From:rdd@netcom.com (Robert Dorsett)Organization:Netcom Online Communications Services (408-241-9760 login: guest)Date:02 Jun 96 16:23:50References:1Followups:1

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In article <airliners.1996.605@ohare.Chicago.COM> Francis.Jambon@imag.fr (Francis JAMBON) writes: >What I do not understand is if a pitot probe is blocked. the indicated >speed must be lower than the air speed, not highter ? Higher. An airspeed indicator basically measures the difference between static pressure and total pressure. These inputs come from two sources: static pressure from a static port, and total pressure from a pitot tube Let's assume that: P_stat = static pressure Dynamic pressure, q, is 1/2 rho * V^2, where rho is density and V is the true airspeed of the airplane. Ptot = q + Pstat. So the airspeed, v, is roughly proportional to Ptot - Pstat. If total pressure is held constant, then you'll note that *a* static value is trapped in the Ptot term, such that the steadily decreasing static pressure in the second term, as the airplane climbs, will result in a steadily increasing airspeed. So this results in velocity being proportional to: P_stat_blocked - P_stat_ambient Where P_stat_blocked is the static pressure when the tube was blocked and P_stat_ambient is the current pressure. You may want to plug some numbers into this: V = sqrt(2*(Ptot - Pstat)/rho)) Or: V = sqrt(2*(P_stat_blocked - P_stat_ambient)/rho)) Just remember to keep the two static terms (the one that goes into the velocity calculation and the one that goes into the Ptot calculation) different, since they will be different in this case (the value in Ptot will be stuck at the value present whenever the pitot tube got clogged). This explains why, as the airplane seems to accelerate, the pilots pull further and further back on the stick, in a futile attempt to slow down on the airplane, until the airplane eventually stalls out. By then, they're at such a high attitude that recovery becomes somewhat doubtful. >Francis JAMBON -- Robert Dorsett Moderator, sci.aeronautics.simulation rdd@netcom.com aero-simulation@wilbur.pr.erau.edu ftp://wilbur.pr.erau.edu/pub/av