

Toronto,Canada
Joined Jul 2002
440 Posts

airspeed vs dynamic air pressure
Given "standard day" conditions, what is the relationship between impact pressure on a pitot tube and the indicated airspeed ?
I found this formuala : velocity = square root of (2 x [impact pressure  static pressure] / air density). To solve for Vel = 1, (1 what ?? .. ft/sec ?), I used Static= 14.7 lb/in^2, air_density= .00237 slug/ft^3 got impact_press = 14.701185 lb/in^2 Is that correct ? (14.701185 lb/in^2 pressure difference = 1 ft/sec ? If so, how would I solve for Vel=XX. I realize it's a "basic" math question, but basic math is not one of my strong points Mike 




Hi Mike,
I use the equation for dynamic pressure(q) where q= 1/2 Rho*V^2 q is in lbs/ft^2, Rho is in slugs/ft^3 or lbssec^2/ft^4 and V is in ft/sec. If we assume 88 ft/sec (60 mph) for V, then q=0.5*0.002377*(88)^2=9.20 lbs/ft^2. which will be the total pressure on a pitot tube. Dick Huang 



Total pressure is static + dynamic (q).
A conventional airspeed indicator subtracts (it's vented to static) static from total to show "airspeed", which is actually the dynamic pressure. The bent tube that runs the needle in the airspeed indicator reads total directly. . The altimeter shows static pressure. 


Toronto,Canada
Joined Jul 2002
440 Posts

Quote:
Thanks Dick. I've read about "q" before, time to review it again Mike 



Cincinnati, OH
Joined Aug 2002
767 Posts

QUOTE]Given "standard day" conditions, what is the relationship between impact pressure on a pitot tube and the indicated airspeed ? [/QUOTE]
An interesting discussion, but why do you ask? In terms of modeling, the important thing to know is "q", calculated as Dick Huang suggests. Seeing that q is a function of the velocity squared, in terms of Standard day, 20 fps (13.6 mph) equates to about 7.6 oz/sq ft, 40 fps to 30.4 oz/sq ft, and and 60 fps to 68.5 oz/sq ft. These are the numbers that relate to stall speeds for a given wing loading, design CL, etc. 


Virginia Beach, Va.
Joined Jan 2002
1,233 Posts

To add a little more to it, 'indicated airspeed' on the dial is not true airspeed. True airspeed is indicated airspeed adjusted for temperature (density). Full scale pilots use indicated airspeed plus outside air temp to compute their true airspeed.
Jimmy 
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