Originally Posted by
ShyTorque
As far as risk goes, I doubt the wearing of a hi-viz vest trumps not wearing a seat belt and coming close to having the top of your head knocked by the blade roots.
Yep, in that video all sorts of things are scary in the way that guy flies and picks up the other guy, but rotor stall is definitely not one of them. The AoA is not critical at all. BTW, in a flare we normally push the collective down, because otherwise you balloon up. We use a flare to stop the ship, not to gain altitude. And one more thing, a flare tends to bring your NR up(!) not down, even if you change nothing on your blade pitch. Chr, if you want to stall the whole disk, in any helicopter, it is associated with loss of NR. Since in a flare you actually can gain NR, there is no risk of stalling all the blades. Again, in normal operation with NR in the green, there is no way known to man, that any helicopter certified today would stall all the blades simultaneously and crash. You overestimate the AoA on the blades in these videos. The AoA on the rotor disk has nothing to do with the AoA on the blades. I can autorotate with an AoA of -90° on the disk (straight down) and the pitch angle of the blades would still be positive. You can go straight down with full power (called VRS, oh good heavens I don't know why I am opening that box of the pandora again) and still not have the whole disk stalled and the NR will still not drop. It is a whole mess aerodynamically with lots of vortices coming and going, but what you are thinking is happening, just does not happen. It is not a plane. There is no such thing as an accelerated stall like in a plane. Blade stall you can have with normal NR when you go too fast or too high and too fast for that altitude. It is called retreating blade stall. But in that case, as the name says, just the blade on the retreating side begins to stall. Which gets your attention because of the very pronounced vibration. Slow down, and everything is good again. Though, whenever you see helicopters do crazy stuff and even mess things up, it is not because the rotor stalled. With the one exception of not controlling the NR correctly. When that happens you see a very pronounced coning angle and then the folding of blades begins. During a Robinson Safety course, we hovered the ship at 70% ( in ground effect). Shakes like hell and is not recommended, no, rather completely forbidden, to do by yourself (don't do this at home, kids) but it still flys and you can bring up the NR again.
There is nothing crazier than seeing Rainer Wilke with the Red Bull BO105 flying aerobatics. Even if he flies upside down, at no moment during his performance is the rotor stalled. Never. It may not produce lift, but there is no complete stall of the disk. And compared to the Red Bull BO105, what anybody else does in these videos is just lame. It might be not safe like that bloke in the video and the way he flies might end in tears one day, but not because the rotor stalled. It is good that you ask these questions and I invite you to ask more questions, but I think with what everybody said until now, that question about the rotor disk stall has been answered.