It is late and I haven't time to ensure that I have double-checked the science bit so in simple terms:
when I built my aerobatic biplane I installed a turn and slip (needle & slip ball) because:-
THE NEEDLE INDICATOR ALWAYS GIVES ME DIRECTION OF YAW - whether erect or inverted.
That is the important bit.
Having experienced most spins - I can recall occasionally checking the needle to ensure that it agrees with the known facts BUT I guess I would have to be totally disorientated to have to use that as my primary recovery tool. I spin regularly but the :-
Yawing to the left/rolling left (erect)
yawing left but rolling right (inverted)
discussions really confuse me - perhaps I am not spatially aware enough to comprehend them whilst in the classroom.
It just makes more sense when it is happening!
This year's BAeA sequences higher than beginners all incorporate spins of increasing complexity - I have tried them all and can not recollect ever having time to peruse the turn&slip!!
GET SOME QUALITY TRG.
Stik