Summarising tech spec:
> ... motor was about 1 HP, 30000+ RPM top speed
> 18-inch diameter, about 200 gram propeller
> via a 6:1 reinforced timing belt drive reduction. Once
> while it was spinning in a test stand at hover speed (4200 RPM),
> instantaneously commanded 0 RPM.
> short[ed] the motor.

> As soon as I hit the enter key... the prop came to an instant complete stop,
> accompanied by a loud thud/twang. No damage to motor, prop, timing
> belt, or pulleys. The delay between command and full stop was totally
> imperceptible to my brain. It was probably not as fast as this SawSafe
> unit, but it couldn't have taken more than a few 10s of milliseconds
> to stop the prop. It really is amazing what electric motors can do!

E&OE

Decided a quick order of magnitude check worthwhile - motor/rotor
braking is of interest.
Gross assumption: mean mass radius = 6" and 0.5mV^2  = adequate energy model.

Calculated and got ~~~ 640 Watt/second which at one HP would take
about 1 second to stop.
Puzzled.
Then I noted you'd said 4200 RPM hover rev and I'd used full speed !!!
Recalc at v^2 = 50 times less and get about 13 Watt-second.
Maybe 20 mS at 1 HP and instantaneous braking under short could be
above rated power at least for part of profile.
Anything under say 40 mS is liable to be close to instantaneous to the eye.

At 20 mS full stop that's still about 2 full turns - so SawStop
technology would be marginal to useless there.
Energy level is not Too bad - nasty injury possible.
At full throttle you'd be "in trouble".
600+ Joule is well above 38 Magnum energy :-(.

E&OE, again

           Russell
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