electrics

[gwright]

i've seen several writings regarding electrics on this list and torque and
p-factor and noise, and many mention that it's all close to the same if you
spin the same prop the same rpms. One thing thats missing is that you
generally never do that with electric power. With gearing, you can use a
much larger and more efficient prop to turn the power into thrust. for
instance, I'm not totally sure what he's using now, but before Jason left
for the worlds, they'd settled on a 22X12 prop (comapared to the 18 or so
inch props used on the large glow engines). If you put the same horsepower
(watts) into a larger prop, you can get more thrust for a given input power,
and if you setup a system to turn the same prop at the same rpms as glow,
you usually end up with an innefficient electric setup requiring a LOT more
input power for the same output (output classified as thrust/pitch speed).
For an example, I'm flying an 88" span 80" long 3D plane that's 9 lbs, on
just under 1000 watts input power (equivalent to an OS32) but geared to spin
a 24X12 prop. that setup makes close to 15 lbs of thrust and flies at just
over 50mph at full bore. Pitch speed is actually a bit higher than desired,
but the 24X12 is the largest E-series prop at the moment from APC. I don't
think you'd ever consider an OS32 for a plane this size, so you see,
horsepower (1HP=753 watts) comparisons isn't really an apples to apples way
of comparing glow to electric. With a larger prop you can do more with less
input power.  Take a 40 size plane for instance with a .46 . you'd use
around an 11X6 prop at some very high revs with equivalent to about 1200
watts input power. with e-power, you generally end up with a 15X8, 15X10, or
something in that range on a "40 size conversion" with around 600 to 700
watts  of input power to get the same thrust and speed as you would with an
11x6 on the glow engine. Going to the larger prop and appropriate gearing
allows you to do the same job while only drawing about 1/2 the power from
the batteries that you would need if you setup a system to turn an 11X6 at
.46 glow type rpms. My funtana that's been mentioned before flies on a 22X12
with about 1800 watts right now (2.5 hp roughly) compared to the OS160 EFI
engine that's intended, spinning an 18X6 at considerably more horesepower
(watts) input. however, we have another one here with that exact glow setup
and the e-power one has FAR more thrust, and even a little more speed at
full throttle. another example of larger prop, less power going in, more
"power" (thrust/speed) coming out.


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