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By Beeza
#344912
A hang glider will pitch up when the wing loading is increased suddenly.
The increase in load can be a from sudden increase in lift, a gust, or the glider rapidly changing direction.
An increase in lift is more immediately noticed than a drop in lift. In sink it can be several moments before the pilot realizes he is going down (flying without a vario)
Moving into increasing lift the harness is immediately squeezed in and the control bar tries to pull forward. Circling off-center in the thermal with periodically changing lift gives a regular, repeated pull upwards on the harness. It could be used as an aid to judge position in the thermal.

Continuing on from the last post, let's say the circuit flown is pear shaped, viewed from above. The narrow radius is nearer the hill, the climb is flatter than the return, and the wider "base" part of the pear is where the glider flies across the "wind" before returning.
The radius represents the angle of bank of the glider, not the actual path flown, which to an observer viewing from above may be more circular or elliptical.
Imagine a line drawn out from the hill side. With the pilot always turning right, trying to center in what he thinks is the thermal, the area to the left of the center line where the glider flies away from the hill, is the upwind leg.
To the right is the downwind leg, where the pilot flies back towards the hill. (This can be compared directly to the Renner/Reichmann demonstration of dynamic soaring).

The pilot has done one circuit, has turned and rolled the glider flat and is starting to head away from the hill again with a good excess of speed.
On entering the thermal (if it is still there), the glider will pitch up as it impacts the gust, the gradient of increasing airflow. The pilot keeps the bar firmly pulled in until the pressure from the control bar pulling against him eases.
The glider may still be pitched up even while the pilot is still grimly holding the control bar in during the climb.
If the climb is very steep both glider and pilot are pitched up, with the bar firmly held in. At the top of the climb if the pilot has scrubbed off of all the excess speed, the nose of the glider drops by itself, and the bar returns to its normal position (just held in, if it is trimmed tail down).
In a "stick thermal"with no gradient, the pilot pulls speed then eases the bar out and pushes it forward slightly to get the glider to climb. Flying in the described gradient, there is no need to push the bar out to climb. So it is not a stick thermal, or a stick thermal imposed on a gradient. The bar is only eased out as the climb slackens.

Just before the climb, the drop from the sharp bank has been suddenly arrested by the pilot banging the glider flat, causing an increase in load, so the glider pitches up. The bar is held in.
Just before that, when the pilot threw the glider up into a steep bank the load increased, causing the glider to pitch up in the turn. Only "up" is more towards the center of the turn. He may have the impression of bouncing of the wall of the thermal if he is flying with speed in hand. The pull on the bar may momentarily ease before the glider goes "flat".
The three load increases follow on from each other and effectively run almost continuously, but the impact from the thermal (if it is still there) is usually the most marked.
So fly back in a tail wind, start to bank, rapidly roll up, bounce, bang flat, hit the thermal, and climb steeply up into the prevailing wind.
The final roll up and roll flat must be one movement.
The maximum bank will be before the apex of the turn (assuming an elliptical pattern), the roll flat will be completed just after the apex, before the glider has turned completely outwards to face the wind.

This is for a small well defined thermal that might be found when a pilot is trying to go from slope soaring to circling in a thermal. The first circuit maps out the thermal roughly, the pattern then closes up, and becomes smaller as the moves become regular and repeated with less hesitation, (possibly becoming more circular). If successful, as the glider climbs, the pattern opens right up again till the pilot can climb no higher. He can maintain height by swooping round in circles, but if he faces into wind like everyone else the glider fairly quickly drops down again.

The pilot is not trying to dynamic soar, he is trying to roughly center in a thermal (real or imagined) without a vario, using the regular repeated change of feel from flying off center. When successful he anticipate what is (should be) coming next (in the sequence)and make adjustments (and adjust) , helped by flying with speed in hand, to keep up the steady rhythm.
By Beeza
#345373
At the top of the climb, if the pilot turns back to the hill with speed in hand (he has tightened the pattern up after going round once), the glider can be banked up on end as it is in the bottom turn, and rolled flat just as quickly. This turn is less loaded than the bottom turn.
Both the bottom and the top (if there is one) steep turns can be compared it to a bird bringing its wing(s) down and forward driving its body mass through the air.
A hang glider only has one wing, so the equivalent of the bird (flap) would be the very quick forceful roll flat after the steep bank in the tailwind.
The air "driven out" helps continue the turn of the glider. If sufficiently forceful, it can help propel the glider round to face into wind ready to impact the gradient. So to the pilot, the maximum bank can appear to be before the apex of the turn

Any glider will be affected by a gust from the side. It will react in roll or yaw (let's ignore pitch) The tail of a 3-axis glider will help it weather vane into the gust, but only as long as the airflow is increasing.
On a hang glider it is mainly the sweepback that provides directional stability, together with that part of the sail with positive dihedral, that can be considered as contributing to the glider's overall sweepback.
The overall sweep back will effectively increase as the glider pitches up.
As it pitches up going into the gradient, the load will increase and depending on impact, the leading edges will bend further distorting the wing. The amount of positive dihedral to negative will increase compared to when the glider is normally loaded, flying straight and level.
All that extra "sweep back" will make it react more to any sideways gust, as long as the gust is increasing. So as the glider climbs at high speed and a higher angle of attack it will be drawn round towards the greater flow of air. In the climb the pilot might feel that his glider is being drawn into the thermal.
This yawing into the "gust" continues the turn round from the roll flat and means the glider can be flat before it faces directly out from the hill.
All the extra dihedral from the loaded and distorted wing increases the stability in roll, so depending on how strong the thermal is, the pilot might have to really throw his weight right over to keep the inner wing down "flat".

Having the control bar nearer chest level in normal flight is better than having it under the pilot's chin. The forward position means that even if the pilot throws his feet over first, it is hard to prevent just twisting on the hang strap, rather than moving the whole body weight as intended.
The pilot is holding the bar in firmly. In the climb the extra body weight can be shared between the hang strap and the control bar if it seems excessive. towards the end of the climb as the (pitch) pressure eases, the bar can be eased out. In the core (or what the pilot judges to be the core) he can fly forward a bit before turning back to the hill, or just turn back if he has found a good rhythm and it seems to be working.
In each climb the pilot is also aiming for the same pressure in roll. The pilot is circling to his right, and off-center in the climb. That is to the left of the core looking out from the hill. If he is too far to the left of the center of the imaginary thermal there will be increased effort required to force it into the core. If he is too far to the right he will find he falls through the centre and automatically his excessive efforts to force the glider in will result in him quickly flicking round and facing back down wind to the hill. To an extent it self-adjusts. Helped with a bit of speed in hand.
By Beeza
#345374
At the top of the climb, if the pilot turns back to the hill with speed in hand (he has tightened the pattern up after going round once), the glider can be banked up on end as it is in the bottom turn, and rolled flat just as quickly. This turn is less loaded than the bottom turn.
Both the bottom and the top (if there is one) steep turns can be compared it to a bird bringing its wing(s) down and forward driving its body mass through the air.
A hang glider only has one wing, so the equivalent of the bird (flap) would be the very quick forceful roll flat after the steep bank in the tailwind.
The air "driven out" helps continue the turn of the glider. If sufficiently forceful, it can help propel the glider round to face into wind ready to impact the gradient. So to the pilot, the maximum bank can appear to be before the apex of the turn

Any glider will be affected by a gust from the side. It will react in roll or yaw (let's ignore pitch) The tail of a 3-axis glider will help it weather vane into the gust, but only as long as the airflow is increasing.
On a hang glider it is mainly the sweepback that provides directional stability, together with that part of the sail with positive dihedral, that can be considered as contributing to the glider's overall sweepback.
The overall sweep back will effectively increase as the glider pitches up.
As it pitches up going into the gradient, the load will increase and depending on impact, the leading edges will bend further distorting the wing. The amount of positive dihedral to negative will increase compared to when the glider is normally loaded, flying straight and level.
All that extra "sweep back" will make it react more to any sideways gust, as long as the gust is increasing. So as the glider climbs at high speed and a higher angle of attack it will be drawn round towards the greater flow of air. In the climb the pilot might feel that his glider is being drawn into the thermal.
This yawing into the "gust" continues the turn round from the roll flat and means the glider can be flat before it faces directly out from the hill.
All the extra dihedral from the loaded and distorted wing increases the stability in roll, so depending on how strong the thermal is, the pilot might have to really throw his weight right over to keep the inner wing down "flat".

Having the control bar nearer chest level in normal flight is better than having it under the pilot's chin. The forward position means that even if the pilot throws his feet over first, it is hard to prevent just twisting on the hang strap, rather than moving the whole body weight as intended.
The pilot is holding the bar in firmly. In the climb the extra body weight can be shared between the hang strap and the control bar if it seems excessive. towards the end of the climb as the (pitch) pressure eases, the bar can be eased out. In the core (or what the pilot judges to be the core) he can fly forward a bit before turning back to the hill, or just turn back if he has found a good rhythm and it seems to be working.
In each climb the pilot is also aiming for the same pressure in roll. The pilot is circling to his right, and off-center in the climb. That is to the left of the core looking out from the hill. If he is too far to the left of the center of the imaginary thermal there will be increased effort required to force it into the core. If he is too far to the right he will find he falls through the centre and automatically his excessive efforts to force the glider in will result in him quickly flicking round and facing back down wind to the hill. To an extent it self-adjusts. Helped with a bit of speed in hand.
By Beeza
#359549
A link to some sketches and notes to go with the previous posts. Had a load more but can't find them.

https://onedrive.live.com/redir?resid=A ... file%2cpdf

(Just tried the link. Might not work. Works from a hotmail account, when emailed......cut and paste? .Might have to find another method)
They were written for my own benefit and use, with no ifs, buts or maybes. Everything was written as if that was the way things were, largely due to a lack of information on the subject. The intention was to modify/correct them later if needed. So some of this may be WRONG.

The egg shaped pattern with one side slightly flattened illustrates the changing angle of bank of the glider, not its actual path through the air. It is not what a viewer would see from the ground. A viewer would probably see something more oval or circular.

It is intended for going from slope soaring to thermalling, and back again (if he can't get positioned in the thermal to climb away).
Flying perfectly centered in the core of a thermal, the hang glider pilot has no "feel" or feedback from the glider to tell him where he is positioned, so unless he is flying with a vario, or is lucky, he is likely to quickly drop out.

The idea was that (without vario) by flying sufficiently off center* it was possible to introduce enough feel for the pilot to almost automatically work out where he was, and quickly correct his position before flying out. (*And orientated the same way to the "wind" as for the Renner Reichmann example)

By repeatedly climbing into wind, pausing at the top, turning back to the hill, rolling into a steeply banked turn as soon as any drop is felt (or anticipated) followed by a sharp roll flat (with the bar pulled in) to take maximum advantage of the increase in lift, the pilot has a constant steady, change of feel that gives him an idea of where he is.

Finding what he thinks could be a thermal and say circling to the right, if a pilot flies out from the hill and is too far to the left of the core it is noticeably harder to push in to where the pilot thinks the core is. (That is pushing in sideways against the lift of the thermal on the inner wing, not pushing the bar out).

If he is too far right and has passed through the core there is little resistance to the turn, and when he pushes sideways in anticipation of making the same effort as last time round, the glider easily rolls up and he finds himself quickly facing back at the hill. After the first circuit it is easy to work up a rhythm and any change is easily noticed.

It was sometimes surprisingly efficient, so the notes were also an attempt to work out why.

(Had a couple of books in French. "Visiteurs du Ciel" and eventually one of Dennis Pagen's. No internet in those days)

A comparison was made, to the way that Helmut Reichmann described Ingo Renner's successful dynamic soaring trials, in one of the books.

A comparison was also made to a paragraph describing how the albatross flies. Was less happy with that one.

The tail down glider became part of it, having got used to/acquired a taste for it, being the heaviest pilot in the group of students.

Landing was perhaps more of a problem than taking off. It was hard to pull speed and maintain it on final approach and then very easy to get the thing oscillating in roll, when moving a hand from the centre of the bar to the upright.

Possibly the best way is to move the hands right down the uprights to pull speed, then quickly and evenly walking them up through the fingers (in ground effect) ready to be ready to flare. The glider wallowed hands off, but did not easily stall, probably because they were student gliders. The tail down aspect, and the "more tiring" aspect encourages the pilot to fly with a bit of speed in hand, and encourages him to grab a handful of control bar before turning.

Tail down the glider might also be more responsive to gusts, which can add to the effort needed to fly, but also means the pilot gets more used to letting the glider do its own thing, which might help show when the air flow is increasing. Finger tip control is out, oafish enthusiasm is in.

It should be possible to have a hang point moveable in flight. Maybe based on some of the contraptions they used to have for pitch assistance before hang points were on the mast. All that garbage that used to slop around as they waddled to the ramp.

Having the control bar forward under the chin instead of at chest height is a disadvantage in my view. it makes it very difficult to chuck the thing up on end when you need to. Even with throwing your feet into the turn first. The tendency is for the body to pivot on the hang point instead of moving across sideways.

A figure of eight pattern (flying this way) might seem a good idea, to avoid turning back to the hill.

Not a good idea in my view. Although never tried it. Probably does not work, although it might for another kind of glider.
So probably best to bin the figure of eight idea, for this kind of flying.

Reasons .....in my view.

- Rolling from bank to bank in a figure of eight is less efficient than just a varying bank in a simple circuit, and introduces a hesitation which upsets the rhythm. (Same as deciding which way to circle. Better to always circle one way and maybe turn a bit more on the initial circuit).

- The "downwind" leg is too long. Renner's drop would have been much shorter and sharper than it looked as it was started with the wind behind him, and he would have been carried further down wind than he would have in still air.

- The pilot might think he can get closer to the hillside as he can easily turn away, making the downwind leg too long and more across the wind than actually downwind.

-It might encourage trying to fly through all the gradient, to the stiller air closer to the hill. Whereas if you are flying directly back to the hill with a bit of speed in hand, you are only going to want to do one thing.

- The pilot is trying to go from slope soaring to thermalling, where there is just one small area of strong lift that he is trying to get into, as smartly as possible.

Although, one of the jokes from one of the experienced pilots who travelled in the van with us, was that in normal thermals the pilot did horizontal circuits and the thermal inclined back over the hill, whereas with my thermals the circuits were inclined and the thermal went straight up.
Being a Johnny foreigner, one was allowed to be a bit odd.

So the fore and aft positioning using this method might not be so good, and although allowance was made for the thermal to "drift" back, it obviously did not seem enough for someone flying with a vario.
So for me there was a question over whether the glider was coming out of the front of the thermal and ending up circling in relief winds or whether the times it did climb straight up, convergence was involved as there were air flows from several valleys and a sea breeze meeting in that area.
In any event, when the glider had climbed as far as it seemed able to go, it stayed up as long as the circling was maintained. Stopping circling and tracking back and forth above the line of the hill, as everyone else was doing, caused the glider to drop below the higher performance gliders.
By Roadrunner
#359565
Hello Guys:

Reading this Dynamic soaring story got me to thinking about a soaring story of my own.

What do you guys think?

You know, I had a fun-one at my local Mountain Flying Site, Mount Diablo. I was flying the site one day alone, this is the sad reality of mine when flying Mount Diablo. It is quite often the case when flying Diablo that I end up flying the Mountain alone. Because other Pilots usually shun the thought of flying the Mountain. I don't know why?

Well there I was, soaring the mountain. after an hour or two I landed at Mt Diablo's State Park's resident L-Z, (Juniper ridge). Well, I was standing there by my Glider, and a couple came over to me and said to me "We have been watching you soar the Mountain" "you fly just like a Bird" I replied to this by saying " I fly better than the Birds, I soar with-out flapping" It was such a fun moment. I enjoyed talking a little trash to these two spectators.

I then got hooked back into my Glider, re-launched and flew some more.

I so want to fly again. Time will tell.

Good By The BIG Guy
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By AndRand
#359567
Beeza wrote:This is supposed to be the "public" link to those notes.

https://onedrive.live.com/redir?resid=A ... 4B0F%21106

Doesn't seem to work any better.

Might not be hitting it with a big enough hammer or something.

Usually works.

Just noticed the Add Attachment button.

See if that does anything.
It works for me so I downloaded the document :thumbsup:
While dynamic soaring thru wind gradient seems quite plausible for me (although I wonder about needed gradient to get advantage of it instead of staying aloft in ridge updraft - and I would try not stationary but kind of spiral along the ridge), I doubt about that offset thermal circling. It looks fine on paper when you know where the core is.
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By aeroexperiments
#359574
Beeza wrote:
Having the control bar forward under the chin instead of at chest height is a disadvantage in my view. it makes it very difficult to chuck the thing up on end when you need to. Even with throwing your feet into the turn first. The tendency is for the body to pivot on the hang point instead of moving across sideways.
Well... I find it much more ergonomic to make roll control inputs when the bar is by my chin rather than back by chest or belly. My arm muscles can't work as well when my hands are further aft and my elbows are more tightly bent. I find it more ergonomic to make roll inputs when flying aerotow with a double V-bridle (i.e. connected to each shoulder and also to keel) than when flying Pro-tow (connected to shoulders only.) The bar is further aft in the latter case. I find it distinctly un-ergonomic to make roll inputs when flying with a Mosquito power harness under power (have only done once), when the bar is much further aft than usual.

Yes, there may be more of a cross-controlling tendency when the bar is well forward-- the pilot's body has more of a tendency to rotate-- but that is easily overcome with no real net penalty.

PS I didn't read all the previous posts in full detail-- not sure what the "tail down" glider is...
By bobknop
#359589
quote-The idea was that (without vario) by flying sufficiently off center* it was possible to introduce enough feel for the pilot to almost automatically work out where he was, and quickly correct his position before flying out. (*And orientated the same way to the "wind" as for the Renner Reichmann example)



As a test I tried this with my vario off,on a hot and nasty day ,bad lapse rate that day.

I trimmed my glider as slow as i dared and just let myself drift around,feeling where to turn.

Some how it worked for that day,after landing at the gliderport someone asked me what my sinkrate was,because to them it looked as if i floated straight up.

Nice thread....This is raw footage of me flying like that,boring to watch.



Regards Bob.
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By AndRand
#359597
aeroexperiments wrote: PS I didn't read all the previous posts in full detail-- not sure what the "tail down" glider is...
It could be translation issue - probably "tail heavy"
By Beeza
#375737
Couldn't find the scribbles and sketches that went with the previous posts. These were A5 copies made a few years ago. They are not sorted out. Some are tracings and half finished notes from books, a good few maybe not so relevant. Some (most) are only half finished, on realisation each time that the approach, applied more to steady state aerodynamics. They might be a bit hard to read, but if nothing else it gives an idea of the time spent on this.
Where notes or drawings are copied (traced), there is usually something on the page to say who the author, artist or source was. (D Pagen, ENAC, AAIA Journal, Journal of Fluid Mechanics JFM, Kasper, Mises, Visiteurs...etc). Apologies if any are not marked.
There are a few more tidied up drawings somewhere , and a list of articles from various journals, sourced from one of the British Museum Libraries, before they moved shop. I'll post them if I find them.

In the "Visiteurs" book, there was a brief description and diagram of albatross flight. That was also compared to flying off-center....through lack of other information, or many other ideas.
There were similarities with albatross flight,(in my view) but a couple of things were not quite right, even with a bit of bending. Soaring into the wind shear at the top of the wave didn't seem to quite correspond to the idea of prevailing wind penetrating into the thermal.
Also, the albatross flying in ground effect before climbing above the wave crest, might only be comparable to some kind of temporary re-arrangement of vortices - leading edge, wing tip, and starting vortex - bolted together to form a ring of sorts, to produce another way to "shove more air down".
The idea of flying up or down wind, or up or down gradient was kept throughout for comparing the albatross flight, the glider thermalling and slope soaring, to make things easier.
To account for the supposed increase in efficiency that must occur if flying off center really is viable, the following possibilities seemed reasonably reasonable, and worth looking at.
In order of likelihood:

1 - Flying off center allows a smaller thermal to be worked. Perfectly centered a given glider would have a steeper bank from a tighter circuit, than the average angle of bank from the slightly larger circuit that is flown, flying off center.

2 - Generally when a wing suddenly and sharply pitches up, the lift greatly increases (momentarily) before stalling. In a favourable gradient the continual slight increase in air flow, delays the onset of stall, even as the glider slows in the climb. Gliders don't seem to stall very often as they pitch up entering a thermal. And student gliders are usually hard to stall, anyway.
The effect might be increased with a glider trimmed to fly tail down in steady flight, when the glider is pitched up (As the load is suddenly and sharply increased, such as when a sharp drop is halted, a gradient impacted, or in a sudden steep turn with speed in hand. Never by pushing out on the bar - the bar is always pulled in to try to counter/dampen the pitch up).

4 - Something along the lines of a Kasper style vortex could form over the glider, to deflect more air .....shove more of it down. Kasper's leading edge vortex, he associated with a kind of "parachuting" drop. In this case it could be linked to the sharp pitch up, on entering the thermal with a decent amount of speed in hand, to scrub off in the climb. A starting vortex could form when a heavily loaded wing suddenly starts producing lift after being very lightly loaded (in a drop for instance, or in some gradients). Bolt it together with wing tip vortices and you could have some kind of ring.

5 - Comparing the way the glider flexes when it is circling off center in the gradient, to the way a bird's wing distorts when it flaps down, and when it climbs, and whether combined with this the pilot can usefully add his own (admittedly small), efforts into the system, to climb faster or stay up longer in weak lift.
Circling off-center the gradient could be viewed as flapping the glider. If so, the pilot might be able to take advantage of that effect
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By Beeza
#375738
One of Derek Piggott's books talks about the benefits of pulling quickly back on the stick in brief gusts. Thermalling off center could be a variation of that practice.


One of Denis Pagen's books had a break up of how a hang glider turns. (French version had an Omega on the cover) Pushing the bar out in the turn, occurs almost without effort when the glider is normally tail down. The steeper and more abrupt the turn, the more "assisted" it seems.
The same book shows a glider circuiting at an angle in a thermal, that is influenced by local relief winds. The glider climbs upwind and drops downwind. What is described here is additional to that effect. As I understood it the pilot, in his example, is still trying to perform a well coordinated, evenly banked turn.
Flying as described here the banking and speed vary continually and repeatedly as the glider goes round. A sharp load increase causes the glider to pitch "up" or turn, not the pilot pushing the bar out.
The sudden load increase makes the hang glider pitch up. The hang glider pilot is perhaps dampening the pitch up by pulling the bar back , and maybe prolonging the climb.
If the thermal has gone walkabout the glider does not pitch up, when the pilot turns back out from the hill. Unless there is another way to suddenly increase the load, and produce the same effect. So if the glider does not pitch up when expected, it's probably best to ease the bar out, and find somewhere else to play before too much height is lost.
A reliable up-draught over an outcrop when slope soaring in firm conditions is a good place to get the feel of it. Approaching the outcrop pick up a little speed along the slope, sharply turn out when behind the outcrop, aiming to time the pitch up with the impact of the gust with the glider "flat" (in roll), as you fly out over the outcrop. Then yaw round almost on the spot, at the top of the climb as the glider slows, to end up facing back along the slope, flying at normal speed.
The roll flat, pitch up and climb has the same feel flying "off center" in a thermal (out from the hill -upwind).

For me, if nothing else, it was an agreeable way to fly. And sometimes it was surprisingly efficient.
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By Beeza
#375739
Last two posts were supposed to be in the "Dynamic Soaring and the Albatross" thread.
By Beeza
#396318
A couple of things maybe worth mentioning again.
After the first climb, the glider flies forward for a few seconds before turning back to the hill. The aim is to fly just off the centre of a small, well defined thermal so that the changing and repeated feel of the glider, gives an indication of where the pilot is positioned, without having a vario. That is, flying in the best combination of gradient and lift, and not just trying to circle in the strongest gradient.
All the various patterns drawn are the same thing, apart from the figure of eight which is a load of old rubbish as far as I'm concerned. In the sketches the radius of the circuit represents the angle of bank, not the path flown. So showing it as D shaped, egg shaped, oval, pear shaped or whatever, it is all the same thing. More elongated to start with going out from the hill, and more circular and maybe compact, as the pilot tightens up the sequence and works up a rhythm (such as with rowing for example)

Separately, all the stuff on leading edge vortices can be put to one side. It is not really relevant as it is not something the pilot has a lot of control over, whether it plays much of a part or not.
In the early 90s just about every volume of journals such as the Journal of Fluid Mechanics and the AIAA journal contained papers with titles along the lines of "Vortex formation on a pitching and translating airfoil" or "Vortex formation on a flexible airfoils at high angles of attack" and so on (from memory).
Using the references supplied at the end of the paper, it was easy to collect quite a pile for later reading. As a result, too much time was probably spent considering it.
On a rigid wing a leading edge vortex needs a lot of energy to drive it. Witold Kasper described his vortex as appearing in a kind of parachute mode, and partly climbed out on the wing to try and disturb it.
With the style of hang glider flying described here, even if it was sometimes present it would be unlikely to be present on every circuit. There would likely be a marked difference in feel, between the times it was present and when it was not, which was not the case. Once moving the feel was pretty much the same each time round, with only small adjustments made if the glider's position needed adjusting a bit in the "thermal". So my view is that it is not involved to any extent that needs worrying about. The number of sketches just reflect the number of vortex papers brought back.

Other papers collected concerned flapping flight, with birds, ornithopters, and rigid and flexible wings.
It seemed a better idea to compare the glider being loaded and unloaded by the gradient with a flapping wing, and looking at whether it was possible for the pilot to enhance the changes and any gains during the cycle.

Flapping wing propulsion was taken as coming from repeatedly distorting the flexible wing so that part of the lift is directed forwards, or part of the air normally shoved downwards when gliding, is shoved backwards when flapping.
Suddenly loading up the wing of glider trimmed tail down automatically causes it to pitch up, increasing lift briefly. It also increases twist so that part of the lift is directed forwards. The pilot does not have to push out, the bar is always pulled in to retain control.
The harder the glider impacts the gradient the more it pitches up and distorts/twists, so while the style of flying can be toned down it can't be smoothed out completely. In favourable circumstances it's a case of flying one way or the other.
A single surface glider is probably better for this, being more sensitive in pitch, as well as being light, easy to throw around and quick in response both to the pilot and changes in the air around it. As well as being more forgiving to "sloppy" flying.
A keel pocket is probably an advantage over a bridle in the aft lower rigging wires, as it allows the sail to more easily move from side to side. Useful when the glider is curving into the gradient at an angle, so the twist is spread over the whole sail and not equal on both sides of the sail.
No allowance was made for the "prevailing wind" veering with height as the glider was following a curved path as it climbed into the gradient so it sorted itself out. However high the glider is expected to climb, going out from the line of the hill, and drop going back.
Going out from the hill with the airflow increasing, the glider pitches up by itself under heavy load and is harder to turn. Going back in, with the airflow from the gradient decreasing, and the angle of attack decreasing with it, the glider rolls up very easily with little effort.
Being slightly flabby, probably with more dihedral it seems to find its own angle of bank, helped perhaps by the pilot heading directly for the hill and also wanting to turn quickly before he is spat out the back of the thermal.
The heavier the bounce on the steep turn the less requirement there is for the pilot to push out the bar, but it probably moves itself out depending on how rubbery the pilots arms are. In some respects it can be an advantage to be physically tired before flying, that way there is less likely to be any wasted effort, less likely to be anything overdone or any "flourishes" in the flying. Whatever it looks like. There can be a lot of effort momentarily but it is kept to the absolute minimum. The glider is otherwise left to sort itself out. Another advantage of a single surface glider.
(In one of the H Aupetit books there was a sketch of Henri Mignet zig-zagging up a thermal, propellor windmilling, with his aircraft being hit by an imaginary tennis racquet at each turn back. Seemed odd when he could have just circled up)
When the pilot has decided where the thermal is and the pattern is tightened up ("closed up"), it is probably closest to being similar to a wing being flapped. With no wasted time, no wasted movement, no wasted effort. Everything follows in quick succession. There's a rhythm to it, like rowing. Any change in feel can suggest a change in position within the thermal and can be quickly corrected by moving closer or further away from the hill, extending or shortening the climb, or moving a bit left or right as the glider climbs back out.
The figure of eight pattern breaks the rhythm and allows the downwind leg to be overly extended. It introduces a hesitation. It would be like a wing pausing in mid flap, or a rower pausing in mid pull or as he raises the oar. It mucks it up.
The "flap" can be anywhere the wing is distorted by increased load.
Or it can be continuing. From the sharply banked turn away from the hill, through the oblique impact on the gradient, through the climb where the inner wing can be shoved up by the air as the glider slows to force the sharp bank and turn back towards the hill side.
It's perhaps a variation on the situation described by Derek Piggott in one of his books where he describes pulling the stick back momentarily in an updraft to gain a height advantage, except in this case with a tail down glider the pitch up is automatic. The pilot responding by pulling in may be slightly delayed depending on how tired his arms are, but the main impact before the climb is usually anticipated with the bar already pulled in.
The gliders we flew (apart from the Altas and the Lancer IV on the nursery slope) had floating cross booms but they were not adjustable in flight. To me it seems that tightening the boom strop in flight moves the sail forward over the pilot increasing any tail down aspect, when really the opposite is required.
Namely a tail down, slightly flabby wing that twists easily for climbing, and a tauter sail with less twist, trimmed more normally, for gliding normally.

The ideal thermal was small and well defined, and in moderate conditions so the prevailing wind did not disperse the thermal, and lower down before it widened. So the sort of thermal that forms when the sun is hot but the surrounding air is still cool , midday/early afternoon, early/late in the summer.
But not too turbulent. It can be a bit grim when the glider is tail down as it is more easily knocked about and harder to keep on track at normal gliding (slope soaring) speeds.
The bigger the difference in temperature between the inside and outside of the thermal the more reasonable it seems to look at it as effectively being like a coherent pipe with the air flowing up it at an angle, rather than as largish uniform blocks of air that are rising with the glider inside, where the glider cannot really be thought of as turning into wind. The weight of the cold air around holds the whole thing together so that it is a small as possible in diameter.
Within the ("pipe") thermal the lighter air will rise to the "front" so looking at a cross section from above, the lift contours will be closer on the (prevailing) windward side. A cross section from the side will show the flow in an airfoil shape, using a given altitude as a starting point. Which is the way they are drawn in the notes. As the day progresses they will spread and be less energetic as the temperatures between inside and outside the thermal become more equal.
That's how I looked at it anyway. It seemed to work very well for me.
Sometimes, not always.
User avatar
By waveview
#396319
Beeza wrote:Last two posts were supposed to be in the "Dynamic Soaring and the Albatross" thread.
Beeza
I observed an Albatross soaring on the ocean out front of my local coastal site recently. These are very large birds and even at a distance of half a mile off the coast it was easy to see the technique/glide path this Albatross was employing, swooping down very close to the face of the wave (wing tip almost touching it) then climbing back up high above the wave in a gradual turn to repeat the cycle over and over without flapping. It appears that few, if any, of the discussions on dynamic soaring by the Albatross consider the speed of the waves and swell. This fast moving hill must be producing some ridge lift energy potential as the air is moved up and over the waves. It is possible that the Albatross is efficiently collecting ridge lift from the ocean wave face?

"Surfline's very own Sean Collins responds:

The speed of travel of the deep water swell group will be 1.5 times the swell period; ie: a 20 second swell will be traveling at 30 Nautical mph. The actual individual waves will be traveling at three times the swell period, so a 20 second swell will have waves moving at up to 60 Nautical mph.

The individual waves will travel to the front of the set as the swell group moves forward; once there, it'll slow down and drift to the back of the pack before moving forward again. Think of it as a conveyor belt turning at 30mph but the whole conveyor belt chassis is actually on the back of a flatbed truck going down the highway at 30mph. So if you had an object attached to the conveyor belt -- while it's turning and being carried by the truck -- the actual speed of the object would be 60mph. (Sound like math yet?)"
http://www.surfline.com/community/whokn ... fm?id=1111
Last edited by waveview on Thu Mar 02, 2017 5:55 am, edited 1 time in total.
User avatar
By mlbco
#396330
Albatross flying on a nearly windless day. My camera was in the wrong mode so the video is overexposed but it's still interesting. I filmed this in January 2017 in Kaikoura New Zealand.

Steve
[youtube]
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User avatar
By magentabluesky
#396336
This is the best demonstration of an albatross Dynamic Soaring that I have come across . . . by Macgellan .

You Tube Albatross
User avatar
By DMarley
#396379
I wish Gary Osoba would put what he's learned and experienced about DS and general soaring into a book. That would be a highly interesting and informative read. Perhaps it would take multiple volumes.
User avatar
By waveview
#396401
The only hang gliding example of dynamic soaring (it ended badly) that I ever witnessed was at a high coastal site called Sand Patch. The female pilot was wire assisted off this 400 ft and steep ridge in strong conditions, even with full forward speed control input on the glider she was driven up and back from launch into the notorious lee side rotor at this site. The pilot now found herself going from a very strong headwind lift to a slight tail wind and sink (from the strong rotor behind launch) while she was still holding full forward control input. The wing accelerated down and forward very rapidly. Just as Jenny pushed out vigorously to reduce the rapid decent and high forward speed of the wing the glider re-entered the very strong headwind and massive lift band just in front of launch. The glider this time climbed very very rapidly but by the time the pilot once again applied full forward speed to arrest this very rapid climb and get the nose down the glider had started the initial stage of a loop and stalled as it went over the top just inverted. The glider then did a partial "Immelmann" type turn and knife edged with one wing tip pointed directly down into the ground from a height of about 70 feet. Fortunately the leading edge and crossbar took the full impact in decelerating the speed somewhat with the pilot luck enough to escape with a nasty gash to the ankle. Without a doubt this was the most spectacular hang gliding crash I have very witnessed over the years.

Dynamic soaring opportunities for hang gliders do exist but I would estimate the risks to be rather high.
Last edited by waveview on Sat Mar 04, 2017 9:08 pm, edited 1 time in total.

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