One other thing, the 9.6V battery pack you have is too big for that motor. The specs on your motor say that the operating voltage should be limited to 8.4V. As far as getting 15+ minute flight times this is dependant on the amp rating (not the voltage) of your battery, how you fly, and the aircraft itself.

You might consider dumping the NiMH in favor of a 2S1P Kokam 1500HD pack; but that is going to li-po technology and has a whole other set of costs associated with it. Otherwise, just follow what the other slow stick guys recommend for either NiCad or NiMH cells.