So, as far as useful load on an aircraft goes--how do gliders play into this? I would assume that they don't count pound-for-pound towards the useful load, but they must hurt t/o performance. How do you go about combining this with useful load, etc to figure out whether or not you're over capacity?
It's already been cleared up, I guess, but I'm surprised you didn't figure it out on your own. If you just watch an aerotow in progress, it becomes real obvious.
While the glider is rolling, the towplane is definitely dealing with additional weight- but only as a tractor, not as an airplane. Once the glider is flying, the glider's wings support its weight (the old trust/drag/lift/weight thing applies to gliders as well as power planes). Fortunately, most gliders will get airborne at significantly lower speeds than the towplanes, so the glider gets up first, allowing the towplane to accelerate better as it approaches flying speed.
Drag, however, is a factor throughout the launch (including the induced drag from the glider's wings lifting their load), so even with the glider flying, the towplane is going to be sacrificing a lot of thrust just to overcome the extra drag. So even the drag can still affect how much runway the towplane needs to get aloft, and its climb performance. And the induced drag will increase with the weight, enough to make a difference.
Having been in the 2-seater with other of various weights in the other seat, and solo, and in the single-seater, I can confirm that. The 200 ft AGL callout (minimum altitude to safely make a 180 back to the runway if the rope breaks or an emergency release is needed) comes a bit later and farther from the threshold when the glider is heavy.
So a tow pilot just has to have a good idea what will happen, and what that will mean based on runway length, density altitude, etc. A little math and a lot of common sense come into play.
For example, with our club's 150 hp Citabria and about 3000 feet of runway to work with, on a typically warm day during the soaring season, our tow pilots won't ever have a passenger in the back seat of the tow plane when towing the 2-seat glider with both seats occupied. They also don't top off the tow plane's tanks before a day of glider launches, preferring to keep the tow plane light and just monitor the fuel carefully. I's not like it would be impossible to go ahead with glider and towplane at their max gross weights... but it would be a stunt, not a normal launch.
I don't know if turbo or the other tow pilots could draw you an accurate chart based on weight, but they know what works, and how to add some safety padding.
The single- seater, on the other hand, produces much less drag on tow and weighs a few hundred pounds less at max gross, so launches of that glider are better opportunities to get a ride in the tow plane.