On Fri, 08 Sep 2017 21:08:42 -0700, larsonchristina wrote:

Perhaps! What I describe is a pitch of the nose down and the tail up.
Longitudinal seesawing (pitching) at the lateral axis.

All aircraft entering a thermal will do this, some more noticeably than
others.

Reasoning: as the glider flies into the thermal it enters a rising air
mass and the further it moves into this air mass, the faster the vertical
movement becomes. The effect on the glider is that its effective AOA is
reduced by the air's vertical velocity (draw a vector diagram and this
becomes obvious), which in turn means that the wing no longer supports
all the glider's weight. The effect of trim means that the glider will
automatically pitch down and speed up to regain lift.

The combination of this small forward acceleration plus the upward
acceleration as to enters progressively faster air is what you feel as
"the surge" when you enter a thermal.

What you see and feel depends on what you're flying. In my Libelle I
don't particularly notice the pitch down, and anyway it may be masked as
I'm pitching up to slow down from inter-thermal cruise speed or even
faster flight through the sink surrounding the thermal, but the surge
forward and up is quite easy to feel.

The most obvious demonstration of this that I've seen was shown by an A/1
competition model glider of around 4 feet span, even when it was 50-60 m
overhead. When entering a strong thermal it would pitch down quite
rapidly and about as steeply as a helicopter does after takeoff when its
pilot is in a hurry to get moving before returning to its normal trimmed
attitude as it entered the core of the thermal. Like all free flight
models of its era, its trim was set before launch and remained so
throughout the flight, so this strong pitch down was not due to trim
changes.


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martin@ | Martin Gregorie
gregorie. | Essex, UK
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