Message-ID: <39986C5B.16F828D@regandesigns.com>
Date: Mon, 14 Aug 2000 17:02:03 -0500
From: Brent Regan <brent@regandesigns.com>
To: Lancair List <lancair.list@olsusa.com>
Subject: RE: Computed AOA
Reply-To: lancair.list@olsusa.com
Mime-Version: 1.0

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After some additional consideration and consultation, have to offer the
following:

First, comparing the PSS AOA to the SFS EFIS-2000 is a little like comparing
a Bose stereo system to a Ferrari with a CD player. The Bose will sound
better but you didn't buy the Ferrari to listen to it's radio. Likewise, if
you are using the AOA function of the EFIS-2000 as a significant portion of
your buy decision then you need to check your medication.

Having said that, lets compare the AOA functions of both systems (bring on
the Prozac).

The PSS (Proprietary Software Systems) AOA device actually calculates the
angle from the relative wind at zero lift to the current relative wind. This
calculation is based on an algorithm which uses Pitot, Static, Upper Wing
and Lower Wing pressures. The algorithm also depends on the pilot
successfully performing a series of calibration steps and maneuvers. The
units displayed are NOT degrees but rather AOA "Units" of non specific
magnitude. The display is either a LCD or LED depending on which system is
compared. The PSS system issues an "Angle Advisory" NOT a Stall Warning in
the form of the audio warning "Angle Angle Push". PSS has not made public
any intentions to certify this system.

The SFS EFIS-2000 calculates the stall speed based on the weight entered by
the pilot (presumably max gross), G loading, airspeed and Vs (clean at
gross). G loading is measured by the accelerometers in the AHRS. The display
consists of a Pitch Limit Indicator (PLI) that appears on the screen above
the Velocity Vector 20 Kts before stall. At 5 Kts before stall the PLI has
moved closer to the Vector and turned red. An audio "Stall, Stall" warning
is also issued at 5 Kts. At 0 Kts to stall the PLI is overlaid on the
Velocity Vector. While theoretically not as accurate as the PSS AOA due to
the difference between the actual aircraft weight and the entered max gross
weight, the SFS system will issue a stall warning between 5 and 15 Kts
before theoretical stall. The SFS EFIS is in the process of being certified
as a primary flight instrument. The symbology and methodology for the entire
system, including the PLI and Stall Warning, has been developed with, and
approved by, the human factors specialists at the FAA. Not that this is a
glowing endorsement. Those same specialists argued that a G meter was not
needed or desired (even though it could be added at no additional cost). Go
figure.

The other advantage of the PSS AOA is that you can nail your best glide
speed for best dead stick performance. The SFS EFIS-2000 also allows you to
do this, but again, via a different approach. The EFIS-2000 moving map
displays a "Dead Stick Glide" circle that is corrected for terrain,
altitude, turns and crosswind. Pitching for the largest glide area will give
you best glide. If you can keep the airport you are trying to get to inside
the circle then you will get there. What else do you need to know?

IMHO the SFS PLI and Stall Warning are adequate for alerting the pilot that
he is entering a dangerous portion of the flight envelope. If the pilot
wants to know the angle of the relative wind from zero lift then you need a
PSS AOA. You should have a stall warning or a PSS AOA or an EFIS-2000 in
your plane. The AOA and EFIS are not mutually exclusive or redundant in
there methods. An analogy to their respective AOA/Stall functionality would
be as a VSI compares to an Altimeter. There is no reason besides cost and
panel space why you couldn't have both.

Jim Frantz has done a fine job in developing the AOA and educating the
experimental community as to the importance of AOA awareness. Likewise, SFS
has defined the state of the art for electronic flight information systems
and offers this technology at a competitive price. I am just happy to be a
pilot that can take advantage of this technology while those poor SOBs do
all the work ;)

Now isn't that just as clear as mud?

Regards
Brent Regan


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