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By magentabluesky
waveview wrote:Dynamic soaring opportunities for hanggliders do exist but I would estimate the risks to be rather high.
There are dynamic soaring opportunities for hanggliders that are not in close proximity to the ground thus reducing the risks.

Taras Kiceniuk, Jr. explains the concept:

Papers and diagrams by Taras Kiceniuk, Jr.

Have fun exploring.

By Beeza
It is probably worth mentioning that a tail down glider is pretty much a pig for slope soaring. Where we flew it wasn't worth attempting to slope soar if conditions were marginal.
If another student managed half a dozen passes on the slope before heading off to land, my glider would probably be able to do about three. It was harder to keep an even speed with the glider tail down, so in practice the glider ends up flying at varying speeds, but generally a bit too fast, which also helps cope with the glider being pushed around in any turbulent air. Well coordinated turns can be difficult, and if the air turns the glider into the hill it's a handful to turn back out. Generally it was better to keep a bit further away from the slope, and fly with speed in hand, although speed is also variable as the pilot tires. If the wind was strong enough to instantly lift the glider, staying further away from the hill didn't matter and slope soaring became viable.
On the way to the landing area for that site, there was a village on a rise, that regularly generated thermals. If a student arrived low over the village, through having spent too long slope soaring, he didn't really then have the height to play around, even if there was lift. By the landing area was a caravan site that occasionally had lift near it. Leaving out the slope soaring in weak lift, quite a few of my flights with that school were five minute top to bottom, with no apparent attempt to do anything.
After takeoff, and trying to get over an outcrop, over to one side (which sometimes had a good updraught), it was usually better to head straight off to the landing area with the aim of passing over the village with as much height as possible, which gave a longer period to fool around if there was a thermal forming over the village. Then head to the caravan site if the village was uncooperative. Then finally down if there was nothing doing.
The presence of convergence lift was always a worry, when it worked, but flying with another school on several different sites, reduced the chances of that being too much of a possibility.

There are a couple of pages showing my view on where the change from elongated circuit , to near circular and back to elongated, fits in the thermal. And a page to repeat the general flying style, and one to show the layout of the site. Leaving out any attempt at an explanation, the "style" of flying can probably be described on a page, or at most two. In the sketch the sharp turn away from the slope represents the instant steep bank, not the actual radius of the turn. As the glider is moving at its fastest, at that point, the radius will be much bigger to someone viewing it from the ground.
So it might be better to put everything already posted suggesting an explanation, to one side, and stick to the idea of this being a means of centring in a thermal (without a vario), by using the regular change of feel of the glider to suggest where the glider is, when deliberately flying slightly off-centre.

A slightly tail down, slightly flabby, single surface glider with the control bar not too far forwards, and the hang point on the keel, is probably ideal to start with. A higher performance, less forgiving, double surface glider might need a bit more care.
Moderate conditions, with a reliable thermal generator in front of the slope is probably also ideal, so that most times the pilot can reasonably hope to quickly move from slope-soaring to thermalling.
Entering the thermal coming towards the slope, with the prevailing wind directly on to the slope, going out from the slope is taken as upwind, and back to the slope is downwind, justified by assuming that there is some kind gradient, and as a result the airflow is slightly increased over the glider going "upwind", and slightly decreased going "downwind", being additional to any change of speed of the glider by the actions of the pilot moving the control bar. Some speed in hand is useful in the climb, if the glider is being turned out as it slows. There may be some slight bank, but the glider is kept as flat as possible at least to the middle of the climb. Dutch roll might be playing a part if the glider is restrained in roll, it might respond in yaw, as the pilot is trying to circle. (Could also be where the "real or imaginary thermal" comes from...the gradient is not imaginary, the thermal may be.....The tail down trimmed glider is impacting the marked gradient, obliquely....it might not actually be tail down when it impacts the gradient....the bar is pulled in in anticipation, second time round....At the very top, the gradient is only just noticeable, which is when the pilot may be tempted to stop circling and track back and forwards along the line of the hill below, the same way as everyone else)
In the example of Derek Piggot in the previous post, pulling back on the stick in momentary updraughts, is more effective by having some speed in hand to scrub off in the climb ( for a hang glider )
The pilot could find that circling one way only in the thermal, is an advantage. For me it was always more automatic to turn right, even if it meant jockeying around a bit at the start of entry.
Side to side positioning is by largely by feel, a fairly constant "shouldering" through to get the glider to turn into the core at the top of the climb is aimed for. Fore and aft position is more by sight, apart from trying not to fall out the back of the thermal, anticipating the sudden drop and turning early. A point on the ground can be selected and some guestimate made for the slope of the thermal. Say ten degrees or so. If it looks like it needs adjusting it is easy to fly a bit forward at the top of the climb, or delay the quick turn when flying back towards the hill. My thermals were generally steeper than everyone else's.
The fast turn away from the hill is an advantage as the glider has the greatest speed and momentum when it is flying through any air that may be rotating round a horizontal axis on the edge of the thermal(see sketch). Speed in hand can be kept all the way round by not quite scrubbing of all the excess speed in the climb, and turning quickly back at the top of each climb, which means the speed back to the hill is increased and a very sharp forceful turn is used to turn away from the hill. A certain amount of brutality is required, but it is not quite as oafish as it looks. It's also a natural reflex. Nobody would drag their heels turning away, if they were belting back to the hillside, even if they are not that close. In a way the gradient supplies the prompts. Although the aim is to anticipate them.
A rhythm develops that gives the feel of "working" the thermal, and automatically helps the timing. Anyone that has had to maintain heavy manual working over a long period, without running out of steam, will recognise it. There are no flourishes, there is no showing off, particularly with arms made of rubber. With a tail down glider, when it is rough, the glider really gets shoved around and it can be a bit grim. It is important not to fly in conditions that are too strong. If the pilot can not assemble the glider and take off without assistance, the conditions are probably too strong. Or the glider needs to be re-trimmed back to normal.
Something has to be held back for the landing with a tail down glider, the pilot has to avoid getting too tired. My flights were not timed as it was important not to linger if the glider dropped to a certain level. It was important to move quickly on to the next stage, particularly when using the alternative take off, higher up the valley. In the summer the sea breeze could penetrate that far inland and make it harder to reach the only viable landing area.
In my view it is also a good idea to have a rule that only two or three attempts are made to get "centred" in anything that the pilot thinks is a thermal. If those attempts fail then it can be better to move on to somewhere else to play around. That helps to avoid spending too long trying to get into the "wall" of the thermal or something else (strong turbulence for example), thinking that it is the core of a very narrow thermal. It's easy to notice a steep climb with the glider "flat". Noticing the sometimes sharp drop when it is steeply heeled over is not so immediate.
A single surface glider, or an almost single surface glider, is more forgiving and probably more fun. A high performance glider (which I've never flown) would probably need a bit more care.
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By red

If the term "tail down glider" means that the glider will go into a stall, when flown hands-off, that is a very dangerous glider. A glider trimmed to stall needs far too much attention from the pilot. Any lapse of attention may result in a loss of control, and maybe a spin.

Trying constantly to fly at an efficient speed (not too fast) will cause fatigue. That fatigue is dangerous to the pilot, both physically and mentally. Often the pilot will fly too fast then, resulting in very short flights. Standing in the landing field does very little to teach a new pilot about flying.

I can write every advantage of flying a tail-heavy glider on your thumbnail, using a large black crayon.

It is far better (safer) that the pilot will push out in lift slightly, and maybe in turns. I do not know why any experienced pilot would tolerate a tail-heavy glider to fly. Most certainly, no student should ever have that option.

Please look at this link on my web page.

A glider that flies hands-off at the correct airspeed is a joy to fly, and it will be very easy to soar. This problem is very important, especially to the safety of low-time pilots.
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By DMarley
Color me stupid, but why in the wide wide world of sports would anyone with any kind of flying knowledge want to trim their glider to such a high angle of attack (at neutral bar pressure) that it is on the kissing edge of stalling, or is actually stalling ("tail heavy")? Is beeza talking about setting his trim at the slightest AoA below stall, or at stall AoA?
https://www.willswing.com/pitch-stabili ... -location/
An increase in pilot weight, and even enough vertical acceleration, can create a situation in which the set trim AoA is increased to the point of mush or stall.
http://freeflightadvice.com/trim-what-i ... r-vg-have/

Beeza's non-succinct description seems to have very little semblance to dynamic soaring, at least from my attempt to discern his meaning. Why, unless the thermal is too small to core, why is he entering and exiting it on every turn? And if it was too small to remain within, what is the possibility of actually finding it again and again? Would it not be better to move on to another thermal? And obviously he is too close to the hard stuff to waste precious altitude in gambling he's going to tap it repeatedly. What about the downdrafts surrounding that small thermal so close to 'the hill'?
Perhaps I'm not understanding his meaning. :ahh: !
By Beeza
Sketches to go with previous post.

Had to use another scanner.
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By Beeza
The side gust soaring in the Icarus engineering Taras Kiceniuk link, looks to be along similar lines in some respects.

Haven't bothered googling this, as it would only confuse the issue. It's too easy to go off at a tangent as I found out when I tried to explain it to someone who flew fibre glass gliders at the time. I could see what he was getting at, he couldn't see what i was getting at.
We flew in different air.

The last two sketches probably do not need to be there. They just looked interesting as I'd just found some more envelopes.

Conditions where we flew were rarely too strong to fly. Probably due to being near the coast. (300 flying days a year, supposedly)
Thermals were relatively moderate, even early on.

Stating the obvious, but none of what has been posted is a suggestion to chuck a glider around in really strong conditions.

My view is still that the glider started off thermalling and ended up slope soaring. Which to me leaves the question of to what extent the thermal was needed in the first place. Some gradient was needed.

Where I ended up flying, parachutists had to be avoided as they would box the glider in on the slope, and with my glider I could not see them coming. There was he odd charmer that would even unroll his parachute in front of the glider while I was ready to start the run. Ended up doing a nil wind take of on a shallow slope, that turned out to be a slight tail wind. My fault entirely.
Only a few bruises and a bent glider (never liked the thing anyway) but that was the end of it.
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By DMarley
All the attached image scans are upside down.
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By DMarley
I'm guessing you don't have any video of your flights?
By Beeza
The Icarus Engineering link given above, is along the same lines in some respects.
Extracts in the order that they appear:
(The comments after each, are my inexpert views or opinions after a quick read through. They are also not following exactly what Taras K is describing, but looking at it specifically from the point of view of a much less efficient hang glider flying off centre in a thermal. My own biased point of view. Perhaps not a fair way to comment on it)

" Getting energy from sink requires negative gees or inverted flight."
Negative gee, doesn't apply to a hang glider, not practical.
"Still, the increase in kinetic energy can far exceed the loss of potential energy due to loss of height."
A hang glider has much more drag and little momentum and inertia, so any gain/ loss of speed must be in a short timeframe and over a smaller distance, and probably needs a bigger gradient of airflow in a given distance.
Downward Gust diagram.
The hang glider is dropping as it turns on end at this point (in the "downward" gust) and travelling fast. (but not nearly as fast as the 3 axis glider). The air might not actually be going down at that point, but from a relative point of view it may be in the lowest lift on the circuit. The pilot is positioning his hang glider on the solid feel of a good climb, the best gradient, which may or may not include descending air. So he may or may not be flying all the way out of a thermal to any descending air. My view is that if a comparison is made to a flapping wing (flapped by the gradient), it isn't desirable to extend the "flap". Everything has to continually change in a steady, regular succession. With a hang glider the range of speed it can fly efficiently at, is much smaller than that of a 3 axis fibre glass glider with its much higher L/D ratio. The hang glider is looking for the best combination of lift and gradient in a space it can reasonably gain and lose the small amount of speed it can efficiently play around with.
"To extract energy in sinking air the glider must push upward on the down gust. "
With the hang glider flat it would just feel as if the support holding it up had suddenly been removed and was dropping into a hole in the air. Any upward push would be minimal. With a glider on end it would be moving in the same direction at the air going down which might very slightly increase the drop of the hang glider. On end it could be more like the action of a flopper stopper used to stop a boat rolling, where the board is on end and moving "down" with the direction of the boat as it rolls. (Glider is pumping its way up the thermal?) maybe the best that can be said is that a sharper "downward" gust accelerates the glider down with a smaller loss of height, so that when it is sharply rolled flat the impact of the "upward" moving air is stronger.
"....which means that any kinetic energy picked up due to gravity is precisely offset by a corresponding loss in potential energy...."
The excessive drag of a hang glider in comparison to a 3axis glider would mean that kinetic energy gained would not precisely offset the loss in potential energy. The circuit or area of operations needs to be kept relatively small for the hang glider.
For me the downward gust by itself means a loss in height. The gradient of airflow is what matters. For an area with given gradient that goes from say plus3 to minus3, it is better to be circling in the part that is also lifting the glider away from the ground, and avoiding as much as possible of any descending air. It would have to be a very tight gradient for the pilot to be circling through all of such a gradient, and he probably would not particularly notice the descending air. This is pretty much stated by in the Sailplane Energetics paper: " To get the most power from the atmosphere we want the air to push our glider in the same direction that the glider is moving as much as possible."
For me we would additionally want to use the shove of the momentum of the air in a gradient to push the glider "up" as much as possible or to push against as the glider maneuvers. So a steep turn bouncing off an increasing airflow would be the opposite of the situation described in the paper, of flying into a downward gust. The load on the hang glider increases so the resulting "pitch up" is directed inwards giving assistance in making the turn correctly, without any spiral dive (A kind of power assist, no need to push the bar out, it does it itself)
The increasing air flow supplies the prompts (as the glider rears up, grab the bar in, throw yourself over to one side to counter the shove from one side), and even some power assistance in the steep turn across the gradient at the top. The heaviness in roll goes as the glider turns across, and the bar moves out by itself.
Lack of airflow supplies assistance in roll going downwind back to the hill, and the lurch/drop as it rolls up into the steep turn, encourages/prompts the pilot to get it flat and pointing away from the hill as soon as possible.

If the air on the side of a thermal is strongly rotational (about a mainly horizontal axis) the pilot probably doesn't want to be turning slowly through it (not that this is something suggested in the paper).
The hang glider wing has the additional advantage that it can seriously distort both in twist of the wing and from side to side, and the pilot can shift his weight to balance the distortion, whereas a fibre glass glider wing mostly just bends ...again adding something that is also not relevant to the paper.
The small amount of energy gained from the system, whether from the air (gradient) or pilot input, would be used to climb faster or stay up higher in weaker lift.
Even if a hang glider pilot can in some way add his own efforts to the system, it isn't going to be nearly enough to maintain height in still air.

Probably went off the subject too much there.

I'll read it properly in the next week or so.
By Beeza
A lot of the Taras K link is the same thing.

Allowances need to be made for the differences of a hang glider , such as avoiding negative G, and the wing being more flexible,but a lot of it is along the same lines.

"How do we make the atmosphere lose energy? By pushing on the air in a direction opposite to its motion. But first let’s clarify our terminology, the energy we are talking about is large-scale kinetic energy due to air motion, which is the kind of energy a sailplane can use. Heat energy and micro-turbulence are of little use (that’s where the sailplane loses energy via drag).

Once again, to make the atmosphere lose energy we push on the air opposite to its direction of motion. As the atmosphere loses energy the sailplane gains it. In what direction can a sailplane push on air? Well-- in any direction.

The wing of a sailplane is designed to push on air in a direction perpendicular to the wing surface and towards the landing gear. The wing can also push in the “negative geeâ€￾ direction (away from the landing gear), but the airfoil is less efficient when used that way.

......By banking and maneuvering the glider we can orient the wing to push air in any direction: up, down or sideways."

Spot on.

Still helps to be trimmed tail down though.....in my opinion.

Maybe add that you have to be pushing against a favorable gradient of air flow.

A uniform block of steadily moving air is of no use.

That's back to the often quoted idea that it makes no difference which way a boat turns in a uniform stream of water. It gets no energy from the stream and it can not seem to turn into the stream, (or into wind), whatever the pilot thinks is happening.

Also a lot less power is needed if the glider pilot just wants to climb faster or stay up higher in weaker lift.

Couple of comments on the Sailplane Energetics paper. Finish it later.

(1) A hang glider cannot dive efficiently, in the same way a sailplane can. The reflexed sail and raised tips work against it.

(2) A steep bank of less than 90 degrees is probably more practical for a hang glider.

(From my point of view, relating to what has been posted previously)

If the hang glider's maximum bank advances as the hang glider's circuit becomes compact ("closed up") then the glider is being rolled flat at the time it is passing through the downward gust, or outer extreme of the gradient, in which case the downward airflow is moving in the same direction as the glider and it assists with the roll flat, also helping to finish the tight turn of the glider, even though it is rolling "flat".

(Taras K: " To get the most power from the atmosphere we want the air to push our glider in the same direction that the glider is moving as much as possible." )

Child on a swing? Move legs with the motion to increase the swing. move legs against the motion to damp the motion out.
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By danmoser
Now the record is 519 mph !!!
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By aeroexperiments
I haven't read all the posts on this thread yet. One brief comment-- any time that you are flying in a significant wind gradient, and you do a "zoom" climb as you turn to face more into to the wind, and then you regain your airspeed later when you are facing less into the wind, you are doing a mild form of dynamic soaring.

Watching a Northern Harrier-- a lightweight hawk with poor energy retention, very unlike an albatross-- I've noticed that it usually climbed when it turned to face into the wind. The harrier was doing laps back and forth low over the terrain, where there was undoubtedly a wind gradient. I'm sure it was getting some degree of energetic benefit from the dynamic soaring effect.

I've never once seen a soaring (or flapping) hawk do the opposite-- systematically dive whenever it turned to face into the wind.

Taking the case of a hang glider soaring over a sand dune in a 20 mph wind, what is the range of altitudes where the wind gradient is likely strong enough for us to get some significant benefit from this effect? Food for thought...

I've come across Kiceniuk's ideas before-- certainly the idea of extracting energy from sink is theoretically sound--

By Beeza
http://www.icarusengineering.com/Dynami ... n-Gust.GIF

Some sketches on the down gust diagram, bent to fit a hang glider and from my point of view.

As a hang glider can't dive efficiently it has to roll up steeply and roll flat as it comes out of the down gust. Which means it has to turn.

The down gust in a thermal may be on the edge of the thermal, but the best lift and gradient combination is further in. (Depending on the size of the thermal)

So rather than the gust being air moving down it may just be the weakest lift flown through in the circuit.
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By Beeza
Found a copy of "Les Visiteurs du Ciel" on the internet. That was the first flying book I had at the time. It had just been published and someone at one of the schools had a copy. Some dynamic soaring related extracts are attached.

My view is that Henri Mignet was flying in a thermal, as he managed to climb, and not in a horizontal gust as he supposed. (Not much difference in the direction of air actually hitting his aircraft, in either case)

The initial sketches I made were combined with the albatross flight description in "Visiteurs", as a means of comparison. Bent to fit, as it were.
These were sent to someone at the time (end of 1990) for comment. They have not been posted here.

The main thing to note is that the horizontal gradient above the wave was drawn just outside the thermal. So the albatross was climbing in two stages. First in the lift close to the wave surface (in ground effect), and then in the wind shear above the crest.

For the hang glider, putting the horizontal gradient on the windward surface of the thermal, meant that the prevailing wind had to blow quite strongly around the thermal column. Which meant that it would probably have been strong enough to disperse the thermal. It also meant that the horizontal gradient had to be exactly the same and at every level that the glider turned back to face the hill.

However many times it was lobbed in the corner, and dragged out a few days or weeks later, it never looked quite right. The lift and horizontal gradient needed to follow on smoothly in a single climb inside the thermal, even though they appear to impact the albatross from directions ninety degrees apart. The air impacting the wing is roughly from the same direction all the way through the climb.

That meant the horizontal gradient outside the thermal can be binned, and the idea that a strong wind flows around the thermal can also be dumped.

Although weaker flow may be possible. A cigar smoker can produce vortex rings that exist for a few moments, and flow through the air so it is maybe not entirely unreasonable to think that to some degree, and in some conditions air can flow around the thermal "column". Just not so strongly that it would be the equivalent of the strong wind flowing over a wave, that the albatross encounters.

Didn't bother redrawing it as the possibility of some arrangement of Vortex lift, or Pilot Input, was more interesting. Plus being obsessed with an albatross seemed a bit Monty Python.

As I have no idea where those sketches ended up, just thought it was worth a mention just in case they turn up and someone thinks "What the **** is that"
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By Beeza
On page 159 of the book there is a photo of a hang glider under heavier than normal load.

How much of that glider is acting effectively as sweep-back, or would be if the air flow was from slightly lower than where it is coming from in the photo?

What would be the effect of a gust from the side? The hang glider flying is not anchored the same way, but it still has some inertia for a moment or two.

In my view this is what the glider looks like, pitched up in the climb. The pilot would be as far forward as possible, with the bar pulled right in.
By blindrodie
I've used the "side gust" factor with my Gentle Lady RC glider. We always called it "rocking up" to gain altitude...

By Beeza
The photo on page 159 brought back memories. Think I even traced it as an illustration.

Then I thought "Ah that's going to need an explanation".

Why use sweepback all the time and not see it from the pivot point, point of view?

There were maybe half a dozen, probably not very good reasons, for seeing it from the point of view of the first part of the glider to impact the increasing airflow. Apart from consistency.

The idea of the proportions of sweepback and dihedral changing as the glider pitched up.
The lack of a tail surface and keel pocket on modern gliders (even though it's the total sideways area behind the pivot that counts).
A leading edge vortex more easily forms on the leading edge of a wing with more sweepback (even though a sail stall starts from the trailing edge)
Just as the first point of a deep V hull hitting the wave is the dihedral/sweepback surface, so the first point of the hang glider hitting the gust/progressive gradient is the leading edge/lower surface.
The lower surface/leading edge distortion on impact seemed to be more of interest, even thoughit's the upper surface that contributes more to the overall deflection of aif flow, shoving more air "down"
Sweep back seems to be an integral part of any description of a Dutch roll (even though not even one period of the sequence is completed).

It was one of those things that was supposed to simplify and unify things, but probably had the exact opposite effect. Then to be consistent, it was continued all the way through.


More Monty Python.

Is the albatross effectively flying in lift when it is flying very close to the water, in ground effect, exactly half way between the trough and crest of a wave, and largely sheltered from the prevailing wind in the lee of the wave?
(The steady prevailing wind is higher than the wave speed, and the waves have built up to a maximum.)

Use of the internet is not allowed.

http://www.dynamic-soaring-for-birds.co ... lysis.html

Best quote: "....I have not attempted to write a single equation. It was a bit like juggling with jellyfish."
By Beeza
Bent drawings.

The description of albatross from the Visiteurs du Ciel book was superimposed on my view of what the hang glider flying off centre was doing, to compare them and try to hammer them in to fitting together. It seemed like a valid way to progress when that was the only information available.

As it was sketched, the albatross circuit partly in a thermal, didn't look quite right. The idea was to finish it as much as possible, leave it a while, then after a few days/weeks see if it could be hammered into a better shape, or dismantled and bolted together differently. The aim being to get them to match.

To start with, to be realistic, the prevailing wind would have to flow around the thermal for there to be an equivalent airflow and gradient to support the albatross when it pops up into wind from behind the crest of a wave. Such a strong wind seemed more likely to disperse the thermal.

Another problem would be having the horizontal gradient of the prevailing wind, appear in exactly the right place each time the glider finished the climb and turned back to the hill. (The "soaring turn across the wind" at the top of the climb on some of the drawings of the circuit, was wording taken directly from the albatross description in the book. Not that different from matching the letters in Taras K's description in the previous post)

Block capitals were used to cut the wording right down, so there was only space for the minimum of words. There was no space for any "perhap's, maybe's, in theory's, in my opinion's" or anything like that. The drawings had to suggest the story, they were more flexible.

The same thing was done with the Renner/Reichmann description.

The Reichmann extract was drawn to show the climb and decent repeated as the glider drifted downwind. One cycle was taken out and the end of the drop, where the lower sharp turn is made, moved upwind to the previous lower turn giving the impression that the pattern would no longer drift down wind.

A bit two dimensional, but it did look similar to the pear shaped pattern drawn for the hang glider. The problem is that it suggests that the glider has to fly a certain distance to recover the energy lost in the climb, rather than (efficiently) drop a certain height. To remain over one spot it the sailplane probably just has to perform a steeper drop and a shallower climb to cover a greater distance upwind.

There were about half a dozen pages along those lines. They have not been posted here but they were sent to someone, and there could be a small chance that they turn up, in which case they should not be regarded as ever having been a statement of what actually "is".

Taking the effect of the increasing airflow at the point of impact in terms of sweepback and dihedral, and not the overall pivoting effect, is along the same lines. It also helped to compare the blob with wings with the blob with one wing, where the other wing does not exist and the main body mass is suspended under the wing rather than between the two wings. The impact would move the jointed flexible wing slightly before it rotates the rest of the contraption, a good part of which does not exist for the hang glider.

The upwind/downwind split of the circuit is still probably a valid way to look at it even if it seems hard to justify for a glider in a thermal, particularly if it is properly centred.

The only bent drawing that, in my view, should not be put to one side (or binned) is the pear shaped pattern itself, with one side slightly flattened. It does not describe what an observer would see, but the changing angle of bank of the glider as it goes round the circuit, and what the pilot thinks he is doing, or what he thinks is actually happening.
It seemed better to draw it as a distorted circuit rather than as just a circle or an oval. The circle/oval being what an observer may see, or what is actually happening.

From the previous post?
Is the albatross effectively flying in lift when it is flying very close to the water, in ground effect, exactly half way between the trough and crest of a wave, and largely sheltered from the prevailing wind in the lee of the wave?


For the albatross climbing close to the water on the lee side of the wave, my view is that it is fling through a band of rising air that blends almost seamlessly into the horizontal moderate gradient of the prevailing wind, blowing across the wave's crest. When it drops with the wind behind it turning steeply, it passes through a band of sinking air.
The bands/patches of rising and sinking air come from the circulation of air that occurs in the trough between two waves. The pocket of air is not flowing with the prevailing wind but rolling along underneath it with the water below and the gradient of flowing air above.
The prevailing wind is a bit faster than the speed the wave is moving so that the winds and gradients that the albatross is flying in are not as strong as someone on a slow moving boat is experiencing in the same place ( when the bird is making overall downwind progress).
Apart from the flight in ground effect, to me anyway, the flight is very similar in several respects to the flight of the hang glider flying off centre in a thermal. It was hoped that the leading edge vortex idea would end up as a replacement for the flight in ground effect, but that could not be hammered into shape.
The bird could even be viewed as being flapped by the "up" and "down" gradient (particularly as its wings lock at the shoulder) and as the thing even sleeps on the wing, maybe it's just being rocked in its cradle.
(The "up" gradient being mainly smooth lift, from the low point to the high point......the air actually hitting the glider does not change much in direction, if a vertical gust becomes a horizontal gust. Or from the bird's point of view a ventral gust with wings level becomes a ventral gust as it climbs above the crest and turns back.
The amount of "down" gradient flown through will depend on where the albatross decides to face back upwind.)
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By DMarley
From spending many hours and sometimes many days at sea in a small boat (35 ft sailboat), I can most definitely tell you that the wind does not reverse itself like your diagram suggests, even if there are very large, peaked crests. But then what is seen are small rotors behind these sharp crests, and the rest of the flow reattaches to the water's surface after the very small rotor. If this was not the case, such as your diagram would suggest, there would be no possible way that small sailing vessels could navigate the larger, smooth-contoured seas without being back-winded behind every wave crest, which is certainly not the case. The wind remains mostly laminar to the wave surface throughout it's length, especially when the crest is less defined with no jetting and a smooth peak. I am talking about my experiences with all shapes and sizes of waves I've encountered, from wind -generated waves ~1 to ~25 ft in height, wavelength ratios of ~2:1 through unmeasurable (gravity waves), and from decaying waves that have smoothly rounded crests to building waves in strong winds that have jetting / breaking crests.
Instead of sailing the seas, I now am a land-bound landlubber on a nice-sized farm with rolling hills that remind me somewhat of the sea. One of our large pastures whose length-wise contour elevation form has 20' deep depression that duplicates a large, 20' mature deep-water wave almost perfectly in wavelength to height ratio and contour. It is situated NW to SE, so the prevailing local winds flow across just as constant-direction winds flow on mature sea waves. I use this as a LZ for RC sailplanes and possibly later on with my HG. I have a series of 6' tall fiberglass rods with ribbon tell-tales tied at the tops. These are spaced every 12 yards from one end of the pasture to the other (~1200' long), along the direction of the prevailing winds. NEVER do these tell-tales show a reversal in wind direction. Yes, one can sea a gradient of wind strength from crest to bottom to crest, but never a reversal, unless the buzzards are circling above because a thermal just lifted off.
In whiling away the hours at sea, watching the sea birds make their way across the waves, I came to the conclusion that the gradient was enough for the birds to conserve energy in the manner that they do.
When sailing, one rarely experiences a wind reversal unless the waves are square (2:1 ratio) and jetting (breaking) with spray flying off indicating the wind separation, but this is only found very close to the lee side of the crest. Never saw any birds fly in such extreme conditions (~30 knots and up).
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