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
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
Does anyone have a conversion chart or formula that explains how to
come up with the right sized motor ?
It makes a bit more sense now, esp with their motors, but Im still a
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
Jim Slaughter wrote:
Basixally it goes roughly like this
Take the engine capacity in cu inches, and multiply by 2000
That's about what WATTS you need.
Its crude, because a geared electrc motor running a big prop slow will
need less than that on s slow flying model, whereas to replicate a high
revving tuned pipe you will need more. but as a starring point thats a
fair rule of thumb.
So being that your C130 is fairly slow and probably WOULD use either
gears, or a slow revving outrunner, you wouldn't need 4x500 watt motors
on it, but more like 4x300W.
A better way to go is to use wttas per lb.
A tanky old C130 needs only about 50W/lb to lumber round the sky
successfully, so if its is say 15lb it would need a total of 750W, or
about 4x200W motors.
THEN you need a tool like Motocalc to work out what gears and props and
pack to use. Or ask someone else who knows.
The problem - or the advantage - of electric is that there are engines
and gearboxes that will operate efficiently over a MUCH wider range of
RPM than a Glo engine. This means that prop selection is non trivial.
For example. perhaps the motor I have used the most is the $5 speed 400.
That will run a range of props at around 90W in/65W out from - say - a
4x4 right up to a 11x11. Depending on how its geared. And what voltage
its used on.
With electric you cannnot think
get right motor
stick one of 4 props on it
You have to think in terms of the whole powertrain..prop,box, motor and
An interesting example for you..60" span WWI biplane. Scale prop is
Weight (estimated 4-6lb) suggests it should fly on about 300watts -
about 1/3bhp shaft after losses, so something like a .15 glo equivalent.
Calculations suggest that the prop needs to turn a shade over 2000 RPM
with a pitch of about 22 inches.
Show me a .15 or .25 engine that can do this!
You would PROBABLY put a 60 or 90 4 stroke in that to swing that size of
prop (20x22). I am putting in a thwacking gearbox and a much lighter
Contrast a tuned pipe 36 or so, able to swing an 8x8 at 18K RPM and
propel a small model at 140mph. THAT actually NEEDS the RPM and the
power, and you are looking at over 1500W of electric to replicate it.
So, we have two extremes..in the first case our slow biplane with a huge
prop, we can use a 300W motor to simulate a 90 4 stroke, in the second
we need a 1.5KW motor to replace a tuned pipe 36 2 stroke.
There is about a 15:1 difference in 'what electric power equals what
size of IC engine'.
If you are truly interested in the detail of all this, I suggest you
take your question over to e.g. rcgroups. Where we can go into it with
pictures, diagrams and calculations that simply cannot be done here.
Either in the power
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
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
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
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.
Wow, Can open, worms everywhere!!! :)
Thanks for taking the time to go into so much detail. It is far from
the simple conversion I thought it might be. I'm not too afraid of the
math as I like to work stuff like this out to be honest, but I really
do need to know what is available to me in terms of equipment - youve
mentioned gearboxes etc which I had no idea existed!! Once I know the
formula and other details such as the watts, pitches and rpm etc I
should be able to work this out 'fairly' simply.
I'll have a look at the forums youve suggested.
I suppose I ought to ask about props while Im here...
Eventually I'd quite like to build a B-17 (quite a large scale one) and
would like to know if triple bladed props are available? Also what do
the numbers on the props mean? My park flier has the numbers 8 x 3.5 on
them. What on earth does this mean?
Can you tell how green I am at this yet?? :)
The Natural Philos> > Hi all,
No greener than me when I re-started the hobby..only about 7 years ago..
8x3.5 is the diameter and pitch of the propellor. The puitch is how far
it would advance in a sort of ideal situation for every revolution.
The 'pitch speed' is the pitch times the RPM. It sets an upper limit on
the model speed. Experience tells us that it needs to be at least twice
stall speed, and 2.5-3 is better, or you end up with a model that no
matter how hard the engines rev, will barely stay in the air.
If you really want to get to the bottom of all this I have some serious
First, take out a free subscription to RCgroups and ask the silly
questions there. Ignore smart alecs who give you a hard time.
Secondly go to
and download the 30 day evaluation copy
and play with it. I'd advise buying it outright - its not very expensive
$39. Its better than any book and most of the algebra is done for you.
Thirdly, the other two items you really need if you want to stray off
the beaten track of 'use this, it worked for me' is a decent tacho, and
a 'whattmeter' The original Astro Whattmeter is a device that goes
between the battery and everything else., and measures voltage and
current and watts, and indeed how many mAh you have drawn..and tells you
what you are pulling. Or at the least some form of decent clamp on DC -
I stress the DC - current meter is almost mandatory. In an ideal world
none of this is necessary, but one mistake and you can blow and
expensive motor, pack and controller. It pays to KNOW you are safe.
Other even better whattmeters are out there. Some download data to a PC
which is useful for keeping a record and drawing pretty graphs and teh like.
Finally, I'd say that doing all the above taught me a lot, taught me how
little I knew when I started, and how little the average glo pilot
knows, or cares about how his stuff actually flies the plane. Not fast
enough on a 25? Stick in a 40. You have three props: a 9x9, a 10x6 and
an 11x4. That's it, Try all three and keep the one you like.
the inrunner referral to 280, 370, 400, etc. goes back to the original
brushed motors that were rated that way. The brushless motors are a quantum
leap over the old brushed. The problem with inrunner motors is they are very
high RPM and very low torque so you hve to use a very small prop on them or
use a gearbox. The Outrunner motors have tremendous torque and no need for a
gearbox. If possible, stay with the outrunners.
The easiest way to determine the motor is not by using watts, but by using
thrust. You know wht the airplane weighs, say 4 lbs. That is ready to fly by
the way. 4 lbs is 64 ounces. So if you can find a motor manufacturer that
gives you thrust numbers or gives you the weight of an airplane the motor
will fly and with which prop, that is the way to go. Forget watts! It varies
so much by what TYPE of airplane it is. a 4 lb trainer will require far less
power to fly it properly, say 60 to 70%. For aerobatics you'll want 80-90%.
For 3D you will want 100-110%.
but you also need speed no good having 100% thrust to weight if it only
pulls the aircraft at 20mph and the stall speed is 30mph or you could
end up with a plane that will prop hang forever but will not fly forwards
Absolute total bollocks.
No it won't.
A helicopter has thrust that exceeds its weight, ..now put wings on a
helicopter and try and turn it sideways. It will not fly. It is not
capable of enough speed to get the wing above stall.
You only need enough thrust to overcome drag and contribute to the
climb. On a slick airframe this could be as little as one tenth of he
aircraft weight..in level flight and not much more for a pretty decent
rate of climb.
Thrust is easy to measure, but relying on the measurement to make the
plane fly is very very foolish. Unless you intend to fly 3D planes you
are far far better advised to look at power to weight, and pitch speed,
and forget the thrust. That will come naturally anyway. Thrust IS more
or less power divided by pitch speed (gross oversimplification, but the
dimensions are correct)
You can get a ton of thrust from a 10 watt motor..as long as you don;t
mind the pitch speed being somewhat slower than a snail on smack.
More absolute bolloxs.
Power to weight is directly related to rate of climb. Sure if you want a
100mph straight up, that is a lot more watts per pound than a parkflyer
that climbs at 300fpm, but that is easy enough to understand.
And given the relationship of drag to speed, the faster you go the more
watts per pound you want as well.
But you will find that no pylon racer is capable of vertical climbs. The
thrust is lower but sustained to very high speeds with a coarse pitched
I repeat, never rely on static thrust to be in indication of anything
beyond a models ability, or inability to hover. Nearly every model I
have has a prop that is NOT optimised for thrust, but for overall
Having set up successful power trains on over 15 models, I know what I
am talking about.
More absolute bollox., Static readings of THRUST don't mean much because
when the airplane gets in the air the prop/motor will unload and you
will get far *worse* performance than the static numbers. Electric
motors of high efficient do not show much RPM increase at all with plane
speed. A *100%* efficient motor would stay at the same RPM regardless of
airspeed..its speed is dictated solely by the applied voltage.
I repeat, rules of thumb learnt and grandad's knee flicking gassers DO
NOT APPLY to electric.
What happens with an electric is that the thrust decays, (and the
current drawn) more or less linearly as the pitch speed is approached
becoming zero at (or around) the pitch speed - depending on how this is
If the pitch speed is no greater than the stall speed the model will
*never* fly on its wing. At best it can be persuaded to flutter around
in a nose up attitude like a hummingbird on methedrine.
No matter how much *static* thrust it has.
This is an unlikely condition with an IC engine, because the power and
RPM regime dictates that only a really fine pitched prop would show this
sort of behaviour. Its an extremely likely scenario with electric
models, because they are designed to swing far bigger props slower.
Precisely. AND I've seen those planes do just that. They call them '3D'
shockies' with > 1:1 thrust to weight on stupidly fine pitched
props..they don't actually FLY at all. Often the CG is so far back that
they are unstable in level flight ANYWAY. They are really just helicopters.
whatever you say. Where do you get your information? Mine comes from a very
sophisticated company called E-Power Test Labs. That use all sorts of dyno
equipment to produce static results and then telemetry to measure 'in the