AF447

[HazelNuts39]

RE: PJ2 (#3572) : agreed also

RE: funfly (P.178 #3567)

Quote:
GPS cannot give you airspeed - it gives you ground speed. It's airspeed that makes aeroplanes fly.
KISS - there can be nothing more basic and therefore reliable for measuring airspeed than an air pressure monitor sticking out into the air itself (pitot tube).
/Unquote

Agreed entirely. However, I would like to add that there are other means of measuring airspeed. Before going into that, let me expose my understanding of the problem with pitot tubes. They need to be ventilated by drain holes to get rid of water or (melted) ice particles. The problem arises when the drain holes are blocked. Let me therefore explain two schemes which don't need drain holes.

As an ex-glider pilot I recall that sailplanes used to have a venturi probe. Whereas the pitot pressure is total pressure equal to static pressure plus dynamic pressure, the pressure measured with such a venturi is static MINUS dynamic pressure. In sailplanes that pressure is fed to a pneumatic rate-of-climb indicator (variometer) which then indicates variation of total energy, that is altitude corrected for airspeed changes. Since the air passes right through the venturi, it doesn't need drains.

Another scheme which I have come across was envisaged to replace the AoA vane (I don't recall why someone would want to do that, except that it had to do with icing and/or ice detectors). It consisted of a small cylinder projecting into the airstream with 2 pressure holes 120 degrees apart at the 1 o'clock and 5 o'clock positions (airflow coming from 9 o'clock). A servomotor would rotate the cylinder about its axis until the pressures at the 1 o'clock and 5 o'clock positions are equal, which means that the 9 o'clock position is the stagnation point indicating the direction of the local airflow, just as an AoA vane would do. I am writing this from distant memory so that details could well be different. The point is that one could measure a pressure that is representative of dynamic pressure on the surface of a suitably shaped body at a pressure hole where the airflow is flowing past it rather than into the hole.

However, just keep in mind that any scheme to replace pitots may well import new problems that are not easy to solve.

regards,
HN39