2 תגובות של WW :
I was not at all surprised that the glider tumbled. My first reaction on watching the video was that if I were trying to get a glider to tumble, that’s pretty much exactly how I’d try to do it. His inputs were virtually perfect, as far as what is required or likely to produce a tumble. He starts with an aggressive, rapid pitch up to a very nose high pitch attitude, then, right at the apex, he pulls in hard to produce a very rapid nose down rotation, and then at the worst (or best) possible moment he pushes out hard to ensure an aft center of gravity location and the most unstable configuration possible for a nose up recovery.
The one thing that does surprise and disturb me is that apparently a significant number of pilots are surprised by the results of this sequence of pilot inputs, and seem to have little understanding of why they should be expected to lead to the result that ensued. A hang glider is a tailless aircraft with relatively low pitch damping, and relatively high pitch authority. This is, in fact, what allows us to perform foot landings in no wind, on an aircraft with a minimum flying speed much faster than we can run. What we effectively do on a no wind landing is a whipstall to a tail slide. It works, because before the tail slide gets going, our feet hit the ground, and the event stops. If the ground were not there, the glider would continue into a very nose down attitude, and possibly all the way to an inverted attitude. Getting the pilot’s weight forward, and keeping it there, can be a way to save the situation without tumbling to a high negative angle, but it’s sort of a last resort, like deploying a parachute, and it depends on keeping the weight as far forward as possible. And, it has downside consequences in that if the pull in is too rapid and not timed correctly, it can accelerate the pitch down rotation.
There is a reason why the glider’s operating limitations state “the pitch angle (should) not exceed 30 degrees nose up or nose down from the horizon.” There is a reason why the U2 owner’s manual says, when recovering from a stalled condition, “Do not pull in rapidly and do not ‘stuff’ the bar.” There is a reason why the U2 owner’s manual states, “The U2 provides the pilot with a high degree of pitch authority, in combination with a very low twist sail. As a result, it is possible to produce a very aggravated and severe stall, the recovery from which may involve very severe pitch down rotation, the pilot going weightless, and the glider recovering via an unpredictable trajectory with a significant altitude loss. Therefore, aggravated stalls should not be induced except on landing flare.” The reason in each case is because these statements provide useful information to the pilot that is relevant to the pilot’s safety. Of course, the pilot is free to ignore this information, but should probably not expect to do so without the possibility of negative consequences.
None of this is unique to the U2 – it applies to the T2 or any low twist high aspect ratio design, and it applies to a somewhat, but not greatly, lesser degree, to any glider.
Sincerely,
Mike Meier
Wills Wing
