This is the beta version 1.2a of the AtmosModeler
Simulator program, and you are invited to participate in the
beta testing. The older
version 1.1f
of the program is still available if you prefer a simpler version.
If you find errors in the program or would like to
suggest improvements, please send an e-mail to
benson@grc.nasa.gov.

This is an interactive program in which you can investigate
changes in the atmosphere and its effects on aerodynamic variables.
It uses mathematical models of the
standard atmosphere of the Earth.
You can find the equations
for the standard Earth atmosphere at this web site in both
English units and metric
units.
Based on your input velocity, the program also calculates the
Mach number,
dynamic pressure, and
stagnation, or
total, temperature
on your aircraft. The stagnation temperature is the temperature of the
airflow at a stagnation point, such as the leading edge of the wing or nose of the aircraft.

If you see only a grey box at the top of this page, be sure that Java is
enabled in your browser. If Java is enabled, and you are using the Windows XP
operating system, you need to get a newer version of Java. Go to this link:
http://www.java.com/en/index.jsp,
try the "Download It Now" button, and then select "Yes" when the download box from Sun
pops up.

The pressure, temperature,
and density of the atmosphere constantly
change. At any one time there are great variations in the properties
of the atmosphere, depending on location around the planet and height
above the surface of the planet. The mathematical models used in this
simulator show an average variation of properties of the
atmosphere at various heights, but not at various locations. The
simulator will not predict the temperature or pressure at any single
location at any time. But it will help us understand the relations
among the values of a given variable at different heights.

The simulator is divided into three main sections:

On the left is the graphic showing the altitude of interest and
the velocity of your aircraft. You can set the
altitude by clicking on the aircraft image, holding the mouse
button down, and moving the aircraft to a new location. The altitude roughly
corresponds to the nose of the aircraft image. The velocity is set by the
slider at the left. Click on the yellow bar and slide it to your
desired velocity.

At the upper right are the input selection buttons and text fields.
You can display the
input and output in either English or metric units. You can also input desired
values of altitude and velocity using the white input boxes. Simply backspace
over the current value, enter a new value, then hit Enter to send the
value to the program.

The lower right portion of the simulator provides output
information. You can display
either the temperature, pressure, density,
speed of sound, dynamic pressure, force ratio,
Mach number, or total temperature on your aircraft in the
output box. Output gauges also display the atmospheric temperature and
pressure. The speed of sound depends on
the type of gas in the atmosphere
and on the square root of the
temperature of the gas.
The dynamic pressure
depends on the gas density and the square of the velocity and is an
important design constraint on aircraft structures.
You can make a comparison of the
aerodynamic force generated on an aircraft at two
altitudes.
The force ratio displayed here compares the aerodynamic force
generated by a given aircraft design, at the specified velocity, at the
selected altitude (and planet) to the force generated by the same
aircraft, at the same velocity, at sea level on the Earth.
You can compare the Mach number
of a vehicle at two altitudes or on different planets.
The Mach number is computed at the specified altitude and velocity.
Since the speed of sound depends on the temperature and
the gas, you will note some important differences in Mach number.
As the Mach number gets closer to (or exceeds) one,
compressibility effects, like shock waves and wave drag, become
more important to the aircraft. Finally, the program determines the
stagnation, or
total temperature, which occurs on the
wing leading or nose of your aircraft. The total temperature depends on the local, atmospheric,
static temperature and on the velocity of the aircraft.

You can download your own copy of this program to run off-line by
clicking on the yellow button: