A circular model with central dihedral, the simple Gymball flies well.. It has lots of stability because of its high dihedral, yet is highly aerobatic because it has large control surfaces. Depending on the amount of throw used the model is flyable for the full range of modellers, from beginners to expert. The Gymball project was the conclusion of using the circular (nutball) shape, loads of dihedral, and big control surfaces. Centre dihedral is used so that the elevons can be cut full width, going out to the tips. Control uses four channels, for the ESC, Rudder and the Elevons.
The build is ultra simple
Only a few parts are needed, 1- the main disc, 2- the fin and rudder, 3- a two-layer lower spline, 4- a triangular three layer motor mounting block with a thin ply plate for fixing the motor
This is the standard indoor version at an 18" span. My one was made from 8mm EPP.
For a larger version, I recommend scaling up to 26" and using a KFm2 build.
The KF top layer is fitted going back to the 50% position on the circle. But first set the dihedral..
For easy fitting, this layer is cut in half and then laminated to the main circle layer once the main layer has the dihedral set..
Construction for all sizes.
Cut the circle. Draw lines down the centre and crosswise. Slot part depth, down the centre line, so the wing can be folded down to the correct 20-degree dihedral angle. Using a gauge fold the wing down by 20 degrees and fill the centre line crack with hot melt (low temperature) glue.. the dihedral is then set. Now is the time to add the pallet strapping or laminating film spar. The strapping is glued using UHU across at the centre line , mirrored both above and below the wing . The strapping was used because it goes nicely across the dihedral break, and when fitted makes for a stiff wing.. that is easier than joining a CF strip.. see the pictures
Cut three triangular motor block pieces. They are later hot glued on to the front of the spline with just a touch of right thrust. 2 to 3 degrees. These will take an undercarriage and a ply motor plate.
Cut two lengths of spline, laminate these together and cut the front end at a 5 degrees down angle . this will set the down thrust. The spline is glued under the wing, but remember to leave the front with a space where the motor block layers can be fitted. Glue the spline in place with UHU and fit the motor block layers with hot glue. fillet the spline joints with hot glue. The motor mount layers are glued individually with 3 degrees of right thrust added. A thin ply plate is hot glued to the front to take the motor.
The simple X style U/C made from CF rods lashed together and CA'd is hot glued in place. Make sure the axle is on square set at 90 to the line of flight.
The fin and rudder are fitted on top using UHU, filleted with hot glue. A CF reinforcing rod is put down the front of the fin, glued in place, it goes through the wing into the spline.
The COG is positioned at 1/4 back on the main wing, whatever size circle you cut. This is a safe spot.
There is additional info on the RC Groups site.