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Date: Tue, 08 Aug 2000 08:25:01 -0500
From: jerry@mc.net
To: lancair.list@olsusa.com
Subject: Re: Rd: AOA
References: <008401c000b3$8d0d9f00$0464a8c0@station4>
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It's my understanding AOA indicators report just that ... AOA ... and AOA is
the resultant of what the wing is lifting.  That lifting is directly related
to gross weight or G loading. AOA doesn't know if the critical angle is
caused by gross weight, G load or angle of bank.  It only sees the angle of
air crossing the front of the wing.  Therefore, any AOA reporting system
will consider all the factors affecting the weight to be lifted and thereby
be seen as "automatically" compensating for all factors of flight ...
including icing, if the probe is still sensing properly.

Jerry Grimmonpre'  LNC2


[Just a few things... AFAIK Jim's AOA indicator is the only one that
actually uses the wing to generate the AOA information and so in this 
regard most of your comments are true.  Since the device is calibrated to
the individual airfoils (flaps up & flaps down) it will, in fact, deliver
reliable AOA information for those airfoils.  When you toss icing into the
picture, however, all bets are off as you are essentially dealing with a 
different airfoil than either of those for which the instrument was 
calibrated.  Since we know that icing is going to degrade the lifting
capability of any airfoil, though, you can still use the instrument for an
approximation of the new angle of attack by by interpreting the displayed
info as having been shifted downward on the display.  IOW, instead of
thinking that the wing will be stalled when the uppermost red light or
bar is illuminated it will most likely be stalled when one of the lower
indications are given.  When operating in this regime, however, it is
important to realize that the pressure taps may well be seeing a very
different pressure picture than what they'd be seeing without the icing
condition, so any indications used would still have to be suspect.  The 
pressure taps are recommended to be located anywhere from 15 to 40% of
MAC, which should keep them behind the icing buildup area (which should
actually stop at about 12%).  Actual icing, however, may well extend 
further back slightly or accrete at an uneven rate above and below the
chord line, and either of these conditions could dramatically alter the
pressures that are being seen at the ports.  For these reasons the 
reliance on reported AOA information during icing would be questionable at
best.  

  <Marv>     ]

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