How does a glider thermal?

On Sep 1, 10:38 pm, 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.

For more viewing, I'd recommend the following collection
http://www.youtube.com/user/kawior10
and a couple of DVD's
Fine Week of Soaring and Windborn from
http://store.ssa.org/cgi/search.cgi?...o s_and_Music

Frank Whiteley

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

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

Ian wrote:

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...

....and, when the air is going up faster than the glider is going down,
then the Glider goes UP. Or, thermal air rising 400 fpm - glider sinking
100 fpm = glider climbing at 300 fpm.

Angle of attack changes only momentarily, while in the transition phase
from the glide in still air to a glide in rising air, in order to
accelerate the mass of the glider to a state of equilibrium within the
rising air mass--assuming a constant airspeed.

If entering a strong area of rising air both the angle of attack and the
airspeed may increase initially but both return to equilibrium as above
without pilot input.


Jack

Right, it's just Galilean relativity. The vertical version of wind
drift or wind correction angle between heading and track. The glider is
always descending through the surrounding air, but if that air is rising
faster than that rate relative to the ground, there is a net gain.

Many airplane pilots find it puzzling that a glide attitude is
maintained while climbing. Also seemingly paradoxical is that spiraling
at a steep angle of bank often results in a better climb rate in a
thermal. Yes, the sink rate of the glider is higher at the higher bank
angle, load factor, and airspeed, but the vertical speed gradient of the
air (faster in the "core") can more than overcome that.

--JHC

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, 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

The the walking down an up escalator analogy (and walking down the
down escalator when in sink) analogy is good, but I suggest the best
way of you finding out is to try gliding...

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

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