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