Date:13 Feb 99 02:26:07From:Matthew Willshee <mjw44@cam.ac.uk>Organization:University of Cambridge, EnglandReferences:1 2 3 4 5

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JF Mezei wrote: > I do not understand. If the static pressure trapped inside the system > were enough to show a V1 speed on the pilot's gauges, shouldn't pilots > have noticed that the gauges were reading V1 while the plane was iddle? Assuming incompressible flow, Bernoulli's equation tells us that: p + 0.5 * rho * V * V = p0 p is static pressure - "barometric pressure" rho is density V is speed p0 is stagnation or total pressure - the pressure you would measure in the air if you slowed it to a stop. 0.5 * rho * V * V is called the dynamic pressure. It is the difference between static and total pressure due to the motion of the air. A pitot tube has two parts - a forward pointing tube brings the air to rest (relative to the plane) and hence the pressure in it is total pressure p0 - a hole pointing at right angles to the direction of travel measures the static pressure p. Subtract p from p0 and you get dynamic pressure 0.5 * rho * V * V - from which you can extract speed (given density which is a function of altitude and temperature, hence IAS versus TAS discussions elsewhere in the group). So now we tape over the static ports - obviously while the plane is on the ground!. We fix our reading of static pressure to sea level atmospheric pressure. Any change in this before takeoff will be negligible so the static ports read the correct pressure on takeoff. There is no airspeed indication problem. At altitude though, our trapped pressure is too high so we calculate dynamic pressure too low and underestimate airspeed. Obviously, it would be much easier to spot taped over dynamic ports, as this value pressure changes as we accelerate. Air is not incompressible, so there are some inaccuracies in using Bernoulli but the argument will be the same in principle with compressible flow. -- Matthew Willshee E-Mail: mjw44@cam.ac.uk Churchill College, Cambridge, CB3 0DS : 96mjw@eng.cam.ac.uk