Block pitots - back to basics
Thinking out loud....
Ok, so a pitot in it's simplest form is a tube facing into the 'liquid' flow, such that the resulting pressure can be measured with a transducer (diaphragm). At a constant (fixed) altitude that's all you need to measure 'speed' (once calibrated). However the measured pressure is a function of two components - there is a static component to the pressure (effectively due to the mass of the column of air pressing down on the diaphragm) and the dynamic component due to movement (effectively the additional pressure due to more air molecules pressing on the diaphragm). So if the thing that needs it's speed measured can change altitude, then we need to measure both dynamic and static pressure, and subtract the later from the former to derive speed irrespective of altitude. So far so good? (Please excuse the layman-esque language, but longer words make my head ache).
Alrighty then... so all's well and good until something blocks the dynamic tube - moisture, bees, ice, whatever - bad things happen.
Considering moisture - water - pretty common on Earth, so what's the solution - drain holes. Now this is where I need some help. Obviously the drain holes MUST be significantly smaller than the inlet port (otherwise there would be no pressure to measure - all the air would flow out the 'drain'...), so in normal operation we have air entering the inlet port and some portion of that flow exiting the drain? Clearly it is possible to calibrate the sensor to compensate for this, so the speed measurement still occurs.
However, block the inlet (ice), with the drain still open and the pressure will drop to static (indicated speed will decrease, at the limit to zero). My assumption is this is the failure mode we are discussing? (Since a blocked DRAIN alone will cause the speed to over-read).
My thought is this - in BOTH malfunction cases the flow inside the pitot has stopped at least through the drain - so why not include a mass-flow sensor right there in the drain outlet? Can we compare the GPS/INS derived speed to the pitot derived output and cross-check this with the mass-flow value, and figure out if we have a sensor problem? If our GPS ground speed is 400 knots and the pitot IAS is 80knots, with mass-flow derived value of 0 then there's a problem? I am presuming that pitot icing occurs fairly rapidly.
Am I oversimplifying?
- GY