Chapter 2 The Atmosphere PDF

CHAPTER 2 The Atmosphere Chapter 2 1  The Atmosphere — Many people wonder how something as big and heavy as an aircraft can stay in the air. The answer lies partly in the air that the aircraft flies through. — An understanding of the Earth’s atmosphere is fundamental to...

CHAPTER 2 The Atmosphere Chapter 2 1  The Atmosphere — Many people wonder how something as big and heavy as an aircraft can stay in the air. The answer lies partly in the air that the aircraft flies through. — An understanding of the Earth’s atmosphere is fundamental to a pilot, since it is not only the air that we breath , but the environment in which an aircraft flies. Chapter 2 2  Introduction — The atmosphere is the medium in which an aircraft operates. It is the properties of the atmosphere, changed by the shape of the wing, that generate the required lift force. — The most important property is air density (the “thickness” of air). — KEY FACT: If air density decreases, the mass of air flowing over the aircraft will decrease. — A given mass flow will generate the required lift force, but a decrease in air density will reduce the mass flow. — To maintain the required lift force if density decreases, the speed of the aircraft through the air must be increased. The increased speed of airflow over the wing will maintain the mass flow and lift force at its required value. Chapter 2 3  The Physical Properties of Air — Air has substance! Air has mass; not very much if compared to other matter, but nevertheless a significant amount. A mass of moving air has considerable kinetic energy; for example, when moving at 100 knots the kinetic energy of air can inflict severe damage to man-made structures. — Air is a compressible fluid and can flow or change its shape when subjected to even minute pressure differences. (Air will flow in the direction of the lower pressure). — The viscosity of air is so low that very small forces can move the molecules in relation to each other. Chapter 2 4  The Physical Properties of Air — When considering the portion of atmosphere in which most aircraft operate (up to 40 000 ft), with increasing altitude the characteristics of air undergo a gradual transition from those at sea level. — Since air is compressible, the lower layers contain much the greater part of the whole mass of the atmosphere. — Pressure falls steadily with increasing altitude, but temperature falls steadily only to about 36 000 ft, above which it then remains constant through the stratosphere. Chapter 2 5 Composition of the atmosphere The Atmosphere is composed of different types of gases: 78% Nitrogen 21% Oxygen 0.95% Argon 0.05% Carbon Dioxide Trace gases include Carbon Monoxide, Helium, Methane, Ozone, and Hydrogen. The relative amounts of the gases in the atmosphere remain constant up to an altitude of 60 km. 6  Structure of the Atmosphere Gravitational separation alters the composition of the atmosphere. For the purposes of Principles of Flight, we will only consider the lower atmosphere. Commercial aircrafts flies in the tropopause surfaces which is 36,090 ft from the sea. Chapter 2 7  Static Pressure The unit for static pressure is N/m2 (Pascal), the symbol is lower case ‘p’. — Static pressure also known as atmospheric pressure, is the result of the weight of the atmosphere pressing down on the air beneath. — Static pressure will exert the same force per square meter on all surfaces of an airplane. The lower the altitude, the greater the force per square meter. — It is called static pressure because of the air’s stationary or static presence. — An aircraft always has static pressure acting upon it. — Newtons per square meter is the SI unit for pressure. 1 N/m2 is called a Pascal and is quite a small unit. In aviation the hectopascal (hPa) is used. (‘hecto’ means 100 and 1 hectopascal is the same as 1 millibar). — Static pressure at a particular altitude will vary from day to day and is about 1000 hPa at sea level. In those countries that measure static pressure in inches of mercury (inHg), sea level static pressure is about 30 inHg. Chapter 2 8  Pressure Change with Altitude Near to the Earth’s surface, the number of air molecules is greater than at altitude. As altitude increases, gravitational acceleration becomes weaker, the number of molecules fewer and the mass of air smaller. Therefore, force per unit area (i.e., pressure), reduces with increasing altitude. Static pressure reduces with altitude. Chapter 2 9  Temperature — The unit for temperature is °C, or ˚K. It is degrees Celsius (or centigrade) when measured relative to the freezing point of water, or Kelvin when measured relative to absolute zero. (0°C is equivalent to 273 ˚K). — Temperature decreases with increasing altitude up to about 36 000 ft. and then remains constant. Chapter 2 10  Air Density The unit for density is kg/m3 and the symbol is the Greek letter ρ [rho]. — Density is ‘mass per unit volume’ (The ‘number’ of air particles in a given space). — Density varies with static pressure, temperature and humidity. — Density decreases if static pressure decreases. — Density decreases if temperature increases. — Density decreases if humidity increases. — Air Density is proportional to pressure and inversely proportional to temperature. This is shown in the ideal gas law formula below. 𝒑 = constant, more usefully it can be said that ρ ∝ p/T 𝑻𝝆 where p = pressure, T = temperature, and ρ = density 11  Air Density Density decreases with increasing altitude because of decreasing static pressure. However, with increasing altitude temperature also decreases, which would tend to increase density, but the effect of decreasing static pressure is dominant. Chapter 2 12  Air Density — Water Vapor Water vapor is also an important constituent of the atmosphere. Water vapor is invisible as a gas. It become visible in its liquid state as clouds and rain. Snow, ice and hail are examples of the solid state of water vapor. The amount of water vapor in air will also affect density. The more water vapor in the air, the lower the density of the air. (The density of water vapor is about 5/8 that of dry air). 13  International Standard Atmosphere (ISA) — The values of temperature, pressure and density are never constant in any given layer of the atmosphere. To enable accurate comparison of aircraft performance and the calibration of pressure instruments, a ‘standard’ atmosphere has been adopted. — The standard atmosphere represents the mean or average properties of the atmosphere. — Europe uses the standard atmosphere defined by the International Civil Aviation Organization (ICAO). — The ICAO standard atmosphere assumes the following mean sea level values: — Temperature 15°C — Pressure 1013.25 hPa — Density 1.225 kg/m3 Chapter 2 14  International Standard Atmosphere (ISA) — The temperature lapse rate is assumed to be uniform at a rate of 2°C per 1000 ft. (1.98°C) from mean sea level up to a height of 36090 ft. (11,000 m) above which the lapse rate becomes zero and the temperature remains constant at -56.5°C. 15 International Standard Atmosphere (ISA) Chapter 2 16  Dynamic Pressure The unit for dynamic pressure is N/m2 and the symbol is lower case ‘q’ or upper case ‘Q’. — Because air has mass, air in motion must possess kinetic energy, and will exert a force per square meter on any object in its path. KE = ½ m V2 — It is called DYNAMIC pressure because the air is moving in relation to the object being considered, in this case an aircraft. — Dynamic pressure is proportional to the density of the air and the square of the speed of the air flowing over the aircraft. An aircraft immersed in moving airflow will therefore experience both static AND dynamic pressure. (Remember, static pressure is always present). Dynamic pressure is common to ALL aerodynamic forces and determines the air loads imposed on an airplane moving through the air. — Dynamic pressure is Q = ½ ρ V2 Chapter 2 17  Key Facts — A pilot needs to know how much dynamic pressure is available, but dynamic pressure cannot be measured on its own because static pressure will always be present. The sum of static and dynamic pressure, in this context, is known as ‘Total’ pressure. It can also be referred to as Stagnation or Pitot pressure Total Pressure = Static Pressure + Dynamic Pressure — This can be re-arranged to show that: Total Pressure - Static Pressure = Dynamic Pressure — The significance of dynamic pressure to the understanding of Principles of Flight cannot be overemphasized. Chapter 2 18  Key Facts — Because dynamic pressure is dependent upon air density and the speed of the aircraft through the air, it is necessary to fully appreciate the factors which affect air density: — Temperature - increasing temperature decreases air density. Changes in air density due to air temperature are significant during all phases of flight. — Static pressure - decreasing static pressure decreases air density. Changes in air density due to static pressure are significant during all phases of flight. — Humidity - increasing humidity decreases air density. Humidity is most significant during take-off and landing. 19  Thank you Chapter 1 20