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Long wing twisting



 
 
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  #11  
Old June 28th 05, 10:44 AM
Andy Blackburn
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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  
Old June 28th 05, 02:06 PM
John Sinclair
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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  
Old June 28th 05, 04:44 PM
Papa3
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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  
Old June 28th 05, 06:42 PM
Andy Blackburn
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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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