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
of a gas equals the
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
and it plays a large role in aerodynamic
A gas is composed
of a large number of molecules which are in constant motion. The
of a gas is a measure of the kinetic energy of the molecules 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 the 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
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)
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
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 is 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.
On this slide you will find typical values of the gas
parameters described above which apply to air. Some parameters, like density,
pressure, temperature and viscosity coefficient can vary
over a wide range of values. The quoted values are for sea level
static conditions for a standard day.
We are all aware that pressure and temperature
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
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
- Re-Living the Wright Way
- Beginner's Guide to Aeronautics
- NASA Home Page