This discussion needs to be divided into two subjects, stall warners and AOA
indicators. They are really two very different devices.

A stall warning device tells you that you've made a mistake. An AOA
indicator can tell you that you are about to make a mistake. A stall warner
is just a on-off binary device. An AOA indicator provides a continuum of
information across the whole speed range.

AOA tells you about safety margins and the rate of change of that margin. A
stall warning device doesn't - unless it's set at a large margin over stall
in which case it will be constantly triggered by turbulence and pilots will
turn it off. A Cessna-style stall warning would drive a pilot nuts if
installed in a glider.

The point that AOA varies across the span when thermalling is a good one.
Any device mounted on the fuselage won't provide data on the flow at the
wing tips. But, modern gliders have wings designed to prevent tip stall so
fuselage mounted AOA sensors are still useful. Damning AOA indicators
because they aren't perfect is a straw man argument. We have no perfect
instruments but they are still useful.

AOA indicators are better than ASI's at low speed where there are large
errors in airspeed indications but ASI are better at high speed for things
like Va and Vne. AOA indicators are faster responding and easier to
interpret. i.e. if the needle is in the yellow, push until it isn't. The
airspeed will take care of itself.

Maneuvering for landing in gusty wind shear is exactly where I'd want an
AOA. It will show the maximum gust induced AOA so the airspeed can be
increased just enough that no gust stalls the glider but not so much that
you'll need to dump a lot of energy in ground effect.


Bill Daniels




"Neal Pfeiffer" wrote in message
et...
Tufts are probably the best way to visualize the unsteady flow near stall
(as long as the tufts are not so large that they affect the flow.) The
tufts show when, where, and how the wing actually stalls. The flight can
be straight ahead, in a turn, with an accelerated entry, ...

The real question is, when does the wing stall and how can information be
passed to the pilot to avoid the stall?

Tufts are not a very practical method in normal operation, but they are
great during the development of a aircraft.

Airspeed has been used widely as the indirect means to avoid stall.
Airspeed is readily available on any glider and by using a relatively
simple weight and bank angle correction, one can determine a safe
airspeed.

AOA vanes on the fuselage are often used on a range of powered aircraft.
On commercial ones, these are often linked electronically to a stick
pusher in the cockpit to inhibit stall.

Some small airplanes use leading-edge vanes on the inboard wing and I've
even seen a small vane on the upper aft portion of the wing on a 1947
Bonanza.

If you are concerned about circling flight, a long-span glider flying in a
moderately-tight circle, a fuselage-mounted AOA or one on the inboard wing
would be somewhat useless, since the inboard wing would stall well before
reaching the critical AOA at the fuselage. So now we need multiple
sensors, one at each wing tip and possibly one or two at the fuselage with
a complex set of electronics and software to sort all the data out and
decide the proper answer to give the pilot.

Then there are the atmospheric effects. Say you're flying along on final
into a strong headwind (common here in Kansas) and you encounter a
significant wind shear. This could be caused by a frontal passage or gust
from a nearby storm that you are trying to avoid, but more likely a simple
wind shadow from a hill, a row of tall trees, a hangar, ... You go from
15 or 20 knots above stall speed to 5 knots below in a matter of a couple
of seconds. What sensor would alert you quickly, reliably, and give you
extra time to react and avoid the stall?

Most of the fatal and serious injury accidents occur while setting up the
landing pattern when the glider is allowed to slow too much and go to too
large an AOA. If I were want to equip my glider with a warning system,
and a purely uncertified one at that since I do know first hand about
certification in my day job, I would use a simple differential pressure
sensor to measure the airspeed from the delta between total and static
pressure and use electronics to trigger an audable warning whenever the
airspeed was less than say 12 knots over 1-g stall (not too loud though,
maybe my wife's voice saying 'Speed Up'). I would only turn this system
on when I enter downwind and am preparing to land. Extra info when most
needed.

Just my 2 cents worth,

..... Neal

Bill Daniels wrote:

Tufts are good and yes, students should see them.

However, tufts show boundary layer behavior not angle of attack and
streamlines. Start with the little smoke tunnel in the classroom and
then show them the tufts in flight.

Bill Daniels