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8/23/17    Home | Articles | Training | Instructor's Corner | Airplanes | Travelogues | PIREPS | For CFIs | ATPs | Pilot for Hire


Air Density Sim
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:

  1. 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.
  2. 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.
  3. 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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