The root edges of the rudder fins have to be sanded at an angle.
Mark the flat root edges shading with the side of a soft pencil.
The picture on the right shows the angle sand about halfway through. The pencil shading gives a better contrast so you can sand the line evenly.
I'd recommend using a sanding block, like the Estes Ultimate Sanding Bar. To reach a 3/4" dihedral height, the sanding angle is not very steep.
On the right, you can see what's left of the pencil line. Sand until the pencil shading is gone - no further.
With a weight on one side of the rudder assembly,
Raise the other outside edge to 3/4".


































