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Physics of the Atmosphere
An understanding of the basic principles of aerodynamics is as important to the aviation maintenance technician as it is to the pilot and the aerospace engineer. The technician is concerned with the strength of an aircraft because of the stresses applied through the forces of aerodynamics when the aircraft is in flight. Often responsible for the repair or restoration of aircraft structures, the technician must know that the repair work will restore the required strength to the parts that are being repaired. There are certain physical laws which describe the behavior of airflow and define the various aerodynamic forces acting on a surface. These principles of aerodynamics provide the foundations for a good understanding of what may be termed the “theory of flight.”
The study of moving air and the force that it produces is referred to as aerodynamics. As studied by the engineer or scientist, aerodynamics involves the use of advanced mathematics and physics; however, this chapter presents only the basic principles of the subject and their application to the flight of aircraft, without the necessity of advanced mathematical analysis. The subject can therefore be more easily understood by you, the student whose primary concern lies with the maintenance, operation, and repair of the aircraft.
Pressure
Atmospheric pressure is usually defined as the force exerted against the earth’s surface by the weight of the air above that surface. Weight is force applied to an area that results in pressure. Force (F) equals area (A) times pressure (P), or F = AP. Therefore, to find the amount of pressure, divide area into force (P = F/A). A column of air (one square inch) extending from sea level to the top of the atmosphere weighs approximately 14.7 pounds; therefore, atmospheric pressure is stated in pounds per square inch (psi). Thus, atmospheric pressure at sea level is 14.7 psi.
Atmospheric pressure is measured with an instrument called a barometer, composed of mercury in a tube that records atmospheric pressure in inches of mercury (Hg).
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| Standard sea level pressure |
The standard measurement in aviation altimeters and U.S. weather reports has been "Hg". However, worldwide weather maps and some non-U.S., manufactured aircraft instruments indicate pressure in millibars (mb), an SI metric unit.
Aviators often interchange references to atmospheric pressure between linear displacement (e.g., inches of mercury) and units of force (e.g., psi). Over the years, meteorology has shifted its use of linear displacement representation of atmospheric pressure to units of force. The unit of force nearly universally used today to represent atmospheric pressure in meteorology is the hectopascal (hPa). A pascal is a SI metric unit that expresses force in Newtons per square meter. A hectoPascal is 100 Pascals. 1 013.2 hPa is equal to 14.7 psi which is equal to 29.92 Hg. (Figure 1-3)
Atmospheric pressure decreases with increasing altitude. The simplest explanation for this is that the column of air that is weighed is shorter. How the pressure changes for a given altitude is shown in Figure 1-4. The decrease in pressure is a rapid one and, at 50 000 feet, the atmospheric pressure has dropped to almost one-tenth of the sea level value.
As an aircraft ascends, atmospheric pressure drops, the quantity of oxygen decreases, and temperature drops. These changes in altitude affect an aircraft’s performance in such areas as lift and engine horsepower. The effects of temperature, altitude, and density of air on aircraft performance are covered in the following paragraphs.
Temperature
Temperature variations in the atmosphere are of concern to aviators. Weather systems produce changes in temperature near the earth’s surface. Temperature also changes as altitude is increased. The troposphere is the lowest layer of the atmosphere. On average, it ranges from the earth’s surface to about 38,000 feet above it. Over the poles, the troposphere extends to only 25,000–30,000 feet and, at the equator, it may extend to around 60,000 feet.
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| The troposphere extends higher above the earth’s surface at the equator than it does at the poles |
Most civilian aviation takes place in the troposphere in which temperature decreases as altitude increases. The rate of change is somewhat constant at about –2 °C or –3.5 °F for every 1,000 feet of increase in altitude. The upper boundary of the troposphere is the tropopause. It is characterized as a zone of relatively constant temperature of –57 °C or –69 °F.
Above the tropopause lies the stratosphere. Temperature increases with altitude in the stratosphere to near 0 °C before decreasing again in the mesosphere, which lies above it. The stratosphere contains the ozone layer that protects the earth’s inhabitants from harmful UV rays. Some civilian flights and numerous military flights occur in the stratosphere.
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| The atmospheric layers with temperature changes depicted by the red line |
When an aircraft is flown at high altitude, it burns less fuel for a given airspeed than it does for the same speed at a lower altitude. This is due to decreased drag that results from the reduction in air density. Bad weather and turbulence can also be avoided by flying in the relatively smooth air above storms and convective activity that occur in the lower troposphere.
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| Cabin environmental systems establish conditions quite different from these found outside the aircraft |
To take advantage of these efficiencies, aircraft are equipped with environmental systems to overcome extreme temperature and pressure levels. While supplemental oxygen and a means of staying warm suffice, aircraft pressurization and air conditioning systems have been developed to make high altitude flight more comfortable.
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:
- Altitude – as altitude increases, density decreases due to decreased atmospheric pressure.
- Temperature – as temperature increases, density decreases due to the volume of air expanding.
- 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).
Density is a term that means weight per unit volume. Since 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 that in the container under lower pressure. The air under greater pressure is twice as dense as that in the other container. For equal weights of air, that which is under the greater pressure occupies only half the volume of that under half the pressure.
The density of gases is governed by the following rules:
- Density varies in direct proportion with the pressure.
- Density varies inversely with the temperature.
Thus, 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.
Changes in density affect the aerodynamic performance of aircraft with the same horsepower, an aircraft can fly faster at a high altitude where the density is low than at a low altitude where the density is great. This is because air offers less resistance to the aircraft when it contains a smaller number of air particles per unit volume.
Humidity
Humidity is caused by the condition of moisture or dampness. Water vapour is always present in the atmosphere and is one of the most important factors in human comfort. The proportion of water vapour in the atmosphere varies widely from place to place, and time to time.
Travelling around Australia in the summer months you would come across large fluctuations of humidity, depending on where you were. In Melbourne the temperature may be 30°C with a humidity of 60%, while in Darwin the temperature may be 30°C with a humidity of 95%. If you were to travel into the outback away from the coast the temperature could fluctuate between 20°C and 50°C, with almost no humidity (the air is very dry).
When the proportion of water vapour is small, the air is said to be dry. When the proportion is significant, the atmosphere is described as moist, damp, wet or humid. Figure 10 below shows that on a humid day air is less dense for a given volume due to water vapour displacing some of the dry air.
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 s the ratio between the amount of moisture in the air to the amount 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.
Composition of the Atmosphere
The atmosphere is the layer of air which envelops the earth and the mixture of gases is commonly called air. It is composed principally of 78 percent nitrogen and 21 percent oxygen. The remaining 1 percent is made up of various gases in smaller quantities. Some of these are important to human life, such as carbon dioxide, water vapor, and ozone.
As altitude increases, the total quantity of all the atmospheric gases reduces rapidly. However, the relative proportions of nitrogen and oxygen remain unchanged up to about 50 miles above the surface of the earth. The percentage of carbon dioxide is also fairly stable. The amounts of water vapor and ozone vary. Up to a height of some 8–9 km water vapour is found in varying quantities. The amount of water vapour in a given mass of air depends on the temperature of the air and whether or not the air has recently passed over large areas of water. The higher the temperature of the air, the higher the amount of water vapour it can hold. Thus, at altitude where the air temperature is least, the air will be dry.
The earth’s atmosphere (Figure 4.90) can be said to consist of five concentric layers. These layers, starting with the layer nearest the surface of the earth, are known as the troposphere, stratosphere, mesosphere, thermosphere and exosphere.
The boundary between the troposphere and stratosphere is known as the tropopause and this boundary varies in height above the earth’s surface from about 7.5 km at the poles to 18 km at the equator. An average value for the tropopause in the “International Standard Atmosphere” (ISA) is around 11 km or 36,000 ft.
The thermosphere and the upper parts of the mesosphere are often referred to as the ionosphere, since in this region ultraviolet radiation is absorbed in a process known as photo-ionization.
In the above zones, changes in temperature, pressure, density and viscosity take place, but of these (aerodynamically at least), only the troposphere and stratosphere are significant. About 75% of the total air mass in the atmosphere is concentrated in the troposphere
International Standard Atmosphere (ISA)
Due to different climatic conditions that exist around the earth, the values of temperature, pressure, density, viscosity and sonic velocity (speed of sound) are not constant for a given height,
Changes in atmosphere can effect:
- Lift.
- Drag.
- Engine performance.
- Component adjustments.
- Instrument adjustment and monitoring.
- The application of surface finishes.
- Manufacture and repair of composite structures.
The ISA has, therefore, been set up to provide a standard for:
- The comparison of aircraft performance; and
- The calibration of aircraft instruments.
The ISA is a “hypothetical” atmosphere based on world average values. Note that the performance of aircraft, their engines and their propellers is dependent on the variables quoted in the ISA. It will be apparent that the performance figures quoted by manufacturers in various parts of the world cannot be taken at face value but must be converted to standard values, using the ISA. If the actual performance of an aircraft is measured under certain conditions of temperature, pressure and density, it is possible to deduce what would have been the performance under the conditions of the ISA, so that it can then be compared with the performance of other aircraft which have similarly been reduced to standard conditions.
The ISA is used to compare aircraft performance and enable the calibration of aircraft instruments.
The sea-level values of some of the more important properties of air contained in the ISA are tabulated below.
Changes in properties of air with altitude
Temperature falls uniformly with height until about 11 km (36,000 ft). This uniform variation in temperature takes place in the troposphere, until a temperature of 216.7 K is reached at the tropopause. This temperature then remains constant in the stratosphere, after which the temperature starts to rise once again.
It is possible to calculate the temperature at a given height h (km) in the troposphere from the simple relationship Th = T0 - Lh, where Th is the temperature at height h (km) above sea level and T 0 and L have the meanings given in the table of properties of air at sea level shown above.
The ISA value of pressure at sea level is given as 1013.2 mb. As height increases, pressure decreases, such that at about 5 km, the pressure has fallen to half its sea-level value and at 15 km it has fallen to approximately one-tenth of its sea-level value.
The ISA value of density at sea level is 1.225 kg/m3. As height increases, density decreases but not as fast as pressure. Such that, at about 6.6 km,
the density has fallen to around half its sea-level value and at about 18 km it has fallen to approximately one-tenth of its sea-level value.
Humidity levels of around 70% water vapour at sea level drop significantly with altitude. Remember that the amount of water vapour a gas can absorb decreases with decrease in temperature.At an altitude of around 18 km the water vapour in the air is approximately 4%. Thus, to ensure passenger comfort during flight, it is essential to maintain the correct humidity level within an aircraft’s environmental control system.
With increase in altitude up to the tropopause; temperature, density, pressure and humidity all decrease.
The International Civil Aviation Organization ISA
As you already know, the International Civil Aviation Organization Standard Atmosphere, as defined in ICAO Document 7488/2, lays down an arbitrary set of conditions, accepted by the international community, as a basis for comparison of aircraft and engine performance parameters and for the calibration of aircraft instruments.
The conditions adopted have been based on those observed in a temperate climate at a latitude of 40◦ North up to an altitude of 105,000 feet. The principle conditions assumed in the ISA are summarized below, for your convenience.
- Temperature = 288.15 K or 15.15◦C
- Pressure = 1013.25 mb or 101325 N/m2
- Density = 1.2256 kg/m3
- Speed of sound = 340.3 m/s
- Gravitational acceleration = 9.80665 m/s2
- Dynamic viscosity = 1.789 × 10-5 N.s/m2
- Temperature lapserate=6.5 K/km or 6.5◦C/km
- Tropopause = 11,000 m, –56.5◦C or 216.5 K.
Note the following Imperial equivalents, which are often quoted:
- Pressure = 14.69 lb/in2
- Speed of sound = 1120 ft/s
- Temperature lapse rate = 1.98◦C per 1000 feet.
- Tropopause = 36,090 feet
- Stratopause = 105,000 feet.
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| The ICAO Standard Atmosphere |
The changes in ICAO Standard Atmosphere with altitude are illustrated in Figure 7.1. Note that the temperature in the upper stratosphere starts to rise again after 65,000 feet at a rate of 0.303◦C per 1000 feet or 0.994◦C per 1000 m. At a height of 105,000 feet, or approximately 32,000 m, the chemosphere is deemed to begin. The chemosphere is the collective name for the mesosphere, thermosphere and exosphere, which were previously identified in atmospheric physics.
