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#1
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I don't think 40/1 is remotely unreasonable for the Perkoz, even with
that "big clunky fixed gear hanging down." It has 20m in wing span and a decent aspect ratio. Compared to the Duo Discus and the DG500/1000 series, it's about 4-5 pts less in best L/D or about a 10% hit in performance. Seems very reasonable to this professional aerodynamicist. |
#2
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On Nov 7, 5:10*pm, KevinFinke wrote:
I don't think 40/1 is remotely unreasonable for the Perkoz... Ten bucks says that if the Idafliegs do a polar it tops out at or below 39. Thanks, Bob K. |
#3
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On Nov 7, 7:51*pm, Bob Kuykendall wrote:
On Nov 7, 5:10*pm, KevinFinke wrote: I don't think 40/1 is remotely unreasonable for the Perkoz... Ten bucks says that if the Idafliegs do a polar it tops out at or below 39. Thanks, Bob K. I'd be curious about the result. Our club Janus C has a 20 meter wing and 'clunky fixed gear'. The manual claims 43:1. I know from experience it's better than my Speed Astir. |
#4
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On Nov 7, 8:10*pm, Uncle Fuzzy wrote:
I'd be curious about the result. *Our club Janus C has a 20 meter wing and 'clunky fixed gear'. *The manual claims 43:1. *I know from experience it's better than my Speed Astir. I'm pretty curious as well. I don't hold Dr. Finke's qualifications, but the calibration I get from comparing manufacturers' claims with Idaflieg and Johnson results leads me to generally approach the manufacturers claims with something between "probably not" and "no nfbsk'n way." That skepticism has stood me in good stead so far. The things I see in the photos and three-view of the SZD that make it questionable in my mind are that the undercarriage does appear to hang down quite a bit from the fairing, and is not the cleanly-faired and mostly enclosed wheel of the Janus and ASK and even of the G103. Also, the degree of taper of the wing leads me to suspect that there will be a non-trivial amount of washout, which could tend to make the wingtip extensions less effective than you might otherwise expect. I seem to recall that the Slingsby 15/17m Dart is an example of such. To the Perkoz's credit, it looks to be a nice clean and well-proportioned machine, so I woudn't be too surprised to lose ten bucks on the wager. Johnson's tests on the 18.2m Janus rang up an impressive 40.9:1, so it seems within reason that the 20m variety could do 43:1. I had thought that Johnson had done the G104, but it's not on the SSA Web Site. It might well be in the hardcopy Johnson compendium, I'll try to remember to check that at work next week. Thanks, Bob K. www.hpaircraft.com/hp-24 |
#5
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Couldn't the wing use multiple airfoils or varying thicknesses to get
around the need for (as much) washout, despite the moderate taper ratio? Those wouldn't be immediately obvious from the visuals or 3- views... *shrug* --Noel |
#6
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Bob,
I fly with Brad Hill, and I'm sure he'll vouch for me. I'll take you up on the 10 dollar bet. My old club had a Jr and a Puchacz. Both planes seemed well constructed and gave honest performance along the lines of their published values. Back in college, I did measure our clubs Ka7 using the techniques outlined by Dick Johnson. I still have some of the equipment. Would you accept an independent measurement if we every get one up here in the Northwest? Kevin The only time I've ever used Dr, was back in college when I did some DJ'ing. I was known then as D.J. Dr. Finkenstein. |
#7
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On Nov 7, 11:36*pm, "noel.wade" wrote:
Couldn't the wing use multiple airfoils or varying thicknesses to get around the need for (as much) washout, despite the moderate taper ratio? *Those wouldn't be immediately obvious from the visuals or 3- views... **shrug* --Noel Multiple airfoils is a slick way to design the desired tip stalling characteristics into a plane. I'm not sure if the Perkoz uses this, but I have noticed it seems to have a good bit of forward sweep to the wing. Forward sweep can certainly be used instead of washout, as demonstrated clearly on the Sisu. Based on independent tests I've seen done on other Polish gliders, I have no reason to doubt the SZD figures for the 54. The Pols have always been rather scientific and less optimistic about their polars and such. -Paul Hanson SZD-59 driver, claimed at 40/1 by SZD (wet w/winglets...) and measured by Johnson at 38/1 (dry w/no winglets...) |
#8
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On 8 Nov, 03:51, Bob Kuykendall wrote:
On Nov 7, 5:10*pm, KevinFinke wrote: I don't think 40/1 is remotely unreasonable for the Perkoz... Ten bucks says that if the Idafliegs do a polar it tops out at or below 39. Perhaps, but if Dick Johnson were still around he could stick some turbulator tape on it and suddenly get 45 ... Ian |
#9
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At 14:52 08 November 2008, Ian wrote:
On 8 Nov, 03:51, Bob Kuykendall wrote: On Nov 7, 5:10=A0pm, KevinFinke wrote: I don't think 40/1 is remotely unreasonable for the Perkoz... Ten bucks says that if the Idafliegs do a polar it tops out at or below 39. Perhaps, but if Dick Johnson were still around he could stick some turbulator tape on it and suddenly get 45 ... Ian Quite the reverse - Dick Johnson tended to find that turbulated gliders performed better without the tape. He also usually found lower performance figures than the manufacturer's (and Idaflieg) polars. John Galloway |
#10
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![]() On Nov 8, 3:15*pm, John Galloway wrote: Quite the reverse - Dick Johnson tended to find that turbulated gliders performed better without the tape. *He also usually found lower performance figures than the manufacturer's (and Idaflieg) polars. John Galloway John - Actually, it depends. On earlier gliders he started finding benefits to the turbulator tape - which is why he developed the practice of sticking it on gliders he tested.... That's because earlier airfoils tended to have laminar separation bubbles, which could be eliminated by forcing the airflow to go turbulent (which, while higher-drag than laminar flow, is usually less-draggy than a bubble). It went on this way until the airfoils developed for some later gliders started to reverse the trend. I attribute that to the advancement of high-end wind-tunnels, CFD, and other computer-modeling techniques that allowed much better prediction and testing of airfoils for laminar bubbles and other boundary-layer effects. However, many of the gliders that are still popular today (and flown in large numbers) use airfoils that can benefit from select turbulation. --Noel |
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