G-Tech Hp Meter Update

[Chuck Tomlinson]

At 07:51 AM 7/19/96 cdt, John Faubion wrote:
>
>I read your response and am curious about parts of it. For example the 
>following equation for air density:
>
>  AirDensity=0.0228645 * barometer(In Hg)/(0.5555* (temp - 32) + 273.15) 
>  
>I understand the barometer and the ((temp - 32) + 273.15) to get degrees Kelvin 
>I'm sure the constants 0.0228645 and the 0.5555 are for unit conversion but how 
>are they derived? 

Sorry it took me awhile to respond to this (the question may be answered already) but the above equation works iff AirDensity is in slugs/ft3 and temp is in degrees Fahrenheit.  The 0.5555 (i.e. 5/9) is part of the conversion from deg F to deg C or Kelvin.

The general form of the (ideal gas) equation is:

GasDensity = AbsPressure * GasMolWt / (UnivGasConst * AbsTemp)

The molecular weight of air is about 28.96.  The Universal Gas Constant is the same for all ideal gases, and pressure and temperature are absolute (i.e. referred to hard vacuum and absolute zero, respectively).

The 0.0228645 term comes out in the wash if you choose slugs/ft3 and deg F (and inHg for pressure).

>Also concerning the next equation:
>
>  AeroDragFactor = 1.07556 * AirDensity * Coeff_Drag * Frontal Area
>
>Again the 1.07556 is probably for unit conversion, how is it derived. This is an
>area that always intrigued me. Thanks for bringing up the topic.

I missed the first part of this thread, but I presume that the AeroDragFactor is used in: AeroDragForce = AeroDragFactor * AeroSpeed^2.

Then 1.07556 is correct iff AeroDragForce is in lbf, AirDensity is in slugs/ft3, FrontalArea is in ft2, *and* AeroSpeed is in mph.  What a mess!

The general (independent of units) form of the equation is:

AeroDragFactor = 1/2 * AirDensity * Coeff_Drag * Frontal Area

<Soapbox> IMHO, units-dependent equations are evil incarnate, especially when combined with the hodge-podge of units typical of non-SI units systems.  The AeroDragForce equation is a prime example.  SI rules!! </Soapbox>  ~8-)

Later.
Chuck Tomlinson <tomlinsc at ix.netcom.com>