Extract from an article by Gene Beggs in Sport Aerobatics of October 1985:
"In my telephone conversation with Bill Kershner, he told me of a spin mode in his Cessna Aerobat from which he was unable to recover using my power-off, hands-off, opposite-rudder method of emergency spin recovery....…
If I placed the aircraft in a spin to the left with the power on as might happen with a student who did not correct for the “P” factor properly, and let it spin with the power on for two turns or more, and then cut the power off, released the yoke completely, and then applied full right rudder and held it, the aircraft would continue to spin and showed no sign of recovery evern after as many as 10 turns! I would then place the heel of my hand on the padded center portion of the yoke and briskly push it forward, and the aircraft would always recover promptly in one additional turn ....
I found that the Cessna 150 would spin very docile, as long as the ailerons were held neutral and the power completely off in either direction .. Trouble developed only in the spin to the left, either when the power was left on or when “in-spin” aileron was applied and the aircraft permitted to turn two turns or more ..."
Cessna produced a booklet in 1978 about spinning and much of that information is provided in this
article in Flight Magazine. Most of this info is also provided in Kershner's book, The Basic Aerobatic Manual.
I see that FAR 23 now (well, for many years) has another spin placard mandated but only newly certified airplanes will have it. Worth noting as it emphasizes a limitation on the number of turns in a spin based on what was tested.
[(d) For acrobatic category airplanes and utility category airplanes approved for spinning, there must be a placard in clear view of the pilot--
(1) Listing the control action for recovery from spinning maneuvers; and
(2) Stating that recovery must be initiated when spiral characteristics appear, or after not more than six turns or not more than any greater number of turns for which the airplane has been certificated.]
Aerobatic category airplanes are normally tested to 6 turns and with a comprehensive spin matrix of configurations and modes so your favourite video on Youtube doesn't count for much. i.e. the recommended maximum number of turns in a spin is 6 (there are physiological effects on the pilot which also support that same limit).
I'm certainly wary of spinning, especially flat spins but I do like instructor trainees to get a taste for how aeroplanes can suddenly misbehave as distinct from the standard spin entry.