Brilliant Frank the penny dropped for me.So few words.So much info. Fred ----- Original Message ----- From: FrankTurley@xxxxxxx To: jhb@xxxxxxxxxxxxx Sent: Tuesday, September 11, 2007 8:06 AM Subject: [jhb] Re: Pireps In a message dated 10/09/2007 21:32:58 GMT Daylight Time, pdodds@xxxxxxxxxxxxx writes: 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 Thanks Peter, I think what you are saying is that the tube loses its efficiency as the angle moves from the direct input. That makes sense. Frank T. ------------------------------------------------------------------------------ No virus found in this incoming message. Checked by AVG Free Edition. Version: 7.5.485 / Virus Database: 269.13.13/998 - Release Date: 10/09/2007 08:48