[jhb] Re: Pireps
- From: "Peter Dodds" <pdodds@xxxxxxxxxxxxx>
- To: jhb@xxxxxxxxxxxxx
- Date: Mon, 10 Sep 2007 21:33 +0100 (BST)
The pitot head is parallel to the chord of the wing. At zero angle of
attack, the air flow is parallel to the wing chord, and the little
molecules are hurtling straight up the hole in the pitot tube. TAS=IAS.
At near stall speed, the angle of attack is over 10 degrees. This means
that the airflow approaching the wing is at an angle of over 10 degrees
to the wing chord from beneath. It is therefore also at an angle of 10
degrees to the pitot tube. Those little molecules hurtling towards it
have to change direction by 10 degrees to go up the pitot tube, so they
bounce off the inner wall and then off the other and so on. Also fewer
of them go up the tube - some bounce away. Bit like blowing across the
top of a beer bottle - not much air goes into the bpttle - most of it
passes over the top.
Eventually those that do get to the end exert pressure on the Air Speed
indicator gubbins. In changing direction and rattling about inside the
tube they also lose energy. The loss of energy and the fewer molecules
getting up there equates to the ASI giving a lower reading than TAS.
Peter
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