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#21
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I flew an LS-3 for many years, which nominally has the same airfoils
as the PIK. When it was newer, I had some "falling out of the sky" experiences, too, although not as dramatic as the PIK drivers reported. Slowing it down and using more positive flap than normal as suggested by several PIK pilots seemed to help. I later kept my wings in a 400 grit sanded surface, and once for a few years at 220 (the latter was very difficult to keep clean, the joke being that one had to sand off the bugs!). The satin finish seemed to help substantially: the water didn't bead up; it "sheeted" instead and flowed off. It still came down faster than usual when it got wet but it wasn't scary and, as one fellow suggested, sometimes it's difficult to separate the effects of rain and storm-associated sink anyway. Chip Bearden |
#23
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Earlier, Chris wrote:
...I remember reading that the late production runs of the PIK-20s were more sloppily built, and that the 17% thickness at the root got much thicker 20-21%... According to this Dick Johnson report, there was a thickness error, but to a much smaller degree: http://www.ssa.org/Johnson/26-1979-01.pdf Where Dick writes: : The PIK-20D factory brochure data : indicate that the wing thickness-to-chord : should be .170 tapering to .150 at the tip. : Our measurements of N19YZs wing showed .176 : at the wing root, .184 at the aileron root, : and .161 at the wing tip, This averages : about .01 t/c greater than it should be, : and perhaps is the reason for the higher drag. Guessing that the PIK-20 side-of-body chord is nominally about 35", the difference in depth between 17.0% and 17.6% amounts to 0.21" (5.3mm). On the other hand, if the actual side-of-body thickness really were on the order of 21%, the difference between 17% and 21% times the estimated 35" chord would be around 1.4" (36mm). I cannot imagine any production sailplane could ever be built with _that_ degree of sloppiness. For me, the real surprise is reading that the aileron root thickness had creeped out to 18.4%. Guessing that the taper from 17% at side-of-body to 15% at the tip is supposed to be distributed linearly, and that the aileron root falls at about the 0.5 semispan, the thickness there should have been about 16%. Guessing further that the chord at the aileron root is about 27.5", the percentage difference amounts to a depth error of around .66" (17mm). Now that's a large error. Thanks, and best regards to all Bob K. http://www.hpaircraft.com/hp-24 |
#24
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Bob,
Wouldn't the Dick Johnson report just report on one example/Serial Number? The one example may not be representative of the worse ones. Maybe the ones built close to specification are similar to the other gliders with the same airfoil, and possibly the thickest ones are the ones that perform really poorly in the rain. Chris (Bob Kuykendall) wrote in message . com... Earlier, Chris wrote: ...I remember reading that the late production runs of the PIK-20s were more sloppily built, and that the 17% thickness at the root got much thicker 20-21%... According to this Dick Johnson report, there was a thickness error, but to a much smaller degree: http://www.ssa.org/Johnson/26-1979-01.pdf Where Dick writes: : The PIK-20D factory brochure data : indicate that the wing thickness-to-chord : should be .170 tapering to .150 at the tip. : Our measurements of N19YZs wing showed .176 : at the wing root, .184 at the aileron root, : and .161 at the wing tip, This averages : about .01 t/c greater than it should be, : and perhaps is the reason for the higher drag. Guessing that the PIK-20 side-of-body chord is nominally about 35", the difference in depth between 17.0% and 17.6% amounts to 0.21" (5.3mm). On the other hand, if the actual side-of-body thickness really were on the order of 21%, the difference between 17% and 21% times the estimated 35" chord would be around 1.4" (36mm). I cannot imagine any production sailplane could ever be built with _that_ degree of sloppiness. For me, the real surprise is reading that the aileron root thickness had creeped out to 18.4%. Guessing that the taper from 17% at side-of-body to 15% at the tip is supposed to be distributed linearly, and that the aileron root falls at about the 0.5 semispan, the thickness there should have been about 16%. Guessing further that the chord at the aileron root is about 27.5", the percentage difference amounts to a depth error of around .66" (17mm). Now that's a large error. Thanks, and best regards to all Bob K. http://www.hpaircraft.com/hp-24 |
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