Bit Bewildered

[Nigel Heather]

I come from an IC background and having recently returned to the hobby I can see a big move towards electric and this is a direction I want to go in, but I find myself quite bewildered by motor specifications.

In the past I might have seen an article which says “I fitted an OS 46 and it flies great”. Now I would look at that and say “well the OS is out of my price league, but I can understand its size and power and choose one of many direct alternatives”.

But with electric motors I find I am unable to look at a motor and find alternatives.

Fir example, say motor X is recommended for a model but hasn’t been in stock any where for ages, how do I go about finding alternatives?

Can anyone point at a beginners guide?

Cheers,

Nigel

[Nigel R]

The easy route is buying a pre-rolled setup from somewhere like 4-max.

4-max do a guide for choosing your own kit.

**LINK**

 

"But with electric motors I find I am unable to look at a motor and find alternatives."

Don't look at the motor. Start with the lipo, that initially tells you far more than the motor size.

Edited By Nigel R on 31/01/2020 11:57:52

[Romeo Whisky]

This might help too.

**LINK**

[Nigel Heather]

Thanks, that is a good link, explains things very clearly, at least until it gets to choosing KV when it goes all vague.

This is my understanding, KV is RPM per volt. So with a 3S battery, a 1000 KV will in theory have a maximum RPM of 11,100 RPM.

Also that you tie that in with prop pitch, so in theory a 10x5 prop will travel a maximum of 55,500 inches every minute, about 53 mph.

I also understand that you can’t just throw any prop and motor together, they need to be well matched.

Get all that, but where do you start. Do you decide what max speed you want and then choose a prop and motor that will deliver that?

If so, is there a rule of thumb for desirable speeds?

Cheers,

Nigel

[Simon Chaddock]

Nigel Heather

The trick to identifying a replacement electric motor is to look at its specification.

Unlike IC, where the choice is now basically down to displacement and price (in the past it wasn't always that easy), an electric motor has more variables. These have to be matched exactly (or within a couple of percent) to achieve a direct replacement.

The basic factors are a motor's max Watts input and the its kV. It is actually as simple as that although this only applies if the same battery voltage is used.

This does of course require that you know what the figures were for the 'not available' motor but they should be available in the description for any current one.

Despite this apparent simplicity actually specifying a motor/prop/battery combination from scratch for a particular air frame is a much more complex issue but then I suspect your 'experience' with IC was not gained overnight.

[SIMON CRAGG]

Another useful tip is to start with the weight of the model.

There are a plethora of formulas out there, but for sport use I have found that you need 150w for every pound in weight, So a lb5 model would need a 750w motor.

Another way of working out a direct replacement for example an ASP .53 is by working out how many hp the engine produces:

ASP .53 = 1.46kw

1.46kw = 1.75hp

1.00 hp = 746w

So ASP replacement needs to be 1305w

There are many variables to all this, but I have converted several models using the above criterea and they have flown a treat.

Good luck!.

[Nigel Heather]

Thanks, I’ve got as far as knowing that I need

Power between 240 and 390 Watts

A 3S 2200mAh battery

Current between 21.6 and 35.1 Amps

Just need to conclude on a KV and a prop combination.

This is for a 3lb, 45” span, sports plane.

Cheers,

Nigel

[Peter Miller]

Posted by Nigel Heather on 31/01/2020 11:40:54:

I come from an IC background and having recently returned to the hobby I can see a big move towards electric and this is a direction I want to go in, but I find myself quite bewildered by motor specifications.

In the past I might have seen an article which says “I fitted an OS 46 and it flies great”. Now I would look at that and say “well the OS is out of my price league, but I can understand its size and power and choose one of many direct alternatives”.

But with electric motors I find I am unable to look at a motor and find alternatives.

Fir example, say motor X is recommended for a model but hasn’t been in stock any where for ages, how do I go about finding alternatives?

Can anyone point at a beginners guide?

Cheers,

 

Nigel

 

Like you I have moved over to electric power and I too am confused but I have a simple answer.

I phone George at 4-Max and tell him what the specs of the models are and he provides the perfect combination of power.

I beat my brains out enough designing without having to work out what I need in the way of assorted component for the power train.

I.C was so simple.I knew exactly what power for what model.

Edited By Peter Miller on 31/01/2020 14:52:15

[Nigel Heather]

Okay, time to come clean. I had already looked at 4-Max so have a suggested motor and ESC.

But I want to understand why that motor, just how my brain works, I like to understand things.

The motor they recommend has a KV of 1070 to pair up with a 12x6 prop.

So theoretical max RPM of 11,877.

That makes sense, but why 1070 rather than 900 or 800.

Would just like to understand why.

Cheers,

Nigel

[Piers Bowlan]

The Late, great, David Burton (aka BEB) wrote a great article in RCM&E (in fact several) on this very subject. If someone has the article perhaps they could post it on the Forum here as it is very helpful for those trying to make sense of it all.

[Denis Watkins]

To be going on with, a quote from the internet Nigel,

And then there is Cost too....

“Kv” refers to the constant velocity of a motor (not to be confused with “kV,” the abbreviation for kilovolt). It is measured by the number of revolutions per minute (rpm) that a motor turns when 1V (one volt) is applied with no load attached to that motor. The Kv rating of a brushless motor is the ratio of the motor’s unloaded rpm to the peak voltage on the wires connected to the coils.

Knowing the Kv rating of a motor will help you determine how fast that motor will rotate when a given voltage is applied to it. For example, a 980Kv motor powered by an 11.1V battery would spin at 10,878 rpm (980 x 11.1) with no load. A change in voltage will change the rpm and will require changing the propeller to avoid overloading the motor. Kv allows you to get a handle on the torque that can be expected from a particular motor. Torque is determined by the number of winds on the armature and the strength of the magnets. A low Kv motor has more winds of thinner wire—it will carry more volts at fewer amps, produce higher torque, and swing a bigger prop. A high Kv motor has fewer winds of thicker wire that carry more amps at fewer volts and spin a smaller prop at high revolutions.

Knowing the Kv rating of a motor is helpful to determine which motor belongs in which aircraft. An FPV racing quad, for example, requires high rpm for high speed, so you would use a high Kv motor and a small-diameter prop. On the other hand, you would use a lower Kv motor in a heavy-lift multirotor because you want to turn a large prop at lower rpm and obtain high torque.

[Bob Cotsford]

The 1070kv is a ballpark figure to get a reasonable size prop for your airframe on 3S, I'd have suggested a 3536 motor between 950 and 1100kv, then it's a bit of trial and error with 10-12" props and a wattmeter to find one that gives the performance you want without stressing the motor. Much as you would with an IC motor. Electric setups are quite flexible in general and within reasonable limits.

With your model specs you could have just looked at the Wot 4 Faom-E power train for inspiration.

[Bob Cotsford]

ps -- pick an ESC rated for 25% higher current than you expect to draw. In your case I'd look for a 40A one.

[PatMc]

Posted by SIMON CRAGG on 31/01/2020 14:26:59:

Another useful tip is to start with the weight of the model.

There are a plethora of formulas out there, but for sport use I have found that you need 150w for every pound in weight, So a lb5 model would need a 750w motor.

Another way of working out a direct replacement for example an ASP .53 is by working out how many hp the engine produces:

ASP .53 = 1.46kw

1.46kw = 1.75hp

1.00 hp = 746w

So ASP replacement needs to be 1305w

There are many variables to all this, but I have converted several models using the above criterea and they have flown a treat.

Good luck!.

Unless I completely misunderstand your maths I'm afraid they make no sense.

ASP .53 = 1.46kw but this is = 1460w.

1.00 hp = 746w - I agree with that

In which case 1460w = 1460/746 hp = 1.96hp

So ASP .53 replacement needs to be 1460w & the maths in between is not needed.

But the figures for the ASP are taken at the shaft, whereas those for electric motors are the input power.
So to deliver the same power at the shaft the wattage of an electric power combo needs to higher.

However the ic engine power peak is usually at very high rpm for 2 strokes & lower (but often still quite high) rpm for 4 strokes.
OTOH the choice of max power rpm of an electric motor can be made by using a high or low kv.
The of max power rpm can also be changed the choice of voltage (number of cells) used, which also changes the actual max power available.

BTW 150W/lb is well over the top for most models. 100w/lb is often quoted but IMO this is still too high for the majority of sports models, 60 - 80W/lb is generally plenty but a lot depends on whether a high kv motor with a small dia prop or a low kv one with a larger dia prop is possible.

 

Edited By PatMc on 31/01/2020 16:02:09

[Max Z]

have a look HERE if you want someone else to do the calcs  (limited free version)

 

Max.

Edited By Max Z on 31/01/2020 16:17:53