I think Eric has it right here. At speeds gliders fly they are subject to
classical (Newtonian) mechanics. Thus, if a glider in still air moves into
rising air (with no change in forward airspeed, to keep it simple), an
increased upward force is required to reduce its downward momentum and
restore normal descent in the new air mass. At constant airspeed this can
only come from increased AOA.

Ed Grens

Eric Greenwell wrote in message
...
In article ,
ess says...

I must respectfully disagree. If you fly into a rising airmass from a
neutral or sinking one your instantaneous sinking speed is reduced by
the vertical velocity of the new air mass - hence the reduced AOA.

The sinking speed is reduced with reference to the ground, but not
with respect to the airmass, which is what the glider flies in. The
glider has entered a new airmass, and for a few seconds is not in
equilibrium with this new airmass.

During the transition, the glider is accelerated upward, even though
it pitches downward (you can feel that acceleration upward!). The lift
on the wing must increase to provide this acceleration, and it
increase because the AOA increases momentarily due to the upward
velocity of the new airmass. The glider pitches downward, attempting
to reduce the AOA back to the trimmed AOA. If the transient AOA were
reduced, as you suggest, the natural stability of the glider would
pitch it upwards as it attempted to regain the AOA for which it was
trimmed.

Equilibrium is obtained when the AOA returns to what it was before,
the glider is no longer accelerated upward, and the sinking speed will
be the same as before (now referenced to the new air mass). Of course,
the sinking speed referenced to the ground will vary with the vertical
velocity of the airmass.
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Eric Greenwell
Richland, WA (USA)