In this video you will see the inside of Dustin's sail and the sprogs as they react to Dustin speeding up. When the VG is loose, you only see the inboard sprog. The cable is loose, which tells you that the sprog isn't doing anything other than sitting there.
Then Dustin pulls the VG on full and you can see both sprogs, although the cable on the outboard sprog is hard to see. You do have to work at seeing it.
You'll notice that the cable on the inboard sprog is loose, although not as loose as it was under no VG. You'll also notice that it gets a little looser as Dustin speeds up. The cable on the outboard sprog is the same, but this is hard to see.
It is easier to see the movement of the loose outboard sprog cable near the end of the video where Dustin goes from 40 to 115 kph.
The point of all this?
1) Pilot education. This is what your sprogs are doing when you are flying fly. There is no need to wind down your sprogs. They are not interfering with your flying fast. Also your sprogs are not interfering with thermaling if you have some VG on when thermaling
2) Sprog Committee education. We don't want them to think:
"...but they [sprogs] only come into play when we fly at very low angles of attack (usually with half or more VG pulled and the bar at least to our waist)."
3) We are interested in being technically correct with any recommendations or rules that might be made by governing bodies. Since all the designers we have spoken with have said the idea of doing sprog measurements is without merit, we wanted to be sure that this idea was taken seriously.
Probably someone with greater understanding of aerodynamic than me should explain this, but anyway, this is how the "generally accepted" theory for what you see in the film goes.
A modern hang gliding profile has a rather high negative pitching moment. You can think of this as the faster we fly the more the sail wants to "twist up" which is exactly what you see in the film. (A side note: to compensate for this negative pitching moment of the profile we have a twist in the sail: the tips of the glider flies at a lower angle of attack than the root). Probably a profile of a modern hang glider in VG tight configuration has a negative pitching moment in the range -0.05 to -0.10
If I got the numbers right (and it was some time since I did this). Very, very roughly at 20 m/s already the twisting load from the negative pitching moment corresponds to 75% of the load that the pilot puts on the glider. In other words, already at 20 m/s it is like if we are flying with a pilot weighting in at 175 Kg instead of 100 Kg. This twisting force from the negative pitching moment adds load on the frame, causing the LE to bend backward, and then the trailing edge comes up. Note: there is also profile drag acting to "pull" the LE back more directly, but this force is rather low in comparison (it is more intuitive to understand though).
So what for the situation with zero G?
Personally I think that the trick is really to understand what happens in the moment when the glider unloads due to turbulence. This is a dynamic situation, and is a little bit difficult to reason about without using differentials (i.e. dCm/dL), but I give it a try (sorry if it turns out incomprehensible).
So lets think about the situation where Dustin speeds along at 100Km/h, VG tight and there being a little bit of slack in the cables of his sprogs as we see in the film. He is in a steady state situation and everything is fine. Suddenly he enters a downdraft (turbulence) and his hang loop goes slack. The glider is now very suddenly flying in more or less a zero G situation.
Two things will happen during this very short period of time:
a) The sail will unload because it no longer needs to support Dustin's weight. This will cause the trailing edge to come down on the sprogs. The glider now has less twist in it and it will want to continue pitching over - which is bad. The more slack Dustin had in his sprog cables to start with the more the trailing edge can come down. Personally I adjust my sprogs to have less slack in the cable full VG than Dustin has - ideally the cables would just come tight at 55 Km/h.
B) because Dusting is hanging in front of the aerodynamic centre of gravity of the glider, as Dustin's hang loop goes slack he actually stops "pulling down the nose" of the glider. This is a good thing because it causes the glider to pitch up again.
So, we have two forces battling each other:
1) If (a) and (b) are balanced not much will happen - the side wires will go slack but the glider will continue. There is no sudden force on the basetube pushing back in your hand. Such a glider is a pleasure to fly because the base bar feels solid in your hands and it is very confidence inspiring even in rough air.
2) If (a) is just a tiny bit bigger that (b) the glider is still safe, but the basetube will tend to push back as we enter turbulence (it will "hunt"). This is uncomfortable if it becomes too pronounced, but probably not too unsafe because of the inertia of the system.
3) If (a) is a bit bigger than (b) we can get a sort of a "luff dive" where we follow a free falling path until the glider decides if it should go upside down or recover. If you ever experience these kind of "free falling feeling" for more than a split second this is a very strong hint that your glider needs higher sprogs.
4) If (a) completely overcomes (b) the glider will fly down and intersect the free falling path of the pilot. A very lethal scenario.
I believe this would be a reasonable explanation for a high speed tumble situation. This is probably what DHV try to measure at the test vehicle at zero G. I have been as far as point (3) above on Brett Hazlett's old glider from Florida flying at King Mountain, Idaho. I was not enjoying it and I will never repeat it.
So far so good. The problem is that if we look at the recent tumbles of Seppi Salvenmoser, Andreas Orgler, and Richi Meier this is probably not the best explanation model. If I have understood correctly all of them were flying slow VG tight in thermals. This is not a high speed tumble situation as discussed above. It is instead a situation that is more complicated and, I believe, the relevant explanation model has more to do with pitch dampening and the aerodynamic work required for the glider to pitch over.
In other words, higher sprogs would (I believe) have had little effect on preventing these tumbles. The major determining factor (I believe) would be choosing to fly VG tight rather that VG loose. But honestly, I am really out in deep water here with my very limited understanding of the mechanisms involved. I am not even sure these situations could be easily analyzed - we will need someone with empirical data and a long experience. As it happens Mike Meier at Wills Wing has both. And he has mentioned (maybe) writing up an article on his findings. I hope he does because to me this would represent the best way forward on the sprog-setting issue.
Now, don't get me wrong: I am all for some sort of regulation of the sprogs, especially to avoid the ultra-low uninformed settings some people are flying with. But if we are concerned with avoiding tumble related accidents maybe there are more efficient ways forward:
1) Continued and improved education of pilots on when not to fly VG tight.
2) Continued and improved education of pilots on how the sprogs work (like more of the very good film we already saw from Dustin and Davis), and why there is no point in lowering them beyond a certain point.
3) Continued and improved sprog measurement during the comp. (Public results, but not enforced settings)
Gerolf has held a number of seminars on the subject, which has been good. The measurement of the sprogs was much appreciated by many pilots. An additional thing would be to put together a full instructional video of "real comp. Gliders/real life films" like the one Davis and Dustin has made so that it is not just "all talk". The educational sessions could be made even more informative by playing with models in smaller workshops. Yet an additional idea would be for CIVL to provide a little camera kit for (voluntarily) filming inside people's sail. I think there is no better cure for the "I need lower sprogs disorder" than seeing your sprog cable lying loose and completely useless inside the sail. Dustin might agree. Further ideas include building a Wiki-tumble-database where all tumble related incidents could be gathered for analysis.
To me, all of these proposals represents better ways to prevent the actual tumbles than enforcing certified sprog settings. That is not to say it should not be done, but only relying on enforcing certified sprog settings alone would seem like a poor idea indeed.
To make the educational aspect work properly, more and better educational material needs to be produced. Preferably both informative, entertaining and professionally produced. Can CIVL provide funding? Are there national funds available to apply from for producing such safety-related material (it would after all not only benefit comp pilots)?
Dustin wrote last Thursday:
It's unlikely you could accurately determine that exact sprog departure level with a plumb since all the wings are so different. I think CIVL should give each pilot a sweet SLR camera to stick in their wings so we can all find that spot on our own wings. And we can keep the camera. Probably cheaper than all the employees they will have to hire to police our happy asses.
