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#31
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#33
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Very thoughtful of Andreas to put this together. Based on 17+ years
and 2000 hours in a 20, I would add only the following thoughts... You may move the flap handle from position 2 to position 4 on take off as soon as the pass the start point of the tow plane. This is where wings typically drop, in the wake turbulence as you enter it at low speed. Once past it, you will find plenty of control authority. I prefer flap position 4 since it lowers the nose, allowing a much improved view of the tow rope. When thermalling, use flap position 4, or drill a hole between positions 3 and 4 if you want less drag. If you need to shift your circle or correct for gusts, move the flap handle to 3 as you make aileron inputs. This will give you a better roll rate. As soon as you have established the desired angle of bank, pop the handle back into positive (3.5 or 4). Martin Gregorie wrote in message . .. On 4 Jul 2004 17:54:39 -0700, (Ventus B) wrote: I have been considering buying an ASW20, ASW20B, or ASW20C. I knew they were champions in their day and still have a lot of admirers. However a few folks from my club say they have some nasty spin characteristics. Specifically, that they have a tendancy to not only immediately spin when stalled, but will go inverted as they spin. Can anyone eloborate or corroborate? I normally only hear good things about the 20. Respectfully, Assuming you haven't seen the handbook yet, the following may answer some of your questions: http://www.gregorie.org/gliding/asw2..._handling.html It was written by Andreas Maurer for a pilot who was converting from a Pegasus: in fact the guy I bought my '20 from. I've found it very useful, especially as I, too, was converting from a Pegasus. IMO it tells you most of what you need to know about the '20 that isn't in the flight manual. |
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#35
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In article ,
Andreas Maurer wrote: On 13 Jul 2004 14:33:53 -0700, (Andy Durbin) wrote: Just catching up with this thread and no-one seems to have mentioned the effect of the flexible wings. I don't have experience in the 20 but I do have a series of photos of a fatal accident that started with a contest finish pull-up and quickly ended up in a spinning impact with the ground. I believe that the increased angle attack caused by the wings returning to normal deflection contributed to the accident. Flexible wings do NOT change their AoA while they are bending - otherwise flutter would start immediately. It's not the amount of bend, it's the change in the bend. Anything that makes the wings move vertically changes the angle of attack to generate a lift force that opposes the movement. That's why gliders roll so slowly, for example. The exception to the above is if the extra angle of attack is sufficient to cause the wing to stall, in which case the lift becomes less and rolling/bending becomes anti-stable. That's what the original poster is talking about, not about twisting of the wing or some such thing. I'd be very surprised though if that was a big enough effect to *cause* the accident. Especially given that things are normally designed so that the wing roots stall considerably before the tips. -- Bruce |
#36
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Andreas Maurer wrote in message
Flexible wings do NOT change their AoA while they are bending - otherwise flutter would start immediately. Bye Andreas Hi Andreas, Wing flex must not result in twisting and wing flex does not change the angle of incidence of any part of the wing. I don’t think this means that wing flex does not change angle of attack. Assume a glider is static on the ground and has the tail raised so that the mean chord is horizontal. Now flex the wings upwards and release them. The wings move downward through the air. The relative air motion is at 90 deg to the mean cord so the angle of attack at the tips is approximately 90 degrees while the wings unflex. Now assume a flight condition that resulted from a high g pull up that approached stall speed. I’ll assume the speed is 40kts, that the wing tips flexed up 6 feet, and that as the pilot pushes forward to avoid stall the wings return to normal deflection in 1 second. The wing tip angle of attack change due to the downward motion can be calculated from the forward speed of 40kts = 67.5 ft per second, and the downward speed of 6 ft per second. Inverse tan of 6 / 67.5 is 5 so the tip angle of attack was increased by 5 degrees as the wings unflexed. The effect reduces to zero at the root where there is no deflection. If the numbers are valid then it remains to be decided if wing flex induced angle of attack changes of this magnitude would have an effect on stall and stall recovery characteristics. I expect that they would. Others disagree with me. Note to other posters - I didn't say this *caused* the accident. I said I believed it was a contributing factor. Andy |
#37
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Andy Durbin wrote:
Now assume a flight condition that resulted from a high g pull up that approached stall speed. I’ll assume the speed is 40kts, that the wing tips flexed up 6 feet, and that as the pilot pushes forward to avoid stall the wings return to normal deflection in 1 second. The wing tip angle of attack change due to the downward motion can be calculated from the forward speed of 40kts = 67.5 ft per second, and the downward speed of 6 ft per second. Inverse tan of 6 / 67.5 is 5 so the tip angle of attack was increased by 5 degrees as the wings unflexed. The effect reduces to zero at the root where there is no deflection. I think the problem here might be picking the right numbers: A high G pull up would no longer be "high G" at 40 knots (near stall speed), as the G loading would already be reduced to 1 G. At one G, the wings will not be flexed upwards. So, I think the wings will return to their normal position during the speed reduction that occurs after the pull-up is initiated; that is, more slowly than the 1 second used in the calculation. To get a 2 G load (a guess - I don't know how much it takes to bend the wings up 6 feet) on the wing that stalls at 40 knots would require 56 knots. Perhaps there would still be some effect, but it would also be reduced by the increased speed used (56 knots). If the numbers are valid then it remains to be decided if wing flex induced angle of attack changes of this magnitude would have an effect on stall and stall recovery characteristics. I expect that they would. Others disagree with me. It might be a difficult effect to determine experimentally: a pilot would have detect that the tip stalled when he reduced the G loading. -- Change "netto" to "net" to email me directly Eric Greenwell Washington State USA |
#38
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#39
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On Thu, 15 Jul 2004 09:02:20 +1200, Bruce Hoult
wrote: Anything that makes the wings move vertically changes the angle of attack to generate a lift force that opposes the movement. That's why gliders roll so slowly, for example. Now I start to see the light... I'd be very surprised though if that was a big enough effect to *cause* the accident. Especially given that things are normally designed so that the wing roots stall considerably before the tips. Not necessarily in a 20 with flap setting 4... Bye Andreas |
#40
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In fact, if you think about it, there would be a change in AoA as the
wings returned to their normal 1g state. The AoA increase at the tips would be greatest and negligible at the roots. How large an increase are we talking about? Pretty darn small. An amusing exercise though. A friend once figured out how thick a layer of material a tire leaves on the road, given normal wear. This seems on the same order. Andreas Maurer wrote in message . .. On 13 Jul 2004 14:33:53 -0700, (Andy Durbin) wrote: Just catching up with this thread and no-one seems to have mentioned the effect of the flexible wings. I don't have experience in the 20 but I do have a series of photos of a fatal accident that started with a contest finish pull-up and quickly ended up in a spinning impact with the ground. I believe that the increased angle attack caused by the wings returning to normal deflection contributed to the accident. Flexible wings do NOT change their AoA while they are bending - otherwise flutter would start immediately. Bye Andreas |
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