 Detail Design 133
9.1 Sheet-metal components 133
9.2 Machined components and large forgings 136
9.3 Notching and stress raisers 139
9.4 Rivets and bolts 144
9.5 Joggling 152
9.6 Clips or cleats 154
9.7 Stringer/frame intersections 155
9.8 Lugs 156
9.9 The ‘stiff path’ 157
9.10 Review of the key points 157
Chapter 10 Composite Materials in Aircraft Structures 159
10.1 What are composites? 159
10.2 The strength of composite materials 161
10.3 Types of structures 162
10.4 Joining composites 164
10.5 Fibres 167
10.6 Resins 169
10.7 Working safely with composites 171
10.8 Review of the key points 172
Contents vii
Chapter 11 Quality and Airworthiness 173
11.1 Quality assurance and quality control 173
11.2 Control 174
11.3 Procedures and systems 175
11.4 Further notes on quality control functions 177
11.5 Airworthiness engineering 179
11.6 Continued airworthiness 180
11.7 Review of the key points 181
11.8 References 181
Chapter 12 Stressing 182
12.1 Introduction 182
12.2 The stressman’s work 183
12.3 Stressing methods 187
12.4 Stress reports 190
12.5 Review of the key points 193
12.6 References 194
Chapter 13 Presentation of Modifications and Repairs 195
13.1 Definitions 195
13.2 The essential paperwork associated
with modifications 197
13.3 Review of the key points 201
13.4 Conclusion 201
13.5 References 202
Appendices 203
Index 209
Chapter 1
Introduction
The aim of this book is to present the principles of aircraft structures to
the interested reader in a manner that is both clear and thorough whilst
avoiding the necessity for complex mathematical formulae.No previous
knowledge of the field is assumed,only the desire to know more.
Like most industries, aviation has various specialist fields which are
often considered to be ‘black arts’ by the uninitiated;the work of the air-
craft structures analyst,or stressman,falls into this category.However,as
with most disciplines,given a little curiosity and some application the basic
principles may be easily understood,at which point much of the mystery
disappears.
There are many people involved in the engineering side of the aircraft
industry,such as draughtsmen,fitters or licensed engineers,who deal with
aircraft structures in their daily lives, yet may have only an incomplete
understanding of why aircraft are designed as they are.For those people,
and others with enquiring minds, reading this book will not make them
stressmen.It will,however,give them a qualitative understanding of the
general principles of aircraft structures, allowing them to ask informed
questions.
The book is written in such a way that individual chapters may be read
independently of each other if a topic is of particular interest to the reader.
However, taken together they provide a logical progression and offer a
thorough introduction to the subject.
The book begins by giving an historical perspective,presenting a brief
outline of the evolution of aircraft structures from the earliest flying
machines to the present day.The one major change in philosophy made
by aircraft structures designers is described.After this the basic shape and
main structural elements of the aeroplane are defined,whilst introducing
the fundamental reason why an aircraft requires a structure at all;that is
to support the various loads applied to it.Subsequent chapters explain the
form which that structure takes in order to efficiently carry those loads,
the materials that are employed to make it and the processes involved in
its construction.There follows an introduction to the principles of corro-
sion protection and there is a distillation of much ‘rule of thumb’ and
‘good engineering practice’ so essential to the sound detail design of air-
craft structures.The regulatory and quality environment within which the
2 Understanding Aircraft Structures
aviation industry functions is outlined.Later chapters allow an insight into
the work of the stressman and describe,in broad terms,the justification
and documentation necessary to gain approval for aircraft modifications.
For anybody intending to become an aircraft structures specialist
(stressman)Understanding Aircraft Structureswill provide a firm founda-
tion upon which to build and will provide explanations of phenomena
which may not be easily found in more formal textbooks.Those who are
simply looking for a broad appreciation of the subject will also find all
that they require in this book.
Chapter 2
History
2.1 Outline
In this chapter we will book at the general development of aircraft struc-
tures over the short period of their history.As with most subjects,knowl-
edge of the steps which led to the present position is a great help in
understanding current problems;later in the book there are more detailed
comments concerning structures as they are now.
Flying machines obviously changed enormously over the 70 years from
the Wright Brothers Flyer at Kittyhawk to Apollo on the Moon, and a
fighter ace of 1918 flew a very different aircraft from that flown by his suc-
cessor today,so a review of the whole development of flying would be a
large task. However, there are many different branches of science and
engineering which make up aeronautics as a whole and when these are
looked at separately, the problem of dealing with them becomes more
manageable.
The main divisions of aeronautical engineering are (a) the science,
which deals with the airflow round the aircraft;(b) the power-plant engi-
neering;(c) the avionics,that is the radios and navigation aids;(d) the air-
frame engineering, where the airframe includes hydraulic and electrical
systems,flying and engine controls,interior furnishings and cargo systems;
and (e) the section which concerns this book,which is the structure.All
these divisions and subdivisions have developed at different rates.Power
plants (engines) for instance have moved with two great strides,and many
years of continuing short but rapid steps. Before the Wright Brothers
could make their first successful aeroplane,the power plant engineers had
to make their first stride and invent an engine which was light and pow-
erful.The next stride was the invention of the jet engine but,in between,
the power of piston engines increased nearly 200 times in just 40 years,
from 12hp (horse power) to over 2000hp,with only a ten times increase
of weight.As we shall see,structures have made only one major funda-
mental jump forward,but that was sufficient to change the whole charac-
ter and appearance of aircraft.
4 Understanding Aircraft Structures
2.2 Wire-braced structures
If we look at the aircraft in Fig.2.1 we can have no doubt about the form
of its construction.The wings and the fore and aft structures carrying the
other covered surfaces were all made of rectangular frames which were
prevented from collapsing (or parallelogramming) by wires stretched
from corner to corner.Although the methods were not original,there were
two imaginative pieces of structural thinking here. Firstly, the idea that
two wings,one above the other,would make a lighter,stronger structure
than the type of wing arrangement suggested by bird flight and,secondly,
the idea that a rectangle could be held in shape with two light wires rather
than with one much heavier diagonal member,like the structure of a farm
gate.At this stage,and for the next 30 years,the major structural material
was wood;at first bamboo and later mainly spruce,a lightweight timber
with very straight grain and medium strength. Strangely enough, balsa
wood,which means so much to the model aircraft enthusiast,was not used
during this period but has been used sometimes since then as a filler,or
core material,in flooring panels for large aircraft.Wire bracing continued
to be used as a major feature of aircraft construction for many years.
Figures 2.2 and 2.3 illustrate its extensive use on early fighters and Fig.2.4
shows it still in evidence into the era of metal aircraft. (Note: these
particular aircraft are illustrated as they show a progression of designs for
the same manufacturer:Sopwith Aviation Co.,which became Hawker in
1920.)
2.2.1 Biplane structures
Biplane structures (Figs 2.1–2.3) dominated aircraft design for many years
and,for certain particular requirements,such as aerobatic aircraft or agri-
cultural crop sprayers,they still appear from time to time.The structural
advantage of the arrangement is that the combination of upper and lower
wing, the interplane vertical members (struts) and the wire bracing
forms a deep,light member which is very rigid and resistant to bending
and twisting.
2.2.2 The change to metal construction
The biplane era lasted until the middle of the 1930s,by which time wooden
construction was being replaced by metal.The similarity of construction
between the biplanes in Figs 2.2 and 2.3 is obvious but the main fuselage
members of the later aircraft are steel tubes, as are the wing spars.
Although wood was still being used very extensively at the time of the
Hawker Fury, by the time that the Hurricane was produced (see Fig.
2.4(a)),the change to metal was almost complete.In spite of this change,
the structures were still wire braced, and the principles of structural
thinking behind the Sopwith Camel of 1917 and the Hurricane of 1935
showed some distinct similarities.Although the structures designer had