Maybe it is simply an ergonomics thing --- I feel more comfortable
circling to the right in clutch situations simply because it is easier
to pull and adjust rather than push and adjust in right turns.

K.P. Termaat wrote:
Thanks Larry for your interesting respons with your links to the articles.

As a matter of fact most members of our National Team (my son is one of
them) tend to circle to the right when low. They don't in effect know why.
They say, and I agree, that it usually feels better in getting up again. So
Peter Gray may be right in saying that most of the thermals have the
tendency to rotate to the left in the northern hemisphere, especially when
low when still having small diameters. Coriolis may be the cause then. Would
be interesting to know the experience in the southern hemisphere.

Talking about a drain and water brings me to the idea of telling that when
low and looking for a thermal I always try to locate small ponds in dry
areas. It looks to me that the water vapor rising from these ponds is an
excellent means of starting a thermal. Water vapor is lighter then air, so
it increases the boyancy of the air over the pond and off it goes. Starting
at about ground level, circling to the right may then generally be the
better option. Any experience with this Larry?

Karel, NL


"Larry Dighera" schreef in bericht
...

On Fri, 26 Dec 2003 09:00:24 GMT, "K.P. Termaat" wrote
in Message-Id: :


Larry wrote:


On an average day, a pilot will
spend a great deal of time attempting to "core" thermals. That
involves mentally visualizing the sailplane's position relative to the
thermal's vertical anticyclone column center, and guiding his
sailplane to circle as near to the center of it as he is able.

Why do you write "anticyclone" Larry.

That was my recollection from my soaring experience in the early '70s.
However, it appears that the word 'cyclone' may have more correct now
that I have done some research:

The National Weather Service Glossary page here
http://www.wrh.noaa.gov/Phoenix/general/glossary/ has this to say:

ANTICYCLONE - A region of high atmospheric pressure around which
winds move in a clockwise direction in the northern hemisphere.
Generally associated with warm and dry weather

CYCLONE - An area of low pressure around which winds blow in a
counter-clockwise fashion in the northern hemisphere. Generally
associated with cool, wet and unsettled weather

On the other hand ...


This would mean that all gliders would circle righthanded to stay as

close

to the core as possible thereby using the headwind of the thermal.

I'll defer to these sources:

http://www.rc-soar.com/tech/thermals.htm
Do thermals rotate?
They do, but not predictably. Even dust devils don't have a
preferred direction of rotation (see Stull, p.449). Thermals are
too small and too short-lived to be affected by the earth's
rotation (Coriolis force) or by the equator/pole thermal gradient.
Their rotation is determined by local terrain. Rotational velocity
in the core of a typical thermal is small compared to the vertical
velocity.


http://www.skynomad.com/articles/athermal10.html
THERMALS

Do Thermals Spin?
by Peter Gray

Of course, dust devils spin, so thermals probably do also, at
least when they're close to the ground. For many years, I was
convinced that dust devils rotated in random directions. However,
based on a few flights in northern Washington State, where I kept
more careful mental notes, I would guess that about 2/3 to 3/4 of
the dust devils there spin counterclockwise, in agreement with the
Coriolis Effect. Contrary to popular belief, the smaller-scale
equivalent, water going down a toilet or drain, is essentially
unaffected, and the rotation, if it is biased toward one spin
direction, is the result of the geometry of each such basin (check
it out for yourself!).

The dust devils formed by thermals seem to be just large
enough to be affected somewhat by the Coriolis Effect. If I have
the choice, I usually opt for circling against the dust devil's
rotation, most often to the right, in case this will produce a
better climb rate by reducing my circling ground speed, thus bank
angle and sinkrate.

However, when I have reversed direction several times in one
climb, I have rarely detected a significant advantage in one
direction. What little angular momentum thermals start with, they
seem to lose through drag in the surrounding air, and they
probably start with little spin anyway. As with water going down a
drain, very little spin momentum at a large radius can translate
to a rapid spin when the radius shrinks to that of a dust devil.


I have never found a meteorological reason to circle in a certain

direction.

Most pilots circle lefthanded just because they were tought to do so in

the

vicinity of their home field. However in x-country flights there is

usually

no preference. In low situations the pilot usually circles in the

direction

he feels most comfortable. Sometimes the gliders feels better when

circling

to the right rather then to the left is my experience.

Karel, NL

Agreed. Circling direction is more often dictated by other gliders in
the thermal than meteorological phenomena and physics.