Step Turn,
Thanks for that very illuminating post. Much food for thought there.
I would like to re-emphasize what you said about the aft CG limit. It is a flight condition that pilots should treat with great respect, particularly since most pilots are probably not very familiar with it. I have many hours in four-seat Cessnas, but I have only had four passengers on two flights, so aft CG in Cessnas is not a condition that I have much experience with.
When I was learning to fly on RAF Chipmunks, weight and balance was not even discussed. The assumption was that no matter who was in the seats, you were within limits. I did multiple-turn spins both dual and solo, but unfortunately I was not sufficiently experienced in those days to notice if there was a difference in spin recovery between dual and solo flights.
However, when I became a glider instructor and did a lot of spin-training with students, I became very aware of the different spin characteristics, depending on the weight of the student and hence the CG location. Most of my spinning was done in Blaniks - a medium performance and relatively draggy glider. In the Blanik, like most two-seat gliders, the instructor is sitting quite close to the CG, so changes in weight in the rear cockpit do not have much of an impact on the CG locations. The weight of the student, being much further forward, has a profound effect on the CG location.
With heavy students and a forward CG location, the Blanik was often reluctant to spin and would often transition to a spiral dive, particularly if the student wasn't holding the stick right on the rear stop.
"Er, see the airspeed increasing! We are unstalled! This is a spiral dive! I HAVE CONTROL!"
On the other hand, with lightweight students, the spin entry was easy and the spin was very stable. Some years ago, my club had a "low mass" female CFI, so during our Spring Checkouts, she would sit in the front seat. The CG location was close to the rear limit and the aircraft flew very differently - much more "twitchy" and as I said, entered and recovered from spins very nicely.
This year, after a eight year absence, I am back in Canada and will be flying much more modern two-seaters this year - ASK-21, DG-1000 and DuoDiscus. I'm looking forward to seeing how they spin and more importantly, how they recover. Being very low-drag, they accelerate very quickly when pointed downward.