BASIC AERODYNAMICS

Physics of the Atmosphere

• Atmosphere and Basic.............................................................................................. 
• Aerodynamics............................................................................................................
• Temperature, Pressure and Altitude........................................................................
•  Density........................................................................................................................
• Humidity.....................................................................................................................
• Absolute Humidity...................................................................................................... 
• Relative Humidity and the Dew Point........................................................................
• International Standard Atmosphere (ISA).................................................................

Atmosphere and Basic Aerodynamics

 As an aircraft operates in the air the properties of air that affect aircraft control and performance must be understood. Air is a mixture of gases composed principally of nitrogen and oxygen. Since air is a combination of gases, it follows the laws of gases. Air is considered a fluid because it answers the definition of a fluid, namely, a substance which may be made to flow or change its shape by the application of moderate pressure. Air has weight, since something lighter than air, such as a balloon filled with helium, will rise in the air. Air is made up of approximately 21% oxygen (O2) and 78% nitrogen (N) by volume, with the remaining 1% being made up from other gases. The ratios of the gases (21%, 78% and 1%) vary little with height although the moisture content drops with increase in altitude. Aerodynamics is the study of the dynamics of gases, or the interaction between moving object and atmosphere causing an airflow around a body. As first a movement of a body (ship) in a water was studies, it is not a surprise that some aviation terms are the same as naval ones – rudder, water line, keel beam, speed measured in knots (nautical miles). The understanding of basic aerodynamics – the possibility of flight, forces acting on aircraft in flight, why aircraft is designed with particular flight control systems, - is important for understanding the maintenance of aircraft systems. As a part of physics (gas laws, fluid dynamics and propagation of sound were studied in Module 2 “Physics”) aerodynamics gives laws determining forces acting on aircraft and its behavior in interaction with atmosphere. 

Temperature, Pressure and Altitude

 Physically atmosphere is considered as a fluid of changing density, pressure and temperature. According to temperature changes with the height above the sea level atmosphere is divided into troposphere, stratosphere, mesosphere and thermosphere . As altitude increases, up to 30,000 feet (about 10 000 m), the temperature, pressure and density of the air decrease. This region is known as the TROPOSPHERE and the upper boundary is the TROPOPAUSE. Being minimal (about −60°C) at tropopause it rises up to −10°C at stratopause, and then decreases to the altitude of about 80-85 kilometers (mesopause). These changes in temperature are very interesting as it is known the temperature of cosmic background is −455°𝐹𝐹 𝑜𝑜𝑜𝑜 − 273𝐾𝐾, but measurement show it depends on place of measurement – being in shadow or not.

More exactly the changes in temperature from sea level up to tropopause are presented . Really the change in temperature from the sea level up to tropopause is almost linear and gives the values of 6.5°𝐶𝐶 for each 1 000 meters or 3.6°𝐹𝐹 per each 1 000 feet. This is called the standard (or average) laps rate.

If a 1-in. square column of air extending from sea level to the “top” of the atmosphere could be weighed, it would be found to weigh about 14.7 lbs. Thus, atmospheric pressure at sea level is 14.7 PSI (pounds per square inch). However, pounds per square inch are rather a crude unit for the measurement of a light substance such as air. Therefore, atmospheric pressure is usually measured in terms of inches of mercury  when measured with a mercury barometer or SI units.

The relation between different units is as follows:

 29.92 in Hg = 1 atm = 14.7 psi = 1013.2 hPa = 760 mm Hg = 1.013 bar 

The last unit is that meteorologists use. 

Density

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 (gas Laws studied previously in M2 Physics): - Density varies in direct proportion with the pressure (under constant temp); - Density varies inversely with the temperature (under constant pressure). 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, turbine 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 the amount of water vapor in the air. The maximum amount of water vapor that air can hold varies with the temperature. The higher the temperature of the air, the more water vapor it can absorb. By itself, water vapor weighs approximately five-eighths as much as an equal amount of perfectly dry air. The last fact is due to difference in water (𝐻𝐻2𝑂𝑂) and main air components (𝑁𝑁2 and 𝑂𝑂2) molecular weights. 𝐻𝐻2𝑂𝑂 molecule molecular weight is 18 whereas 𝑁𝑁2 molecular weight – 28 (∼78% of air), and 𝑂𝑂2 molecular weight – 32 (∼20% of air). Therefore, when air contains water vapor it is not as heavy as air containing no moisture. 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.

Absolute Humidity- The number of grams of water vapor per 1 𝑚𝑚3 of the atmosphere.

Relative Humidity and the Dew Point

Existing water vapor pressure of the atmosphere, expressed as a percentage of the saturated water vapor pressure at the same temperature. Air temperature drops as we rise in altitude above the surface. At some point the air temp drops to the dew point of the air at which point the water vapor in the air condenses into liquid water, and this water we see condensed onto specs of dust in the air makes up the clouds. All air contains some amount of water vapor, varying from just a fraction of a percent (by weight) for cold dry desert air, on up to some 3% for hot steaming jungles. As the properties of air and water vapor are essentially independent, the property of the water – water vapor equilibrium at various temperatures is of great importance, but not the properties of the air.Having a jar of dry air, and poured some water into the bottom of it, water molecules on the surface will evaporate from the surface and periodically condense back into the surface until it reaches some equilibrium value where evaporation and condensation is equal.