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#1
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High altitude flutter - Vne
Interesting subject needs new thread.
It strikes me that a glider designed to soar in weak thermals is well off its design point when running fast in a wave XC or, for that matter, in strong thermal conditions. Ballast helps, of course, but the basic airframe aerodynamics are wrong for extreme conditions. Flutter shouldn't be allowed to become the limiting condition. There were hints that Klaus Ohlman's Nimbus 4DM was modified to increase Va and Vne for the record flights. It was never made clear just what those mods were. With today's records being set in extremely strong conditions, an increase in Va and/or Vne seems like it would be more important than any increase in the already high L/D max. |
#2
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High altitude flutter - Vne
bildan wrote:
Interesting subject needs new thread. It strikes me that a glider designed to soar in weak thermals is well off its design point when running fast in a wave XC or, for that matter, in strong thermal conditions. Ballast helps, of course, but the basic airframe aerodynamics are wrong for extreme conditions. Flutter shouldn't be allowed to become the limiting condition. There were hints that Klaus Ohlman's Nimbus 4DM was modified to increase Va and Vne for the record flights. It was never made clear just what those mods were. With today's records being set in extremely strong conditions, an increase in Va and/or Vne seems like it would be more important than any increase in the already high L/D max. Time to order your DuckHawk - 200 Kt Vne, 160 knot Va, 10.75 lbs/sqft - just what you need for those high-speed wave flights! -- Eric Greenwell - Washington State, USA * Change "netto" to "net" to email me directly * Updated! "Transponders in Sailplanes" http://tinyurl.com/y739x4 * New Jan '08 - sections on Mode S, TPAS, ADS-B, Flarm, more * "A Guide to Self-launching Sailplane Operation" at www.motorglider.org |
#3
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High altitude flutter - Vne
Time to order your DuckHawk - 200 Kt Vne, 160 knot Va, 10.75 lbs/sqft -
just what you need for those high-speed wave flights! But whats the L/D at 160 & 200 Knots? 10 & 5 ? Allan |
#4
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High altitude flutter - Vne
On Jan 2, 10:52*pm, Allan wrote:
Time to order your DuckHawk - 200 Kt Vne, 160 knot Va, 10.75 lbs/sqft - just what you need for those high-speed wave flights! But whats the L/D at 160 & 200 Knots? 10 & 5 ? Allan L/D? When the nose pushed down at 160 knots, crabbing into a 50+ knot headwind and you've got +5 knots of up in wave you won't care... I can't wait to see what magic Greg Cole pulls off with the Duck Hawk. Darryl |
#5
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High altitude flutter - Vne
Darryl Ramm wrote:
On Jan 2, 10:52 pm, Allan wrote: Time to order your DuckHawk - 200 Kt Vne, 160 knot Va, 10.75 lbs/sqft - just what you need for those high-speed wave flights! But whats the L/D at 160 & 200 Knots? 10 & 5 ? Allan L/D? When the nose pushed down at 160 knots, crabbing into a 50+ knot headwind and you've got +5 knots of up in wave you won't care... I can't wait to see what magic Greg Cole pulls off with the Duck Hawk. Darryl has it right - the limit for high speed wave flights is Vne, not L/D. But, extrapolating from an ASW 27 B polar gives ~20:1 at 160 knots. Attempting to extrapolate to 200 knots is pointless, so we'll have to wait for Windward Performance to publish a curve. -- Eric Greenwell - Washington State, USA * Change "netto" to "net" to email me directly * Updated! "Transponders in Sailplanes" http://tinyurl.com/y739x4 * New Jan '08 - sections on Mode S, TPAS, ADS-B, Flarm, more * "A Guide to Self-launching Sailplane Operation" at www.motorglider.org |
#6
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High altitude flutter - Vne
Darryl Ramm wrote:
On Jan 2, 10:52 pm, Allan wrote: Time to order your DuckHawk - 200 Kt Vne, 160 knot Va, 10.75 lbs/sqft - just what you need for those high-speed wave flights! But whats the L/D at 160 & 200 Knots? 10 & 5 ? Allan L/D? When the nose pushed down at 160 knots, crabbing into a 50+ knot headwind and you've got +5 knots of up in wave you won't care... I can't wait to see what magic Greg Cole pulls off with the Duck Hawk. Darryl That would be mighty strong wave to have you consistently going up at an airspeed of 200 knots. |
#7
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High altitude flutter - Vne
On Jan 3, 9:46*am, Greg Arnold wrote:
Darryl Ramm wrote: On Jan 2, 10:52 pm, Allan wrote: Time to order your DuckHawk - 200 Kt Vne, 160 knot Va, 10.75 lbs/sqft - just what you need for those high-speed wave flights! But whats the L/D at 160 & 200 Knots? 10 & 5 ? Allan L/D? When the nose pushed down at 160 knots, crabbing into a 50+ knot headwind and you've got +5 knots of up in wave you won't care... I can't wait to see what magic Greg Cole pulls off with the Duck Hawk. Darryl That would be mighty strong wave to have you consistently going up at an airspeed of 200 knots. I've listened on channel 9 while a westbound United flight crew was surprised they were climbing in wave near Denver with a 160 kt headwind on the nose. Net speed had to be well over 200kt and wing loading well over 10.75 lbs/sqft! The big problem with high altitude and wave flight is the constriction of your flight envelope. At Minden, I discovered ambudant ballast in a G-102 during a wave flight pre-take- off check. Ship had been flown the day before by a petite pilot (I'm about 200 lbs with wave flight gear). Calculated my flight envelope at the ballasted weight after the flight - not much between stall and flutter. Made me appreciate the A12 and U2 pilots who often had +/- 3 kts or less [Shadow Flights by Curtis Peebles, pg 96]. Also made me appreciate my insistance on going through my check-list even tho the tow plane was ready! |
#8
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High altitude flutter - Vne
On Fri, 2 Jan 2009 17:10:34 -0800 (PST), bildan
wrote: There were hints that Klaus Ohlman's Nimbus 4DM was modified to increase Va and Vne for the record flights. It was never made clear just what those mods were. It's got fully balanced control surfaces. |
#9
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High altitude flutter - Vne
Andreas Maurer wrote:
There were hints that Klaus Ohlman's Nimbus 4DM was modified to increase Va and Vne for the record flights. It was never made clear just what those mods were. It's got fully balanced control surfaces. Which you usually don't want on gliders, because it makes the controls heavier (see DG-1000 vs. Duo). |
#10
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High altitude flutter - Vne
On Jan 3, 9:00*am, Eric Greenwell wrote:
Darryl Ramm wrote: On Jan 2, 10:52 pm, Allan wrote: Time to order your DuckHawk - 200 Kt Vne, 160 knot Va, 10.75 lbs/sqft - just what you need for those high-speed wave flights! But whats the L/D at 160 & 200 Knots? 10 & 5 ? Allan L/D? When the nose pushed down at 160 knots, crabbing into a 50+ knot headwind and you've got +5 knots of up in wave you won't care... I can't wait to see what magic Greg Cole pulls off with the Duck Hawk. Darryl has it right - the limit for high speed wave flights is Vne, not L/D. But, extrapolating from an ASW 27 B polar gives ~20:1 at 160 knots. Attempting to extrapolate to 200 knots is pointless, so we'll have to wait for Windward Performance to publish a curve. Just to finish the thought - you only need 8 knots of lift to maintain altitude at 20:1 and 160 kts. I've never had a Vne issue in thermal soaring, even at 18,000', though I have been occasionally concerned about it in strong lift approaching cloudbase. Even a great cloudstreet is unlikely to have sustained lift that strong so if you are keeping decent clearance from cloudbase you can usually let your altitude vary rather than running up the airspeed. I could easily imagine it being more of an issue in wave, particularly for those folks running wave under IFR above 18,000'. Wouldn't the 200 kt Vne be from sea level up to some limited altitude? If so, you probably don't need to figure the L/D at 200 kts IAS for wave flying - you'll be flying no faster than Va in wave above the upper teens to low twenties (depending on how high the 200 kts is good for), so 8 kts of up will be the strongest lift in which you'll be able to hold altitude (versus 6-7 kts for, say, an ASW-27). I will need to get used to the idea of flying that fast in a glider that weighs 300 lbs empty. 9B |
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