Date: Thu, 10 Aug 2000 16:44:01 -0400
From: James Frantz <LFrantz@compuserve.com>
Subject: Computed AOAs
Sender: James Frantz <LFrantz@compuserve.com>
To: "INTERNET:lancair.list@olsusa.com" <lancair.list@olsusa.com>
Message-ID: <200008101644_MC2-AF5B-E767@compuserve.com>
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>Does the SFS unit actually know how many G's you're pulling?  If so, the 
accuracy you credit it with should be good enough for stall warning, even
if 
it assumes a gross weight.  If not, it's no better than an airspeed
indicator.<

Aerodynamically, either a vane based AOA unit or a pressure based unit
gives good AOA results.  A problem with moving vanes and pressure probes is
that they must be installed well ahead of the wing in order to be in
undisturbed air.  This is tricky for tractor driven aircraft such as our
Lancairs and requires the installation of a long mast from which the
vane/probe is attached.  The advantages of the wing pressure tap approach
is obvious. 

I'm not familiar with the SFS computed AOA so I am not commenting on its
function specifically.

Stall speed, best L/D, best approach speed, max endurance and Vx generally
vary with the square root of the aircrafts gross weight and other things. 
For a Lancair the GW can vary by about 40%.  Thus these speeds will vary by
about 17% due to gross weight changes alone.  FAR 23.207  requires that
stall warners activate at not less than 5 knots but more than the greater
of  10 knots or 15% of the stalling speed.  This points out one of several 
problems with computing AOAs.  The resulting accuracy of computed AOAs,
which in the case for Lancairs could have errors of 17% due to GW alone, 
will not meet the FARs. There are other problems that contribute further to
the problem of computing AOA increasing errors even further.

However, I am a firm beliver in getting some type of stall warner on our
aircraft and computed stall warners (computed AOAs) are much better than
nothing.  The facts are that we (experimentals) are running nearly double
the fatality rate due to stall spins as compared to Part 23 certified
aircraft.  There are several reasons for the differences but the fact that
Part 23 aircraft are required to have stall warners cannot be over looked
and is significant in my opinion.  When the Navy and Marines converted to
AOA during approach, they cut their fatality rate in half that first year
getting their pilots back aboard the ship! 

Nonetheless, true full range AOAs, such as the AOA Pro and Sport, are the
ultimate aerodynamic instrument becasue they are self compensating for GW,
CG, temperature, altitude, bank angle, "G" loading, attitude, flight path
angle, humidity, etc. etc.  

Jim Frantz
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