AME 108 Student Resource PDF

Student Resource Subject B-8: Aerodynamics Copyright © 2016 Aviation Australia All rights reserved. No part of this document may be reproduced, transferred, sold, or otherwise disposed of, without the written permission of Aviation Australia. 2016-09-05 B-8 Aerodynamics Page 2 of 8 Training Material Only CONTENTS Study Resources 3 Learning Outcomes 5 Physics of the Atmosphere B-8.1 Aerodynamics B-8.2 Theory of Flight B-8.3 Flight Stability and Dynamics B-8.4 2016-09-05 B-8 Aerodynamics Page 3 of 8 Training Material Only STUDY RESOURCES Jeppesen A&P Technician General Textbook Jeppesen A&P Technician Airframe Textbook Aircraft Engineering Principles – Dingle & Tooley. Mechanics of Flight – A. C. Kermode B-8 Student Resource 2016-09-05 B-8 Aerodynamics Page 4 of 8 Training Material Only DEFINITIONS Define To describe the nature or basic qualities of. To state the precise meaning of (a word or sense of a word). State Specify in words or writing. To set forth in words; declare. Identify To establish the identity of. List Itemise. Describe Represent in words enabling hearer or reader to form an idea of an object or process. To tell the facts, details, or particulars of something verbally or in writing. Explain Make known in detail. Offer reason for cause and effect. 2016-09-05 B-8 Aerodynamics Page 5 of 8 Training Material Only INTRODUCTION The purpose of this subject is to familiarise you with basic aerodynamics and the theory of flight. It also covers flight controls and conditions which affect the aerodynamics of aircraft. On completion of the following topics you will be able to: Topic 8.1 Physics of the Atmosphere Describe the application of the International Standard Atmosphere (ISA) to aerodynamics. Describe the following characteristics associated with the atmosphere: Composition, Pressure and temperature distribution effects of altitude and Effects of humidity, temperature and pressure on density. Topic 8.2 Aerodynamics Describe airflow around a body in relation to the following terms: Boundary layer Laminar and turbulent flow Free stream flow Relative airflow Upwash and downwash Vortices and stagnation Describe the following terms and list their interaction with related forces: Camber Chord Mean Aerodynamic Chord (MAC) Profile (Parasite) Drag Induced Drag Centre of Pressure Angle of Attack Wash In and Wash Out Fineness Ratio Wing Shape and Aspect Ratio Describe the relationship between thrust, weight and aerodynamic resultant. 2016-09-05 B-8 Aerodynamics Page 6 of 8 Training Material Only Describe how lift and drag are generated and define the following associated terms: Angle of Attack Lift Coefficient Drag Coefficient Polar Curve Stall Describe aerofoil contamination including ice, snow and frost. Describe the relationships between: Ground speed (GS) True air speed (TAS) Indicated air speed (IAS) Topic 8.3 Theory of Flight Describe the relationship between lift, weight, thrust and drag. Describe glide ratio. Describe steady state flight and define performance. Describe the theory of the turn. Describe load factor and its influence on stalling, flight envelope and structural limitations. Describe methods of lift augmentation. 2016-09-05 B-8 Aerodynamics Page 7 of 8 Training Material Only Topic 8.4 Flight Stability and Dynamics Describe the following types of flight stability (active and passive): Longitudinal Lateral Directional In relation to longitudinal, lateral, and directional stability, be able to state: The axis about which they apply The aircraft structural features that provide stability about that axis. Describe flight stability including: Anhedral Dihedral Asymmetric power Dynamic stability Longitudinal dihedral Static stability Torque effect Ground effect 2016-09-05 B-8 Aerodynamics Page 8 of 8 Training Material Only TOPIC 8.1: PHYSICS OF THE ATMOSPHERE Table of Contents TOPIC 8.1: PHYSICS OF THE ATMOSPHERE.......................................................................................... 1 Introduction......................................................................................................................................... 2 Composition of the Atmosphere...................................................................................................... 2 The Physical Composition of the Atmosphere............................................................................. 3 The Pressure of the Atmosphere.................................................................................................. 4 Temperature Changes in the Atmosphere................................................................................... 4 Troposphere.................................................................................................................................. 6 Tropopause................................................................................................................................... 6 Stratosphere................................................................................................................................. 6 Air Density........................................................................................................................................ 8 Air Density with Altitude Changes................................................................................................ 8 Water Vapour............................................................................................................................... 9 Humidity..................................................................................................................................... 10 Dew Point.................................................................................................................................... 11 International Standard Atmosphere (ISA)...................................................................................... 12 ISA Standard Conditions............................................................................................................. 12 Pressure Altitude........................................................................................................................ 13 2016-05-25 B-8.1 Physics of the Atmosphere Page 1 of 14 Training Material Only INTRODUCTION The atmosphere is the life giving substance which surrounds our planet earth. We rely on it to provide adequate gases to sustain life and a climate which is suitable for us to perform our everyday activities. Most of the atmosphere exists within a height of 10 km above the earth, and it is within this region that all weather and climatic conditions are generated. This topic will discuss in relation to the atmosphere: Composition; Pressure and temperature distribution effects of altitude; Effect of humidity and pressure on density; ISA standard conditions. Composition of the Atmosphere The atmosphere is a complex and ever changing mixture, commonly called air. The air is a mixture of gases, but also contains quantities of foreign matter, such as pollen, dust, bacteria, soot, volcanic ash and dust from outer space. The proportions of gases in the atmosphere are shown below in Figure 1. The remaining 0.003% is made up of microscopic quantities of other gases such as neon, helium, krypton, ozone, etc. Figure 1: Gas composition graph The nature of the atmosphere may vary considerably from day to day at any given place, and may also vary from place to place at any given time. Because of these variations and because aircraft move from one place to another quickly, they continually experience changes in the air in which they fly. The characteristics of the atmosphere have important effects on the operation and maintenance of aircraft. Aircraft performance and forces such as lift, drag, and engine power are affected by changes in densities which result from variations in atmospheric pressure, temperature or humidity. 2016-05-25 B-8.1 Physics of the Atmosphere Page 2 of 14 Training Material Only Many maintenance operations are also affected by atmospheric conditions: Ground running of engines; Adjusting components; Adjusting and monitoring instruments; Applying surface finishes, i.e. paint. The Physical Composition of the Atmosphere The atmosphere is classified into regions based on the variation of temperature with altitude. These regions are: Troposphere; Tropopause; Stratosphere; Mesosphere; Thermosphere (Ionosphere). Aircraft fly only in the Troposphere and the lowest part of the Stratosphere. Civil aircraft would rarely exceed altitudes of 45,000 feet (nearly 14 km). Figure 2: Physical Composition of the Atmosphere 2016-05-25 B-8.1 Physics of the Atmosphere Page 3 of 14 Training Material Only The Pressure of the Atmosphere The weight of air above any surface causes pressure at that surface. The average pressure at sea- level due to the weight of the atmosphere is about 14.7 psi (1013.25 hPa [hectopascals]). This pressure is referred to as ‘one atmosphere’. The higher we ascend in the atmosphere, the less will be the weight above us. Therefore, the pressure will be less. Figure 3: Pressure of the atmosphere Temperature Changes in the Atmosphere As we ascend in the atmosphere, there is a gradual decrease in temperature. The temperature drops at a steady rate called the ‘lapse rate’. The lapse rate at a given place varies from day to day and even during each day. The lapse rate reduces the temperature by about 2 deg. C for each 1000ft in height, up to 36,000 feet (11,000m). Above 36,000ft (11,000m) the temperature remains nearly constant until the outer regions of the atmosphere are reached. Figure 4: Temperature v Altitude 2016-05-25 B-8.1 Physics of the Atmosphere Page 4 of 14 Training Material Only The atmosphere is classified into regions based on the variation of temperature with altitude as shown in Figure 5. Air temperature undergoes considerable change as altitude increases: Troposphere -gradual temperature decrease Tropopause -temperature approx. constant Stratosphere -gradual temperature increase Mesosphere - gradual temperature decrease Thermosphere (ionosphere) - rapid temperature increase Figure 5: Effects of altitude on temperature The composition of the atmosphere (oxygen, nitrogen etc.) remains almost constant from sea level up, but its density diminishes rapidly with altitude. For example, at approximately 30 000 feet (10 km), it is too thin to support respiration and at 60,000 feet there is not enough oxygen to support combustion. Note: Aircraft altitude is still measured in feet. Civil aircraft normally fly at altitudes up to 45,000 feet (14 km). Although the atmosphere is divided into several regions, we will only be covering the three closest to the earth’s surface, these being: Troposphere Tropopause Stratosphere 2016-05-25 B-8.1 Physics of the Atmosphere Page 5 of 14 Training Material Only Troposphere The troposphere is the layer in which we live and in which most aircraft fly. It is characterised by large changes in temperature, humidity and by generally turbulent conditions. Nearly all cloud formations are within the troposphere and approximately three quarters of the total weight of the atmosphere is within it. It extends from the surface of the earth to where the temperature ceases to decrease with altitude (roughly 36,000 feet). In the troposphere, for every 1,000 feet increase in altitude, the temperature drops approximately 2°C (lapse rate). Tropopause The tropopause is defined as the point in the atmosphere at which the decrease in temperature (with increasing altitude), abruptly ceases. The tropopause is located at the top of the troposphere and the start of the stratosphere. The temperature at the tropopause is around a chilling -57°C. The tropopause is not at a constant altitude above the earth. At the poles it can be as low as 28,000 feet, while over the equator it can be as high as 55,000 feet. These heights may vary due to seasonal changes which cause temperature fluctuations. However, the average of approximately 36,000 feet is taken to be the tropopause. At this height, the atmospheric pressure is approximately 3 psi or 15 the sea-level pressure. The troposphere is also characterised by a rapid drop in atmospheric pressure. The pressure drops from approximately 15 psi at sea level to 3 psi at 36,000 feet. Stratosphere The atmospheric layer extending from the tropopause up to an average altitude of between 50 to 55 kilometres is termed the stratosphere. Pressure continues to drop from 3 psi at the tropopause to about 0.015 psi at the top of the stratosphere. The temperature remains almost constant at -57°C, forming an isothermal layer from the tropopause up to an altitude of 20 kilometres (70,000 feet). Between 20 kilometres and approximately 32 kilometres the temperature begins to slowly rise. Above an altitude of 32 kilometres, the temperature starts to increase more rapidly. The temperature rise ceases at around 0°C, between the altitudes of 50 to 55 kilometres. This point is called the stratopause - Refer to Figure 6. 2016-05-25 B-8.1 Physics of the Atmosphere Page 6 of 14 Training Material Only Figure 6: Atmospheric conditions 2016-05-25 B-8.1 Physics of the Atmosphere Page 7 of 14 Training Material Only Air Density Density is described as mass per unit of volume of a substance. Density is of great importance when studying aerodynamics because of its effects on an aircraft or aerofoil. Three factors affect air density: 1. Altitude – as altitude increases, density decreases due to decreased atmospheric pressure. 2. Temperature – as temperature increases, density decreases due to the volume of air expanding. 3. Humidity – as humidity increases, density decreases due to a decreased molecular weight in a given volume (relatively lighter water vapour molecules displace oxygen, nitrogen, etc. molecules). Air Density with Altitude Changes In the troposphere, the air is warmest nearest the surface of the Earth. As altitude increases: Air temperature decreases Air density decreases Air pressure decreases The decrease in air pressure has a greater effect on air density than the decrease in temperature. Therefore, the air becomes less dense with increasing altitude. Air is under greater pressure at the earth’s surface. It is denser because it is compressed. It becomes less dense with increasing altitude. Aircraft and engine performance is decreased if air density is decreased. These effects are illustrated below in Figure 7. Figure 7: Effects of Temperature on Air Density 2016-05-25 B-8.1 Physics of the Atmosphere Page 8 of 14 Training Material Only Half of all air molecules mass are found below 5500 m (18,000 feet) altitude - Figure 8. Figure 8: Molecule Mass and Altitude Air density has a major effect on an aircraft in flight. At high altitude (less air density), a greater speed and distance can be achieved because of reduced resistance (drag). Figure 9: Air Density Effect on Aircraft in Flight Water Vapour Water vapour makes up only a very small fraction of the total mass of air but it has a major effect on flight. Because water vapour is only 63% as heavy as air, it soon mixes with air and lowers air density. This less dense air near the Earth’s surface rises and cools until its temperature drops to where it can no longer hold the water as a vapour. The water condenses out to become a liquid, the liquid forms very tiny droplets small enough to be supported by the moving air currents. This forms clouds. 2016-05-25 B-8.1 Physics of the Atmosphere Page 9 of 14 Training Material Only Humidity Humidity is caused by the condition of moisture or dampness. Water vapour is always present in the atmosphere and is one of the most important factors in human comfort. The proportion of water vapour in the atmosphere varies widely from place to place, and time to time. Travelling around Australia in the summer months you would come across large fluctuations of humidity, depending on where you were. In Melbourne the temperature may be 30°C with a humidity of 60%, while in Darwin the temperature may be 30°C with a humidity of 95%. If you were to travel into the outback away from the coast the temperature could fluctuate between 20°C and 50°C, with almost no humidity (the air is very dry). When the proportion of water vapour is small, the air is said to be dry. When the proportion is significant, the atmosphere is described as moist, damp, wet or humid. Figure 10 below shows that on a humid day air is less dense for a given volume due to water vapour displacing some of the dry air. Figure 10: Water vapour Humidity can be stated as: Absolute humidity Relative humidity Absolute humidity refers to the actual amount of water vapour in a mixture of air and water. The amount of water the air can hold varies with air temperature. The higher the air temperature the more water vapour the air can hold. Relative humidity is the ratio between the amounts of moisture in the air to that that would be present if the air were saturated. For example, a relative humidity of 75% means that the air is holding 75% of the total water vapour it is capable of holding. Relative humidity has a dramatic effect on aircraft performance because of its effect on air density. In equal volumes, water vapour weighs 62% as much as air. This means that in high humidity conditions the density of the air is less than that of dry air. 2016-05-25 B-8.1 Physics of the Atmosphere Page 10 of 14 Training Material Only Dew Point The amount of water vapour present in the air can be measured by blowing air over a wet-bulb and a dry-bulb thermometer. The different in readings between the two thermometers is compared on a chart to find the relative humidity. This measurement is the ratio of how much water vapour the air will hold at a given temperature. For practical application in aviation, temperature and dew point are used more often than relative humidity to measure the amount of water vapour in the air. Dew point is the temperature to which the air must be lowered before the water vapour condenses out and becomes liquid water. Figure 11: Morning dew 2016-05-25 B-8.1 Physics of the Atmosphere Page 11 of 14 Training Material Only International Standard Atmosphere (ISA) Changing atmospheric conditions cause significant changes in the performance of aircraft. As the atmosphere’s temperature, pressure, and density vary from place to place and from day to day, it became necessary to develop a standard set of conditions to which performance of an aircraft could be measured. For this reason, an International Standard Atmosphere (ISA) was adopted. The ISA was formulated by the National Advisory Committee for Aeronautics (NACA), now called National Aeronautics and Space Administration (NASA). The International Civil Aviation Organisation (ICAO) now administers the ISA, and you may therefore find reference to ICAO (SA) Standard Atmosphere in some publications. Aircraft performance is measured under actual atmospheric conditions. This actual performance can be compared to an ideal performance by recording parameters and correcting them to ISA conditions using graphs and charts. ISA Standard Conditions The set of standard conditions is known as the International Standard Atmosphere (ISA). ISA defines precise values of: Lapse rate; Tropopause height. It also defines sea-level values for: Air pressure; Air temperature; Air density. ISA values for the above are: Lapse rate (reduces by) 1.98/1000 feet (6.49ºC / I000 m). Tropopause height is at 11 000 m (36,000 feet). Mean sea-level pressure is: o 1013.25 hectopascals (hPa). o 14.69 pounds per square inch (psi). o 29.92 inches mercury (in Hg). Mean sea-level temperature is 15ºCelsius. Mean sea level humidity is zero (0%). Gravity (g) is 9.809 m/second2 (32.174 feet/second2). These values are referred to as ISA “Standard Day”. The reason 15°C (when the air is perfectly dry) is used is because it is the average condition prevailing at latitude 45° north. 2016-05-25 B-8.1 Physics of the Atmosphere Page 12 of 14 Training Material Only Changes in atmosphere can effect: Lift; Drag; Engine performance; Component adjustments; Instrument adjustment and monitoring; The application of surface finishes; Manufacture and repair of composite structures. Pressure Altitude Pressure altitude is the altitude in the standard atmosphere corresponding to a particular value of air pressure. The aircraft altimeter is essentially a sensitive barometer calibrated to indicate altitude in the standard atmosphere. With the altimeter of an aircraft set at 1013.2 hPa (29.92 inches Hg), the dial will indicate the number of feet above or below a level where 1013.2 hPa exists, not necessarily above or below sea level, unless standard day conditions exist. In general, the altimeter will indicate the altitude at which the existing pressure would be considered standard pressure. The symbol H is used to indicate pressure altitude. 2016-05-25 B-8.1 Physics of the Atmosphere Page 13 of 14 Training Material Only This page intentionally blank 2016-05-25 B-8.1 Physics of the Atmosphere Page 14 of 14 Training Material Only B-8.2 Aerodynamics Table of Contents Airflow.............................................................................................................................................. 3 Free Stream Airflow...................................................................................................................... 4 Friction.......................................................................................................................................... 4 Boundary Layer............................................................................................................................. 5 Laminar Boundary Layer............................................................................................................... 5 Transition Point:........................................................................................................................... 6 Stagnation point........................................................................................................................... 7 Separation Points.......................................................................................................................... 7 Wake............................................................................................................................................. 7 Relative Airflow............................................................................................................................. 8 Coanda Effect................................................................................................................................ 8 Upwash......................................................................................................................................... 9 Downwash.................................................................................................................................... 9 Vortices........................................................................................................................................... 10 Wing Tip Vortices........................................................................................................................ 10 Aerofoils......................................................................................................................................... 11 Chord Line................................................................................................................................... 11 Camber........................................................................................................................................ 11 Mean Camber............................................................................................................................. 12 Maximum Camber...................................................................................................................... 12 Fineness Ratio............................................................................................................................. 12 Aerofoil Shapes........................................................................................................................... 12 High Lift Aerofoils....................................................................................................................... 13 General Purpose Aerofoils.......................................................................................................... 13 High-Speed Aerofoils.................................................................................................................. 14 Aspect Ratio.................................................................................................................................... 15 Aspect Ratio and Maximum Lift Coefficient............................................................................... 16 Aspect Ratio and Induced Drag.................................................................................................. 16 Wing Planforms.......................................................................................................................... 17 Mean Aerodynamic Chord (MAC).............................................................................................. 17 Generation of Lift........................................................................................................................... 18 Angle of Incidence...................................................................................................................... 18 2014/11/10 B-8.2 Aerodynamics Page 1 of 36 Training Material Only Angle of Attack............................................................................................................................ 18 Centre of Pressure CP.................................................................................................................. 18 Pressure Distribution.................................................................................................................. 19 Lift Coefficient............................................................................................................................. 20 Resultant Lift............................................................................................................................... 22 Drag................................................................................................................................................ 22 Parasite Drag............................................................................................................................... 22 Form Drag................................................................................................................................... 23 Skin Friction................................................................................................................................ 24 Interference Drag........................................................................................................................ 24 Induced Drag............................................................................................................................... 25 Lift/Drag - The Polar Curve............................................................................................................. 26 Conditions of Flight........................................................................................................................ 27 Straight and Level Flight............................................................................................................. 27 Aerodynamic Forces................................................................................................................... 27 Drag Curves................................................................................................................................. 28 The Stall...................................................................................................................................... 29 Wash-in and Washout................................................................................................................ 30 Aircraft Speed................................................................................................................................. 31 Knot............................................................................................................................................. 31 Airspeed Indication..................................................................................................................... 32 Indicated Airspeed...................................................................................................................... 32 True Airspeed.............................................................................................................................. 33 Ground speed................................................................................................................................. 34 Icing Effects.................................................................................................................................... 35 2014/11/10 B-8.2 Aerodynamics Page 2 of 36 Training Material Only Airflow Air is a viscous fluid. As you can see in Figure 1, air behaves differently when it moves, or when a body moves through it, at speeds below the speed of sound and at speeds above the speed of sound. Because air is invisible, it is difficult to understand what happens in flight. Figure 1 Airflow disturbance 2014/11/10 B-8.2 Aerodynamics Page 3 of 36 Training Material Only Free Stream Airflow The free stream airflow around a shape is the clean flow, distant enough to be unaffected by the body passing through it, and does not change direction. Lines which show the direction of the flow are called ‘streamlines’. A body shaped to produce the least possible resistance is called a ‘streamline shape’. The amount of free stream air is directly relative to the resistance applied to the airflow (DRAG). Resistance creates turbulence (Figure 2) - the greater the resistance, the greater the turbulence; therefore, the further the locality of the free stream air. The amount of drag depends on when the airflow separates. The airflow around the ball has remained attached for longer. Figure 2 Air Flow Resistance Friction Skin friction is caused by the resistance which is set up when relative motion exists between the surface of a body and the air; contact between the two gives rise to a layer of retarded air in immediate contact with the surface over which it is passing. This layer is known as the boundary layer and the amount of drag arising from it is determined by the nature and thickness of the flow in the layer. 2014/11/10 B-8.2 Aerodynamics Page 4 of 36 Training Material Only Boundary Layer The boundary layer is the layer of air adjacent to the surface of the body. The air velocity in the boundary layer varies from zero on the surface of the aerofoil to the velocity of the free stream at the outer edge of the boundary layer (Figure 3). Figure 3 Boundary Layer The boundary layer is caused by the viscosity of the air sticking to the surface of the wing and the succeeding layers of air. The thickness of the boundary layer is relative to the velocity, and depends on the type of flow. Laminar Boundary Layer The amount of drag produced depends on whether the flow in the boundary layer is laminar or turbulent. Laminar flow is an orderly motion in which successive strata of air particles slide past each other in much the same way as the action of a pack of cards when thrown along a flat surface. If we could ensure a laminar boundary layer over the whole surface of a wing the skin friction would be reduced to about one-tenth of its value on a conventional type of wing. As the speed increases the boundary layer becomes turbulent and the drag becomes greater. The usual tendency is for the boundary layer to start by being laminar over the surface near the leading edge of a body, but there comes a point, called the transition point, when the layer tends to break away from the surface and become turbulent and thicker. 2014/11/10 B-8.2 Aerodynamics Page 5 of 36 Training Material Only The boundary layer differs from the free air stream in that the particles of air are rotating as they move rearwards. Those on the upper surface move in a clockwise direction and those below move in an anti-clockwise, in exactly the same way as ball bearings when rolled along a surface - refer Figure 4. Figure 4 Laminar and Turbulent Airflow Transition Point: That point on the wing at which the boundary layer changes from laminar to turbulent flow is called the transition point. Because the increase in drag resulting from a turbulent boundary layer is considerable, care is taken to preserve laminar flow over as much of the wing as possible, for example in a true laminar flow wing shown in Figure 5. Skin friction is a major source of drag at high speeds and it is one of the most difficult to reduce. It can never be eliminated completely. Figure 5 Transition Point As the speed increases the transition point tends to come further forward, so more of the boundary layer becomes turbulent and the skin friction becomes greater. If this much is understood it will be obvious that the main purpose of research work has been to discover why the transition point moves forward, and how its movement can be controlled so as to maintain laminar flow over as much of the surface as possible. 2014/11/10 B-8.2 Aerodynamics Page 6 of 36 Training Material Only Stagnation point The stagnation point, as shown below in Figure 6 is that point at which the air is brought to rest by the leading edge and the point from which the boundary layer originates. The stagnation point is also the first point of contact of relative airflow, or, the point on the leading edge of an aerofoil where the airflow divides. Some airflow goes over the wing and some goes under the wing. Separation Points The separation points are the points on the wing at which the boundary layers break away from the surface. Wake The wake consists of the unsteady rotational flow, resulting from separation of the boundary layers from the wing, and which tends to be dragged behind the trailing edge. For a chord of seven feet the wake is about four to five inches in depth during flight at small angles of attack. Refer Figure 6 Figure 6 Airflow Separation 2014/11/10 B-8.2 Aerodynamics Page 7 of 36 Training Material Only Relative Airflow Relative Airflow (US Relative wind) is the direction of the airflow with respect to the wing. If a wing moves forward horizontally, the relative airflow moves backward horizontally. Relative airflow is parallel to and opposite the flight path of the aeroplane. (Figure 7). Figure 7 Relative Airflow Coanda Effect Viscosity is defined as a fluid’s resistance to flow. One of the consequences of this is the tendency of a viscous fluid to follow a reasonable curvature of, for example, the back of a spoon, or the top surface of a wing - Figure 8. Figure 8 Coanda Effect 2014/11/10 B-8.2 Aerodynamics Page 8 of 36 Training Material Only Upwash Figure 9 shows that, in advance of the wing, the streamlines of air curve upwards towards the top surface. Upwash, as this is called, is an inherent feature of any surface which is giving lift and exists because air always tends to flow towards an area of low pressure. The deeper the low- pressure region is, the greater the amount of upwash. Figure 9 Up-wash Downwash Hydrodynamics is similar to aerodynamics except for the fluid used. When a person water-skis, the towing boat must have enough speed through the water that the ski will continually force down enough water to equal the weight of the skier. When the rope is released, the skier slows down sinks into the water. An aeroplane generates its lift in the same way as the water ski. The aeroplane is propelled through the air by its powerplant, and as the air passes over the lift-producing surfaces, called the aerofoils, it is deflected downward. This downward deflection or down-washing of the air has an opposing effect, that of pushing upward on the aeroplane. There is nothing mysterious about this down-washing action. In fact, any inclined plane will force air downward, but, the shape of the aeroplane wing makes this down-washing action more efficient. This downwash should not be confused with the downwards flow caused be wing tip vortices. 2014/11/10 B-8.2 Aerodynamics Page 9 of 36 Training Material Only Vortices When the airflow over the top surface of a wing meets with the airflow over the lower surfaces at the trailing edge they are flowing at different angles to each other. This causes eddies or vortices rotating clockwise (viewed from the rear) from the left wing, and counter-clockwise from the right wing - Figure 10. All the vortices on one side tend to join up and form one large vortex at each wing tip. These are called Wing-tip Vortices. Vortices occur continuously while an aeroplane is flying. The central core of the vortex is made visible by the condensation of moisture caused by the decrease of pressure and temperature, in the vortex. These visible (and sometimes audible!) trails from the wing tips should not be confused with the vapour trails caused by condensation trails left by hot exhaust gases at high altitudes. This downward flow must not be confused with the ordinary downwash. Figure 10 Cause of vortices Wing Tip Vortices Wingtip vortices are caused by the higher pressure air beneath the wing leaking around the wingtip and mixing with the low pressure air above the wing. (Figure 11). This causes a spiral or vortex that trails behind each wingtip whenever lift is being produced. The influence vortices have on flow extends well beyond their central core, modifying the whole flow pattern. The trailing vortices have a strong influence on lift, drag and handling properties of the aircraft. Wake turbulence is mainly due to these wingtip vortices. Figure 11 Wing Tip Vortices 2014/11/10 B-8.2 Aerodynamics Page 10 of 36 Training Material Only Aerofoils Figure 12 shows the names assigned to parts of an aerofoil. Figure 12 Aerofoil Nomenclature Chord Line The chord of the aerofoil is the straight line joining the leading edge to the trailing edge. (Figure 13). It is used as an arbitrary reference line when measuring the angular position of the wing in relation to the airflow. Figure 13 Chord Line Camber Camber is defined as the curvature of an aerofoil surface or an aerofoil section from the leading edge to the trailing edge. (Figure 14). The degree or amount of camber is expressed as the ratio of the maximum departure of the curve from the chord to the chord length. An aerofoil having a double convex curvature means that it has camber above and below the chord line. Upper camber refers to the curve on the upper surface of an aerofoil, and lower camber refers to the curve of the lower surface. Figure 14 Camber 2014/11/10 B-8.2 Aerodynamics Page 11 of 36 Training Material Only Mean Camber Mean camber is the curved line that forms the equal distance between the upper surface of the wing and the lower surface. (Figure 15). Camber is positive, when the departure from the straight line (the chord line) is upward and negative when it is downward. When the upper and lower cambers of an aerofoil are the same, the aerofoil is said to be symmetrical. Figure 15 Mean Camber Maximum Camber Maximum camber is the maximum or greatest distance between the chord line and the mean camber line. Fineness Ratio The fineness ratio is a measure of the thickness of the aerofoil. t There is also a thickness ratio of where t is breadth and c is the length. (Figure 16). c Figure 16 Fineness Ratio Aerofoil Shapes The performance of an aerofoil is governed by its contour. Generally, aerofoils can be divided into three classes: High lift. General purpose. High speed. Figure 17 Aerofoil Shapes 2014/11/10 B-8.2 Aerodynamics Page 12 of 36 Training Material Only High Lift Aerofoils t There sections employ a high ratio (15%), a pronounced camber, and a well-rounded leading c edge (Figure 18). Their maximum thickness is at about 25 per cent to 30 per cent of the chord aft of the leading edge. The greater the camber is, i.e. the amount of curvature of the mean camber line, the greater the shift of centre of pressure for a given change in the angle of attack. The range of movement of the Centre of Pressure (CP) is therefore large on a high-lift section. This movement can be greatly decreased by reflexing upwards the trailing edge of the wing, but some lift is lost as a result. Sections of this type are used mainly on sailplanes and other aircraft where a high Coefficient of Lift (CL) all important and speed a secondary consideration. Figure 18 High Lift Aerofoil General Purpose Aerofoils t These sections employ a lower ratio (10%), less camber and a sharper leading edge than those c of a high-lift type (Figure 19), but their maximum thickness is still at about 25 per cent to 30 per cent of the chord aft of the leading edge. The lower t ratio results in less drag and a lower CL than c those of a high-lift aerofoil. Sections of this type are used on aircraft whose duties require speeds which, although higher than those previously mentioned, are not high enough to subject the aerofoil to the effects of compressibility. Figure 19 General Purpose Aerofoil 2014/11/10 B-8.2 Aerodynamics Page 13 of 36 Training Material Only High-Speed Aerofoils t These sections employ a very low ratio (7%), no camber, and a sharp leading edge (Figure 20). c Their maximum thickness is at about the 50 per cent chord point. Most of these sections lie in the 5 per cent to 10 per cent tic ratio band, but even thinner sections have been used on research aircraft. The reason for this is the overriding requirement for low drag; naturally the thinner sections have low maximum-lift coefficients. High-speed aerofoils are symmetrical about the chord fine; some sections are wedge-shaped whilst others consist of arcs of a circle placed symmetrically about the chord line. Figure 20 High-Speed Aerofoil 2014/11/10 B-8.2 Aerodynamics Page 14 of 36 Training Material Only Aspect Ratio Any plan form can be described briefly, but well enough to give a rough idea of its performance, by its aspect ratio. The aspect ratio of a wing is found by dividing the square of the wing span by the area of the wing i.e. Thus, if a wing has an area of 250 square feet and a span of 30 feet, the aspect ratio is 3.6. Another wing with the same span but with an area of 150 square feet would have an aspect ratio of 6. Aspect ratio can also be found by dividing the span by the mean chord of the wing. For example, a span of 50 feet with a mean chord of 5 feet gives an aspect ratio of 10. From the foregoing, it can be concluded that the smaller the mean chord in relation to the span the higher the aspect ratio. The dimensions of the wing-tip vortices and therefore the amount of induced drag can be reduced considerably by increasing the aspect ratio. Figure 21 shows three wings of the same area but with different aspect ratios. The wing with the higher aspect ratio forms smaller wing-tip vortices than the others because a smaller proportion of the total area is involved in the process of spilling air from the lower to the upper surface. Consequently, the rate of spilling or circulation around the tips of high aspect ratio wings is less. The high aspect ratio wing can be said to be more efficient, from the point of view of low induced drag. Since the total drag of a wing is the sum of the profile and induced drags, and the induced drag changes with aspect ratio, the total drag also changes with aspect ratio. The graph shows the effect of aspect ratio on the total drag of two wings of different aspect ratios over the working range of angles of attack. Figure 21 Aspect Ratio 2014/11/10 B-8.2 Aerodynamics Page 15 of 36 Training Material Only Aspect Ratio and Maximum Lift Coefficient The maximum lift coefficient (CL) obtained from a given wing area and aerofoil section is almost unaffected by aspect ratio. However, there is a tendency for the CL max to decrease as the aspect ratio is reduced, becoming most noticeable at very low aspect ratios of about 2 to 3 (Figure 22). Therefore, stalling speeds for a given wing loading are not very seriously affected by a reduction in the aspect ratio. Figure 22 Aspect Ratio and Maximum Lift Coefficient Aspect Ratio and Induced Drag The relationship for the induced drag coefficient emphasises the need for a high aspect ratio for aircraft which are continually operated at high lift coefficients. On the other hand, long thin wings increase structural weight and eventually a compromise has to be reached. Aircraft developed for very high speed flight operate at relatively low lift coefficients and require great aerodynamic cleanliness. These aircraft, in consequence, usually have low aspect ratio planforms. The limiting factor in the use of high aspect ratio is the difficulty of providing sufficient strength for the wings without the excessive weight which neutralises the advantage gained. Broadly, it can be said that the lower the cruising speed of the aircraft the higher the aspect ratio that can be usefully employed. Refer to: Figure 23 Aspect Ratio and Induced Drag. Figure 23 Aspect Ratio and Induced Drag 2014/11/10 B-8.2 Aerodynamics Page 16 of 36 Training Material Only Wing Planforms Planform refers to the shape of the aeroplanes’ wing when viewed from above or below: Rectangular is the cheapest to build. Elliptical is most efficient. Tapered is a compromise. Sweepback is for high speed. Figure 24 Wing Planforms Mean Aerodynamic Chord (MAC). Certain aerodynamic and weight-and-balance characteristics are referenced as a percent of the wing chord. However, when a wing is tapered, the chord is not uniform across the entire wing span. For this reason these characteristics are referenced as a percent of the mean aerodynamic chord (MAC). The mean aerodynamic chord is the chord drawn through the centre of the area of the aerofoil; that is, equal amounts of wing area will lie on both sides of the MAC. (Figure 25) Often, the MAC is confused with the average chord. As an example, the pointed-tip delta wing would have an average chord equal to one-half the root chord but a MAC equal to two-thirds of the root chord. Figure 25 Mean Aerodynamic Chord (MAC) 2014/11/10 B-8.2 Aerodynamics Page 17 of 36 Training Material Only Generation of Lift Angle of Incidence The angle of incidence is the acute angle which the wing chord makes with the longitudinal axis of the aircraft, when the wing is attached to fuselage. (Figure 26). This angle is fixed in manufacture and does not change. Figure 26 Angle of Incidence Angle of Attack The angle of inclination between the aerofoil chord and the relative airflow is of great importance. This angle is called the “Angle of Attack” (A of A; Figure 27). For most aerofoils, lift increases as angle of attack increases from zero, at a slightly negative angle, to maximum lift at about 15 degrees. Above about 15 degrees angle of attack lift will very rapidly drop to zero again, where the aerofoil is said to have stalled. This applies to wings, propeller blades, helicopter rotor blades and jet engine fan, compressor and turbine blades. Figure 27 Angle of Attack Centre of Pressure CP The centre of pressure is a point along the wing chord line where lift is considered to be concentrated. For this reason, the centre of pressure is often referred to as the centre of lift. During flight, this point along the chord line changes position with different flight attitudes. It moves forward as the angle of attack increases and aft as the angle of attack decreases. As a result, pitching tendencies created by the position of the centre of lift in relation to the Centre of Gravity (CG) vary. For example, with a high angle of attack and the centre of lift in a forward position (closer to the CG) the nose-down pitching tendency is decreased. The position of the centre of gravity in relation to the centre of lift is a critical factor in longitudinal stability. 2014/11/10 B-8.2 Aerodynamics Page 18 of 36 Training Material Only Pressure Distribution Figure 28 below illustrates the pressure distribution over an aerofoil at an angle of attack of 4 deg. It shows that the decrease in pressure on the upper surface is greater than the increase in pressure on the lower surface, also the pressure is not evenly distributed and both pressures are greater on the forward portion of the aerofoil. Although both surfaces contribute, it is the upper surface, by means of its lower pressure which provides the greater part of the lift at some angles of attack as much as 80%. Figure 28 Pressure Distribution The location and direction in which the centre of pressure will move depends upon the shape of the aerofoil section and the angle of attack. The centre of pressure (CP) is the point at which the resultant force intersects the chord of an aerofoil. Lift acts from the centre of pressure, or, stated another way, the centre of pressure is the centre of lift. The location and direction in which the resultant will point depends upon the shape of the aerofoil section and the angle at which it is set to the airstream. Throughout most of the flight range, that is, at the usual angles of attack, the CP moves forward as the angle of attack increases and backward as the angle of attack decreases. 2014/11/10 B-8.2 Aerodynamics Page 19 of 36 Training Material Only As illustrated in Figure 29 the resultant intersects the chord line or centre of pressure at progressively forward locations as the angle of attack is increased. Figure 29 Pressure Distribution/Angles of Attack The centre of pressure is generally located at approximately the 25% chord position for most aerofoils. On an aerofoil with a 60 inch chord, this would locate the centre of pressure at 15 inches aft from the leading edge. Lift Coefficient When several wings of the same geometrical shape and area, but with different aerofoil sections, are compared at a given angle of attack and air speed, the lift obtained from each wing varies the exact amount of lift depending on the aerofoil section used. Generally, at subsonic speeds at a given angle of attack, the greater the amount of lift obtained from a given wing; conversely, the flatter the camber and the thinner the wing the less the lift. This difference is due to the greater accelerating effect on the air stream of pronounced camber, resulting in a larger reduction in pressure. The measure of the lifting effectiveness, or power of wing under a given set of conditions, is its lift coefficient or CL. The CL is not constant but varies with the angle of attack. Furthermore, various aerodynamic aids can be used to increase the CL and thus raise the lifting effectiveness of a wing. 2014/11/10 B-8.2 Aerodynamics Page 20 of 36 Training Material Only All else being equal, the higher the CL the lower is the minimum speed at which a given wing can produce a required lift. The formula for calculating the lift is: The graph (Figure 30) shows that as angle of attack increases, so does coefficient in lift up to a maximum of about 15 degrees. Figure 30 Lift Coefficient 2 Since the expression ½ ρ V S (ρ Greek letter Rho) applies to all aerodynamic forces, it is sufficient, when considering increases or decreases of lift under a given set of conditions, to refer to the increase or decrease of the lift coefficient alone. Thus an increased CL implies an increased lift, and vice versa. When a wing aerofoil combination is placed in an air stream at a given angle of attack, and the speed of this stream is then progressively increased, the lift increases in proportion to the square of the speed as shown by the lift formula. At higher subsonic speeds the rate at which the lift has been increasing, in accordance with the V2 law, begins to fall appreciably. This effect is caused by the compressible nature of the air which, although negligible at lower subsonic speeds, begins to play an important part at the higher subsonic speeds. Compressibility, as this is called, brings with it a reduction in the CL and hence a falling off in the rate of increase of lift, owing to fundamental changes in the nature of the airflow. Summarising: The two math factors affecting co-efficient of lift are: Aerofoil shape; Angle of attack. Note1: Velocity only affects the lift force of the aerofoil not the coefficient of lift. Note2: Rho (Greek letter ρ) - air density at standard day (0.02378 slugs per cubic foot). 2014/11/10 B-8.2 Aerodynamics Page 21 of 36 Training Material Only Resultant Lift The resultant lift produced by an aerofoil is the net force produced perpendicular to the relative airflow. The resultant drag incurred by an aerofoil is the net force produced parallel to the relative airflow. Refer Figure 31. LIFT TOTAL AERODYNAMIC REACTION DRAG Figure 31 Resultant Lift Drag Drag is caused by any aircraft surface that deflects or interferes with the smooth airflow around the aeroplane. A highly cambered, large surface area wing creates more drag (and lift) than a small, moderately cambered wing. If you increase airspeed, or angle of attack, you increase drag (and lift). Drag acts in opposition to the direction of flight, opposes the forward-acting force of thrust, and limits the forward speed of the aeroplane. Drag is broadly classified as either parasite or induced. Parasite Drag Parasite drag (Figure 32) includes all drag created by the aeroplane, except that drag directly associated with the production of lift. It is created by the disruption of the flow of air around the aeroplane’s surfaces. Parasite drag normally is divided into three types: Form drag. Skin friction drag. Interference drag. Each type of parasite drag varies with the speed of the aeroplane. The combined effect of all parasite drag varies proportionately to the square of the airspeed. In other words, if airspeed is doubled, parasite drag increases by a factor of four. 2014/11/10 B-8.2 Aerodynamics Page 22 of 36 Training Material Only Figure 32 Parasite Drag Form Drag Form drag is created by any structure which protrudes into the relative airflow. (Figure 33) The amount of drag created is related to both the size and shape of the structure. For example, a square strut creates substantially more drag than a smooth or rounded strut. Streamlining reduces form drag. Figure 33 Form Drag 2014/11/10 B-8.2 Aerodynamics Page 23 of 36 Training Material Only Skin Friction Skin friction drag is caused by the roughness of the aeroplane’s surfaces. Even though these surfaces may appear smooth, under a microscope they may be quite rough. (Figure 34). A thin layer of air clings to these rough surfaces and creates small eddies which contribute to drag. Figure 34 Skin Friction Interference Drag Interference drag occurs when varied currents of air over an aeroplane meet and interact. This interaction creates additional drag. One example of this type of drag is the mixing of the air where the wing and fuselage join - Figure 35. Each type of parasite drag varies with the speed of the aeroplane. Figure 35 Interference Drag 2014/11/10 B-8.2 Aerodynamics Page 24 of 36 Training Material Only Induced Drag Induced drag is the main by-product of the production of lift. It is directly related to the angle of attack of the wing. The greater the angle, the greater the induced drag. Since the wing usually is at a low angle of attack at high speed, and a high angle of attack at low speed, the relationship of induced drag to speed also can be plotted. Refer to Figure 36 and Figure 37 below. Figure 36 Induced Drag Figure 37 Induced Drag 2014/11/10 B-8.2 Aerodynamics Page 25 of 36 Training Material Only Lift/Drag - The Polar Curve The polar curve is a name given to mathematical representation of data involving lift/drag/speed/angle of attack. The lift/drag ratio increases rapidly up to an angle of attack of about 4°. Lift may be between 12 to 20 times the drag, the exact figure depending on the aerofoil used. At larger angles the L/D ratio decreases steadily. At the stall the L/D is about 6 - refer Figure 38. Figure 38 Lift/Drag Ratio 2014/11/10 B-8.2 Aerodynamics Page 26 of 36 Training Material Only Conditions of Flight Straight and Level Flight For an aeroplane to remain in straight and level flight, the amount of lift is dependent on airspeed and the angle of attack. At low airspeed the aircraft has a large angle of attack. At high airspeed the angle of attack can be reduced. The graph (Figure 39) shows that at the maximum coefficient of lift, any further increase in angle of attack will cause a stall, and with that, rapid decrease in coefficient of lift. Figure 39 Maximum Coefficient of Lift Aerodynamic Forces Aircraft flight is controlled by adjusting the relationship between the four aerodynamic forces (Figure 40): Lift is the component of the aerodynamic reaction perpendicular to the relative airflow; Drag is the component of the aerodynamic reaction parallel to the relative airflow; Weight is due to gravity; Thrust is produced by the power plant. Figure 40 The Four Aerodynamic Forces 2014/11/10 B-8.2 Aerodynamics Page 27 of 36 Training Material Only For constant speed, straight and level flight: Lift equals Weight. Thrust equals Drag. To accelerate or climb, thrust must be added to change the equilibrium. To descend, thrust is reduced. Drag Curves In Figure 41, at low speeds, induced drag is high due to the large vortices created at high angle of attack. At high speeds, parasite drag dominates. Total drag starts high, decreases to a minimum, and then increases towards the aircraft’s maximum speed. Figure 41 Drag Curve 2014/11/10 B-8.2 Aerodynamics Page 28 of 36 Training Material Only The Stall A stall is caused by the separation of airflow from the wing’s upper surface. (Figure 42) This results in a rapid decrease in lift. For a given aeroplane, a stall always occurs at the same angle of attack, regardless of air speed, flight attitude, or weight. This is the stalling or critical angle of attack. It is important to remember that an aeroplane can stall at any airspeed, in any flight attitude, or at any weight. For a specific aerofoil, the stall always occurs at the same angle of attack but can occur at any speed. At critical angle of attack (about 15 degrees): Airflow separates Wing stalls Aircraft loses height Figure 42 The Stall 2014/11/10 B-8.2 Aerodynamics Page 29 of 36 Training Material Only Wash-in and Washout Many aeroplanes are designed with a greater angle of incidence at the root of the wing than at the tip; this characteristic of a wing is called washout. The purpose of washout is to improve the stability of the aircraft as it approaches a stall condition. The section of the wing near the fuselage will stall before the outer section, thus enabling the pilot to maintain good control and reducing the tendency of the aircraft to “fall off’ on one wing. If a wing is designed so that the angle of incidence is greater at the tip than at the root, the characteristic is called washin. (Figure 43). A difference in the washout and washin of the right and left wings of an aircraft is used to compensate for propeller torque. Propeller torque causes the aircraft to roll in a direction opposite that of the propeller rotation. To compensate for this, the right wing is rigged or designed with a smaller angle of incidence at the tip than that of the left wing. Thus, the right wing is washed out more than the left. Figure 43 Wash-in and Washout 2014/11/10 B-8.2 Aerodynamics Page 30 of 36 Training Material Only Aircraft Speed Knot A knot is a measure of speed, and equates to one nautical mile per hour. The international nautical mile was defined by the First International Extraordinary Hydrographic Conference, Monaco (1929) as exactly 1852 metres. This is the only definition in widespread current use, and is the only one accepted by the International Hydrographic Organisation and by the International Bureau of Weights and Measures (BIPM). Before 1929, different countries had different definitions, and the Soviet Union, the United Kingdom and the United States did not immediately accept the international value. Both the Imperial and U.S. definitions of the nautical mile were based on the length of one minute of arc ( 1 degree) along a great circle of a hypothetical sphere (Figure 44). 60 The United States Nautical Mile was defined as 1853.248 metres: It was abandoned in favour of the International Nautical Mile in 1954. The Imperial (UK) Nautical Mile, also known as the Admiralty Mile, was defined in terms of the knot such that one nautical mile was exactly 6080 feet (1853.184m). It was abandoned in 1970. The nautical mile has now been standardised as 1853 metres exactly. Figure 44 One nautical mile A knot is also a measure of subsonic airspeed. Multiply knots by 1.15 to find statute miles per hour; Multiply knots by 1.85 to find kilometres per hour; Divide miles per hour by 0.87 to find knots; Divide kilometres per hour by 0.54 to find knots. For example, to find the approximate speed in kilometres/hour of an aircraft flying at 250 knots: 250 knots x 1.85 = 462.5 km/h 2014/11/10 B-8.2 Aerodynamics Page 31 of 36 Training Material Only Airspeed Indication The dynamic pressure of the relative airflow is the most commonly used measure of aircraft airspeed. This pressure is used to position the pointer of an airspeed indicator. As with any other fluid, the dynamic pressure of airflow is ½ ρ V2, where ρ is air density and V is velocity. Since air density (ρ) decreases with altitude, then for a constant velocity (V) the indicated airspeed must also fall off with altitude. There are several ways of recording aircraft speed. Three common indications are: Indicated Air Speed (IAS); Ground Speed (GS); True Air speed (TAS). Indicated Airspeed Airspeed indicators (Figure 45) can be calibrated to read the ‘true airspeed’ at only one value of air density. It is universal that they are calibrated to read true airspeed in standard density air at sea level (ISA). It follows that with increase in altitude, the indicated airspeed of an aircraft drops below its true airspeed. At 40 000 feet the indicated airspeed is only half the true airspeed. Indicated airspeed is important to the pilot because it is a gauge of the lift and other aerodynamic forces acting on the aircraft. This is because the indicated airspeed is aerodynamic pressure. Thus, an aeroplane stalls at the same indicated airspeed close to sea level, or at 40 000 feet, even though at the higher altitude the true airspeed is twice the indicated airspeed. If an aeroplane stalls at an indicated airspeed of 70 knots, the true airspeed at which it stalls varies from 70 knots at sea level up to 140 knots at 40 000 feet. Figure 45 Airspeed Indicator 2014/11/10 B-8.2 Aerodynamics Page 32 of 36 Training Material Only True Airspeed A thorough understanding of true airspeed is absolutely critical for navigational purposes. In many aircraft, pilots have a simple computer ( Figure 46) that calculates true airspeed when they input indicated airspeed, altitude, and ambient temperature. Altitude and ambient temperature give a close approximation of density, which the pilot cannot readily determine. Figure 46 Flight Computer A few aircraft have true airspeed indicators that automatically compute and display the true airspeed. Figure 47 True Speed Indicator 2014/11/10 B-8.2 Aerodynamics Page 33 of 36 Training Material Only Ground speed True airspeed may not be an aircraft’s actual speed over the ground. If there is a headwind of 50 knots, the ground speed is true airspeed minus 50 knots. With a tailwind of 50 knots, the ground speed is true airspeed plus 50 knots - Figure 48. Figure 48 Ground speed 2014/11/10 B-8.2 Aerodynamics Page 34 of 36 Training Material Only Icing Effects Rain, snow, and ice can have a detrimental effect on flight (Figure 49). Under certain atmospheric conditions, ice can build rapidly on aerofoils and engine air inlets. Ice on an aircraft affects its performance and efficiency in many ways: Increases drag and reduces lift; Causes destructive vibration; Hampers true instrument readings; Control surfaces become unbalanced or frozen; Fixed slots are filled and movable slots jammed; Radio reception is hampered; Engine performance is affected; Stalling speed increases. Figure 49 The effects of icing Contamination caused by ice, snow and frost can alter the aerofoil shape. Ice build-up can change the effective chord line. It can also alter the upper and lower camber of the aerofoil - Figure 50. Figure 50 Ice build-up 2014/11/10 B-8.2 Aerodynamics Page 35 of 36 Training Material Only This page intentionally blank 2014/11/10 B-8.2 Aerodynamics Page 36 of 36 Training Material Only Topic 8.3: Theory of Flight Table of Contents Topic 8.3: Theory of Flight................................................................................................................... 2 Aerodynamic Forces......................................................................................................................... 2 Four Forces of Flight..................................................................................................................... 2 Lift..................................................................................................................................................... 3 Weight.............................................................................................................................................. 4 Centre of Gravity.............................................................................................................................. 4 Adverse Forward Centre of Gravity.............................................................................................. 5 Adverse Aft Centre of Gravity....................................................................................................... 6 Centre of Gravity Limits................................................................................................................ 6 Straight and Level Flight................................................................................................................... 7 Forces in a Climb........................................................................................................................... 8 Forces in a Descent....................................................................................................................... 8 Forces in a Glide............................................................................................................................ 9 The Glide........................................................................................................................................... 9 Glide Angle.................................................................................................................................. 10 Theory of the Turn.......................................................................................................................... 10 Centrifugal Force and Centripetal Force.................................................................................... 10 Turning Flight.............................................................................................................................. 11 Sideslip........................................................................................................................................ 11 Skidding....................................................................................................................................... 12 Balanced Turn............................................................................................................................. 12 Wing Loading.............................................................................................................................. 13 Load Factor................................................................................................................................. 13 “g” Limit...................................................................................................................................... 14 Lift Augmentation........................................................................................................................... 15 Lift Augmentation - Slots............................................................................................................ 16 Lift Augmentation – Slats............................................................................................................ 16 2014/11/10 B-8.3 Theory of Flight Page 1 of 18 Training Material Only TOPIC 8.3: THEORY OF FLIGHT Aerodynamic Forces Four Forces of Flight During flight, there are four forces acting on an airplane. They are lift, weight, thrust, and drag Figure 1. Lift is the upward force created by the effect of airflow as it passes over and under the wings. It supports the airplane in flight. Weight opposes lift. It is caused by the downward pull of gravity. Thrust is the forward force which propels the airplane through the air. It varies with the amount of engine power being used. Drag, which is a backward, or retarding, force that limits the speed of the airplane opposes thrust. Figure 1 Four Forces of Flight The arrows which show the forces acting on an airplane are often called vectors. The magnitude of a vector is indicated by the arrow's length, while the direction is shown by the arrow's orientation - Figure 2. When two or more forces act on an object at the same time, they combine to create a resultant. Figure 2 Vectors 2014/11/10 B-8.3 Theory of Flight Page 2 of 18 Training Material Only Lift Lift is the key aerodynamic force. It is the force that opposes weight. In straight-and-level, un- accelerated flight, when weight and lift are equal, an airplane is in a state of equilibrium. If the other aerodynamic factors remain constant, the airplane neither gains nor loses altitude - Figure 3. During flight, the pressures on the upper and lower surfaces of the wing are not the same. Although several factors contribute to this difference, the shape of the wing is the principle one. The wing is designed to divide the airflow into areas of high pressure below the wing and areas of comparatively lower pressure above the wing. This pressure differential, which is created by movement of air about the wing, is the primary source of lift. Figure 3 State of Equilibrium 2014/11/10 B-8.3 Theory of Flight Page 3 of 18 Training Material Only Weight Weight has a definite relationship with lift, and thrust with drag. This relationship is quite simple, but very important in understanding the aerodynamics of flying. As stated previously, lift is the upward force on the wing acting perpendicular to the relative airflow. Lift is required to counteract the aircraft’s weight, caused by the force of gravity acting on the mass of the aircraft. This weight force acts downward through a point called the centre of gravity (Figure 4) which is the point at which all the weight of the aircraft is considered to be concentrated. Figure 4 Centre of Gravity (CG) and Centre of Pressure (CP) When the lift force is in equilibrium with the weight force, the aircraft neither gains nor loses altitude and can be considered to be in straight and level flight at a constant airspeed. The lift will act through the centre of pressure, which will depend on the position of the wings; so the designer must be careful to place the wings in the correct position along the fuselage. But the problem is complicated by the fact that a change in the angle of attack means a movement of the lift, and usually in the unstable direction. If the angle of attack is increased the pitching moment about the centre of gravity will become more nose-up, and tend to increase the angle even further. Centre of Gravity Centre of gravity is of major importance in an aircraft for its position has a great bearing upon stability. The centre of gravity is determined by the general design of the aircraft. The designer estimates how far the centre of pressure will travel and will fix the centre of gravity in front of the centre of pressure for the corresponding flight speed in order to provide an adequate restoring moment for flight equilibrium - (Figure 4). 2014/11/10 B-8.3 Theory of Flight Page 4 of 18 Training Material Only Adverse Forward Centre of Gravity When too much weight is toward the forward part of the aeroplane, the centre of gravity (CG) is shifted forward (Figure 5) and any one of the following conditions may exist or they may occur in combinations at the same time: Increased fuel consumption; Increased power for any given speed; Increased tendency to dive, especially with power off; Increased difficulty in raising the nose of the aeroplane when landing; Increased oscillation tendency; Increased stresses on the nose wheel; Increased danger during flap operation; Development of dangerous spin characteristics. Figure 5 Adverse Forward CG 2014/11/10 B-8.3 Theory of Flight Page 5 of 18 Training Material Only Adverse Aft Centre of Gravity When too much weight is toward the tail of the aeroplane (Figure 6), any one of the following conditions may exist or they may occur in combination: Decreased flying speed; Decreased range; Increased strain on the pilot during instrument flight; Increased danger of stall; Dangerous spin characteristics; Reduction of long range optimum speed; Poor stability; Increased danger if tail assembly is damaged; Poor landing characteristics. A study of the above listed conditions will reveal that most of them could lead to an accident with a resulting loss of life and destruction of the aeroplane. Figure 6 Adverse Aft CG Centre of Gravity Limits Each aeroplane type has its own centre of gravity limits, typically from about 15 per cent to 40 per cent of the wing chord (Figure 7). These will have been established by design and from the aircraft’s flight handling characteristics. The actual centre of gravity of a loaded aeroplane varies in flight as fuel is used or as people move along the cabin. The centre of pressure and the centre of gravity will rarely coincide, resulting in either a nose up or a nose down pitching moment. Figure 7 CG Limits 2014/11/10 B-8.3 Theory of Flight Page 6 of 18 Training Material Only Straight and Level Flight Lift and drag are components of the total aerodynamic force acting upon the wing. This total force is called the resultant. Each tiny portion of the wing in flight has small forces acting upon it. The force acting on one small portion of the wing is different in magnitude and direction from all the other small forces acting upon all the other portions of the wing. By considering the magnitude, direction, and location of each of these small forces, it is possible to add them all together into one resultant force. This resultant force has magnitude, direction, and location with respect to the wing. The resultant force on an aerofoil flying at a specified speed and angle of attack can be shown as a single entity possessing both magnitude and direction. It is also possible to break the resultant down into two major components (lift and drag), with magnitudes in two directions. In aerodynamics these forces are discussed as having directions perpendicular and parallel to the relative airflow. The component of the resultant force which acts perpendicular to the relative airflow is lift. The component of the resultant force which acts parallel to the relative airflow is called drag. The resultant forces are broken down into separate component forces. (Figure 8) Recall that in straight and level flight at a constant air speed, Lift = Weight and Thrust = Drag. To accelerate or climb, thrust must be added to change the equilibrium; To descend, thrust is reduced. Figure 8 Four Forces 2014/11/10 B-8.3 Theory of Flight Page 7 of 18 Training Material Only Forces in a Climb In a steady climb, thrust must balance the drag plus a portion of the weight. Lift is less than weight; Thrust is greater than drag. To operate at the maximum angle of climb possible we need the biggest possible value of thrust minus drag. If the thrust minus the drag is equal to the weight we have vertical climb. If thrust minus drag is greater than the weight then the aircraft will be in an accelerating rather than steady climb. As the climbing angle increases, lift proportionally decreases (w cos θ), therefore more thrust is required (Figure 9). Figure 9 Increasing thrust to climb Forces in a Descent In a powered descent, thrust may be reduced as gravity supplies some of the energy. Lift is less than weight; Drag is balanced by the reduced thrust and a part of the weight. As the aeroplane descends, weight is once again greater than lift and thrust is reduced to allow gravity to pull the aircraft towards the Earth. (Figure 10) Figure 10 Decreasing thrust to descend 2014/11/10 B-8.3 Theory of Flight Page 8 of 18 Training Material Only Forces in a Glide In a glide there is no thrust, and the pilot adopts the descent angle that gives the best lift to drag ratio (L/D) and the lowest rate of descent. This occurs at the minimum drag speed. (Figure 11) Glide Ratio = L/D. Gravity provides all of the energy to remain flying. Lift is less than weight. Figure 11 Forces in a Glide The Glide In a glide, thrust is removed from the four forces. In a steady glide the aeroplane must be kept is a state of equilibrium by lift, drag and weight. Lift and drag must be exactly opposite to the weight. Lift is at right angles to the glide path; Drag acts rearwards, parallel to the glide path. If an aeroplane is to glide as far as possible, the Angle of Attack (AoA) during the glide must produce the maximum lift/drag ratio (L/D). If the pilot attempts to glide at an AoA greater or less than the best L/D ratio, the glide path will be steeper. The pilot has to maintain the best L/D ratio. There is no way that the pilot can extend the glide beyond the best L/D ratio. (Figure 12) Figure 12 Glide L/D Ratio 2014/11/10 B-8.3 Theory of Flight Page 9 of 18 Training Material Only Glide Angle If a glider is in a steady (constant velocity and no acceleration) descent, it loses altitude as it travels. The glider's flight path is a simple straight line, shown as the inclined line in the figure. The flight path intersects the ground at an angle ‘a’, called the glide angle. If we know the distance flown and the altitude change, we can calculate the glide angle using trigonometry. The tangent (tan) of the glide angle ‘a’ is equal to the change in height ‘h’ divided by the distance flown ‘d’: tan(a) = h / d Figure 13 Glide Angle Theory of the Turn Centrifugal Force and Centripetal Force Everyone is familiar with the fact that a weight attached to the end of a cord and spun around (Figure 14) will produce a force tending to cause the weight to fly outward from the centre of the circle. This outward pull is called centrifugal force. There is an equal and opposite force pulling the weight inward and preventing it from flying outward; this is called centripetal force. From Newton’s first law of motion we know that a body in motion tends to continue in motion in a straight line. Hence, when we cause a body to move in a circular path, a continuous force must be applied to keep the body in the circular path. This is centripetal force. Figure 14 Centrifugal Force and Centripetal Force 2014/11/10 B-8.3 Theory of Flight Page 10 of 18 Training Material Only Turning Flight Before an aeroplane turns it must overcome inertia; the tendency to continue in a straight line. The necessary turning force is created by banking the aeroplane so that the direction of lift is inclined. Now, one component of lift still acts vertically to oppose weight, just as it did in straight- and-level flight, while another acts horizontally. To maintain altitude, lift must be increased by increasing back pressure and, therefore, the angle of attack, until the vertical component of lift equals weight. The horizontal component of lift, called centripetal force, is directed inward, toward the centre of rotation. It is this centre-seeking force which causes the aeroplane to turn. Centripetal force is opposed by centrifugal force, which acts outward from the centre of rotation. When the opposing forces are balanced, the aeroplane maintains a constant rate of turn, without gaining or losing altitude. Refer to Figure 15 Figure 15 Turning Flight Sideslip In normal flight and in a correct bank the airflow will come from straight ahead (neglecting any local effects from the propeller slip-stream). If the bank is too great, the aeroplane will sideslip inwards and the aeroplane, and pilot if in an open cockpit, will feel the airflow coming from the inside of the turn (Figure 16). Figure 16 Sideslip 2014/11/10 B-8.3 Theory of Flight Page 11 of 18 Training Material Only Skidding In an aeroplane, there is no such thing as a flat turn. If the bank is too small, the aeroplane will skid outward caused by the centrifugal force generated in the turn. The pilot will feel the airflow come from the outside of the turn. (Figure 17) Figure 17 Skidding Balanced Turn During a correct bank the pilot will sit without any feeling of sliding either inwards or outwards. In fact, the pilot will be sitting tighter on the seat than ever, his/her effective weight being magnified in the same proportions as the lift. If the pilot weighs 70kg in normal flight, that 70kg will feel like 700 kg when banking at 84½ degrees. The relative airflow will be coming head-on. (Figure 18) Figure 18 Balanced Turn 2014/11/10 B-8.3 Theory of Flight Page 12 of 18 Training Material Only Wing Loading Wing loading is the all-up-weight (AUW) of the aircraft divided by the wing area, i.e. the amount of the total weight carried by unit area of the wing. It is usually given in pounds per square foot (lb/ft2), or kilograms (kg) per square metre (kg/m2). An aeroplane with a lower wing loading will have a lower minimum speed than one with a high wing loading. A ‘light aircraft’ may have a high wing loading and therefore a high landing speed. It is not a question of weight, but of weight compared to wing area that governs minimum speed. An aeroplane with a lower wing loading will have a lower stalling speed. An aeroplane with a higher wing loading will have a higher stalling speed. As more weight is added to the aircraft, e.g. passengers and baggage, its wing loading, minimum speed and stalling speed will increase (Figure 19). Figure 19 Wing Loading Load Factor In a turn the resultant lift has two components: Vertical component. Horizontal component. It is the horizontal component of lift that provides centripetal force. Weight will always act vertically towards the centre of the Earth and opposite the vertical component of lift. It will be obvious from the calculations below that the lift on the wings during the turn is greater than the lift which the wings have to supply during straight flight. It is also noticeable that the lift increases considerably with the angle of bank. This means that all the lift bracing of the aeroplane, such as the struts and spars, will have to carry loads considerably greater than those of straight flight. Minimum speed, the stalling speed, can be determined mathematically by using the formula: W/L = cos θ, or L = W/cos θ. Where: L = lift W = weight θ = Angle of bank 2014/11/10 B-8.3 Theory of Flight Page 13 of 18 Training Material Only With a stalling speed of 60 miles per hour (mph) in level flight: At 60 degrees angle of bank, lift = 2xW, stalling speed 85 mph (2g turn); At 70.5 degrees angle of bank, lift = 3xW, stalling speed 104 mph (3g turn); At 75.5 degrees angle of bank, lift = 4xW, stalling speed 120 mph (4g turn); At 80.25 degrees angle of bank, lift = 10xW, stalling speed 190 mph (10g turn). Regardless of the angle of bank the lift on the wings must be provided by CL½ ρ V2S. It follows, therefore, that the value of CL½ ρ V2S must be greater during a turn than during normal flight, and this must be achieved either by increasing the velocity or increasing the value of CL. Thus it follows that the stalling speed, which means the speed at the maximum value of CL must go up in a turn. It will go up in proportion to the square root of the wing loading. For example, a 4g turn or pull-up will double the stall speed. (Figure 20) Figure 20 Resultant Lift in a Turn “g” Limit As CL increases, the stress on the airframe increases. All aircraft will have a specified “g” limit to avoid structural damage. Often the positive “g” limit is higher than the negative “g” limit (Figure 21). It is also possible that a particular aircraft could be designed to reach its critical angle of attack and stall before it reaches a dangerous load factor. Another structural limitation is gross weight. There will always be a maximum weight abov

AME 108 Student Resource PDF

Document Details

Tags

Related

Summary

Full Transcript

Upgrade to continue