photofly,
Interesting post with respect to replacing CAS with Dynamic Pressure. You must have been referring to some vintage aircraft where attempt was made to use Dynamic Pressure to convert it to CAS (Even 1950's text books describe it as archaic). These olden days aircraft used the following formula for instrument calibration -
V = SQR (2q / Rho)
Where-
q = DYNAMIC Pressure
Rho = Air Density
V = True Air Speed
Nice sentimental stuff which actually produced DAS (Density Air Speed), now
THAT's an "invented" and meaningless speed. For the low and slow Piper Cubs and Gipsy Moths it was a good enough substitute for TAS.
Unfortunately for the well-meaning folk who thought they were measuring Dynamic Pressure,
ALL pitot tubes 'capture' Impact Pressure, even that on the Piper Cub. They were assuming that air was incompressible, but it is.
Now, if we want to get serious about airspeed measurement (as aircraft have moved on somewhat since the Piper Cub), we must consider
IMPACT PRESSURE.
The calibration formula for this is -
Vc = SQR ((Y/( Y-1) X p0/qc X [(qc/p0 +1)^((Y-1)/ Y) - 1] X 2qc/Rho0)
Where-
Vc = Calibrated Air speed (ft/sec)
qc = IMPACT Pressure (lb/ft²) ........ What we’re talking about!
Rho0 = Sea Level Air Density (0.0023769 Slugs/ft³)
p0 = Sea Level Air Pressure (2116.2 lb/ft²)
Y = A constant for Air = 1.4 (Ratio of specific heat of air at constant pressure to specific heat of air at constant volume) (Y=Gamma)
Now this is good for Sea Level, for which the ASI is calibrated. At Altitude we need to replace p0 with ps (the static pressure at the particular altitude).
Now, I'm not trying to blind anyone with science, and it aint rocket science, but you wanted Dynamic Pressure, and there it is embedded in the formula! qc is impact pressure, and SQR(2qc/Rho0) is the compressibility factor (known as f), which if applied in reverse yields ...... Dynamic Pressure
So go and do the algebra and the maths to exract to extract your preferred Dynamic Pressure, and produce a table or simple computer programme to give you the more useful figures that you seek
Here's a nice simple one for you to pracice on. An aircraft I recently flew had a dynamic pressure limit of 79.4 lb/ft² for full flap exension. What was the CAS limit for full flaps?
A piece of cake!
Regards,
Old Smokey