As usual, having a pause for thought has resulted in a plausible cause for the very different tests results between yesterday and today.
Both days I was using the cheaper, lower powered motors with plastic props. The only difference was yesterday the battery was charged to perhaps 11.5V – today nearer to 12.2V.
Imagine that the combination of heavier plastic blades (rather than carbon) combined with higher battery charge level creates sufficient angular momentum in the blades that the motor rotor overshoots the motor stator coils it’s supposed to just reach and instead it continues on to the next set. Depending on the number of coils, that would yield a signifiant increase in RPM and therefore lift.
I’d never had problems with the carbon blades; being so light they simply don’t carry anything like as much angular momentum. And the motors they’re attached to have higher Kv value (980 vs 920) and much stronger magnets meaning they had a tighter grip on these lighter blades. The result: lower angular momentum and closer control leading to much reduced risk of overshoot beyond the expected stator coil.
But in these tests, with heavier props and lower powered motors, it seems the motors simply do not have quite enough oomph to restrain the props angular momentum.
Convincing enough for me; time to move back to the better motors and carbon blades.