Thermal Releasing

[mingram]

Cross-posting from the PG group.

[Danny Brotto]

Cool. I remember being set up with about 6 other hang gliders at the top of the Heber site in Utah. It was a shallow, rolling launch with nil wind blowing. The gliders started rustling and someone yelled “Thermal!”. We all ran for our gliders but the thermal caused nothing more than a little shake in the gliders… except for my glider bag that was on the ground laid out straight underneath my glider. I watch as the bag was snatched away behind the glider and lifted up. It gently danced in the sky to about 100 ft. for 30 seconds or so. It was gently deposited about 150 ft away.

I went on to have a very nice flight with great scenery a bit later.

Danny Brotto

[deveil]

was that plastic sheeting that's used as a sort of green house for new plants in fields or was it one of Cristo's installations gone awry? (yuk,yuk)
remember him?
http://christojeanneclaude.net/vc.shtml

pretty cool.

[brianvh]

thought I should point out that it's rotating clockwise: at that small scale conservation of angular momentum is more important than coriolis.

[deveil]

dude, i don't know who's mind is more warped, yours or mine.







don't answer that.

[cthornberger]

I was wondering about that point as well, Brian. What factors go into the initiation of the rotation direction then?

[John Dullahan]

"Minigram,"

Thanks for forwarding the fascinating graphic display of thermal activity close to the ground.

Here's an interesting take from Bruce Abell, an Australian who held a sailplane senior rating (highly respected in soaring circles) and still flies model aircraft: http://soartech-aero.com/Thermals.htm He states thermals rotate counterclockwise in the northern hemisphere and clockwise in the southern hemisphere, but the rotational direction can be changed by ground features, the wind, or interaction with other thermals. (Observed from directly above, an anticlockwise rotating thermal will be rotating clockwise if observed from directly below - so Brian, Danny, and comp pilots, is there a standard perspective from viewing thermals: above or below?)

In any event, it stands to reason that we should fly against the rotating air. This will reduce the ground speed, which will reduce the centrifugal force and the possibility of being "spat out" of the thermal. So what to do if the first pilot in the thermal, who establishes the direction for other pilots joining him, is flying with the direction of the thermal rotation and increases the possibility of being "spat out?" Safety protocol demands that we join the herd, or find another thermal.

Bruce also observed "thermal meisters" (pelicans) changing direction in thermals, presumably to maximize their rate of climb. I have never experienced a thermal changing direction while I was in it (but then I am more adept at finding sink holes than thermals), and I am probably not sensitive enough to the change, if one existed, to have noticed it. Thoughts?

John

[deveil]

one of the things that i thought was cool was how the plastic, instead of the whole thing ballooning up as if a horizontal wind got under an edge, was being pulled horizontally across the ground surface and toward the center before it was lifted into the air, just as would be expected with a thermal.

[deveil]

comp pilots and others of similar proficiency aside, i think most recreational pilots tend toward just mostly liking to be turning in one direction or the other . this is also the reason some pilots do their dbfs from one side of a field versus the other. doesn't actually address your question except for one wondering why the first guy in would be circling one direction or the other .

[brianvh]

"I was wondering about that point as well, Brian. What factors go into the initiation of the rotation direction then?"

The coriolis effect is directly involved in rotation only if the period of rotation is a significant fraction of a day (earth's rotation). For small rotation you are usually just concentrating existing angular momentum. In you toilet the flush direction comes from the direction the water is injected or the shape of the bowl. In the shower, let the water spin one way down the drain, then move the shower head or turn it off and swish with your foot to reverse direction.

In meteorology the local environment might have air swirling in one direction or the other, but on large scales the coriolis effect sets it going CCW, which can be concentrated down to smaller scales by convergence. Tornados typically form in pairs with reverse spin, but the larger scale environment means one will die out first: ~60% of the surviving tornadoes end up CCW.