- Thu Mar 30, 2017 5:25 am
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.