I wanted to mention something about CG, trim and airfoils I haven't seen mentioned in regards to the SO.

The SO is designed for a Clark Y airfoil. The Clark y airfoil is called a flat bottomed airfoil, but it is only flat in its rear section. The forward section lifts off of the bottom line.

If you draw a line through the furthest forward part of the leading edge to the furthest aft point on the trailing edge of a Clark Y, it will form a line that is angled up compared to the bottom flat part.

This is the "chord" line. The angle that this line makes with the fuselage centerline is called the "angle of incidence".

For a Clark Y when used on a SO type plane the angle of incidence is a positive amount -- a few degrees above horizontal.

A couple of things -- some builders don't cut a true Clark Y -- they just cut the top surface and leave the bottom flat. This will lower the angle of incidence of the wing to zero degrees. Such a plane will need to fly faster or, at a higher angle to the line of flight to provide the same amount of lift. It will likely therefore either require more elevator up trim, or a more rearward center of gravity to maintain this angle in flight.

A solution to this for true flat bottomed airfoils would be to mount the wing a few degrees positive on the fuselage -- this will reduce the amount of elevator trim needed. A small wedge set to the proper angle of incidence under the wing would trim the plane better.

I have found that even with the 15% Clark Y I use, that some up elevator trim is needed for level flight.

Another point for those, who, like me cut a true Clark Y:

Because the wing is actually already at a positive angle, the location of the CG is not accurately determined by measuring from a vertical line dropped from the leading edge to the bottom of the plane.

From a practical standpoint, it can be done this way if the measurement is in inches, not a chord percentage, and supplied by the designer -- he has corrected for the difference.

But if a precentage of wing chord is specified, it would actually need to be measured along a line parallel with the wing chord line. Since the botom of the fuselage isn't parallel, to the chord -- it isn't accurate.

The practical side of this on the SO is that planes with a true Clark Y will fly with a more forward CG location, in inches measured along the bottom, than a true flat bottomed foil will. The difference could be a fair amount -- and this explains why different people will find their SO planes fly with different CG locations (measured along the bottom).

I hope this solves the mystery of different results by different builders on the same type of plane.