FergWorld : Training & Reference : Air Density Simulator
Courtesy NASA
This is an interactive program in which you can investigate
changes in the atmosphere. It uses mathematical models of the
standard atmosphere of the Earth and Mars.
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 can also demonstrate the relative magnitude of the
variables on the Earth and Mars.
The simulator is divided into three main sections:
 On the left is the input for altitude. You can set the
altitude by clicking on the airplane image, holding the mouse
button down, and moving the airplane to a new location.
 At the upper right are the selection buttons. You can choose
to look at airplanes on Earth or Mars, and you can display the
input and output in either English or metric units.
 The lower right portion of the simulator provides output
information. You will see the exact altitude that you selected in
an output box and a choice of output variables. You can display
either the temperature, pressure, density,
sound speed, lift ratio, or Mach number in an
output box. Output gauges also display the temperature and
pressure. The speed of sound depends on
the type of gas in the atmosphere (nitrogen and oxygen for the
Earth and carbon dioxide for Mars) and on the square root of the
temperature of the gas. You can also make a comparison of the
lift generated by an aircraft at two
altitudes. The lift ratio displayed here compares the lift
generated by a given aircraft design, at a given velocity, at a
selected altitude (and planet) to the lift generated by the same
aircraft, at the same velocity, at sea level on the Earth. And
finally, you can compare the Mach number
of an aircraft at two altitudes or on different planets. For
simplicity, a reference speed of 800 ft/sec. (545 mph or
244 meters/sec.) has been chosen, and the computed Mach number is
displayed. 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.
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