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Old Nov 26, 2012, 09:51 AM
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more input

post 30
thank you, Dave. great data!
so, maybe after all, thermals turn.
oh well, then turning left at the northern hemisphere was not wrong?
that's what i have been doing (and still do), and getting into thermals fine, thank you very much.
edit: an additional comment: from that post, it seems that sometimes thermals could change rotation if joining a larger 1 that turns the other way, so we should keep an eye on that. just in case the plane suddenly starts to tell us that.
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Old Nov 26, 2012, 10:36 AM
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facing the flow?

still, there is something that puzzles me. in 1 of his paragraphs of the post (http://www.soartech-aero.com/Thermals.htm) he says:
'When flying full-size gliders, I found that sometimes the aircraft would fight me and want to get out of the Thermal, while at other times the flight was very smooth and I finally came to the conclusion that, if I was flying WITH the direction of rotation, the Thermal would try to spit me out, whereas flying AGAINST the direction of rotation, i.e. into it, then I would find the flight was smooth and the glider tended to be drawn in towards the center of the Thermal where the rising air (read Lift!) was strongest. Under calm conditions, then, I would usually turn to Port (left) in the southern hemisphere when I thought I'd entered a Thermal but would change direction if the aircraft was fighting to get out of it.'

he flies in australia, where things go the other way, including the rotation of storms (and thermals, he thinks), and he concludes that, as thermals turn right down there he turns left (facing them).
what's going on? why then we turn left up here? has any1 tried both ways and seen that 1 of these ways helps his plane to climb more?
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Old Nov 26, 2012, 12:02 PM
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It would seem to me to be better to fly against the flow since that gives us higher airspeed and lower angular momentum. We could achieve a tighter turning radius without stalling a tip. My experince with thermals while hang gliding is that usually they don't "want" you in them and sometimes, if you strayed into the side near the top, they were very impolite. Some pilots called thermalling "air wrestling". I am sure there is almost no preference: Northern or Southern hemisphere so we should be open to the performance of the glider as well as the rules of the air.
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Old Nov 26, 2012, 01:50 PM
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tailwind or headwind

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Originally Posted by rrcdoug View Post
It would seem to me to be better to fly against the flow since that gives us higher airspeed and lower angular momentum. We could achieve a tighter turning radius without stalling a tip. My experince with thermals while hang gliding is that usually they don't "want" you in them and sometimes, if you strayed into the side near the top, they were very impolite. Some pilots called thermalling "air wrestling". I am sure there is almost no preference: Northern or Southern hemisphere so we should be open to the performance of the glider as well as the rules of the air.
although i have been trimming my free flight planes to turn left, and now that i can control them too, i still am not sure what is all of this. that's why i started this thread, hopping that, somehow, we could reach that elusive truth.
what i would like to hear is if some1 has experience flying into a thermal both ways and which way he climbs better.
regarding your comment on flying against the flow and getting higher speed, i am under the impression that it is the other way, that you go faster if going same way. it is like flying upwind or downwind: if you go downwind you go faster, right?
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Old Nov 27, 2012, 02:11 AM
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Airspeed vs ground-speed

Phil, I am sure going upwind in the thermal would be the preferred direction. Having lower angular velocity at minimum sink speed means we can fly "flatter"
turns. The lower angular velocity means less centripetal force so we don't waste as much lift pulling the glider around in the turn.

If we fly with the direction of flow or rotation we have a tailwind. We will have to fly a little faster around the circle to maintain our minimum sink speed, and now the centripetal force is greater, requiring a greater bank angle and more of the precious lift, raising our minimum sink speed and increasing sink rate

I have soared in many thermals. I have switched directions many times. Usually it was because I felt the thermal had "given me the slip" and I thought it might be "over there". If I then started to climb better, I attributed that to better centering in the thermal rather than the direction I was circling in.

We already have evidence in this thread that thermals small enough to matter to our little sailplanes rotate in both directions and often not very rapidly. Dust devils provide a chance to test theory. Unfortunately when I was given the chance to test it I blew it completely. I flew the glider into the dust devil at about 20- 30 ft altitude, an act of desperation! The glider rolled inverted... and started to climb. I cannot remember which way I turned in that thermal after I got the glider upright nor do I remember which way the dust devil was rotating so alas I have failed you.
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Old Nov 27, 2012, 06:08 PM
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Since the plane is always flying in an air mass (the surrounding air) its air speed has nothing to do with which way the air mass is moving relative to the ground. The plane will rise in a thermal because the air mass that it is in is going up not because of which way it is rotating.
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Old Nov 27, 2012, 06:54 PM
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Airspeed vs angular velocity.

Although I generally agree with Gary that airspeed has nothing to do with wind, this is a special case similar to DS where the wind is assumed to have a circular character. This means our aircraft is continually flying into a wind gradient. The wind is always shifting to the right or left depending on the rotation of the thermal.

My own belief is that this thermal rotation is often insignificant and Gary's assertion would then hold true.

If the thermal rotation was significant then I am holding that flying against the rotating airmass (thermal) would be beneficial because the aircraft would have less angular velocity and it would require less force to keep it in a given turn radius.

Another case of airspeed being influenced by wind direction is when entering the wind gradient near the ground. If flying upwind, the airspeed will rapidly decline. If traveling downwind, airspeed will rise and the aircraft will float on and on and into the fence.
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Old Nov 29, 2012, 01:39 AM
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ground speed: changes when into a thermal?

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Originally Posted by Gary Evans View Post
Since the plane is always flying in an air mass (the surrounding air) its air speed has nothing to do with which way the air mass is moving relative to the ground. The plane will rise in a thermal because the air mass that it is in is going up not because of which way it is rotating.
agreed. but we have to get into details: in full size aviation you talk about 'indicated airspeed', meaning what the speedometer indicates: your airspeed relative to the wind. that is, you get readings relative to the wind, not the ground. now, if facing a 20 mph wind, your speed relative to the ground will be 20 mph less than the indicated in your speedometer, and if flying with the wind, your speed relative to the ground will be 20 mph higher.
now, flying into a thermal: there are 2 opinions regarding horizontal flow of air within a thermal: 1 that says that there is a horizontal component, and another that says that there is only a vertical flow.
i never gave it a thought until now that i opened this thread, and i have been debating myself on the subject.
but the point here is that, if there is any horizontal wind within a thermal and you face it, it will slow you down relative to the ground (and you can see that reduction in speed relative to the ground), or if you fly with it (tailwind), the ground speed would be higher (again, you can see that).
and this is the key factor: for all that i have watched (and this has been thousands of flights into thermals), the ground speed of a plane within a thermal is not higher or slower than when it is not into a thermal, and that is enough proof to me that there is no horizontal flow within a thermal (see my #9 post. i said: 'any which way is ok. am beginning to see the light, thanks to all of your advice'.am reaching the conclusion that thermals just go up and there is no turn left or right inside of them, so you turn any way you find is better to stay put).

but this is my personal perception, and this is exactly what i am asking all of you guys to tell me here based on your personal experience, not suppositions:

if you see that a plane slows down-or accelerates-when flying into a thermal.

thank you very much.
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Old Nov 29, 2012, 08:57 AM
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Originally Posted by phil alvirez View Post
i never gave it a thought until now that i opened this thread, and i have been debating myself on the subject.
but the point here is that, if there is any horizontal wind within a thermal and you face it, it will slow you down relative to the ground (and you can see that reduction in speed relative to the ground), or if you fly with it (tailwind), the ground speed would be higher (again, you can see that).
and this is the key factor: for all that i have watched (and this has been thousands of flights into thermals), the ground speed of a plane within a thermal is not higher or slower than when it is not into a thermal, and that is enough proof to me that there is no horizontal flow within a thermal (see my #9 post. i said: 'any which way is ok. am beginning to see the light, thanks to all of your advice'.am reaching the conclusion that thermals just go up and there is no turn left or right inside of them, so you turn any way you find is better to stay put).

but this is my personal perception, and this is exactly what i am asking all of you guys to tell me here based on your personal experience, not suppositions:

if you see that a plane slows down-or accelerates-when flying into a thermal.

thank you very much.
Not sure I follow you but it seems to be saying that thermals do not rotate? I believe the overwhelming opinion would be that they often do at least near the ground because it is visually apparent from the rotating debris. Aloft is another question and in spite of 10 plus years thermaling full sized sailplanes I really don't know as the rotating speed if there is any would be so slight as to be undetectable from a cockpit.
When thermaling you are circling so assuming the entire 360-degree rotation was the same speed there would be no apparent difference in the average ground speed. A rising thermal is also dynamic in all dimensions which would make any measurement suspect at best.
I have been in a small thermal that was still pulling in air at the ground and I could clearly see the rotation in the bottom part due to the dust but in the upper portion that visual clue was gone so who knows. There is so much complex air movement within a thermal that you can only guess at its size, shape and core/s, which is also changing as it rises. The best you can do IMO is to find the strongest area of lift and try to stay in it. The simple rule of turn in lift and fly straight in sink is a big step in achieving that objective.
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Old Nov 29, 2012, 09:44 AM
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do thermals rotate?

thank you for your great comments. and coming from some1 that flies full size sailplanes and therefore has expertise on the subject is of great significance too. what you mention about the debris near the ground rotating, i have noticed only from dust devils, that may not behave the same way as thermals. perhaps thermals are animals of different breed. i said 'perhaps'.
when i have seen debris near the ground with thermals, it does not rotate, just goes up in an unorganized fashion. on the other hand, when i have seen debris near the ground in a dust devil, it definitely rotates-and fast. therefore my suspicion that dust devils and thermals are of a very different origin and behavior.
as you may see when surfing through these posts, i have been shifting from 1 belief to the other regarding if thermals rotate-or not, until the final comment i made recently (post 38), based on observations: if the plane gains-or looses-speed when entering a thermal. but this can be easier to detect in models, due to their smaller size, turning circle, and sensibility to weaker thermals than those found at higher altitude. and the small thermal that you mention could be more the exception than the rule.
still, the more comments by fellows that have some experience on the matter, the closer we will get to the elusive truth.
and this brings us again to the question: if you see that a plane slows down-or accelerates-when flying into a thermal.
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Old Nov 29, 2012, 10:28 AM
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At their inception a thermal and dust devil are doing the same thing, which is pulling in warmer surrounding air, and rising which on the ground is easy to see and classify. If the lapse rate is good enough that rising air may continue upward and at some undermined altitude may be considered a thermal. Even at ground level a dust devil may or may not be rotating.
There is enough difference in definitions to keep everyone happy but I think the common definition of a thermal is simply a centralized area of rising air. There are a lot of things that create rising air of totally different shapes so it would not be surprising that you may not know exactly what you have encountered. For example you could be circling in a huge area of rising air and but since it has the same updraft speed all the way around you continue circling while envisioning it as a classic round thermal.
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Old Nov 29, 2012, 10:49 AM
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The best clues I can think of to visualize what a thermal looks like are cloud shapes and smoke rising from fires. Niether "rotate" enough to really notice. The only rising air I've seen that rotates are dust devils and that is only near the ground.

As far as speeding up or slowing down upon entering a thermal I suspect this is only observable when flying in wind and piloting standing on the ground versus being in the cockpit (fullscale). When flying in calm conditions I dont see any change in speed other than switching from cruise or reflex into thermal flight mode and it doesnt matter which direction I am circling in.

A more important discussion would be how to best fly in wind and how to stay with the core and not outfly it to either the upwind or downwind side. Cores can be elusive things sometimes. In XC flying in wind is an essential skill. I'll admit it is much easier with a vario.

Steve
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Old Nov 29, 2012, 11:44 AM
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Quote:
Originally Posted by Gary Evans View Post
At their inception a thermal and dust devil are doing the same thing, which is pulling in warmer surrounding air, and rising which on the ground is easy to see and classify. If the lapse rate is good enough that rising air may continue upward and at some undermined altitude may be considered a thermal. Even at ground level a dust devil may or may not be rotating.
There is enough difference in definitions to keep everyone happy but I think the common definition of a thermal is simply a centralized area of rising air. There are a lot of things that create rising air of totally different shapes so it would not be surprising that you may not know exactly what you have encountered. For example you could be circling in a huge area of rising air and but since it has the same updraft speed all the way around you continue circling while envisioning it as a classic round thermal.
ok, let's get into the definition of thermal: it is hot air. and therefore hot air rises. there are other kinds of rising air (updrafts?) due to other conditions. but a thermal is hot, rising air.
my interest is in learning if a plane slows down or speeds up when getting into a thermal. only a thermal.
thanks
from wiki i found this on the definition of thermals:
A thermal column (or thermal) is a column of rising air in the lower altitudes of the Earth's atmosphere. Thermals are created by the uneven heating of the Earth's surface from solar radiation, and are an example of convection, specifically atmospheric convection. The Sun warms the ground, which in turn warms the air directly above it.[1] Dark earth, urban areas and roadways are good sources of thermals.
The warmer air expands, becoming less dense than the surrounding air mass. The mass of lighter air rises, and as it does, it cools due to its expansion at lower high-altitude of 125415748 pressures. It stops rising when it has cooled to the same temperature as the surrounding air. Associated with a thermal is a downward flow surrounding the thermal column. The downward moving exterior is caused by colder air being displaced at the top of the thermal.
The size and strength of thermals are influenced by the properties of the lower atmosphere (the troposphere). Generally, when the air is cold, bubbles of warm air are formed by the ground heating the air above it and can rise like a hot air balloon. The air is then said to be unstable. If there is a warm layer of air higher up, an inversion can prevent thermals from rising high and the air is said to be stable.
Thermals are often indicated by the presence of visible cumulus clouds at the apex of the thermal. When a steady wind is present thermals and their respective cumulus clouds can align in rows oriented with wind direction, sometimes referred to as "cloud streets" by soaring and glider pilots. Cumulus clouds are formed by the rising air in a thermal as it ascends and cools, until the water vapor in the air begins to condense into visible droplets. The condensing water releases latent heat energy allowing the air to rise higher. Very unstable air can reach the level of free convection (LFC) and thus rise to great heights condensing large quantities of water and so forming showers or even thunderstorms.
Thermals are one of the many sources of lift used by soaring birds and gliders to soar.
Thermals on the sun typically form hexagonal prisms (Bénard cells).
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Old Nov 29, 2012, 11:54 AM
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The best clues I can think of to visualize what a thermal looks like are cloud shapes and smoke rising from fires. Niether "rotate" enough to really notice. The only rising air I've seen that rotates are dust devils and that is only near the ground.

As far as speeding up or slowing down upon entering a thermal I suspect this is only observable when flying in wind and piloting standing on the ground versus being in the cockpit (fullscale). When flying in calm conditions I dont see any change in speed other than switching from cruise or reflex into thermal flight mode and it doesnt matter which direction I am circling in.

A more important discussion would be how to best fly in wind and how to stay with the core and not outfly it to either the upwind or downwind side. Cores can be elusive things sometimes. In XC flying in wind is an essential skill. I'll admit it is much easier with a vario.

Steve
thank you very much for your comment. this is what i am asking. you said: 'When flying in calm conditions I dont see any change in speed other than switching from cruise or reflex into thermal flight mode and it doesnt matter which direction I am circling in'.
thanks again.
regarding your suggestion on how to stay with the core, why not? let's do it. please bring your comments.
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Old Nov 29, 2012, 02:38 PM
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ok, let's get into the definition of thermal: it is hot air. and therefore hot air rises. there are other kinds of rising air (updrafts?) due to other conditions. but a thermal is hot, rising air.
It doesn't have to be hot air to initially rise or continue to rising but only to be warmer than the surrounding air. Thermals can originate from a water surface triggered by the temperature difference caused by cloud shadows.
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