Bent up wings on Schempp-Hirth and Jonkers glider

Can an aerodynamicists explain the reasoning behind the bent up wing tips of the Schempp-Hirth and Jonkers gliders. I thought Schempp first started this on the Nimbus4 purely to keep the outboard tips from getting scraped, but now the tips are bent on shorter wing birds. Someone fairly knowledgeable once told me the the bent tips actually hurt performance in the run but help in climb. I just wanted to get a bt more educated not the reasoning. I noticed the Quintus has bent tips but the same wing on the Antares 23 were straight.

If we go back to theory, the perfect wing is a double super-ellipse (Lamé curve):
http://en.wikipedia.org/wiki/Superellipse

Both the top view and the dihedral of the wing should have the shape of a super-ellipse for lowest induced drag for a given bending moment (structural weight) and wetted area (profile drag at high speeds). Both the A350 and the Dreamliner are very close to this ideal:
wallpaperswide.com/download/boeing_787_dreamliner-wallpaper-1920x1200.jpg

Another plus compared to a wing without dihedral and winglets is that the interference drag between the winglet and the wing is much reduced.

Such a gradually curved wing is impossible to build because all the control surfaces would have a bend in them. The wing with sections progressively canted more and more (polyhedral) is a good compromise.

It also helps with flutter apparantly. The Vortex shedding frequency of the various sections make the critical flutter speed for such a wing higher, allowing either a higher VNE, or a less stiff and thus lighter wing structure.

I don't know if it was the extra tip dihedral or some other aspect of the design, but I found the Discus 2 to be the easiest glider I've ever thermalled - like it was on rails. It wasn't shabby on the run either.

Mike

On Monday, May 18, 2015 at 1:34:15 PM UTC-7, J. Nieuwenhuize wrote:
If we go back to theory, the perfect wing is a double super-ellipse (Lamé curve):
http://en.wikipedia.org/wiki/Superellipse

Both the top view and the dihedral of the wing should have the shape of a super-ellipse for lowest induced drag for a given bending moment (structural weight) and wetted area (profile drag at high speeds). Both the A350 and the Dreamliner are very close to this ideal:
wallpaperswide.com/download/boeing_787_dreamliner-wallpaper-1920x1200.jpg

Another plus compared to a wing without dihedral and winglets is that the interference drag between the winglet and the wing is much reduced.

Such a gradually curved wing is impossible to build because all the control surfaces would have a bend in them. The wing with sections progressively canted more and more (polyhedral) is a good compromise.

It also helps with flutter apparantly. The Vortex shedding frequency of the various sections make the critical flutter speed for such a wing higher, allowing either a higher VNE, or a less stiff and thus lighter wing structure.
h

Elliptical polyhedral is not part of any drag theory I ever learned studying aerodynamics. The wing planform, airfoils, twist and the use of winglets are used together to optimize the tradeoff between parasitic and induced drag while maintaining desirable handling and stall characteristics. My sense is that use of dihedral (or polyhedral) is mostly motivated by handling (and perhaps ground clearance) considerations rather than performance considerations. They may also think it looks cool.

9B

On Monday, May 18, 2015 at 3:56:47 PM UTC-4, Jonathan St. Cloud wrote:
Can an aerodynamicists explain the reasoning behind the bent up wing tips of the Schempp-Hirth and Jonkers gliders. I thought Schempp first started this on the Nimbus4 purely to keep the outboard tips from getting scraped, but now the tips are bent on shorter wing birds. Someone fairly knowledgeable once told me the the bent tips actually hurt performance in the run but help in climb. I just wanted to get a bt more educated not the reasoning. I noticed the Quintus has bent tips but the same wing on the Antares 23 were straight.

Here is a theory: When you are banked at 40° when thermalling the inside
wing tip is closer to the core than the outside wingtip. With the strong dihedral of the wingtip this puts the bank angle of the wing tip closest to the center of the thermal at say ~28° instead of the ~40° so the lift vector is closer to perpendicular to the lift = better climb rate.

What i don't understand is that when flying straight/level with an effectively high dihedral the lift vector of both wings are pointing perpendicular to the wings, so the vectors are not pointing up, but rather inward. They balance each other out of course but seems like if they were perpendicular to gravity they would be most efficient. Isn't dihedral costly on performance, but helps on handling?

Curious if anyone else makes any sense of this. I'm more asking than telling.
Chris
Not an aerodynamic expert!

Op dinsdag 19 mei 2015 04:38:19 UTC+2 schreef Andy Blackburn:
On Monday, May 18, 2015 at 1:34:15 PM UTC-7, J. Nieuwenhuize wrote:
If we go back to theory, the perfect wing is a double super-ellipse (Lamé curve):
http://en.wikipedia.org/wiki/Superellipse

Both the top view and the dihedral of the wing should have the shape of a super-ellipse for lowest induced drag for a given bending moment (structural weight) and wetted area (profile drag at high speeds). Both the A350 and the Dreamliner are very close to this ideal:
wallpaperswide.com/download/boeing_787_dreamliner-wallpaper-1920x1200.jpg

Another plus compared to a wing without dihedral and winglets is that the interference drag between the winglet and the wing is much reduced.

Such a gradually curved wing is impossible to build because all the control surfaces would have a bend in them. The wing with sections progressively canted more and more (polyhedral) is a good compromise.

It also helps with flutter apparantly. The Vortex shedding frequency of the various sections make the critical flutter speed for such a wing higher, allowing either a higher VNE, or a less stiff and thus lighter wing structure.
h

Elliptical polyhedral is not part of any drag theory I ever learned studying aerodynamics. The wing planform, airfoils, twist and the use of winglets are used together to optimize the tradeoff between parasitic and induced drag while maintaining desirable handling and stall characteristics. My sense is that use of dihedral (or polyhedral) is mostly motivated by handling (and perhaps ground clearance) considerations rather than performance considerations. They may also think it looks cool.

9B

No, it's not just a matter of handling and cool looks.

Most universities don't go further than lifting line theory. The name already gives away that is has it's issues; it's a 2D theory.
It's not for cosmetic reasons that the first generation of airliners and bizjets that can make use of new understanding and new construction methods all converge to polyhedral wings where the winglets are blended in the wing design.

On Monday, May 18, 2015 at 8:00:29 PM UTC-7, wrote:
On Monday, May 18, 2015 at 3:56:47 PM UTC-4, Jonathan St. Cloud wrote:
Can an aerodynamicists explain the reasoning behind the bent up wing tips of the Schempp-Hirth and Jonkers gliders. I thought Schempp first started this on the Nimbus4 purely to keep the outboard tips from getting scraped, but now the tips are bent on shorter wing birds. Someone fairly knowledgeable once told me the the bent tips actually hurt performance in the run but help in climb. I just wanted to get a bt more educated not the reasoning. I noticed the Quintus has bent tips but the same wing on the Antares 23 were straight.

Here is a theory: When you are banked at 40° when thermalling the inside
wing tip is closer to the core than the outside wingtip. With the strong dihedral of the wingtip this puts the bank angle of the wing tip closest to the center of the thermal at say ~28° instead of the ~40° so the lift vector is closer to perpendicular to the lift = better climb rate.

What i don't understand is that when flying straight/level with an effectively high dihedral the lift vector of both wings are pointing perpendicular to the wings, so the vectors are not pointing up, but rather inward. They balance each other out of course but seems like if they were perpendicular to gravity they would be most efficient. Isn't dihedral costly on performance, but helps on handling?

Curious if anyone else makes any sense of this. I'm more asking than telling.
Chris
Not an aerodynamic expert!

Where to begin.

In your example the inner wing is going slower and produces slightly less lift (everything else being equal) and the outer tip is going slightly faster and produces more lift, so by your logic the extra dihedral overall would go towards producing more inward lift than upward lift.

Of course none of that really explains use of polyhedral versus dihedral. Both are used to create spiral mode stability. Simple V dihedral is easier to manufacture, whereas polyhedral will produce similar spiral stability with more of the wing carrying a lower dihedral angle overall because of the longer moment arm out at the wingtip. In either case, the dihedral effect occurs through the coupling of roll and sideslip.

It's really not first and foremost a performance thing. The angles involved are pretty small in the first place so the cosine for the dihedral angle will be close to 1 and the sine will be close to 0, meaning the performance effect will be tiny. You mention something like 12 degrees of dihedral in your example, but that much would produce a pronounced dutch roll and would be a pretty unpleasant to fly airplane. More typical is a couple of degrees..

Also keep in mind how angle of attack is produced by pitching the aircraft. The whole idea of producing lift parallel to the span requires either a fixed angle of incidence relative to the direction of the travel (such as for winglets) or lift that is created as a function of sideslip to create spiral stability (as is the case for dihedral and polyhedral). Wikipedia has a decent explanation. http://en.wikipedia.org/wiki/Dihedral_(aeronautics).

9B

On Mon, 18 May 2015 19:38:18 -0700, Andy Blackburn wrote:

Elliptical polyhedral is not part of any drag theory I ever learned
studying aerodynamics. The wing planform, airfoils, twist and the use of
winglets are used together to optimize the tradeoff between parasitic
and induced drag while maintaining desirable handling and stall
characteristics. My sense is that use of dihedral (or polyhedral) is
mostly motivated by handling (and perhaps ground clearance)
considerations rather than performance considerations. They may also
think it looks cool.

Elliptical polyedral and planform have been described as the ideal and
used for years in the design of high performance free flight competition
models. There are references going back to the early '60s: Jim Baguley's
articles on F1A design in Aeromodeller, several articles in the annual
NFFS Symposium reports since 1968. These suggested that approximating an
elliptical area distribution minimises tip drag, while doing the same for
polyhedral minimises the tip height and hence the total wing area for a
given projected area, with the added benefit that, because polyhedral
minimises the angle between adjacent panels, it also minimises
interference drag. Six panel wings have been common in the F1ABC classes
for the last 15-20 years.

But then, as Will Schueman said, this is to be expected since the model
design/build generation time is much shorter than that for sailplanes:
6-12 months vs 5+ years, so more rapid evolution is to be expected.


--
martin@ | Martin Gregorie
gregorie. | Essex, UK
org |

All those arguments seem not to consider flexibility, watch at an open class glider wings at high Cl....

Carlo

On Tuesday, May 19, 2015 at 1:32:02 AM UTC-7, Martin Gregorie wrote:
On Mon, 18 May 2015 19:38:18 -0700, Andy Blackburn wrote:

Elliptical polyhedral is not part of any drag theory I ever learned
studying aerodynamics. The wing planform, airfoils, twist and the use of
winglets are used together to optimize the tradeoff between parasitic
and induced drag while maintaining desirable handling and stall
characteristics. My sense is that use of dihedral (or polyhedral) is
mostly motivated by handling (and perhaps ground clearance)
considerations rather than performance considerations. They may also
think it looks cool.

Elliptical polyedral and planform have been described as the ideal and
used for years in the design of high performance free flight competition
models. There are references going back to the early '60s: Jim Baguley's
articles on F1A design in Aeromodeller, several articles in the annual
NFFS Symposium reports since 1968. These suggested that approximating an
elliptical area distribution minimises tip drag, while doing the same for
polyhedral minimises the tip height and hence the total wing area for a
given projected area, with the added benefit that, because polyhedral
minimises the angle between adjacent panels, it also minimises
interference drag. Six panel wings have been common in the F1ABC classes
for the last 15-20 years.

But then, as Will Schueman said, this is to be expected since the model
design/build generation time is much shorter than that for sailplanes:
6-12 months vs 5+ years, so more rapid evolution is to be expected.


--
martin@ | Martin Gregorie
gregorie. | Essex, UK
org |

I have to admit to having a bit of trouble with the idea that having two steps in the polyhedral of 3 degrees each followed by a 84 degree angle for the winglet has much impact on interference drag. Gliders with span limits (for class or structural weight considerations) still generally have a vertical winglet at the tip rather than the Boeing-style flat raked tip (though a winglet and a span extension have similar effects on wingtip vortex and induced drag reduction for slightly different tradeoffs in bending moment).

I can accept the idea that raking the tip near the winglet affects spanwise flow and may have some beneficial effect on the transition. We've known about the potential benefits of sweeping the leading edge since Will Schuemann started modifying his ASW-12 and probably before that. If interference drag at the winglet junction were the big factor everyone would have LS-8-style winglets. I suspect the radius to reduce interference drag at these Reynolds numbers is measured in inches, not tens of yards.

I also get that polyhedral may give you similar handling for less wetted area than v-dihedral and that this may have become more attractive with the advent of stiffer carbon wings that don't give you dihedral through bending as much, but seriously, it has to be a fraction of a percent since we are talking about needing more polyhedral at the tip to yield similar spiral stability to low single-digit dihedral at the root. IMHO the additional tip clearance may throw enough weight in favor of the polyhedral design to make it worth the additional construction complexity.

You certainly are seeing it in multiple designs now.

9B