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#21
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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. -- Change "netto" to "net" to email me directly Eric Greenwell Washington State USA |
#22
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Quote:
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. |
#23
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Bronze Badge question
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) |
#24
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Bronze Badge question
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 -- Change "netto" to "net" to email me directly Eric Greenwell Washington State USA |
#25
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Bronze Badge question
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 |
#26
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Bronze Badge question
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. -- Change "netto" to "net" to email me directly Eric Greenwell Washington State USA |
#27
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Bronze Badge question
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) |
#28
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Bronze Badge question
They are weak thermals we have in New Mexico.
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#29
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Bronze Badge question
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? -- Change "netto" to "net" to email me directly Eric Greenwell Washington State USA |
#30
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Bronze Badge question
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. |
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