Hi Russell,

Your final result sounds about right. Note that the 30000+ RPM I
mentioned is at the motor shaft. There was a 6:1 reduction to the prop
shaft. Our full-throttle prop speed was about 5700 RPM. Note that this
is actually 80% more thrust (because thrust goes as rpm^2) and 2.5
times the power (as power goes as rpm^3). Actually, I think that the
electrical input power to the motor was about 400W at hover, which
would mean 1kW at full power. Our batteries would not allow full power
for very long - it was only used for maneuvers. This machine overall
weighed about 6.5 kg and could accelerate upward at about 1 G. Prop
tip speed at hover was 100 m/sec. It was very scary to be around - we
used a 1 cm thick polycarbonate police riot shield whenever we needed
to get close while it was hovering.

I looked up the APC 18x6W prop we used and it only weighed 136 grams.
However, there was also the motor rotor moment of inertia, which,
although much less on an absolute scale, gets multiplied by the gear
ratio squared (6*6=3D36). So, a 200g at 6 inch radius estimate is
probably fair.

The 1HP rating for the motor is not a peak power limit but rather a
thermal limit for typical flight durations. The motor is perfectly
capable of MUCH more (maybe 10x) peak power for very short periods of
time. Shorting the motor at a prop shaft speed of 4200RPM (25200 RPM
at the motor shaft) will generate several kilowatts of electrical
power in the initial stages of braking. To give you an idea of the
current involved in shorting this motor, in an earlier test, shorting
it through a 10 cm long, 2mm wide PCB trace caused the trace to
explode (several cm of it were completely gone from the PCB surface).
I had to add external wires to the board as a quick hack to beef up
the current handling ability.

Sean





On Tue, Jul 13, 2010 at 11:45 AM, RussellMc <apptechnz@gmail.com> wrote:
> 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 s=
top,
>> 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 =3D 6" and 0.5mV^2 =A0=3D adequate ene=
rgy 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 =3D 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 ey=
e.
>
> 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
>
> =A0 =A0 =A0 =A0 =A0 Russell
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