Polyhedral? Dihedral? Anhedral?
 

Actually, I have found that most civic aircrafts are installed dihedral wing. But I am curious that is polyhedral the same as dihedral?

Besides, as anherdral can increase manoeurability of the aircrafts (mostly on military planes), how does it do it? What is the theory of increasing manoeurability?

Can anyone help me on this issue?

thanks;)

Most low-wing aircraft have some dihedral for roll stability. The 747-8 has a sort of continuous polyhedral, which seems like a sort of refinement of ordinary dihedral, and also gives more ground clearance for outboard engines.

High-wing aircraft probably do not need dihedral, and may in fact be too stable, so some anhedral may be used to counter that.

I know of one high-wing aircraft where the designer used a very small amount of dihedral for cosmetic reasons - no technical reason at all! :cool:

http://en.wikipedia.org/wiki/Dihedral

"An alternative to dihedral for the wing as a whole is to cant the wingtips or outer section of the wing upwards instead. This is called polyhedral and has the same effect. It is commonly seen on gliders, and some other aircraft."

Anne, Cath, Dai and Polly....

The 4 members of the Hedral family.

As I was told in about 1959!

Polyhedral's hardly new; the Blackburn Skua/Roc had upturned wingtips, if memory serves me.

It's not a question of "anhedral increasing manoeuvrability" - it's a question of designing the configuration to have the optimal balance of stability derivatives - one of which is Cl-beta, aka "dihedral effect", which unsurprisingly is affected by the dihedral/anhedral angle, among other things. For some aircraft anhedral may help, for others it would make something worse. As usual, the only real answer is "it depends..."

Can you say a bit more about "optimal balance" please? Is it a question of the ratio of various derivatives, or something more complex?

If you take the various derivatives and their contributions to things like the frequency and damping of the various aircraft 'modes' - Dutch Roll, Phugoid, etc., you'll find that with some small simplifications you can characterise most of the characteristics with relatively simple expressions (sometimes ratios, or difference, or whatever) of the derivatives.

Experience and HQ research shows that certain characteristics are favourable - much of the MIL-F-8785 requirements comes from that kind of background. So with a target range for, say, Dutch Roll Frequency, and knowing what derivatives dominate this parameter, I can derive desirable values (or ranges of values) for the derivatives.

The complication is that it's rare to be able to satisfy all the design requirements at once, even if all you consider is HQ, and if the context of the whole design is considered it becomes as always a matter of compromises.

For the specific of dihedral/anhedral, it can be shown that the ratio between dihedral effect Cl-beta and directional stability Cn-beta is important in determining both Dutch Roll and Spiral stability; depending on which is more important and on the directional stability, there will be an optimal value of Cl-beta, which may influence the choice of geometric dihedral/anhedral.