Polyhedral rules

[Bohh33]

Hi all,

I'm looking to make a glider with polyhedral using my foam cutter. I was wondering if there are any rules of thumb to be followed regards to how long the polyhedral tips should be and how far they should be angled?

Thanks!

[nmasters]

You should be able to learn what you need somewhere in this search http://www.google.com/search?hl=en&q=drela+%22Equivalent+Dihedral+Angle+%22

[gyrocptr]

Here's a "rule-of-thumb" from free-flight.
Dihedral-break at wing-center, and an outboard dihedral-break at 2/3 of each of the wing half-spans, as measured from the wing-center.
Decide what your total dihedral angle will be. (e.g. Common dihedral is 6 degrees in each side of the wing.)
Place 1/3 of the total dihedral at the wing center. Place 2/3 of the total dihedral at each outboard dihedral-break. (e.g. if the total dihedral in each half-span is 6 degrees: center-wing dihedral will be 2 degrees on each side of the wing-center; outboard dihedral-break will be an additional 4 degrees.)
My understanding is that the idea behind this configuration is to approximate an elliptical, dihedral-shape.

[Bohh33]

Great! Thanks a lot guys. That's exactly the type of information I was looking for.

Thanks a lot!

[BMatthews]

I've seen lots of variations in both directions and they all work well. Just try to make it so it is "visually pleasing" and it'll be fine.

Although if you were to look at a half ellipse and set the panels so that they sort of follow the ellipse as well as is practical you'll likely end up with a visually pleasing ratio of inner to outer panels and angles that looks a little nicer and maybe works a little better than some other options.

[Texas Buzzard]

Eliptical Wings are for Spitfires. Tapered wings are not for models.

Sure you can get away with those pointy wings if performance is not primary.

Yes, they look good and sometimes are pretty, but look at the winners over the last 40 years.
.................................................. ................
If someone has been using the word eliptical to describe dihedral then that is another story. There have been very few model aircraft built with an eliptical dihedral. If one does build a truely elipticalized dihedral then the spars, T.E. and L.E. have to be laminated. This makes for a strong set of spars alright!
Once I built a wing this way, but for the extra work I could not see an advantage when the bird was in the air. It just made me "feel" good to do it.

Go back and take a look at what Goldberg and Ron St. Jean did in FF with normal FF wings. Like has been said before, "There is no magic bullet". Follow what has worked and build without warps. What applies to full-scale doesn't always apply to low Rn models.

[Brandano]

Oh, performance wise the elliptical wings are ideal. The British planes used elliptical wings to get their marvelous speed, and once the Germans sniffed it out they started rounding the wingtips on their planes pretty quickly. The lift distribution along the spar on an elliptical wing is close to the best to obtain the ideal LD ratio. What suffers is the handling. Both elliptical and strongly tapered wings tend to have harsh asymmetrical stalls, that are usually cured by adding washout (and therefore adding back in the drag that the wing platform had taken away).

[BMatthews]

Um.... elliptical dihedral? Not wing planforms.... :D

[Texas Buzzard]

Um.... elliptical dihedral? Not wing planforms.... :D
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>

Do you remember a F.F. (Class B) named the "SPOOK"and Bill Winter's "Wog"?a

Beautiful planes were built and admired. The Spook had an odd dihedral. Coming from the fuse the wing was turned up at a 45 degree angle for a short way then on to polydihedral.

Can't think of the model that appeared in MAN that require the building board to be bent to that eliptical curve so the laminated spars could be built. Yes that Class A FF had eliptical dihedral. The year may have been 1960. I thot that was cool so I built ONE 60" eliptically curved dihedral wing. It wasn't any better than the wing of the Zeek.

I guess you are talking about the planform of the wing. To me that means how it looks when viewed from above.

I am talking about how the wing looks from head-on. In the late 1960's there was a famous Glider made ready to fly ( sold for a high price) that had eliptical dihedreal. It was very popular - sorry but can't call the name. Slopers liked it a lot.

[mnowell129]

I am talking about how the wing looks from head-on. In the late 1960's there was a famous Glider made ready to fly ( sold for a high price) that had eliptical dihedreal. It was very popular - sorry but can't call the name. Slopers liked it a lot.
Hobie Hawk

[MCarlton]

I made an HLG with elliptical dihedral once, it was a bit 'orrible, somehow it always felt like it was sliding around in turns and not "locking in", a bit like dutch roll somehow. All a bit spongy feeling and needed careful application of opposite rudder coming out of a turn not to go "wallowing" off in a drunken looking waltz

[JetPlaneFlyer]

Here is a link to a spread sheet produced by a very clever guy called John Barker: http://www.fileden.com/files/2006/11/11/368005/Dihedral.xls

It analyses two panel and three panel polyhedral wings and gives the equivalent ‘V’ dihedral angle plus the vertical lift component for the polyhedral and ‘V’ dihedral layouts.

Generally polyhedral produces one percent or less increase in vertical lift component compared to the equivalent ‘V’ dihedral. This theoretical gain in efficiency could quite easily be lost due to increased weight and increased interference drag of the cranked wing. Suffice to say real world gains in efficiency will be very small to non existant.

Steve