A key component to soaring is the air mass the sailplane flies in.
Also, there is an energy source producing lift, either a warm air
thermal (thermal lift), or the wind rising as it meets an obstacle
such as a hill or a line of mountains (ridge lift). We will limit our
discussion to describing thermal soaring.
We will be using the electric motor to launch our sailplane to
altitude. Once at altitude we shut down the motor and the
sailplane will soar, eventually to return to earth until we use
the motor to climb again. How then does a sailplane stay aloft
for long periods of time and travel long distances? Some force
has to provide sufficient lift to overcome gravity when the
motor is not used.
One such force is the thermal. The thermal is simply a column
of rising warm air. Warm air is lighter (less dense) than cooler
air and thus rises. The term "differential heating" is used to
describe the generation of thermals. Descending cool air is
known as "sink."
The principle of warm vs cool air is used by balloonists to
launch and fly their hot air balloons. They create and trap warm
air inside the balloon envelope, and the warm air displaces the
cool air, causing the balloon to inflate and rise until air begins
to cool inside the envelope. The balloonist simply uses a
propane heater to warm the air again and the balloon rises again
or maintains its altitude.
Nature generates thermals by the sun heating darker ground or
objects more that lighter colored surfaces. The dark object
absorbs the sun's heat becoming warm and thus warming the
air above it.
For a thermal to be formed, the sun (or a heat source, such as a
hot metal roof, factory, etc.,) will heat the ground or surrounding
air in one location faster or warmer than the surrounding air. The
warm ground will warm the air above it and cause the air to
begin to rise. Rising warm air can take on the form of a column
or a funnel. Usually the part of the thermal near the ground is
small and expands outward as it rises in altitude.
Since the warming of air is usually a much smaller area than
the total area, the thermal updraft will be faster than the cooler
downdraft motion of air. This cooler downdraft of air is referred
to as "sink" and causes glider flights to be of a much shorter
duration as the lift generated by the wing is overcome by the
downward motion of the air.
To stay aloft one's task is to move from one thermal to another,
utilizing the lift created by rising warm air. In level flight, a glider
continuously descends in relation to the surrounding air. The
only way to sustain flight in a glider beyond the sink time in still
air (without a motor) is to fly in an air mass that is rising at a
rate greater than the sink rate of the glider.
Thermals usually cannot be seen. (An exception is a "dust
devil—a small thermal that has picked up dust making it
visible.) One can sometimes "feel" the presence of a thermal.
A breath of air in an otherwise calm spot indicates the presence
of a thermal. A shift in the wind (in a light breeze) probably indi-
cates airflow into a thermal. And one can watch for the graceful
soaring of birds, such as hawks and eagles to locate the pres-
ence of thermals.
Sometimes the wind will cause the thermal to bend or break
causing a warm air bubble that slowly travels downwind as it
rises. Thermals can vary in strength, rising at speeds of a few
hundred to over a thousand feet per minute.
Thermal Forms (Column)
16
Section 9: Thermal Soaring
