Message-ID: <001301c004a1$19a58ea0$0464a8c0@station4>
From: "John Barrett" <2thman@olympus.net>
To: "Lancair Mail List" <lancair.list@olsusa.com>
Subject: RE: Computed AOA
Date: Sat, 12 Aug 2000 13:04:44 -0700
Reply-To: lancair.list@olsusa.com
Mime-Version: 1.0

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OK Skip - we agree on several things - we both used to be carrier pilots and
we know that a measured AOA is most accurate.  Do you and I know that
Frantz's AOA computer and system is absolutely accurate in all areas of the
envelope?  We trust that he engineered it to be accurate within needed
tolerances so that when it warns us to push the stick forward, we DO JUST
THAT, or we expect the airplane to stall - and soon.  We're told that it's
only accurate with a clean airframe and only with gear and flaps fully
retracted or fully extended - nothing in between - no ice, no wheels
down/flaps up, no partial flaps.  We agree that there are several important
inputs to AOA measurement, whether done by a vane, Jim's computer or the SFS
computer.  Those include weight, G forces, density altitude, angle of bank,
aircraft configuration and probably some others.  Both computers measure
most of these factors adequately IMHO.  If you got the idea that SFS uses
only G forces to measure AOA, you're missing the message.  The SFS EFIS 2000
is a very sophisticated system that relies on AHRS (solid state gyro) input
for much of its output.  It does utilize all the parameters I listed above
(and proabably others I don't know about) to arrive at AOA with the
exception of weight variances, as I understand it.

Your comment about a computed AOA not being as accurate as the real thing is
certainly valid and it's important to ask yourself why you would trust Jim
Frantz's computer over another.  Personally, I believe his instrument is
highly accurate and reliable, but I'm taking it on faith because I've never
flown with it or seen its accuracy demonstrated.  The same is true for the
SFS to some degree, because I have not actually performed stalls with it.
However I have flown it in two different aircraft - a Centurion and the
LIVP.  The owner of the IVP tells me it is very accurate for his airplane in
all configurations.  For me, the proof is in the pudding and having seen it
function, I'm very confident in it.

I believe we lose sight of what the instrument does for us sometimes.  If
the instrument is programmed to give a verbal warning of imminent stall at,
say 5 knots above the actual stall, are we going to complain if it's off two
or three knots on the high side?  What if it's programmed to tell you when
you're just one knot away from stall?  Would you accept that in an
instrument?  I wouldn't.  I don't know anyone who can control airspeed
within one knot.  In terms of graphic representation, what are you going to
use it for?  Do you want it to tell you when you're flying at best L/D in
cruise?  If so, then you'd like Jim's computer better than SFS, becuase you
only see the graphic AOA curve as you approach stall AOA with the SFS
system.

In the Navy, the idea was to keep the AOA centered on the donut between the
high and low chevron during approaches (the instrument was never used in
cruise - only on approach) because that AOA gave you best L/D giving you
enough residual lift to correct if you drifted off the glidepath.   I recall
flying backseat in an A4 and the front seat guy slowed the a/c down to a
very nosehigh angle of attack going into Miramar.  The red chevron was
blinking brightly at several hundred feet above the runway.  I remember this
because it scared the hell out of me.  I thought we were seconds away from a
crash and burn.  After talking with some more seasoned F8 pilots in my
squadron, I learned what I should have already known - that the red chevron
was not an indication that stall was imminent but simply that you were
flying closer to stall AOA than you would on a carrier approach.  To my
knowledge, the airplanes we flew didn't even have stall warning devices
other than the AOA which only had the indicators I mentioned above.

Please don't take any of my comments as a criticism of the Frantz AOA.  I'm
only cautioning LML readers not to jump to negative conclusions about the
SFS EFIS until they've seen it in action.  I have.   Quite a bit actually,
and I can tell you it is one impressive system - including but not limited
to it's AOA measuring capabilities.

 Skip Slater wrote:
>As an old carrier pilot, I have to take exception to SFS's argument and
>agree with Jim Franz that a computed AOA which uses a constant weight just
>isn't as accurate as the real thing.  Measuring G forces is a poor
>substitute for a system which measures the amount of lift a wing is
>creating.  As we know, a wing can stall at several G's or it can stall at
>one.  Airspeed, weight and density altitude are all factors that also all
>come into play.  Finally, you have to question the accuracy of the weight
>that the user puts into the system; (garbage in, garbage out.)  The
>difference between the two types of systems (computed vs. real) can
>certainly be well beyond what I'd consider "inconsequential".


>If you ever approach the corner of the envelope, an actual AOA taken from
>the wing, a vane or other sensor is the only way you're going to have an
>accurate, real-time presentation of your L/D max.  A computed AOA is little
>better than a published stall speed; it's right sometimes, but a real AOA
is
>right ALL the time.


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