



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. 

Last edited by Sparky Paul; Feb 02, 2003 at 01:50 PM.




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






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. 





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 


Thread Tools  
Similar Threads  
Category  Thread  Thread Starter  Forum  Replies  Last Post 
air pressure needed for AMT air start ??  wojtek  Fuel Jet Talk  15  Nov 22, 2004 07:17 PM  
Land "vs." Air Frequency  tenover  Electric Plane Talk  155  Nov 18, 2004 04:45 AM  
air pressure behind spinner?  nupeswv  Modeling Science  2  Oct 11, 2004 03:28 PM  
MF480.....static vs dynamic  stegla  Electric Ducted Fan Jet Talk  12  May 30, 2002 04:04 PM  
airspeed vs groundspeed vs flutter.  DeaninMilwaukee  Power Systems  13  Apr 11, 2002 12:29 PM 