Bent up wings on Schempp-Hirth and Jonkers glider

Op dinsdag 19 mei 2015 17:33:31 UTC+2 schreef Andy Blackburn:
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

Comparing a planar (no dihedral) wing with a winglet to a "perfect" super-elliptic continously changing polyhedral wing we're talking on the order of a percent lower induced drag and another percent less drag due to not having to suffer interference drag. With the present polyhedral wings, you're probably down to less than half that. That's under 1% less induced drag, or about half a percent less drag at low speeds.

This is an interesting result. Having seen the work in a bit more detail, having a big radius (or a continously changing polyhedral) most certainly pays off, especially at the high Cl's we fly a considerable amount of time at:
http://www.apollocanard.com/4_blended%20winglets.htm

On Tuesday, May 19, 2015 at 12:12:49 PM UTC-7, J. Nieuwenhuize wrote:
Op dinsdag 19 mei 2015 17:33:31 UTC+2 schreef Andy Blackburn:
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

Comparing a planar (no dihedral) wing with a winglet to a "perfect" super-elliptic continously changing polyhedral wing we're talking on the order of a percent lower induced drag and another percent less drag due to not having to suffer interference drag. With the present polyhedral wings, you're probably down to less than half that. That's under 1% less induced drag, or about half a percent less drag at low speeds.

This is an interesting result. Having seen the work in a bit more detail, having a big radius (or a continously changing polyhedral) most certainly pays off, especially at the high Cl's we fly a considerable amount of time at:
http://www.apollocanard.com/4_blended%20winglets.htm

6-11" radius seems reasonable for interference drag reduction and if I look at the JS-1C 3-view it seems to have a winglet transition with around 8-10 cm radius (and a smaller chord than the apollo canard).

That doesn't explain the OP question about polyhedral. The JS-1C 3-view appears to show polyhedral breaks at 6, 7.5 and 9.5 meters with radii of on the order of 5-10 meters, not centimeters, so it would appear two 50-100 times too big to be of practical use in reducing interference drag, plus the reduction of the included angle between the wing tip and the winglet is too minor that this is the reason when you can reduce the interference drag simply by upping the transition radius to the winglet by a centimeter or two.

I remain skeptical that polyhedral in these modern wings is motivated by winglet aerodynamics or a drive for meaningful induced drag reduction. At least the arguments posted so far are not compelling on the point.

But I have a high need to verify things...

9B

On Tue, 19 May 2015 08:33:29 -0700, Andy Blackburn wrote:

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.

They're not at all common, and don't seem to help a lot in F1ABC class
models and are not commonly used. There's have been a few F1Cs using t5hem
but that's about it: I can't thing of an A or B that tried them, but they
probably don't add much in RN number range (40,000 - 100,000). What does
seem to work is the Hoerner tip in conjunction with a swept LE on the
outermost wing panels. as well as the effects Schueman described, fairing
the upper surface so it meets the lower surface at an acute angle, raking
the tip at least 30 degrees and making it meet the TE at a sharp point
all helps to reduce the tip vortex, maybe moves it outboard a bit and
certainly stabilises its location, all of which seem to help thermalling
performance and certainly don't hinder the model's ability to self centre
in thermals.

I notice that the F5B and F5F boys are using similar wing planforms to
current FF models, i.e usinh Hoerner tips rather than winglets and are
using them at a very much higher RN, combined with interesting low drag
aerofoils that differ radically from anything I've seen on either F1ABC
models or our bigger toys. They obviously glide and thermal well and do
so at surprisingly high airspeeds.


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).

That's interesting. Maybe, if the class rules impose a span limit its
better to accept the additional surface drag of the vertical tiplet
because of the extra area you get from the blunter tip?



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