An erect spin is caused when positive alpha is exceeded and a wing slows sufficiently as not to cause lift.
An inverted spin occurs when negative alpha is exceeded and a wing slows sufficiently as not to cause lift.
So I can be upside down in attitude, pull the stick back TOO far, hoof in some rudder and I will be spinning errect.
I can be upside down and push TOO far, hoof in some rudder and be spinning inverted.
I can therefore be upright and push - voila inverted spin as critical -ve alpha exceeded.
Stik,
for the upside down, pull the stick too far + rudder spin, this will be an erect spin, but only for a split second as long as you continue upwards. Won't gravity come into play pretty quickly in any normal aircraft and then you'll end up in a yawed spiral dive downwards..ie not a spin or possibly a transition to an erect spin?
So for practical purposes the only spins that are sustainable are erect (stick back) when you are the correct way up and inverted (stick forward) when you are upside down?
Unless I have misunderstood something (not unknown).
ps, no inverted spins at all at Fenland this weekend.