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Spring thermals
If the wind at launch changes direction radically in strong conditions, beware.
First, an accident that I (the original author of this web site) witnessed in 1975 at Monk’s Down in north Dorset, England.
One of my flying companions launched his standard Rogallo into the 20+ mph wind, which had just switched direction to across the slope. (We were doing the best we could with the knowledge we had at the time.) The glider lifted off, rose to about 15 ft above the hill and then the wing half nearest the hill went completely slack.
The standard Rogallo had no battens. It was just loose fabric that kept its aerodynamic shape by the pressure of the air flow – or not in this case. Just after he took off, the wind suddenly swung round to directly up the hill. In other words, he had launched into strong thermic turbulence.
The glider struck the hillside and the pilot (in a seat harness, which was the norm in those days) sustained a broken thigh bone, which put him out of action for the rest of the year.
Incidentally, battens in the sail would confer no advantage in such a scenario. It is just that, in the absence of battens, that side of the sail being completely stalled was visually obvious.
The second event is from reports in Whole Air, August 1984 and October 1984 editions.
Chris Smith of Chattanooga, Tennessee, reported that his launch seemed good initially. The launch assists agreed that all was normal until the moment Smith committed to flight, when the wind suddenly changed direction radically, too late to stop the launch. The moment the glider became airborne, it was slapped by a down-draft, which is evident by the curvature of the sail in the photo. Smith crashed into the scrub and rocks directly below the ramp and was lucky to sustain only a broken arm.
The photographer’s identity is not recorded, but much info about the accident was supplied by pilot and witness Chris Voith.
Hang Gliding History 