Swing RAST

Talk by Michael Nesler, May 7th, 2017:

(German presentation on Youtube) www.youtube.com/watch?v=m9MjH2LkY6I

 

A year with RAST – Experience and findings

Last year I presented the RAST system, which was a new concept. I’m not sure if anyone present today was here for that. We have been inventing and testing a system that makes paragliding safer, and offers some other refinements as well.

Now that we have been experimenting with it properly for a year, and have even manufactured some series gliders featuring it, I have been asked to talk about what we have actually achieved during this time, how the year went, what we discovered in the way of advantages and disadvantages, in other words, simply to give a brief overview.

Just by way of introduction, for anyone who doesn’t know or hasn’t heard about the system, we came up with the idea of dividing the glider into two sections, in a lengthwise direction. We have a front section – and here you can pretty much see the partition – in yellow – which seals off the front and the rear sections …

… in other words, to start with, when you inflate the glider, the front section fills first and then, after a brief delay, the air goes into the rear section and, if there is any kind of malfunction, the rear section initially stays inflated.

Now, something that has often been asked in recent years is if there wasn’t already a rigid system in the past? People are always saying it. And I did have some involvement in that. In earlier times we made gliders with pressurised air bars, inflated, which then generally burst above 3,000 m … and other things.
Then we tried all sorts of things with poles in the trailing edge, i.e. tent poles, made of aluminium, carbon rods, then there was fibreglass such as LTK with the Panther. And we kept on trying ways to make gliders rigid.

And it took a while before we realised that making the glider rigid actually achieves exactly the opposite to what we want. Sure, it no longer collapses, but instead it flies towards you or under you. The paraglider is actually an ingenious device, so it dissipates the energy, by becoming deformed instead of flying somewhere where it shouldn’t go.

So, as with a hang-glider, it can of course happen if it doesn’t yield, that the energy has to go somewhere, then it can tuck or tumble – that means it swings around. And we have to cope with that.

With the paraglider, it is true that everyone would be happy with the idea that a paraglider can no longer collapse. But then we would pay for that by the glider diverting the energy differently, instead of collapsing, as I said before. Then it would either fly towards me, somersault, fly under me, then at some point I’m caught up. And that’s not what you want.

That’s why some time ago we completely gave up again on making them rigid. But now we have the situation that the biggest problem with the paraglider, if it collapses, is that the canopy completely empties. We know the collapses are harmless, if the trailing edge is not affected by the collapse or front stall, i.e. if the trailing edge at least to the middle is still intact.

Then the idea was that we would have to find or invent a system where the trailing edge always remains intact, that in practice the air there cannot get out and at the front where the issue is, i.e. where the lift is created and the momentum is created, that the section can still collapse, empty and thereby discharge the energy if the glider gets out of control.

And this has been working very well. You see at the top a conventional division without a partition, without RAST, and below a dividing partition is incorporated, which may or may not have valves depending on what is needed, and which allows the air into the rear section after some delay and lets it out again even more slowly.

That means that if something dynamic happens, such as a collapse etc. then it takes a while before the rear empties, the trailing edge remains intact, there is less rotation and most importantly less forward momentum.

What we expected from RAST:

• improved inflation behaviour on launch
  This has proved relatively good in practice. I’ll come back to this point…
• improving the problem of the pilot being launched unintentionally in strong winds
• the glider “aligning itself to the slope” in a tail wind
• resistance to large-scale collapses
• preventing the dreaded rosette on a front stall

We have now drawn up a table, because we have made a large number of prototypes and carried out many test flights in various classes. We have tested out virtually everything from EN-A to EN-D, with and without the partition.

Which really works and can be mastered by every pilot?

1. doesn’t work
2.  works only under certain conditions (e.g. if the pilot can cope with it)
3. works to some degree
4. works satisfactorily
5. works 100% reliably

We have already made some series gliders, and we have of course analysed market feedback. It is obviously difficult being the only company on the market for the moment to have a completely new system. One encounters a good deal of resistance from the other manufacturers and distributors, that’s normal, we were expecting it too. And that’s why we were actually expecting market feedback to be mixed…surprisingly, however, it was extremely positive, even from companies who are in direct competition with us.

They said to work on it a little more, and in a year we’ll talk about whether we maybe continue it together. Meantime we’re at least to the point where there are at least two major manufacturers wanting to work with us on it. Let’s see what the year brings in that respect, that would of course be interesting, because this shouldn’t be something which just one company has, rather it’s really about we as pilots also getting the benefit. It doesn’t matter what brand is on it now. …

We have of course made the beginner wing with basic RAST ….

(MITO) … which is now trend-setting when it comes to launch behaviour, very comfortable in extreme flying manoeuvres and there were absolutely no complaints with regard to stiffness or that it somehow attracted negative attention. On the contrary, the people who are using it for training are very satisfied.

Market feedback so far:

Series gliders:

• MITO with RAST 1.0 → launch, extreme flight behaviour, no complaints
• TWIN RS Tandem → launch (pilots need to adjust somewhat, thermals and control very good)
• APUS RS Miniwing with RAST 2.0→ 14, 16, 18 certified with extremely long speed travel, excellent feedback (Kilimanjaro, Freestyle)
• TRINITY RS 23 → first acro-glider with certification
• ARCUS RS The first time that a low-end EN-B glider on the market has RAST.
• MIRAGE RS The first Speedwing on the market has RAST.

It was a little different with the tandem. The tandem is an experiment. With the tandem, we have incorporated RAST in such a way that it also has a bearing on its characteristics in thermal flying, not only on launch and safety. Another point is that tandem pilots of course like to launch by running like mad and trying to get the wing above them as quickly as possible. Then it usually happens that the glider overshoots and is braked hard ,and you do of course see a good deal of video footage of tandem launches where the brakes are nearly at 100%, because the glider is simply too far forwards. RAST doesn’t like that. In other words, if I overbrake, then the glider is not able to inflate at the rear and the take-off run becomes a bit longer. Then you simply have to tell people “take your time when launching, no stress –it’s inflated at the front so it will hold”.… except you are braking at the back. And interestingly, you believe it if you’ve tried it a couple of times. As a tandem pilot you are less likely to believe it. And then the take-off run suddenly becomes shorter again.

(Audience question: Winch) … yes a lot. Flatland Paragliding is as it were part of our extended test team, which tests it all on winch. And it is also a matter here of simply inflating comfortably and then off you go. And on winch it doesn’t matter whether or not the rear section is inflated, because it is actually already holding at the front. Because you are of course under tension.

….then we have the miniwings with RAST 2.0. 2.0 just means that there are additional valves in the partition which, as soon as there is any kind of disturbance, seal off the rear section further, so it is even harder for the air to get out …
surprisingly with that we have certified 14, 16 and 18 m² up to 100 – 110 kg, partly in the B- and partly in the C-category, and even though they are 14 or 16 m², which were really overloaded. Particularly with the small gliders, this is what was most surprising (you’ll see a few photos later and I can also show a short video): 1) it is extremely difficult to get major collapses, you have to use both hands and full throttle because RAST of course is working against it…and 2) if the wing disappears, reinflation to just a few metres is less than one second.
You also have to get a bit accustomed to the fact that a glider, if it completely disappears, flies straight quite normally again after a second, but it works. But that it also just a bit of adjustment.

And what the RAST system has also allowed or enabled us to do … we have for the very first time in history, I believe – if I’m not mistaken – a certified pure acro-glider.
This has the RAST system and it was of interest not only for safety aspects, so that collapses and front stalls etc. fall within the certification limits, but with the TRINITY RS with RAST, it is possible to fly manoeuvres which are not even possible with a normal glider. That means I can, for example, if the glider is still under me, start a heli from a dynamic full stall and go into the heli without any transition. Or I can simply send it out of a heli and immediately go into a SAT, without the glider briefly having to gain momentum. Because the rear section does not empty and momentum is not lost.

Then we still have the ARCUS RS in the pipeline. This is the first time that a low-end B glider with RAST will be available. Just certified, at least in size M, and we’ll see what happens from there.

What we weren’t expecting:

People who don’t have RAST do whatever they can to run it down:

Before anyone was even able to test RAST!

The opposition from the industry is unexpected: instead of other manufacturers being interested in it, to begin with there was only criticism (with two exceptions).

This shows that, for the market, it is not really about safety and improvements for pilots but, as always, only about pure profit.
Reinforcement has never worked … this is not wrong as an argument, but in this case it can’t be strictly applied because of course the front section can collapse and only the rear section is somewhat more rigid.

doesn’t work … all I can say to that is try it for yourself! I can’t convince you or prove it. You simply have to try it for yourself.

not much changes in the certification … this is also a small problem with which we have to deal … it is hard with RAST to cause collapses which come within the measurement field, i.e. I have to pull down harshly, I have to use tricks or ploys, I have to pull down the A- and B-risers on one side simultaneously just to cause the collapse, and either it is too small or it is beyond the line and too big. However the situation is such that I can only get a certification if I come within the collapse field. And then it gets difficult for the test pilots – is it better to take the one that’s too small or the one that’s too big? And we are then in some instances 2 categories apart. And this means it is also to some extent the case that the time we are now investing is not about whether RAST is working better or worse, but rather we have to find a way that I still somehow come within the ramge for certification and RAST still works well. And that involves firstly the positioning of RAST, how far to the rear or the front to position it and secondly how sealed the valves are. If I make the valves completely sealed and I position the RAST between B and C, then there are hardly any more collapses, but nor can I get any gliders certified. And now I gradually move towards the C level and try to find a compromise somewhere. This has worked relatively well for us with the new ARCUS RS, as we have upper limit all A and lower limit we have 1 or 2B – I believe – and this is also typical of RAST … the more weight is on it, the more stable it becomes.

Now let’s look at point 1 – Inflation behaviour on launch…

Better inflation behaviour on launch – 1.

Typical for RAST: the whole canopy is inflated at the front and the rear section is still empty, and that works very reliably. The interesting thing is that if you inflate a glider and it is inflated fully at the front but empty at the rear, as in the photo, then this has an interesting effect: it inflates, is stable, stays above the pilot’s head and it does not overshoot, because in that moment the profile is auto-stable, i.e. it is still not inflated at the back, this is a type of reflex profile …

This has several advantages:

1. it does not launch the pilot unintentionally, regardless of how strong the wind is

2. it doesn’t overshoot

3. regardless of how powerfully the student or the pilot runs, you will no longer have the effect that the centre collapses because the rear section fills first and the air does not go in quickly enough at the front. Here it is completely the opposite. I can also run full speed, it promptly inflates fully at the front, nevertheless rises above the pilot and it normally doesn’t overshoot. This has worked well so far.

The same thing in strong winds …

You see above that the profile is still empty at the back in the inflation phase – this is a type of “self-stabilising profile”, and in the middle it is then fully inflated, and then it really starts to carry properly.
We’re talking here about ½ – 1 second which you save on launch, but that is quite a considerable time on launching, before the profile carries. This is usually the exact length of time the pilot needs to turn around comfortably.

And this also works really well! We were in South America for 5 weeks with extremely windy launch sites, and in particular, in some instances we were doing cliff launches. And it was very pleasant not to be immediately launched off whenever the wing was inflated.

You can see this again here: the front section is fully inflated and the rear section is still slightly empty. The pilot is running directly towards the glider…it is also important if it is very windy on launch to always go towards the glider, relieve the pressure, so it rises comfortably.

Inflate the glider firmly and run up gently, there’s no more stress on launch. If it’s fully inflated at the front, then it will carry.

The next point …

What happens now with a tailwind, if it needs more until it inflates….

We somehow came across this by chance. We were at Stubai during the winter, where there is of course always a tailwind with snow…and we thought, hmm, this is going to be exciting now, launching here with a tailwind. And we launched at the same time with 3 tandems, with RAST, and interestingly enough we were airborne after three steps. And anyhow we thought: this isn’t what we’re used to…we filmed the whole thing from the side and worked out that in a tailwind the normal gliders try to line themselves up with the gradient of the slope …

In a tailwind, “aligning itself with the direction of the slope” – 2

In a tailwind, “aligning itself with the direction of the slope” – 2

…just like the orange glider here without RAST … it is just always a bit further ahead, the wind is coming from above downwards, following the slope, and thus the canopy adjusts itself to its normal glide angle as well. That’s why the canopy (without RAST) overtakes us if we’re inflating on a slope regardless of how fast we run, it is always further ahead, I have to brake and I have to run after it like a madman. This is completely normal, we’re all used to it.

With RAST …. Interestingly, if the RAST remains more or less sealed, I inflate the glider but it doesn’t overtake me. It stays in the angle above my head, the profile is auto-stable, it can’t easily follow the slope…. This means, firstly, I don’t have to run off, I can instead take my time and, secondly, I am actually running down the slope with, comparatively speaking, a very high angle of attack, i.e. instead of the 5-8 degrees to which the glider normally orients itself, we have 10-20 degrees, without having really braked already, and it becomes airborne much much sooner. I can’t tell you everything here of course, the only way to really believe it is to have a look some time or try it for yourself…for me this is the most astonishing effect of RAST.

The next point… once again relating to safety…

RAST is not a miracle cure for collapses. Nor should it be, because the glider should of course dissipate energy, it should collapse, it should go into a front stall, if you fly anywhere or carry out manoeuvres where there is simply too much energy. The glider has to dampen it. And naturally there are limits for every safety system or for every stable system. If you exceed the limits, then it’s no longer as it should be.

And we have two different possibilities:

If I fly actively, then every time I apply the brakes, the internal pressure in the rear section increases, i.e. by active braking I make the rear section much much more stable than it would normally be. And at that moment when it shakes, when it somehow gets really turbulent, and I go actively on the brakes, I increase the pressure still further and, even if it has a frontal collapse, it almost can’t collapse through to the back so that the trailing edge collapses with it. We tried that often, and it always worked well.

It is different with passive flying. If I do nothing with the brakes, then I now have at the rear a section with increased pressure, the collapses are indeed extremely delayed, they are not as big as they would normally be if one does absolutely nothing, but I cannot prevent them completely. So if I now fly into a lee area without applying the brakes, at some point it will also collapse. The collapse will perhaps be 1/3 smaller than normal, i.e. than without RAST, but it will still happen. If, on the other hand, I actively apply the brakes, then the leading edge will perhaps dip a little, but otherwise nothing much will happen.

That’s why I also have here 1 – 3 … with active flying, I can actively prevent anything happening. With passive flying, I have to rely on the fact that the glider somehow works.

Resistance to major collapses – 1 to 3:

With active flying – 1:
– the pressure in the rear section can be increased by braking
With passive flying – 3:
– the pressure is not actively increased, major collapses are extremely delayed, but not prevented.

Collapses:

Comparison of the different collapses (superimposed):

• green: normal flight
• red: during a disturbance

Here are the valves photographed from inside. We also have a video of this. Note how it is open above and below, the air goes through.

RAST shuts as soon as the pressure below is higher, then it sits against the top surface and bottom surface and effectively closes the rear section. No more air can get out.

Collapses then look like this:

Here you see the typical RAST folding angle. The trailing edge remains completely intact. This is a 14m² glider flown with 100kg all up. And it held, to the extent it ever would.

Here once again graphics to show the lines of collapse:

Red is a conventional glider without RAST – the usual collapse, where the trailing edge collapses back at least to the centre and accordingly the remaining surface area is clearly different. First the red…

Green is a glider with RAST. The green one is collapsed just as far at the front, but at the back RAST keeps the surface open to some extent. And this gives me ⅓ to ½ more surface area than I would have with a collapse on a conventional glider.

(Audience question: Isn’t it necessary for certification that the trailing edge collapses as well?)
Michael: Exactly, that is the biggest problem we have with type-test certification, that the test pilots have to force a collapse in such a way that it goes beyond the trailing edge.
(But the gliders can get certification?) Yes, you just have to use some tricks or ploys. You use both hands, you pull down A and B, use full throttle etc. ….but it is a bit difficult at the moment. For the lower classes, it still goes through the wide cells, if you take time, it empties. So it is easier if you pull down slowly. But at the moment it is difficult with low wing depth, because either it becomes an asymmetric front stall, because you’re turning everything around, or it is much too small because the trailing edge doesn’t go too.

Again, typical, photographed from behind: collapse, RAST stays inflated at the rear.

This is a front stall: This brings us to the next topic – preventing a rosette on a front stall

The worst thing that can happen is a front rosette on a front stall. That means that both wing-tips come forwards, usually they then meet at the front and can then tangle…and then you have a problem!

The front stall should actually happen in such a way that the ears go to the back, that’s the safest configuration. Because then the centre opens first, the ears then follow and inflate again and there is actually no momentum inside. This works best of all if the trailing edge again stays intact.

Here you see RAST from the pilot’s perspective … as he pulled down. Then you see the edge again. At the front it empties nicely, as it should, and the trailing edge simply remains intact.

Front stall … exactly the same … typical folding angle.

At the front everything has gone, dampens the energy, and the trailing edge remains intact.
Same thing as it opens. Now it starts to inflate in the front, at the rear it has of course emptied a little after a while. And the great thing is that it inflates much more quickly at the front, because there’s less volume to fill, than if it were the whole canopy.

This is now a front stall full throttle
… and completely inverted. Here you see how the glider completely somersaults, but the trailing edge still stays intact, for a short time. And the time between the photos is 0 – 1.17 seconds. Here the glider is already partly inflated, the trailing edge is again completely inflated and then it flies again. That means there is less than one second from total shutdown with somersault until the centre at the back is full again.

Once again a front stall, where the trailing edge remains intact: