Bronze Badge question

Denis wrote:

And also, why would you get blown out of a thermal? Since you are
circling in it, shouldn't you drift at it's speed, instead of the wind
speed?


THat's perfectly true if the thermal is vertical, which should be the
case if it is not triggered by a ground feature, and with no significant
wind shear. In the other case, as you climb slower than the air in the
thermal, you will get under it and have to correct upwind to find it
again (another solution for us impure pilots is start the engine to
climb again in the original part of the thermal without ajust our
circling ;-) )

How high do you think this effect (the acceleration of the thermal
until it matches the wind speed) persists? I would expect the thermal
drift to match the wind speed in less than 2000 feet agl, based on
observations of dust devils in our area.


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Eric Greenwell
Washington State
USA

Originally Posted by Eric Greenwell

Denis wrote:

And also, why would you get blown out of a thermal? Since you are
circling in it, shouldn't you drift at it's speed, instead of the wind
speed?


THat's perfectly true if the thermal is vertical, which should be the
case if it is not triggered by a ground feature, and with no significant
wind shear. In the other case, as you climb slower than the air in the
thermal, you will get under it and have to correct upwind to find it
again (another solution for us impure pilots is start the engine to
climb again in the original part of the thermal without ajust our
circling ;-) )

How high do you think this effect (the acceleration of the thermal
until it matches the wind speed) persists? I would expect the thermal
drift to match the wind speed in less than 2000 feet agl, based on
observations of dust devils in our area.


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Eric Greenwell
Washington State
USA

[color=blue]



I beleive the windspeed to be changing through height gradient, sometimes decreasing, usually increasing with altitude, thus a rising thermal mass will always be chasing the speed of the wind, not meeting it.
Thus we can often experience thermic wave, atop our thermal.

Just my 2c.

I'm curious to know your contest record. Also curious to know what
Cambridge computer you use. I thought I had the latest one.

Andy (GY)

bagmaker wrote:
Eric Greenwell Wrote:

How high do you think this effect (the acceleration of the thermal
until it matches the wind speed) persists? I would expect the thermal
drift to match the wind speed in less than 2000 feet agl, based on
observations of dust devils in our area.


I beleive the windspeed to be changing through height gradient,
sometimes decreasing, usually increasing with altitude, thus a rising
thermal mass will always be chasing the speed of the wind, not meeting
it.

True, but the variations I see are usually small enough, that I think
the thermal quickly adjusts so the difference is always less than 2
knots. I picked 2 knots because that is the kind of difference I
sometimes see in the wind measurements between circling and cruising
flight. But, perhaps there is a way to measure this:

* the pilot leaves the thermal
* he begins to circle again as soon as he is out of the lift
* after about 4 circles, he continues on his flight
* post-flight, the wind drift in the thermal and out of the thermal can
be compared from the flight record

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Eric Greenwell
Washington State
USA

Eric Greenwell a écrit :
Denis wrote:

And also, why would you get blown out of a thermal? Since you are
circling in it, shouldn't you drift at it's speed, instead of the
wind speed?


THat's perfectly true if the thermal is vertical, which should be the
case if it is not triggered by a ground feature, and with no
significant wind shear. In the other case, as you climb slower than
the air in the thermal, you will get under it and have to correct
upwind to find it again (another solution for us impure pilots is
start the engine to climb again in the original part of the thermal
without ajust our circling ;-) )

How high do you think this effect (the acceleration of the thermal until
it matches the wind speed) persists? I would expect the thermal drift to
match the wind speed in less than 2000 feet agl, based on observations
of dust devils in our area.

I don't really know. I found up to 10 kmh difference but it might be measuring errors.

But even if the thermal moves at wind speed, if it is continuously climbing from the same ground point, it will be oblique (downwind from its triggering point) hence the need to correct upwind from time to time...

Denis

Denis wrote:
Eric Greenwell a écrit :

Denis wrote:

And also, why would you get blown out of a thermal? Since you are
circling in it, shouldn't you drift at it's speed, instead of the
wind speed?



THat's perfectly true if the thermal is vertical, which should be the
case if it is not triggered by a ground feature, and with no
significant wind shear. In the other case, as you climb slower than
the air in the thermal, you will get under it and have to correct
upwind to find it again (another solution for us impure pilots is
start the engine to climb again in the original part of the thermal
without ajust our circling ;-) )


How high do you think this effect (the acceleration of the thermal
until it matches the wind speed) persists? I would expect the thermal
drift to match the wind speed in less than 2000 feet agl, based on
observations of dust devils in our area.


I don't really know. I found up to 10 kmh difference but it might be
measuring errors.

But even if the thermal moves at wind speed, if it is continuously
climbing from the same ground point, it will be oblique (downwind from
its triggering point) hence the need to correct upwind from time to time...

It would be oblique with respect to the airmass until it had the same
speed; then, it would be vertical. It would always look oblique to
someone looking at the thermal (if it had dust or gliders in it, for
example) while standing on the ground. Once the thermal and the airmass
are moving at the same speed, there would be no need to correct upwind.

What I'm not sure about is how long it takes for the thermal to speed up
to the airmass speed. What do you think of the idea of circling for a
few turns after leaving a thermal? I think this might let us measure the
speed difference, if any, between a thermal and the airmass.


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Eric Greenwell
Washington State
USA

T o d d P a t t i s t wrote:

Imagine a 10 knot thermal....

What's "a ten-knot thermal"?


Jack
(in N. IL)

They are weak thermals we have in New Mexico.

T o d d P a t t i s t wrote:
Eric Greenwell wrote:


Once the thermal and the airmass
are moving at the same speed, there would be no need to correct upwind.


Imagine a 10 knot thermal being continuously generated from
a quarry or other warm spot on the ground. Assume a 10 knot
steady breeze with no speed change with altitude (no wind
shear). In one tenth of an hour (6 minutes) the thermal
will have risen to one nautical mile high (6,000') and its
top will have drifted one nautical mile downwind of the
quarry.

Now imagine a glider at 6,000' that began to circle (in
sink) directly over the quarry when the thermal started.
The glider has a 2.5 knot descent rate when turning. In the
absence of the thermal, in the same six minutes, the glider
would be circling about 1500' lower and have drifted the
same one nautical mile downwind of the quarry. Clearly, the
descending downwind angled path of the glider (dropping from
6000' to 4500') and the rising downwind track of the thermal
(rising from 0' to 6000') must cross, so what happens at
that point?

The answer is simply that the glider begins to rise as it
descends into the rising air. However, it does not rise as
fast as the thermal. It's still descending at the 2.5 knot
descent rate relative to the rising 10 knot thermal. Each
instant that the glider is in the rising air, it is
descending slightly in the thermal, and each bit of descent
takes it into air that left the ground later and was
slightly farther upwind relative to where the glider
started.

This is where this model is wrong. What you describe is true near the
ground, where the airmass speed exceeds the thermal source (the ground
point) speed by 10 knots. At 1000', the airmass speed is still 10 knots,
but the thermal speed is now (for example) 5 knots because the the wind
has accelerated it; i.e., the airmass above 1000' is being fed by a
_moving_ source, not a stationary one.

At some point (I suggest 2000') the thermal has accelerated to the same
horizontal speed as the air mass. At that point, the airmass above 2000'
is being fed by a thermal source (the airmass at 2000') that is moving
at the same speed it is.

Eventually the glider drops out the bottom of the
angled downwind, rising, path of the thermal (provided the
glider makes no centering corrections) and it continues its
downward and downwind drifting path, having been delayed as
its descending path crossed the thermal's rising path.

As long as the glider enters the thermal above 2000' (in this case), it
will not drop out of thermal, since the thermal is moving at the wind
speed. In fact, this is usually the case I encounter, because most of my
thermals do not require an upwind correction.

So far, no one has commented on my suggestion we measure the difference
in the wind speed and the thermal drift by circling a few times after we
leave a thermal, then comparing the drift from the flight trace later.
Does anyone have a better idea?

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Eric Greenwell
Washington State
USA

Jack wrote:
T o d d P a t t i s t wrote:

Imagine a 10 knot thermal....


What's "a ten-knot thermal"?


Jack
(in N. IL)
Fair, but not booming...

Best anyone in our club has achieved was 10m/s - on the averager - At Gariep.
That's booming.

--
Bruce Greeff
Std Cirrus #57
I'm no-T at the address above.