I had (have) 7.5 and upgraded to RF8 and purchased it with the controller because my old controller was well used. As it turns out, I dislike the layout of the new controller. For one, they put the flaps on a 3-position switch, and it requires removing my hand from the stick to activate it. The older controller had the flaps on a rotary knob, well within reach of a finger while my thumb was on the stick and also it was proportional.
I had to program the new controller to put the flaps on one of the slide switches. It works, but it is sort of upside down in operation. Not intuitive although I have gotten used to it.
Just my $0.02
My transmitter has a pot knob on the top right corner -- pretty small knob (maybe 3.8" in dia). Is this the knob you are speaking about?