IC and Electric

[Richard Harris]

I agree with PeterF in that, if you want a like for like comparison then careful selection of motor KV is a must to replicate prop size/pitch and power to match that of the IC engine that once powered the model.

Something I also consider is the centre of mass of the model (ready to fly), with a mounted IC engine the cylinder could be in any configuration to suit its purpose (vertical, side, inverted etc), this in turn sets the position of the fuel tank, silencer etc. To replicate this CG position when converting to electric is not easy. Without significant butchery we are limited to where we can place our Lipo and in turn it could raise or lower this CG position. If the thrust line is 'as per' the ic's then it could effect pitch trim through the throttle curve, possibly worsen the effect with a shorter moment arm or tailplane with a limited area.

One of the beauties of electric flight is that once set the centre of mass never alters as there is no fuel burnt off.

Rich

[Peter Beeney]

supertigrefan, Dave Hess’s post above is brilliant, a careful study of this will give you a very good start.

The motor is delivering max torque at zero rpm, what it’s not delivering is any actual useful power at all. We are looking at just mechanical power at the prop shaft, this is obtained by multiplying the rpm’s by the torque, power is the rate at which work is done and the rpm’s give the time element. In this situation the revs per minute equal zero, any number multiplied by zero equals zero, hence no power! And, indeed, for all the reasons in Dave’s post, this is not really a good place to be for very long, at any time… it tends to get a bit hot under the collar… …or should that be the rotor…

A very brief of brief scribbles I’m afraid, but I hope there’s at least some sort of clue in there.

Another clue regarding the heating element (sorry) of the story is the resistance , or lack of it, of the motor. All tied up with the current’s ‘Is it pushed, or was it shoved’ little debate.

I’m sure it has to be pushed…

PB

[Martin Harris - Moderator]

Semantics really but voltage can be referred to as electromotive force so that infers the voltage pushes...

[Chris Walby]

....way off topic, best start a new thread.

[Bob Cotsford]

Posted by Martin Harris on 20/12/2018 13:26:46:

Semantics really but voltage can be referred to as electromotive force so that infers the voltage pushes...

but then I seem to recall from chemistry classes several decades ago that it is electrons filling higher energy valency bands or something along those lines, so is electricity in fact pulled and not pushed?

[Former Member]

[This posting has been removed]

[Cuban8]

Posted by Daithi O Buitigh on 20/12/2018 00:49:29:

Congratulations, you have just failed City and Guilds 224 (Electronic servicing)

Those were the days, 1980 -1985 three nights a week at the local tech....passed with distinctions and then on to HTC for another year!

Well, if I don't blow my own trumpet, no one else will.

[Peter Beeney]

supertigrefan, Yes, max torque when stalled because because max current is flowing, creating a max magnetic field around the stator coils. This is so because the motor is not turning, therefore there is no motional or back emf to oppose the applied voltage from the battery.

Powered flight. The secret is in the word powered. Power is the prop shaft actually turning. In very general terms, the faster, the better; within the design of the motor, that is. The torque is required to maintain the turning motion with the addition of a load. In general terms, the stronger, the better. So for powered flight, torque is absolutely essential.

When the load on on the motor increases the revs fall slightly. So the back emf also falls in an exact proportion. Therefore the motor’s net voltage increases by the same exact proportion, but as it’s coils resistance alway stay the same the current flow will to increase by the same exact proportion… The extra amps increase the magnetic field strength and thus the torque, holding the speed at this level; however, it can’t increase speed again though, because the back emf would then increase again, reducing the current flow again. It’s all a careful electrical and mechanical balancing act. Reducing the load returns everything back to square one.

Perhaps not the most elegant explanation I guess, but I hope most of the basics are in there somewhere.

Keep on sparkin’…

PB

Edited By Peter Beeney on 20/12/2018 15:40:39

[Geoff S]

I suppose high torque with no movement is analogous to applying a spanner to a very tight nut. You can be applying a lot of torque with no movement.

The trolley buses I used to travel on as a child had a series wound element on their compound wound brushed motors (no permanent magnets both field and armatures were wound components) which have maximum torque at zero rpm and had very high acceleration as they set off.

I really think the only difference between ic and electric motor power sources is psychological probably reinforced by the audible feedback more apparent on ic engines. As just about all F3A aircraft these days are electric and pattern flying needs accurate control of the engine/motor and both types are successful.

Geoff

[Chris Walby]

Geoff, sorry but I have to disagree.

Mine was the same model, powered by electric and then IC, there was a definitely something between the set up's.

My money is still on the difference regarding centre of mass for the respective components as the C of G's were the same. There was little difference in low speed performance and it was only occasional use at WOT (not the sort of model that improves with speed). The model seemed more responsive with IC with being more stable.

I am not surprised as it was originally designed for IC and the electric conversion looked a bit of an afterthought. For me it only applies to this model and if another model the design is for electric then it will be a different story.

I suppose if more models are specifically designed for electric, then the IC powered may be less harmonious to the same extent (noticeable, but not much else).

Or perhaps its just in my mind

PS thanks to all these that contributed and Merry Christmas

[Bob Cotsford]

The only difference that I've noticed in my conversions has been the instant motor acceleration available with electric with notable torque effects on lighter models. My 1 1/2 strutter will turn 90 degrees on the spot when slamming open the throttle, something that didn't happen with ic, even when over-powering the model.

[J D 8]

Makes it more like the full size then,although if you slam open a rotary engine a cut is more likely.

[Dave Hess]

Posted by Daithi O Buitigh on 20/12/2018 00:49:29:

"A motor doesn’t draw current. Instead, using layman's terms, the current is pushed through by the battery."

Congratulations, you have just failed City and Guilds 224 (Electronic servicing)

A load draws current - a source provides voltage. Consider a river: the current is the rate it flows at and the 'voltage' is the pressure from the source pushing it along.

Oh by the way, all transmitters have throttle trim so you should be able to set 'zero' throttle to a slow idle if you want to

A motor draws current as much as the air sucks the water out of your hose-pipe when you wash you car or fill your pond.

It's pressure difference that moves the water and voltage diference that moves electrons (current). You can always argue that the low pressure is sucking rather than the high pressure pushing, but that's pretty weird unless you're a scientist, in which case you're probably weird. Nothing wrong with being weird though. We're all weird in our own way.

[PatMc]

Posted by Dave Hess on 20/12/2018 00:03:48:

The amount of voltage generated depends on the motor's speed and its Kv, and it's in the opposte direction to the battery voltage. A motor with a Kv of 1000 will generate 1v per 1000 rpm, so at 6000 rpm, it's generating 6v.

Rightly or wrongly, by convention the Kv figure quoted for model motors refers to the rpm/volt applied not volts generated/rpm.
Thus an unloaded 1000Kv motor will turn at 10,000 rpm when 10v is applied.

The back emf = applied volts - (current x resistance of the windings)

Edited By PatMc on 20/12/2018 22:13:11

[Dave Hess]

Posted by PatMc on 20/12/2018 22:08:13:

Posted by Dave Hess on 20/12/2018 00:03:48:

The amount of voltage generated depends on the motor's speed and its Kv, and it's in the opposte direction to the battery voltage. A motor with a Kv of 1000 will generate 1v per 1000 rpm, so at 6000 rpm, it's generating 6v.

Rightly or wrongly, by convention the Kv figure quoted for model motors refers to the rpm/volt applied not volts generated/rpm.
Thus an unloaded 1000Kv motor will turn at 10,000 rpm when 10v is applied.

The back emf = applied volts - (current x resistance of the windings)

Edited By PatMc on 20/12/2018 22:13:11

That's just saying the same thing the other way round. The problem is if you explain it your way round, you can't see why the current goes up when the motor slows down.

[Dave Hess]

Posted by Frank Skilbeck on 20/12/2018 12:02:13:

Not very scientific, but I've just got back from flying my Super Flying Models Fokker D7, I set the telemetry to give a regular amp readout every 3 seconds. With the throttle fixed to give 24 amps in level flight, putting the model into a steep climb the amps increased to just over 26 amps as the model came to the stall, in a dive the amps dropped to around 22 amps. Note very scientific but you can see how the motor draws more/less amps as the load changes.

That's quite correct an exactly what I explained. The motor slows down when under load, so less back emf is generated and the higher net voltage pushes more current through the motor.

The motor has no switch, valve or other mechanism to adjust its own current, so it has no way of changing how much current it draws. All that happens is that little gates open in the ESC many times a second and eletrons get shoved through by the battery voltage from one battery terminal to the other. They get hindered by resistance in the wires and the back emf trying to push them back the other way, but it doesn't push as hard as the battery, except at the theoretical maximum speed of the motor, which is why it maxes out.

Edited By Dave Hess on 20/12/2018 22:38:52

[PatMc]

Posted by Dave Hess on 20/12/2018 22:21:37:

Posted by PatMc on 20/12/2018 22:08:13:

Posted by Dave Hess on 20/12/2018 00:03:48:

The amount of voltage generated depends on the motor's speed and its Kv, and it's in the opposte direction to the battery voltage. A motor with a Kv of 1000 will generate 1v per 1000 rpm, so at 6000 rpm, it's generating 6v.

Rightly or wrongly, by convention the Kv figure quoted for model motors refers to the rpm/volt applied not volts generated/rpm.
Thus an unloaded 1000Kv motor will turn at 10,000 rpm when 10v is applied.

The back emf = applied volts - (current x resistance of the windings)

Edited By PatMc on 20/12/2018 22:13:11

That's just saying the same thing the other way round. The problem is if you explain it your way round, you can't see why the current goes up when the motor slows down.

No, it's not.
It's pointing out that you're misinterpeting what Kv means in the convention applied to model motors.

If we were saying the same thing then applied voltage would be equal to back emf & no current would flow - perpetual motion would result.

Edited By PatMc on 21/12/2018 12:17:14

[Former Member]

[This posting has been removed]

[Ron Gray]

Gosh, whilst I do fly leccy powered models all this talk of back emf, electrons swimming up and down the wires, Kv means this not that, I'm glad that I also fly IC where I only have to worry about the prop size and very occasional tweaks to the carb jets.Of course I can fly longer and have a much better 'noise' experience as well - just saying!

[PatMc]

How come this thread is missing from the "Latest Posts" though it's still in "All Things Flying", albeit minus minus Ron Gray's post at 12:41:09 ?

Now it's back (still sans Ron's post) since I posted this query.

Edited By PatMc on 21/12/2018 14:58:03

[Former Member]

[This posting has been removed]

[Rocker]

The one thing I am finding between IC and Electric ,is every time I buy a new electric model I always seem to have the wrong batteries for it where has I.C one fuel does all !!!!

[PatMc]

Posted by Rocker on 21/12/2018 15:05:25:

The one thing I am finding between IC and Electric ,is every time I buy a new electric model I always seem to have the wrong batteries for it where has I.C one fuel does all !!!!

Really ?

Then why are all these or these listed ?

[eflightray]

Posted by Rocker on 21/12/2018 15:05:25:

The one thing I am finding between IC and Electric ,is every time I buy a new electric model I always seem to have the wrong batteries for it where has I.C one fuel does all !!!!

No, you are buying the wrong plane for the batteries, (electric fuel), you already have.

[Ron Gray]

Posted by Rocker on 21/12/2018 15:05:25:

The one thing I am finding between IC and Electric ,is every time I buy a new electric model I always seem to have the wrong batteries for it where has I.C one fuel does all !!!!

Well as long as you have 1S, 2S, 3S, 4S, 5S and 6S you'll be OK (ish)