Depending on how you taper the wing it can have a positive drag reduction at higher angles of attack. The Schumann (sp?) planform works on model sailplanes very well .

Taper in the model airplane world has to be carefully moderated overall due more to the effects of reduction in the Reynolds numbers that occur when the wing uses a strong taper. Strong in this case being in the range of 60% tip chord. Smaller amounts of taper are not so damaging. Even larger taper ratios with the tips being at 50% or less of the root require strong additions of washout to help control tip stalling at lower speeds.

There's no real numbers on all this other than what you have already available as a student. It's lessons we have learned as modelers and passed on via magazine writeups and forums such as this. The lack of hard numerical analysis comes from the fact that no lives are riding on the outcome and there's no big savings in fuel costs either. We just do not fly in a steady state for long enough for any of that to matter. Now if you are designing a micro or at least mini UAV given the reference to a 9 inch chord then this may all change.

Analysys done by unremembered gentlemen that reported their analysis in the old Soartech journals published by Herk Stokely showed that for smaller model sizes the optimum aspect ratio alters radically from sailplanes at 120 inch span and sailplanes of 2 meter span. The reason being that it was more important from a speed range standpoint to keep the Reynolds number higher by not similarly reducing the wing chord than it was to minimize the induced drag by keeping the aspect ratio high on the smaller models. Aerodynamically the former solution may be better for a craft designed to cruise at a consistent speed for most of its life in the air but from a practical RC sailplane perspective the speed range was more important as long as the overall efficiency isn't compromised to a great extent.