bellcrank and leadouts
Glad to have engineers in on this. In flight, the lines form an accelerated catenary due to their air drag. For the non-math types, a catenary curve is the form of a rope suspended at both ends. Suspension bridges, okay?
But, for that example, the 'load' is simply the 'rope's' weight. Added loads shift the bottom of the sag. Or not, if the expected load is designed for...
The air drag on our flying lines depends on their velocity - actually V^2 - and the drag of their shape. as the model turns one lap, the lines near the handle have practically no velocity. Out at the wingtip they fly the same speed as the model. Instead of a symmetrical sag, the sag grows larger (with the increasing drag load) on the way to the wingtip.
Line pull, at neutral elevator, is balanced between the two lines. You can think of pull as one force acting along the sag as it reaches the leadout guides. Maneuver loads change this only temporarily and the center of pull is still between the two lines.
If you want to reduce friction and bending of the leadouts, this mid-line - at neutral - should point at the bellcrank pivot at the CG and the leadout guides should aim the pull leadouts so they don't rub on the guides. Not totally possible, but we can come close... This is the only benefit of placing the bellcrank pivot at a certain location. We can tolerate some slight rubbing in the guides and bending of flex cable leadouts if we can't fit the bellcrank perfectly.
The sag can be calculated by the line diameter, by how many lines and by the speed at the leadout guides. The pull "axis" between the leadouts at the CG is usually about 3° to 5° behind a right angle line in the fuselage at the CG out towards the inboard wingtip. Having the CG a bit forward of the quarter chord does help keep the nose out.
With modern designs, not much is needed. Excess engine out thrust (angle out from fuselage centerline) can cause problems. Same for excess right rudder (in normal counter clockwise flight.) Too much right yaw is draggy; forces the fuselage out of best alignment to the air it flies through. For a model that should stunt, that can mess up sharp corners.
Hinging? When the rudder and/or leadout to CG conditions - or both - are far off, the model is forced into a definite yaw position. Drag from a sharp turn can pull the lines straight,.rapidly removing the yaw. That moves one wingtip faster and slows the other. Lift also goes by the square of speed, so that causes a big difference in lift between the two halves of the span. The faster panel lifts more; the lower one less. Works like ailerons! CL doesn't (usually) need ailerons.
In a bad hinging situation, fliers have seen the "more lift" wing almost in plan view. If it puts the model that much 'out of shape' (clean and streamlined to the air it flies through) imagine how much it affects stability, controllability and the flier's nerves!
This is usually more problem when trying sharp corners... It really helps to have all the big forces pass through a single point - the fore/aft, spanwise and vertical CG. Then they don't have lever arms around the CG to cause direction changes. Again - impossible to have perfect for all speeds and conditions, but close is very good.