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Wing Contour?



 
 
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  #21  
Old August 4th 19, 06:26 PM posted to rec.aviation.soaring
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Default Wing Contour?

On Tuesday, July 30, 2019 at 11:00:34 PM UTC-5, wrote:
Can anyone tell me specifically how to tell how closely a wing is following its intended shape or to know if it has "issues"? Also, if I'm sanding, polishing or refinishing it, how do I accurately correct for any deficiencies? Asking for a friend... : )


My primary reason for starting this thread was to help me know how to evaluate whether an FX-67 winged glider has suffered from "spar shrinkage" and if so, how much and how badly it will impact the performance. I've heard so much about and that it can profoundly decrease the performance so I'm trying to figure out whether to worry too much about it. As a relative newcomer to working with composites I'm not sure I could actually fix it...or might make it worse.

Thoughts?
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  #23  
Old August 4th 19, 07:28 PM posted to rec.aviation.soaring
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Default Wing Contour?

On Sunday, August 4, 2019 at 6:26:30 PM UTC+1, wrote:
On Tuesday, July 30, 2019 at 11:00:34 PM UTC-5, wrote:
Can anyone tell me specifically how to tell how closely a wing is following its intended shape or to know if it has "issues"? Also, if I'm sanding, polishing or refinishing it, how do I accurately correct for any deficiencies? Asking for a friend... : )


My primary reason for starting this thread was to help me know how to evaluate whether an FX-67 winged glider has suffered from "spar shrinkage" and if so, how much and how badly it will impact the performance. I've heard so much about and that it can profoundly decrease the performance so I'm trying to figure out whether to worry too much about it. As a relative newcomer to working with composites I'm not sure I could actually fix it...or might make it worse.

Thoughts?


If its spar shrinkage you are worried about you don't need a template; if it is significant then it will be easily visible and palpable along the spar cap line. If you can see that and feel that it is worth the effort for cosmetic reasons you could fill and smooth that area. It is not likely to be worthwhile worrying about it aerodynamically on an old Wortmann FX-67K-150/170 wing as that aerofoil has an inherent property of rather early airflow separation on the top surface. It was, IMHO, not a very good section as shown by the fact that ASW 17s and 20s with older 1962 Wortmann sections performed better than all the comparable gliders with 67 sections and also were not so susceptible to bugs and rain
  #24  
Old August 5th 19, 03:13 PM posted to rec.aviation.soaring
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Default Wing Contour?

I bought a near new LS-7 for 39k because the word was, “I had a climb problem!” Got it home and in the shop and found a big spar depression on the bottom of both wings! Couldn’t bring myself to jump on this new wing with 40 grit, so I carefully marked out the low areas which were behind the spar up to an inch wide and 7 to 10 feet long! Masked the low areas off and then sanded them with 220 grit to give the gelcoat “tooth”. I carefully leveled the wing both fore/aft and span-wise.. Mixed up a batch of gelcoat and brushed in everything with repeated coats until it was thicker than the depressions. Gave it a minimum cure time, about 4 hours as I remember, then wet sanded the edges with 400 grit while the stuff was green and very soft, switching to 600, then 800 as the depression disappeared. Buffed everything out after completely cured. She climbed with everyone else as far as I could see. Loved that bird, should have never sold it!
JJ
  #25  
Old August 12th 19, 09:28 PM posted to rec.aviation.soaring
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Default Wing Contour?

I contoured my LS-3 wings after 3 years because there was a flat spot over the spars. Made a big difference in glide, especially at high speed (impressed ASW 20 drivers asked me what I'd done). I thought I could feel it and the sanding soon confirmed it.

A few years later, I actually profiled the area over the spar on the upper surface. I should have used templates but wrote a program that allowed me to do it with a dial gauge. Yes, it can be done. If you know the dimensions of the dial gauge and the profile, then moving it X mm should cause the radius of curvature to change Y amount and the dial gauge to reflect that. If it changes Z amount, there's something wrong, and you can eventually figure out what. Anyway, we sprayed over the spar and built it back up and profiled it in that one area. Again, made a big difference. This time it was the ASW 20 C drivers asking me what I'd done.

Years later, I contoured it one more time but the profile hadn't changed much; the shrinkage over the spar caps apparently occurred fairly quickly. I could still climb and run with anything of that generation (Ventus, LS-6, ASW 20 C/B). The airfoil was more sensitive to rain but it didn't fall out of the sky like some brands were reputed to. But I tended to follow the advice of the PIK drivers: i.e., slow it up and add a little more flap than usual. And until I sold it, I kept the gel coat unwaxed and in 400 grit so the water didn't bead up.

I spoke to one guy (Jim Cox) who actually profiled the whole wing on his LS-3. He said the leading edge was too blunt, especially on the outer portion and he had to build that up. I believe that's what P3/ACA discovered on their LS-3, which they're refinishing. Jim had the same remarkable experience I did in terms of better performance. Would my glider have been even better if I'd done the whole wing? I don't know.

Most don't recall that Dick Johnson's flight tests of the LS-3 and ASW 20 showed essentially identical performance when new, which I confirmed. But the LS-3 deteriorated over several years. Not completely coincidentally, most competitive pilots moved over to the '20 after a year or two and the LS-3 became a forgotten airplane, with a few exceptions.

There's no doubt that a competent reprofiling job will help gliders like the LS-3. But that's a fair amount of work. IMO, it's only worth it if a) you want a great Sports Class machine and b) you're refinishing it anyway. The original LS-3 still climbs great and flies beautifully regardless of the spar cap shrinkage.

My comments apply only to the LS-3, not the later LS-3a with the split aileron/flap. Apparently the wing molds warped a bit and some later 3a models ended up with thicker wing sections. The factory disputed this.

UH's comments about pulling templates from the existing airfoil before you start a refinish job are spot on. When he managed/assisted with my ASW 24 refinish in his shop last year, we pulled templates for the whole wing AND made new ones for the "B mod" leading edge for the outer section. I don't know how much difference it made but my glider goes at least as well now as it did before so I'm happy. The templates didn't take that long to make with UH's expertise and alacrity.

Yes, you can usually get more bang for the buck by becoming a better pilot. But if you have trouble keeping up with your buddies and/or you're interested in Sports Class, it can be worth tuning an older ship like the LS-3. If you have time and some expertise (or able friends), it can also be good for the soul.

Chip Bearden
 




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