is there aerodynamicaly proven technique or for calculating dihedral, i m not asking "how to measure the dihedral of a given wing".??
Thanks

i had a short of discussion [link=http://www.indiarc.com/forum/index.php?topic=2890.0]discussion[/link]here which you can follow,
as you can see there my question is unanswered , so i thought i shud post my query to this father forum

thanks guys

Of course there is, but it may not be simple. There are several balancing of torques. For example, take the fin/rudder...

It is above the CG usually and so acts like half a wing. In sideslip, there would be a tendency to roll. A sideforce from the left may want to turn the nose to the left, but will cause a right roll. The effective angle of attack of the wing due to sideslip with dihedral is a simple formula,

angle of attack change = dihedral angle * sin ( yaw angle )

You would need to figure out the resulting rolling torque and balance it against the fin/rudder.

In a turn, there is also the effect of the outer wing travelling faster than the inner one. It's a small effect, but must be accounted for as well.

If your goal is to optimize the dihedral angle to obtain no yaw/roll coupling, a low wing layout normally requires about one degree or so of dihedral, and a shoulder wing layout a roughly equal amount of anhedral. This is because of the influence that the fuselage exerts on the wing when the airplane is yawed. For example, in a yaw to the right, the air pressure is higher on the left side of the fuselage, which tends to produce a downforce on the left wing, and an equal upforce on the right wing, causing a low wing airplane to tend to roll left. The result is adverse roll, which can be nullified by the correct dihedral, which depends on the depth of the fuselage and the wing shape. In a high wing layout, the rolling tendency is in the opposite direction, a proverse roll, in the same direction as the yaw, requiring anhedral to remove.

Getting the dihedral/anhedral exactly right is difficult. Dihedral is difficult to change after the airplane is built, unless it has strut-braced wings. Some competition aerobatic pilots, after flight testing, slightly bend the joiner tube between the panels of a two-piece wing, until all yaw/roll coupling is removed.

Harsh:

If you are into derivatives and calculus, you may find the mathematical response to your question here:

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

For a not so mathematical approach, listing the many factors involved:

http://en.wikipedia.org/wiki/Dihedral_(aircraft)

I would like to learn how did you arrive to the conclusion that you presented in the forum of reference regarding relation between dihedral and wing span.
It seems to be that it should be a direct relation, instead of inverse.

The amount you need is determined by the design and the number of control surfaces. Even then there's no one single "right" amount. Instead each model will do well with a rather broad range of dihedral angles.

A rudder-elevator glider requires a lot of dihedral and it's best located mostly at the tips. Hence the use of tip or polyhedral with many R-E only designs. With an R-E design the dihedral is not just there for stability but also to provide the yaw coupled rolling action. A higher amount of dihedral will result in a sharper rolling response to a given amount of rudder induced yaw angle. But put in too much and efficiency suffers. It's something that needs to be set mostly by trial and error to find the amount that works to give a reasonable rudder to roll response so the control inputs are rapid enough for the pilot.

On low wing stunt models the dihedral angle can avoid knife edge rolling to the top side of the model. The old Top Flite Contender with it's flat and low wing was bad for this. Many have written that adding just a mere 3/8 to 1/2 inch of dihedral per side would have taken this effect away. Still others have written that the rudder area was just too high and a lower centralizing of the rudder area would have corrected this trait as well.

Trainers use a lot more dihedral than sport designs because it provides more ability for the model to self right itself from a pilot induced disturbance. Free flight models use generous amounts to achieve a high degree of stability for not just stability but to aid with the power to glide transition and avoid loosing a lot of the height gained in the climb due to a bad transition.

From none to generous polyhedral. All of these are right and the option you choose is best determined by the type of model you are designing. But along with this vast range of correct values based on usage comes the fact that there's no one equation to find the "right" amount since each application has it's own correct amount.

In the end you're best to pick the angle by examination and copying proven successful designs of the same style. Or in the case of very minor amounts used in RC pattern competition designs to just plan on cutting and re-joining if you find you need to add some roll couple via dihedral where it's not convienient to mix it in with one of the computer transmitters.