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Atmosphere Calculator
Based On Ilan Kroo's (Stanford)


Select unit system:
English Metric


Altitude   ft
Speed   ft/sec
Reference Length ft


deg R
Speed of Sound
lb sec/ft^2
Mach Number
Dynamic Pressure
Critical Cp
Vacuum Cp
Reynolds Number
Laminar Cf
Turbulent Cf

Notes on the Aerocalc Entries

This calculator generates the value of various common functions in aerodynamic calculations for the standard atmosphere. The Standard Atmosphere (officially "The U.S. Standard Atmosphere 1976", used world wide) is dry, that is it has 0% humidity. The temperature is defined as a nearly linear function of altitude with the temperature at sea level being 59°F and dropping about 3.5°F each 1000 feet. Because of this the answers will be somewhat different from the day you fly. In general the density will be less and the speed of sound will be higher on a typical flying day in the US than at the same pressure in the standard atmosphere.

The temperature is given in degrees Rankine (absolute). This is

Rankine = Fahrenheit + 459.67°

The units of density and pressure are identical to those found in most US textbooks if the 'English' option is taken.

The viscosity is the dynamic viscosity. Some texts use the kinematic viscosity which is the dynamic viscosity divided by density.

The Mach number is the velocity divided by the speed of sound. For modelers this is of interest in propeller design and performance.

The dynamic pressure is used in many aerodynamic calculations. It is often called "q"

q = 0.5*density*V*V

The next two are pressure coefficients. A pressure coefficient is the difference between the ambient pressure and the local pressure divided by the dynamic pressure. The critical pressure coefficient is for a pressure so low that the flow must be at Mach number 1. The second is for a pure vacuum. These calculations are for compressible flow and would be useful primarily in high Mach number situations, such as propeller airfoils in speed, free flight, and (to a lesser extent) racing.

The Reynolds number is the ratio of inertia forces to viscous forces. The Reynolds number for the flow around a piece of shot dropped in a can of 30 weight oil is very low. For the flow around a 747 wing at cruise it is very high.

The skin friction coefficients are for laminar and turbulent boundary layers. These are always higher for models than for their full-size cousins.


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