John,

Check your gmail account for an offline note.

Wayne
http://www.soaridaho.com/Schreder

"jcarlyle" wrote in message
...
Thanks for a very clear explanation, Michael. I don't think any salt
is needed!

The only thing I'm left unsure about is AOA behavior vs. ASI behavior
in a turbulent thermal. I buy that AOA eliminates worrying about bank
angle and ballast - just set the attitude for best min sink AOA and
try to keep it there. But I wonder how much an AOA meter (say one
based upon pressure ports) would vary in a thermal vis a vis an ASI.
I've seen plus/minus 5 kts in ASI. If this translated in several
degrees in AOA, it might make the AOA as useless as the ASI.

-John

Michael Ash wrote:
Right, the AOA increases. On a very short timescale, when you drop into
slower wind, your airspeed decreases. This then decreases the amount of
lift your wings are developing. The lift they create is now less than
your
weight, so you begin to drop. As you drop, the relative wind becomes more
vertical, increasing your AOA and the lift created by the wings. Once the
AOA increases to the point where the lift balances out your weight, you
reach a steady state again at a slower airspeed and higher AOA. If the
AOA
reaches the stall angle before the lift increases to match your weight,
you'll stall, and this is why you should carry plenty of extra speed in
that situation.

Turbulence would bounce it around similar; if you get hit with a gust
from
below the AOA will momentarily become larger, increasing the lift from
the
wings. You then begin to climb until your vertical speed matches the gust
and your AOA goes back to what it was. Similarly with a gust from above,
where you start to drop.

I think the advantage in a thermal is that the optimal airspeed changes
with bank, whereas the optimal AOA doesn't change. Instead of trying to
figure out the best airspeed to track as you keep altering your bank to
center the thermal, you can just track a single AOA all the time.

For landing I think the advantage is just that you can stick a warning
signal around the stall AOA to remind you to stop hauling back on the
stick when you get too close to a stall.