H/Handle, Great start on the idea of the catenary curve...

Suspension bridges and telephone or power lines don't usually face a uniform increase in wind speed from one pole to the next. That, however, is the standard case for our flying lines. Speed is distance divided by time, right? Miles per hour... feet per second... We don't have a simple catenary curve, we have an accelerated catenary. That fits with Howard's comment that the "bottom of the belly" (sounds anatomical!) of the lines - er, try it this way - the point where the lines are furthest back from the line-of-sight from handle to model is about 2/3 out from the handle (40/60 is 2/3, right?).

We also have to deal with one other problem: Speed changes uniflormly from the handle to the model, but drag changes with the square of the speed.

Lift and drag - aerodynamically -calculate from the square of velocity. If the model is flying, say, 60 miles per hour, the point halfway out the lines to the model, obviously, is "flying" (moving, anyway) 30 miles per hour. Full distance to the model is twice that half distance. Well, duhhh...

If the lines could lift, the lift at the model would be four times its value at the halfway point on the lines. Same way with the air drag on the lines. The model is twice as far as at the halfway point, so is going twice as fast. Twice squared is four times as much...

So, the angle the pull force aims when it gets to the leadouts is affected, too. Their curve - if you could look down on the lines from above during a flight - tightens as you get nearer the leadouts. The natural state is that the CG will align to that pull force direction, as aimed through the leadout guides. IMHO, the leadouts should be lined up so they practically float free inside the guides in level or straight flight. Any change in direction, whether we add handle input, or the model rolls or yaws for whatever reason, moves the leadouts some, at least. That aims the pull force away from the CG, which tries to get back in alignment.

Some studies I've played with suggest that this is a very powerful stabilizing factor for our models. That's why I'd like it aimed RIGHT for simple flight. That's the center of the different moves we can make in maneuvers and altitudes. With any luck at all, we'd have about the same range of deviation from simple CG-aligned flight for outside and inside turns. I like that.