Are stabilators effective for controlling roll and pitch for a plane with a high aspect ratio wing (i.e sailplane)? This question comes from an argument that my friend and I won't let go. My contention is that if the horizontal stabilizers are of adequate size and aspect ratio that their moment arm would be large enough to induce a lateral roll. He says no chance, to induce that kind of rolling moment, the tail would not be able to also produce the downforce necessary to keep the plane flying straight and level. Help!

The tail can do both, provide a rolling moment (but very modestly) and the down force.
A high aspect ratio tail is subject to flutter and excessive bending due to the down force.

On a more or less normal sailplane design the tail surfaces are far too short in span to be able to control the roll at all let alone effectively. There just is too little leverage to be able to control that long span of the wings.

Even on a regular sport power model they are not spaced out far enough to work decently. A fellow with a relatively low aspect ratio bipe wanted to put all this work into just the two tail surfaces and used a larger stab and elevators as ailevators. Apparaently it didn't work worth a hoot and he dumped it trying to make it fly This was a 30 inch span bip with darn near a 14 inch span elevator if that is any indication.

To even stand a chance of being effective your stabilators would need to have a span of at least about 50% of the wing's span. And that would be a pretty darn funny looking model.

A flying buddy and I are kicking a speed design around that uses (among several other things) all flying stabilators as the only control surfaces. Our design is optimized to manage and vector the pressure wave generated by the main wing and direct it to the stabilators, so we can make use of the higher density airflow. That being said, we then modeled it on realflight and found it only marginally controlable (even though the horizontal tail is nearly 50% of an already very short wingspan) due to the minimal roll response to control input. Now that is sort of what we expected, and the whole purpose of this plane is to drop out of the stratosphere and make a level speed run across the field. If it takes 1000 feet to make a 180 degree turn, that's OK with us, but would obviously be unacceptable for a sport plane. However, stabilators are used as compliments to the regular flight control surfaces on many, if not all, modern jet fighter designs, and could certainly ne used in conjunction with normal aileron function ti help improve roll rate.