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#11
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At 12:30 27 June 2005, John Sinclair wrote:
My thought was that wing twisting (digging in) might have gotten them (Ivens & Engen) into the spiral dive in the first place. Once you get a big ship pointed down with the speed near red-line and increasing, you might very well have already had the snitz. We probably can't say definitively one way or the other. Many big ships have a tendency to digging in at the stall even without any aeroelastic (twisting) effect. This can be exacerbated by a pilot trying to pick the wing up through use of aileron rather than rudder. As you say, once the nose gets pointed down in a big ship you can have very little time to get things straightened out before you're past some limit or other. Generally you need pretty high Q before the aileron can produce enough moment to twist the wing. This could happen once the spiral is established and speed has built up, but the root cause of the spiral is likely related to a more common stall/wing drop sequence. In the Minden accident there wasn't any asymmetric wing bending reported, but there was quite pronounced symmetric bending observed leading to structural failure. It's not clear to me that you can un-twist a wing without slowing down, so I'd be hard pressed to come up with the sequence of events that would have the glider go directly from an asymmetric wing bending to symmetric bending/failure. Notwithstanding the specifics of that accident, the general warning about aeroelastic effect - particularly in sailplanes over 20m in span - is noteworthy. 9B |
#12
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At 10:00 28 June 2005, Andy Blackburn wrote:
At 12:30 27 June 2005, John Sinclair wrote: My thought was that wing twisting (digging in) might have gotten them (Ivens & Engen) into the spiral dive in the first place. Once you get a big ship pointed down with the speed near red-line and increasing, you might very well have already had the snitz. We probably can't say definitively one way or the other. Many big ships have a tendency to digging in at the stall even without any aeroelastic (twisting) effect. This can be exacerbated by a pilot trying to pick the wing up through use of aileron rather than rudder. As you say, once the nose gets pointed down in a big ship you can have very little time to get things straightened out before you're past some limit or other. Generally you need pretty high Q before the aileron can produce enough moment to twist the wing. This could happen once the spiral is established and speed has built up, but the root cause of the spiral is likely related to a more common stall/wing drop sequence. In the Minden accident there wasn't any asymmetric wing bending reported, but there was quite pronounced symmetric bending observed leading to structural failure. It's not clear to me that you can un-twist a wing without slowing down, so I'd be hard pressed to come up with the sequence of events that would have the glider go directly from an asymmetric wing bending to symmetric bending/failure. Notwithstanding the specifics of that accident, the general warning about aeroelastic effect - particularly in sailplanes over 20m in span - is noteworthy. 9B That was very good, 9B. Give yourself a big hug from old JJ. BTW, good job at Parowan. I would have given you a run for the money if only my wheel hadn't rolled about 12 miles short of the finish cylinder, on the first day. I enjoyed the contest, Parowan is an awesome place to fly. Hope the Region 9 guys will continue to have regionals and nationals there. Cheers, JJ |
#13
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Andy Blackburn wrote: At 12:30 27 June 2005, John Sinclair wrote: Generally you need pretty high Q before the aileron can produce enough moment to twist the wing. This could happen once the spiral is established and speed has built up, but the root cause of the spiral is likely related to a more common stall/wing drop sequence. In the Minden accident there wasn't any asymmetric wing bending reported, but there was quite pronounced symmetric bending observed leading to structural failure. It's not clear to me that you can un-twist a wing without slowing down, so I'd be hard pressed to come up with the sequence of events that would have the glider go directly from an asymmetric wing bending to symmetric bending/failure. Notwithstanding the specifics of that accident, the general warning about aeroelastic effect - particularly in sailplanes over 20m in span - is noteworthy. 9B Not meaning to be glib about this serious subject, but it reminds me of a video I once saw. They were clearing out the flight research lab at the university, and I came across a short film of flight testing on the Goodyear Inflatoplane. For the unitiated, this was (as the name implies) an inflatable aircraft designed to provide a platform for forward recon to the army and potentially recovery to downed aviators. There's an air-to-air sequence during stability tests that showcases bending and twisting moments like nothing else I've ever seen. It ought to be required viewing. The inflatoplane had a VNE somewhere in the 60-70 mph range if I recall, and these tests were done near that range. In one shot, the left wing (I think) tucks briefly like a folded napkin. In this wing, however, the structure was elastic enough that it basically "sproinged" back into shape. I spoke with a guy who was a test pilot on the project, and he seemed to be of the opinion that springy wings weren't such a great idea after all :-)) P3 |
#14
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At 13:24 28 June 2005, John Sinclair wrote:
That was very good, 9B. Give yourself a big hug from old JJ. BTW, good job at Parowan. I would have given you a run for the money if only my wheel hadn't rolled about 12 miles short of the finish cylinder, on the first day. I enjoyed the contest, Parowan is an awesome place to fly. Hope the Region 9 guys will continue to have regionals and nationals there. Thanks JJ. Seems like many of the top competitors this year could have used the 'drop a day' rule. Every day was a different challenge. Ditto on the continuing use of this beautiful site for regionals and nationals - without a local club groups from out of town will need to adopt it. I have heard rumblings that Region 9 has an interest in Moriarty for next year - so who will take the Parowan baton from the very capable Mifflin group? 9B |
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