CASA B-08 Basic Aerodynamics 2022 PDF

MODULE 08 Category B1 and B2 Licences CASA B-08 Basic Aerodynamics  Copyright © 2020 Aviation Australia All rights reserved. No part of this document may be reproduced, transferred, sold or otherwise disposed of, without the...

MODULE 08 Category B1 and B2 Licences CASA B-08 Basic Aerodynamics  Copyright © 2020 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. CONTROLLED DOCUMENT 2022-05-20 B-08 Basic Aerodynamics Page 2 of 113 CASA Part 66 - Training Materials Only  Knowledge Levels Category A, B1, B2 and C Aircraft Maintenance Licence Basic knowledge for categories A, B1 and B2 are indicated by the allocation of knowledge levels indicators (1, 2 or 3) against each applicable subject. Category C applicants must meet either the category B1 or the category B2 basic knowledge levels. The knowledge level indicators are defined as follows: LEVEL 1 Objectives: The applicant should be familiar with the basic elements of the subject. The applicant should be able to give a simple description of the whole subject, using common words and examples. The applicant should be able to use typical terms. LEVEL 2 A general knowledge of the theoretical and practical aspects of the subject. An ability to apply that knowledge. Objectives: The applicant should be able to understand the theoretical fundamentals of the subject. The applicant should be able to give a general description of the subject using, as appropriate, typical examples. The applicant should be able to use mathematical formulae in conjunction with physical laws describing the subject. The applicant should be able to read and understand sketches, drawings and schematics describing the subject. The applicant should be able to apply his knowledge in a practical manner using detailed procedures. LEVEL 3 A detailed knowledge of the theoretical and practical aspects of the subject. A capacity to combine and apply the separate elements of knowledge in a logical and comprehensive manner. Objectives: The applicant should know the theory of the subject and interrelationships with other subjects. The applicant should be able to give a detailed description of the subject using theoretical fundamentals and specific examples. The applicant should understand and be able to use mathematical formulae related to the subject. The applicant should be able to read, understand and prepare sketches, simple drawings and schematics describing the subject. The applicant should be able to apply his knowledge in a practical manner using manufacturer's instructions. The applicant should be able to interpret results from various sources and measurements and apply corrective action where appropriate. 2022-05-20 B-08 Basic Aerodynamics Page 3 of 113 CASA Part 66 - Training Materials Only  Table of Contents Physics of the Atmosphere (8.1) 7 Learning Objectives 7 Fundamentals of the Atmosphere 8 Atmospheric Layers 8 Composition of the Atmosphere 8 Atmospheric Regions 9 Atmospheric Conditions 12 Air Pressure 12 Air Density 12 Air Temperature 13 Viscosity 14 Humidity 15 Air Density 18 International Standard Atmosphere (ISA) 20 Definition 20 ISA Standard Conditions 20 Pressure and Density Altitude 21 Aerodynamics I (8.2) 23 Learning Objectives 23 Airflow Around a Body 24 Free Stream Airflow 24 Laminar Flow 24 Turbulent Flow 25 Boundary Layer 26 Relative Airflow 30 Introduction 30 Upwash and Downwash 30 Vortices 31 Wake 31 Wing-Tip Vortices 32 Bernoulli’s Principle 33 Aerofoil 35 Introduction 35 Chord Line 35 2022-05-20 B-08 Basic Aerodynamics Page 4 of 113 CASA Part 66 - Training Materials Only  Camber 36 Fineness Ratio 37 Angle of Attack 37 Angle of Incidence 38 Aerofoil Shapes 38 Centre of Pressure 39 Aerodynamics II (8.2) 43 Learning Objectives 43 Generation of Lift 44 Introduction 44 Stalling Angle 45 Generation of Drag 47 Drag 47 Lift Coefficient 50 Aerodynamic Curves 53 Aerodynamic Forces 55 Lift, Drag, Weight and Thrust 55 Wing Shape 55 Wing Geometry 56 Aspect Ratio and Maximum Lift Coefficient 58 Mean Aerodynamic Chord (MAC) 59 Wash In and Wash Out 60 Icing Effects 61 Theory of Flight (8.3) 64 Learning Objectives 64 Aircraft Aerodynamics 65 The Four Forces of Aerodynamics 65 Parts of an Aeroplane 66 Movement of Primary Flight Controls 66 Aircraft Axes 67 Aerodynamics of Flight 70 Lift and Weight 70 Centre of Gravity 70 Adverse Forward Centre of Gravity 72 Adverse Aft Centre of Gravity 73 Centre of Gravity Limits 74 Straight-and-Level Flight 74 2022-05-20 B-08 Basic Aerodynamics Page 5 of 113 CASA Part 66 - Training Materials Only  Forces in a Climb 75 Forces in a Descent 78 Forces in a Glide 78 Theory of the Turn 81 Centrifugal Force and Centripetal Force 81 Turns 81 Wing Loading 84 Introduction 84 Load Factor 84 Load Factor Limits 85 Lift Augmentation 87 High Lift Devices 87 Trailing Edge Flaps 87 Leading Edge Slats 88 Leading Edge Slots 89 Effects of Flaps and Slats on Coefficient of Lift (CL) 90 Flight Stability and Dynamics (8.4) 92 Learning Objectives 92 Aircraft Stability 93 Introduction 93 Static Stability 93 Dynamic Stability 95 Aeroplane Axes (Stability) 98 The Three Axes 98 Centre of Gravity 98 Stability About the Axes 99 Longitudinal Stability 101 Sideslip or Yawing 102 Lateral Stability 104 Oscillatory Instability 107 Introduction 107 Dutch Roll 107 The Pendulum Effect 108 Torque Effect 109 Ground Effect 110 Spiral Instability 111 Active Stability 112 2022-05-20 B-08 Basic Aerodynamics Page 6 of 113 CASA Part 66 - Training Materials Only  Physics of the Atmosphere (8.1) Learning Objectives 8.1.1 Interpret ISA conditions for temperature, pressure, humidity and density at sea level and identify basic changes in atmosphere with altitude (Level 2). 2022-05-20 B-08 Basic Aerodynamics Page 7 of 113 CASA Part 66 - Training Materials Only  Fundamentals of the Atmosphere Atmospheric Layers An atmosphere is a layer or a set of layers of gases surrounding a planet. It is within this region that all weather and climatic conditions are generated. Aviation Australia Layers of the atmosphere 2022-05-20 B-08 Basic Aerodynamics Page 8 of 113 CASA Part 66 - Training Materials Only  Composition of the Atmosphere The atmosphere is composed of 78% nitrogen, 21% oxygen and 1% other gases, e.g. carbon dioxide. All gases have physical properties such as pressure, density and temperature, which can vary within the atmosphere. Because of these variations, the performance of an aircraft will vary. 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. Composition of gases in the atmosphere 2022-05-20 B-08 Basic Aerodynamics Page 9 of 113 CASA Part 66 - Training Materials Only  Atmospheric Regions The atmosphere is classified into regions: troposphere, tropopause, stratosphere, mesosphere and thermosphere (ionosphere). Classification is based on the variation of temperature with altitude. Aircraft fly only in the troposphere and in the lowest part of the stratosphere. Layers of the atmosphere 2022-05-20 B-08 Basic Aerodynamics Page 10 of 113 CASA Part 66 - Training Materials Only  Troposphere The troposphere is the layer in which we live and in which most aircraft fly. It extends from the surface upward to the tropopause. The troposphere contains water vapour that causes clouds and accounts for what we call weather. In the troposphere, for every 1000 ft increase in altitude, the temperature drops approximately 2 ⁰C. This phenomenon is called the lapse rate. Tropospheric Layer Tropopause The tropopause is defined as the point in the atmosphere where the temperature is consistent regardless of altitude. The tropopause is located at the top of the troposphere and the start of the stratosphere. Its temperature is around a chilling -57 ⁰C. The tropopause occurs at approximately 20 000 ft over the poles and at approximately 60 000 ft above the equator. The International Standard Atmosphere (ISA) assumes that the average height of the tropopause is 36 000 ft. Stratosphere The atmospheric layer extending above from the tropopause is called the stratosphere. There is no water vapour in the stratosphere, and therefore no weather. 2022-05-20 B-08 Basic Aerodynamics Page 11 of 113 CASA Part 66 - Training Materials Only  Atmospheric Conditions Air Pressure 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 represented by the values displayed in the Sea Level Atmospheric Measurement table. Value Unit 14.7 PSI 29.92 inches of mercury (Hg) 760 mm of Hg 1013.25 millibars or hecto Pascal Mercury barometers are primarily used for measuring atmospheric pressure. A barometer is a device that consists of an upside-down tube filled with mercury that is in a vessel full of mercury. The atmosphere (or the air) pushes down on the mercury in the vessel, which in turn pushes the mercury in the tube up. As the pressure of the surrounding air decreases or increases, the mercury column lowers or rises correspondingly. At sea level, the height of the mercury in the tube measures standard day condition at 29.92 in. Hg or 1013.25 mbar. 2022-05-20 B-08 Basic Aerodynamics Page 12 of 113 CASA Part 66 - Training Materials Only  Air Density Density is mass per unit of volume of a substance. It is a property of air which makes all flight possible. The lower the density of the surrounding atmosphere, the more difficult flight becomes. Air at high altitudes with low pressure is less dense than air at low altitudes with higher pressure. The higher we ascend in the atmosphere, the lower the weight of the atmosphere above us. Therefore, the pressure will decrease. Note: Density increases as pressure increases and if temperature decreases. Air Pressure 2022-05-20 B-08 Basic Aerodynamics Page 13 of 113 CASA Part 66 - Training Materials Only  Air Temperature As we ascend in the atmosphere, there is a gradual decrease in temperature. In the troposphere the temperature drops at a steady rate called the lapse rate (2⁰C for every 1000 feet increase in altitude). The rate of temperature decrease does not alter until about 36000 ft. This is where tropopause is reached and quite suddenly the temperature ceases to fall, remaining practically constant to the outer limits of the stratosphere. Temperature decreases as altitude increases. 2022-05-20 B-08 Basic Aerodynamics Page 14 of 113 CASA Part 66 - Training Materials Only  Viscosity The viscosity of air is important in aerodynamics because air tends to "stick" to any surface over which it flows, slowing down the motion of the air. Air viscosity 2022-05-20 B-08 Basic Aerodynamics Page 15 of 113 CASA Part 66 - Training Materials Only  Humidity Humidity is the condition of moisture or dampness due to the amount of water vapour present in the air. The proportion of water vapour in the atmosphere varies depending on the temperature. When the proportion of water vapour is small, the air is said to be dry air. When the proportion is significant, the atmosphere is described as humid air. The higher the temperature of air, the more water vapour it can absorb. The density of the air varies with humidity. On a humid day, air is less dense due to water vapour displacing some of the dry air. Water vapour weighs approximately 5/8ths (five eighths) as much as an equal volume of perfectly dry air. Comparison of dry air and water vapour displacing air molecules in humid air. 2022-05-20 B-08 Basic Aerodynamics Page 16 of 113 CASA Part 66 - Training Materials Only  Absolute Humidity This refers to the actual amount of water vapour in a mixture of air and water. The higher the air temperature the more water vapour the air can hold. A hygrometer is an instrument used to measure the amount of water vapor in the air. Relative Humidity This is the ratio of the amount of moisture in the air to the amount that would be present if the air were saturated. Relative humidity has a dramatic effect on aircraft performance because of its effect on air density. For example, a relative humidity of 75% means the air is holding 75% of the total water vapour it can hold. Note: Air is most dense when it is perfectly dry. 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. 2022-05-20 B-08 Basic Aerodynamics Page 17 of 113 CASA Part 66 - Training Materials Only  Dew Point Dew point is the temperature to which the air must be lowered before the water vapour condenses out and becomes liquid water. The dew point is the temperature to which air must be cooled to become saturated without changing the pressure. 2022-05-20 B-08 Basic Aerodynamics Page 18 of 113 CASA Part 66 - Training Materials Only  Air Density Pressure, Temperature, Altitude Three factors affect air density: Pressure - As atmospheric pressure decreases, air density decreases. Temperature - As temperature increases, density decreases due to the volume of air expanding. Altitude - As altitude increases, air temperature decreases and 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. The affect of altitude on pressure. 2022-05-20 B-08 Basic Aerodynamics Page 19 of 113 CASA Part 66 - Training Materials Only  International Standard Atmosphere (ISA) Definition The International Civil Aviation Organisation (ICAO) administers the 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 was necessary to develop a standard set of conditions to which performance of an aircraft could be measured, and the ISA was adopted. 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 Conditions 2022-05-20 B-08 Basic Aerodynamics Page 20 of 113 CASA Part 66 - Training Materials Only  ISA Standard Conditions The values in the ISA Reference Values table are referred to as ISA Standard Day. Property Value Lapse rate 2 ⁰C/1000 feet Tropopause height 36 000 feet Sea level pressure 1013.25hPa = 29.92 in Hg = 14.7 psi Sea level temperature 15 ⁰C Gravity (g) 32.174 ft/sec² = 9.81 m/s² On many occasions the ground temperature and pressure are not exactly equal to ISA standard conditions. Where local sea level temperature is above 15 ⁰C, an ISA+ model is used. In this model the complete atmosphere is incremented by the temperature difference between the current sea level temperature and the standard value of 15 ⁰C. Temperatures at all levels of the ISA model are incremented by 5 ⁰C. For example, on a 20 ⁰C day, an ISA+ model is used. ISA temperature correction 2022-05-20 B-08 Basic Aerodynamics Page 21 of 113 CASA Part 66 - Training Materials Only  Pressure and Density Altitude Pressure altitude is the indicated altitude when an altimeter is set to 29.92 in. Hg (1013 hPa in other parts of the world). It is primarily used in aircraft performance calculations and in high-altitude flight. High Density Altitude = Decreased Performance Density altitude is an indicator of aircraft performance. The published performance criteria in the Pilot’s Operating Handbook (POH) are generally based on standard atmospheric conditions at sea level. Your aircraft will not perform according to “book numbers” unless the conditions are the same as those used to develop the published performance criteria. © Aviation Australia Pressure and density altitudes 2022-05-20 B-08 Basic Aerodynamics Page 22 of 113 CASA Part 66 - Training Materials Only  Aerodynamics I (8.2) Learning Objectives 8.2.1 Describe airflow characteristics as air flows around various shapes (Level 2). 8.2.2.1 Explain the meaning of the terms laminar flow, turbulent flow, boundary layer, free stream flow and stagnation as it relates to airflow (Level 2). 8.2.2.2 Explain relative airflow, up wash, downwash, vortices and how vortices are formed (Level 2). 8.2.3.1 Explain the term camber and calculate the mean camber line on a given aerofoil (Level 2). 8.2.3.2 Explain the term chord and identify a chord line on a given aerofoil (Level 2). 8.2.3.3 Explain the terms fineness ratio, angle of attack and centre of pressure (Level 2). 8.2.4.2 Explain the term resultant force with respect to lift (Level 2). 2022-05-20 B-08 Basic Aerodynamics Page 23 of 113 CASA Part 66 - Training Materials Only  Airflow Around a Body Free Stream Airflow To understand airflow around a body, let us first learn some terms and principles related to airflow. Air is invisible and it is difficult to understand what happens in flight; therefore, to be able to observe airflows around object, the flows must be made visible. A technique often used in wind tunnels is to introduce smoke in front of the aerofoil that is being tested. The smoke comes from regularly spaced point sources, and the wind flow in the tunnel spreads it out into parallel lines called streamlines. The streamlines make it possible to visualise the airflow over the aerofoil. When the lines continue smoothly over and past the airflow, they show that the flow remains laminar and that the aerofoil is creating very little drag. When the streamlines show more chaotic, turbulent flow, they indicate that the aerofoil is creating more drag. 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. © Aviation Australia Airflow around a body 2022-05-20 B-08 Basic Aerodynamics Page 24 of 113 CASA Part 66 - Training Materials Only  Laminar Flow Laminar flow is fluid flow in which the streamlines maintain a uniform parallel separation with no turbulence. It is shown as parallel straight lines on a flow diagram. Laminar flow Turbulent Flow Turbulent flow is random motion of fluid with unpredictable fluctuations and vortices. There are no streamlines present. At a certain velocity, laminar flow becomes turbulent flow. This velocity is called the critical velocity. Turbulent flow 2022-05-20 B-08 Basic Aerodynamics Page 25 of 113 CASA Part 66 - Training Materials Only  A flat plate has 100% resistance to airflow and a sphere has 50% resistance to airflow. An ovoid shape has 15% resistance to airflow and a streamlined shape (aerofoil) has 5% resistance to airflow. 2022-05-20 B-08 Basic Aerodynamics Page 26 of 113 CASA Part 66 - Training Materials Only  Boundary Layer The boundary layer is a very thin layer of air lying over the surface of the wing and all other surfaces of the aeroplane. It begins at the stagnation area. Boundary layer on the surface of a body The stagnation point is where the air is brought to rest by the leading edge and where the boundary layer originates. It 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. Because air has viscosity, this layer of air tends to adhere to the wing. 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. 2022-05-20 B-08 Basic Aerodynamics Page 27 of 113 CASA Part 66 - Training Materials Only  Boundary layer airflow on the surface of an aerofoil 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. 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 anti-clockwise. The transition point is the point on the wing at which the boundary layer changes from laminar to turbulent flow. As the speed increases, the transition point tends to move further forward, so more of the boundary layer becomes turbulent and the skin friction becomes greater. Separation points are the points on the wing at which the boundary layers break away from the surface. 2022-05-20 B-08 Basic Aerodynamics Page 28 of 113 CASA Part 66 - Training Materials Only  Boundary layer and transition region 2022-05-20 B-08 Basic Aerodynamics Page 29 of 113 CASA Part 66 - Training Materials Only  Relative Airflow Introduction Relative airflow is the movement of the air relative to the aircraft (or aerofoil). 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. It does not depend on the aircraft’s flight attitude or on the direction and speed of the wind. It does depend on the aircraft’s direction of travel. When the aircraft wing is moving forward and upward, the relative airflow is backwards and downwards. Relative airflow 2022-05-20 B-08 Basic Aerodynamics Page 30 of 113 CASA Part 66 - Training Materials Only  Upwash and Downwash The upwash is an area in front of the leading edge of an aerofoil where the airflow tends to move upwards. The downwash is an area behind the trailing edge of an aerofoil where the airflow tends to move downwards. © Aviation Australia Upwash and Downwash 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 anti-clockwise from the right wing (viewed from the rear). 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. Vortices 2022-05-20 B-08 Basic Aerodynamics Page 31 of 113 CASA Part 66 - Training Materials Only  Wake Wake turbulence is a disturbance in the atmosphere that forms behind an aircraft as it passes through the air. Wake turbulence 2022-05-20 B-08 Basic Aerodynamics Page 32 of 113 CASA Part 66 - Training Materials Only  Wing-Tip Vortices Wing tip vortices are caused by the higher-pressure air beneath the wing leaking around the wing tip and mixing with the low-pressure air above the wing. Wake turbulence is mainly due to these wing-tip vortices. Wing tip vortices 2022-05-20 B-08 Basic Aerodynamics Page 33 of 113 CASA Part 66 - Training Materials Only  Bernoulli’s Principle Bernoulli’s principle helps explain that an aircraft can achieve lift because of the shape of its wings. A practical application of Bernoulli’s principle is the venturi tube. The venturi tube has an air inlet that narrows to a throat (constricted point) and an outlet section that increases in diameter toward the rear. The diameter of the outlet is the same as that of the inlet. The mass of air entering the tube must exactly equal the mass exiting the tube. At the constriction, the speed must increase to allow the same amount of air to pass in the same amount of time as in all other parts of the tube. When the air speeds up, the pressure also decreases. Past the constriction, the airflow slows, and the pressure increases. Wings are shaped so that that air flows faster over the top of the wing and slower underneath. Fast- moving air equals low air pressure, while slow-moving air equals high air pressure. The high air pressure underneath the wings will therefore push the aircraft up through the lower air pressure. Several theories describe how a lifting force is generated by the action of air in motion past an aerofoil. Whatever the theory, the lift force results from a difference between the pressure's action on the upper and lower surfaces. Venturi tube 2022-05-20 B-08 Basic Aerodynamics Page 34 of 113 CASA Part 66 - Training Materials Only  Aerofoil Introduction Aerofoil is the term used to describe the characteristic shape of the cross-section of an aircraft wing whose purpose is to generate lift. Let us look at aerofoil sections. Comparison of airflow and pressure in a venturi and around an aerofoil 2022-05-20 B-08 Basic Aerodynamics Page 35 of 113 CASA Part 66 - Training Materials Only  Chord Line The chord of the aerofoil is the straight line joining the leading edge to the trailing edge. It is used as an arbitrary reference line when measuring the angular position of the wing in relation to the airflow. © Aviation Australia Aerofoil Chord Line and Mean Camber Line Camber Camber is defined as the curvature of an aerofoil section from the leading edge to the trailing edge. 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. Upper camber refers to the curve on the upper surface of an aerofoil, and lower camber refers to the curve of the lower surface. Aerofoil 2022-05-20 B-08 Basic Aerodynamics Page 36 of 113 CASA Part 66 - Training Materials 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. 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. 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, the ratio of length to breadth. Fineness ratio 2022-05-20 B-08 Basic Aerodynamics Page 37 of 113 CASA Part 66 - Training Materials Only  Angle of Attack Angle of attack is the angle between the chord line of the aerofoil and the free-stream flow. This means for a given airspeed, lift increases with an increase in the angle of attack, but only up to the stalling. Above stalling, 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 and helicopter rotor blades as well as to jet engine fan, compressor and turbine blades. Angle of Attack 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 the fuselage. This angle is fixed in manufacture and does not change. Angle of Incidence 2022-05-20 B-08 Basic Aerodynamics Page 38 of 113 CASA Part 66 - Training Materials Only  Aerofoil Shapes Symmetrical Aerofoil An aerofoil that has the same shape on both sides of its centre line. Non-symmetrical Aerofoil An aerofoil whose shape on either side of the chord is not the same. Aerofoil shapes 2022-05-20 B-08 Basic Aerodynamics Page 39 of 113 CASA Part 66 - Training Materials Only  Centre of Pressure All the pressure differences between the top and bottom surfaces of the aerofoil can be added together to produce the Total Air Reaction, which acts at a point called the Centre of Pressure (also referred to as CoP or C of P). Centre of Pressure Note: Total Air Reaction is also called resultant force, which is the vector sum of the magnitude and direction of the aerofoil’s reaction to the airflow. The component of resultant force that is perpendicular to the relative airflow is lift. The component that is parallel to relative airflow is the drag induced by the generation of lift. 2022-05-20 B-08 Basic Aerodynamics Page 40 of 113 CASA Part 66 - Training Materials Only  Air pressure distribution around an aerofoil The Cp is the point at which the resultant force intersects the chord of an aerofoil. For this reason, the CoP is also 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. PCB Piezotronics Model aircraft with multiple sensors in wind tunnel 2022-05-20 B-08 Basic Aerodynamics Page 41 of 113 CASA Part 66 - Training Materials Only  The location and direction in which the CoP will move depends on the shape of the aerofoil section and the angle of attack. The CoP is generally located at approximately the 25% chord position for most aerofoils. For example, on an aerofoil with a 60-in. chord, the centre of pressure is located at 15 in. aft from the leading edge. © Aviation Australia Pressure sensing around an aerofoil 2022-05-20 B-08 Basic Aerodynamics Page 42 of 113 CASA Part 66 - Training Materials Only  Aerodynamics II (8.2) Learning Objectives 8.2.3.4 Explain the terms Induced drag and Parasite drag (Level 2). 8.2.3.5 Describe common wing shapes and their use (Level 2). 8.2.3.6 Explain the terms aspect ratio, wash in and wash out (Level 2). 8.2.4.1 Explain the terms thrust and weight (Level 2). 8.2.5.1 Describe the role of wing angle of attack, wing shape and the lift coefficient in the generation of lift (Level 2). 8.2.5.2 Describe the role of wing shape and drag coefficient in the generation of drag (Level 2). 8.2.5.3 Describe the aerodynamic polar curve and how it may be used (Level 2). 8.2.5.4 Explain aerodynamic stall and the resultant lift immediately before and after the point of stall (Level 2). 8.2.6 Describe the aerodynamic effects of aerofoil contamination including ice, snow or frost (Level 2). 2022-05-20 B-08 Basic Aerodynamics Page 43 of 113 CASA Part 66 - Training Materials Only  Generation of Lift Introduction A few explanations for lift include Newton’s Third Law or angle of attack and the Bernoulli principle. These explanations work together to explain how lift is produced. Newton’s Third Law of motion states that for every action, there is an equal and opposite reaction. Based on this law, wings are forced upwards because they are tilted, pushing air downwards. This tilt is the angle of attack or the angle at which the wing meets the airflow. As air flows over the surface of a wing, it sticks slightly to the surface it is flowing past and follows the shape. If the wing is angled correctly, the air is deflected downwards. The action of the wing on the air forces the air downwards, while the reaction is the air pushing the wing upwards. Generation of lift (angle of attack) The amount of lift depends on the speed of the air around the wing and the density of the air. To produce more lift, the object must speed up and/or increase the angle of attack of the wing (by pushing the aircraft’s tail downwards). Speeding up causes the wings to force more air downwards so lift is increased. Increasing the angle of attack causes the air flowing over the top to turn downwards even more, and the air meeting the lower surface is also deflected downwards more, increasing lift. There is a limit to how large the angle of attack may be. If it is too great, the flow of air over the top of the wing is no longer smooth and the lift suddenly decreases. According to Bernoulli’s principle, the faster air moves, the less air pressure it generates. Normally, air moves along smoothly in streams, but airflow is disturbed when a wing moves through it, and the air divides and flows around the wing. The top surface of the wing is curved (aerofoil shape). The air moving across the top of the wing goes faster than the air travelling under the bottom. Because it’s moving faster, the air on top of the wing exerts less air pressure on the wing than the air below the wing. In other words, air below the wing pushes on the wing more than air above the wing. This difference in pressure combines with the lift from the angle of attack to give even more lift. 2022-05-20 B-08 Basic Aerodynamics Page 44 of 113 CASA Part 66 - Training Materials Only  Generation of lift (air pressure) Stalling Angle A stall is caused by the separation of airflow from the wing’s upper surface. This results in a rapid decrease in lift. As the angle of attack increases, the airflow has more difficulty remaining laminar on the top surface. There is often a transition point on the top of the aerofoil where the airflow changes from laminar to turbulent, and this may be the case for the whole speed range of the flight. Note: As the angle of attack increases, this point will move forward. Lift vs Angle of Attack 2022-05-20 B-08 Basic Aerodynamics Page 45 of 113 CASA Part 66 - Training Materials Only  A stall always occurs at the same angle of attack, regardless of air speed, flight attitude or weight. The Critical Angle of Attack (Stalling Angle) is the AOA at which maximum lift is generated. The aerofoil stalls at this point. It is important to remember that an aeroplane can stall at any airspeed, in any flight attitude, or at any weight. Note: Critical angle of attack normally occurs at about 15° and is when: Airflow separates Wing stalls Aircraft loses height. Aerofoil Stalling 2022-05-20 B-08 Basic Aerodynamics Page 46 of 113 CASA Part 66 - Training Materials Only  Generation of Drag Drag Drag is caused by any aircraft surface that deflects or interferes with the smooth airflow around the aeroplane. 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 can be classified into two main headings: Induced drag – drag due to lift Parasite drag – drag due to the viscosity of the air. Induced Drag Induced drag is caused by the lift generated by the wings. It is directly related to the angle of attack of the wing. The greater the angle, the greater the induced drag. Air flowing over the top of a wing tends to flow inwards because the decreased pressure over the top surface is less than the pressure outside the wing tip. Below the wing, the air flows outwards because the pressure below the wing is greater than that outside the wing tip. The direct consequence of this, as far as the wing tips are concerned, is that there is a continual spilling of air upwards around the wing tip. Aviation Australia Spilling of air upwards The streams of air from above and below the wing are flowing at an angle to each other as they meet along the trailing edge of the wing. They combine to form vortices which, when viewed from the rear, rotate clockwise from the left wing and anti-clockwise from the right. The tendency is for these vortices to move outwards towards the wing tip, joining as they do so. By the time the wing tip is reached, one large wing-tip vortex has formed. Most of these vortices are, of course, completely invisible, but in very humid air, the central core of a vortex may become visible because the air pressure within its centre has reduced and has therefore cooled sufficiently for condensation to occur. 2022-05-20 B-08 Basic Aerodynamics Page 47 of 113 CASA Part 66 - Training Materials Only  Wing Tip vs Vortices Parasite Drag Parasite or parasitic drag is all drag that is not associated with the production of lift. Parasite drag is created by the disruption of the airflow around the aeroplane’s surfaces. Parasite drag can be classified into the following main types. Form Drag Form drag is the result of the aerodynamic resistance to motion due to the shape of the aircraft. It is created by any structure which protrudes into the relative airflow. The amount of drag created is related to both the size and shape of the structure. It is essential that form drag be reduced to a minimum in all those parts of aircraft exposed to air. This can be done by shaping them so that the flow of air past them is as smooth as possible, which is accomplished by streamlining all the exposed parts. Form drag 2022-05-20 B-08 Basic Aerodynamics Page 48 of 113 CASA Part 66 - Training Materials Only  Skin Friction Drag Skin friction drag is due to the smoothness or roughness of the surfaces of the aircraft. Even though these surfaces may appear smooth, under a microscope they may be quite rough. A thin layer of air clings to these rough surfaces and creates small eddies, which contribute to drag. Skin friction drag Interference Drag Interference drag may occur where surfaces with different characteristics meet (e.g. wing and fuselage). It occurs when varied currents of air over an aeroplane meet and interact. Interference drag 2022-05-20 B-08 Basic Aerodynamics Page 49 of 113 CASA Part 66 - Training Materials Only  Total Drag Parasite drag increases with the square of the airspeed, while induced drag, as a function of lift, is greatest when maximum lift is being developed, usually at low speeds. There is an airspeed at which total drag is at a minimum, and in theory, this is the maximum range speed; however, flight at this speed is unstable because a small decrease in speed results in an increase in drag and a further fall in speed. The further decrease in speed causes yet more drag and, without the addition of thrust or initiation of a descent, could result in a stall or loss of control. Aviation Australia Total Drag In practice, for stable flight, maximum range is achieved at a speed a little above the minimum drag speed, where a small speed decrease results in a reduction in drag. At low speeds, induced drag is high due to the large vortices created at a high angle of attack. Comparison of Jet and Propeller driven aircraft Drag Curves 2022-05-20 B-08 Basic Aerodynamics Page 50 of 113 CASA Part 66 - Training Materials Only  Lift Coefficient The aircraft generates lift by moving quickly through the air. There is a high-pressure region underneath and a very low-pressure region on top of the wing. The difference in these pressure forces creates lift on the wing. The lift produced will be proportional to the: Shape of the aerofoil Area of the aerofoil Air density Speed of the air relative to the aerofoil surfaces Angle between the aerofoil and the relative airflow (angle of attack). DENSITY WING SUR FACE AREA LIFT = C Lx 1 /2 v2 s ANGLE OF ATTACK WING SHAPE SPEED © Aviation Australia Formula for Lift CL: Coefficient of Lift - This is affected by the angle of attack and the shape of the wing. 1/2 ρ: one half times rho (density) - Rho relates to the density of the air at the level and in the conditions in which you are currently flying. v2: velocity squared - Velocity relates to the speed at which you are flying. Notice its effect is squared, so it has a significant impact on the creation of lift. s: surface area of a wing - This is the area of the wing in square feet or square metres. 2022-05-20 B-08 Basic Aerodynamics Page 51 of 113 CASA Part 66 - Training Materials Only  Resultant force 2022-05-20 B-08 Basic Aerodynamics Page 52 of 113 CASA Part 66 - Training Materials Only  Drag Coefficient In moving through the air, an aircraft experiences a drag force. This force is made up of several distinct components: T otal Drag F orce (D) = I nduced Drag + F orm Drag + F riction Drag + I nterf erence Dr The following equation is simply a rearrangement of the above drag equation where we solve for the drag coefficient in terms of the other variables. 2 ρ × V D = Cd × × A 2 Variable Identity D Drag Cd Drag Coefficient p (rho symbol) Density of air V Velocity A Area 2022-05-20 B-08 Basic Aerodynamics Page 53 of 113 CASA Part 66 - Training Materials Only  Aerodynamic Curves The aerodynamic curve graph shows the curve produced and indicates a point of intersection at about a 4⁰ angle of attack. This is known as the optimum angle of attack. It is the angle that produces the best lift-to-drag (L/D) ratio. The angle created where the wing is fixed to the fuselage is called the angle of incidence. Aerodynamic Curve Graph 2022-05-20 B-08 Basic Aerodynamics Page 54 of 113 CASA Part 66 - Training Materials Only  Aerodynamic Forces Lift, Drag, Weight and Thrust Aircraft flight is controlled by adjusting the relationship between the four aerodynamic forces: Lift - perpendicular to the relative airflow Drag - parallel to the relative airflow Weight - due to gravity Thrust - produced by the power plant. © Aviation Australia Aerodynamic forces For constant speed and straight and level flight, lift equals weight and thrust equals drag. 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. 2022-05-20 B-08 Basic Aerodynamics Page 55 of 113 CASA Part 66 - Training Materials Only  Wing Shape Aircraft wings are aerofoils that create lift when moved rapidly through the air. Aircraft designers have created a variety of wings with different aerodynamic properties. Attached to the body of an aircraft at different angles, these wings come in different shapes. Wing configurations vary to provide different flight characteristics. The amount of lift an aircraft generates, control at different operating speeds, stability and balance all change as the aircraft wing’s shape is changed. Planform refers to the shape of an aeroplane's wing when viewed from above or below. Here are a few wing shape variations: Rectangular – cheapest to build Elliptical – most efficient Tapered – compromise between cost and efficiency Sweepback – highest speed. Wing shape 2022-05-20 B-08 Basic Aerodynamics Page 56 of 113 CASA Part 66 - Training Materials Only  Wing Geometry NASA Glenn Research Center Wing Geometry Chord Straight line joining the leading edge and trailing edge of an aerofoil. Wingspan Distance from one wing tip to the other wing tip. Wing Area The wing area is the projected area of the planform and is bounded by the leading and trailing edges and the wing tips. Note: The wing area is not the total surface area of the wing. The total surface area includes both upper and lower surfaces. 2022-05-20 B-08 Basic Aerodynamics Page 57 of 113 CASA Part 66 - Training Materials Only  Aspect Ratio The aspect ratio of a wing is found by dividing the square of the wingspan by the area of the wing, i.e. 2 Span Span AR = or Area Chord Thus, if a wing has an area of 250 sq ft and a span of 30 ft, the aspect ratio is 3.6. Another wing with the same span but with an area of 150 ft2 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 ft with a mean chord of 5 ft gives an aspect ratio of 10. Note: 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. Wing Description applied to an Aircraft 2022-05-20 B-08 Basic Aerodynamics Page 58 of 113 CASA Part 66 - Training Materials 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. Aspect Ratio and Maximum Lift Coefficient Aspect Ratio and Induced Drag A high-aspect-ratio wing will produce less induced drag than a wing of low aspect ratio because there is less air disturbance at the tip of a longer, thinner wing. Induced drag can therefore be said to be inversely proportional to aspect ratio. Aspect Ratio and Induced Drag 2022-05-20 B-08 Basic Aerodynamics Page 59 of 113 CASA Part 66 - Training Materials Only  Mean Aerodynamic Chord (MAC) When a wing is tapered, the chord is not uniform across the entire wingspan. The mean aerodynamic chord is the chord drawn through the centre of the area of the aerofoil. Tip Chord Mean Aerodynamic Chord (MAC) Root Chord Mean Aerodynamic Chord (MAC) Note: Equal amounts of wing area will lie on both sides of the MAC. MAC and theoretical wing 2022-05-20 B-08 Basic Aerodynamics Page 60 of 113 CASA Part 66 - Training Materials Only  Wash In and Wash Out Wash out is a greater angle of incidence at the root of the wing than at the tip. If a wing is designed so that the angle of incidence is greater at the tip than at the root, the characteristic is called wash in. The purpose of wash out 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. Wash in and Wash out A difference in the wash out and wash in 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. 2022-05-20 B-08 Basic Aerodynamics Page 61 of 113 CASA Part 66 - Training Materials Only  Icing Effects In-flight icing is one of the greatest dangers of cold weather flying because it causes airflow disruption that decreases the aircraft’s control and performance. As ice forms on the leading edge of an aircraft’s wing, it causes the wing to stall at a lower angle of attack and at a higher airspeed. Ice build up on a wing profile Even a thin layer of ice can have a large effect on wing stall. In many cases, increasing speed is required to maintain level flight. Ice can also build up unevenly between the two wings, which may throw the aircraft off balance and cause roll control issues. Effect of icing on the camber and lift curve 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. For example, 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 cambers of the aerofoil. 2022-05-20 B-08 Basic Aerodynamics Page 62 of 113 CASA Part 66 - Training Materials Only  The effects of icing 2022-05-20 B-08 Basic Aerodynamics Page 63 of 113 CASA Part 66 - Training Materials Only  Theory of Flight (8.3) Learning Objectives 8.3.1 Explain the relationship between lift, weight, thrust and drag and the effect of equilibrium in opposing forces (Level 2). 8.3.2 Explain glide ratio and describe how best glide angle is achieved with best lift/drag ratio (Level 2). 8.3.3.1 Explain the force relationships when an aircraft is in straight and level flight, or climbing/descending/turning at a constant rate (Level 2). 8.3.3.2 Describe measures of aircraft performance in steady state flight (Level 2). 8.3.4 Explain the forces apparent when an aircraft is turning and the results of force imbalance or poor turn coordination (Level 2). 8.3.5.1 Describe the correlation between wing loading and aircraft stall speed (Level 2). 8.3.5.2 Describe how load factor affects aircraft operating parameters, such as minimum speeds, ceiling and manoeuvering limits (Level 2). 8.3.5.3 Describe how load factor influences aircraft operational and structural limits, such as takeoff and landing weights, positive and negative G-forces and payload (Level 2). 8.3.6 Describe methods of lift augmentation (Level 2). 2022-05-20 B-08 Basic Aerodynamics Page 64 of 113 CASA Part 66 - Training Materials Only  Aircraft Aerodynamics The Four Forces of Aerodynamics During flight, four main forces act on an aeroplane: lift, weight, thrust and drag. © Aviation Australia Lift, weight, thrust and drag forces Lift Lift is upward force created by the effect of airflow as it passes over and under the wings. It supports the aeroplane in flight and acts at right angles to the free-stream flow through the Centre of Pressure (C of P). Weight Weight opposes lift and is caused by the downward pull of gravity. All the mass of the aircraft is said to act through the Centre of Gravity (C of G) of the aircraft. Thrust Thrust is forward force which propels the aeroplane through the air. It varies with the amount of engine power being used and acts through the centre line of the jet engine or the propeller spinner. 2022-05-20 B-08 Basic Aerodynamics Page 65 of 113 CASA Part 66 - Training Materials Only  Drag Drag is backward, or retarding, force that limits the speed of the aeroplane and opposes thrust. It acts backwards at right angles to the lift and resists the forward motion of the aircraft. Parts of an Aeroplane Each part of the aeroplane has a different function. The body of an aeroplane is called the fuselage, and the wings, tail and engine are attached to it. The wing is a horizontal aerofoil with the ailerons and flaps hinged to it, which produces lift. Parts of Aeroplane 2022-05-20 B-08 Basic Aerodynamics Page 66 of 113 CASA Part 66 - Training Materials Only  Movement of Primary Flight Controls The ailerons, which are the surfaces on the outside of the wing that swing up and down, control the roll of the aeroplane. When the right aileron swings up, the left aileron swings down and vice versa. The elevator swings up and down, which controls the pitch of the aeroplane. The rudder swings left and right, which controls the yaw of the aeroplane. The cockpit houses all the controls and instruments. Primary Flight Controls 2022-05-20 B-08 Basic Aerodynamics Page 67 of 113 CASA Part 66 - Training Materials Only  Aircraft Axes The simplest way to understand the axes is to think of them as long rods passing through the aircraft, where each will intersect the other two at a point called the centre of gravity. The aircraft can rotate around all three axes simultaneously, or it can rotate around just one axis. Stability of the aircraft is the combination of forces that act around these three axes to keep the pitch attitude of the aircraft in a normal level flight attitude, the wings level and the nose of the aircraft directionally straight along the desired path of flight. Aircraft Axes Lateral Axis The axis that extends crosswise (wing tip through wing tip) is called the lateral axis, and rotation about this axis is called pitch. Longitudinal Axis The axis that extends lengthwise (nose through tail) is call the longitudinal axis, and rotation about this axis is called roll. 2022-05-20 B-08 Basic Aerodynamics Page 68 of 113 CASA Part 66 - Training Materials Only  Vertical Axis The axis that passes vertically through the center of gravity (when the aircraft is in level flight) is called the vertical axis, and rotation about this axis is called yaw. 2022-05-20 B-08 Basic Aerodynamics Page 69 of 113 CASA Part 66 - Training Materials Only  Aerodynamics of Flight Lift and Weight Lift Lift is the key aerodynamic force. It is the force that opposes weight. In straight-and-level, unaccelerated flight, when weight and lift are equal, an aeroplane is in a state of equilibrium. If the other aerodynamic factors remain constant, the aeroplane neither gains nor loses altitude. 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 principal 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. © Aviation Australia Lift is the key aerodynamic force Weight Weight has a definite relationship with lift, and thrust has a definite relationship with drag. Lift is the upward force on the wing, acting perpendicular to the relative airflow. Lift is required to counteract the aircraft’s weight, which is caused by the force of gravity acting on the mass of the aircraft. 2022-05-20 B-08 Basic Aerodynamics Page 70 of 113 CASA Part 66 - Training Materials Only  Centre of Gravity This weight force acts downward through a point called the centre of gravity which is the point at which all the weight of the aircraft is considered to be concentrated. The lift acts 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 causes a movement of the lift. 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 (CG) and Centre of Pressure (CP) Centre of gravity is of major importance in an aircraft, for its position has great bearing on 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 fixes 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. The ideal arrangement is where the following conditions are both met: The centre of gravity is forward of the centre of pressure, which produces a nose-down couple, and the thrust line is lower than the centre of drag, which produces a nose-up couple. Each couple opposes the other and they cancel each other out. 2022-05-20 B-08 Basic Aerodynamics Page 71 of 113 CASA Part 66 - Training Materials Only  © Aviation Australia Ideal arrangement of aerodynamic forces Adverse Forward Centre of Gravity When too much weight is toward the forward part of the aeroplane, the CG is shifted forward. Adverse forward centre of gravity 2022-05-20 B-08 Basic Aerodynamics Page 72 of 113 CASA Part 66 - Training Materials Only  Any one of the following conditions may exist: Increased fuel consumption Increased power for any given speed Increased tendency to dive, especially with power off Increased oscillation tendency Increased stresses on the nose wheel Increased danger during flap operation Development of dangerous spin characteristics Increased difficulty in raising the nose of the aeroplane when landing. Adverse Aft Centre of Gravity When too much weight is toward the tail of the aeroplane, the centre of gravity is shifted backwards. Adverse Aft Centre of Gravity 2022-05-20 B-08 Basic Aerodynamics Page 73 of 113 CASA Part 66 - Training Materials Only  Any one of the following conditions may exist: Poor stability 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 Increased danger if tail assembly is damaged Poor landing characteristics. A study of the above listed conditions reveals that most of them could lead to an accident with a resulting loss of life and destruction of the aeroplane. Centre of Gravity Limits Each aeroplane type has its own centre of gravity limits, typically from about 15% to 40% of the wing chord. These have been established by design and based on 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. Centre of Gravity Limits 2022-05-20 B-08 Basic Aerodynamics Page 74 of 113 CASA Part 66 - Training Materials Only  Straight-and-Level Flight In straight-and-level flight and at a constant air speed: Lift = Weight and Thrust = Drag To change the equilibrium: To accelerate or climb, thrust must be added. To descend, thrust is reduced. © Aviation Australia Constant forces of flight 2022-05-20 B-08 Basic Aerodynamics Page 75 of 113 CASA Part 66 - Training Materials Only  Forces in a Climb During the transition from straight-and-level flight to a climb, a change in lift occurs when back elevator pressure is first applied. Raising the aircraft’s nose increases the Angle of Attack (AoA) and momentarily increases the lift. Lift at this moment is now greater than weight and starts the aircraft climbing. If the climb is entered with no change in power setting, the airspeed gradually diminishes because the thrust required to maintain a given airspeed in level flight is insufficient to maintain the same airspeed in a climb. When the flight path is inclined upwards, a component of the aircraft’s weight acts in the same direction as, and parallel to, the total drag of the aircraft, thereby increasing the total effective drag. Consequently, the total effective drag is greater than the power, and the airspeed decreases. Generally, the forces of thrust and drag, and lift and weight, again become balanced when the airspeed stabilises, but at a value lower than in straight-and-level flight at the same power setting. Since the aircraft’s weight is acting not only downwards but rearwards with drag while in a climb, additional power is required to maintain the same airspeed as in level flight. The amount of power depends on the angle of climb. When the climb is established steeply enough that insufficient power is available, a slower speed will result. Forces in climb Note: In a steady climb, thrust must balance the drag plus a portion of the weight, lift is less than weight and thrust is greater than drag. 2022-05-20 B-08 Basic Aerodynamics Page 76 of 113 CASA Part 66 - Training Materials Only  Changes in lift during climb entry Changes in speed during climb entry To operate at the maximum angle of climb possible, we need the biggest possible value of thrust minus drag. If thrust minus 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 a steady climb. 2022-05-20 B-08 Basic Aerodynamics Page 77 of 113 CASA Part 66 - Training Materials Only  As the climbing angle increases, lift proportionally decreases (w cos θ), and therefore more thrust is required. Forces in a Descent As in climbs, the forces that act on the aircraft go through definite changes when a descent is entered from straight-and-level flight. For the following example, the aircraft is descending at the same power used in straight-and-level flight. As forward pressure is applied to the control yoke to initiate the descent, the angle of attack is decreased momentarily. Initially, the momentum of the aircraft causes the aircraft to briefly continue along the same flight path. In this instant, the angle of attack decreases, causing the total lift to decrease. With weight now greater than lift, the aircraft begins to descend. At the same time, the flight path goes from level to descending. As the aeroplane descends, weight is once again greater than lift and thrust is reduced to allow gravity to pull the aircraft towards Earth. Note: In a powered descent, thrust may be reduced as gravity supplies some of the energy, lift is less than weight, and drag is balanced by the reduced thrust and a part of the weight. Forces in Descent 2022-05-20 B-08 Basic Aerodynamics Page 78 of 113 CASA Part 66 - Training Materials Only  Forces in a Glide In a glide, thrust is removed from among the four forces. In a steady glide, the aeroplane must be kept in a state of equilibrium by lift, drag and weight. Lift and drag must be exactly opposite to the weight. Forces in a Glide If an aeroplane is to glide as far as possible, the angle of attack during the glide must produce the maximum lift/drag ratio (L/D). If the pilot attempts to glide at an angle of attack 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, and there is no way the pilot can extend the glide beyond that ratio. The pilot adopts the descent angle that gives the best L/D ratio or the lowest rate of descent. Gliding at a speed greater than the best glide speed will cause the parasitic drag to increase. Similarly, gliding at a speed less than the best glide speed will cause the induced drag to increase. Glide Ratio The glide ratio is expressed as the ratio of the horizontal distance travelled (d) to the vertical height descended (h): d Glide Ratio = h 2022-05-20 B-08 Basic Aerodynamics Page 79 of 113 CASA Part 66 - Training Materials Only  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’: h tan(a) = d Glide Lift to Drag Ratio (L / D) Glide Angle 2022-05-20 B-08 Basic Aerodynamics Page 80 of 113 CASA Part 66 - Training Materials Only  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 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, which 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. Centrifugal Force and Centripetal Force (Turning Flight) 2022-05-20 B-08 Basic Aerodynamics Page 81 of 113 CASA Part 66 - Training Materials Only  Turns 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 the other 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 the weight. The horizontal component of lift, called centripetal force, is directed inwards, 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 outwards from the centre of rotation. When the opposing forces are balanced, the aeroplane maintains a constant rate of turn without gaining or losing altitude. 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 slipstream). 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. 2022-05-20 B-08 Basic Aerodynamics Page 82 of 113 CASA Part 66 - Training Materials Only  Skidding In an aeroplane, there is no such thing as a flat turn. If the bank is too small, the aeroplane will skid outwards, caused by the centrifugal force generated in the turn. The pilot will feel the airflow come from the outside of the turn. 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, with their effective weight magnified in the same proportions as the lift. Types of Turns 2022-05-20 B-08 Basic Aerodynamics Page 83 of 113 CASA Part 66 - Training Materials Only  Wing Loading Introduction Wing loading is the all-up weight (AUW) of the aircraft divided by the wing area. Wing Loading = Weight / Wing Area Wing Loading That is, it equals the amount of the total weight carried per unit area of the wing. It is usually given in pounds per square foot (lb/ft2) and kilograms (kg) per square metre (kg/m2). Wing loading is an important factor in determining the minimum speed at which the aircraft can fly without stalling. An aeroplane With a lower wing loading will have a lower stalling speed With a higher wing loading will have a higher stalling speed With a lower wing loading will have a lower minimum speed than one with a high wing loading. As more weight is added to the aircraft, e.g. passengers and baggage, its wing loading, minimum speed and stalling speed will increase. 2022-05-20 B-08 Basic Aerodynamics Page 84 of 113 CASA Part 66 - Training Materials Only  Load Factor Load factor is the ratio of load imposed on an aircraft structure to the weight of the structure itself. Load factor can be expressed as: imposed load Load F actor (n) = aircraf t weight It is usually given in g (gravity units). For example, 2 g is a load that is twice the weight of the aircraft. Aircraft are fitted with a vertical accelerometer that measures the imposed load factor during flight. In straight and level flight, the load imposed on the airframe is equal to aircraft weight, thus n = 1 g. The load factor in a normal turn is greater than 1 g. A load factor of less than 1 g (including a negative load factor) is possible during certain manoeuvres and during turbulence. The designer must ensure that the aircraft structure is strong enough to withstand load factors greater than +1 g as well as negative load factors. Load Factor 2022-05-20 B-08 Basic Aerodynamics Page 85 of 113 CASA Part 66 - Training Materials Only  Load Factor Limits As load factor increases, the stress on the airframe increases. All aircraft will have a specified g limit to avoid structural damage. Certification standards set minimum standards of airframe strength. Some aircraft are designed to reach their critical angle of attack and stall before reaching a dangerous load factor. The regulations may permit such an aircraft to operate with a lower g limit. Effect of Load Factor on Stall Speed The safety margin above stall becomes narrower during a turn because of the increased load factor. The increased angle of attack through the turn means the aircraft is flying closer to the angle of attack at which it stalls. Bank Angle vs Load Factor There is also a narrower margin between the actual speed of the aircraft through the turn and the speed at which the aircraft will stall if the pilot fails to maintain airspeed. 2022-05-20 B-08 Basic Aerodynamics Page 86 of 113 CASA Part 66 - Training Materials Only  Lift Augmentation High Lift Devices High-lift devices are movable surfaces or, in some cases, stationary components that are designed to increase lift during some phases or conditions of flight. High-lift devices are most frequently utilised during the take-off and initial climb and the approach and landing phases of flight, but may also be used in any other low-speed situation. The incorporation of high-lift devices allows aircraft designers to reduce the overall size and surface area of the wing, reducing its drag and making the aircraft more fuel efficient during the cruise phase of flight. The most common high lift devices are: Trailing edge flaps Leading edge slats Leading edge slots. © Aviation Australia High lift devices 2022-05-20 B-08 Basic Aerodynamics Page 87 of 113 CASA Part 66 - Training Materials Only  Trailing Edge Flaps Trailing edge flaps are designed to increase the lift of the wing and decrease the stall speed. This allows the aeroplane to fly at reduced speed while maintaining sufficient control. One method of lift augmentation is altering the effective shape of the aerofoil by movable trailing edge flaps. Trailing Edge Flaps The ability to fly slowly is particularly important during the approach and landing phases. For example, an approach with full flaps allows the aircraft to fly slowly and at a fairly steep descent angle without gaining airspeed. This allows for touchdown at a slower speed. Flap Effects 2022-05-20 B-08 Basic Aerodynamics Page 88 of 113 CASA Part 66 - Training Materials Only  Leading Edge Slats Slats are extendable high lift devices on the leading edge of the wings of some fixed-wing aircraft. Their purpose is to increase lift during low-speed operations such as take-off, initial climb, approach and landing. They accomplish this by increasing both the surface area and the camber of the wing by deploying outwards and drooping downwards from the leading edge. Slats 2022-05-20 B-08 Basic Aerodynamics Page 89 of 113 CASA Part 66 - Training Materials Only  Leading Edge Slots A stall occurs when the angle of attack becomes so great that the energy in the air flowing over the wing can no longer pull air down to the surface. The boundary layer thickens and becomes turbulent, and airflow separates from the surface. This separation can be delayed until a higher angle of attack is achieved by increasing the energy of the air flowing over the surface. One way to do this is by installing a slot in the leading edge of the wing. This slot is simply a duct for air to flow from below the wing to the top. Once there, the air is directed over the surface in a high-velocity stream. A slot in the leading edge of the wing is a fixed duct and is not necessary at all angles of attack. A slat extends to form a slot. Some slats can be automatically deployed at a predetermined angle of attack; other types can be mechanically deployed by the pilot when needed. Slat Slots are typically placed ahead of the aileron to keep the outer portion of the wing flying after the root has stalled. This maintains aileron effectiveness and provides lateral control during most of the stall. Slot 2022-05-20 B-08 Basic Aerodynamics Page 90 of 113 CASA Part 66 - Training Materials Only  Effects of Flaps and Slats on Coefficient of Lift (CL) When the flaps/slats are lowered and the camber is increased, both the lift and drag are increased. Most flap/slat installations are designed in such a way that the first half of the flap/slat extension increases the lift more than the drag, and partial flaps/slats are used for take-off. Lowering the flaps/slats all the way increases the drag more than the lift, and full flaps are used for landing. The Effect of Flaps and Slats on CL 2022-05-20 B-08 Basic Aerodynamics Page 91 of 113 CASA Part 66 - Training Materials Only  Flight Stability and Dynamics (8.4) Learning Objectives 8.4.1.1 Describe common methods used for passive longitudinal, lateral and directional stability of an aircraft (Level 2). 8.4.1.2 Describe common methods used for active longitudinal, lateral and directional stability of an aircraft (Level 2). 2022-05-20 B-08 Basic Aerodynamics Page 92 of 113 CASA Part 66 - Training Materials Only  Aircraft Stability Introduction Although no aircraft is completely stable, all aircraft must have desirable stability and handling characteristics. This quality is essential throughout a wide range of flight conditions: during climbs, descents and turns, and at both high and low airspeeds. The stability of an aeroplane means its ability to return to some particular condition of flight (after having been slightly