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Wortmann vs Eppler



 
 
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  #1  
Old January 28th 19, 09:17 AM posted to rec.aviation.soaring
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Default Wortmann vs Eppler

Hi, first time caller here, I note the first flight of the Pik-20 was in Oct-73, the Mosquito and Mini Nimbus in Sep 76. Grob came out with their Speed Astir in Apr 78. All competed in the 15 metre class. One major difference between them was the first 3 had the FX67k170/150 as a wing airfoil, whereas the last one, Speed Astir, had the Eppler 662. If you run an XFoil analysis of these two sections, (Profili provides a nice front end and visualisation tool), you'll see that the code predicts funny flows at the leading edge of the Wortmann section, (a reason for the bug problem maybe). Conversely the Eppler section is well behaved, i.e. the flows around the leading edge are well behaved. Both foils deliver very similar performance. So my question is, was the Eppler 662 directly a demand from someone asking Eppler to better the FX67k150 from Wortmann? Secondly, is one aspect of the development of airfoils simply getting laminar flow over greater and greater portions of the foil, (excepting that there are flow stability under varying conditions issues)? Regards Mark
  #2  
Old January 28th 19, 02:47 PM posted to rec.aviation.soaring
Mike C
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Default Wortmann vs Eppler

On Monday, January 28, 2019 at 1:17:19 AM UTC-7, nzhills wrote:
Hi, first time caller here, I note the first flight of the Pik-20 was in Oct-73, the Mosquito and Mini Nimbus in Sep 76. Grob came out with their Speed Astir in Apr 78. All competed in the 15 metre class. One major difference between them was the first 3 had the FX67k170/150 as a wing airfoil, whereas the last one, Speed Astir, had the Eppler 662. If you run an XFoil analysis of these two sections, (Profili provides a nice front end and visualisation tool), you'll see that the code predicts funny flows at the leading edge of the Wortmann section, (a reason for the bug problem maybe). Conversely the Eppler section is well behaved, i.e. the flows around the leading edge are well behaved. Both foils deliver very similar performance. So my question is, was the Eppler 662 directly a demand from someone asking Eppler to better the FX67k150 from Wortmann? Secondly, is one aspect of the development of airfoils simply getting laminar flow over greater and greater portions of the foil, (excepting that there are flow stability under varying conditions issues)? Regards Mark


It may not be Eppler vs. Wortmann but Eppler vs. Drela.

Grob gliders used Eppler airfoils so I doubt (guessing) that it was a response for a better airfoil than the FX series airfoils you mentioned.

Another point to consider, if a person uses Dr. Eppler's 'Profil' program they would get different results than they would using Dr. Drela's 'XFoil' program. Martin Hepperle has a short comparison of the two programs that mentions a potential problem with leading edge analysis when using XFoil.

https://www.mh-aerotools.de/airfoils/index.htm

To add to the reality of 1960 era airfoils, airfoil accuracy in the 60' and 70's seems to have been a hit and miss proposition. When I tried to fit computer generated CNC cut templates on a PIK 20B, at no station was the airfoil close to being accurate. Using a station template on a profiled Mini Nimbus showed a very accurate FX 150K profile, but the template when put on a Mosquito, with the original un-profiled wing did not fit at all.

You may find it interesting to read the Johnson articles on modifying his PIK 20 to the proper leading edge profile, to get a better picture of the problem and resulting performance improvements.

Airfoil analysis may be one of the dark arts.

Mike


  #3  
Old January 29th 19, 12:33 AM posted to rec.aviation.soaring
Segelflieger
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Default Wortmann vs Eppler

On Monday, January 28, 2019 at 3:17:19 AM UTC-5, nzhills wrote:
Hi, first time caller here, I note the first flight of the Pik-20 was in Oct-73, the Mosquito and Mini Nimbus in Sep 76. Grob came out with their Speed Astir in Apr 78. All competed in the 15 metre class. One major difference between them was the first 3 had the FX67k170/150 as a wing airfoil, whereas the last one, Speed Astir, had the Eppler 662. If you run an XFoil analysis of these two sections, (Profili provides a nice front end and visualisation tool), you'll see that the code predicts funny flows at the leading edge of the Wortmann section, (a reason for the bug problem maybe). Conversely the Eppler section is well behaved, i.e. the flows around the leading edge are well behaved. Both foils deliver very similar performance. So my question is, was the Eppler 662 directly a demand from someone asking Eppler to better the FX67k150 from Wortmann? Secondly, is one aspect of the development of airfoils simply getting laminar flow over greater and greater portions of the foil, (excepting that there are flow stability under varying conditions issues)? Regards Mark


$0.02: Both Eppler and Wortman have greatly contributed to the advancement of modern glider design with their airfoil design concepts.
Today, world class designers use 3D Computational Fluid Dynamics (CFD) using some form of Navier Stokes Equations, simulating the flow around the entire aircraft, even with the near wake, flap edge, and tip vortices.
  #4  
Old January 29th 19, 01:27 AM posted to rec.aviation.soaring
Charlie Quebec
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Default Wortmann vs Eppler

Not all 70’s gliders are that bad, Put laser cut templates on my 1980 DG200 with original gelcoat, and it was close to perfect. The PIK is known to be terrible in this aspect, one that I know of is 19% thick rather than 17%. Back in the day a couple of guys I know reprofiled a Mossie and a Mini, with very good results.
  #5  
Old January 29th 19, 06:52 AM posted to rec.aviation.soaring
David Kraus
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Default Wortmann vs Eppler

Sometimes xflr5 have problem with convergence of solution, you have to change input conditions to get it right. One video tutorial about it
https://youtu.be/O4qlA_hjORc
  #6  
Old January 29th 19, 07:29 AM posted to rec.aviation.soaring
krasw
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Default Wortmann vs Eppler

Moulds for 70's gliders were not very stable, first serial numbers might have FX67-K170 and last ones "-K190" because of the deformation.


On Monday, 28 January 2019 15:47:27 UTC+2, Mike C wrote:
On Monday, January 28, 2019 at 1:17:19 AM UTC-7, nzhills wrote:
Hi, first time caller here, I note the first flight of the Pik-20 was in Oct-73, the Mosquito and Mini Nimbus in Sep 76. Grob came out with their Speed Astir in Apr 78. All competed in the 15 metre class. One major difference between them was the first 3 had the FX67k170/150 as a wing airfoil, whereas the last one, Speed Astir, had the Eppler 662. If you run an XFoil analysis of these two sections, (Profili provides a nice front end and visualisation tool), you'll see that the code predicts funny flows at the leading edge of the Wortmann section, (a reason for the bug problem maybe). Conversely the Eppler section is well behaved, i.e. the flows around the leading edge are well behaved. Both foils deliver very similar performance. So my question is, was the Eppler 662 directly a demand from someone asking Eppler to better the FX67k150 from Wortmann? Secondly, is one aspect of the development of airfoils simply getting laminar flow over greater and greater portions of the foil, (excepting that there are flow stability under varying conditions issues)? Regards Mark


It may not be Eppler vs. Wortmann but Eppler vs. Drela.

Grob gliders used Eppler airfoils so I doubt (guessing) that it was a response for a better airfoil than the FX series airfoils you mentioned.

Another point to consider, if a person uses Dr. Eppler's 'Profil' program they would get different results than they would using Dr. Drela's 'XFoil' program. Martin Hepperle has a short comparison of the two programs that mentions a potential problem with leading edge analysis when using XFoil.

https://www.mh-aerotools.de/airfoils/index.htm

To add to the reality of 1960 era airfoils, airfoil accuracy in the 60' and 70's seems to have been a hit and miss proposition. When I tried to fit computer generated CNC cut templates on a PIK 20B, at no station was the airfoil close to being accurate. Using a station template on a profiled Mini Nimbus showed a very accurate FX 150K profile, but the template when put on a Mosquito, with the original un-profiled wing did not fit at all.

You may find it interesting to read the Johnson articles on modifying his PIK 20 to the proper leading edge profile, to get a better picture of the problem and resulting performance improvements.

Airfoil analysis may be one of the dark arts.

Mike


  #7  
Old January 29th 19, 07:42 PM posted to rec.aviation.soaring
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Default Wortmann vs Eppler

Mike, there is I believe a major clinker when discussing the Pik 20 wing, known as before and after the fire. As in fire in the factory, which affected the molds which were repaired. The post-fire wings were off a bit from the Wortmann design.
Or at least that is what I was told about my nice Pik which was from before the fire. It was flown sometimes by a friend who was a long time Pik driver who had one from after the fire. He always said mine flew a lot better than his.
So on the basis of this clearly scientific evidence (LoL) I believe the story about the fire.
To deviate from this thread's origin let me add that my Pik was just as troubled by rain -- the water balled up and ran along the leading edge and the sink rate went way up. A Finnish friend who had at one time been a partner in Ingo Renner's former Pik told me the solution -- just put the flaps down around 8 degrees. I experimented one time and his trick worked.




On Monday, January 28, 2019 at 8:47:27 AM UTC-5, Mike C wrote:
On Monday, January 28, 2019 at 1:17:19 AM UTC-7, nzhills wrote:
Hi, first time caller here, I note the first flight of the Pik-20 was in Oct-73, the Mosquito and Mini Nimbus in Sep 76. Grob came out with their Speed Astir in Apr 78. All competed in the 15 metre class. One major difference between them was the first 3 had the FX67k170/150 as a wing airfoil, whereas the last one, Speed Astir, had the Eppler 662. If you run an XFoil analysis of these two sections, (Profili provides a nice front end and visualisation tool), you'll see that the code predicts funny flows at the leading edge of the Wortmann section, (a reason for the bug problem maybe). Conversely the Eppler section is well behaved, i.e. the flows around the leading edge are well behaved. Both foils deliver very similar performance. So my question is, was the Eppler 662 directly a demand from someone asking Eppler to better the FX67k150 from Wortmann? Secondly, is one aspect of the development of airfoils simply getting laminar flow over greater and greater portions of the foil, (excepting that there are flow stability under varying conditions issues)? Regards Mark


It may not be Eppler vs. Wortmann but Eppler vs. Drela.

Grob gliders used Eppler airfoils so I doubt (guessing) that it was a response for a better airfoil than the FX series airfoils you mentioned.

Another point to consider, if a person uses Dr. Eppler's 'Profil' program they would get different results than they would using Dr. Drela's 'XFoil' program. Martin Hepperle has a short comparison of the two programs that mentions a potential problem with leading edge analysis when using XFoil.

https://www.mh-aerotools.de/airfoils/index.htm

To add to the reality of 1960 era airfoils, airfoil accuracy in the 60' and 70's seems to have been a hit and miss proposition. When I tried to fit computer generated CNC cut templates on a PIK 20B, at no station was the airfoil close to being accurate. Using a station template on a profiled Mini Nimbus showed a very accurate FX 150K profile, but the template when put on a Mosquito, with the original un-profiled wing did not fit at all.

You may find it interesting to read the Johnson articles on modifying his PIK 20 to the proper leading edge profile, to get a better picture of the problem and resulting performance improvements.

Airfoil analysis may be one of the dark arts.

Mike


  #8  
Old January 29th 19, 08:54 PM posted to rec.aviation.soaring
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Default Wortmann vs Eppler

I think the subject is better stated as "Wortmann and Eppler", not versus. But one time Bruce Carmichael, Paul MacCready and I were discussing how Dr. Wortmann's sections seemed to enjoy more usage and success than Eppler's. When the discussion turned to "why", Carmichael said: "Don't forget that Wortmann had the tunnel".

Eppler's genius was evident upon bringing us the inverse design method- a remarkable achievement. Something which was then ingeniously expanded upon by Mark Drela when wake effects could also be taken into account. However, as Carmichael wisely pointed out there's great value in empirical results. This still applies to the best CFD. Irv Culver was fond of reminding: "One test flight is worth a thousand expert opinions", which I suppose is in a similar vein and stated only as Irv could.

It's sad to think that all these great contributors are now gone. As is Dennis Brown, a flight test engineer from Boeing who spent untold hours contouring his Mosquito wings to near perfection. He saw very little benefit, if any, after careful testing and measurement. Doesn't mean others haven't enjoyed good improvement with other situations. Dennis is gone now too. I miss these guys.

-Gary Osoba

  #9  
Old January 30th 19, 12:31 AM posted to rec.aviation.soaring
Mike C
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Default Wortmann vs Eppler

On Tuesday, January 29, 2019 at 11:42:27 AM UTC-7, wrote:
Mike, there is I believe a major clinker when discussing the Pik 20 wing, known as before and after the fire. As in fire in the factory, which affected the molds which were repaired. The post-fire wings were off a bit from the Wortmann design.
Or at least that is what I was told about my nice Pik which was from before the fire. It was flown sometimes by a friend who was a long time Pik driver who had one from after the fire. He always said mine flew a lot better than his.
So on the basis of this clearly scientific evidence (LoL) I believe the story about the fire.
To deviate from this thread's origin let me add that my Pik was just as troubled by rain -- the water balled up and ran along the leading edge and the sink rate went way up. A Finnish friend who had at one time been a partner in Ingo Renner's former Pik told me the solution -- just put the flaps down around 8 degrees. I experimented one time and his trick worked.




On Monday, January 28, 2019 at 8:47:27 AM UTC-5, Mike C wrote:
On Monday, January 28, 2019 at 1:17:19 AM UTC-7, nzhills wrote:
Hi, first time caller here, I note the first flight of the Pik-20 was in Oct-73, the Mosquito and Mini Nimbus in Sep 76. Grob came out with their Speed Astir in Apr 78. All competed in the 15 metre class. One major difference between them was the first 3 had the FX67k170/150 as a wing airfoil, whereas the last one, Speed Astir, had the Eppler 662. If you run an XFoil analysis of these two sections, (Profili provides a nice front end and visualisation tool), you'll see that the code predicts funny flows at the leading edge of the Wortmann section, (a reason for the bug problem maybe). Conversely the Eppler section is well behaved, i.e. the flows around the leading edge are well behaved. Both foils deliver very similar performance. So my question is, was the Eppler 662 directly a demand from someone asking Eppler to better the FX67k150 from Wortmann? Secondly, is one aspect of the development of airfoils simply getting laminar flow over greater and greater portions of the foil, (excepting that there are flow stability under varying conditions issues)? Regards Mark


It may not be Eppler vs. Wortmann but Eppler vs. Drela.

Grob gliders used Eppler airfoils so I doubt (guessing) that it was a response for a better airfoil than the FX series airfoils you mentioned.

Another point to consider, if a person uses Dr. Eppler's 'Profil' program they would get different results than they would using Dr. Drela's 'XFoil' program. Martin Hepperle has a short comparison of the two programs that mentions a potential problem with leading edge analysis when using XFoil.

https://www.mh-aerotools.de/airfoils/index.htm

To add to the reality of 1960 era airfoils, airfoil accuracy in the 60' and 70's seems to have been a hit and miss proposition. When I tried to fit computer generated CNC cut templates on a PIK 20B, at no station was the airfoil close to being accurate. Using a station template on a profiled Mini Nimbus showed a very accurate FX 150K profile, but the template when put on a Mosquito, with the original un-profiled wing did not fit at all.

You may find it interesting to read the Johnson articles on modifying his PIK 20 to the proper leading edge profile, to get a better picture of the problem and resulting performance improvements.

Airfoil analysis may be one of the dark arts.

Mike


Howard - I think,

Concerning the PIK, I remember our conversation about this a few years ago and it was in the back of my mind when I wrote what I wrote. I do not know if the one I measured was pre or post fire.

Anyway, there are gliders out there with varying degrees of airfoil accuracy.

Hope you are well and enjoying yourself.

Mike

 




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