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.
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: