A graphical version of this slide is available which gives the standard day values of all of the flow properties.

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 of a gas equals the force exerted by the gas divided by the surface area on which the force is exerted. The temperature of a gas is a measure of how hot or cold the gas is. Gas is composed of a large number of molecules, and the sum of the mass of all the molecules is equal to the mass of the gas. 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, which is the volume divided by the mass. When a gas is moving, it is more convenient to use the density 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. 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. A fundamental understanding of thermodynamics is very important in describing the operation of propulsion systems.

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 Pascals or 14.7 pounds per square inch

Temperature: 15 degrees Celsius or 59 degress Farenheit

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 flow parameters like the speed of sound. You will learn that jet engines do not produce as much thrust on hot, muggy days as on cold, crisp days, and that lift, drag, and thrust vary greatly with altitude.

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 by Tom Benson
Please send suggestions/corrections to: benson@grc.nasa.gov

Last Updated Fri, Jul 11 09:04:07 AM EDT 2008 by Ruth Petersen, 9/9/99