Post by k***@rhodesfamily.co.ukThanks Jim...
It makes a bit more sense now, esp with their motors, but Im still a
little confused...
I hear the terms 280, 370 and 400 etc with regards inrunner motors. Is
there any way of quickly converting between these brushed and brushless
motors?
Not exactly.
As I explained in the last post, its a can of worms that is a delight
for those who enjoy technical detail, and a right royal pain for those
who just want to bolt something on and fly it.
In *general* a motor of a given size and weight is *broadly* comparable
in power input and output to another similar sized motor. BUT outrunners
are multipole devices..they develop their power at lower RPM and have
more torque. So the prop choices will be very different unless the
inrunner is geared.
Also, efficiency makes a deal of difference. I think it was Matthew Orme
who said that the actual MOTOR is more equivalent to an IC CRANKSHAFT.
Its only PART of the solution. The pack is usually the heavier part, and
really its almost better to think of the pack as being the starting
point. Motors of high efficiency can develop enormous power for their
size as well. Since the limitation is heat in nearly all cases, a motor
that is 100% efficient can deliver any power you want without getting hot.
Now motors of up to 90% efficiency are definitely available. If you take
a given size at 90% efficiency, and compare it with a motor at - say -
70% efficiency which is about the 'average' the 90% efficient motor can
handle THREE TIMES the power of the 70% one..IF it has a big fat battery
pack to supply the juice.
In most of my models the pack is the dominant weight, and cost. I start
from there really.
The algorithm I would use with a model like the C130 is this
Find out the airframe weight and the wing area.
That enables a stab at power and stall speed to be obtained.
Now select a pack and 4 right power sort of motors and add that weight
in and see if it still makes sense. Going for about 60W/lb for a model
like that.
Being as how its big and slow, then find a gearbox and prop that draws
the right amount of power, and has a static pitch sped at least twice
stall speed..maybe 3 times. That will give a good flight pattern with
enough thrust to get it off the ground, and into a decent climb, but
still able to be throttled back for an efficient and scale like cruise.
I would use Motocalc (www.motocalc.com) to do these early guesstimations.
Once the power train is fairly well arrived at. I would get a range of
props and a meter, and confirm the power levels, and a tacho to confirm
RPM.
If all is within reasonable limits, I'd then fly the thing and try a few
'close' props in flight to find the best ones.
If it were me doing the conversion on a 4x .25 scale model, I would
actually choose cheap old fashioned brushed motors - probably speed 600
or car race motors (buggy motors) as you only need one cheap speed
controller - brushless motors need one per motor - and suitable deep
ratio gearboxes (probably 3:1 or 4:1)to get RPM down to about 4-6k and
then use a large diameter very coarse pitched propellor - probably a 4
blade Varioprop, somwhere in th 9-11" diameter and similar pitch area..
- to get somewhere near scale appearance.
Current is going to be massive..something like 25A per motor so the
speed controller will be around 100A capability, and those sorts pf
motors run well off about 12v, so I'd probably go for a 8000mAh 3s LIPO
pack (around 11v). Not a cheap item.
That should fly a model up to around 20lb AUW, though I'd say it would
be happier down around 15lb.
However it would NOT surprise me if the actual model couldn't be made a
lot lighter and run on a lot less. I hate seeing scale models of large
lumbering slow aircraft rushing around the sky like fighter planes and
sounding like them too. You don't need massive structures to mount
electric motors on, and neither do you need to build them like tanks.
And they fly much more realistically and float in to gentle landings if
you keep them light.. They can't of course fly in more than modest
winds, but that is the price you pay for scale appearances.
Post by k***@rhodesfamily.co.ukKarl
Post by Jim Slaughtergo to www.flyhurricane.com and look in the Forums. They have some really
good explanations of how to arrive at the correct size electric motor.
Post by k***@rhodesfamily.co.ukHi all,
Ive been modelling for years and used to work for a model engineering
company. I say this to show that I can model, but what I would like to
know most of all is what size electric motors replace what glow
engines?
I'm new to rc modelling and have decided to go down the electric route
as it seems to be cheaper and cleaner, but I dont know what size motors
to buy (all the buying will be done online) when the kits say the need
such and such a size glow engine? I'm looking at a small C130 which
needs 4 x .25 glow engines. A smaller version needs 4 x .051 glow
engines.
Does anyone have a conversion chart or formula that explains how to
come up with the right sized motor ?
Thanks