E3D,

What determines which line does what depends on both the location of the elevator horn and the side of the bellcrank used to move the pushrod.

Jim Walker, probably, established the traditional layout, in which the pushrod connects outboard of the bellcrank pivot. In that case, if the elevator horn is above the elevator, pulling the front line causes UP elevator. Vice versa, if the elev horn is underneath, the front line causes DOWN elevator.

Downunder mentions gyroscopic precession effects. Think of a toy gyroscope. Spin it up and try to tilt the axis. It tries to turn as if it were pushed on the side 90° 'later' in the direction of flywheel rotation. The faster you try to tilt the axis, the more force it generates.

For trainers, or models less maneuverable than fun or contest stunters, this is usually no problem. For stunters with large, hefty props turning considerable RPM, often with fairly heavy spinners mounted, it can be a problem. The gyro effect - for standard rotation engines, flying counterclockwise when upright - UP gyro 'tends' to swing the nose out, and vice versa.

When we apply control motion at the handle, we want to overcome airloads on the control surfaces. That takes a shift of the pull force. In level flight, the lines each carry half the total pull. A sharp corner needs the loaded line to carry more force than the 'other' line. Half the shift is added to the 'pulled' line, and half eased off the 'other' line.

This shifts the point the 'middle' of the total pull force aims into the model. In level flight, it's like two kids who weigh the same on a seesaw, as against one heavy kid and one light kid. With unequal weight kids, the heavier kid sits closer to the pivot, and the lighter one further, than when the weights were equal.

So, with UP line forward, this pull shift moves forward, and partly counters the gyro tendency to turn the nose out. Works just as well on DOWN control - shifting rearward , opposite to the gyro tendency to swing the nose in...

Again, this is important for very maneuverable stunters, and much less so for planes that fly less demandingly. The complication is that to get UP-line forward, you DO have to either take the pushrod to the top of the elevator, or flip the bellcrank over so that the pushrod connects inboard of the pivot. Most stunt models keep the elev horn under the tail surfaces, so the bellcrank gets flipped over.