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#11
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High altitude flutter - Vne
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#12
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High altitude flutter - Vne
On Jan 3, 4:34*pm, Greg Arnold wrote:
wrote: 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 am skeptical about 20:1 at 160 knots. *I think the best Dick Johnson flight test was the ASH-26 at 9 pounds wing loading, and it was below 20:1 at 120 knots. *A more realistic figure might be 10:1 at 160 knots, which would require 16 knots of lift to maintain altitude. Good catch. My curve fit to the factory polar of my ASW-27 shows 20:1 at 150 kts and max gross, but that seems wildly optimistic. Looking at a couple of final glides in relatively smooth air show less than 20:1 at 120 kts. So the summary point would be that it would have to be one heck of a wave day or you'd have to have an IFR flight plan to fly higher to really need much above 150 kts of Vne on a cross country flight - at least from a performance perspective. 9B |
#14
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High altitude flutter - Vne
Greg Arnold wrote:
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 am skeptical about 20:1 at 160 knots. I think the best Dick Johnson flight test was the ASH-26 at 9 pounds wing loading, and it was below 20:1 at 120 knots. A more realistic figure might be 10:1 at 160 knots, which would require 16 knots of lift to maintain altitude. You can get the ASW 27 B curve from Schleicher's site and draw your own line. It's guess, because the curve only goes to 210 kph, but the curve I drew "looks right", using the 55.6 kg/m2 line as the starting point. I'm sure it's not 10:1, but certainly between than 15:1 and 20:1. The ASH 26 is not a good choice for comparison because it has the same aspect ratio as the 27, and only goes to 45 kg/m2 (the 9 pounds you mentioned). -- 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 |
#15
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High altitude flutter - Vne
wrote:
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. I agree, but I saw the spar stub mockups when I was at Windward Performance in June. It's comforting that they are massive. Part of the reason for the light weight is the wing is only 80 square feet, compared to 99 on the ASW 27. High temperature cured pre-preg allows significantly lighter weight than wet layups done by hand, too. -- 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 |
#16
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High altitude flutter - Vne
Eric Greenwell wrote:
Greg Arnold wrote: 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 am skeptical about 20:1 at 160 knots. I think the best Dick Johnson flight test was the ASH-26 at 9 pounds wing loading, and it was below 20:1 at 120 knots. A more realistic figure might be 10:1 at 160 knots, which would require 16 knots of lift to maintain altitude. You can get the ASW 27 B curve from Schleicher's site and draw your own line. I am skeptical about the accuracy of that curve! Didn't they claim best L/D of 48? It's guess, because the curve only goes to 210 kph, but the curve I drew "looks right", using the 55.6 kg/m2 line as the starting point. Can you extrapolate that way? Just as unflapped ships suffer a noticeable fall off in performance above 80 to 85 knots, flapped ships must have a similar speed at which their wings aren't working very well (120 knots?). I'm sure it's not 10:1, but certainly between than 15:1 and 20:1. When the Duckhawk comes out, we will put you 15 to 20 nm out over the ocean at 6,000', and see if you want to fly at 160 knots towards the coast! The ASH 26 is not a good choice for comparison because it has the same aspect ratio as the 27, and only goes to 45 kg/m2 (the 9 pounds you mentioned). It is the best data point we have, I believe. |
#17
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High altitude flutter - Vne
On Jan 3, 7:36*pm, Eric Greenwell wrote:
You can get the ASW 27 B curve from Schleicher's site and draw your own line. It's guess, because the curve only goes to 210 kph, but the curve I drew "looks right", using the 55.6 kg/m2 line as the starting point. I'm sure it's not 10:1, but certainly between than 15:1 and 20:1. Based on a look at some actual flight logs I'd guess that at redline (151 kts) the ASW-27 would get something like 16:1. A quadratic curve fit to the factory polar shows 20:1. If you extrapolate out to the aforementioned 160kts (above redline for the -27) the quadratic curve off the factory polar yields 18:1, My guess at reality would be more like 14:1 at that speed (and 11.5 lbs/sqft). We'll see, but I doubt the Duckhawk will have the same performance as the -27 at equivalent wing loadings and I also doubt it'll be able to get to much above 10 lbs/sq ft wing loading because it starts out 230 lbs lighter empty and the wing is too small to carry enough water ballast to get above 10 lbs. Soooo, if I had to bet I'd guess an L/D below 15:1 rather than above 15:1 at max gross and 160 kts. Even 20:1 is enough to cruise in strong lift without gaining altitude, so I think the main point has been established - that you probably don't gain much by trying to optimize a design for wave XC in the 15 meter class. 9B |
#18
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High altitude flutter - Vne
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#19
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High altitude flutter - Vne
Greg Arnold wrote:
You can get the ASW 27 B curve from Schleicher's site and draw your own line. I am skeptical about the accuracy of that curve! Didn't they claim best L/D of 48? Maybe we don't have the same curve? The one I got from their download page has a line labeled E=48. http://www.alexander-schleicher.de/s...spekte/27E.pdf It's guess, because the curve only goes to 210 kph, but the curve I drew "looks right", using the 55.6 kg/m2 line as the starting point. Can you extrapolate that way? Just as unflapped ships suffer a noticeable fall off in performance above 80 to 85 knots, flapped ships must have a similar speed at which their wings aren't working very well (120 knots?). That's certainly part of the problem trying to extrapolate. A bigger problem is I put my dot on the graph paper in the wrong place when I calculated from the 36.6 kg/m2 curve, so now the curve looks like a 14:1 glide at 160 knots IAS. I'm sure it's not 10:1, but certainly between than 15:1 and 20:1. 14:1 is the new 15:1! When the Duckhawk comes out, we will put you 15 to 20 nm out over the ocean at 6,000', and see if you want to fly at 160 knots towards the coast! Apparently, I would slow down a bit sooner than I first anticipated, doing the last nm in ground (water?) effect -- 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 |
#20
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High altitude flutter - Vne
Eric Greenwell wrote:
Greg Arnold wrote: You can get the ASW 27 B curve from Schleicher's site and draw your own line. I am skeptical about the accuracy of that curve! Didn't they claim best L/D of 48? Maybe we don't have the same curve? The one I got from their download page has a line labeled E=48. The point I was trying to make, maybe not very well, is that their claim of 48 probably is unreasonably optimistic (it was measured at 45.6 by the Akaflieg). Thus, the rest of their polar curve probably also is too optimistic. http://www.alexander-schleicher.de/s...spekte/27E.pdf It's guess, because the curve only goes to 210 kph, but the curve I drew "looks right", using the 55.6 kg/m2 line as the starting point. Can you extrapolate that way? Just as unflapped ships suffer a noticeable fall off in performance above 80 to 85 knots, flapped ships must have a similar speed at which their wings aren't working very well (120 knots?). That's certainly part of the problem trying to extrapolate. A bigger problem is I put my dot on the graph paper in the wrong place when I calculated from the 36.6 kg/m2 curve, so now the curve looks like a 14:1 glide at 160 knots IAS. I'm sure it's not 10:1, but certainly between than 15:1 and 20:1. 14:1 is the new 15:1! And if their polar curve has some "salesmanship" in it, and allowing for a possible drastic dropoff in performance at high speeds, even 14:1 is too high. |
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