On Mon, 18 Aug 2003 10:53:46 -0700, Eric Greenwell
wrote:

In article ,
says...

Not so at the instant the airmass is entered.
After some time - about 0.75 to 2.5 seconds the glider will be close
enough to equilibrium in the new airmass.

Indeed.

I misstated what I meant to say: that the instantaneous position is
that the glider has its original velocity vector but is now in rising
air, so has a reduced AOA until its in equilibrium with the rising
air.

Rising air comes from beneath the wing, and increases the AOA.

Kindly draw the vector diagram before continuing. You'll see that the
sinking speed velocity vector points down and the rising air vector
points up. Simple vector addition says that the rising air velocity is
subtracted from the sinking speed because their directions are
opposite.

The
increased AOA produces an increased lift, so the glider accelerates
upward. If the AOA decreased, the glider would begin to sink, and we
know that is not the case in rising air!

The glider rises because the rising air is rising faster than the
glider's sinking speed. That's nothing to do with the AOA, which in
any case as the same as before once the glider is stabilised in the
rising air. There is a perceived (and actual) acceleration as the
glider's vertical velocity is decreased but this too has nothing to do
with what the AOA may or may not do during the transition from one
stable state to the next. If you prefer, the acceleration is the
transient result of changing the frame of reference from one air mass
to another.

I'm attempting to account for the AOA changes during that transition.
Fixed trim models definitely pitch down on entry to a thermal.
Transient AOA decrease explains this rather well and I find this
explanation also works for the sensations I experience in a glider.

You have a different account of what happens. That's fair enough, BUT
so far you haven't used it to explain why my model pitches down on
entering a thermal or why a glider surges forward on entering a
thermal and until you can do that I have to regard your account as
incomplete.

--
martin@ : Martin Gregorie
gregorie : Harlow, UK
demon :
co : Zappa fan & glider pilot
uk :