Dr.havoc

What are polyhedral wings? What makes the different then normal wings?

marcellus

A polyhedral wing has dihedral along it's length, usually about 2/3rds of the way towards the tip. The tip section is angled up.
They tend to be more stable in flight than a straight dihedral wing and turn with rudder only in flatter circles and are generally sensitive to rudder input. They are often used for thermal soarers, enabling flat and gentle turns. A Gentle Lady is a good example.
A polyhedral sailplane is not usually suitable for anything but elevator driven aerobatics or simple rudder initiated stall turns.

BMatthews

In purest terms the poly part of the name means many. So if you look at a polyhedral wing you'll see that there's typically three or more panels with dihedral angle joints in the wing. This is in addition to any short center section area that is just used to let the wing sit flat on the fuselage. In such cases this short center section is considered to be just one break.

There used to be special names to describe various sorts of dihedral. Typicaly V dihedral is just one joint or a short center section in the middle. Tip dihedral was generally considered to be a flat center with angled up tip panels. Polyhedral was then left to describe wings with a V dihedral in the center two panels and additional upturned tips for outer panels. These days it's not uncommon to see 5 panel wings with a flat center, middle dihedralled panels and outer tip panels as well.

Dihedral of any sort works hand in hand with rudder control to roll the model into the turn when there are no ailerons. It also helps to restore the model to level flight when the rudder control is neutralized.

Why polyhedral rather than simpler to build V dihedral? By moving the majority of the dihedral out to the outer panel area you get more leverage for the dihedral effect to work with so you don't need as much dihedral overall. It also leaves the flatter inner panels in hopefully a better attitude to generate more lift upwards than off to the side as you get with higher dihedral angles. But along with this separating of lift duties comes some additional turbulence and drag at the angle joints. So like most things in life it's a tradeoff.

Contrary to Marcellus' post it does not make the model turn flatter than without dihedral. Any model of a given size and weight and wing area needs the same bank angle to turn at a given turn radius regardless of dihedral or not. If it does not bank that much then it's side slipping and creating lots of drag. Also if you get enough speed up or have an electric motor up front it's quite possible to do nice barrel rolls using rudder and elevator in conjunction with the dihedral. So it's not just for flying upright. Sorry for correcting you Marcellus.

marcellus

Bruce - I stand corrected!!

It might have been better if I'd said that polyhedrals are not designed with roll axis aerobatics in mind. My polyhedral HLG will barrel roll without a motor up front but I don't think it would win an aerobatic comp!

Without getting into aerodynamics, which I am not able to argue, the poly tips give you some built in bank and in practice a poly will give you nice flat turns a lot easier than a dihedral ship.

rscarawa

Dihedral and Polyhedrals main purpose in RE gliders is to enable the machine to roll. Without it, there would be 0 roll control for turning or returning to level flight (roll axis wise).


Scot

BMatthews

This is very true. The rolling action from the dihedral is further out on the tips and due to the higher angles it is also much stronger a force for a given yaw angle. The stronger force acting over a longer leverage arm makes the model roll into the turn far quicker or with less drag inducing yaw than with a simple V dihedral wing that has the same overall dihedral amount at the tips. This is not to say that a simple V dihedral model won't roll fast. It will. But it takes a lot more yaw angle to make it respond with the same roll rate. And that yaw is creating a LOT of drag.