can conversion values

[Bob Cotsford]

as a newbie to electrics, I find the comparison between the old style brushed can motors and modern brushless confusing. Say a plan or kit states it's for a 200, 400, 480, or 600 motor, how do you convert that in terms of brushless? For example, my old Orange Box potters round on a 15 year old 540 and 7 cells, so what would be the modern equivalent? Is a comparison to , say, 250W brushless valid? I'm not expecting to get optimised performance here, just a guide to getting in the right ball park.

Additionaly, what would my 300 W 1000kv motor be the equivalent of? I bought it originaly to power a soarer, but fancy something along the lines of a Gemini - would it be man enough on3S?  Any other aeobatic suggestions?

[Bruce Richards]

I have just put together a Gemini for my son and have spent quite a lot of time considering a power set for it. I think your 300w 1000kva motor should be fine on 3s for the Gemini. What weight is the motor. I have read that to archive a good balance point you need 130-160g motor. You can always add weight to the nose if your motor is lighter than this but who wants to add weight to a light foam model!

Sorry I am too new to the hobby to know anything abut brushed motor sizes. 

[Bob Cotsford]

according to the BRC website it's 102gms, it's the HiModel A2814-8 -:

"NEW! HiModel A2814-8 (300W) 1000Kv Brushless Outrunner Motor supplied with Alloy motor mount & Prop Adaptor.

This powerful brushless outrunner motor is designed to get maximum performance on lower cell counts IE: 8 Cells NiMH/NiCad or 3S LiPo, bringing the cost of the project down! The motor shaft can be moved to exit the other end of the motor if required.

We include a BRC HOBBIES PROP DATA SHEET for this motor to give you an idea of what prop to use in your model depending on what batteries you use.

Each motor is included with a Alloy Prop Adapter and a Radial Alloy Motor Mount!

SPECS:
* Type: 2814-8T
* Dimension: D35 x L36mm
* Weight: 102g (3.6oz)
* Motor shaft dia: 4mm
* Kv: 1000 RPM
* Efficiency: >80%
* Voltage: 8-10 Cells Ni-xx or 2-3S LiPo
* Cont Current: 8-25A
* Max Current: 30A (10 sec bursts)

As this motor has a Kv rating (rpm per volt) of 1000 it will swing larger props (10-12") at lower rpm making it ideal for Trainers, Scale, Vintage or Warbird type models on lower cell counts upto 5 lbs."

up to 5lbs it says - what chance of it lifting my Veron 1 1/2 Strutter then?  Might just give it a go to try the system out
 

[Alistair Taylor]

Hiya Bob

I know what you mean about can motors v brushless motors - it's a nightmare trying to get a handle on how they compare to each other!

I tend to bear the following rules of thumb in mind when trying to compare them:

1)  A direct drive can motor is a bit like a high KV brushless - both are happiest spinning a tiny prop very quickly (or EDF)

2) A can motor with a gearbox is like a lower KV brushless - Happier spinning big props fairly quickly, or slowly.

3) brushless motors are more efficient - i.e. more of the energy that is put in to the motor comes out as power to the prop (instead of heat and noise).

4) 100 watts per pound is a good starting point for successful flight.

5) Big props are more efficient than little props, but little props are better for flying quickly

What this all means is that there is more than one way to skin a cat....i.e.

Starting from the airframe - you have a 3lb plane, you need a 300W motor. if it's a small slippery plane, you will want a high revving small prop - in which case use direct drive (Speed 600) can motor or high Kv brushless. If it's a big, lightly loaded, slow plane (like a glider) a slower spinning bigger prop will be more efficient - so use a gearbox or lower KV brushless.

OR

Starting from the motor - You've been running a 540 motor on six cells at 25 amps. So that,s (erm) about 175 watts.  If you replace this setup with a 175 watt brushless setup, you will have a better flying plane than before because of the improved efficiency of the brushless motor (assuming you're using the same cells).

OR

Starting from the prop, your speed 600 can motor spins an 8x4.5 prop at lots of RPMs, but your electric glider climbs like a brick. You could add a gearbox and spin a bigger prop more slowly for greater efficiency (not forgetting that the gearbox will also sap some power) or switch to a low KV brushless spinning the same prop.

Speaking in ballpark figures (roughly):

Speed 600 can = around  200 - 300 watts max. Speed 400 can = 100 - 150 watts max

However, you'll not get much more than 50% efficiency out of these motors in direct drive mode (and this will get worse as the brushes wear out) so your prop is only getting half the power that goes in.

What this means in the real world is - replacing the direct drive can motor in a glider (tiny prop, crap performance) with a low kv brushless motor spinning a bigger prop will net you better prop effciency, better motor efficiency, and the possibility of switching to a smaller/lighter battery pack, as your glider's power train will give you same performance as previously on less amps.

 Sorry this is a bit rambly, I guess the short answer is buy a whattmeter and spend the afternoons when the weather is crap testing motors in the back garden - beats watching antiques roadshow....

AlistairT

[Tim Mackey]

[Former Member]

[This posting has been removed]

[Bob Cotsford]

"Starting from the prop, your speed 600 can motor spins an 8x4.5 prop at lots of RPMs, but your electric glider climbs like a brick. You could add a gearbox and spin a bigger prop more slowly for greater efficiency (not forgetting that the gearbox will also sap some power) or switch to a low KV brushless spinning the same prop."

you've seen it fly Eric!

So for sports aerobatics I could really do with a bit higher kv than the 1000kv unit I've got? That's what I half expected as it seems happiest on a 12*6.  Well, it was bought to tug a 100" soarer up, but with the weather at present lightly loaded thermal soarers hardly seem appropriate.