# Properties of Air – Text Version

Air is a mixture of gases, 78% nitrogen and 21% oxygen with traces of water vapor, carbon dioxide, argon, and various other components. We usually model air as a uniform (no variation or fluctuation) gas with properties that are averaged from all the individual components.

Any gas has certain properties that we can detect with our senses. The values and relations of the properties define the state of the gas. The pressure (p) of a gas equals the perpendicular (normal) force exerted by the gas divided by the surface area on which the force is exerted. A gas can also exert a tangential (shearing) force on a surface, which acts like friction between solid surfaces. This “sticky” property of the gas is called the viscosity (mu) and it plays a large role in aerodynamic drag. A gas is composed^M of a large number of molecules which are in constant motion. The temperature (T) of a gas is a measure of the kinetic energy of the gas. The sum of the mass of all the molecules is equal to the mass of the gas.

A gas occupies some volume in three dimensional space. For a given pressure and temperature, the volume depends directly on the amount of gas. Since the mass and volume are directly related, we can express both the mass and volume by a single variable. When working with a static (unmoving) gas, it is convenient to use specific volume (v), which is the volume divided by the mass. When a gas is moving, it is more convenient to use the density (r) of a gas, which is the mass divided by the volume the gas occupies. Either variable can be used to define the state of the gas, since they are reciprocals.

The density (specific volume), pressure, and temperature of a gas are related to each other through the equation of state. There is a universal gas constant which relates these variables and the molecular weight of any gas. Including the value of the molecular weight, we can define a particular gas constant (R) for air. The state of a gas can be changed by external processes, and the reaction of the gas can be predicted using the laws of thermodynamics. Studies of the zeroth and first laws introduce the idea of the heat capacity of a substance. The specific heat of a gas is a measure of the amount of energy necessary to raise the temperature of the gas by a single degree. Since the amount depends on the process used to raise the temperature, there a specific heat (cv) coefficient for a constant volume process, and a different valued coefficient for a constant pressure process (cp). The ratio of these coefficients is denoted by the greek letter gamma and appears in many thermodynamic equations.

Typical values of the density, pressure, and temperature of air at sea level static conditions for a standard day are:

Density: 1.229 kilogram per cubic meter or .00237 slug per cubic feet

Specific Volume: .814 cubic meters per kilogram or 422 cubic feet per slug

Pressure: 101.3 kilo Newtons per square meter or 14.7 pounds per square inch

Temperature: 15 degrees Celsius or 59 degrees Farenheit

Absolute Temperature: 288 degrees Kelvin or 519 degrees Rankine

Viscosity: 1.73 time 10^-5 Newton-second per square meter or 3.62 times 10^-7 pound-second per square foot.

Gas Constant: .286 Joules per gram per degree Kelvin or 53.5 foot-pounds per pound per degree Rankine.

Specific heat at constant volume: .715 Joules per gram per degree Kelvin or .17 BTU’s per pound per degree Rankine.

Ratio of specific heats: 1.4

We are all aware that pressure and temperature (and density) of the air depend on your location on the earth and the season of the year. And while it is hotter in some seasons than others, pressure and temperature change day to day, hour to hour, sometimes even minute to minute (during severe weather). The values presented on the slide are simply average values used by engineers to design machines. That’s why they are called standard values. We also know that all of the state-of-the-gas variables will change with altitude, which is why the typical values are given at sea level, static conditions. Because the gravity of the earth holds the atmosphere to the surface, as altitude increases, air density, pressure, and temperature (for lower altitudes) decrease. In deep space, the density is almost zero. The variation of the air from the standard can be very important since it affects the aerodynamic lift and drag. For the same geometry, the lift and drag decrease with altitude. The Wright brothers never had to worry about these variations from standard conditions because their aircraft seldom flew higher than 100 feet off the ground and they never flew in Denver or any other cities located at high altitude.

A quick look at the table shows that there are two quoted values for the temperature. The lower value in each column is the absolute temperature referenced to absolute zero. There are two different English units for energy (ft-lbs and BTU’s), which is part of the “fun” of working in English units.

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