Modifying the wing of a GP Spirit 2 Meter

[aeajr]

This thread is about modifying the polyhedral characteristics of the wing of a Great Planes Spirit 2 Meter.

CURRENT PLANE

I have a Great Planes Spirit 2 Meter sailplane that I have been flying since July. I like the plane very much and do not plan to make any changes to this plane other than adding the spoilers, which is almost finished.
http://www.greatplanes.com/airplanes/gpma1045.html


SECOND PLANE to be modified

I just purchased a kit to build a second Spirit, but I am thinking of making a modification and I would like to get your opinion of how it would change the flying characteristics of the plane. I ask that you not take this opportunity to suggest that I should have purchased a different plane. That is not the question and is not relevant to the discussion. However if there is another plane that has this kind of design, using it as a reference would be fine.

MODIFICATION PLANNED

The Spirit has a polyhedral wing made up of two wing halves. The two wing halves, each one meter or about 39.5 inches, are joined at the fuselage with a wing joiner that introduces the initial dihedral. Then, 23.5" out there is a break adding more dihedral for a 16" wing tip giving the polyhedral effect.

If I took the initial angle out of the wing joint giving me a flat wing at the fuselage and left in the dihedral at the wing tip, how would that impact the flying chrematistics of the plane?

Would it float better but be less stable?

Would it make the plane faster or slower?

Would it improve or hurt the wind penetration?

This is the only modification I have planned for the kit. However if you have a building suggestion or experience that would make the wing stronger for winch launches I would appreciate your advice here as well.

WHY?

The second Spirit is a learning project, taking a plane I already know, building it from a kit and making a modification that will give me two similar but different planes. I will have spoilers in both planes operated by servos in the wings.

MY FLYING BACKGROUND

I am very new to RC flying. I started in March with an Aerobird parkflyer. I really enjoy that plane and have about 100 flights on it. In July I purchased a RTF Spirit Select. I have about 50 hi-start launches on this plane and really enjoy this plane very much. My launches are confident and I am working on my thermaling technique.

Thanks for your help.

[Dave Robelen]

Hi,
First, the change in dihedral yu suggest would have no discernable effects on lift or drag. The model would float as well, and penetrate just as well as the original. The difference would come in two areas. Reducing the dihedral will make the model less stable in roll. This will show as a tendency to fall off on a wing in turbulence, and require more intensive piloting.
You do not mention if you are using the rudder as the primary control, but if so, a reduction of this magnitude would likely make the model very hard to control. In order to turn and bank with the rudder, the model requires dihedral to initiate a bank with a yaw. The design dihedral is generally carefully blended with the vertical tail area to give the desired amount of spiral stability. Reducing the dihedral and leaving the tail alone will have the effect of being very difficult to get the model out of a nose down turn with rudder.
Generally, sailplane pilots move to aileron control when the dihedral is reduced to compensate for the reduced effectiveness of the rudder. That original dihedral tends to act like an autopilot in roll, and can be very handy when you have the model high, and the bank angle is tough to see easily.
Cheers, Dave