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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