Discussion:
glow engine to electric motor comparisons
(too old to reply)
k***@rhodesfamily.co.uk
2007-01-05 19:38:57 UTC
Permalink
Hi 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
Jim Slaughter
2007-01-05 19:51:51 UTC
Permalink
go 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.uk
Hi 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
k***@rhodesfamily.co.uk
2007-01-05 20:05:48 UTC
Permalink
Thanks 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?

Karl
Post by Jim Slaughter
go 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.uk
Hi 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
The Natural Philosopher
2007-01-06 11:25:12 UTC
Permalink
Post by k***@rhodesfamily.co.uk
Thanks 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.uk
Karl
Post by Jim Slaughter
go 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.uk
Hi 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
Jim Slaughter
2007-01-06 23:54:43 UTC
Permalink
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.
Post by The Natural Philosopher
Post by k***@rhodesfamily.co.uk
Thanks 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.uk
Karl
Post by Jim Slaughter
go 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.uk
Hi 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
The Natural Philosopher
2007-01-06 10:58:51 UTC
Permalink
Post by k***@rhodesfamily.co.uk
Hi 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 ?
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
go fly.

You have to think in terms of the whole powertrain..prop,box, motor and
pack.

An interesting example for you..60" span WWI biplane. Scale prop is
around 20"

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
electric motor.

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 http://www.rcgroups.com/power-systems-13/ or scale
http://www.rcgroups.com/scale-electric-planes-10/ fora.
Post by k***@rhodesfamily.co.uk
Thanks
k***@rhodesfamily.co.uk
2007-01-06 15:57:00 UTC
Permalink
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?? :)
Post by The Natural Philosopher
Post by k***@rhodesfamily.co.uk
Hi 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 ?
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
go fly.
You have to think in terms of the whole powertrain..prop,box, motor and
pack.
An interesting example for you..60" span WWI biplane. Scale prop is
around 20"
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
electric motor.
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 http://www.rcgroups.com/power-systems-13/ or scale
http://www.rcgroups.com/scale-electric-planes-10/ fora.
Post by k***@rhodesfamily.co.uk
Thanks
The Natural Philosopher
2007-01-06 17:14:53 UTC
Permalink
Post by k***@rhodesfamily.co.uk
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?? :)
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
recommendations.

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 www.motocalc.com 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.
Post by k***@rhodesfamily.co.uk
Post by The Natural Philosopher
Post by k***@rhodesfamily.co.uk
Hi 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 ?
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
go fly.
You have to think in terms of the whole powertrain..prop,box, motor and
pack.
An interesting example for you..60" span WWI biplane. Scale prop is
around 20"
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
electric motor.
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 http://www.rcgroups.com/power-systems-13/ or scale
http://www.rcgroups.com/scale-electric-planes-10/ fora.
Post by k***@rhodesfamily.co.uk
Thanks
Jim Slaughter
2007-01-06 23:59:11 UTC
Permalink
Remember, static readings of pitch speed don't mean much. When the airplane
gets in the air, the prop/motor will 'unload' and you will get far better
performance than the static numbers.
Post by The Natural Philosopher
Post by k***@rhodesfamily.co.uk
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?? :)
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
recommendations.
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 www.motocalc.com 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.
Post by k***@rhodesfamily.co.uk
Post by The Natural Philosopher
Post by k***@rhodesfamily.co.uk
Hi 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 ?
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
go fly.
You have to think in terms of the whole powertrain..prop,box, motor and
pack.
An interesting example for you..60" span WWI biplane. Scale prop is
around 20"
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
electric motor.
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 http://www.rcgroups.com/power-systems-13/ or scale
http://www.rcgroups.com/scale-electric-planes-10/ fora.
Post by k***@rhodesfamily.co.uk
Thanks
The Natural Philosopher
2007-01-07 13:58:04 UTC
Permalink
Post by Jim Slaughter
Remember, static readings of pitch speed don't mean much. When the airplane
gets in the air, the prop/motor will 'unload' and you will get far better
performance than the static numbers.
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
defined.

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.
Jim Slaughter
2007-01-07 17:35:14 UTC
Permalink
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
air' data.
Post by The Natural Philosopher
Post by Jim Slaughter
Remember, static readings of pitch speed don't mean much. When the
airplane gets in the air, the prop/motor will 'unload' and you will get
far better performance than the static numbers.
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 defined.
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.
The Natural Philosopher
2007-01-07 19:08:18 UTC
Permalink
Post by Jim Slaughter
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
air' data.
Oh, a degree in engineering and electrical sciences..the experiences of
a thousand people or so on the e-zone who fly electric models and pass
their experience on..plus test data done of 30 motors and props or so
over a few months..

I could continue..
Jim Slaughter
2007-01-06 23:57:54 UTC
Permalink
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%.
Post by The Natural Philosopher
Post by k***@rhodesfamily.co.uk
Hi 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 ?
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
go fly.
You have to think in terms of the whole powertrain..prop,box, motor and
pack.
An interesting example for you..60" span WWI biplane. Scale prop is around
20"
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
electric motor.
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 http://www.rcgroups.com/power-systems-13/ or scale
http://www.rcgroups.com/scale-electric-planes-10/ fora.
Post by k***@rhodesfamily.co.uk
Thanks
funfly3
2007-01-07 11:02:05 UTC
Permalink
Post by Jim Slaughter
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
Post by Jim Slaughter
Post by The Natural Philosopher
Post by k***@rhodesfamily.co.uk
Hi 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 ?
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
go fly.
You have to think in terms of the whole powertrain..prop,box, motor and
pack.
An interesting example for you..60" span WWI biplane. Scale prop is around
20"
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
electric motor.
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 http://www.rcgroups.com/power-systems-13/ or scale
http://www.rcgroups.com/scale-electric-planes-10/ fora.
Post by k***@rhodesfamily.co.uk
Thanks
The Natural Philosopher
2007-01-07 14:00:27 UTC
Permalink
Post by funfly3
Post by Jim Slaughter
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
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.
Ed Cregger
2007-01-07 14:36:25 UTC
Permalink
Post by The Natural Philosopher
Post by funfly3
Post by Jim Slaughter
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
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.
Are you implying that those constantly hovering 3D flyers aren't really good
airplane pilots because they're not really "flying" on the wings anyway? <G>

Ed Cregger
funfly3
2007-01-07 15:48:27 UTC
Permalink
Post by Ed Cregger
Post by The Natural Philosopher
Post by funfly3
Post by Jim Slaughter
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
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.
Are you implying that those constantly hovering 3D flyers aren't really good
airplane pilots because they're not really "flying" on the wings anyway? <G>
Ed Cregger
how the hell do you get to that question from his answer? the 3d pilots
skill was never in question
Jim Slaughter
2007-01-07 17:36:53 UTC
Permalink
Yeah, my thought exactly. It takes some skill to just hover!
Post by funfly3
Post by Ed Cregger
Post by The Natural Philosopher
Post by funfly3
Post by Jim Slaughter
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
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.
Are you implying that those constantly hovering 3D flyers aren't really
good airplane pilots because they're not really "flying" on the wings
anyway? <G>
Ed Cregger
how the hell do you get to that question from his answer? the 3d pilots
skill was never in question
Morgans
2007-01-08 00:11:55 UTC
Permalink
Post by Jim Slaughter
Yeah, my thought exactly. It takes some skill to just hover!
Oh, I can disprove that!

A long time back, before I became the "consummate" skilled RC pilot that I
am now, (harrumph! <g>) and I was trying to takeoff in grass that was too
long for the short wheels and low belly of the little .15 powered Lazy Bee.
I finally broke ground almost at the runway's end, and had to pull up, or
crack up in some very tallll weeds.

The plane was not ready to fly yet, but I made it, anyway. Did I say that
it wasn't ready to fly yet? <g>

It went into mushing flight, and I had but a few inches between the wheels
and the weeds. I pulled back more. And more. I was keeping my altitude,
and it was by this time pointed almost straight up, hanging on the prop.

I stirred the sticks madly, keeping it from settling into the weeds. A few
of the guys standing around were starting to turn their heads, pointing, and
nudging to the next, saying, "Hey, look at this! I didn't know he could do
a torque roll! Why didn't you do it a little closer to you, so you could
see what it was doing?"

One of them asked, "Hey, when did you learn to do that?"

"Right now!" I replied. "It sorta' just happened!"

Well, finally, luck caught up, just as skill ran out. It finally flopped
over into the weeds, after hovering for nearly a minute, I guess, but I was
really sweating it. No damage, since it had no speed, and the grass was
tall.

So really, not much skill is needed to hover. Lots of luck can take skill's
place, at least for a minute, or so! <g>

Skill is preferable though, I think! ;-)
--
Jim in NC
Ed Cregger
2007-01-08 01:58:01 UTC
Permalink
Post by funfly3
Post by Ed Cregger
Post by The Natural Philosopher
Post by funfly3
Post by Jim Slaughter
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
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.
Are you implying that those constantly hovering 3D flyers aren't really
good airplane pilots because they're not really "flying" on the wings
anyway? <G>
Ed Cregger
how the hell do you get to that question from his answer? the 3d pilots
skill was never in question
Lighten up some. I'm just teasin' ya'll. <G>

Ed Cregger
funfly3
2007-01-08 09:03:09 UTC
Permalink
Post by Ed Cregger
Post by funfly3
Post by Ed Cregger
Post by The Natural Philosopher
Post by funfly3
Post by Jim Slaughter
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
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.
Are you implying that those constantly hovering 3D flyers aren't really
good airplane pilots because they're not really "flying" on the wings
anyway? <G>
Ed Cregger
how the hell do you get to that question from his answer? the 3d pilots
skill was never in question
Lighten up some. I'm just teasin' ya'll. <G>
Ed Cregger
trouble is it just looks like your picking a fight :-)
Ed Cregger
2007-01-08 11:35:57 UTC
Permalink
Post by funfly3
Post by Ed Cregger
Post by funfly3
Post by Ed Cregger
Post by The Natural Philosopher
Post by funfly3
Post by Jim Slaughter
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
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.
Are you implying that those constantly hovering 3D flyers aren't really
good airplane pilots because they're not really "flying" on the wings
anyway? <G>
Ed Cregger
how the hell do you get to that question from his answer? the 3d pilots
skill was never in question
Lighten up some. I'm just teasin' ya'll. <G>
Ed Cregger
trouble is it just looks like your picking a fight :-)
Yep, I have that problem once in a while - but I am just teasing.

Ed Cregger
The Natural Philosopher
2007-01-07 19:05:53 UTC
Permalink
Post by Ed Cregger
Post by The Natural Philosopher
Post by funfly3
Post by Jim Slaughter
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
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.
Are you implying that those constantly hovering 3D flyers aren't really good
airplane pilots because they're not really "flying" on the wings anyway? <G>
Right. They are indifferent *helicopter* pilots.
Post by Ed Cregger
Ed Cregger
Jim Slaughter
2007-01-07 17:35:53 UTC
Permalink
Yes, you are correct. Desired speed must also be a factor. Sorry, forgot
that part!
Post by funfly3
Post by Jim Slaughter
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
Post by Jim Slaughter
Post by The Natural Philosopher
Post by k***@rhodesfamily.co.uk
Hi 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 ?
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
go fly.
You have to think in terms of the whole powertrain..prop,box, motor and
pack.
An interesting example for you..60" span WWI biplane. Scale prop is
around 20"
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
electric motor.
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 http://www.rcgroups.com/power-systems-13/ or scale
http://www.rcgroups.com/scale-electric-planes-10/ fora.
Post by k***@rhodesfamily.co.uk
Thanks
The Natural Philosopher
2007-01-07 13:48:38 UTC
Permalink
Post by Jim Slaughter
The easiest way to determine the motor is not by using watts, but by using
thrust.
Total bollocks.

Absolute total bollocks.
Post by Jim Slaughter
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
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.
Post by Jim Slaughter
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%.
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
prop.

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
performance.

Having set up successful power trains on over 15 models, I know what I
am talking about.
Six_O'Clock_High
2007-01-07 15:47:00 UTC
Permalink
"The Natural Philosopher" <***@b.c> wrote in message news:***@demeter.uk.clara.net...
SNIP TO SAVE BANDWITH
Having set up successful power trains on over 15 models, I know what I am
talking about.
FIFTEEN whole models?
The Natural Philosopher
2007-01-07 19:06:28 UTC
Permalink
Post by Six_O'Clock_High
SNIP TO SAVE BANDWITH
Having set up successful power trains on over 15 models, I know what I am
talking about.
FIFTEEN whole models?
Ive only been back round 4 years in the hobby.


How many electric planes have YOU set up?
Six_O'Clock_High
2007-01-13 14:43:45 UTC
Permalink
Post by The Natural Philosopher
Post by Six_O'Clock_High
SNIP TO SAVE BANDWITH
Post by The Natural Philosopher
Having set up successful power trains on over 15 models, I know what I
am talking about.
FIFTEEN whole models?
Ive only been back round 4 years in the hobby.
How many electric planes have YOU set up?
I fly wet and the electrics are a sideline that I dabble in from time to
time as a joke. That being said, the direct answer to your question is six
electrics but enough wet birds to stock a small hobby shop. Smallest wet
engine in my active use collection is a .15 glow and the largest a 3E -
75i..

I fly a lot, so my active engine collection is diverse. I used to fly OS
ABC engines a lot, the definition of a worn out engine was when the prop
would free wheel while deadstick These days when my Saitos begin loosing
compression, I sell them and move on.
The Natural Philosopher
2007-01-13 21:13:57 UTC
Permalink
Post by Six_O'Clock_High
Post by The Natural Philosopher
Post by Six_O'Clock_High
SNIP TO SAVE BANDWITH
Post by The Natural Philosopher
Having set up successful power trains on over 15 models, I know what I
am talking about.
FIFTEEN whole models?
Ive only been back round 4 years in the hobby.
How many electric planes have YOU set up?
I fly wet and the electrics are a sideline that I dabble in from time to
time as a joke. That being said, the direct answer to your question is six
electrics but enough wet birds to stock a small hobby shop. Smallest wet
engine in my active use collection is a .15 glow and the largest a 3E -
75i..
I fly a lot, so my active engine collection is diverse. I used to fly OS
ABC engines a lot, the definition of a worn out engine was when the prop
would free wheel while deadstick These days when my Saitos begin loosing
compression, I sell them and move on.
Yeah, but how many of those planes did you BUILD YOURSELF and how many
are actually your own designs?
Jim Slaughter
2007-01-07 17:42:04 UTC
Permalink
You are amazing. Thrust, not watts! Watts is a smokescreen. You can't simply
say it takes X# of watts to fly X airplane! Total BS! It takes less thrust
to fly a simple trainer successfuly than a scale or aerobatic airplane of
the same weight! Using your watts per pound formula is pure bunk. In the gas
world, a trainer weighing 5 lbs could easily be flown on a .25 motor while a
5 lbs scale job would need a .40 and a 5 lbs 3D aerobat would do better with
something even bigger still to get the right performance. THRUST is a
function of pitch also. You can get more thrust (and speed) from a 8" pitch
prop than a 6" pitch prop at the same RPM!
Post by The Natural Philosopher
Post by Jim Slaughter
The easiest way to determine the motor is not by using watts, but by
using thrust.
Total bollocks.
Absolute total bollocks.
Post by Jim Slaughter
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
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.
Post by Jim Slaughter
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%.
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
prop.
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 performance.
Having set up successful power trains on over 15 models, I know what I am
talking about.
Jim Slaughter
2007-01-07 17:46:03 UTC
Permalink
15 models? Now THERE is a real test! Try about 500 models of every size and
description for me alone and probably over a 1000 for E-Power Test Labs. You
know, you CAN learn something new! Ya think?

Besides that, discussions of helicopters in this dialogue is totally bogus!
Totally different technology, etc. No bearing at all. All helicopters use
heavily geared motors.
Post by The Natural Philosopher
Post by Jim Slaughter
The easiest way to determine the motor is not by using watts, but by
using thrust.
Total bollocks.
Absolute total bollocks.
Post by Jim Slaughter
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
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.
Post by Jim Slaughter
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%.
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
prop.
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 performance.
Having set up successful power trains on over 15 models, I know what I am
talking about.
The Natural Philosopher
2007-01-07 19:09:39 UTC
Permalink
Post by Jim Slaughter
15 models? Now THERE is a real test! Try about 500 models of every size and
description for me alone and probably over a 1000 for E-Power Test Labs. You
know, you CAN learn something new! Ya think?
I suggest then, that you do.
Post by Jim Slaughter
Besides that, discussions of helicopters in this dialogue is totally bogus!
Totally different technology, etc. No bearing at all. All helicopters use
heavily geared motors.
So do a lot of electric planes.

Stop weaselling.
Dr. zara
2007-01-07 17:51:14 UTC
Permalink
Post by k***@rhodesfamily.co.uk
Hi 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
If you go to Horizon Hobbies website and look at the E-Flight series of
motors, they are numbered to the corresponding glow engines. IE: Power 46 =
46 Glow, the Power 60 = 60 Glow. You need about 150 Watts per pound for
real good performance. The page also tells you how much battery power you
need.
Jim Slaughter
2007-01-07 18:08:35 UTC
Permalink
The Horizon formula works in general. But their numbers are also pretty
exaggerated.
Post by Dr. zara
Post by k***@rhodesfamily.co.uk
Hi 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
If you go to Horizon Hobbies website and look at the E-Flight series of
motors, they are numbered to the corresponding glow engines. IE: Power 46
= 46 Glow, the Power 60 = 60 Glow. You need about 150 Watts per pound for
real good performance. The page also tells you how much battery power you
need.
The Natural Philosopher
2007-01-07 19:10:58 UTC
Permalink
Post by Dr. zara
Post by k***@rhodesfamily.co.uk
Hi 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
If you go to Horizon Hobbies website and look at the E-Flight series of
motors, they are numbered to the corresponding glow engines. IE: Power 46 =
46 Glow, the Power 60 = 60 Glow. You need about 150 Watts per pound for
real good performance. The page also tells you how much battery power you
need.
You need about 5KW per pound to take a F16 vertically.

You need about 3W per pound to keep a man powered aircraft aloft.

Tell me what "good performance" means..
Morgans
2007-01-08 00:18:27 UTC
Permalink
Post by The Natural Philosopher
You need about 5KW per pound to take a F16 vertically.
You need about 3W per pound to keep a man powered aircraft aloft.
Tell me what "good performance" means..
You are skipping so many variables, that those numbers don't have a thing to
do with the others.
--
Jim in NC
Jim Slaughter
2007-01-09 18:53:04 UTC
Permalink
Absolutely correct! That's why the WATTS formula just doesn't work!
Post by Morgans
Post by The Natural Philosopher
You need about 5KW per pound to take a F16 vertically.
You need about 3W per pound to keep a man powered aircraft aloft.
Tell me what "good performance" means..
You are skipping so many variables, that those numbers don't have a thing
to do with the others.
--
Jim in NC
The Natural Philosopher
2007-01-10 09:33:00 UTC
Permalink
Post by Jim Slaughter
Absolutely correct! That's why the WATTS formula just doesn't work!
Ah, but it does..you just have to answer the question for yourself.

The thrust formula doesn't work either, unless you answer the question
'at what speed'

At least one can GUARANTEE that 50W/lb with a pitch speed more than
twice the stall speed will actually fly.

There is no thrust figure that will GUARANTEE flight

At best it will GUARANTEE a hover.

The same thrust can be obtained with widely different powers..do you
really think a 50W plane is going to fly the same as a 500W plane, just
because they produce the same thrust?

Power sets an absolute upper limit on the rate of climb and the top
speed of a given aircraft.

Power is the rate of gain of potential energy in the climb. Power is
MEASURED in lb feet per second, as well as bhp and watts...

Power is drag times speed , in level flight.

Saying that thrust is what determines how a plane flies is as silly as
saying that the tractive effort on the back axle of a stationary car
will determine how fast it goes and how fast it will accelerate..its
only true for the first instant. You will, for example, get the very
best traction out of a 100bhp tractor..infinitely better than a 3000bhp
drag car..but the car will do 230mph, the tractor a mere 15..

I challenge you to put a top fuel car back to back with a bulldozer, and
have the top fuel car wing the 'tug of war'.. ;)

Power to weight governs a cars acceleration, and an aircrafts rate of
climb, and power to drag governs the top speed of both. With aircraft on
the wing, the drag is, at medium speeds, more or less proportional to
weight. Only at super speeds does the actual profile drag dominate, and
the induced drag fall stay relatively constant.

If you want the actual figures, 1 watt per lb of output power equates to
44.25 feet per minute climb rate. If your unpowered sink rate is less
that 120 feet per minute, 3W/lb out of the prop will keep you aloft
indefinitely.

Conversely if you want to achieve a 30 mph vertical climb, that's about
60W/lb out of the prop. Allowing for about 50% inefficiency in the power
train, thats about 120W/lb.

Now measuring thrust will at some point possibly determine that you HAVE
that 50% efficiency, and are not just stirring the air to no avail, but
it cannot get away from Newtons laws, which tell you that if you do NOT
have that power to weight you will NEVER achieve that sort of climb.

likewise, if you want a plane that will break the sound barrier going
straight up, you need at least an output power of 20 times that - 1200w/lb.

In reality, because of drag, it's got to be more than that.

Static thrust is completely meaningless, because once the plane starts
to move, that thrust changes. possibly dramatically.
Post by Jim Slaughter
Post by Morgans
Post by The Natural Philosopher
You need about 5KW per pound to take a F16 vertically.
You need about 3W per pound to keep a man powered aircraft aloft.
Tell me what "good performance" means..
You are skipping so many variables, that those numbers don't have a thing
to do with the others.
--
Jim in NC
Karl
2007-01-12 10:44:04 UTC
Permalink
Hmmm,

Can open, worms everywhere!

:)
Post by The Natural Philosopher
Post by Jim Slaughter
Absolutely correct! That's why the WATTS formula just doesn't work!
Ah, but it does..you just have to answer the question for yourself.
The thrust formula doesn't work either, unless you answer the question
'at what speed'
At least one can GUARANTEE that 50W/lb with a pitch speed more than
twice the stall speed will actually fly.
There is no thrust figure that will GUARANTEE flight
At best it will GUARANTEE a hover.
The same thrust can be obtained with widely different powers..do you
really think a 50W plane is going to fly the same as a 500W plane, just
because they produce the same thrust?
Power sets an absolute upper limit on the rate of climb and the top
speed of a given aircraft.
Power is the rate of gain of potential energy in the climb. Power is
MEASURED in lb feet per second, as well as bhp and watts...
Power is drag times speed , in level flight.
Saying that thrust is what determines how a plane flies is as silly as
saying that the tractive effort on the back axle of a stationary car
will determine how fast it goes and how fast it will accelerate..its
only true for the first instant. You will, for example, get the very
best traction out of a 100bhp tractor..infinitely better than a 3000bhp
drag car..but the car will do 230mph, the tractor a mere 15..
I challenge you to put a top fuel car back to back with a bulldozer, and
have the top fuel car wing the 'tug of war'.. ;)
Power to weight governs a cars acceleration, and an aircrafts rate of
climb, and power to drag governs the top speed of both. With aircraft on
the wing, the drag is, at medium speeds, more or less proportional to
weight. Only at super speeds does the actual profile drag dominate, and
the induced drag fall stay relatively constant.
If you want the actual figures, 1 watt per lb of output power equates to
44.25 feet per minute climb rate. If your unpowered sink rate is less
that 120 feet per minute, 3W/lb out of the prop will keep you aloft
indefinitely.
Conversely if you want to achieve a 30 mph vertical climb, that's about
60W/lb out of the prop. Allowing for about 50% inefficiency in the power
train, thats about 120W/lb.
Now measuring thrust will at some point possibly determine that you HAVE
that 50% efficiency, and are not just stirring the air to no avail, but
it cannot get away from Newtons laws, which tell you that if you do NOT
have that power to weight you will NEVER achieve that sort of climb.
likewise, if you want a plane that will break the sound barrier going
straight up, you need at least an output power of 20 times that - 1200w/lb.
In reality, because of drag, it's got to be more than that.
Static thrust is completely meaningless, because once the plane starts
to move, that thrust changes. possibly dramatically.
Post by Jim Slaughter
Post by Morgans
Post by The Natural Philosopher
You need about 5KW per pound to take a F16 vertically.
You need about 3W per pound to keep a man powered aircraft aloft.
Tell me what "good performance" means..
You are skipping so many variables, that those numbers don't have a thing
to do with the others.
--
Jim in NC
Paul Ryan
2007-01-20 19:53:45 UTC
Permalink
Post by The Natural Philosopher
Post by Dr. zara
Post by k***@rhodesfamily.co.uk
Hi 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
If you go to Horizon Hobbies website and look at the E-Flight series
Power 46 = 46 Glow, the Power 60 = 60 Glow. You need about 150 Watts
per pound for real good performance. The page also tells you how much
battery power you need.
You need about 5KW per pound to take a F16 vertically.
You need about 3W per pound to keep a man powered aircraft aloft.
Tell me what "good performance" means..
Wait a minute- you can take a 5 pound plane vertically easily and
consistently with a one horsepower engine, about 3/4 of a kW- so this is
a good rule of thumb.
Maybe some f16 you heard about somewhere had 5 kW per pound, which I
doubt, but it certainly doesn't _take_ that much to make a plane fly
vertically.
Check your facts and your attitude. Anyone who makes as many typos as
do you, "Natural Philosopher," has no business quoting anything to anybody!
The Natural Philosopher
2007-01-21 01:24:32 UTC
Permalink
Post by Paul Ryan
Post by The Natural Philosopher
Post by Dr. zara
Post by k***@rhodesfamily.co.uk
Hi 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
If you go to Horizon Hobbies website and look at the E-Flight series
Power 46 = 46 Glow, the Power 60 = 60 Glow. You need about 150 Watts
per pound for real good performance. The page also tells you how
much battery power you need.
You need about 5KW per pound to take a F16 vertically.
You need about 3W per pound to keep a man powered aircraft aloft.
Tell me what "good performance" means..
Wait a minute- you can take a 5 pound plane vertically easily and
consistently with a one horsepower engine, about 3/4 of a kW- so this is
a good rule of thumb.
Maybe some f16 you heard about somewhere had 5 kW per pound, which I
doubt, but it certainly doesn't _take_ that much to make a plane fly
vertically.
It does if you want to do it at mach 1+ Do the sums. Rather than
criticise someone else who actually has done them.

Watts per pound is rate of climb. You can take an elephant straight up
with a 10W motor as long as you don't mind it dying before its got more
than an inch from the floor.
Post by Paul Ryan
Check your facts and your attitude. Anyone who makes as many typos
as do you, "Natural Philosopher," has no business quoting anything to
anybody!
Fuck off. That suit you? Spelt correctly enough?
Six_O'Clock_High
2007-01-22 16:31:20 UTC
Permalink
Post by The Natural Philosopher
Post by Paul Ryan
Post by The Natural Philosopher
Post by Dr. zara
Post by k***@rhodesfamily.co.uk
Hi 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
If you go to Horizon Hobbies website and look at the E-Flight series of
motors, they are numbered to the corresponding glow engines. IE: Power
46 = 46 Glow, the Power 60 = 60 Glow. You need about 150 Watts per
pound for real good performance. The page also tells you how much
battery power you need.
You need about 5KW per pound to take a F16 vertically.
You need about 3W per pound to keep a man powered aircraft aloft.
Tell me what "good performance" means..
Wait a minute- you can take a 5 pound plane vertically easily and
consistently with a one horsepower engine, about 3/4 of a kW- so this is
a good rule of thumb.
Maybe some f16 you heard about somewhere had 5 kW per pound, which I
doubt, but it certainly doesn't _take_ that much to make a plane fly
vertically.
It does if you want to do it at mach 1+ Do the sums. Rather than
criticise someone else who actually has done them.
Watts per pound is rate of climb. You can take an elephant straight up
with a 10W motor as long as you don't mind it dying before its got more
than an inch from the floor.
Post by Paul Ryan
Check your facts and your attitude. Anyone who makes as many typos
as do you, "Natural Philosopher," has no business quoting anything to
anybody!
Fuck off. That suit you? Spelt correctly enough?
Hey DH (been a while since you signed a post with that one)
You are far from perfect. Very few in a modeling forum speak of a rider
scale bird when deciding what power is needed for the models we fly. In
most cases we use significantly more power than the rider scale stuff does.
In short, I think your numbers have too much brown porridge in them. Most
real modelers understand that flying a model at mach 1 + is impossible due
to limitations in human physiology.
The Natural Philosopher
2007-01-23 01:34:05 UTC
Permalink
Post by Six_O'Clock_High
Post by The Natural Philosopher
Post by Paul Ryan
Post by The Natural Philosopher
Post by Dr. zara
Post by k***@rhodesfamily.co.uk
Hi 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
If you go to Horizon Hobbies website and look at the E-Flight series of
motors, they are numbered to the corresponding glow engines. IE: Power
46 = 46 Glow, the Power 60 = 60 Glow. You need about 150 Watts per
pound for real good performance. The page also tells you how much
battery power you need.
You need about 5KW per pound to take a F16 vertically.
You need about 3W per pound to keep a man powered aircraft aloft.
Tell me what "good performance" means..
Wait a minute- you can take a 5 pound plane vertically easily and
consistently with a one horsepower engine, about 3/4 of a kW- so this is
a good rule of thumb.
Maybe some f16 you heard about somewhere had 5 kW per pound, which I
doubt, but it certainly doesn't _take_ that much to make a plane fly
vertically.
It does if you want to do it at mach 1+ Do the sums. Rather than
criticise someone else who actually has done them.
Watts per pound is rate of climb. You can take an elephant straight up
with a 10W motor as long as you don't mind it dying before its got more
than an inch from the floor.
Post by Paul Ryan
Check your facts and your attitude. Anyone who makes as many typos
as do you, "Natural Philosopher," has no business quoting anything to
anybody!
Fuck off. That suit you? Spelt correctly enough?
Hey DH (been a while since you signed a post with that one)
You are far from perfect. Very few in a modeling forum speak of a rider
scale bird when deciding what power is needed for the models we fly. In
most cases we use significantly more power than the rider scale stuff does.
In short, I think your numbers have too much brown porridge in them. Most
real modelers understand that flying a model at mach 1 + is impossible due
to limitations in human physiology.
I said an F16. A real one
Do keep up there at the back.
Six_O'Clock_High
2007-01-26 03:16:09 UTC
Permalink
Post by The Natural Philosopher
Post by Six_O'Clock_High
Post by The Natural Philosopher
Post by Paul Ryan
Post by The Natural Philosopher
Post by Dr. zara
Post by k***@rhodesfamily.co.uk
Hi 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
If you go to Horizon Hobbies website and look at the E-Flight series
Power 46 = 46 Glow, the Power 60 = 60 Glow. You need about 150 Watts
per pound for real good performance. The page also tells you how
much battery power you need.
You need about 5KW per pound to take a F16 vertically.
You need about 3W per pound to keep a man powered aircraft aloft.
Tell me what "good performance" means..
Wait a minute- you can take a 5 pound plane vertically easily and
consistently with a one horsepower engine, about 3/4 of a kW- so this
is a good rule of thumb.
Maybe some f16 you heard about somewhere had 5 kW per pound, which I
doubt, but it certainly doesn't _take_ that much to make a plane fly
vertically.
It does if you want to do it at mach 1+ Do the sums. Rather than
criticise someone else who actually has done them.
Watts per pound is rate of climb. You can take an elephant straight up
with a 10W motor as long as you don't mind it dying before its got more
than an inch from the floor.
Post by Paul Ryan
Check your facts and your attitude. Anyone who makes as many typos
as do you, "Natural Philosopher," has no business quoting anything to
anybody!
Fuck off. That suit you? Spelt correctly enough?
Hey DH (been a while since you signed a post with that one)
You are far from perfect. Very few in a modeling forum speak of a rider
scale bird when deciding what power is needed for the models we fly. In
most cases we use significantly more power than the rider scale stuff
does. In short, I think your numbers have too much brown porridge in
them. Most real modelers understand that flying a model at mach 1 + is
impossible due to limitations in human physiology.
I said an F16. A real one
Do keep up there at the back.
Hey DH,
I was astounded to note that you did not even bother to read what was said
rather launched right into another attack. I guess reading was not one of
your best subjects...

I see your intellectual might impressed Uncle Pauly also.

Paul Ryan
2007-01-23 04:33:17 UTC
Permalink
Post by The Natural Philosopher
Post by Paul Ryan
Post by The Natural Philosopher
Post by Dr. zara
Post by k***@rhodesfamily.co.uk
Hi 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
If you go to Horizon Hobbies website and look at the E-Flight series
Power 46 = 46 Glow, the Power 60 = 60 Glow. You need about 150
Watts per pound for real good performance. The page also tells you
how much battery power you need.
You need about 5KW per pound to take a F16 vertically.
You need about 3W per pound to keep a man powered aircraft aloft.
Tell me what "good performance" means..
Wait a minute- you can take a 5 pound plane vertically easily and
consistently with a one horsepower engine, about 3/4 of a kW- so this
is a good rule of thumb.
Maybe some f16 you heard about somewhere had 5 kW per pound, which I
doubt, but it certainly doesn't _take_ that much to make a plane fly
vertically.
It does if you want to do it at mach 1+ Do the sums. Rather than
criticise someone else who actually has done them.
Watts per pound is rate of climb. You can take an elephant straight up
with a 10W motor as long as you don't mind it dying before its got more
than an inch from the floor.
Not when you're moving air as your means of propulsion you can't...
Post by The Natural Philosopher
Post by Paul Ryan
Check your facts and your attitude. Anyone who makes as many
typos as do you, "Natural Philosopher," has no business quoting
anything to anybody!
Fuck off. That suit you? Spelt correctly enough?
Wow- your college degree is doing you a lot of good. we're all
impressed.
It ain't that difficult- Uncle Pauly
Continue reading on narkive:
Loading...