Physics of the Atmosphere (8.1) PDF - Aviation Australia

```markdown # Physics of the Atmosphere (8.1) 8.1 Interpret ISA conditions for temperature, pressure, humidity and density at sea level and identify basic changes in atmosphere with altitude (Level 2) # Fundamentals of the Atmosphere ## Introduction to Physics of the Atmosphere The atmosphere is th...

```markdown # Physics of the Atmosphere (8.1) 8.1 Interpret ISA conditions for temperature, pressure, humidity and density at sea level and identify basic changes in atmosphere with altitude (Level 2) # Fundamentals of the Atmosphere ## Introduction to Physics of the Atmosphere 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: * Nitrogen 78% * Oxygen 21% * CO2, Argon and other Gases 1% The remaining 0.003% is made up of microscopic quantities of other gases such as neon, helium, krypton, ozone, etc. **Composition of gases in the atmosphere** The nature of the atmosphere may vary considerably from day to day and from place to place at any given place. 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. 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 ft (nearly 14 km). The image shows the concentric layers of the atmosphere surrounding planet earth: Troposphere, Tropopause, Stratosphere, Mesosphere, Thermosphere. **Physical Composition of the Atmosphere** ## 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 hectopascals (hPa)). The higher we ascend in the atmosphere, the less will be the weight above us. Therefore, the pressure will be less. **Pressure of the atmosphere** | Altitude in 1000 FT | Pressure in PSI | | ----------- | ----------- | | 0 | 14 | | 10 | 12 | | 20 | 9 | | 30 | 6 | | 40 | 4 | | 50 | 3 | | 60 | 2 | | 70 | 1 | ## 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 °C for each 1000 ft in height, up to 36 000 ft (11000 m). Above 36 000 ft (11000 m) the temperature remains nearly constant until the outer regions of the atmosphere are reached. **Temperature v Altitude** | Altitude x 1000 FT | Temperature in DEG. CELSIUS | | ----------- | ----------- | | 0 | 15 | | 10 | -5 | | 20 | -25 | | 30 | -45 | | 40 | -57 (36, 40, 50, 60, 70) | The atmosphere is classified into regions based on the variation of temperature with altitude as shown in the image below. Air temperature undergoes considerable change as altitude increases: * Troposphere - gradual temperature decrease. * Tropopause - temperature remains approximately constant. * Stratosphere - gradual temperature increase. * Mesosphere - gradual temperature decrease. * Thermosphere (ionosphere) - rapid temperature increase. The image shows a diagram with the description that air expands and cools as it rises to 15,000 ft reaching -10°C. Descending air compresses and warms ending at 5,000 ft reaching +20°C. **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 ft (10 km), it's too thin to support respiration. **Note:** Aircraft altitude is still measured in feet (ft). Civil aircraft normally fly at altitudes up to 45 000 ft (14 km). Although the atmosphere is divided into several regions, we will only be covering the three closest to the Earth's surface. These are: * Troposphere * Tropopause * Stratosphere. ### 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 ft). In the troposphere, for every 1000 ft 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 ft, while over the equator it can be as high as 55 000 ft. These heights may vary due to seasonal changes which cause temperature fluctuations. However, the average of approximately 36 000 ft is taken to be the tropopause. At this height, the atmospheric pressure is approximately 3 psi or 1/5 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 ft. ### Stratosphere The atmospheric layer extending from the tropopause up to an average altitude of between 50 to 55 km 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 km (70 000 ft). Between 20 km and approximately 32 km, the temperature begins to slowly rise. Above an altitude of 32 km, the temperature starts to increase more rapidly. The temperature rise ceases at around 0 °C, between the altitudes of 50 to 55 km. This point is called the stratopause. **Atmospheric conditions** | Speed of sound m/s | Density kg/m³ | Press kN/m² | Temp. K | Height above sea-level km | Temp. °C | Press mb | Relative density P/Po | Relative pressure P/Po | | ----------- | ----------- | ----------- | ----------- | ----------- | ----------- | ----------- | ----------- | ----------- | | 299 | 0.040 | 2.5 | 299 | 25 | -52 | 28 | 0.036 | 0.028 | | 298 | 0.047 | 3.0 | 221 | 24 | -54 | 35 | 0.046 | 0.035 | | 298 | 0.055 | 3.5 | 220 | 23 | -55 | 45 | 0.059 | 0.044 | | 296 | 0.076 | 4.7 | 218 | 21 | -56 | 57 | 0.075 | 0.056 | | 295 | 0.089 | 5.5 | 217 | 20 | -56 | 72 | 0.095 | 0.071 | | 295 | 0.122 | 7.6 | 217 | 18 | -56 | 92 | 0.121 | 0.091 | | 295 | 0.165 | 12.1 | 217 | 15 | -56 | 117 | 0.153 | 0.115 | | 295 | 0.228 | 14.2 | 217 | 14 | -56 | 148 | 0.194 | 0.145 | | 295 | 0.267 | 16.6 | 217 | 13 | -56 | 188 | 0.247 | 0.186 | | 295 | 0.312 | 19.4 | 217 | 12 | -54 | 239 | 0.311 | 0.235 | | 299 | 0.414 | 26.5 | 223 | 10 | -44 | 301 | 0.375 | 0.298 | | 304 | 0.467 | 30.8 | 230 | 9 | -34 | 377 | 0.449 | 0.372 | | 308 | 0.525 | 35.7 | 235 | 8 | -25 | 466 | 0.533 | 0.460 | | 317 | 0.650 | 47.2 | 249 | 6 | -15 | 572 | 0.629 | 0.565 | | 325 | 0.819 | 61.7 | 262 | 4 | -5 | 697 | 0.739 | 0.688 | | 333 | 79.5 | 275 | 5 | | 853 | 0.862 | 0.832 | | 340 | 101.3 | 288 | 15 | | 1013 | 1.000 | 1.000 | ## Air Density ### Introduction to 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. The image shows a diagram with the description that air expands and cools as it rises to 15,000 ft reaching -10°C. Descending air compresses and warms ending at 5,000 ft reaching +20°C. **Effects of altitude on temperature** Half of all air molecule mass is found below 5500 m (18000 ft) altitude. **Molecule mass and altitude** The diagram shows the distribution of air molecule mass as it relates to altitude. 164000 ft = 99.9% OF MASS BELOW THIS LEVEL 53000 ft= 90% OF/MASS BELOW THIS LEVEL 18000 ft = 50% OF MASS BELOW THIS LEVEL 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). On the left there is a series of large circles representing molecules of air. An airplane passes through them with the description: GREATER AIR SPEED AND DISTANCE HIGH ALTITUDE - LESS DENSITY On the right, there are smaller circles representing the increased density of air at lower altitudes, with an airplane with the description: SAME POWER LESS SPEED LESS DISTANCE **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. ## 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 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. The diagram 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. **DRY AIR** (Picture of air particles) **HUMID AIR** (Picture of air and water particles) **Water vapour** Humidity can be stated as: * Absolute humidity * Relative humidity. ### Absolute 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 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. ### 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. The image shows dew on the windows of an aircraft. **Morning dew** ## International Standard Atmosphere ### Introduction to International Standard Atmosphere 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 ft (6.49°C/1000 m) * Tropopause height is at 11 000 m (36 000 ft) * Mean sea-level pressure is: * 1013.25 hectopascals (hPa) * 14.69 pounds per square inch (psi) * 29.92 inches mercury (in Hg). * Mean sea-level temperature is 15 °C * Mean sea-level humidity is zero (0%) * Gravity ($g$) is 9.809 m/s² (32.174 ft/s²). These values are referred to as ISA "Standard Day". The reason 15 °C (when air is perfectly dry) is used is because it's the average condition prevailing at latitude 45° North. ## Pressure Altitude Pressure altitude is the altitude in the standard atmosphere corresponding to a particular value of air pressure. Pressure altitude is the indicated altitude when an altimeter is set to 29.92 in Hg (1013 hPa). 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. ## Density Altitude Density altitude is a measure of how thick the air is. This is based on three factors: * Atmospheric pressure * Temperature * Humidity. The technical definition of density altitude is pressure altitude, adjusted for non-standard temperature. What that really means is that on hot days, the air is much thinner, or less dense, than it is on cold days. Why does that matter? It's a big factor in the aeroplane's performance, because when the air surrounding the plane is less dense, it means the wings, propeller, and engine will have a lot less performance, and it will take more time to get airborne during take off. ## Aerodynamics I (8.2) 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) ```