EASA M08-1 Physics Atmosphere.pdf

PHYSICS OF THE ATMOSPHERE PART-66 SYLLABUS LEVELS...

PHYSICS OF THE ATMOSPHERE PART-66 SYLLABUS LEVELS CERTIFICATION CATEGORY ¦ B1 B2 Sub-Module 01 PHYSICS OF THE ATMOSPHERE Knowledge Requirements 8.1 - Physics of the Atmosphere International Standard Atmosphere (ISA), application to aerodynamics. 2 2 Level 2 A general knowledge of the theoretical and practical aspects of the subject and an ability to apply that knowledge. Objectives: (a) The applicant should be able to understand the theoretical fundamentals of the subject. (b) The applicant should be able to give a general description of the subject using, as appropriate, typical examples. (c) The applicant should be able to use mathematical formula in conjunction with physical laws describing the subject. (d) The applicant should be able to read and understand sketches, drawings and schematics describing the subject. (e) The applicant should be able to apply his knowledge in a practical manner using detailed procedures. Module 08 - Basic Aerodynamics 1.1 BASIC AERODYNAMICS Three topics that are directly related to the manufacture, about aerodynamics. The technician must be able to operation, and repair of aircraft are: aerodynamics, understand the relationships between how an aircraft aircraft assembly, and rigging. Each of these subject performs in flight and its reaction to the forces acting areas, though studied separately, eventually connect to on its structural parts. Understanding why aircraft provide a scientific and physical understanding of how are designed with particular types of primary and an aircraft is prepared for flight. A logical place to start secondary control systems and why the surfaces must with these three topics is the study of basic aerodynamics. be aerodynamically smooth becomes essential when By studying aerodynamics, a person becomes familiar maintaining today’s complex aircraft. with the fundamentals of aircraft flight. The theory of flight should be described in terms of the Aerodynamics is the study of the dynamics of gases. The laws of flight because what happens to an aircraft when interaction between a moving object and the atmosphere it flies is not based upon assumptions, but upon a series is the primary interest in this module. The movement of of facts. Aerodynamics is a study of laws which have an object and its reaction to the air flow around it can been proven to be the physical reasons why an airplane be seen when watching water passing the bow of a ship. f lies. The term aerodynamics is derived from the The major difference between water and air is that air combination of two Greek words: "aero," meaning air, is compressible and water is incompressible. The action and "dyne," meaning force of power. Thus, when "aero" of the airflow over a body is a large part of the study of joins "dynamics" the result is "aerodynamics"—the study aerodynamics. Some common aircraft terms, such as of objects in motion through the air and the forces that rudder, hull, water line, and keel beam, were borrowed produce or change such motion. from nautical terms. Aerodynamically, an aircraft can be def ined as an M a ny te x t b o ok s h av e b e en w r it ten a b out t he object traveling through space that is affected by the aerodynamics of aircraft f light. It is not necessary changes in atmospheric conditions. To state it another for an airframe and powerplant technician to be as way, aerodynamics covers the relationships between the knowledgeable as an aeronautical design engineer aircraft, relative wind, and atmosphere. PHYSICS OF THE ATMOSPHERE Before examining the fundamental laws of f light, PRESSURE several basic facts must be considered. An aircraft Atmospheric pressure is usually def ined as the force operates in the air. Therefore, those properties of air exerted against the earth’s surface by the weight of the that affect the control and performance of an aircraft air above that surface. Weight is force applied to an area must be understood. that results in pressure. Force (F) equals area (A) times pressure (P), or F = AP. Therefore, to find the amount of The air in the earth’s atmosphere is composed mostly pressure, divide area into force (P = F/A). A column of of nitrogen and oxygen. Air is considered a f luid air (one square inch) extending from sea level to the top because it fits the definition of a substance that has the of the atmosphere weighs approximately 14.7 pounds; ability to flow or assume the shape of the container therefore, atmospheric pressure is stated in pounds per in which it is enclosed. If the container is heated, square inch (psi). Thus, atmospheric pressure at sea level pressure increases; if cooled, the pressure decreases. is 14.7 psi. (Figure 1-1) The weight of air is heaviest at sea level where it has been compressed by all of the air above. This Atmospheric pressure is measured with an instrument compression of air is called atmospheric pressure. called a barometer, composed of mercury in a tube that records atmospheric pressure in inches of mercury (Hg). (Figure 1-2) 1.2 Module 08 - Basic Aerodynamics  Height of Earth’s Thermosphere atmosphere (about 50 miles) PHYSICS OF THE 1 inch 350 km 1 inch ATMOSPHERE Mesosphere Entire air column weight 14.7lb 90 km Ozone layer 50 km Stratosphere Tropopause Troposphere 18 km 14 km Sea level Earth Figure 1-1. The weight exerted by a 1 square inch column of air stretching from sea level to the top of the atmosphere is what is measured when it is said that atmospheric pressure is equal to 14.7 pounds per square inch. The standard measurement in aviation altimeters and U.S. weather reports has been "Hg". However, world- Vacuum Inches of Millibars Standard Standard Sea Level Mercury Sea Level wide weather maps and some non-U.S., manufactured Pressure 30 1016 Pressure 29.92" Hg 1013 aircraft instruments indicate pressure in millibars (mb), 25 847 mb 20 677 an SI metric unit. 15 508 10 339 Aviators often interchange references to atmospheric 5 170 pressure between linear displacement (e.g., inches of 0 0 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 1" force. The unit of force nearly universally used today to represent atmospheric pressure in meteorology is 1" the hectopascal (hPa). A pascal is a SI metric unit that expresses force in Newtons per square meter. A 1" hectoPascal is 100 Pascals. 1 013.2 hPa is equal to 14.7 0.491 lb Mercury psi which is equal to 29.92 Hg. (Figure 1-3) Figure 1-2. Barometer used to measure atmospheric pressure. Atmospheric Pressure Standard atmospheric pressure at sea level is also known as 1 atmosphere, or 1 atm. The following measurements of standard atmospheric pressure are all equal to each other. 1 atm = 14.7 psi = 29.92 in Hg = 1013.2 hPa = 1013.2 mb = 760 mm Hg (or 101325 (prounds per (inches of (millimeters (atmosphere) newtons per (millibars) square inch) mercury) of mercury) square meters) Figure 1-3. Various equivalent representations of atmospheric pressure at sea level. Module 08 - Basic Aerodynamics 1.3 Atmospheric pressure decreases with increasing altitude. The simplest explanation for this is that the column of 100 000 air that is weighed is shorter. How the pressure changes for a given altitude is shown in Figure 1-4. The decrease 80 000 in pressure is a rapid one and, at 50 000 feet, the Altitude (feet) 60 000 atmospheric pressure has dropped to almost one-tenth of the sea level value. 40 000 As an aircraft ascends, atmospheric pressure drops, the 20 000 quantity of oxygen decreases, and temperature drops. These changes in altitude affect an aircraft’s performance Sea level 0 2 4 6 8 10 12 14 in such areas as lift and engine horsepower. The effects Pressure (pounds per square inch) of temperature, altitude, and density of air on aircraft performance are covered in the following paragraphs. Figure 1-4. Atmospheric pressure decreasing with altitude. At sea level the pressure is 14.7 psi, while at 40 000 feet, as the dotted lines show, the pressure is only 2.72 psi. DENSITY Density is weight per unit of volume. Since air is a Thus, air at high altitudes is less dense than air at low mixture of gases, it can be compressed. If the air in one altitudes, and a mass of hot air is less dense than a mass container is under half as much pressure as an equal of cool air. Changes in density affect the aerodynamic amount of air in an identical container, the air under performance of aircraft with the same horsepower. An greater pressure is twice as dense as that in the other aircraft can fly faster at a high altitude where the air container. For the equal weight of air, that which is density is low than at a low altitude where the density under the greater pressure occupies only half the volume is greater. This is because air offers less resistance to of that under half the pressure. the aircraft when it contains a smaller number of air particles per unit of volume. The density of gases is governed by the following rules: 1. Density varies in direct proportion with the pressure. 2. Density varies inversely with the temperature. HUMIDITY Humidity is the amount of water vapor in the air. The By itself, water vapor weighs approximately five-eighths maximum amount of water vapor that air can hold varies as much as an equal amount of perfectly dr y air. with the temperature. The higher the temperature of the Therefore, when air contains water vapor, it is not as air, the more water vapor it can absorb. heavy as dry air containing no moisture. 1. Absolute humidity is the weight of water vapor in a unit volume of air. 2. Relative humidity is the ratio, in percent, of the moisture actually in the air to the moisture it would hold if it were saturated at the same temperature and pressure. 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 takeoff on damp days than it does on dry days. 1.4 Module 08 - Basic Aerodynamics TEMPERATURE AND ALTITUDE PHYSICS OF THE Temperature variations in the atmosphere are of concern ATMOSPHERE to aviators. Weather systems produce changes in temperature near the earth’s surface. Temperature also 100 000–116 000 feet changes as altitude is increased. The troposphere is the lowest layer of the atmosphere. On average, it ranges 137 000–153 000 feet from the earth’s surface to about 38 000 feet above it. 25 000–30 000 feet Over the poles, the troposphere extends to only 25 000 - N 30 000 feet and, at the equator, it may extend to around Equator 60 000 feet. This oblong nature of the troposphere is S Troposphere 55 000–65 000 feet illustrated in Figure 1-5. Stratosphere Most civilian aviation takes place in the troposphere in which temperature decreases as altitude increases. The Mesosphere rate of change is somewhat constant at about –2 °C or Thermosphere –3.5 °F for every 1 000 feet of increase in altitude. The upper boundary of the troposphere is the tropopause. Figure 1-5. The troposphere extends higher above the earth’s surface at It is characterized as a zone of relatively constant the equator than it does at the poles. temperature of –57 °C or –69 °F. Above the tropopause lies the stratosphere. Temperature 90 increases with altitude in the stratosphere to near 0 °C before decreasing again in the mesosphere, which lies 80 above it. The stratosphere contains the ozone layer that protects the earth’s inhabitants from harmful UV Thermosphere (Ultraviolet) rays. Some civilian flights and numerous 70 military flights occur in the stratosphere. 60 Figure 1-6 diagrams the temperature variations in different layers of the atmosphere. 160 000 ft Mesopause 50 Height (km) As stated, density varies inversely with temperature or, Tem per Mesosphere atu 40 as temperature increases, air density decreases. This re phenomenon explains why on very warm days, aircraft takeoff performance decreases. The air available for 160 000 ft Stratopause 30 combustion is less dense. Air with low density contains Stratosphere less total oxygen to combine with the fuel. 20 Ozone layer 10 38 000 ft Tropopause Mt. Everest Troposphere –100 –80 –60 –40 –20 0 20 40 50 °C –140 –100 –60 –20 0 20 60 100 120 °F Temperature Figure 1-6. The atmospheric layers with temperature changes depicted by the red line. Module 08 - Basic Aerodynamics 1.5 INTERNATIONAL STANDARD ATMOSPHERE The atmosphere is never at rest. Pressure, temperature, Civil Aviation Organization (ICAO), International humidity, and density of the air are continuously Organization for Standardization (ISO), and various changing. To provide a basis for theoretical calculations, governments establish and publish the values known as performance comparisons and instrumentation parity, the International Standard Atmosphere. (Figure 1-7) standard values for these and other characteristic of the atmosphere have been developed. International ALTITUDE TEMPERATURE PRESSURE DENSITY Feet °F °C psi hPa slug/ft³ kg/m³ Sea Level 59 15 14.67 1013.53 0.002378 1.23 1000 55.4 13 14.17 977.16 0.002309 1.19 2000 51.9 11 13.66 941.82 0.002242 1.15 3000 48.3 9.1 13.17 908.11 0.002176 1.12 4000 44.7 7.1 12.69 874.94 0.002112 1.09 5000 41.2 5.1 12.05 843.07 0.002049 1.06 6000 37.6 3.1 11.78 812.2 0.001988 1.02 7000 34 1.1 11.34 781.85 0.001928 0.99 8000 30.5 -0.9 10.92 752.91 0.001869 0.96 9000 26.9 -2.8 10.5 724.28 0.001812 0.93 10 000 23.3 -4.8 10.11 697.06 0.001756 0.9 15 000 5.5 -14.7 8.3 571.82 0.001496 0.77 20 000 -12.3 -24.6 6.75 465.4 0.001267 0.65 25 000 -30.2 -34.5 5.46 376.01 0.001066 0.55 30 000 -48 -44.4 4.37 301.3 0.000891 0.46 35 000 -65.8 -54.3 3.47 238.42 0.000738 0.38 40 000 -69.7 -56.5 2.72 187.54 0.000587 0.3 45 000 -69.7 -56.5 2.15 147.48 0.000462 0.24 50 000 -69.7 -56.5 1.68 115.83 0.000362 0.19 Figure 1-7. The International Standard Atmosphere. 1.6 Module 08 - Basic Aerodynamics