Fred wrote:
Just got asked this question, didn't have a quick and easy answer.
How
do you explain it?

Airspeed is not defined as the speed of the glider nor is it the speed
of the wind moving over and around it. Airspeed is the speed of an
aircraft relative to the air in which it is flying. It is the speed of
the relative airflow that is aerodynamically influencing the aircraft.

When the glider is flying in still air all of its airspeed is generated
by its motion thru the air. To move thru the air it must over come
drag. When the glider in slightly downward flight experiences a thermal
part of its airspeed is now caused by the motion of air around the
glider. To remain still in moving air you must overcome drag.

The only thing the glider has to overcome the drag caused by moving air
is its reluctance to change speed (inertia). The glider is being held
in the upward airflow by inertia. As drag overcomes the gliders inertia
it starts to accelerate upward with the airflow. This reduces the
effect of the thermal on the airspeed and aerodynamics because the
glider is now moving with the upward flow and produces no relative
motion between the two.

The glider in downward flight in still air is supported by an
aerodynamic resistance force that opposes gravity called lift. Also
like lift in particular circumstances drag can resist motion and if
that motion is downward drag is upward also resisting gravity. Lift and
drag slows its descent in still air and cause its assent while in a
thermal.

The upward acceleration of the flying glider in a thermal entry is
caused 100 percent by the component of the relative airflow caused by
the thermal. It requires a force to accelerate the glider upward. Lets
see what aerodynamic force is most accurately defined as the
aerodynamic force that is in the direction of the relative airflow that
caused it? That's right drag.

It is true that angle of attack goes up causing more lift but as far as
accelerating the glider upward this extra lift is negated by the fact
that the direction of this lift moves farther away from the upward
direction. This extra lift comes with extra drag and its direction is
more in the upward direction as a result of the thermal. The thermal
not only increases the airspeed it changes its direction.