Model Forum / Radio Controlled / Air Models / June 2004

Go to the model motors web site and look at typical power draws, and use
the power conversion stuff taking 1bhp=750watts=.40 cu in as a rough guide.

The bigger axi 41 series - are .40 motor equivalent, the 2820 are in the
400 watt area,  and that equates to roughly .25-.36 sort of capacity.
The smallest 2808 is something like an 049.

Another way to get this all working is to calculate all up weight and go
for the old watts per lb rule.

Roughly 50 watts per pound will get you 'vintage' style flying. 100
watts per pound is first class sport flying, 150W per pound will net you
either hovering and 3d, *or* in excess of three times stall speed
velocities (bit not both on the same set-up) and is up there with decent
lightweight glo engines, but to match the sort of tuned racing IC
engines you need to go up over 200W/lb, which is just doable. We
reckoned a 200mph electric plane was just doable, but would be around a
thousand quid to build.

Finally, its very possible to run a motor effectively at many power
levels, so don't be misled into thinking 'this motor will be as good as
that I C engine'

In reality the power pack - especially in Nickel based - will be a far
more critical and heavy item.

Finally, its often possible by judicious tuning of pro motor and gearbox
and pack to extract a performance from an electric plane that would
simply not work on an IC plane. IC engines like to be in the 8-20K RPM
band, Electric motors like to be in the 10-50K RPM band. Propellors can
vary from just a few thousand RPM for a steep climbing but slow model,
to maybe 20K for an out an out racer. So gearboxes are almost mandatory
on many electric motors that don't peak until well into 30K rpm or more.

Once you have a box, you might as well go for optimum prop RPM, which
for a typical sport model tends to be between 5k and 15kRPM

On an IC engine the tendency would be to go for a much larger capacity 4
stroke swinging a prop that doesn't really let the motor develop full
power, but does have bags of low speed thrust.

Other differences between electric and IC are that IC throttles rather
badly, but electric - especially brushed - throttles down to extremely
low RPM, and all electric can be stopped and restarted in flight too.

Electric motors don't in general speed up as much as the prop unloads
either. This may mean that a combination that spins the same prop at the
same speed on the ground, may be slower in top speed than the IC
equivalent in flight.

Which is to say, its very hard to exactly duplicate IC performance
curves in electric models. In some ways you will be pleasantly
surprised, in other ways potentially disappointed.
The other aspect is the cost aspect. The lowest cost IC model tends to
be around 25-40 size, whereas the lowest cost electrics tend to be in
the 049-25 sort of size: To get a 40 type plane in the air is usually a
couple of hundred quid in power train for best performance. Whereas I am
getting very good sort of 15 sized plane with exhilarating performance
from a power train costing about £80. (cheap motor expensive batteries)

Down at 049 sorts of levels I can do well on a 25 quid pack of LIPOS, a
20 quid speed controller and a £5 motor with an £8 gearbox. Not sure how
that compares with a RC 049 engine, mount,  tank, and throttle servo,
but the fuel is cheap :=)