Understanding Aircraft Structures

8/13/2019 Understanding Aircraft Structures

1/224


2/224

John Cutler 1981, 1992, 1999 John Cutler and Jeremy Liber 2005

Editorial offices:Blackwell Publishing Ltd, 9600 Garsington Road, Oxford OX4 2DQ, UK

Tel:+44 (0)1865 776868

Blackwell Publishing Inc., 350 Main Street, Malden, MA 02148-5020, USATel:+1 781 388 8250

Blackwell Publishing Asia Pty Ltd, 550 Swanston Street, Carlton, Victoria 3053, AustraliaTel:+61 (0)3 8359 1011

The right of the Author to be identified as the Author of this Work has been asserted in accordancewith the Copyright, Designs and Patents Act 1988.

All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, ortransmitted, in any form or by any means, electronic, mechanical, photocopying, recording orotherwise, except as permitted by the UK Copyright, Designs and Patents Act 1988, without the priorpermission of the publisher.

First published in Great Britain by Granada Publishing 1981Second edition published by Blackwell Scientific Publications 1992Third edition published by Blackwell Science Ltd 1999Fourth edition published by Blackwell Publishing Ltd 2005

ISBN-10: 1-4051-2032-0ISBN-13: 978-1-4051-2032-6

Library of Congress Cataloging-in-Publication DataCutler, 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 2005629.13431dc222005048077

A catalogue record for this title is available from the British Library

Set in 10 on 12 pt Timesby SNP Best-set Typesetter Ltd., Hong KongPrinted and bound in India

by Replika Press Pvt Ltd, Kundli

The publishers policy is to use permanent paper from mills that operate a sustainable forestry policy,and which has been manufactured from pulp processed using acid-free and elementary chlorine-freepractices. Furthermore, the publisher ensures that the text paper and cover board used have metacceptable environmental accreditation standards.

For further information on Blackwell Publishing, visit our website:www.blackwellpublishing.com


3/224

Preface

The fourth edition of Understanding Aircraft Structures builds, naturallyenough, 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 worldas a whole tends to change at a more noticeable pace and many of thesechanges impinge upon the realm of aircraft structures. The major changesince 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 AviationSafety Agency (EASA). This change is reflected in the chapter describinghow to apply for approval of an aircraft modification and, to a lesserextent, in the section on quality and airworthiness.

The broad range of subjects which support, and interact with, the workof the aircraft structures designer is covered in greater detail than beforeand the opportunity has been taken to provide a round up of the keypoints at the end of each chapter. In their own studies the authors havefound that this kind of review at the end of a chapter greatly helps thelearning process.

Finally, in recent years we have seen the European Airbus organisationsurpass the great Boeing Company in terms of numbers of aircraft sold.However, the American aircraft industry remains the dominant force inthe world of aviation. Its influence is such that, probably, the majority ofthe industry continues to work in imperial rather than SI units. For thisreason, and after much soul-searching, it has been decided to retain bothimperial and SI units in this book.


4/224

Contents

Preface ix

Chapter 1 Introduction 1Chapter 2 History 3

2.1 Outline 32.2 Wire-braced structures 42.3 Semi-monocoque structures 92.4 Sandwich structures 142.5 Review of the key points 15

Chapter 3 Parts of the Aircraft 293.1 Terms connected with flight 29

3.2 Terms connected with control 313.3 Terms connected with high-lift devices 323.4 Terms associated with the shape and

dimensions of the aircraft 323.5 Review of the key points 37

Chapter 4 Loads on the Aircraft 384.1 General flight forces 384.2 Acceleration loads 444.3 Further aerodynamic loads 48

4.4 Other loads 504.5 Further load factors 514.6 Loads acting on the whole aircraft 534.7 Review of the key points 554.8 References 56

Chapter 5 The Form of Structures 575.1 Structure relative to aircraft design 575.2 Historic form of structure 575.3 General form of structure 59

5.4 The basic load systems in structures 595.5 The forms of stress in materials 635.6 Bending and torsion 725.7 Compression 85
http://20326_pref.pdf/http://20326_01.pdf/http://20326_02.pdf/http://20326_02.pdf/http://20326_02.pdf/http://20326_02.pdf/http://20326_02.pdf/http://20326_02.pdf/http://20326_03.pdf/http://20326_03.pdf/http://20326_03.pdf/http://20326_03.pdf/http://20326_03.pdf/http://20326_03.pdf/http://20326_04.pdf/http://20326_04.pdf/http://20326_04.pdf/http://20326_04.pdf/http://20326_04.pdf/http://20326_04.pdf/http://20326_04.pdf/http://20326_04.pdf/http://20326_04.pdf/http://20326_05.pdf/http://20326_05.pdf/http://20326_05.pdf/http://20326_05.pdf/http://20326_05.pdf/http://20326_05.pdf/http://20326_05.pdf/http://20326_05.pdf/http://20326_05.pdf/http://20326_05.pdf/http://20326_05.pdf/http://20326_05.pdf/http://20326_05.pdf/http://20326_05.pdf/http://20326_05.pdf/http://20326_05.pdf/http://20326_04.pdf/http://20326_04.pdf/http://20326_04.pdf/http://20326_04.pdf/http://20326_04.pdf/http://20326_04.pdf/http://20326_04.pdf/http://20326_04.pdf/http://20326_04.pdf/http://20326_03.pdf/http://20326_03.pdf/http://20326_03.pdf/http://20326_03.pdf/http://20326_03.pdf/http://20326_03.pdf/http://20326_02.pdf/http://20326_02.pdf/http://20326_02.pdf/http://20326_02.pdf/http://20326_02.pdf/http://20326_02.pdf/http://20326_01.pdf/http://20326_pref.pdf/


5/224

5.8 The whole structure 875.9 Review of the key points 875.10 References 88

Chapter 6 Materials 89

6.1 Choice of materials 896.2 Material properties 926.3 Smart structures (and materials) 1016.4 Cost as a property of a material 1036.5 Heat treatment 1056.6 Reference number for materials 1066.7 Review of the key points 1096.8 References 110

Chapter 7 Processes 112

7.1 Introduction 1127.2 Manufacturing 1127.3 Jointing 1197.4 Review of the key points 1237.5 References 123

Chapter 8 Corrosion and Protective Treatments 1248.1 Nature of corrosion 1248.2 Causes of corrosion 1268.3 Protection against corrosion 128

8.4 Review of the key points 1328.5 Reference 132

Chapter 9 Detail Design 1339.1 Sheet-metal components 1339.2 Machined components and large forgings 1369.3 Notching and stress raisers 1399.4 Rivets and bolts 1449.5 Joggling 1529.6 Clips or cleats 154

9.7 Stringer/frame intersections 1559.8 Lugs 1569.9 The stiff path 1579.10 Review of the key points 157

Chapter 10 Composite Materials in Aircraft Structures 15910.1 What are composites? 15910.2 The strength of composite materials 16110.3 Types of structures 16210.4 Joining composites 164

10.5 Fibres 16710.6 Resins 16910.7 Working safely with composites 17110.8 Review of the key points 172

vi Contents
http://20326_05.pdf/http://20326_05.pdf/http://20326_05.pdf/http://20326_06.pdf/http://20326_06.pdf/http://20326_06.pdf/http://20326_06.pdf/http://20326_06.pdf/http://20326_06.pdf/http://20326_06.pdf/http://20326_06.pdf/http://20326_06.pdf/http://20326_07.pdf/http://20326_07.pdf/http://20326_07.pdf/http://20326_07.pdf/http://20326_07.pdf/http://20326_07.pdf/http://20326_08.pdf/http://20326_08.pdf/http://20326_08.pdf/http://20326_08.pdf/http://20326_08.pdf/http://20326_08.pdf/http://20326_09.pdf/http://20326_09.pdf/http://20326_09.pdf/http://20326_09.pdf/http://20326_09.pdf/http://20326_09.pdf/http://20326_09.pdf/http://20326_09.pdf/http://20326_09.pdf/http://20326_09.pdf/http://20326_09.pdf/http://20326_10.pdf/http://20326_10.pdf/http://20326_10.pdf/http://20326_10.pdf/http://20326_10.pdf/http://20326_10.pdf/http://20326_10.pdf/http://20326_10.pdf/http://20326_10.pdf/http://20326_10.pdf/http://20326_10.pdf/http://20326_10.pdf/http://20326_10.pdf/http://20326_10.pdf/http://20326_10.pdf/http://20326_10.pdf/http://20326_10.pdf/http://20326_10.pdf/http://20326_09.pdf/http://20326_09.pdf/http://20326_09.pdf/http://20326_09.pdf/http://20326_09.pdf/http://20326_09.pdf/http://20326_09.pdf/http://20326_09.pdf/http://20326_09.pdf/http://20326_09.pdf/http://20326_09.pdf/http://20326_08.pdf/http://20326_08.pdf/http://20326_08.pdf/http://20326_08.pdf/http://20326_08.pdf/http://20326_08.pdf/http://20326_07.pdf/http://20326_07.pdf/http://20326_07.pdf/http://20326_07.pdf/http://20326_07.pdf/http://20326_07.pdf/http://20326_06.pdf/http://20326_06.pdf/http://20326_06.pdf/http://20326_06.pdf/http://20326_06.pdf/http://20326_06.pdf/http://20326_06.pdf/http://20326_06.pdf/http://20326_06.pdf/http://20326_05.pdf/http://20326_05.pdf/http://20326_05.pdf/


6/224

Chapter 11 Quality and Airworthiness 17311.1 Quality assurance and quality control 17311.2 Control 17411.3 Procedures and systems 175

11.4 Further notes on quality control functions 17711.5 Airworthiness engineering 17911.6 Continued airworthiness 18011.7 Review of the key points 18111.8 References 181

Chapter 12 Stressing 18212.1 Introduction 18212.2 The stressmans work 18312.3 Stressing methods 18712.4 Stress reports 19012.5 Review of the key points 19312.6 References 194

Chapter 13 Presentation of Modifications and Repairs 19513.1 Definitions 19513.2 The essential paperwork associated

with modifications 19713.3 Review of the key points 20113.4 Conclusion 20113.5 References 202

Appendices 203Index 209

Contents vii
http://20326_11.pdf/http://20326_11.pdf/http://20326_11.pdf/http://20326_11.pdf/http://20326_11.pdf/http://20326_11.pdf/http://20326_11.pdf/http://20326_11.pdf/http://20326_11.pdf/http://20326_12.pdf/http://20326_12.pdf/http://20326_12.pdf/http://20326_12.pdf/http://20326_12.pdf/http://20326_12.pdf/http://20326_12.pdf/http://20326_13.pdf/http://20326_13.pdf/http://20326_13.pdf/http://20326_13.pdf/http://20326_13.pdf/http://20326_13.pdf/http://20326_apdx01.pdf/http://20326_indx.pdf/http://20326_indx.pdf/http://20326_apdx01.pdf/http://20326_13.pdf/http://20326_13.pdf/http://20326_13.pdf/http://20326_13.pdf/http://20326_13.pdf/http://20326_13.pdf/http://20326_12.pdf/http://20326_12.pdf/http://20326_12.pdf/http://20326_12.pdf/http://20326_12.pdf/http://20326_12.pdf/http://20326_12.pdf/http://20326_11.pdf/http://20326_11.pdf/http://20326_11.pdf/http://20326_11.pdf/http://20326_11.pdf/http://20326_11.pdf/http://20326_11.pdf/http://20326_11.pdf/http://20326_11.pdf/


7/224

Chapter 1

Introduction

The aim of this book is to present the principles of aircraft structures tothe interested reader in a manner that is both clear and thorough whilst

avoiding the necessity for complex mathematical formulae. No previousknowledge of the field is assumed, only the desire to know more.

Like most industries, aviation has various specialist fields which areoften considered to be black arts by the uninitiated; the work of the air-craft structures analyst, or stressman, falls into this category. However, aswith most disciplines, given a little curiosity and some application the basicprinciples may be easily understood, at which point much of the mysterydisappears.

There are many people involved in the engineering side of the aircraftindustry, such as draughtsmen, fitters or licensed engineers, who deal with

aircraft structures in their daily lives, yet may have only an incompleteunderstanding of why aircraft are designed as they are. For those people,and others with enquiring minds, reading this book will not make themstressmen. It will, however, give them a qualitative understanding of thegeneral principles of aircraft structures, allowing them to ask informedquestions.

The book is written in such a way that individual chapters may be readindependently of each other if a topic is of particular interest to the reader.However, taken together they provide a logical progression and offer athorough introduction to the subject.

The book begins by giving an historical perspective, presenting a briefoutline of the evolution of aircraft structures from the earliest flyingmachines to the present day. The one major change in philosophy madeby aircraft structures designers is described.After this the basic shape andmain structural elements of the aeroplane are defined, whilst introducingthe fundamental reason why an aircraft requires a structure at all; that isto support the various loads applied to it. Subsequent chapters explain theform 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 andgood engineering practice so essential to the sound detail design of air-craft structures. The regulatory and quality environment within which the


8/224

aviation industry functions is outlined. Later chapters allow an insight intothe work of the stressman and describe, in broad terms, the justificationand documentation necessary to gain approval for aircraft modifications.

For anybody intending to become an aircraft structures specialist

(stressman) Understanding Aircraft Structures will provide a firm founda-tion upon which to build and will provide explanations of phenomenawhich may not be easily found in more formal textbooks. Those who aresimply looking for a broad appreciation of the subject will also find allthat they require in this book.

2 Understanding Aircraft Structures


9/224

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 inunderstanding current problems; later in the book there are more detailedcomments concerning structures as they are now.

Flying machines obviously changed enormously over the 70 years fromthe 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 alarge task. However, there are many different branches of science andengineering which make up aeronautics as a whole and when these arelooked at separately, the problem of dealing with them becomes moremanageable.

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 electricalsystems, flying and engine controls, interior furnishings and cargo systems;and (e) the section which concerns this book, which is the structure. Allthese divisions and subdivisions have developed at different rates. Powerplants (engines) for instance have moved with two great strides, and manyyears of continuing short but rapid steps. Before the Wright Brotherscould make their first successful aeroplane, the power plant engineers hadto 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 increaseof 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.


10/224

2.2 Wire-braced structures

If we look at the aircraft in Fig. 2.1 we can have no doubt about the formof its construction. The wings and the fore and aft structures carrying the

other covered surfaces were all made of rectangular frames which wereprevented from collapsing (or parallelogramming) by wires stretchedfrom corner to corner.Although the methods were not original, there weretwo imaginative pieces of structural thinking here. Firstly, the idea thattwo wings, one above the other, would make a lighter, stronger structurethan 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 ratherthan with one much heavier diagonal member, like the structure of a farmgate. At this stage, and for the next 30 years, the major structural materialwas wood; at first bamboo and later mainly spruce, a lightweight timber

with very straight grain and medium strength. Strangely enough, balsawood, which means so much to the model aircraft enthusiast,was not usedduring this period but has been used sometimes since then as a filler, orcore material, in flooring panels for large aircraft.Wire bracing continuedto 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. 2.4shows it still in evidence into the era of metal aircraft. (Note: theseparticular aircraft are illustrated as they show a progression of designs forthe same manufacturer: Sopwith Aviation Co., which became Hawker in1920.)

2.2.1 Biplane structures

Biplane structures (Figs 2.12.3) dominated aircraft design for many yearsand, for certain particular requirements, such as aerobatic aircraft or agri-cultural crop sprayers, they still appear from time to time. The structuraladvantage of the arrangement is that the combination of upper and lowerwing, the interplane vertical members (struts) and the wire bracingforms a deep, light member which is very rigid and resistant to bendingand twisting.

2.2.2 The change to metal construction

The biplane era lasted until the middle of the 1930s, by which time woodenconstruction was being replaced by metal. The similarity of constructionbetween the biplanes in Figs 2.2 and 2.3 is obvious but the main fuselagemembers of the later aircraft are steel tubes, as are the wing spars.Although wood was still being used very extensively at the time of theHawker 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 structuralthinking behind the Sopwith Camel of 1917 and the Hurricane of 1935showed some distinct similarities. Although the structures designer had



11/224

Fig. 2.1 Wright Flyer (1903).


12/224

been pushed into accepting the problems of designing monoplane wingsas part of the progress towards better flying performance, the main fuse-lage structure of both aircraft was four longitudinal members calledlongerons and wire bracing was still being used extensively in the Hurri-cane. The side panels of the later fuselage were braced on the Warrengirder principle, but this could not be called a great leap forward. Thisparticular aircraft was designed at a period when some very rapidadvances were being made in aircraft performance, and the airforces ofthe world were expanding in clear preparation for the coming SecondWorld War, so some safe conservative thinking on the design of the struc-ture was understandable. However, the difficulty of combining metal con-

struction with traditional fabric covering in a monoplane wing producedthe strange design shown in Fig. 2.4(b). This was very short lived and allbut the first few Hurricanes had metal-covered wings, but it is worth seeinghow the unfamiliar problems of preventing bending and twisting in a


Fig. 2.2 Sopwith Camel (1917).


13/224

Fig. 2.3 Hawker Fury (1931).


14/224

Fig. 2.4(a) Hawker Hurricane (1935).


15/224

monoplane wing were tackled by one designer in the interim periodbetween biplane construction and the type of structure we use today.

2.3 Semi monocoque structures

An essential feature of the fuselages discussed so far was the internalcross-bracing which, although acceptable for some types of aircraft, wasdefinitely in the way when fuselages became large enough to carry pas-sengers seated internally. A much more suitable type of construction hadin fact been flying for some while.When we look back into aviation history,we refer to the flying boat without perhaps recognising just how well itwas named. The fuselages of the early wooden flying boats were made tothe highest standards of yacht construction with bent wood frames anddouble or triple ply skins, carvel built (smooth exterior) with a clear var-nished finish. This method of construction presented a much more openand usable fuselage interior. The wooden frames and plywood skinspassed from flying boats to land planes and the particular building tech-nique produced some quite shapely aeroplanes compared with the very

square slab-sided shapes which were the hallmark of the wire-bracedfuselage. The main advantage of the boat type of construction from theengineering point of view was, and is, that the skin forms an integralworking part of the structure, unlike the covering of the braced fuselage,

History 9

Fig. 2.4(b) Hawker Hurricane wing construction. (From Aircraft Production courtesy ofIPC Transport Press.)


16/224

which is a complete framework and which would still be just as strongwithout the fabric. Because the skin is working, the boat constructioncame to be called stressed skin; it is also sometimes called semi mono-coque. (Coque is the French word for shell and also the word meaning

hull (of boats). Mono in French implies all in one piece or integral. Semiis an English addition because the construction uses an internal frame-work which is not a feature of pure monocoque.)

The adoption of this type of construction, with its material changedfrom wood to metal, constitutes the one major fundamental stride forwardthat aircraft structures have taken since the earliest days of flying.The sim-plicity and elegance of this method of design and the influence it broughtto bear on the shape and appearance of aircraft is shown in Fig. 2.5. TheDouglas DC.3 was not the first all-metal stressed-skin aircraft, but it wasa brilliant example of a new technology converted into successful engi-

neering; it set standards and methods of structural design which havealready lasted for decades and look as if they will go on for generations.

2.3.1 Stressed-skin wings

After the fuselage shells came the stressed-skin wings. The Americanaviation industry, by 1930, was already showing signs of dominance in thebuilding of civil aircraft and was gaining stressed skin know how withlarge-capacity fuselages. European industry was more concerned withmilitary flying and secrecy, and continued to design braced structures for

perhaps two generations of aircraft (about five to seven years at that time)longer than they should have done.To be fair, there was a problem of size.If a DC.3 wing was scaled down to a size suitable for a fighter aircraft,the sheet aluminium used for the construction tended to become un-manageably thin and flimsy. However, some European design offices,notably Messerschmitt and Supermarine, faced up to the problemand were making efficient stressed-skin wings for small aircraft by themid-1930s.

2.3.2 Later developmentsThe later illustrations in this chapter, Figs 2.6 onwards, show the main-stream of structural development since the DC.3. There has been anumber of different structural methods invented over the years but thearrangement of vertical members (called ribs in the wings and frames orrings in the fuselage) and longitudinal members (called stringers) has beenthe established convention for some years.

A glance at these post-1955 designs shows very clearly that the struc-tures of big aircraft are not just little aircraft structures scaled up (or the

opposite way round). In fact, whatever the size of the aircraft, the fuse-lage frames are always about 500mm (20in.) apart and have between75mm (3in.) and 150mm (6in.) deep cross-sections. This means that bigaircraft have many structural members and small aircraft have few. This



17/224

Fig. 2.5 Douglas DC.2/3 (1933).


18/224

situation is not altogether what one would expect, and although scalingup a small structure to make a large one would give a strange result,scaling down from large to small may be seen to have more possibilitieswhen the resultant small spaces between frames and stringers are com-pared with the centre cores of the composite materials mentioned inSection 2.4.

Some other differences between the aircraft illustrated are discussed inChapter 9, and we will restrict this chapter to saying that the develop-

ments which have taken place in recent years have been mainly towards(a) a reduction of the number of rivets in the aircraft, either by machin-ing large pieces of structure from solid (see Fig. 9.12), or by the adhesivebonding assembly of components and (b) reducing the effects of minor


Fig. 2.6(a) Starship fuselage structure. (Courtesy of Beech AircraftCorporation.)

Fig. 2.6(b) Beech Starship. (Courtesy of Beech Aircraft Corporation.)


19/224

Fig. 2.7 British Aerospace HS 7


20/224

structural damage, either by providing sufficient members so that thefailure of one is not disastrous (fail-safe structure), or by improving accessfor easy inspection of structures in service.

The methods of design and manufacture for this stressed-skin type of

construction are now so well understood by the manufacturers that thedesign of the structure for a new medium-size, medium-performanceairliner could be regarded as a routine exercise, but the constantcommercial pressure to reduce weight provides a strong incentive forimprovement.Another area of major interest and innovation for the struc-tures engineer is the search for methods of improving the reliability ofstructures; that is, reducing the time spent on the ground when minorstructural defects are looked for and repaired. The British Aerospace146/Avro RJ (shown in Fig. 2.10) is very interesting in this respect and thedesigners took a great deal of trouble to reduce the number of members

and the number of places where structural deterioration, such as fatiguecracking or corrosion, may start and not be seen at inspections.

2.4 Sandwich structures

One of the main problems and limitations of the skin material in stressed-skin construction is its lack of rigidity. As we shall see later in this book,skins often have to be made thicker than they might otherwise need to bebecause of a tendency to crumple under some types of load. A strip of

paper illustrates this problem very well, one can pull it but not push it.A way of providing thin sheets with rigidity is to make a sandwich with

one very thin sheet, a layer of very light but fairly rigid core material,and another very thin sheet, all bonded together with an appropriateadhesive. As with conventional semi monocoque structures, wooden con-struction led the way with sandwich structures. The famous and elegantde Havilland Mosquito of 1940 was built with plywood skins either sideof a balsa-wood core. In todays major structures, a metal core ofhoneycomb-like cells is recognised as the most suitable core for metal-faced sandwich (see Figs 2.13 and 2.14 and Appendix 3).

2.4.1 Advances in sandwich construction

Although the advantages of honeycomb sandwich have been recognisedfor many years and flat boards in metal and plastics are a standardproduct, shaped products such as those in Fig. 2.14(b) are difficult to makeand therefore expensive. An appreciable percentage of Boeing 747 and777 structure is sandwich construction (Fig 2.14(c)) but increasing use ofcomposite material in other forms has mitigated against sandwich in the

sense that glass- or carbon-reinforced plastic skin is thicker than metalskin and less prone to buckling (see Chapter 11). The Airbus A340 shownin Fig. 10.3 (page 168) has a high percentage of composite, including sand-wich structure, but the rate of adoption of honeycomb sandwich is slower



21/224

than many structures designers would wish.The concept of shell structuresthat are as smooth inside as they are outside, and small fuselages with aratio of structure thickness to outside diameter which is as good as thatof large fuselages, was a dream rather than a reality when this book was

first published in 1981. Since then the Beech Starship (Fig. 2.6, page 12),which is clearly highly innovative in many ways, has been designed witha fuselage which exactly displays the advantages of a honeycomb shell,yet even this advanced aircraft has now ceased production and has beenwithdrawn from service.

Compare Fig. 2.6 with Fig. 5.2 and the extent of the advance which hasbeen made becomes apparent. Innovation on this scale clearly involvesmassive investment in research, design, tooling and courage and althoughthe existing structural conventions will remain for some time, the Starshipfuselage provides an elegant example of the benefits of sandwich

construction.A British innovation some years ago avoided one of the problems

of straightforward honeycomb core material. By a modification of thehexagon pattern a core was made which could be formed around panelswhich were double curved, i.e. dome shaped. This facility is not possiblewith a conventional core, which has to be carved to shape from a largeblock.

2.5 Review of the key points

The structures of early aircraft were comprised of open wooden (or latermetal) frames with diagonal wire bracing. The covering, if there was any,served no real structural purpose (its role being mainly aerodynamic).Theone fundamental step forward made in the field of aircraft structures wasthe replacement of this open form of structure by the stressed skin (orsemi monocoque) form, the key difference being that in the latter the cov-ering (or skin) is now a key structural member. Since the 1930s this typeof construction has predominated and continues to do so to this day what-ever the material used.

History 15


22/224


Fig. 2.8(a) De Havilland Canada Twin Otter. (Courtesy of The de Havilland Aircraft of Canada Ltd.)


23/224

History 17


24/224


Fig. 2.8(b) De Havilland Canada Dash 7. (Courtesy of The de Havilland Aircraft of Canada Ltd.)


25/224

History 19


26/224


Fig. 2.9 Structural detail. Airbus A300B. (Courtesy of Airbus Industry S.A.)


27/224

History 21


28/224


No stringersno cleats

Fig. 2.10 The British Aerospace 146. (Courtesy of British Aerospace.)


29/224

History 23

3 ribs in C/S

2 spars

146

2 panels top

4 panels bottom

146

CL

CL


30/224

Fig. 2.11 Boeing 747. (Courtesy of The Boeing Commercial Airplane Co


31/224

Fig. 2.12 HS 125. (Courtesy of British Aerospace.)


32/224


Fig. 2.13 Typical flat panel methods. (Courtesy of Ciba-Geigy, Bonded Structures Division,

Cambridge.)


33/224

History 27

Fig. 2.14(a) Laying up a sandwich panel. (Courtesy of Ciba-Geigy, Bonded Structures

Division, Cambridge.)

Fig. 2.14(b) Typical sandwich construction. (Courtesy of Ciba-Geigy, Bonded StructuresDivision, Cambridge.)


34/224

Fig. 2.14(c) Typical use of sandwich structure. (Courtesy of the Boeing Commercial Airplane Com


35/224

Chapter 3

Parts of the Aircraft

3.1 Terms connected with flight

There are many words used to describe parts of the aircraft and its oper-ation which, although not directly the concern of the structures engineer,will be used in the text and therefore must be clearly in mind before westart. Later chapters will examine and define terms and expressions whichare particularly relevant to aircraft structures.

The study of how an aircraft flies is called the science of aerodynamicsand the engineers who practise it are aerodynamicists. Immediately wehave used a word which requires some explanation; the word aircraft.Specifically aircraft covers all heavier-than-air flying machines, i.e. otherthan balloons or airships. Thus aircraft covers aeroplanes (or airplanes

in America), be they powered or un-powered, which have wings that arefixed relative to the rest of the structure, helicopters with wings that rotateabove the main structure in order to produce lift and thrust and auto-gyros with wings that rotate above the main structure in order to producelift but not thrust. Structurally they all follow the same principles andso, throughout this book, we generally use the term aircraft for the sakeof simplicity.

Aircraft engines or power plants are internal combustion engines, thatis the basic fuel is burnt inside the engine and not externally as it wouldbe in the case of a steam engine with an external boiler. The fuel is eithergasolene (Avgas) or kerosene (Avtur). Power plants are either pistonengines which drive propellers, or turbines which operate in one of thethree following ways.A pure jet turbine is a producer of high-pressure gaswhich is expelled backwards to thrust the aircraft forwards. Prop-jets areturbine engines driving propellers of the same pattern as those driven bypiston engines, and by-pass engines are turbines which combine a pure jetfunction with driving a multibladed propeller enclosed in a large circularduct.

Aircraft are kept up in the air and can fly because of lift produced by

the wing or mainplane. We deal with lift in more detail in Chapter 4 butfor the moment we will accept that a flat strip of rigid material pushededgeways through the air will generate lift if its forward or leading edgeis slightly higher than its rearwards or trailing edge. Provided that the


36/224

angle of the plate to the airflow is kept below about 1015 the greaterthe angle becomes, the higher the lift becomes. Also, if the angle is keptconstant but the speed through the air is increased, then more lift is gen-erated. (Some simple but quite informative experiments on lift and other

properties of flat plates or model wings can be made by a passenger in amotor vehicle holding the test piece out of a window, but the time andplace for such activities should be selected with some care.)

If we imagine an aircraft flying straight and level at a constant speedthe lift produced by (or generated by) the wing must equal the weight ofthe aircraft. If the lift exceeds the weight, the aircraft will be pushed higherand if the weight exceeds the lift the aircraft will sink.As the speed is con-stant the lift is maintained at the correct amount by keeping the angle ofthe wing to the airflow at a constant correct amount. In conventional air-craft this is achieved by the action of the tailplane. (Note here that main-

planes can be called wings but the horizontal tail surface, although verysimilar in shape and construction to a wing, is never referred to as a tailwing.) Other names for the tailplane are stabiliser, horizontal stabiliser orhorizontal tail and it works in the following way. If the mainplane angleincreases slightly the body of the aircraft is rotated in a tail downnoseup direction and the tailplane is also given an increased angle to theairflow. This increased angle produces a lift on the tail which levers thebody and with it the mainplane back to their original angles. A nose downattitude is similarly corrected by the tail working the opposite way andpushing the rear of the aircraft down.

This tailplane action is continuous and produces the quality calledsta-bility.The degree (or quantity) of stability is decided by the designer whenhe considers the specification of the whole aircraft. Clearly a single-seataircraft intended for competition aerobatics and quick manoeuvres doesnot want to be too stable, and equally clearly a private aircraft intendedfor social air touring without the aid of expensive automatic pilot equip-ment needs to be stable so that the human pilot is not too busy just main-taining level flight. Achieving the correct amount of stability is a difficultproblem for the aerodynamicist.

We have already seen that some parts of the aircraft can have morethan one acceptable name and Fig. 3.1 summarises the situation so far.This figure points out an interesting piece of aeronautical use of language.A bird has two wings but a conventional aeroplane is a monoplane, i.e.it has one wing. On the other hand, in the assembly shop of an aircraftmanufacturing plant, when one side of a wing is being attached to a fuse-lage the action will be called bolting on a wing or something similar, itwill not be bolting on half a wing. Some English names of aircraft partshave a French origin, reflecting the heavy French influence on early aero-nautical research. In some books, although the use of the name has

declined, the whole tail unit, that is the extreme rear fuselage, the verti-cal stabiliser and the horizontal stabiliser together with the control sur-faces (see below), are referred to as the empennage. We shall notice otherwords with a French influence later.



37/224

3.2 Terms connected with control

When the aircraft is flying it needs to be controlled by the pilot and wethink of it being controlled about three axes of movement (Fig. 3.2).

One axis is an imaginary pivot line stretching from wing tip to wing tipand rotation about this axis which causes nose up or nose down move-ment is calledpitching. In engineering terms the pitching axis is also calledthe YY axis.

The imaginary horizontal line extending from the extreme nose of theaircraft to the extreme tail is called the rolling, or XX, axis and movementabout this axis is called rolling.

The third axis is vertical and passes through the intersection of the othertwo. Rotation about this ZZ axis is calledyaw and hence ZZ is the yawaxis.

Figure 3.3 shows the controls which effect the movements about thethree axes. Pitching is controlled by the elevators. Roll is controlled by the

ailerons. Yaw is controlled by the rudder. These three are called controlsurfaces and are usually balanced as shown in the figure. Aerodynamicbalance reduces the effort required to move the surface and the mass (orstatic) balance avoidsflutter, which is discussed in Chapter 4.

Parts of the Aircraft 31

Fig. 3.1 Parts of the monoplane.


38/224

A further small control surface is attached to the trailing edge of the

main control surface. These small surfaces are called tabs. The operationand purpose of trim tabs and servotabs can be investigated in any bookdealing specifically with aerodynamics.

3.3 Terms connected with high-lift devices

The majority of aircraft are equipped with additional surfaces on theirwings to improve lift at slow speed. Figure 3.3 shows the names of sometypical high-lift devices which go under the general group name of flaps.Again, it would be better for the reader who is interested in aerodynam-ics to investigate the function and working of flaps for himself. So far asthis book is concerned, it is sufficient to say thatslots, which are the gapsbetween the flaps and the main wing, smooth out airflow and delay thestall (see also Section 4.6) and flaps either increase camber (see Fig. 3.4),or increase wing area, or both.

3.4 Terms associated with the shape and dimensions of the aircraft

The names of the leading dimensions are shown in Fig. 3.4. The usualAmerican practice (and as shown in the figures in this section) is to quoteleading dimensions in feet and inches but other dimensions, even large


Fig. 3.2 Aeroplane axes.


39/224

distances such as door openings, in inches. Europe and other parts of theworld tend to quote dimensions in metres and/or millimetres.

Figure 3.5 shows the method of describing by drawing the complex

shapes of an aircraft. The technique was inherited from the boat-buildingindustry, which accounts for the name waterline for any horizontal sectionor cut. In the diagram the three sectional cuts are each shown as a singledivision, but in fact a large number of cuts would be required to describe


Fig. 3.3 Controls and flaps.


40/224

a shape accurately. Any point on the surface of a solid shape will have awaterline, a section line and a buttock line passing through it. Point P onthe diagram is an example. The problem for the engineer specifying theshape is that the points either side of P, that is points WW, BB and SS,must all make a smooth or fair curve with P and at the same time agreewith another set of waterlines, butts and sections of their own.The processof achieving this agreement is usually computerised but in the past somevery elegant aircraft have been shaped by having their lines drawn outfull size by the mould loft department. Each of the cuts illustrated iscrossed by the other two cuts which show as lines, and if the various divi-sions are made at regular intervals the shapes will each be covered by agrid pattern. In Fig. 3.5 the section illustrated is crossed by water andbuttock lines which can then be dimensioned to define the section outline.

Sections are taken on station lines which are labelled as to their dis-tance from the aircraft datum, so that on an EH101 helicopter for instance,station 2850 would be 2850mm from the datum which is some distanceahead of the aircrafts nose.

Waterlines are similarly labelled so that WL-

505 would be 505mmbelow (shown by the minus sign) the aircrafts horizontal datum. Thisdatum is an arbitrarily positioned line; in the case of small piston-enginedaircraft it is often on the centre of the propeller shaft, and on large air-


Fig. 3.4 Dimensions of aircraft.


41/224


Fig. 3.5 Lines drawing.


42/224


Fig. 3.6 Alternative methods of describing wing geometry.


43/224

craft with circular section fuselages it is usually a line joining the centresof the circles in the main part of the fuselage; but note here that at noseand tail the centre of the fuselage sections would be below or above thehorizontal datum.

Buttock lines are labelled by their distance from the fuselage centreline, so that RBL 300 would be a butt taken at 300 mm to the right (star-board side) of the aircraft centre line.

In general, fuselage frames (see Chapter 2) lie on stations, but there areno other fuselage structure parts that exactly follow the major loft lines.Wing ribs are correctly defined by butt lines, but unfortunately some com-panies confuse the situation by labelling wing rib positions as stations.Alternative methods of describing wing geometry are illustrated in Fig.3.6; no one is more correct than any other but the differences are oftenconfusing to the student trying to memorise and understand a mass of new

words.


The major parts of an aircraft structure are:

The wing, or mainplane, which generates the lift required to keep theaircraft up in the air. The wing usually has moveable ailerons let intothe outboard (i.e. furthest from the fuselage) trailing edge to provide

roll control. It may also have flaps on the inboard trailing edge to helpthe aircraft fly more slowly when landing.

The fuselage, or body, which tends to link all of the other main partsand also provides space for the crew and payload.

Empennage, or tail feathers, usually consisting of a vertical fixed finand moveable rudder and a horizontal fixed tailplane and moveableelevator.

Any part of an aircraft can be pin-pointed in space relative to a pre-determined datum (or reference) point. This is achieved by defining itsstation (its distance aft, or forward, of the datum), its waterline (its dis-tance above or below the datum) and its buttock line (its distance left orright of the datum).



44/224

Chapter 4

Loads on the Aircraft

Before considering the loads imposed on the structure of the aircraft, itwill be as well if we remind ourselves of some of the engineering conceptsand the associated terminology.

4.1 General flight forces

The method by which an aircraft flies is well known: air passing over thewing lifts by suction, which is a pressure reduction. This pressure reduc-tion over the top of the wing is assisted by an upward pressure under thewing. However, although the aircraft flies by passing through the air, it isconvenient for the engineer to consider that the aircraft is standing stilland that the air is moving past it. These ideas are illustrated in Fig. 4.1.Figure 4.1(b) shows a piece of paper lifting when air is blown over it.

Figure 4.1(c) is a diagram of a wing section with airflow represented bylines. Note the direction of the airflow directly behind the wing. Figure4.1(d) shows diagramatically a slightly different concept of lift on a wingwhich is more convenient for the structures engineer. The lines are notnow meant to represent airflow, they represent direction of air pressureonto the wing.The line of pressure or force behind the wing is in the samedirection as the deflected airflow in Fig. 4.1(c). Because the airflow hasbeen deflected from its straight path, a force must have been applied tomake it change direction. In the diagram this force is represented by theline R. From the principles shown in Fig. 4.1(d) we can divide the appliedforce R into two parts, a vertical part which we call Wand a horizontalpart which we call T. W is provided for us by the weight of the aircraftand Tby the thrust of the engine. To further clarify the situation we dis-pense with the original lines of air forces and replace them with forcesmore directly opposing Wand T. These forces we call liftand drag and welabel them L and D. Lift opposed by weight, and drag opposed by thrust,represent the balanced forces which are established by the change indirection of the airflow.

4.1.1 Resolution of forces

This concept of dividing forces into their component parts, as we dividedR into the components Wand T, is most important and basic for students


45/224

who wish to go on to a serious understanding of the methods of engi-neering associated with the design of aircraft structures. Figure 4.2 illus-trates some of these ways of dividing and joining forces; people

not familiar with the principles should study this with some care. Beforeproceeding any further, we should also remind ourselves of some of theprinciples associated with turning moments, balance and the transfer ofloads.

Loads on the Aircraft 39

Fig. 4.1 Forces created by airflow over surfaces.


46/224


47/224


Fig. 4.3 Moments.


48/224

the picture is still not complete. It is only completed by a vertical reactionat the pivot, as shown in (d). Four units of upwards acting force are essen-tial to oppose the four units acting downwards. In fact, the downwardforces cannot exist without the opposition (or reaction).

If we now redraw (a) in the way shown in (e) we can make it completeby adding a moment and a reaction:

In Fig. 4.3(c) the lever is balanced because the moments each side of thepivot are equal, and the reaction of four units is composed of the threeunits in Fig. 4.3 plus one unit from the lever to the left of the pivot.

Looking at the pivot in (e) on its own, as drawn in (f), the system offorces which is acting at the pivot is as shown. The applied load andmoment are the white arrows and the reacting force and moment are theblack arrows. (Note here that between (e) and (f) the system has beenlabelled in lbf and lbf in. These read as pounds force and pounds forceinches, or just pound inches.Students more used to other units may wishto relabel the diagrams so that lbf becomes N (newton) and lbf in becomes

Nm (newton metre).)Note also that the two white arrows are the 3lbf load which is away at

the right-hand end of the lever, and this illustrates the important prin-ciple that a load can be transferred (relocated) but in its new position itmust be accompanied by a moment.

To further illustrate this last point, consider the beam in the two dia-grams (g) and (h). The reactions at the wall have been omitted to simplifythe drawing. Elements of the beam to the left of the plane of section XXwill feel exactly the same load in each case. An engineer examining thestrength of the beam at section XX would call 28lbfin the bendingmoment(MXX) and 7lbf theshear(SXX).

The last diagram (i) illustrates one possible way of applying a momentat the end of a lever to balance a load. This method is used later in thischapter, see Fig. 4.4.

4.1.2 Balancing forces acting on the aircraft

Returning to our consideration of how the aircraft flies, we have a wingwith lift and drag which we attach to a body as in Fig. 4.4. We said earlier

that the lift balances the weight of the aircraft but it is not sensible toassume that these two forces will be directly opposing one another;without any doubt they will at times be offset from one another as shownin the diagram. This immediately produces an out of balance moment

The moment load distance

= 3 2

= 6

The reaction the load

= 3 but note the direction

M( ) =

=

( )



49/224


Fig. 4.4 Flying loads.

which we attend to by the load on the tailplane. By the same argument,thrust and drag are not immediately opposite one another and we alsobalance those out by the tailplane. All the forces on the aircraft that wehave considered so far come together and balance out in Fig. 4.4(c). This


50/224

concept of the whole aeroplane as a free stationary body surrounded bya system of balancing loads and reactions is a concept which the aircraftengineer also applies to every small part of the structure as he comes toexamine it in detail.

Now consider the wing as viewed from the front; it will have lift along

its whole length. For reasons of airflow, which we, as structural engineersand not aerodynamicists, need only accept without question, there is morelift towards the middle of the wing than at the tips, and the lift is in factdistributed as shown in Fig. 4.5. Of course this lift cannot exist on its own,it must be balanced by something, and it is balanced by the weight of theaircraft, labelled Win the diagram.

4.2 Acceleration loads

So far we have considered the aeroplane as flying straight and level. Obvi-ously this is not always so, and since we have already said that we wish toconsider it as a free body at rest under the influence of the system of bal-anced loads and reactions, we need a system for recognising the forceswhich accompany a manoeuvre, which is the name for any disturbance ofstraight and level flight.

Firstly, we must be convinced that a manoeuvre always involves accel-eration. Imagine, for instance, an aircraft flying due north which then turns90 and heads due east. Before the turn the aircraft had no speed towards

the east but after making the turn it has some speed towards the east,therefore between the two conditions it must have received an accelera-tion. To achieve this acceleration a force was applied to the aircraft which


Fig. 4.5 Distribution of lift.


51/224

had to be resisted by the structure. A phenomenon which is familiar to allof us illustrates a method of introducing these accelerating forces into thefree stationary body idea. Imagine being in a passenger lift or elevator;just by eye we cannot tell whether or not the elevator is moving. In fact,

if it is moving at a steady speed, it is difficult for us to tell from the loadin our legs whether the elevator is going up or down. It is only when theelevator stops or starts that the positive or negative acceleration appearsto change our weight and and the load in our legs is altered. If the eleva-tor is accelerating upwards the load in our legs is greater because theinertia forces in our body act in a downwards direction. (Inertia is theresistance of a body to any change in its motion. If the body is standingstill, it requires some external effort to move it; if it is moving, it will con-tinue to move in a straight line and will resist the force needed to stop itor change its direction.) This is just another instance of the action and

opposite reaction rule. In this case, acceleration or accelerating force actsone way and inertia the opposite way.

The analogy with the elevator is easy to understand when accelerationis up and down, and because we cannot see outside the elevator it is easyto accept the free stationary body idea. When the acceleration is hori-zontal, finding an analogy is more difficult. While travelling at a steadyspeed on a smooth road in a motor car, it may be difficult to admit thatthe loads acting on our body are exactly the same as if the car was sta-tionary, but they are.The loads only change during acceleration away froma stop (positive or forward acceleration); during deceleration by braking

(which is negative acceleration or the same as rearward acceleration); orduring sideways acceleration when turning a corner. Again the accelera-tion one way, inertia the other way rule applies, so that, if the car turnsto the left, some force (the action of the front-wheel tyres) has acted tothe left and accelerated the car the same way, but the passenger has torestrain himself from being thrown to the right.

The essential point in the above discussion is that when the elevator orthe car either stops or starts or changes direction (that is, manoeuvres),the passengers weight appears to increase in the opposite direction to theforce directing the acceleration. This is always so. Do not be confused bythe apparent contradiction of this truth when the passenger lift starts todescend. The situation then is that the acceleration in the downwardsdirection is small and although the passengers weight does increase in theupwards direction the increase is not enough to completely overcome hisnormal weight due to gravity. If the acceleration downwards became greatenough, the occupant of the lift would need to push upwards on the roofto keep his feet on the floor and the greater the acceleration the harderthe push would have to be. In an aircraft the same situations apply, andstructural engineers deal with them by saying that during a manoeuvre

the apparent weight of everything in the aircraft is increased by a factor(n) of the weight due to gravity (g). In some cases the factor n is deter-mined by calculation but usually a figure is specified by government



52/224

legislation working through its own Airworthiness Authority. (See thenote in Fig. 4.6 for the various names given to n.)

4.2.1 Using Vn diagrams

The presentation of the Airworthiness Requirement for the inertia factoror load factor is usually by the Vn diagram, which is in a form shown inFig. 4.6. Working from the European Aviation Safety Agency (EASA)Certification Specification 25 paragraph 337 (CS-25.337) for aeroplaneswith a maximum take-off weight of 5700kg (12500lb) or greater, for anaircraft weighing 22690kg (50000lb) or more n is 2.5 and for an aircraftweighing 6350kg (14000lb) n is 3.1. Figure 4.6 describes a manoeuvringenvelope and particular combinations of speed and load within the enve-lope are called Cases.A similar Vn diagram, which describes a gust enve-lope, is used to present Airworthiness Requirements of the effect of air

currents likely to be met by the aircraft. The air currents, or gusts, have anaccelerating effect and increase apparent weights in exactly the same wayas ordinary manoeuvres; they may also operate in any direction and ingeneral they become more and more severe with higher speeds.


Fig. 4.6 Vn diagram (the manoeuvring envelope).


53/224

4.2.2 Emergency alighting loads

Another group of load factors specified by Airworthiness Authorities arethe crash loads or emergency alighting loads. In the current issue of theEASA CS-25 for large aeroplanes these are required to be from nine (9)

timesg forward to 1.5g rearwards, 6g downwards to 3g upwards and 3gsideways. Helicopters and light aeroplanes have somewhat different setsof inertia requirements quoted in the appropriate certification specifi-cation. This method of specifying the loads along three axes mutually atright angles raises a further interesting point. Inertias are not always soconveniently applied and the section of the now obsolete British CivilAirworthiness Requirements which dealt with emergency alightinginsisted that the prescribed loads should be taken as acting together,although the combined load need not exceed 9g. Bearing in mind therandom nature of emergency landings (such as wheels up in a ploughedfield) and the consequent random directions of the imposed loads, theBCAR principle does not seem too ridiculous. It is sometimes asked whya particular figure (such as 9g) is set, the arguments being that the normalpassenger travelling in an aircraft would be unable to withstand the effectsof a 9g deceleration and therefore the requirement should be set at alower figure; or alternatively that by increasing the requirement to 12gand ensuring a stronger structure, more passengers would survive a crash.These arguments might well be true, but a figure must be set somewhereand the Airworthiness Authorities consider carefully the statistics avail-

able and apply their expertise before fixing a factor at a certain level.Likemost aspects of aircraft design, there is an element of compromiseinvolved. It would be easy to set a figure which produced a very strong,safe aeroplane that was too heavy to fly, and the figure chosen is a balanceof careful thought with the bias, no doubt, on the side of passenger safety.Some of the arguments used in choosing a figure are set out in TyesHandbook of Aeronautics.

In case this question of load factors is not absolutely clear, we will con-sider the specific example of a piece of equipment bolted down to the floorof an aircraft and subjected to the emergency alighting conditions men-

tioned above. If the piece of equipment is a rectangular box of 10 lbweight, to satisfy the requirement of 4.5g downwards the floor will haveto be strong enough to support 45lbf (read forty-five pounds force)pushing down, i.e. 10 4.5g. (Note that 4.5g is just a factor with no dimen-sions but it does alter a weight to a force.) Similarly, the 9g forwardsrequirement means that the attachments must resist 90lbf parallel to thefloor. If we are working in SI units the arithmetic is slightly more compli-cated. A weight of 1 kg at an acceleration of 1g produces 1kgf (read onekilogramme force) but since the unit of force in SI units is the newton, wemust convert. (The conversion factor is 1kgf = 9.81N.) So

therefore

7 15 75kg at 2.25 g kgf = .



54/224

For a quick and rough calculation use a factor of 10 and say:

(Another useful conversion factor is 1lbf = 4.45N.)

4.3 Further aerodynamic loads

4.3.1 Lift and drag

In Fig. 4.4 the load positions shown are exaggerated to illustrate the ideaof the aircraft in balance. If this book was written about aerodynamics we

should be a lot more careful about lift and drag and how and where theyact, but as we are concerned with structures a few more ideas on air loadswill suffice.

Figure 4.1(c) showed a wing section (or airfoil or aerofoil) at an anglea (read alpha) to an air stream.This angle is called the angle of attack andthere is a similar angle called the angle of incidence which is measured tothe horizontal datum of the aircraft (see Fig. 3.4).

As the angle of attack is increased from its usual working or cruisingspeed attitude of about 2, the lift and drag will increase. At about 15 theairflow will no longer follow round the upper surface, the wing will stall

which means that the lift significantly reduces, the point at which it actsmoves aft and drag rises significantly: in essence the wing stops usefullyflying. Conversely, if the angle of attack is reduced, lift will decrease tozero at a small negative angle and then start to produce the negative liftwhich allows aerobatic aircraft to fly upside down. While the lift and dragare changing, the point at which they effectively act, which is called thecentre of pressure (or CP), is also changing. At the angle of zero lift it is,for some wings, right back near the trailing edge and it moves forward asthe angle is increased, usually to about a third of the wing chord distance.To clarify this situation, the aerodynamicists quote the lift and drag asacting through one imaginary point for all angles of attack and add avarying moment to take care of the CP shift. This imaginary point isusually at 25% of the wing chord and is called the quarter chord point.

Lift also varies with wing area, air density and aircraft speed (in fact asspeed squared, i.e. twice the speed, four times the lift). It is also affectedby aspect ratio.

For an absolutely rectangular wing, aspect ratio (AR) = span chord,so a short stubby wing has a low aspect ratio and a long, slender wing, likea sailplane, has a high aspect ratio. Now, if we think about the airflow past

a short wing with the air pressure reducing as it passes over the wing, wecan imagine this reduced pressure pulling some air in from the ends. Theeffect of this is to change the line of the airflow from a direct path betweenleading and trailing edges to a longer path slightly along the wing. With

15 75 157. kgf N which is correct to less than 2%= ( )

15 75 9 81 154 5. . .kgf N =



55/224


56/224

the size and weight of the aircraft they are mounted on; it is less obviousthat, for the same size aircraft, control loads are altered by the weightbeing lifted. Control loads also vary with the rate of response of the air-craft; for instance, it takes more aileron load to roll an aircraft quickly

than to roll it slowly. Methods of assessing all these loads are found in theAirworthiness Requirements publications (see Section 13.5).Ailerons cause special problems: in the 1920s they effectively started a

whole new science which became the study of aeroelasticity.If we say that Fig. 4.7 shows an aileron out on the tip of a long, flexible

wing, by deflecting the control down we are hoping to increase the lift atthat wing tip and cause the aircraft to roll. However, because the addi-tional lift is so far back and the wing is flexible, the whole cross-section inthe diagram will be twisted anticlockwise until the stiffness of the struc-ture stops it. If the rotation is sufficient, the lift on the wing, far from being

increased by the control movement, will actually be decreased,causing theaircraft to roll the opposite way to that expected. David B. Thurston in hisvery excellent book, Design for Flying, says of this control reversal effect. . . [it] could provide quite a bit of activity in the cockpit . . ..

We took an aileron down case to illustrate the effect but aileron upshows a possible, even worse effect where forced rotation of the wingsection causes a local stall, whereupon the unloaded wing springs backuntil the airflow restores itself and the process starts over again. This isone form of a phenomenon calledflutterwhich can be induced in all sortsof ways.The study of flutter is complex but well documented and modern

aircraft do not suffer the catastrophic failures created by flutter in earlyaircraft before its various causes were recognised.The general name,aero-elasticity, is given to the study of these problems of control reversal andflutter, together with control surface balancing, resonance of vibrationsand any other effect which follows from the fact that aircraft structuresare springy and actually change their shape under load.Work in this areais probably the most difficult that the aircraft stressman and designer haveto deal with.As we said of flutter, a lot of study has gone into the subject,especially mathematical analysis which has been made possible by thecomputer power now available. Unfortunately, there are no rule ofthumb methods for the certain elimination of aeroelastic problems. TheAirworthiness Authorities lay down requirements and test procedureswhich eliminate the possibility of danger to the airline passenger or theprivate pilot, but it is in the nature of aircraft design that performance andtechnology must be pressed forward right to their limits and that is whereaeroelastic problems abound.

4.4 Other loads

So far we have concentrated entirely on flying loads, apart from a mentionof crash loads, but there are others which affect the structure. The loadsdue to pressurisation are important, especially if the fuselage is not circu-



57/224

lar. A fuselage with a non-circular cross-section will try to become circu-lar under internal pressure and impose bending loads on the fuselageframes. The double bubble style of fuselage is a clever exception, withthe shape maintained by tension in the floor beams (the term floor joists

is not normally used in aircraft structure).A tragic but instructive accident illustrated an interesting pressurisationstructural problem.In this accident the lower fuselage below the floor wassuddenly and totally depressurised by the opening of an unlocked cargodoor during flight. The floor beams, which were parallel-sided, constant-section bending members then collapsed; in fact, they tried to adopt ashape which was part of a circle. No structures engineer would criticiseanother for not foreseeing this illustration of the power of pressurisationloads, which led, interestingly, to the immediate adoption of automaticpressure-equalising valves between compartments.

Landing loads also need to be considered.Airworthiness Requirementsgive advice and assistance with the assessment of landing loads which canhave a large influence on the structure. For instance on a normally con-figured twin-engined aircraft in flight, the weight of the engines, the fuel,the main undercarriage and the wing itself, are all nicely spread out alongthe wing and directly supported by the lift. On the ground, and worse stillduring a heavy landing, all those same weights are propped up on the topof the undercarriage leg which is a more difficult situation for the wingstructure to deal with.

High-speed flying produces another type of structural load which needs

a classification of its own because it exists in addition to the flight andmanoeuvring loads. This loading is an effect of heat produced by high-speed flight and the distortion which the heat causes.

As the aircraft forces its way through the air,parts of the structure,espe-cially the surface skinning, are warmed and at very high speeds becomequite hot.At Mach 2 (read mark two), that is at twice the speed of sound,some areas reach 150C. This type of heating, called kinetic heatingbecause of its association with movement, produces different tempera-tures on different parts of the structure.The temperature variations causeexpansions which are different from member to member, and thus theinternal loads that we are discussing are produced: members trying toexpand because of the heat are being forcibly restrained by coolermembers. (Note that both hot and cold members are loaded.) Althoughthis idea is not easy to understand without a more general appreciationof the form of structures, it is an important source of load in the aircraftand will be referred to again in Chapter 12.

4.5 Further load factors

Assessment of the loads on each piece of structure under investigationconstitutes more than half of the stressmans job.The notes in this chapterhave dealt with major loads, details of which would be supplied to the



58/224

structures department by the aerodynamics and wind tunnel depart-ments (and also, during a development programme, by the flight testdepartment).

Once the major loads have been established the designers and stress-

men have to break each load down and distribute it through the structure;for instance, they might find that they cannot make two hinges each strongenough to carry half an elevator load and therefore they will need to usethree hinges.

In a later chapter we shall discussproofand ultimate stress (see Section5.5), which are, respectively, working strength and breaking strength for amaterial. The majority of loads derived from specifications and Airwor-thiness Requirements are called limit loads or working loads. To comparethese with the abilities of the material they must first be multiplied by aproof factorto give aproof load. This proof factor is also specified in Air-

worthiness Requirements (it is usually 1.0 so that proof load is numeri-cally the same as limit load). The proof load is a load that the structure isexpected to cope with repeatedly and without distress. Crash case loads(or emergency alighting loads) are usually ultimate loads, that is thestructure must be strong enough to withstand the load once, but it is thenunderstood to be too stretched or distorted for further service.

The connection between proof and ultimate loads applied to a struc-ture, and the working and breaking strengths for the material of which thestructure is made, is clear and will be made clearer after reading Chapter5.There is a further connection between proof and ultimate called the ulti-

mate factor. This is specified in the Airworthiness Requirements and formost civil aircraft applications is required to be 1.5. The way it works isshown in the following simple example.

A stressman is investigating a bracket supporting a piece of equipmentweighing 10lb. He checks with the Airworthiness Requirements and findsthat (for one Case):

the limited load factor (which is quoted as a

manoeuvring factor) = 2.5 g

the proof factor = 1.0

and the ultimate factor = 1.5

therefore the proof load on the bracket is to be taken as = 10lb 2.5 g

= 25lbf

and the ultimate load on the bracket is to be taken as = 25lbf 1.5

= 37.5 lb

Strictly speaking the stressman should now make two sets of calculations,one set comparing 25lbf with the proof stress that is the allowable proof

stress of the material of the bracket, and the other set comparing 37.5 lbfwith the ultimate stress and allowable ultimate stress of the material. Inpractice, he would be investigating a number of components all made ofthe same metal and he would know the relationship between its proof and



59/224

ultimate stresses. If, for instance, he knew that the proof stress was greaterthan two thirds of the ultimate stress he would know that by basing hiscalculations on the ultimate loads and the ultimate stress he would alsobe covering the proof conditions. All these factors are confusing on first

acquaintance and there are more to come. Each makes its contribution tothe designers task of using structural material efficiently, which meansusing it economically with safety. Reserve factors are defined on page 191but before leaving the topic of strength requirements mention must bemade of the fitting factor. This is a requirement of CS-25.635 and meansthat the final fitting which joins two members together must have an addi-tional strength reserve above its normal proof and ultimate factors (seeabove). Fittings such as control surface hinge blocks, wing attachments,equipment mountings, etc., are required to have proof and ultimatereserve factors of not less than 1.15.

4.6 Loads acting on the whole aircraft

With the aid of diagrams we will consider the arrangement of loads onthe whole aircraft in four cases, cruising straight and level, turning, stallingand landing.

The loading situation in the straight and level cruise condition is asshown in Fig. 4.8.

In Section 4.2 we discussed inertia factors (load factors) which are

written as ng. In the cruise the inertia factor is 1g and applies to theweights of all the elements of structure and equipment which constitutethe whole aircraft. Sometimes the inertia factor is referred to as an accel-eration factor or acceleration coefficient.There is nothing wrong with such


Fig. 4.8 Cruising flight.


60/224

a descriptive name except that in the straight and level condition we are

left to explain the statement . . . the acceleration factor is 1g, whenclearly the aircraft is not being accelerated in any direction. Rememberthat whether it is called inertia,load or acceleration it is always a factorand as such is a device used by engineers to bring into calculation theapparent increase in weight of objects which are being accelerated. As amultiplying factor it represents an unaltered situation when it is 1.0.

When an aircraft is turning it suffers an apparent weight increase dueto the centrifugal force which is trying to make it go straight on insteadof round the curved path of the turn. In a correctly executed manoeuvrethe loads balance as shown in Fig. 4.9.

Stalling occurs when the lift surfaces of the aircraft are no longer ableto provide enough lift to balance the weight, and the speed of the aircraftat which this occurs is called the stalling speed. Even this is not simple,because we can have one stalling speed with a clean wing andanother lower speed when flaps, slats and any other of a selection of lift-improving devices are added.We also have the situation described above,where during a turn the apparent weight of the aircraft is increased, sothat the ability of the wing to provide enough lift will disappear at a ratherhigher speed in a turn than when the aircraft is flying straight and level.

The aerodynamic action of stalling is a breakdown of the airflow over thetop surface of the wing, as shown in Fig. 4.1 and, except for some high-speed situations, occurs at an angle of attack of about 15.

The structural load situation is that immediately after the stalling of thewing the aircraft starts to pitch nose down and, more or less, free fall; that


Fig. 4.9 Loads in a turn.


61/224

is all of the individual parts of the aircraft appear weightless. In Section4.1.2 we spoke of the free body concept with all the loads in balance, sowe might expect that when the lift disappears the opposing and balancingweight will also disappear. In fact the ideal situation is not quite realisedas, although the wing has stalled, it is still producing a small amount of liftbut at a point further aft than prior to the stall. The weight now signifi-cantly exceeds the available lift and is also acting forward of this lift thustending to pitch the aircraft nose downwards and,with the aid of the downforce on the tailplane, producing an overall acceleration earthwards. Inthe stall case the unbalanced tailplane load causes a nose down pitching

rotation which aerodynamically allows the aircraft speed to build up andstable flying conditions to be restored (see Fig. 4.10). Structurally the loadsinvolved on the main structure in the stall are not usually of any signifi-cance but the loads on control surfaces and flaps may be important.

Landing loads are a major consideration in the design of the structureand are involved with the designed cushioning characteristics of theundercarriage. The stiffer the springs of the undercarriage the more rapidthe vertical deceleration of the landing aircraft and the higher the loadsinvolved on the structure. The Airworthiness Requirements are verydescriptive and demanding on this subject. Helicopters and, even moreobviously, deck-landing aircraft generate high landing loads.

If we look back at Figs. 4.74.9 we can see that the general loading onany major part of the aircraft structure is a bending or a twisting load orboth. This general loading pattern is shown in Fig. 4.11 and will be dis-cussed in the next chapter.


The principal forces acting upon an aircraft are lift, weight, thrust anddrag. In straight and level flight lift equals weight and thrust equals drag.It can be useful to resolve a force into two other forces at 90 to eachother. For example the force produced by a wing moving through the airis resolved into:


Fig. 4.10 Loads out of balance in a stall.


62/224

lift acting at 90 to the direction of travel; and drag acting parallel to the direction of travel.

A force acting at a point displaced from a support will produce a turning

moment in addition to the direct force. Moment is force multiplied by thedistance from the support (see Fig. 4.3).When an aircraft (or any other object for that matter) is in a steady

state, i.e. not accelerating in any direction, although it might be moving ata steady speed, all forces and all moments acting on the aircraft are inbalance.For an aircraft to change direction (manoeuvre) some of the loadsacting upon the aircraft must be increased and this will cause the aircraftto accelerate. The increased loads are often quoted as a multiple of thenormal force due to gravity, e.g.an aircraft might perform a 3g turn wherethe loads acting upon the aircraft are three times those experienced in

straight and level flight. Airworthiness codes usually specify what factors(multiples of nominal gravity) to apply when designing the structure.

4.8 References

Carpenter, C. Flightwise. Shrewsbury: Airlife Publishing.

Thurston, D.B. Design for Flying. New York: McGraw-Hill.(See also references for Chapter 13.)

Tye, W. Handbook of Aeronautics, No. 1, Structural Principles and Data,Part 1 Structural airworthiness, 4th edn. London: Pitman.


Fig. 4.11 Loads on the wing.


63/224

Chapter 5

The Form of Structures

5.1 Structure relative to aircraft design

In the last chapter we looked at the loads which the aircraft has to carry;in this chapter we consider how the structure is arranged to support theloads (or, more exactly, how the structure links the loads to the reactions).

Perhaps the most significant thing about a piece of aircraft structure isthat it is a compromise. Some years ago an amusing cartoon was printedshowing how different people in the aircraft business thought of an aero-plane.To the commercial operator it was a flying box for carrying peopleor freight; to the avionics engineer it was just a means of getting his radaraerials and navigation aids airborne; to the aerodynamicist it was a slim,shining and beautiful creation with no unsightly access panels or skin

joints or rivet heads, not even room for passengers; to the structural engi-neer it was a few simple robust members reminiscent of a railway bridgeor a tower crane. Like all good cartoons this was not totally ridiculous. Anaeroplane is all of these things but not one thing alone, so compromiseshave to be made.The structures engineer is constantly compromising withhis own requirements; for example, discarding methods or materials whichwould ideally satisfy one set of conditions but which would be unsuitablefor another set, or correcting an apparently satisfactory design whichproves to be impossible to manufacture. However, he mainly needs toadjust his ideas to satisfy both the aerodynamicist, who (broadly speak-ing) specifies the external shape of the aircraft, and the payload, that isthe passengers, freight or armament, which, together with the fuel, deter-mine the internal dimensions.

5.2 Historic form of structure

In the early days of flying the aircraft structure or airframe had almostonly one requirement: it was the minimum which the designer (who was

probably also the test pilot) thought adequate to keep the wings intactand support the control surfaces and the pilot (see Fig. 2.1). Later, thebasic structure was covered with fairing which improved the streamlineshape without contributing to the strength (see Fig. 2.3). Wings, of course,


64/224


Fig. 5.1 Structural boxes or tubes.


65/224

had from the beginning been covered with fabric or paper, but this cov-ering did not form part of the structure (except that very locally it carriedair loads to the ribs). The third phase, which is the method used almostuniversally on modern aircraft, incorporated the fairings into the load-

carrying structure or, looking at it another way, pushed the structure rightout so that it filled the aerodynamic shape. This type of structure, which,as we said in Chapter 2, is called stressed skin or semi monocoque, isefficient in the engineering sense that it has material or components per-forming more than one function. In some aircraft the wing skins performthree functions: they transmit the air loads which provide lift, they makethe tube or box which is the strength of the wing (see Section 5.6) andthey are also the walls of the fuel tanks. Although it is very difficult toimagine a better type of structure for aircraft, it is always possible thatone will be invented. There is even now a lower limit of size and weight

of aircraft below which the stressed skin would become so thin that itwould be unmanageable. Man-powered aircraft, for instance, are so lightthat the structure must belong to the second phase of structural develop-ment because the covering is too flimsy to be made load carrying.

5.3 General form of structure

The aircraft structure in its various phases of development has alwaysconsisted of an assembly of large built-up tubes or boxes. These tubes are

most clearly obvious in the stressed-skin type of structure,which from nowwill be considered as the conventional or normal type. Figure 5.1(a) illus-trates the tubes in their simplest form, and Fig. 5.1(b) shows the refine-ment which is the basis of the vast majority of aircraft, from single seatersto wide-bodied passenger airliners and supersonic bombers. The conven-tional method of construction is shown in Fig. 5.2(a) and (b), with anearlier type of construction illustrated in Fig. 5.3. This earlier constructiondoes not so obviously form a tube but its construction allows it to carrythe same types of load as the conventional structure. These are bendingand torsion loads, and before proceeding further it would be as well todefine these and some of the other terms used by structures engineers.

5.4 The basic load systems in structures

There are three basic combinations of load and resistance to load:

shear tension

compression

It is hard to find aircraft examples of these load/load resistance systemswhere the load application can quite clearly be called one of the above

The Form of Structures 59


66/224

without any element of another. The examples in Figs 5.4 and 5.5 (pages62 and 63) are close to being pure.

In Fig. 5.4(a) we can say that the bolt is in pureshearthrough faces AAand BB; that is, the bolt is resisting the pull by its reluctance to divide inthe way shown in Fig. 5.4(b). In the example illustrated in Fig. 5.5 the boltis in tension and will eventually divide as in Fig. 5.5(b). The act of divisionis called a failure.

Compression is when the load application is the opposite of tension (seeFig. 5.6) and a piece of structure failing in compression is most difficult toshow in a pure form because the start of a failure

Documents

Understanding Aircraft Structures