British Airways Global Learning Academy - Basic Aerodynamics - POTA (PDF)

British Airways Global Learning Academy – Basic Aerodynamics - POTA 8.1 – Physics Of The Atmosphere Composition of the Earths’ Atmosphere Before discussing the fundamentals of the theory of flight, there are several basic ideas that must be considered. An aircraft operates in the air, therefore, the...

British Airways Global Learning Academy – Basic Aerodynamics - POTA 8.1 – Physics Of The Atmosphere Composition of the Earths’ Atmosphere Before discussing the fundamentals of the theory of flight, there are several basic ideas that must be considered. An aircraft operates in the air, therefore, the properties of air that affect aircraft control and performance must be understood. The air that surrounds the earth is a physical mixture of gases, and therefore follows the laws of gasses, it is also considered a fluid because it can be made to flow and change its shape even when a small pressure is applied. This mass of air is known as the atmosphere, which is a large mass of air extending upward hundreds of miles (approximately 500 miles or 804 km). So everything on the earth’s surface is under pressure due to the weight of the air. The pressure applied equals the weight of a column of air one square inch in cross sectional area that extends to the top of the atmosphere. This pressure equals 14.69 psi on the earth’s surface at sea level. Fig 1 – Composition Of The Atmosphere Air consists of several separate gases and the percentage as shown remains constant regardless of their altitude. The gasses referred to are shown in Figure 1 The region known as the Tropopause is a region, or boundary layer of atmosphere, somewhere between 20,000 feet and 60,000 feet above sea level. It is a portion of the troposphere and the stratosphere and is the region of the atmosphere where most fixed wing aircraft are confined to fly shown in figure 2. The ionosphere and exosphere are of little importance for commercial aircraft. The atmosphere is divided into four concentric gas layers starting from the earth’s surface and extending upward we have: a) b) c) d) Troposphere Stratosphere Ionosphere Exosphere 2 - (Approx. 7 miles /35,000 ft above sea level). - (Approx. 32 miles /160,000 ft above sea level). - (Approx. 200 miles above sea level). - (Approx. 500 miles above sea level). Module 8B ETBN 0492 October 2023 Edition 4 Possible Q Basic Aerodynamics – Physics Of The Atmosphere British Airways Global Learning Academy – Basic Aerodynamics - POTA The Tropopause marks the ceiling of the troposphere. It is found at levels varying from around 20,000ft over the Polar Regions to 60,000ft over the equator as seen in Figure 3. In the temperate latitudes, roughly mid-way between the tropics and the poles, the average altitude of the Tropopause is around 35000ft. The Tropopause is higher in summer than in winter, and it rises and falls as high- pressure and low pressure systems move under it. Fig 3 – Troposphere Altitude Variations Figure 2 – Earth’s Troposphere And Stratosphere Module 8B ETBN 0492 October 2023 Edition 5 Basic Aerodynamics – Physics Of The Atmosphere British Airways Global Learning Academy – Basic Aerodynamics - POTA International Standard Atmosphere (ISA) In order to have a reference for all aerodynamic computations, an organisation known as the International Civil Aeronautics Organisation (ICAO) has agreed upon an International Standard Atmosphere (ISA). The pressures, temperatures and densities in a standard atmosphere serve only as a reference. When all aerodynamic computations are related to this standard, a meaningful comparison of flight test data between aircraft can be made. The standard atmospheric conditions are based on a sea level temperature of 15°C or 59°F or 288K and a barometric pressure of 29.92 inches of mercury (inHg), or 1013.25 millibars (mb); also when we observe the chart we see the pressures, temperatures and densities all decrease with an increase in altitude. Temperature 15°C (59°F, 288.15K) Pressure 14.69 lbs/sq in (psi) 1013.25 millibars (mb) 29.92 inHg The ISA declares that the air pressure at sea level is 14.69 psi at a temperature of 15°C (59°F) and 6.75 psi at 20,000 feet indicating a 2 psi decrease for every 5,000 feet. Density Air density at Sea Level, although not on the chart in Figure 5, is 1.225kg/m3 (0.77lbs/ft3) 1.225 kg/m3 (0.77 lbs/ft3) Fig 4 -ISA Summary The standard temperature lapse rate, is 1.98°C or 3.5°F decrease in temperature every 1,000 feet in altitude up to 40,000 feet. However, air pressure is not constant; there are high and low pressure areas. The high-pressure air moves toward the low-pressure air, we witness this phenomenon as wind. Module 8B ETBN 0492 October 2023 Edition 6 Basic Aerodynamics – Physics Of The Atmosphere British Airways Global Learning Academy – Basic Aerodynamics - POTA Figure 5 – ISA Chart Module 8B ETBN 0492 October 2023 Edition 7 Basic Aerodynamics – Physics Of The Atmosphere British Airways Global Learning Academy – Basic Aerodynamics - POTA Pressure In the aviation industry there are four ways of referring to pressure: a) b) c) d) At the surface, pressure reduces at about one millibar per 27ft. By 20,000ft the rate of pressure reduction is about one millibar per 50ft, and by 30,000ft is about one millibar per 75ft Pounds per square inch (psi) Inches of mercury (inHg) Millibars (mb) Pascal (pa) Pressure is essentially caused by earth’s gravity pulling a column of air towards the earth’s surface. As the column of air nears the surface, the pressure increases over the given area, due to the air density and mass as seen in Figure 6. Gauge Pressure Pressures expressed in pounds per square inch (psi) usually do not take into consideration atmospheric pressure and refer to pressure in excess of atmospheric pressure. These pressures are generally read from a gauge and can be referred to as gauge pressure or psig. Fuel and oil pressures are measured in gauge pressure and indicate the amount a pump raises the pressure of the liquid above atmospheric pressure. You can see this when you open the valve on an oxygen cylinder. When this is done the oxygen rushes out until the pressure inside the cylinder equals that of the atmosphere or 14.69 psi. However, once the pressure equalises, the gauge on the cylinder reads zero. The same can be seen when you pump tyres, a flat tyre or empty tyre will indicate zero on the gauge. The reduction of pressure with height does not take place at a constant rate (Fig 6a). Module 8B ETBN 0492 October 2023 Edition Fig 6 – Air Pressure And The Effect Of Gravity 8 Basic Aerodynamics – Physics Of The Atmosphere British Airways Global Learning Academy – Basic Aerodynamics - POTA Absolute Pressure Absolute Pressure is pressure referenced from zero rather than from atmospheric pressure, it is known as absolute pressure, or psia. By definition, absolute pressure is gauge pressure plus atmospheric pressure. When a tube is filled with mercury and inverted in a bowl, the mercury drops in the tube until the atmospheric pressure exerted on the mercury in the bowl equals the weight of the mercury in the tube. Standard atmospheric pressure can support a column of mercury that is 29.92 inches tall. Fig 6a – Relationship Between Altitude And Pressure Figure 7 Gauge Pressure Module 8B ETBN 0492 October 2023 Edition 9 Basic Aerodynamics – Physics Of The Atmosphere British Airways Global Learning Academy – Basic Aerodynamics - POTA An altimeter (seen in Figure 9) measures absolute pressure and displays the result in feet above sea level. In aircraft, the absolute pressure within an engine induction system is indicated on a manifold pressure gauge. However, when the engine is not running, the gauge reads the existing atmospheric pressure. Another good example of this is the altimeter and readings are in feet rather than in inches of mercury. The metric unit of measure for barometric pressure is millibars. One millibar is approximately equal to 0.0295 inches of mercury. Figure 8 – Pressure Measurement Under standard sea-level conditions atmospheric pressure supports a column of 29.92 inches, or 760 millimetres high. Figure 9 – Altimeter (Three Pointer) Module 8B ETBN 0492 October 2023 Edition 10 Basic Aerodynamics – Physics Of The Atmosphere British Airways Global Learning Academy – Basic Aerodynamics - POTA Static And Dynamic Pressures Imagine a tube at rest. The tube is experiencing a pressure from the air around it. This pressure is known as the static pressure, and is the pressure exerted on all of us by the atmosphere all day and every day. Now let’s move the tube at 100Kts. There is now an additional pressure caused by the movement of the tube through the air. This pressure is known as the dynamic pressure, (pitot). Dynamic pressure increases as the tube’s speed increases; and will also increase if air density is increased. Dynamic pressure is given by: Fig 10 – Static And Dynamic (Pitot) Pressure Total Pressure Total pressure is the sum of all pressures acting on a surface. Normally this refers to static plus dynamic pressure. Module 8B ETBN 0492 October 2023 Edition 11 Basic Aerodynamics – Physics Of The Atmosphere British Airways Global Learning Academy – Basic Aerodynamics - POTA Temperature Air Density The temperature also varies throughout the atmosphere, and under normal circumstances it decreases with height. In the troposphere, the average rate of temperature change is about 1.98°C (2°C) for every 1000ft of altitude change. The rate of temperature change through the atmosphere is known as the lapse rate. Density is a term that means the mass of a material per unit volume and it varies with temperature and pressure in both liquids and gases. Two factors that are necessary to figure the density of a substance are mass and volume. Temperature is more critical when measuring the density of gases than when measuring the density of solids or liquids. The basic formula to find density is: There are two temperature scales with which you must be familiar. They are the Fahrenheit and the Celsius scales. The Celsius scale has 100 divisions between the freezing and boiling points of pure water. The Fahrenheit scale is not based on the freezing and boiling points of water so on this scale water freezes at 32° F and boils at 212°F. The standard temperature for all aerodynamic computations is 15°C or 59°F. 𝐷𝑒𝑛𝑠𝑖𝑡𝑦 = 𝑀𝑎𝑠𝑠 𝑉𝑜𝑙𝑢𝑚𝑒 Because air is a mixture of gases, it can be compressed. If the air in one container is under one-half as much pressure as the air in another identical container, the air under the greater pressure weighs twice as much as the air in the container under low pressure. The air under greater pressure is twice as dense as that in the other container. The density of gases is governed by the following rules: 1. Density varies in direct proportion with the pressure. 2. Density varies in inverse proportion with the temperature. Therefore air at high altitudes is less dense than air at low altitudes, and a mass of hot air is less dense than a mass of cool air. Air Density and Lift Changes in density affect the aerodynamic performance of an aircraft. Using the same thrust or power setting, an aircraft can fly faster at high altitudes where the density is low, than it can at lower altitudes where the density of the air is much greater. Figure 11 – UK VFR Temperature Conversion Module 8B ETBN 0492 October 2023 Edition 12 Basic Aerodynamics – Physics Of The Atmosphere British Airways Global Learning Academy – Basic Aerodynamics - POTA This is because air offers less resistance to the aircraft when it contains a smaller number of air particles per unit volume. On hot days, the density of the air is less than on cold days. On wet days, the density is less than on dry days. However, you will see that the amount of lift generated depends upon the shape of the aerofoil, the angle of attack, and the air speed. Another factor that affects lift is the air density. Lift varies directly with air density. Also density decreases or becomes less with an increase in altitude. When the density is low, the lift will also be comparatively lower. Therefore, the airspeed must increase as the density decreases in order to maintain the aircraft at the same angle of attack in level flight. The amount of lift produced by an aerofoil depends on the number of pounds of air it can push down. At 18,000 feet, where the density of the air is just half as much as at sea level, an aircraft would have to travel approximately 1.5 times as fast as it would at sea level to maintain altitude. Therefore aircraft can’t fly too high and it is between 30,000 feet and 45,000 feet that most commercial aircraft fly. It is at this altitude that the best power to weight ratio and lift coefficient is obtained. Humidity As we know, the air is seldom completely dry. It does contain a little moisture, either in the form of fog, or water vapour. Fog consists of minute droplets of water held in suspension by the air. Clouds are composed of fog. The moisture in the air is often referred to as humidity, and the maximum amount of water vapour that air can hold varies with the temperature. The higher the temperature of the air, the more water vapour it can absorb. By itself, water vapour weighs approximately 5/8 as much as an equal amount of perfectly dry air. Therefore, when air contains water vapour it is not as heavy as air containing no moisture. Remember, pure dry air contains about 78% nitrogen and 21% oxygen. Fig 12 – Air Density And Lift Module 8B ETBN 0492 October 2023 Edition 13 Basic Aerodynamics – Physics Of The Atmosphere British Airways Global Learning Academy – Basic Aerodynamics - POTA Assuming that the temperature and pressure remain the same, the density of the air varies inversely with the humidity. On damp days the air density is less than on dry days. For this reason, an aircraft requires a longer runway for take-off on damp days than it does on dry days. Absolute Humidity Absolute humidity refers to the actual amount of water vapour in a mixture of air and water. Relative Humidity Refers to the ratio between the amounts of moisture in the air, to the amount that would be present, if the air were saturated. Fog and humidity both affect the performance of an aircraft. For example, because humid air is less dense than dry air, the allowable take-off gross weight of an aircraft is generally reduced when operating in humid conditions. Dew Point This is the temperature of the air where it can no longer hold any more water vapour. When the dew point is reached, the air contains 100% of the moisture it can hold at that temperature and is now said to be saturated. If the temperature drops below the dew point, condensation occurs. Secondly, because water vapour is incombustible, its presence in the atmosphere results in a loss of engine power output. This power loss is more on piston engines than on turbine engines. Figure 14 – Dew Point And Cloud Formation Fig 13 – Illustration Of Water Vapour In Air Module 8B ETBN 0492 October 2023 Edition 14 Basic Aerodynamics – Physics Of The Atmosphere British Airways Global Learning Academy – Basic Aerodynamics - POTA Acceleration Due to Gravity As the force is inversely proportional to the distance squared this means the attraction at the poles is greater than at the equator. If a small body is sufficiently close to the earth to experience the pull due to gravity the attraction will cause the body to move toward the surface of the earth. The acceleration is constant, i.e. gravity produces a constant acceleration of 9.81 m/s2 or 32.2 ft/s2 regardless of the mass of the body. 1. At the poles g = 32.3 ft/s2 2. In the U. K. g = 32.2 ft/s2 3. At the equator g = 32.1 ft/s2 If a body is allowed to fall freely under the influence of gravity it will suffer constant acceleration due to gravity. The term “freely” is used to indicate that there is no force resisting its downward movement. You can now see that flight through the atmosphere is not as simple as one thinks, and many graphs and calculations must be made and specialised equipment used like the highly sophisticated fuel control units used on modern commercial aircraft. In this condition the speed will continue to build up. In practice however, there is a force resisting the downward motion provided by the atmosphere, i.e. the friction due to movement through the air. These calculations and equipment have to take Pressure, Temperature, Density and Humidity into account when calculating the best power to weight ratio and the correct altitude for our commercial aircraft to fly safely and economically This limits the build-up of speed until a point is reached where retardation due to air friction is equal to the acceleration due to gravity, and no further acceleration occurs. The body has then reached its terminal velocity. Viscosity In streamline motion, layers of fluid travel at different speeds (like traffic in the lanes of a motor-way) and slide over one another. Forces which are in essence equivalent to frictional forces are set up between two layers of a fluid which are in relative motion. This is called viscosity. Because the acceleration due to gravity is constant and applies to all bodies irrespective of their mass, different bodies would undergo the same acceleration when allowed to fall freely, for example a stone and a feather both falling in a vacuum will reach the bottom together but have different accelerations when falling through the atmosphere. If you stir a cup of tea which has tea leaves floating on top so that a circular motion takes place you will see that the leaves near the centre revolve more quickly than do those near the edge. Acceleration (g) due to gravity, although said to be constant, does vary slightly at different parts of the earth’s surface. This is because the distance from the surface to the centre of the earth is not exactly the same at all points. The earth is flatter at the poles than at the equator. Module 8B ETBN 0492 October 2023 Edition If we imagine the liquid to consist of a number of concentric vertical thin cylinders, there is relative motion between these cylinders, the liquid in contact with the side of the cup being at rest. 15 Basic Aerodynamics – Physics Of The Atmosphere British Airways Global Learning Academy – Basic Aerodynamics - POTA Frictional forces exist between these cylinders, when they move relatively to each other, which eventually stop the motion. In a highly viscous liquid such as glycerine the motion ceases almost at once. The higher the viscosity of a liquid then the longer the time it will take to pour out of a container. An example of this situation could be an aircraft wing skin traveling through stationary air where the moving boundary is the wing skin and the fixed boundary is the stationary air, a small distance away from the skin. Viscosity is a property of both liquids and gases. Viscosity in gases can be explained in terms of molecular movement. Distance from stationary boundary y Very briefly, in the case of a gas it is due to molecules moving from the slower moving layers into the faster moving layers, and from the faster moving layers to the slower moving layers. The net result of this is that more momentum is carried one way than the other, which means that forces between the layers retard the faster moving layers and accelerate the slower moving ones. Moving boundary Velocity = v Stationary boundary Velocity = 0 We define viscosity as: “The property of a fluid that offers resistance to the relative motion of its molecules”. The energy losses due to friction within a fluid are dependent on its viscosity. As a fluid moves, there is developed a shear stress in it, the magnitude of which depends on the viscosity of the fluid. Figure 15 – Velocity Change Between Fixed And Moving Boundaries The diagram illustrates the concept of velocity change in a fluid by showing a thin layer of fluid (a boundary layer) sandwiched between a fixed and moving boundary. The boundary layer is a thin layer of fluid between a fixed and moving boundary, across which a velocity change takes place. Module 8B ETBN 0492 October 2023 Edition 16 Basic Aerodynamics – Physics Of The Atmosphere