How does a glider thermal?

On Sun, 02 Sep 2007 00:29:34 -0700, Ian
wrote:

On 2 Sep, 07:28, Allen Smith wrote:
Frank,
thanks for all this information!
however I would like to know of the aerodynamic features of soaring,
what happens to the angle of attack relative wind etc... when
soaring

Think of a slinky spring going down stairs.

Now think of a slinky spring going down an "up" escalator.

In short - nothing fancy happens to the angle of attack. The glider
keeps on going down through the air.

Ian

That is one damn fine metaphor! I'm gonna save that for future
use...;-)

rj

On Sep 2, 2:03 am, kestrel254 wrote:
On Sep 2, 7:28 am, Allen Smith wrote:

Frank,
thanks for all this information!
however I would like to know of the aerodynamic features of soaring,
what happens to the angle of attack relative wind etc... when
soaring

Allen
When a glider is thermalling it is circling in a bubble of rising air.
This tends to be shaped like a doughnut (vortex ring) with the centre
part going up and the outsides going down. This is why you often fly
through sink to get to the strongest lift. Quite often there is a
"whoosh" when you enter the core of the thermal and the airspeed will
show an increase. Sometimes it will push on one of the wings, changing
the angle of bank which you have to correct.
In the UK, the thermals are often smaller than the turning circles of
the glider so the vario reading for rate of climb will vary around the
circling turn that the glider makes in the thermal.
Changes in the angle of attack which a glider makes when try to keep
the speed constant and angles of attack to keep the angle of bank
constant will affect the rate of sink of the glider but to get the
best rate of climb, one will try and circle as near the core as
possible. By having the glider trimmed right and by having an audio
output on the (electronic) vario, one can safely circle a glider in a
thermal with other gliders and hardly need to look at the instrument
panel at all. A common mistake made by early soaring pilots is to fly
with their eyes glued to the vario and forget to lookout.
It is often easy to see where the core of a thermal is by watching the
relative position of the other gliders as you all circle round in the
lift. You position relative to theirs will change as you fly through
the sink on the outside of the thermal and the core.
HTH
George

Although the British Thermal model is a good thinking exercise, radar
and LIDAR measurements show it to be a bit more complex

http://cires.colorado.edu/~angevine/thermals_2006.pdf
http://lidar.ssec.wisc.edu/papers/akp_thes/node6.htm

Frank Whiteley

On Sep 2, 12:38 am, Allen Smith wrote:
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.

Hi Allen,

glad to see a power pilot thinking further ahead than the spinning
prop ;-)
All the explanations are great but the best way to figure it out is to
try it. Go to www.ssa.org, the web-site of the Soaring Society of
America and check out the interactive map under the 'Where to fly'
tab. Contact the nearest soaring operation, hang out for a day and
take a ride.
Soaring is at least as complex than power-flying; it just takes a
different skill-set. Keep in mind: currently, you are learning to fly
a 'potential' glider. Knowing a thing or two about soaring may come in
handy one day should the engine decide to quit.
Good luck,
Uli Neumann

Soaring is essentially like sailing in 3D. You are using the power up
updrafts, whether caused by heating of the earth (thermals), or winds
blowing against mountains (ridge lift and mountain wave) to provide the
power to stay aloft. The basic aerodynamics are the same as in a power
plane with the engine turned off. The big difference is that sailplanes
have much better glide ratios due to their much wider wingspans and the lack
of drag provided by an inop engine.

Mike Schumann

"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.





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