http://ozreport.com/13.37#1

I wrote earlier about tails for flex wing gliders in relation to a tucking and tumbling incident at Manila on an Aeros glider. Aeros apparently has a number of gliders with tails at the DHV for testing. But the difference in sprog settings for the Combat L 13 07 with and without a tail is miniscule so it is unclear what the point of the tail would be. There is a little greater difference in sprog settings with and without the tail for the 09 version.

The argument against tails is that the wing tips are as far back as a tail would be given a normal keel length. Given that, there is no reason why you could expect a damping effect of a tail, that the outer wing area could not give you. I wrote earlier that you would need a longer keel.

You can see a long discussion of this here.