Need recommendations on dihedral angle
 

I am starting a new glider project and would appreciate recommendations or opinions on the dihedral angle. I am exploring two options:

1: Single dihedral break at the center (shown on plans)
2: Two breaks with a flat center panel (slight dihedral break at tip panel giving a 71" center section, flat, with a 35.5" tip section, this is where the break would be)

The kit wing comes in four sections, left and right center section and left and right tips. All sections measure 35.5" in length
Wing specs are as follows:

WS: 142" Plans show 78mm rise from center to tip PER SIDE, 156mm total for dihedral
Root Chord: 9.5"
Tip Chord: 4.5"
Straight TE with the typical LE taper

Sailplane is full function Ail/Flap and V-tail

If I go stock I will build it with the angle shown on plans. If I build the straight center section and break at the tips, what angle would you folks suggest?

Thank you all in advance!

RE: Need recommendations on dihedral angle
 

Tip dihedral is more effective than straight, so you could divide the single value for each side so that the total is the same, some middle section dihedral, and the rest at the tip, or raise the middle section some, and the tips some, but the total at the tip being less than it would if the wing were straight-dihedralled.
The ratio of middle to tip... what looks good.
I think this is more art than science.

RE: Need recommendations on dihedral angle
 

You are most likely 100% correct on it being "more art than science". Guess I was just hoping someone would dazzle me with some hard to decipher formula. I was thinking along the same lines as you suggested but feel better hearing it from someone else.

Thank you Tall Paul

RE: Need recommendations on dihedral angle
 

What do you want the dihedral to do? That must be answered first. A free flight sailplane must be able to recover from any upset so requires a good bit of dihedral. One with aileron control needs hardly any.

Since it is a kit, it’s not unreasonable to assume that the designer specified the dihedral that gives the best compromise between control response and lateral stability. You can’t go wrong building it as designed.

Dihedral is simply a way to make an airplane roll away from a sideslip. It’s essential for free flight, necessary for rudder only control, and a handling quality thing for aircraft with direct roll control (ailerons).

Whether to use straight dihedral or break the wing in several places is largely a matter of art. Whatever looks good to you.

RE: Need recommendations on dihedral angle
 

Polyhedral is normally considered to be more aerodynamically efficient (read: less drag) than simple dihedral, since it is a closer approach to the ultimate, an elliptically dihedralled wing, which looks just gorgeous, but is tricky to build but warps are harder to detect and remove.

RE: Need recommendations on dihedral angle
 

To LouW,

Thank you for the input. I am pretty familiar with basic aerodynamics and know what dihedral is for. Guess I needed to be more specific in what I am trying to do. I know I can't go wrong with factory recommendations but what I am wondering is that if I go with the two midpoint breaks would I best be served by approximating the factory "total" dihedral, less or more? In the past I have usually gone with factory or less angle and have always been pleased with the handling. On the other hand this is my first glider that would have the breaks about mid panel of each side as opposed to say a Falcon 880 or that series that had more of a turned up tip. I tend to prefer my sailplanes to be on the shy side of dihedral, not looking for "super-stable".

RE: Need recommendations on dihedral angle
 

Sorry Phil but I'm going to call foul on this one. The angle break in the wing causes a lot of drag since it's generally accepted that the air flow over the wing has a spanwise component. For straight and level flying the dihedral breaks add drag by causing turbulence around the joints.

However when it's time to turn then it MAY be a different story. The tips being angled more means the rolling couple is stronger than with a simple V wing for the same amount of yaw. Because of this you do not need to yaw as far to roll into the desired turn. It would need to be proven through testing but I suspect that under turning conditions where the model is rolling into or out of the turn then a tip or poly wing would have less drag than a V wing. But for everywhere else the simple V wins the drag war.

Sean, if you plan on sticking with the aileron funcition on this model you don't need any dihedral but if you keep the single V angle as shown it'll produce an easier to fly model when it's out at the limits of your vision and you're flying mostly on instinct and autopilot. I vote for sticking to the original V so the wing does not have even a slight dihedral break to cause any disturbance in the airflow.

RE: Need recommendations on dihedral angle
 

Most FAI free flight sailplanes (F1A, F1B, F1H... ) these days use a flat center panel with raised tips. Dihedral at the tips is usually 15-20 degrees.

I think one reason polyhedral is thought to be more "efficient" than dihedral is that the angle at each break will be smaller, so the added turbulence should be less. However, there are more breaks... so who knows. I have seen several models with 3 or more polyhedral "breaks," although I have never bothered to build one like that.

My F1B model box is built to fit a single break at 15deg, so they all get built that way. ;)

Personally, I think a single break at the fuselage looks ugly. I much prefer a flat center section or a 3-break wing.

-David