Sender: <marv@lancaironline.net> (Marvin Kaye)
To: lml@lancaironline.net
Date: Wed, 24 Sep 2003 20:43:31 -0400
Message-ID: <redirect-2602483@logan.com>
From: "Jerry Fisher" <jerryfisher@charter.net>
Subject: AOA Indications
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First of all, like everyone else, I was distressed to see so much work
destroyed in Ron's accident, and just pleased that he got off so lightly
personally.  However it does bring up some interesting issues about ASIs and
AOAs.

>From the outset I intended to fit an AOA indicator in my 360, having used
them for many years on military fighters.  I see 3 reasons for wanting one:

1.  Providing warning of the approach to the stall in both level and
accelerated flight.
2.  Helping get maximum performance from the airplane (optimum approach
angle, max range, max endurance, best glide angle etc).
3.  Offering a back-up in the event of airspeed indicator failure.

For an airplane like the 360, the last of these can be very significant, as
we have just seen.  I decided from the outset against the "standard" AOA
system from Proprietary Software Systems because it does not offer any
back-up if the pitot lines leak or get blocked, which is much more likely
than a gauge failure.  As I understand it, this system compares differential
pressure between the upper and lower wing ports with pitot presuure to
compute alpha.  As a result, accurate pitot pressure is essential for the
system to give accurate readings.  This could be particularly deceptive if a
pitot problem is subtle (e.g. a slight leak) so that the pilot does not know
which to trust, when the real answer is neither.

I looked at a number of other systems.  The best seemed to be the Rite Angle
III (http://www.riteangle.com/), and the Lift Reserve unit
(http://www.liftreserve.com/).  The Lift Reserve system uses differential
pressures from the top and bottom of a fixed probe projecting at an angle
from the lower wing surface.  It is simlar in some ways to the PSS system,
but without the pitot pressure input.  The only trouble is that, as I
understand it, the stall angle indicated may vary with speed, precisely
because there is no comparison of the upper/lower differential pressures
with pitot pressure.  In other words the indicated alpha for a high speed
accelerated stall will differ somewhat from that for a level unaccelerated
stall.  The company calls it a Lift Reserve indicator, which in my opinion
fudges the issue.

The Rite Angle system is simpler.  It uses a vane mounted under the wing
which swivels to follow the relative airflow, and which drives the cockpit
indicator, which is similar to the PSS indicator.  It also makes provision
for changes in AOA with wing flap deployment.  From the web site photos,
someone (Rob Wolfe?) has fitted one to a 360.  To me the advantage of
independence from the pitot system far outweighs any concern over the use of
an external vane, which is similar to those used on loads of fighters and
airliners.  It helps that the price is lower, at $545, too.

The other possibilities include the Dynon system, but that requires you to
use their electronic flight instrumentation system and a special pitot tube,
and the much simpler and cheaper ACI system
(http://www.stallwarning.co.uk/), but that is just a stall warner, nothing
more.

I had therefore decided on the Rite Angle system.  Rob, is that your
aircraft on their web site, and if so, what is your experience with the
system?

Jerry Fisher