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#31
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I am not disputing the facts, I am disputing the explanation.
Mike MU-2 "Andy Durbin" wrote in message om... "Mike Rapoport" wrote in message link.net... You will find less lift over water of any kind, even if it is contained in vegetation. The best lift is always over the highest, dryest, darkest surface around. The water vapor idea is...well...it is hard to find a place to start...but it won't work Mike MU-2 I hope you come and fly with us in Arizona some time. After a few miles of cross country in the blue you may come to appreciate the thermals triggered by the small ponds known as cattle tanks. Been using them for over 15 years and no theororetical analysis will convince me they dont work. Andy (GY) |
#32
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Mike Rapoport wrote:
"K.P. Termaat" wrote in message ... Thanks Larry for your interesting respons with your links to the articles. 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 You will find less lift over water of any kind, even if it is contained in vegetation. The best lift is always over the highest, dryest, darkest surface around. The water vapor idea is...well...it is hard to find a place to start...but it won't work Mike MU-2 As I just posted in the "water vapour" thread, I think you have it all back the front. Have seen similar effects over the small dams on farms here too. My idea is that the air over the water cools by evaporating water out of the pond. In so doing it looses more heat and hence contracts more (gets denser) than it gains buoyancy by water vapour increase, ie, it gets both colder and denser overall than the surrounding surface air. As the dense pool of air becomes greater, it spreads out, ie, sort of collapses on itself, and pushes out over the edges of the pond / dam, particularly down slope over the dam wall, creating a miniature equivalent of a valley wind in the creek or down the slope, thus acting as a wedge trigger to lift the warm dry air off the ground. |
#33
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Frank, your description of Wayne's model is closest to my own
observation. There is perceptible vorticity in cumulus clouds, both on a macro and micro scale (within the limits of the thermal), that is, you can perceive a slow turning of the cloud as a whole, while numerous smaller vortices are encompassed within it. I will read Wayne's work with interest. To those that ascribe to the theory that there is no rotation of the thermal, I would begin with the prompt to "look up." Best if you can take 20 to 30 minutes and watch a thermal throughout its life. |
#34
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Mark,
Are you referring to a sinuousness or serpentine bending in the vertical path of the core? That is, bending like some narrow cored tornados? |
#35
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I am not disputing the facts, I am disputing the explanation. OK, Its time for my Minden pond theory. We have a small pond about 5 miles east of the airport that consistently produces thermals. JJ's explanation; Heated air that is slowely moving over the ground by the wind, suddenly comes to the cooler pond edge. This cool edge of the pond acts as a trigger that forces the heated air to break loose and start rising and that's why thermals can be found at the edge of small ponds. JJ Sinclair |
#36
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On Mon, 29 Dec 2003 14:11:24 GMT, "William W. Plummer"
wrote in Message-Id: gCWHb.163786$8y1.490686@attbi_s52: I used to wonder why the default for holds is to the right but the default for VFR patterns at airports is left. --Bill |
#37
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That is how I found my best thermal ever. Flying my HP16T at a Region
8 contest in Eric's stomping grounds. It was about 2 hours into a 3 Hour post task and front started moving into the area. I decided that getting home might score better than trying to make the minimum time and landing out so I headed back to the airport under the cloud deck. The Glide was totally smooth and I arrived back at the airport at about 1500 AGL. (~3000MSL) on the far side of the airport the sun was shining and I could see two large dust devils up on the hill from the airport. As I approached closer dust devil I could see cheat grass, tumble weeds and garbage bags floating around in it. I put my left wing into the dust devil and turned hard left. My 10kt vario pegged. I switched the scale to 20kts and it pegged again. It would occasionall drop to as low as 16kts. I went from 3000MSL to 11000MSL in about 4 minutes averaging right at 20kts. I had to stop the climb due to cloud bases. From there I did a final glide out to a turnpoint and back to the airport to finish within a few minutes of the 3 hour minimum. Looking out on the wing I could see cheat grass draped all along the leading edge of the wing. I am sure it didn't due anything good to my glide ratio. As I rolled to a stop at the airport all the Cheat grass dropped off the wing onto the tarmac, which made for a great story "There I was, so low that..." Brian HP16T Eric Greenwell wrote in message ... Casey Wilson wrote: That said, I did once, inadvertantly fly into a dust-devil. I NEVER want to do that again. If I had seen any dust indication that it was there I would definitely have avoided it in the first place. No dust, no dust devil! But, of course, the thermal can still be there. Coming into one low can be dangerous, but up here in eastern Washington State, we use them frequently, especially on blue days. They are usually quite tame. Only the biggest are potentially dangerous, and then only when "near" the ground (say, less then 1500 feet AGL). What you are flying makes a difference, too: a 1-26 is going to be tossed around a lot more than an ASW 20 with ballast. Flying faster than the normal thermalling speed helps quite a bit if the thermal is rough. |
#38
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On the other hand I can't tell you how many times(numerous) I
intercepted a dust devil at 1000-1500 AGL and climbed out at less then 1kt or even did not climb. Most times however I get 3-6 kts out of them. I have see dust devils go to 7-8000 feet up. Hate to tell the one gentleman this that wouldn't fly into a dust devil, but if he flys using thermals he is just flying into dustless dust devils, As far as I can tell the only difference is if it is lifting air over an area were it can pick dust up or not. Brian HP16T Eric Greenwell wrote in message ... Casey Wilson wrote: That said, I did once, inadvertantly fly into a dust-devil. I NEVER want to do that again. If I had seen any dust indication that it was there I would definitely have avoided it in the first place. No dust, no dust devil! But, of course, the thermal can still be there. Coming into one low can be dangerous, but up here in eastern Washington State, we use them frequently, especially on blue days. They are usually quite tame. Only the biggest are potentially dangerous, and then only when "near" the ground (say, less then 1500 feet AGL). What you are flying makes a difference, too: a 1-26 is going to be tossed around a lot more than an ASW 20 with ballast. Flying faster than the normal thermalling speed helps quite a bit if the thermal is rough. |
#39
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"Chris OCallaghan" wrote in message om... Frank, your description of Wayne's model is closest to my own observation. There is perceptible vorticity in cumulus clouds, both on a macro and micro scale (within the limits of the thermal), that is, you can perceive a slow turning of the cloud as a whole, while numerous smaller vortices are encompassed within it. I will read Wayne's work with interest. To those that ascribe to the theory that there is no rotation of the thermal, I would begin with the prompt to "look up." Best if you can take 20 to 30 minutes and watch a thermal throughout its life. There's a later article in one of the modeling magazines complete with the graphics (unfortunately I've don't recall these being reproduced on the web) and more analysis. We'd asked to see if it could be re-published in soaring, but apparently the those rights weren't available. Wayne is an RC enthusiast AFAIK, the thermal modeling he showed us at the club talk indicated plumes at higher altitudes than mentioned in the link. I believe the CIRES team returned to the same cubic mile of air for 4-5 years and I think it was in Illinois or perhaps Indiana (have to dig back), so he modeling of thermals deals with some specific topography and weather patterns. Funding was always an issue. He did say that the most reliable thermal finder would need to 'see' detritous in the air. Frank |
#40
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Eric,
Look up! I know, it's pretty impolite of me. I assure you it is nothing personal against you in particular. Many, many pilots share your opinion. I guess I find it very frustrating to have to explain the obvious. Or. more to the point, to have to argue for the sake of the obvious. My preference would be to move to the next level, to discuss what the real effects of vorticity are on soaring flight. Instead, we wallow about trying to apply models to decide the truth about something easily observed. Granted, our attention spans are short, but spending a half hour on your back watching the development of a cumulus cloud can be very instructive. But you needn't even do this if you are observant during your climbs. You will note that around the edges of the cloud you'll occasionally see vortices forming when the light hits them right. Or if there is clag below cloud base, it will often have a discernable rotation. When observed, they often herald localized cores that can be exploited. More than once I've noted vorticity on the edege of a large cloud, shifted my circle to it, and been rewarded with much improved lift. Spending some time on your back, you will observe that not only are there localized vortices, but that the entire system slowly rotates. Sometimes it's easier to see this than at others. However, patience is required since the rotation is very slow, but nontheless discernable. Best to start with a wisp and watch its full development. Choose one as close to directly overhead as possible. The closer your view to directly beneath, the more obvious the cloud rotation becomes. I was hoping that the accelerated cloudscapes in Going the Distance would make this obvious, but most of the clouds are shot at very low angles. This makes discering the rotation more difficult as it is overpowered by the vertical development and dissipation of the cloud. By viewing from directly beneath, you won't be distracted by this. Perhaps someone with the appropriate video recording capabilities could do some time lapse photography and offer it up for review. |
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