polyhedral - flat center section or angled?

[fprintf]

On my Highlander wing there is an angled center section and then the tips are upturned a little further still.

On other polyhedral gliders I have seen flat center sections with upturned tips, or even multiple outboard sections (ala BubbleDancer) at increasing angles.

As I will soon have a foam cutter completed I am going to start a little experimentation with wings for my Highlander fuse (either that or complete the Terminator I started a long time ago). What could I expect by making a flat center section and either single or multiple polyhedral breaks further out on the wing?

[boomerace]

Originally posted by fprintf
On my Highlander wing there is an angled center section and then the tips are upturned a little further still.

On other polyhedral gliders I have seen flat center sections with upturned tips, or even multiple outboard sections (ala BubbleDancer) at increasing angles.

As I will soon have a foam cutter completed I am going to start a little experimentation with wings for my Highlander fuse (either that or complete the Terminator I started a long time ago). What could I expect by making a flat center section and either single or multiple polyhedral breaks further out on the wing?

Polyhedral - Definition
Polyhedral is an adjective, describing an entity which has the form of a polyhedron. Polyhedron is a combination of the prefix poly ("many"), and suffix hedron ("solid with n-number of faces"). Therefore, a polyhedron is a solid with many faces.
Other than that I have always defined it as middle wing dihedral and mid wing dihedral breaks. Draw your own conclusions as any plane with mid wing breaks seems to be called poly!:confused:
boomer

[BMatthews]

You're quite right. By definition anything with more than one break is polyhedral. But the old books that I learned from broke this down into V dihedral, tip dihedral where the center is flat and the tips are angled up and poly dihedral or polyhedral where all the panels are angled up with the tips being more angled. And then we have the current flat center with mutiple panels on either side. I've just been calling these by the number of panels, ie: 5 panel polyhedral. I don't think anyone has a 7 panel poly wing yet. Technically this is still polyhedral but convention says that "polyhedral" is a 4 panel wing. Oh, and none of these takes into account the little center panel, if there is one, over the fuselage. These are just ignored in describing the dihedral.

[Daemon]

Hmm. I've always seen folks refer to anything with only one break as dihedral and more than one break, polyhedral. No more complex than that. So flat with a break on either wing, or three breaks total, both polyhedral.

ian

[BMatthews]

I think free flighters make more of a distinction about this since they use it more. Sort of like Eskimos have many more names for different types of snow than the rest of us. And since I have a strong free flight background......

[Hankstone]

Break out the geometry book. Dihedral is the angle at which two planes meet (geometric planes, not airplanes), poly means "many" i.e. more than one angle... I would think this applies to wings...

But I'm just an Accountant, what the heck do I know?

Hank

[stephen.s1]

Buck up Stuart, someone'll answer your question...
:)

[boomerace]

Originally posted by stephen.s1
Buck up Stuart, someone'll answer your question...
:)
Sorry but it's impossible to answer without knowing airfoil, wing cord and span, where the wing breaks are located and amount of dihedral, all up expected weight of plane........:rolleyes: And even with that info the answer isn't simple!
boomer

[Ollie]

Consider a wing of four equal panels and a uniform lift distribution neglecting tip losses. With gull dihedral and flat outer panels in one case and with flat center panels and equal dihedral in the outer panels. The rolling moment due to yaw would be three times greater in the second case than in the first case because of longer moment arm in the second case. This illustrates that dihedral is least effective at the center and most effective at the wing tips.

Dr. Drela has suggested that a good distribution of dihedral is one that is a piecewise linear aproximation of a circle.

Another consideration is that, the greater the angle of a panel to the horizontal, the smaller the angle of yaw that will produce a stall. So, there are conflicting objectives to be resolved. The geometry of a yawed and banked polyhedral wing in a turn is fairly complex and calculating the angle of attack at various stations along such a washed out wing can be a real challenge. However, such a computation is necessary to reveal the facts of how polyhedral will actually behave.

[boomerace]

Originally posted by Ollie
Consider a wing of four equal panels and a uniform lift distribution neglecting tip losses. With gull dihedral and flat outer panels in one case and with flat center panels and equal dihedral in the outer panels. The rolling moment due to yaw would be three times greater in the second case than in the first case because of longer moment arm in the second case. This illustrates that dihedral is least effective at the center and most effective at the wing tips.

Dr. Drela has suggested that a good distribution of dihedral is one that is a piecewise linear aproximation of a circle.

Another consideration is that, the greater the angle of a panel to the horizontal, the smaller the angle of yaw that will produce a stall. So, there are conflicting objectives to be resolved. The geometry of a yawed and banked polyhedral wing in a turn is fairly complex and calculating the angle of attack at various stations along such a washed out wing can be a real challenge. However, such a computation is necessary to reveal the facts of how polyhedral will actually behave.

elegantly put!:D
I usually just work it out to approximate a design that has been successful before and build and test the sucker! OR TLAR (that looks about right!)
boomer

[T. Lyttle]

Works, but I prefer SWAG myself...

Its good to look at what works for the type of flying you do; an A1 flies very slowly and is very sensitive to trim at that speed. Slopers want ailerons with little dihedral for obvious reasons. As far as thermal planes are concerned, your foam cutter will get a workout as you try dihedral polyhedral, flat center, center break, (for a challenge, try barrel; think Hobie here:D ); in order to keep your analysis correct, you will need a constant airfoil for all wings... oh, no, another can of worms...;)

[Daemon]

Or if you're really clever, you can make up a curved jig, and bend the wing into a curve when you wet it, lay the glass on and bag it instead of introducing any breaks at all. Seen it done with a DLG.

ian

[Ollie]

I think it is instructive to note that the Hobie Hawk, with elliptical dihedral required about three or four degrees of washout to avoid tip stalling entering low speed moderately banked turns. I atribute the need for this much washout to the very high dihedral angles at the tips. So much washout reduces high speed glide angle significantly because the tips are lifting down while the rest of the wing lifts up. This produces significant induced drag at high speeds. With the highly cambered airfoil the profile drag of the washed out tips at high speed was also very high indeed.

My point in this example is that there are trade offs to be considered and a balance between conflicting objectives must be found for best results. Unless you know the side effects of each design decision and are willing to accept them, it is best to stay with the narrow path of familiar configurations.

[fprintf]

Ok, thanks guys! I think I'll just build a couple different ones based on the TLAR theory - it is cheap enough once I get these cores just about right.