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Old Jun 27, 2002, 05:52 AM
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Joined Jun 2002
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which way ..if any do thermals turn??

I have recently hit thermals which send ny plane turning right and left!!!!! i thought they had some natural law or something to only turn right or counter clockwise...i think im wrong???...also when my plane .."the ripmax coyote" is turned or bounced by this wind,should i pull up?? turn,throttle up/down or off??also ...how far "in diameter" approx. are these things. ive caught a few ....abviously by luck!!! can yhou help
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Old Jun 27, 2002, 10:15 AM
aka: Dances with Buzzards
ICTHRMLS's Avatar
TX
Joined Jun 2002
454 Posts
Ahh - the elusive thermal.... I'm sure there are "experts" that will better answer the question of which direction a thermal rotates (counterclockwise above the equator when viewed from above I think) but the real question is which way do we turn the plane. There really isn't one proper direction although when circling together in a group of planes it is wise to be all going the same direction. When flying solo, one suggestion would be to turn the direction of the raised wingtip - if lift raises the right wing turn right. This should help you find the center of the thrmal quicker resulting in stronger lift.

Thermal size will vary greatly depending on altitude - the higher the larger. Also, outside this area of lift will be a corresponding area of sink - to be avoided if possible. However there is no one size fits all thermals as they all vary greatly. That's what make finding them so kewl.

I can also recommend a book by Dave Thornberg (sp?) with a title similar to "The Old Buzzard's Soaring Handbook" - a great and funny book which I guarantee will improve your thermaling after reading it. Good Luck..........
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Old Jun 27, 2002, 10:52 AM
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Rocky Mount/Wilson, North Carolina, United States
Joined Mar 2002
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Quote:
Originally posted by ICTHRMLS

I can also recommend a book by Dave Thornberg (sp?) with a title similar to "The Old Buzzard's Soaring Handbook" - a great and funny book which I guarantee will improve your thermaling after reading it. Good Luck..........
'Nuff said.
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Old Jun 28, 2002, 11:22 AM
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Michael Heer's Avatar
Stockton, Ca. USA
Joined Apr 2001
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Size does matter

A few years ago I was at Visalia on a Friday and a very large dust devil came through the field. It was probably about 90 feet across and several handtoss gliders were directed by their pilots to the dust devil. The majority of the planes were immediately thrown out by this monstor before they could penetrate the devil but a couple hitting it straight on were able to get in and they were sucked up immediately. One plane flew in a circle from right to left and climbed about 200 feet and flew out of it and another started spinning in the devil and was thrown out after only gaining about fifty feet. That is the only time I can remember seeing the direction the wind was turning when a plane was sucked up. I don't recommend anyone try and fly in a dust devil.
Generally a thermal will try and push your plane away if you fly through the edge of it and you simply do a 3/4 circle back into the direction from which you were pushed away. If you hit it straight on you will notice your tail pick up, your nose point slightly down yet your plane will climb in that position. Try to center on the lift and remember that the lift is flowing with the breeze in the same direction and speed as the breeze.
On rather still days with only gentle breezes you can feel a thermal coming or passing nearby as the air is drawn towards the thermal which is sucking it up. If the breeze has been from your left to right all day and suddenly it is from your right to your left there is most likely a thermal coming at you from your left.
The bigger the thermal the stronger it will be and the more tell tale signs that might be available to direct you to it. These include. circling birds climbing, switch in breeze direction mentioned above, we also watch for spiders floating on a bit of web and where they go in the breeze.
On some days we get thermals that are so large we don't even have to circle we just fly around the sky and go up. Better catch the ride when it comes as these big thermals have a wide band of sink around them. You don't want to launch after one of these large thermals has gone through as you will most likely have a very short flight.
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Last edited by Michael Heer; Jun 28, 2002 at 11:27 AM.
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Old Jun 28, 2002, 12:19 PM
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Tony Oliver's Avatar
North-East England
Joined Mar 2002
3,398 Posts
Thermal direction

Now you've started something!
This subject comes up occasionally and there seems no straight answer, but a lot of unsubstantiated theories.
I prefer to fly in right-hand circles, but if there's not much lift around and the port wing lifts, I'll turn left and keep doing so. It seems more important to centre the lift than fly in either direction.
The other aspect which this subject provokes (so now it's my turn to do it) is the question of shape of a thermal.
One theory is that it's a doughnut shape (toroid?) which climbs but has a 'sinking centre'. (?) This is also modified in arguments by suggesting that the whole thing does rotate, as mentioned by ICTHRMLS, depending if you're north or south of the equator. (what if you're on the equator?).

Tony
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Old Jun 28, 2002, 01:34 PM
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USA, AZ, Tucson
Joined Apr 2002
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Coriolis anyone?

which hemisphere are you in?
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Old Jun 28, 2002, 10:19 PM
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United States, AL, Madison
Joined May 2002
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read this

http://w3.iac.net/~feguy/soaring_symposia/69-dos.html
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Old Jun 29, 2002, 03:44 AM
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Tony Oliver's Avatar
North-East England
Joined Mar 2002
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Thanks, JonStone!

I guess that Puerto Rico is close to the equator so it looks as if I was right (or left, come to think of it!).
I particularly liked item 20 of your reference. I soon stopped following others who were often circling madly. I would fly over to find that they were circling in sink.
From a model flying point of view, another thing to be considered when others are flying is the direction to go when there's not a lot of lift about. Unless you've got obvious lift, downwind drift is unwise unless the height gain will allow a return upwind to the landing area. Upwind will give the opportunity to find lift first and outclimb the others while they drop out - your model must be able to move around the sky without appreciable height loss ('undercambered' wings are generally only able to do this in calm weather, being essentially a single speed machine). If in sink, upwind is an unknown if no other indicators are present - go crosswind quickly to get out of the sink but look for some signs of more positive air - even if it means following a circling model.
Don't stick with it if not in obvious lift - there are good reasons for taking chances and minimising losses by doing your own thing. We all have our feelings on which way to go, and instinct eventually comes into it.
Thie is a list of generalisations, but holds up until you have evidence from your own experience.

Cheers

Tony
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Old Jun 29, 2002, 07:38 PM
Simon Templar didn't R/C
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Massachussetts, USA
Joined Jun 2002
163 Posts
I remember reading somewhere that the coriolis force doesn't really play an part on such a small rotating airmass. I figure 50-50 change it spins either clockwise or counter-clockwise.

Of course, you typical low pressure system, hurricanes and typhoons are another matter.
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Old Jul 03, 2002, 12:14 AM
ASK
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Espanola, NM USA
Joined Jul 2002
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Plane turned by thermal...

1. Glider turned both right and left by thermals...
The direction your plane is turned depends on how your glider enters the thermal, not on which direction the thermal is rotating. If your glider contacts the outside of a rising column, it will initiate a roll by one wing rising faster than the other. Depending on your control input in response, and the design of your plane, this can initiate a turn.

2. How should I respond to the turn?
Two choices: One, let the glider continue the turn the thermal helped initiate. This will be away from the thermal, with you bring the glider 270+ degrees around to reenter the rising column. Two, try to counter the force that is "kicking you out" by turning directly back into the lift. If the lift is strong, this can be difficult to do in the time before you have passed the rising area.

3. What size are thermals?
These can be a few feet across, or span over a mile in diameter. In the desert West where dust devils indicate the center of some thermals, I have seen (and walked up and touched) a 2-foot wide, straight spinning cylinder of dust. It was caused by convergence conditions (mixing of cool ocean and warm desert air), rose over a thousand feet over the mountain peak on which it sat, and stayed in the same place for 20 minutes!

On the huge side of things, some friends and I watched a dust devil marking a thermal lift off of a large dry lake bed we had towed up from (hangliders) a few minutes before. It spanned the narrow dimension of the lake bed, and that was over a mile and a half. We watched from 6000 ft. AGL, and couldn't stop gawking at it. In seconds, we saw a large volumn of clear air fill with a huge spinning cloud of dust.
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Old Jul 04, 2002, 09:59 PM
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SoarNeck's Avatar
Calgary, Alberta
Joined Nov 2001
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Once you're centered in the core of a known patch of lift, try flying circles in both directions. Stick with the direction where the sailplane is flying slowest (most upwind), and you're more likely to gain altitude more quickly.
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Old Jul 05, 2002, 11:10 AM
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Ron Cichowski's Avatar
Carver, MA USA
Joined Jun 2000
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Quote:
Originally posted by SoarNeck
Once you're centered in the core of a known patch of lift, try flying circles in both directions. Stick with the direction where the sailplane is flying slowest (most upwind), and you're more likely to gain altitude more quickly.
If you are sinking at 60' per minute and the air mass is rising at 160' per minute why would the direction of flight make any difference?

You will show an apparent speed increase in one direction if the air mass is rotating but your airspeed and consequently sink rate should be the same in either direction.

Ron
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Old Jul 05, 2002, 02:53 PM
aka: Dances with Buzzards
ICTHRMLS's Avatar
TX
Joined Jun 2002
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As long as you are going UP the rate of climb is not a big deal - how many contests have you seen where the object is to rise the fastest?? Energy managment throughout the turn is vastly more important than which way the thermal rotates.... if indeed the particular thermal you are in is rotating. Even if you are maintaining a constant altitude you are still going up relative to the surrounding air.
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Old Jul 05, 2002, 05:00 PM
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Punta Gorda, FL
Joined Apr 2002
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Thermals' Affect on Models

Being bounced around by thermals usually has little to do with the thermal's rotation or not. The difficulty in entering and centering in thermals has mainly to do with the vertical wind shear which lifts the part of the model closest to the center of the thermal. Growing cumulus clouds mark the top of thermals. Such clouds seldom show any signs of rotation.

The angular momentum of the air mass entering the thermal at its base is what determines the speed and direction of the thermal's rotation. The coreolis force is weak in air masses and affects the direction of rotation very little. The angular momentum is preserved: as the radius of rotation decreases the speed of rotation increases. Small fast rotating thermals near the ground also tend to produce very strong lift. The larger and less violent the thermal, the easier it is to fly in.

Flying with or against the direction of rotation affects the angle of bank for a given size circle. Flying with the direction of rotation requires a higher angle of bank and results in a reduced rate of climb. For most thermals, the speed of rotation at the size of circles we use results in little difference in angle of bank and the direction of rotation can be ignored. Only the best and most daring pilots have the skill to fly thermals that are small and powerful enough for the direction of rotation to matter much. Such thermals usually make their direction of rotation evident by the swirling debris they pick up.
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Old Jul 05, 2002, 07:49 PM
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Calgary, Alberta
Joined Nov 2001
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RE: As long as you are going UP the rate of climb is not a big deal - how many contests have you seen where the object is to rise the fastest??


The last one I flew in! I was flying in an F3J contest in Mission, and there was a large group of trees downwind of the field. I found myself scratching for lift down low (20-30 ft)with my F3B Eraser a couple of times, and if I hadn't found the core of a couple of light patches of lift, I wouldn't have climbed above the trees before I got blown downwind into them.

Same contest...later on the first day. If you couldn't center lift and climb efficiently, you got blown too far downwind too quickly. There were flights (one in particular) when I would climb for two minutes, only to spend the next seven trying to flight my way back upwind. This is an F3B machine I'm talking about too...it covers ground in a hurry.

Anyway, climbing past people in a thermal is fun It's like showing off without being dangerous.
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