There are obvious dangers in flying a hang glider upside-down: If you miss-judge your speed or pitch-rate you might over-stressing the wing structure on the pull-up, possibly causing catastrophic failure. If you miss-manage your energy you might come to a stop, going straight up or upside-down, tail-slide and tumble. If you lose control close to terrain you might hit the ground. If you do not manage your practice space wisely you might collide with another flyer.
There is another hazard which I experienced myself in 1988, and that I have seen and heard reports of lately. The scenario is this: You are inverted at or somewhat past the apex of your maneuver. It does not matter if you have plenty of energy and you are just pulling in for speed for your next maneuver, or you are slow, or you have stopped completely. You “ball-up” (move your body’s mass maximum forward) asking the glider for its max speed.
The result of this can be a lost of positive pitch pressure on the control bar. As most will agree, that is a disconcerting feeling, especially at 90 plus mph heading straight for the ground. We hang glider pilots like to, and should feel some positive pitch pressure any time we are going faster than trim. It reminds us that our stability system is working. To find unexpectedly, no pressure in this unusual situation that obviously requires quick response will cause most of us to push out too abruptly and too far.
While teaching aerobatics in Norway recently I observed an incident that alerted me to the possibility that this potential danger might be getting worse as glider designs get slipperier. One of my pupils towed up and did his first two loops individually, ie. he performed one loop, leveled out to normal flight, and then dove and did another. He was flying a popular modern topless wing. On his next flight he decided to do consecutive loops. Sometime after apex of the first loop, but before his post-dive became vertical he pulled in all the way for the second loop.
Later, when he was safely on the ground he reported that he had felt no pitch pressure when he went to let the bar out for the pull-up. He pushed the bar out and the glider pitched up abruptly pulling many Gs. We on the ground heard a clunk. The sound was made by his basetube hitting his full-faced helmet and chin when the excessive G-force pushed his head downward as the basetube went by, and/or by most of the batten toggles breaking on the right side, releasing the rib tension and/or the breaking of his harness backplate. He went into a nice looking 180 degree right rolling maneuver and pulled out into normal flight.
He landed safely but his glider had to be taken apart for a complete inspection. His keel was dented under the hang-point perch. His vario mount was broken and his chin was sore. One could speculate that the helmet’s chin guard prevented in-flight loss of consciousness which likely would have resulted in tragedy. He might have pulled 6 or 7 Gs or more? That the airframe did not fail is testament to the strength of the glider; however, the pilot’s weight was less than average.
I believe that in this situation the glider is essentially in free-fall and is not really behaving like a wing. It is falling vertically and not generating lift because its angle of attack is zero. If the pilot is able to push out on the bar, against his own mass and raise the AOA(angle of attack); now the wing is generating lift instantaneously but at an AOA that is too high for the extreme speed. This results in positive pitch pressure that is beyond the ability of normal humans to resist. The bastube moves forward violently despite the pilot’s best efforts to hold it in, and the glider pitches up uncontrollably.
On a good day the glider survives the G-loading, climbs out nearly vertically and the pilot recovers to normal flight as the speed decays. That is what happened to me in 1988 on the Mystic Special. The Norwegian pilot represents a middle scenario in which the glider sustains some airframe damage, but the pilot lands safely. The Norwegians told me of yet another pilot who had survived this series of events.
The worst case is that the pilot gets knocked out by the basetube, and/or the glider fails catastrophically from the excessive G-loading. An unconscious pilot is unable to deploy the reserve. Perhaps there have been some unexplained incidents in the past that were caused by this phenomenon.
I do not know if pushing out more slowly will change the outcome. It is possible that once you have accelerated enough to lose pitch pressure, there is no good way back to normal flight mode.
No hang glider testing authority requires pitching moment tests at the extreme speeds seen in aerobatic flying. It would be difficult, expensive and dangerous to the occupants of the test vehicle. To pass the tests the gliders would end up heavier, more expensive and less fun to fly. There is a reason the gliders are marked with a VNE (velocity to never exceed).
It is important to understand that you do not need to go fast enough to lose pitch pressure in order to have a life-threatening event on the pull-up. Any time you are in a “warp-speed” dive and you let the bar out too quickly, the pitch pressure can spike resulting in an uncontrollable and abrupt pull-up. This has been referred to as “positive pitch divergence”.
So, even in this more “normal” scenario, all the afore mentioned perils are possible: the basetube hitting you in the chin or face, the rapid bleed-off of energy causing a stall going straight up, and damage to, or failure of the airframe.
In the early days of looping flex-wing hang gliders one had to go to extraordinary means to gain the speed necessary to go over the top with a little extra energy. This involved pushing out to mush, then quickly hauling the bar all the way back while scrunching up into a “ball” to get as much mass as possible as far forward as possible. In modern aerobatic hang gliding it appears that a new skill is required. This would be: knowing how to avoid going too fast.
First published in March 2008 Hang Gliding & Paragliding Magazine
I wrote the preceding as a supplement to my article “The Ups and Downs of Freestyle Hang Gliding” first published in the July 1994 issue of Hang Gliding Magazine, and viewable at johnheiney.com.