What fraction (percentage) of the control surface should one put forward of the hinge line for aerodynamic balance?

I seem to remember a number like 10%...

From : Don Stackhouse

That's a tricky one. It depends on a number of factors, such as the shapes of both the counterbalance and the control surface, and the amount of counterbalancing effect you want to achieve.

Probably the most straightforward case is when you want to move the entire hinge line aft of the leading edge of the control surface, such as the rudder on a DC-3. In that case the aerodynamic center of the control surface is at 25% of its chord, just like the case of an all-flying horizontal tail. If you put it there, you should have near zero force required to keep it deflected for moderate to large deflections. However, in this case we have a fixed flying surface ahead of it, so there is a centering force for small deflections, but that drops off once the leading edge of the control surface gets clear of the trailing edge of the surface in front of it.

In full-scale aircraft it's desirable to have some stick force required to move the control surface, to give the controls adequate "feel". Controls with zero centering force tend to be way too oversensitive. In some cases the underbalance the control surface, in others (such as the all-flying stabilator on many light aircraft) they hinge the tail panel at 25% chord, but add an "anti-servo tab" to the trailing edge, This deflects in the same direction as the stabilator but to a greater angle, so the forces on the tab tend to push the control surface back to neutral and provide the necessary stick force.

Figuring the counterbalance effect for a surface with a "spade" counterbalance (such as on the ailerons of many aerobatic aircraft) or an overhang (such as the rudder of a J-3 Cub) gets a lot trickier. You can try to figure the effective mean aerodynamic chord and aerodynamic center of the entire control surface, then make sure your hinge line is ahead of that. However, that method, tedious as it is, still doesn't include some of the weird 3-d flow effects present in a low aspect ratio shape such as a counterbalance, so you can still be subject to unpleasant surprises. In an R/C model the safest bet is often to just make sure you have a big enough servo, make the counterbalance small enough that you won't have to worry about the control surface trying to "snatch", and don't rely on the counterbalance for too much help.

Don Stackhouse
DJ Aerotech