How does a glider thermal?

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.

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.

Glad you find it interesting.

Generally, sailplanes have minimum sink rates from approximately 90fpm
to about 175fpm. Due to atmospheric instability and solar heating,
buoyant plumes of air will rise in columns, often combining into
larger plumes. Depending on the surface, type of vegetation or
coloring, slope, winds, and moisture content, the vertical velocity
and diameter and height of these plumes will vary widely. This is
what we call lift. In some areas, it may become usable for soaring
flight as early as 9 or 9:30am locally. Late morning or early
afternoon is a more common time for reliable lift to form. Sometimes,
convergence zones and other factors may delay local lift until mid to
late afternoon. The tops of the plumes, commonly call thermals, are
often marked by cumulus clouds. However, thermals may also be present
on 'blue' days. Like soaring birds, gliders need to find and circle
within the plume to gain altitude. Reaching the top of a climb, the
savvy pilot will have already considered the next two or three
developing thermals near the goal course, if cross-country is the goal
of the flight, and cruise in that direction. Lift generally builds
during the day, so starting out with 2-3knots of climb is okay as long
as the forecast indicates stronger lift on course. 1knot is 100fpm,
so weak lift may only sustain flight. However, in the US west,
thermal lift above 20knots has been experienced. 6-12knots is not
uncommon, thus allowing pilots to achieve cross-country average speeds
approaching 100mph at times.

L/D is what allows gliders to connect thermals together. L/D of about
23:1 is about the minimum for completing closed course thermal flights
that have an into wind leg, like a triangle. 30:1 L/D makes cross
country flying more reasonable. 40:1 L/D makes many long flights
possible as it keeps more landing options within reach. 50:1 L/D is
becoming more common in modern gliders with the super ships reaching
60:1 and 70:1. That's where the imagination runs wild.

By comparison, an SEL training plane like a Tomahawk or C150/152 has
about 3-3.5 minutes from 2000agl to arrival in an engine out
situation, so you pick the first reasonable spot and fly to it. A
40:1 glider at 2000agl has nearly 200 square miles of options for a
landing spot and is 15 minutes or more from arrival should the pilot
not find any more lift.

Visit http://www.onlinecontest.org/olc-2.0...ringId=1&today
to view flights submitted to the Online Contest. You may have to pick
a date. You can then pick a continent, country, or region to find a
list of flight. Then click the Info button on the right of a
particular flight. This will show you a map with the flight route
shown and an altitude/terrain chart. The files may be downloaded and
the flight replayed through a viewer. You can download a trial
version of SeeYou here http://naviter.si/index.php?option=c...9&Itemid =213

Others may tell you it's just f**king magic, which it is, of course.

Soaring, try it, you'll like it,

Frank Whiteley

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

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

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

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.