Principles of Flight PDF - JAA PPL Study Guide 2010

 Principles of Flight Airframe Limitations Theory of Flight Aeroplane Performance 5  Copyright © Oxford Aviation Academy Ltd 2010. All Rights Reserved. This text book is to be used only for the purposes of private study...

 Principles of Flight Airframe Limitations Theory of Flight Aeroplane Performance 5  Copyright © Oxford Aviation Academy Ltd 2010. All Rights Reserved. This text book is to be used only for the purposes of private study by individuals and may not be reproduced in any form or medium, copied, stored in a retrieval system, lent, hired, rented, transmitted, or adapted in whole or part without the prior written consent of Oxford Aviation Academy Limited. Copyright in all documents and materials bound within these covers or attached hereto, excluding that material which is reproduced by the kind permission of third parties and acknowledged as such, belongs exclusively to Oxford Aviation Academy Limited. Certain copyright material is reproduced with the permission of the International Civil Aviation Organisation, the United Kingdom Civil Aviation Authority and the Joint Aviation Authorites (JAA). This text book has been written and published as a reference work for student pilots with the aims of helping them prepare for the PPL theoretical knowledge examinations, and to provide them with the aviation knowledge they require to become safe and competent pilots of light aeroplanes. The book is not a flying training manual and nothing in this book should be regarded as constituting practical flying instruction. In practical flying matters, students must always be guided by their instructor. Oxford Aviation Academy Limited excludes all liability for any loss or damage incurred as a result of any reliance on all or part of this book except for any liability for death or personal injury resulting from negligence on the part of Oxford Aviation Academy Limited or any other liability which may not legally be excluded. This book has been produced by Oxford Aviation Academy. Production Team Subject Specialists - Principles of Flight: Les Fellows, Jon Hedges, Laurie Knight Subject Specialist - Aeroplane Performance: Steve Francis Contributors: Rhodri Davies, Steve Partridge, Glyn Rees, Lesley Smith, Roger Smith, Rick Harland Created and Compiled by: James Kenny Editor: Les Fellows, Rick Harland Cover Design by: Chris Hill Cover Photo by: Mike Jorgensen First Published by: Oxford Aviation Academy, Oxford, England, 2010 Printed in Singapore by: KHL Printing Co. Pte Ltd Contact Details: OAAmedia Oxford Aviation Academy Oxford Airport Kidlington Oxford OX5 1QX England Tel: +44 (0)1865 844290 Email: [email protected] Innovative learning solutions for www.oaamedia.com ISBN 978-0-9555177-4-7 www.oaa.com  TABLE OF CONTENTS GENERAL FOREWORD v TO THE PILOT xiii PRINCIPLES OF FLIGHT COMPONENT PARTS OF THE AEROPLANE 1 CHAPTER 1: PHYSICAL DEFINITIONS 5 CHAPTER 2: THE ATMOSPHERE 17 CHAPTER 3: LIFT 31 CHAPTER 4: MORE ABOUT AIRFLOW AND AEROFOILS 67 CHAPTER 5: DRAG 89 CHAPTER 6: THE LIFT/DRAG RATIO 117 CHAPTER 7: WEIGHT 127 CHAPTER 8: PROPELLER THRUST 141 CHAPTER 9: THE FOUR FORCES AND TURNING FLIGHT 175 CHAPTER 10: LIFT AUGMENTATION 199 CHAPTER 11: STABILITY 221 CHAPTER 12: FLIGHT CONTROLS AND TRIMMING 253 CHAPTER 13: THE STALL AND SPIN 287 CHAPTER 14: FLIGHT AND GROUND LIMITATIONS 319 PRINCIPLES OF FLIGHT SYLLABUS 351 ANSWERS TO PRINCIPLES OF FLIGHT QUESTIONS 353 PRINCIPLES OF FLIGHT INDEX 359 iii Aircraft Technical Book Company http://www.actechbooks.com  AEROPLANE PERFORMANCE CHAPTER 1: INTRODUCTION 1 CHAPTER 2: TAKE-OFF 5 CHAPTER 3: CLIMB 35 CHAPTER 4: EN-ROUTE PERFORMANCE 65 CHAPTER 5: LANDING 89 AEROPLANE PERFORMANCE SYLLABUS 107 ANSWERS TO AEROPLANE PERFORMANCE QUESTIONS 109 AEROPLANE PERFORMANCE INDEX 113 iv Aircraft Technical Book Company http://www.actechbooks.com  FOREWORD FOREWORD TO THE SECOND EDITION. INTRODUCTION. Whether you are planning to fly microlights, space shuttles, gliders, combat aircraft, airliners or light aircraft, it is essential that you have a firm grasp of the theoretical knowledge which underpins practical piloting skills. This Oxford Aviation Academy “Skills for Flight” series of text books covers the fundamental theory with which all pilots must come to grips from the very beginning of their pilot training, and which must remain with them throughout their flying career, if they are to be masters of the art and science of flight. JOINT AVIATION AUTHORITIES PILOTS’ LICENCES. Joint Aviation Authorities (JAA) pilot licences were first introduced in Europe in 1999. By 2006, almost every JAA member state, including all the major countries of Europe, had adopted this new, pan-European licensing system at Air Transport Pilot’s Licence, Commercial Pilot’s Licence and Private Pilot’s Licence levels, and many other countries, world-wide, had expressed interest in aligning their training with the JAA pilot training syllabi. These syllabi, and the regulations governing the award and the renewal of licences, are defined by the JAA’s licensing agency, ‘Joint Aviation Requirements - Flight Crew Licensing’, (JAR-FCL). JAR-FCL training syllabi are published in a document called ‘JAR-FCL 1.’ The United Kingdom Civil Aviation Authority (UK CAA) is one of the founder authorities within the JAA. The UK CAA has been administering examinations and skills tests for the issue of JAA licences since the year 2000, on behalf of JAR-FCL. The Private Pilot’s Licence (PPL), then, issued by the UK CAA, is a JAA licence which is accepted as proof of a pilot’s qualifications throughout all JAA member states. Currently, the JAA member states are: United Kingdom, Denmark, Iceland, Switzerland, France, Sweden, Netherlands, Belgium, Romania, Spain, Finland, Ireland, Malta, Norway, Czech Republic, Slovenia, Germany, Portugal, Greece, Italy, Turkey, Croatia, Poland, Austria, Estonia, Lithuania, Cyprus, Hungary, Luxembourg, Monaco, Slovakia. As a licence which is also fully compliant with the licensing recommendations of the International Civil Aviation Organisation (ICAO), the JAA PPL is also valid in most other parts of the world. The JAA PPL in the UK has replaced the full UK PPL, formerly issued solely under the authority of the UK CAA. Issue of the JAA PPL is dependent on the student pilot having completed the requisite training and passed the appropriate theoretical knowledge and practical flying skills tests detailed in JAR-FCL 1. In the UK, the CAA is responsible for ensuring that these requirements are met before any licence is issued. v Aircraft Technical Book Company http://www.actechbooks.com FOREWORD EUROPEAN AVIATION SAFETY AGENCY. With the establishment of the European Aviation Safety Agency (EASA), it is envisaged that JAA flight crew licensing and examining competency will be absorbed into the EASA organisation. It is possible that, when this change has taken place, the PPL may even change its title again, with the name “EASA” replacing “JAA”. However, we do not yet know this for certain. In the UK, such a step would require the British Government to review and, where necessary, revise the Civil Aviation Act. But, whatever the future of the title of the PPL, the JAA pilot’s licence syllabuses are unlikely to change fundamentally, in the short term. So, for the moment, the JAA Licence remains, and any change in nomenclature is likely to be just that: a change in name only. OXFORD AVIATION ACADEMY AND OAAMEDIA. Oxford Aviation Academy (OAA) is one of the world’s leading professional pilot schools. It has been in operation for over forty years and has trained more than 15 000 professional pilots for over 80 airlines, world-wide. OAA was the first pilot school in the United Kingdom to be granted approval to train for the JAA ATPL. OAA led and coordinated the joint-European effort to produce the JAR-FCL ATPL Learning Objectives which are now published by the JAA, itself, as a guide to the theoretical knowledge requirements of ATPL training. OAA’s experience in European licensing, at all levels, and in the use of advanced training technologies, led OAA’s training material production unit, OAAmedia, to conceive, create and produce multimedia, computer-based training for ATPL students preparing for JAA theoretical knowledge examinations by distance learning. Subsequently, OAAmedia extended its range of computer-based training CD-ROMs to cover PPL and post-PPL studies. This present series of text books is designed to complement OAAmedia’s successful PPL CD-ROMs in helping student pilots prepare for the theoretical knowledge examinations of the JAA PPL and beyond, as well as to provide students with the aviation knowledge they require to become safe and competent pilots. The OAA expertise embodied in this series of books means that students working towards the JAA PPL have access to top-quality, up-to-date, study material at an affordable cost. Those students who aspire to becoming professional pilots will find that this series of PPL books takes them some way beyond PPL towards the knowledge required for professional pilot licences. THE JAA PRIVATE PILOT’S LICENCE (AEROPLANES). The following information on the Joint Aviation Authorities Private Pilot’s Licence (Aeroplanes); (JAA PPL(A)) is for your guidance only. Full details of flying training, theoretical knowledge training and the corresponding tests and examinations are contained in the JAA document: JAR–FCL 1, SUBPART C – PRIVATE PILOT LICENCE (Aeroplanes) – PPL(A). The privileges of the JAA PPL (A) allow you to fly as pilot-in-command, or co-pilot, of any aircraft for which an appropriate rating is held, but not for remuneration, or on revenue-earning flights. vi Aircraft Technical Book Company http://www.actechbooks.com  FOREWORD For United Kingdom based students, full details of JAA PPL (A) training and examinations can be found in the CAA publication, Licensing Administration Standards Operating Requirements Safety (LASORS), copies of which can be accessed through the CAA’s Flight Crew Licensing website. Flying Training. The JAA PPL (A) can be gained by completing a course of a minimum of 45 hours flying training with a training organisation registered with the appropriate National Aviation Authority (the Civil Aviation Authority, in the case of the United Kingdom). Flying instruction must normally include: 25 hours dual Instruction on aeroplanes. 10 hours supervised solo flight time on aeroplanes, which must include 5 hours solo cross-country flight time, including one cross-country flight of at least 150 nautical miles (270km), during which full-stop landings at two different aerodromes, other than the aerodrome of departure, are to be made. The required flying instructional time may be reduced by a maximum of 10 hours for those students with appropriate flying experience on other types of aircraft. The flying test (Skills Test), comprising navigation and general skills tests, is to be taken within 6 months of completing flying instruction. All sections of the Skills Test must be taken within a period of 6 months. A successfully completed Skills Test has a period of validity of 12 months for the purposes of licence issue. Theoretical Knowledge Examinations. The procedures for the conduct of the JAA PPL (A) theoretical knowledge examinations will be determined by the National Aviation Authority of the state concerned, (the Civil Aviation Authority, in the case of the United Kingdom). The JAA theoretical knowledge examination must comprise the following 9 subjects: Air Law, Aircraft General Knowledge, Flight Performance and Planning, Human Performance and Limitations, Meteorology, Navigation, Operational Procedures, Principles of Flight, Communication. A single examination paper may cover several subjects. The combination of subjects and the examination paper titles, as administered by the UK CAA, are, at present: 1. Air Law and Operational Procedures. 2. Human Performance and Limitations. 3. Navigation & Radio Aids. 4. Meteorology. 5. Aircraft (General) & Principles of Flight. 6. Flight Performance and Planning. 7. JAR-FCL Communications (PPL) (i.e. Radiotelephony Communications). The majority of the questions are multiple choice. In the United Kingdom, examinations vii Aircraft Technical Book Company http://www.actechbooks.com FOREWORD are normally conducted by the Flying Training Organisation or Registered Facility at which a student pilot carries out his training. The pass mark in all subjects is 75%. For the purpose of the issue of a JAA PPL(A), a pass in the theoretical knowledge examinations will be accepted during the 24 month period immediately following the date of successfully completing all of the theoretical knowledge examinations. Medical Requirements. An applicant for a JAR-FCL PPL(A) must hold a valid JAR-FCL Class 1 or Class 2 Medical Certificate. THE UNITED KINGDOM NATIONAL PRIVATE PILOT’S LICENCE (AEROPLANES). One of the aims of the United Kingdom National Private Pilot’s Licence (UK NPPL) is to make it easier for the recreational flyer to obtain a PPL than it would be if the requirements of the standard JAA-PPL had to be met. The regulations governing medical fitness are also different between the UK NPPL and the JAA PPL. Full details of the regulations governing the training for, issue of, and privileges of the UK NPPL may be found by consulting LASORS and the Air Navigation Order. Most UK flying club websites also give details of this licence. Basically, the holder of a UK NPPL is restricted to flight in a simple, UK-registered, single piston-engine aeroplane (including motor gliders and microlights) whose Maximum Authorized Take-off Weight does not exceed 2000 kg. Flight is normally permitted in UK airspace only, by day, and in accordance with the Visual Flight Rules. Flying Training. Currently, 32 hours of flying training is required for the issue of a UK NPPL (A), of which 22 hours are to be dual instruction, and 10 hours to be supervised solo flying time. There are separate general and navigation skills tests. Theoretical Knowledge Examinations. The UK NPPL theoretical knowledge syllabus and ground examinations are the same as for the JAA PPL (A). This series of books, therefore, is also suitable for student pilots preparing for the UK NPPL. THE UNITED KINGDOM FLIGHT RADIOTELEPHONY OPERATOR’S LICENCE. Although there is a written paper on Radiotelephony Communications in the JAA PPL theoretical knowledge examinations, pilots in the United Kingdom, and in most other countries, who wish to operate airborne radio equipment will need to take a separate practical test for the award of a Flight Radiotelephony Operators Licence (FRTOL). For United Kingdom based students, full details of the FRTOL are contained in LASORS. viii Aircraft Technical Book Company http://www.actechbooks.com  FOREWORD NOTES ON CONTENT AND TEXT. Technical Content. The technical content of this OAA series of pilot training text books aims to reach the standard required by the theoretical knowledge syllabus of the JAA Private Pilot’s Licence (Aeroplanes), (JAA PPL(A)). This is the minimum standard that has been aimed at. The subject content of several of the volumes in the series exceeds PPL standard. However, all questions and their answers, as well as the margin notes, are aimed specifically at the JAA PPL (A) ground examinations. An indication of the technical level covered by each text book is given on the rear cover and in individual subject prefaces. The books deal predominantly with single piston-engine aeroplane operations. Questions and Answers. Questions appear at the end of each chapter in order that readers may test themselves on the individual subtopics of the main subject(s) covered by each book. The questions are of the same format as the questions asked in the JAA PPL (A) theoretical knowledge examinations, as administered by the UK CAA. All questions are multiple-choice, containing four answer options, one of which is the correct answer, with the remaining three options being incorrect ‘distracters’. Students Working for a Non-JAA PPL. JAA licence training syllabi follow the basic structure of ICAO-recommended training, so even if the national PPL you are working towards is not issued by a JAA member state, this series of text books should provide virtually all the training material you need. Theoretical knowledge examinations for the JAA PPL are, however, administered nationally, so there will always be country-specific aspects to JAA PPL examinations. ‘Air Law’ is the most obvious subject where country-specific content is likely to remain; the other subject is ‘Navigation’, where charts will most probably depict the terrain of the country concerned. As mentioned elsewhere in this Foreword, this series of books is also suitable for student pilots preparing for the United Kingdom National Private Pilot’s Licence (UK NPPL). The theoretical examination syllabus and examinations for the UK NPPL are currently identical to those for the JAA PPL. Student Helicopter Pilots. Of the seven book in this series, the following are suitable for student helicopters pilots working towards the JAA PPL (H), the UK NPPL (H) or the equivalent national licence: Volume 1: ‘Air Law & Operational Procedures’; Volume 2: ‘Human Performance’; Volume 3: ‘Navigation & Radio Aids’; Volume 4: ‘Meteorology’, and Volume 7: ‘Radiotelephony’. The OAAmedia Website. If any errors of content are identified in these books, or if there are any JAA PPL (A) theoretical knowledge syllabus changes, Oxford Aviation Academy’s aim is to record those changes on the product support pages of the OAAmedia website, at: www.oaamedia.com ix Aircraft Technical Book Company http://www.actechbooks.com FOREWORD Grammatical Note. It is standard grammatical convention in the English language, as well as in most other languages of Indo-European origin, that a single person of unspecified gender should be referred to by the appropriate form of the masculine singular pronoun, he, him, or his. This convention has been used throughout this series of books in order to avoid the pitfalls of usage that have crept into some modern works which contain frequent and distracting repetitions of he or she, him or her, etc, or where the ungrammatical use of they, and related pronouns, is resorted to. In accordance with the teachings of English grammar, the use, in this series of books, of a masculine pronoun to refer to a single person of unspecified gender does not imply that the person is of the male sex. Margin Notes. You will notice that margin notes appear on some pages in these books, identified by one of two icons: a key or a set of wings. The key icon identifies a note which the authors judge to be a key point in the understanding of a subject; the wings identify what the authors judge to be a point of airmanship. The UK Theoretical Knowledge Examination Papers. The UK CAA sets examination papers to test JAA PPL (A) theoretical knowledge either as single-subject papers or as papers in which two subjects are combined. Two examination papers currently cover two subjects each: Aircraft (General) & Principles of Flight: The ‘Aircraft (General) & Principles of Flight’ examination paper, as its title suggests, covers ‘Principles of Flight’ and those subjects which deal with the aeroplane as a machine, ‘Airframes’, ‘Engines’, ‘Propellers’ and ‘Instrumentation’, which JAR-FCL groups under the title ‘Aircraft General Knowledge’. Flight Performance & Planning: The examination paper entitled ‘Flight Performance & Planning’ covers both ‘Aeroplane Performance, and ‘Mass & Balance’. When preparing for the two examinations named above, using this Oxford series of text books, you will need Volume 5, ‘Principles of Flight’, which includes ‘Aeroplane Performance’, and Volume 6, ‘Aeroplanes’, which includes ‘Mass & Balance’ as well as ‘Airframes’, ‘Engines’, ‘Propellers’, and ‘Instrumentation’. So to prepare for the ‘Aircraft (General) & Principles of Flight’ examination, you need to take the ‘Aeroplanes’ infomation from Volume 6 and the ‘Principles of Flight’ information from Volume 5. When you are preparing for the ‘Flight Performance & Planning’ examination you need to take the ‘Aeroplane Performance’ information from Volume 5 and the ‘Mass & Balance’ information from Volume 6. It has been necessary to arrange the books in this way for reasons of space and subject logic. The titles of the rest of the volumes in the series correspond with the titles of the examinations. The situation is summed up for you in the table on the following page: x Aircraft Technical Book Company http://www.actechbooks.com  FOREWORD JAA Theoretical Examination Papers Corresponding Oxford Book Title Air Law and Operational Procedures Volume 1: Air Law Human Performance and Limitations Volume 2: Human Performance Navigation and Radio Aids Volume 3: Navigation Meteorology Volume 4: Meteorology Aircraft (General) and Principles of Flight Volume 5: Principles of Flight Volume 6: Aeroplanes Flight Performance and Planning Volume 5: Aeroplane Performance Volume 6: Mass and Balance JAR-FCL Communications (PPL) Volume 7: Radiotelephony Regulatory Changes. Finally, so that you may stay abreast of any changes in the flying and ground training requirements pertaining to pilot licences which may be introduced by your national aviation authority, be sure to consult, from time to time, the relevant publications issued by the authority. In the United Kingdom, the Civil Aviation Publication, LASORS, is worth looking at regularly. It is currently accessible, on-line, on the CAA website at www.caa.co.uk. Oxford, England June 2010 xi Aircraft Technical Book Company http://www.actechbooks.com  xii Aircraft Technical Book Company http://www.actechbooks.com TO THE PILOT. This book comprises two main sections: ‘Principles of Flight’ and ‘Aeroplane Performance’. For those who fly, a thorough knowledge of the Principles of Flight – also referred to as Aerodynamics - is essential if they are fully to appreciate the flight characteristics of their aircraft, and become safe and proficient pilots. One of the main aims of the Principles of Flight section of the book, therefore, is to provide both student and qualified pilots with study material which will enable them to learn as effectively and enjoyably as possible the scientific principles upon which flight itself depends, and to acquire an understanding of the nature of the physical forces in play when an aeroplane is being manoeuvred. Complementary to the study of the Principles of Flight, in terms of acquiring an understanding of the way an aeroplane flies, is the study of Aeroplane Performance, which is the subject of the second section of this book. The Subject of Aeroplane Performance deals with all the main phases of flight: take-off, climb, cruise, descent and landing. In Aeroplane Performance, it is not only aerodynamic factors but also engine power and thrust considerations, along with atmospheric conditions, which must be borne in mind in order to explain an aeroplane’s overall performance. As a pilot, you must at all times have a clear perspective on your aircraft’s capabilities. A knowledge of Aeroplane Performance is crucial to your understanding of the performance potential of your aircraft and, what is more important from the flight safety point of view, its performance limitations. It is essential for you to know how well, or how badly, your aircraft performs in the various phases of flight: what take-off distance it requires, how well it will climb, how far it will fly, how long it can remain airborne, and so on. While we have attempted, in this book, to relate the subjects of Principles of Flight and Aeroplane Performance to the practical aspects of piloting, it is a further primary aim of the book to prepare the student pilot for the PPL theoretical knowledge examinations. Appropriate emphasis, therefore, has been given to the pure theory of the two subjects which is demanded by the PPL ground examinations. The depth and scope of the treatment of both subjects are such that this book should also provide a sound introduction to the subjects of Principles of Flight and Aeroplane Performance for those students who are preparing for examinations at professional pilot level. Finally, be aware that, at PPL level, the United Kingdom Civil Aviation Authority (UK CAA) examines the subject of Principles of Flight along with the subject of Aircraft Systems in one paper, currently entitled Aircraft (General) and Principles of Flight, while Aeroplane Performance and Mass & Balance are examined by the UK CAA in a separate paper called Flight Performance & Planning. Volumes 5 and 6 of this series of books, entitled respectively ‘Principles of Flight’ and ‘Aeroplanes’, will prepare you for both examinations. xiii Aircraft Technical Book Company http://www.actechbooks.com  xiv Aircraft Technical Book Company http://www.actechbooks.com ComPonenT PArTs oF The AeroPLAne 1 Aircraft Technical Book Company http://www.actechbooks.com COMPONENT PARTS OF THE AEROPLANE 2 Aircraft Technical Book Company http://www.actechbooks.com  COMPONENT PARTS OF THE AEROPLANE THE PIPER PA28 WARRIOR. 3 Aircraft Technical Book Company http://www.actechbooks.com COMPONENT PARTS OF THE AEROPLANE THE MAULE M-7. 4 Aircraft Technical Book Company http://www.actechbooks.com  ChAPTer 1 PhYsICAL deFInITIons 5 Aircraft Technical Book Company http://www.actechbooks.com CHAPTER 1: PHYSICAL DEFINITIONS 6 Aircraft Technical Book Company http://www.actechbooks.com  CHAPTER 1: PHYSICAL DEFINITIONS PRINCIPLES OF FLIGHT AND THE LAWS OF NATURE. For our purposes, as pilots, Principles of Flight may be defined as the study of the forces generated by, and acting on, an aircraft in motion. Figure 1.1 Principles of Flight is a branch of applied science. Principles of Flight is, essentially, a branch of applied science. Though you will not need to have studied science to learn enough about Principles of Flight to pass the PPL theoretical knowledge examination, if you wish to master the subject of Principles of Flight thoroughly, you need to possess a basic knowledge of another applied science, the science of Physics. Physics is another name for natural science; that is, the science which explains the way matter and energy interact in nature. The Physics which governs the everyday phenomena and occurrences with which we are all familiar, including the phenomenon of flight, deals with such things as motion, mass, momentum, force, work, energy etc. If you have understood Physics or Combined Science at school or college, you will have enough knowledge of Physics to follow the subject matter of this book. But, in case you have never taken Physics at school, or do not remember much about the subject, we summarise in this chapter certain physical definitions and fundamental laws of Physics which are referred to in our treatment of the Principles of Flight. The definitions and laws are not listed alphabetically, but rather in logical groups of interconnected concepts. Bear these physical fundamentals in mind as you work through this book, and refer to this section, as and when you wish. If you would like to learn more about the physical fundamentals that we summarise here, any school text book on Physics should provide what you are looking for. Alternatively, you may wish to work through the self - teach, interactive CD-ROM, Essential Physics 2, conceived and produced by OAAmedia as pre-course study material for student professional pilots. Details of that material can be found on the OAAmedia website at www.oaamedia.com. Figure 1.2 Essential Physics 2 is a self-teach, interactive CD-ROM dealing with Forces, Motion, Energy and Astronomy. 7 Aircraft Technical Book Company http://www.actechbooks.com CHAPTER 1: PHYSICAL DEFINITIONS BASIC SCIENTIFIC DEFINITIONS AND LAWS. BODY: a 3-dimensional object, possessing mass. MASS: the amount of matter in a body. The standard unit of mass is the kilogram (kg). WEIGHT: Weight is the force acting on a body of a given mass by virtue of the presence of that body in a gravitational field. The standard unit of weight is the Newton (N). The force of weight acting on a body will impart an acceleration to the body towards the centre of the gravitational field. All bodies within the gravitational field of the Earth, near the Earth’s surface, will be accelerated towards the centre of the Earth at 9.81 metres per second per second (9.81 metres/sec2), a value of acceleration referred to by the symbol g and regarded as being constant up to an altitude above the surface of the Earth well above the greatest altitude at which aircraft fly. The relationship between weight, mass and g, the acceleration due to gravity, is given by the formula weight (N) = mass (kg) × g (9.81 metres/sec2) Weight is given in Newtons (N) if the mass is in kilograms and acceleration is in metres/sec2. The weight of a body always acts towards the centre of the gravitational field in which the body is located. On Earth, the weight of a body always acts vertically downwards towards the centre of the Earth. MOMENTUM: Momentum is the name given to the physical property possessed by a body by virtue of its mass and the velocity at which it is travelling. Momentum = mass × velocity. The units of momentum are kilogram-metres per second, which are simply the units of mass (kilograms) and velocity (metres per second) multiplied together. Momentum is often defined as a measure of how difficult it is to stop a moving object. A body which is stationary possesses no momentum, no matter how massive it is. Momentum is one of the most fundamental concepts in science. Momentum is related to another fundamental physical concept, force, by Newton’s Laws of Motion. Newton’s Laws of Motion teach us that any moving body will continue to move in a straight line with uniform velocity unless acted upon by a force (Newton’s 1st Law) and that the momentum of a body will, thus, remain constant unless the body is acted upon by a resultant force. When a resultant force acts on a body, over a given period of time, it causes a change in the momentum of the body. Newton’s 2nd Law teaches us that the rate of change of that momentum is proportional to the magnitude of the applied resultant force. FORCE: A force is a push or a pull. The force acting on a body is related to the momentum of that body by Newton’s Laws of Motion, as described above. The standard unit of force is called the Newton, in honour of the English physicist Sir Isaac Newton. Force has both direction and magnitude; force is, therefore, a vector quantity (see page 15). 8 Aircraft Technical Book Company http://www.actechbooks.com  CHAPTER 1: PHYSICAL DEFINITIONS FRICTION: Friction is a special kind of force which acts to stop two materials from sliding over each other. Between sandpaper and wood, friction is high, which is why the surface of both materials is worn away by friction. Conversely, there is so little friction between skates and ice that the skater can glide effortlessly across the ice. Between the various moving metal parts of a reciprocating internal combustion engine, the friction generated is so high that the engine would quickly seize and the moving parts fuse together, if a lubricant such as oil were not introduced between the moving parts to keep friction to a minimum. The magnitude of the force of friction acting between two surfaces is dependent on the nature of the two surfaces in contact with each other and the force which pushes the two surfaces together. VISCOSITY: Viscosity is a measure of the resistance of a fluid to deformation when a force is applied to it. Viscosity may also be thought of as the resistance of a fluid to flow. Viscosity is sometimes considered as being the thickness of a fluid, which again is an indication of its resistance to pouring. Viscosity may also be seen as a measure of fluid friction. It follows, then, that water and petrol have low viscosity, while treacle and heavy-duty oil have a relatively high viscosity. MOMENTS: The word moment is used to describe a turning force which acts on a body, causing the object to rotate about a pivot. In order for a moment to be present, a force must act at a distance from the pivot, and act perpendicularly to a straight line passing through the pivot. The magnitude of a moment is calculated by multiplying the magnitude of the force which tends to cause a body to turn by the perpendicular distance between the force and the pivot point about which the body tends to turn. While the scientific unit of a moment is the Newton-metre, in aircraft mass and balance calculations, you will mostly meet pound-inches and kilogram-metres. For a body to be in equilibrium, the sum of the moments acting on a body must be zero. That is that the sum of the moments acting in a clockwise direction must equal the sum of the moments acting on a body in an anti-clockwise direction. The diagram below depicts two moments acting in opposite directions on a beam. The two moments cancel each other out so the resultant moment is zero and the beam is in equilibrium. Figure 1.3 Two moments of 400 lb-ins balancing each other out. The beam is in equilibrium. 9 Aircraft Technical Book Company http://www.actechbooks.com CHAPTER 1: PHYSICAL DEFINITIONS TORQUE: In the study of engines and propellers, a moment or turning force is also sometimes called torque. Torque can be thought of as rotational force which causes a change in rotational motion. Torque, like a moment, is defined by the linear force causing a rotation about a pivot multiplied by the perpendicular distance from the pivot of the point at which the force is applied. The standard units of torque are Newton-metres, as for moments. You may also meet older units such as foot- pounds, pound-inches, kilogram-metres etc. Figure 1.4 A torque of 100 Newtons × 0.2 metres = 20 Newton-metres applied to a nut via a spanner. The application of torque to a mechanism will cause a rotational acceleration until the applied torque is balanced by an equal and opposite resistive torque from the mechanism, when the mechanism will continue to rotate with constant angular speed. COUPLES: A couple is a special case of a moment where two equal forces act along parallel lines but in opposite directions and cause a body to tend to rotate about a pivot point. If only one couple is acting on a body, the body will rotate, accelerating until an equal and opposite couple is generated, when the body will continue to rotate with constant angular speed. The magnitude of a couple is calculated by multiplying the magnitude of one of the forces which tends to cause a body to turn by the perpendicular distance Figure 1.5 between the two forces. A couple of 4 Newton-metres EQUILIBRIUM: Equilibrium is a Latin word meaning balance. In science, a body is said to be in equilibrium (in balance) when all the external forces acting on the body, and all the turning moments acting on the body cancel one another out. Another way of saying this is that a body is in equilibrium when the vector sum of all the forces and moments acting on the body is zero. When a body is in equilibrium, it must be 10 Aircraft Technical Book Company http://www.actechbooks.com  CHAPTER 1: PHYSICAL DEFINITIONS either at rest or moving in a straight line at constant speed (i.e. travelling at constant velocity). If a body is not in equilibrium, it will be subject to a resultant force and will be accelerating in some way. You may be able to get an idea of the principle of equilibrium from travelling in a car. If you are travelling in a straight line, at constant speed, you will feel exactly the same forces acting on your body as if you were at rest. At constant velocity, there is no resultant force acting on you (except your weight due to the force of gravity), and you are in equilibrium. If the car gathers speed rapidly, you experience acceleration; if the car brakes suddenly, you experience deceleration. (Deceleration is simply the name given to a negative acceleration.) In both cases, a resultant force must be acting; you are, therefore, not in equilibrium and you feel the resultant force either pressing you back into your seat, if acceleration is present, or causing you to move forwards, if the car is decelerating when braking sharply. PRESSURE: Pressure is basically an applied force spread over a contact area. If a small force is spread over a large area, the pressure is said to be low, whereas if the same small force acts over a very small area, then the pressure can be high. Pressure is defined as force per unit area. To find the magnitude of pressure, we divide the magnitude of the force, acting perpendicularly (normally) to a contact area, by the contact area, itself. Force acting normal to the surface Pressure = Contact area of surface A typical unit of pressure, which you might well expect to come out of the above formula, is the Newton/metre2. The SI (Système International) unit of pressure is the Pascal. One Pascal (Pa) is equal to One Newton/metre2. By international agreement, meteorologists use, as the preferred unit of pressure, the hectopascal (hPa) which is equal to 100 Pa or 100 Newtons/ metre2. The hPa is also the standard unit of pressure used in the European (JAA) aviation world. Sea-level pressure in the ICAO Standard Atmosphere (ISA) is 101 325 Newtons/ metre2 or 1013.2 hPa. Quantitively equivalent to the hectopascal (hPa) is the millibar (mb). In Britain, and many other countries outside Europe, the aviation world uses the millibar as the standard unit of pressure. So ISA sea-level pressure may also be expressed as 1013.2 mb. Pressure may also be expressed in other units such as pounds per square inch (lb/ in2), or even as the height of a column of Mercury (Hg) which may be supported by an applied pressure in the arm of a U-tube or manometer. In the United States of America, the world of aviation uses inches of Mercury to express pressure. ISA sea- level pressure in inches of Mercury is 29.92 ins Hg. 11 Aircraft Technical Book Company http://www.actechbooks.com CHAPTER 1: PHYSICAL DEFINITIONS STATIC PRESSURE: Static pressure is the term used in aviation to refer to the pressure sensed by a body by virtue of its being immersed in the Earth’s atmosphere. Of course, this type of pressure would be felt by a body immersed in any kind of fluid: liquid or gas. Static pressure acts on all bodies whether they are moving or stationary (i.e. static). The magnitude of the static pressure acting on a body depends on the density of the fluid in which the body is immersed and the body’s depth below the surface of the fluid. So, the static pressure experienced by a body in the Earth’s atmosphere is the pressure caused by the weight of air above the body pressing in on the body. The higher a body rises in the atmosphere, away from the Earth’s surface, the lower will be the static pressure. As we mention above, atmospheric static pressure, at sea-level, in the ICAO Standard Atmosphere, is 1013.2 hPa, 1013.2 mb, 29.92 in Hg, 101 325 Newtons/metre2 or 14.7 lbs/ in2. Static pressure is exerted in all directions over the entire area of the surface of any object immersed in a gas or liquid. DYNAMIC PRESSURE: If a body is moving through a gas or liquid, in addition to the static pressure acting on it, the body also experiences a pressure resulting from its motion. This additional pressure is called dynamic pressure. So, while a stationary body within a gas or liquid will experience static pressure, the frontal surfaces of a moving body will experience both static pressure and dynamic pressure. Dynamic pressure is exerted on an aircraft moving through air because the air is brought to rest on the cross sectional area of the frontal surfaces of the aircraft. In being brought to rest, the air applies a force on the object. If we divide that force by the frontal cross sectional area, we can measure the dynamic pressure being exerted. The total pressure experienced by the frontal surfaces of an aircraft in flight, therefore, is equal to dynamic pressure plus static pressure. Those parts of the aircraft which are not exposed to the airflow, and against which the air is not brought to rest, will experience only static pressure. WORK: When a force moves through a distance, work is done. The amount of work done is calculated by multiplying the force by the distance through which it moves. The standard unit of force is the Newton and the standard unit of distance is the metre; so if a 200 Newton force moves through 10 metres, 2 000 Newton-metres of work is done. One Newton-metre is called a Joule, in honour of the English Physicist James Prescott Joule, so 2 000 Newton-metres of work done may also be expressed as 2 000 Joules. If you wish to lift a child weighing 30 kg (scientifically speaking, of mass 30 kg) onto the seat of a chair ½ metre from the ground, on Earth, you first need to find the force pulling the child to the ground (his weight) and which you have to counter in order to lift him. On Earth, the relationship between mass in kilograms and force in Newtons is the acceleration due to gravity which is approximately 10 metres/sec2. A 30 kg child, therefore, weighs, in scientific units, 300 Newtons. So to lift the child onto the chair, you would have to do 150 Newton-metres or 150 Joules of work (300 N × ½ m). To lift the child twice as high, you would have to do twice the amount of work. 12 Aircraft Technical Book Company http://www.actechbooks.com  CHAPTER 1: PHYSICAL DEFINITIONS POWER: Power is defined as being the rate of doing work. Looking again at the simple case of a 30 kg child being lifted through ½ metre, if one person lifts the child slowly while another performs the same task quickly, both people are doing 150 Joules of work, but the person who performs that work quicker develops the greater power. High-power engines perform work at a higher rate than low-power engines. Thus, an aircraft, with engines of low power may transport a given load across the Atlantic in a given time. An aircraft with very powerful engines may transport the same load over the same distance in a shorter time. Both aircraft do the same amount of work, but the aircraft which performs that work at the higher rate is the more powerful aircraft of the two. ENERGY: Energy is not the most simple concept when analysed scientifically, but for our purposes we can describe it with a high degree of accuracy as being the ability to do work. Substances and phenomena such as gasoline (petrol), heat, movement, position, stretched or compressed springs, batteries etc have the ability to do work when they are part of a system designed to perform work. Although it is difficult to measure energy absolutely, it is not difficult to measure the physical effects of energy, such as heat and movement. Phenomena such as heat and movement can be used, therefore, to calculate an actual value for other forms of energy. Energy has the units Joules, the same units as can be used for work. A body or system which possesses 150 Joules of energy is able to do 150 Joules of work. (Note that whereas either Newton-metres or Joules can be used as the units of work, only Joules are used to measure energy.) One form of energy that we are especially concerned with in flying is kinetic energy. Kinetic energy is the energy possessed by a body by virtue of its movement. When a moving body is brought to a halt, work is done on the object stopping the moving body, by virtue of the moving body’s kinetic energy. In the case of a car colliding with a wall or other solid object, the deformation to the car’s structure is evidence of the work done in stopping the car. Obviously, an aircraft in flight possesses kinetic energy. The air moving over the aircraft’s structure, by virtue of its relative motion with the aircraft, also possesses kinetic energy. When moving air is brought to a stop in the pitot tube of an airspeed indicator system, the work done by the air can be used to set in motion a mechanism which indicates to the pilot how fast the aircraft is moving through the air. THE CONSERVATION OF ENERGY: The fact that the total energy in a closed system always remains constant gives us the law of the conservation of energy. The law of the conservation of energy teaches us that energy cannot be created or destroyed; it can only be converted from one form to another. The law of the conservation of energy is a natural law of immense importance, because it applies to almost every aspect of Physics. You may not be surprised to learn that many people are still unsure of the exact meaning of this law. For instance, you will often hear people talking about “burning energy”, or “using up energy”. Words of this kind are misleading because they imply that energy is being destroyed. 13 Aircraft Technical Book Company http://www.actechbooks.com CHAPTER 1: PHYSICAL DEFINITIONS But if energy cannot be created or destroyed, how does energy do work? The answer lies in the explanation that while the total energy of a body or system cannot be used up, one or more types of energy within a body or system can be converted to another type of energy. For example, the chemical energy contained in an aircraft’s fuel is converted to heat energy in the aircraft’s engine or engines. That heat energy is then converted to mechanical energy which turns a propeller. The propeller accelerates air rearwards generating kinetic energy; the temperature of the accelerated air will be raised by a measurable amount, adding to the air’s heat energy. This heat energy will be dissipated throughout the atmosphere, but in whatever closed system (e.g. the engine or the atmosphere) is being observed, the total energy remains the same. James Joule found that when work was done on an object the amount of heat absorbed by the object could be calculated, and that, for a given amount of mechanical work done, the heat absorbed by the object was always the same, thereby equating work done with energy transfer or conversion. MOTION – NEWTON’S LAWS: Every body which is in motion at speeds well below the speed of light (speed of light = 299 792 458 metres per second, or about 186 000 miles per second ) is subject to Newton’s Laws of Motion. Newton’s Laws illustrate the relationship between the motion of a body and the forces acting on the body. Newton’s Laws are a good basis for the explanation of the motion of everyday objects such as cricket balls, motor vehicles and aircraft, under the conditions normally experienced on Earth. Newton’s 1st Law of Motion: A body at rest remains at rest, and a body in motion continues to move in a straight line with a constant speed unless a resultant external force acts upon the body. In other words, an object which is at rest will not move until an external resultant force acts upon it, and an object which is in motion will move at constant velocity (i.e. without changing speed or direction) as long as no external resultant force acts upon it. Newton’s 2nd Law of Motion: The rate of change of momentum of a body is directly proportional to the applied external resultant force and takes place in the direction in which the force acts. Newton’s 3rd Law of Motion: To every action (applied external resultant force) there is an equal and opposite reaction (equal force in the opposite direction). Action and reaction take place on different bodies. In other words, if object A exerts a force on object B, then object B exerts a force of the same magnitude on object A, in the opposite direction. INERTIA: Inertia is that quality of a body which resists any change in velocity. Thus, a body’s inertia means that if the body is in a state of rest, it will tend to remain at rest, and if the body is travelling with uniform motion in a straight line, it will tend to continue at that constant velocity. This is, in fact, a state of affairs described by Newton’s 1st Law, (see above), which is sometimes known as the law of inertia. The amount of inertia possessed by a body is a function of the body’s mass. Thus, a body possessing great mass will have a greater inertia than a body of small mass. 14 Aircraft Technical Book Company http://www.actechbooks.com  CHAPTER 1: PHYSICAL DEFINITIONS SCALAR QUANTITIES: A scalar quantity is a quantity, usually expressed in terms of a number with units, which possesses magnitude ( i.e. size) only. Scalar quantities may be compared with vector quantities which have both magnitude and direction; (see below). Mass is a scalar quantity. When expressing the mass of a body, in kilograms or pounds, for instance, we are describing magnitude only; mass is the amount of matter possessed by a body and cannot be defined in terms of direction. Speed is another scalar quantity (measured in units such as metres per second, miles per hour, knots etc). When we talk about the speed of an aircraft, we describe only how fast or slowly it is moving. Although the aircraft will necessarily be moving in a certain direction, that direction is not defined by the scalar quantity: speed. VECTOR QUANTITIES: In comparison to a scalar quantity which gives us information on magnitude only, a vector quantity contains information on both magnitude and direction. In science, force and velocity are vector quantities. Vector quantities are often represented graphically by arrows. If, for instance, we wish to represent a force acting in a particular direction, an arrow is drawn to a scale such that the length of the arrow indicates the magnitude of the force (a 40 Newton force would be represented by an arrow twice as long as an arrow representing a 20 Newton force) and the orientation of the arrow indicates the direction in which the force is acting. In a similar manner, when representing the velocity of a body by an arrow (for instance, the velocity of an aircraft in a navigation problem), the length of the arrow indicates the magnitude of the velocity (i.e. the aircraft’s speed, either through the air or over the ground) and the orientation of the arrow (e.g. North, North-West, South-East etc) indicates the direction of flight of the aircraft. The arrows, themselves, because they indicate magnitude and direction, are often referred to as vectors. SPEED: See Scalar Quantities, above. When speed is mentioned, in science, it refers only to how fast or how slowly a body is moving; no information is given about direction. In aviation studies, and particularly in navigation, a difference is made between aircraft speeds such as Figure 1.6 Three vectors in a triangle True Airspeed (the actual speed of the of velocities, indicating magnitude and direction: Wind Velocity (W/V) , an aircraft’s aircraft relative to the air), its Indicated Heading and True Airspeed (HDG/TAS) Airspeed (the speed read from the and the aircrafts Track over the ground and aircraft’s Airspeed Indicator), and Groundspeed (TR/GS). Ground Speed (the speed at which the aircraft moves over the Earth’s surface). These speeds are explained in the relevant chapters of this book. VELOCITY: See Vector Quantities, above. In science and navigation, when considering the velocity of a body, velocity is expressed in terms of both magnitude and direction. For instance, when planning a cross-country flight, a pilot will define 15 Aircraft Technical Book Company http://www.actechbooks.com CHAPTER 1: PHYSICAL DEFINITIONS the expected progress of his aircraft in terms of speed and direction. He will, for instance, plan to follow a track defined by its bearing measured relative to True or Magnetic North, say 045 degrees (North-East), and plan to fly at a given indicated airspeed, say 110 knots. In defining the progress of his aircraft in this way, the pilot has defined the velocity of his aircraft. Be aware that, in everyday language, you will often hear the word velocity used to express magnitude only (i.e. speed). Everyday words often differ in detail and accuracy from words used technically and scientifically. The important thing for you, as a pilot, is that you appreciate and understand these differences. ACCELERATION: Acceleration is a vector quantity which defines rate of change in velocity. In everyday language, the word acceleration is used to mean a rate of increase in velocity, but, in science, it can mean either rate of increase in velocity or a rate of decrease in velocity. A rate of decrease in velocity is a negative acceleration and is commonly referred to as a deceleration. CIRCULAR MOTION: Circular motion is motion following a circular path. The important thing to grasp about circular motion is that although a body, such as an aircraft in turning flight, may be moving around the circumference of a circle at a constant linear speed (for an aircraft, say, at 70 knots in a turn) the velocity of the body is continually changing because the direction of motion is continually changing. As velocity is continually changing, a body moving in a circle is also, by definition, continually accelerating. The direction of this acceleration (called centripetal acceleration) is towards the centre of the circle. If the centripetal acceleration were not present, the body would fly off in a straight line in accordance with Newton’s 1st Law. Circular motion can have as its units either linear units such as metres per second or miles per hour, which describe how fast or slow a body is moving around the circumferential path, or may be described using units such as degrees per second or radians per second which define change of angular displacement. When dealing with engines and propellers, common units are revolutions per minute (rpm). CONVENTIONS USED IN EQUATIONS: Although this book on Principles of Flight does not contain much mathematics, a few mathematical and scientific equations necessarily appear in a subject of this nature. When equations are included, certain symbols are used such as ρ for air density, CL for coefficient of lift, S for wing area, v for velocity, and so on. Be aware that when symbols are written together such as: Lift = CL½ρv2S, the symbols CL½ρv2S are considered as being multiplied together. So, CL½ρv2S means exactly the same as CL × ½ × ρ × v2 × S So, if letters in an equation are placed next to one another in this way: Lift = CL½ρv2S, their meaning is exactly the same as if multiplication signs were placed between them. 16 Aircraft Technical Book Company http://www.actechbooks.com  ChAPTer 2 The ATmosPhere 17 Aircraft Technical Book Company http://www.actechbooks.com CHAPTER 2: THE ATMOSPHERE 18 Aircraft Technical Book Company http://www.actechbooks.com  CHAPTER 2: THE ATMOSPHERE THE ATMOSPHERE. Aircraft. All aircraft, by the very definition of the word, can fly only when immersed in the air. Lighter-than-air craft such as hot-air balloons are called aerostats, while heavier- than-air craft which require relative movement between the air and their lifting surfaces are called aerodynes. Of the aerodynes, fixed-wing craft are called generically aeroplanes. In the word aeroplane, the word-element plane refers to the mainplanes, more commonly known as wings, and the tail-plane, which the Americans often refer to as the horizontal stabiliser. The fin of an aeroplane is a plane, too; the Americans call it a vertical stabiliser. So, you see, plane has a particular technical meaning when referring to aircraft, but the word plane certainly does not refer to the complete aircraft. The complete aircraft may, though, be called an aeroplane. In everyday speech you will often hear people talking about “passenger planes” and “military planes”. But because you are a pilot, you might choose to use the more correct words; it is a personal choice. Rotary-wing craft are known collectively as helicopters, from the Greek pteron meaning wing and the Greek heliko - from helix, meaning spiral. (Leonardo da Vinci’s unsuccessful late-15th century design for a vertical take-off flying machine featured a rotating spiral wing). In this book on Principles of Flight we shall be considering the flight of aeroplanes only. Throughout the book, the words aeroplane and aircraft will be used synonymously. The Composition of the Atmosphere. The Principal Gases. As it is the relative movement of aeroplanes and air which generates the aerodynamic forces which enable an aircraft to fly, we may logically begin our study of the Principles of Flight by examining the nature of the Earth’s atmosphere. The gaseous atmosphere which surrounds our Earth is similar to a giant ocean of air. The light aircraft flown by most private-pilot licence-holders operate in the lower 10 000 feet of the atmosphere, whereas jet airliners regularly fly at altitudes up to about 40 000 feet. The total depth of the atmosphere has been calculated to be about 500 miles (800 km), but about 90% of the mass of air lies in the lower 50 000 feet (9 miles or 15 km) of the atmosphere. 19 Aircraft Technical Book Company http://www.actechbooks.com  CHAPTER 2: THE ATMOSPHERE The air in our atmosphere is made up primarily of Nitrogen (78%) and Oxygen (21%). (See Figure 2.1) The remaining The air in the 1% consists mainly of Argon and atmosphere is made up Carbon Dioxide, with traces of Carbon primarily of Monoxide, Helium, Methane, Hydrogen Nitrogen (78%) and Oxygen and Ozone. It is this mixture of gases (21%). The remaining 1% is which not only enables an aeroplane mainly Carbon Dioxide and to fly but which also makes up the air Argon. which sustains human life and enables the combustion of fuel to take place to drive piston engines and gas turbine engines. Water Vapour and Humidity. Atmospheric air also contains a small amount of water vapour of varying Figure 2.1 The four main gases which make quantity. The measure of the amount of up the Atmosphere. water vapour contained in an air mass is termed humidity. Meteorologists measure humidity in several ways: for example, mass of water vapour per unit volume of air (say, 5 gm/m³), or mass of water vapour per unit mass of air (say, 3 gm/kg). As we have said, atmospheric air contains very little water vapour (it is never more than 4% by volume), but the influence of this water vapour is significant, especially on our weather. Despite the presence of water vapour in the air, the air is normally invisible because water vapour can exist in the air as an invisible gas. The higher the temperature of the air, the more water the air can hold in its gaseous form. As temperature decreases, the air can hold progressively less water vapour, and eventually water condenses out onto microscopic impurities (hygroscopic nuclei) in the air, or onto surfaces in contact with the air. This is why you can see your breath on a cold day, why breathing onto a cold glass surface will cause the glass to mist up, and, of course, it is the reason why clouds form. When air When air can no longer hold any more water vapour as gas, the air is said to have cools to its reached its saturation point. The temperature of air at its saturation point, that is, saturation the air temperature at which water vapour condenses out to water, is called the dew point, or dew point. The more water vapour there is in the air, the higher the dew point will be. And, point, the invisible gas water vapour condenses out to its liquid state, and cloud is formed. Figure 2.2 Air is normally invisible but condensed water vapour in the air is visible as cloud. 20 Aircraft Technical Book Company http://www.actechbooks.com  CHAPTER 2: THE ATMOSPHERE of course, on the Earth’s surface, at an airfield for instance, for any given content of Relative water vapour, the nearer the dew point is to the actual temperature of the air, the humidity is greater the danger that the saturation point will be reached, causing condensation to the ratio of occur and mist or fog to ensue. the amount of water vapour present in the air Even unsaturated air, as it rises in the atmosphere, under whatever influence, will to the amount of water vapour cool, and its temperature decreases sufficiently for the rising air eventually to reach the air can hold at the same its saturation point. At the saturation point, cloud is formed. Mist and fog, of course, temperature. are just cases of low level cloud. If the visibility is less than 1000 m, the condition is termed fog, and if 1000 m or more, mist. When temperature You will often hear the term relative humidity used in aviation circles. Relative humidity falls to the is an expression of the ratio of the amount of water vapour actually present in the air dewpoint, to the amount of water vapour the air can “hold” at any given temperature. When the relative humidity is 100%, and temperature of the air falls to the dew point, relative humidity will become 100%. The water vapour condenses out air will then be saturated and the water vapour will condense out, changing from the into its liquid state. gaseous to the liquid state. Water vapour is lighter than the same volume of dry air at equal pressure. Therefore, If the water for a given temperature and pressure, a mixture of air and water vapour will be less vapour content dense if the water vapour content is high than if the water vapour content is low (see of air increases, Figure 2.3). air density will decrease at constant temperature and pressure. Figure 2.3 Moist air is less dense than dry air. Air Pressure and Air Density. The atmosphere was first formed when its gases were released from the Earth during the Earth’s formation, over 4 billion years ago. The gases which now make up the air of our atmosphere were prevented from escaping into space by the Earth’s force of gravity, so, over an unknown period of time, molecules of air spread out to cover the entire surface of our planet. The gravitational force acting between objects is proportional to the mass of those objects, but gets weaker as the distance between the objects increases, so many more air molecules are held in contact with the Earth’s surface than are present in the higher reaches of the atmosphere. This fact and the fact that the air near the surface is compressed by the weight of the mass of air above it mean that air pressure and air density are greatest near the surface of the Earth, and decrease with increasing altitude. 21 Aircraft Technical Book Company http://www.actechbooks.com  CHAPTER 2: THE ATMOSPHERE Air density A useful analogy of the variation of and air pressure and density with altitude is pressure to consider foam rubber blocks piled decrease with on top of one another. If we consider increasing altitude. any one of the blocks, we can see, in our mind’s eye, that it is compressed by an amount which is proportional to At constant the number and weight of blocks above temperature, it, and that the maximum compression an increase in is experienced by the block at the air pressure very bottom of the pile. Similarly, air causes an increase in air pressure and air density are highest at density. the Earth’s surface. (See Figure 2.4). At constant Air density refers to the number of air pressure, an molecules contained within a given increase in air volume of air and is measured in terms Figure 2.4 Air Density and Pressure decrease of mass per unit volume. The standard with increasing altitude. temperature will cause a decrease in air units of air density are kilograms per density. cubic metre. The greater the pressure acting on a given volume of air, the greater the number of air molecules that are contained within that volume. Consequently, air density is directly proportional to pressure. When a given mass of air is heated at constant pressure it expands and its volume increases. Because of this increase in volume, the molecules of air are contained within a larger space and, thus, the mass per unit volume of the air – that is the density of the air – decreases. Air density, then, is inversely proportional to temperature; that is, it decreases with increasing temperature. In general, both engine and flight performance decrease with decreasing air density which is why pilots need to be especially careful in their performance calculations when operating from airfields which are “hot and high”: on the continent of Africa, for example. Both pressure and temperature decrease with increasing altitude. But although a decrease in pressure will cause density to decrease while a decrease in temperature causes density to increase, the effect of the decreasing pressure on air density is the greater. Air density is of considerable importance for the measurement of aircraft performance. Lift, service ceiling, and the relationship between true and indicated airspeed all depend on air density. If air density is low, not only will the lift generated by the wings be less for any given true airspeed, but the power output of the engine will be lower too. Consequently, in low density conditions at an airfield (e.g. high temperature, and high airfield elevation), longer take-off runs will be required for an aircraft of any given take-off mass. Pressure is a description of the way in which a force is spread over a contact area. Pressure is defined as “force per unit area”. In Principles of Flight, the pressure exerted by the atmosphere on objects immersed in it, when neither the air nor the object is in motion, is known as atmospheric pressure or static pressure. The standard unit of pressure is the Newton per square metre, but, in Principles of Flight you will rarely, if ever, hear pressure expressed in those units. In Britain and, especially, the United States, you might still hear pressure expressed, generally, in pounds per square 22 Aircraft Technical Book Company http://www.actechbooks.com  CHAPTER 2: THE ATMOSPHERE inch. In engineering, the bar or millibar is often used, as is the Pascal or hectopascal. The millibar and the hectopascal are also used in Meteorology and Altimetry. In the United States, inches of mercury are the units of pressure in Altimetry. Like air density, atmospheric pressure (static pressure) decreases with increasing altitude. Though the pressure exerted by the atmosphere at the Earth’s surface varies from day to day for reasons you will learn about in Meteorology, at sea level, atmospheric pressure is in the order of 100 000 Newtons per square metre, 1 bar, 1 000 millibars, 1 000 hectopascal, 14.7 pounds per square inch or 30 inches of Mercury. The pressure of the atmosphere acts in all directions, acting on every square inch of every object immersed in it. (See Figure 2.5) For instance, a 6 foot (1.83 metres) human being on the surface of the Earth carries a total load of over ten tons. This is, of course, the equivalent of a force of 14.7 lbs (6.7 kg force) acting on every square inch of his body surface. But despite the fact that atmospheric pressure on the surface of the Earth is very high, we do not notice the pressure, because the pressure inside our own bodies balances this atmospheric pressure. But when the pressure inside any hollow object is less than atmospheric pressure, the difference in pressure can be withstood only by the strength of the object’s structure. You have probably all witnessed during school Physics lessons that an empty tin can will collapse if the air inside it is removed. Atmospheric pressure, also known as static pressure, acts in all directions on a body immersed in the atmosphere. Figure 2.5 Atmospheric pressure acts in all directions. We see then that air possess mass, and that the force of gravity acting on that mass gives air weight which is the ultimate reason why our atmosphere exerts a pressure on objects immersed in it, and why the pressure and the density of the air decrease with altitude. As you will learn in subsequent chapters, these are the properties of air which enable aeroplanes to fly. Variations in atmospheric pressure and density, along with variations in humidity, have a significant influence on aircraft performance and on the functioning of flight instruments, as you will learn in the Aeroplanes (General) volume of this series. 23 Aircraft Technical Book Company http://www.actechbooks.com CHAPTER 2: THE ATMOSPHERE The Temperature of the Atmosphere. The temperature of atmospheric air, like air density and atmospheric pressure, also decreases with increasing altitude. The air is not heated directly by the sun. The sun’s short wave radiation passes through the atmosphere without heat being absorbed by the air. The Earth’s surface, however, is heated up by solar radiation, and it is the Earth which heats up the air in contact with, and near, its surface by conduction, convection and long-wave radiation. Not surprisingly, then, it is the lowest layer of atmospheric air which is heated through its proximity to the Earth’s surface and it is in that lowest layer where a clear and steady decrease in temperature with increasing altitude occurs. (See Figure 2.7.) The lowest layer of the atmosphere is known as the Troposphere, from the Greek word tropos meaning mixing or turning, which undoubtedly refers to the fact that it is in the Troposphere that temperature and pressure changes cause the meeting and mixing of air which gives rise to our weather. Almost all of the Earth’s weather occurs in the Troposphere, so if you find yourself flying in an airliner on a European route, at 38 000 feet, you are indeed, in all probability, flying above the weather. The Troposphere rises from the Earth’s surface to about 50 000 feet over the Equator, 25 000 feet over the Poles, and about 36 000 feet at mid-latitudes. The Troposphere contains approximately 75% of the total mass of the atmosphere, and all of the water vapour. Figure 2.6 The various layers of the atmosphere with approximate heights in kilometres, one kilometre being 3 281 feet. The boundary between the Troposphere and the layer immediately above it, the Stratosphere, is called the Tropopause. At the Tropopause, the temperature is around - 56.5º Celsius ( - 69º Fahrenheit), and this temperature remains constant to an altitude of about 18 miles, or 35 kilometres. At altitudes greater than that, the temperature begins to rise again. But 18 miles high is 95 000 feet, so we will end our account of temperature variation with altitude there, and leave these higher regions to astronauts. The Atmosphere and Flight. The important facts to retain about the physical properties of the atmosphere in terms of your study of the Principles of Flight, is that air has mass, and that the pressure, density, temperature and relative humidity of the air change in certain circumstances. 24 Aircraft Technical Book Company http://www.actechbooks.com  CHAPTER 2: THE ATMOSPHERE Another property of the air which is important in Principles of Flight is its viscosity. The viscosity of air is a measure of its resistance to flow because of a kind of internal friction acting between the air molecules as they move relative to one another. The viscosity of a fluid is often being described when we talk of a fluid being thick or thin. Air and water have a low viscosity and might be described as thin, whereas treacle and tar have a high viscosity and are seen as thick fluids. The viscosity of air, then, is low, but air does possess a measurable degree of viscosity and this viscosity has consequences for an aircraft in flight. The ICAO Standard Atmosphere (ISA). Changes of air pressure, air density, air temperature and humidity within the atmosphere greatly affect the performance of an aircraft in flight as well as the readings of certain flight instruments. In the real atmosphere, of course, these properties are Changes of changing continuously with altitude, with passing time and from place to place. In pressure, order, therefore, that aerodynamicists, aircraft manufacturers and engineers might density, and have a set of standard values for pressure, density temperature etc, against which humidity of to measure aircraft performance and to calibrate instruments, a so-called standard the air, all affect aircraft atmosphere was defined by the International Civil Aviation Organisation (ICAO) in performance. 1964. The ICAO Standard Atmosphere, generally known by its initials ISA, shows a standard variation of pressure, temperature, density, and viscosity, with altitude. ISA, then, serves as an international standard reference so that, when dealing with the measurement of aircraft performance and the calibration of instruments, everyone can be sure that they are working to the same set of atmospheric conditions. The ICAO Standard Atmosphere, with its significant values for the variation of Density, and temperature, pressure and density with altitude, is illustrated at Figure 2.7. Mean pressure Sea Level air pressure in the ICAO Standard Atmosphere (which we will, henceforth decrease with in this volume, refer to as ISA) is 1013.2 millibars (1013.2 hectopascals) or 29.92 altitude, and inches of Mercury. The ISA temperature at Mean Sea Level is 15º Celsius. In the temperature decreases with ISA, temperature decreases with altitude at approximately 2º Celsius for every altitude up to the tropopause. 1 000 ft. Figure 2.7 The ICAO Standard Atmosphere. Any values for atmospheric pressure, density and temperature given in this volume will be ISA values. It is important that you remember, though, that the actual values for atmospheric pressure, density and temperature which prevail on any given day 25 Aircraft Technical Book Company http://www.actechbooks.com CHAPTER 2: THE ATMOSPHERE are inevitably different from the ISA values. (It would be in the order of a million to one chance that all actual values were the same as ISA values.) Consequently, as the calibrations of flight instruments, such as the altimeter and the air speed indicator, as well as manufacturers’ figures for aircraft performance, assume that an aircraft is flying in ISA conditions, it is very important, when reading instruments and measuring aircraft performance, on an actual flight, that pilots and engineers understand the effect that the atmosphere’s deviation from ISA conditions has on the information they are reading. The topic of ISA Deviations is dealt with in detail in the Meteorology and Aircraft (General) volumes in this series. The ISA sea level pressure of 1013.2 millibars is also the altimeter subscale setting, which a pilot selects when reading his altitude in terms of Flight Level. Flight Levels are also known as Pressure Altitudes. The Measurement of Temperature. Before we leave our brief look at the atmosphere, there is one more observation that needs to be made on the measurement of temperature. The standard unit of measurement of temperature in the aviation world, outside the United States, is degrees Celsius (formerly Centigrade). However, the Fahrenheit scale was the primary scale of temperature measurement for non-scientific purposes in most English-speaking countries until the 1960s. Consequently, you will meet degrees Fahrenheit frequently in the United States, and still occasionally in Britain. So, although aviation meteorological reports and forecasts mention temperature in degress Celsius, it is still useful to be able to convert between the two scales. In degrees Fahrenheit, water freezes at 32ºF and boils at 212ºF; in degrees Celsius, 0ºC degrees is the freezing point of water, and 100ºC its boiling point. So in the Fahrenheit scale there are 180º between the boiling points of water, whereas, in the Celsius scale, there are, of course, 100º. Therefore, one Fahrenheit degree is only 5/9 the value of a Celsius degree, (100/180 = 5/9). The formulae for converting from one scale to the other are: Conversion from To Formula Fahrenheit Celsius ºC = (ºF - 32) × 5/9 Celsius Fahrenheit ºF = (ºC × 9/5) + 32 26 Aircraft Technical Book Company http://www.actechbooks.com  CHAPTER 2: THE ATMOSPHERE QUESTIONS Representative PPL - type questions to test your theoretical knowledge of The Atmosphere. 1. Density: a. reduces as altitude increases b. is unaffected by temperature change c. increases with altitude increase d. reduces with temperature reduction 2. The presence of water vapour: a. in air will increase its density b. in the atmosphere will increase the power output of a piston engine c. in the atmosphere will increase the amount of lift generated by an aircraft for a given true airspeed d. in air will reduce its density 3. Atmospheric pressure: a. acts only vertically downwards b. is measured in Pascals per square inch c. acts in all directions d. increases with altitude 4. The air pressure that acts on anything immersed in it: a. is also known as Dynamic Pressure b. is also known as Static Pressure c. is greater at altitude than at sea level d. is also known as Total Pressure 5. What properties of the Earth’s atmosphere most influence the performance of aircraft? a. Its carbon dioxide content, temperature, pressure and humidity b. Its oxygen content, pressure, and water vapour content c. Its water vapour content, temperature, pressure and density d. Its nitrogen content, oxygen content, temperature and pressure 6. A piston engine aircraft flies in that layer of the atmosphere called: a. the Stratosphere b. the Troposphere c. the Mesosphere d. the Tropopause 27 Aircraft Technical Book Company http://www.actechbooks.com CHAPTER 2: THE ATMOSPHERE QUESTIONS 7. The respective percentages of the four most abundant gases that make up the atmosphere are? a. Nitrogen 78% Oxygen 21% Argon 0.95% Carbon Dioxide 0.05% b. Oxygen 78% Nitrogen 21% Argon 0.95% Carbon Dioxide 0.05% c. Nitrogen 78% Oxygen 21% Argon 0.95% Carbon Monoxide 0.05% d. Oxygen 78% Nitrogen 21% Argon 0.95% Carbon Monoxide 0.05% 8. When considering the changes in density of the air with altitude, which of the following four options is correct? a. The temperature increase with increasing altitude causes density to increase b. The reduction in pressure with increasing altitude causes density to reduce c. The temperature reduction with increasing altitude causes density to increase d. The increase in pressure with increasing altitude causes density to reduce 9. Assuming that the pressure at sea level is ISA, but the temperature is 10°C higher than ISA, the density will be: a. as per ISA b. greater than ISA c. less than ISA d. unaffected 10. Which of the following options contains the main constituent gases of the Earth’s atmosphere? a. Hydrogen, Carbon Dioxide and Helium b. Nitrogen, Oxygen and Water Vapour c. Nitrogen, Argon and Carbon Dioxide d. Helium, Nitrogen and Carbon Monoxide 11. Complete the following sentence to give the most correct statement. At constant air temperature and volume, if the pressure of the air increases: a. its density will decrease b. its density will be unaffected because the volume remains constant c. its density will be unaffected because the temperature remains constant d. its density will increase 28 Aircraft Technical Book Company http://www.actechbooks.com  CHAPTER 2: THE ATMOSPHERE QUESTIONS 12. What is the definition of Relative Humidity? a. The amount of water vapour present in a mass of air, at any temperature, expressed as a percentage of the maximum amount of water vapour that the air could support at the ISA sea-level temperature b. The amount of water vapour present in a mass of air relative to the density of air c. The amount of water vapour present in a mass of air expressed as a percentage of the maximum amount of water vapour that the air can support at the same tempera