How does a glider thermal?

Have you noticed that on a day with Cumulus clouds that it is hard to hold
altitude? You may have it nailed, then suddenly you are 200 ft too high, or
low. You have not changed power or trim or moved the yoke. You are flying
through thermals.

When you fly through air going up, your angle of attack increases (often 6
degrees or more), even though your attitude does not change (much).
Therefore your coefficient of lift increases which increases your total
lift. The forces are now unbalanced and you accelerate upward (climb). If
you could stay in this rising air you would continue to climb. Since the
areas of rising are (usually) roughly circular , if your turn circle is
smaller than the rising air circlular area, you can stay in it! Voila!, you
are thermalling!

The hard part to grasp initially is that you are climbing with respect to an
inertial frame of reference (the earth), but you are descending with
reference to the small parcel of air you inhabit, i.e. the thermal. It will
sink in, eventually.

If you could see the air, it might look like a pot of boiling water on a
stove . with columns of bubbles from the hot spots. The cumulus clouds
represent the tops of these columns.

--
Hartley Falbaum


"Allen Smith" wrote in message
...
Hello,
I just watched a movie online (Youtube) about gliders and saw one
thermaling and gaining altitude. How is this is done? I am a presolo
power pilot (Just finished with my 12th hour) and have been taugt
that a power plane will climb when excess power is availible.
(Steepen the angle of attack, and the airplane will start climbing
steady when all 4 forces are equal) How does a glider do that, since
there is no engine? Does the thermal change the relative wind?
(Which can\'t because the airplane creates the relative wind as it
flys through the air, right?)

Really interesting stuff.