How do you compare electric motors

[PatMc]

Nigel, I've just had a look at the Easy Street instruction manual. The motor listed in the manual is this 1200 kv version not the 1000kv version.
HK UK warehouse have this 1250 kv version with the same mount orientation as the Quantum, this older series 1250 kv version , and there's an 1185 kv one as well.

[Peter Beeney]

Nigel,

There is a Quantum II 25 speed model, here, that might be something else to look at. kV 1250, 50 amp continuous current, a 4 cell battery, 60A ESC and a 7 x 7 prop recommended.

If this is worth anything I would check the unloaded kV with a tacho on 4 fully charged cells to make sure the plate value is correct, it usually is but certainly not always, and with this motor that should result in about 21,000 rpm. Then I’d try a 7 x 7 APC i/c prop and check the revs again. These will certainly be lower, but hopefully not too much and I’d also check the current with a clip on ammeter. After a bit of a prolonged run I’d check everything to make sure there are no hot spots. If all is ok I’d try it in the air. If this set up doesn’t really suit the Easy Street, a bit too quick maybe, then I could change the prop for say an 8 x 6 or even a 9 x 5 for roughly the same power requirements. But I’d check each time to see what was going on to make sure I’m still within all limits. Fitting a bigger prop can sometimes add to the load and hence the heating effect if I’m not careful.

If the ammeter now reads 40 amps say then this is about 600 watts, which I suspect would compare fairly well with a good 2.5cc i/c engine. Although the engine power is perhaps generally more accurate because it’s power measurement figure is what the crankshaft is actually producing whereas my electrical measurement is just that of the heat produced in the whole circuit; and they don’t always amount to the same thing.

If I did the same exercise with a 1,000 kV motor I would of course start with a figure of 16,800 rpm. Or 12,000 for 3 cells. 600 watts translates to around 0.8 HP.

Good luck.

PB

[Nigel Heather]

Posted by Peter Beeney on 01/09/2018 23:50:37:

Nigel,

There is a Quantum II 25 speed model, here, that might be something else to look at. kV 1250, 50 amp continuous current, a 4 cell battery, 60A ESC and a 7 x 7 prop recommended.

If this is worth anything I would check the unloaded kV with a tacho on 4 fully charged cells to make sure the plate value is correct, it usually is but certainly not always, and with this motor that should result in about 21,000 rpm. Then I’d try a 7 x 7 APC i/c prop and check the revs again. These will certainly be lower, but hopefully not too much and I’d also check the current with a clip on ammeter. After a bit of a prolonged run I’d check everything to make sure there are no hot spots. If all is ok I’d try it in the air. If this set up doesn’t really suit the Easy Street, a bit too quick maybe, then I could change the prop for say an 8 x 6 or even a 9 x 5 for roughly the same power requirements. But I’d check each time to see what was going on to make sure I’m still within all limits. Fitting a bigger prop can sometimes add to the load and hence the heating effect if I’m not careful.

If the ammeter now reads 40 amps say then this is about 600 watts, which I suspect would compare fairly well with a good 2.5cc i/c engine. Although the engine power is perhaps generally more accurate because it’s power measurement figure is what the crankshaft is actually producing whereas my electrical measurement is just that of the heat produced in the whole circuit; and they don’t always amount to the same thing.

If I did the same exercise with a 1,000 kV motor I would of course start with a figure of 16,800 rpm. Or 12,000 for 3 cells. 600 watts translates to around 0.8 HP.

Good luck.

PB

Many thanks. The link you provided is for the Qunatum II 20 not the Quantum II 25. They do sell the 25 but it is out of stock, as it is most places.

This is still a twinkle in the eye, I’ve not bought anything yet and I may not. Thos has come about through some research into possible planes that will fly well on 3S 2200 batteries that I already have for a Trex 450 helicopter.

If I did get the Easy Street I wouldn’t fly 4S because (a) I already have a bunch of suitable 3S and (b) I have seen it flying on 3S and it is plenty lively enough for me, on 4S it must be ballistic.

The thing that prompted this thread was thinking that if the Quantum II 25 were unavailable when I needed it what would I get instead and at that point I realised it wasn’t very straight-forward to look at another manufacturer and say “that will do nicely, I’ll have that one instead”.

Cheers,

Nigel

[Chris Walby]

All confusing stuff, could just give George a call at 4-Max discuss what you want to do and I am sue he could recommend a motor, esc and prop set up to suit your specific requirements (3S)

Just depends on how much time you want to spend working out what you need or up at the flying field.

[Former Member]

[This posting has been removed]

[Nigel Heather]

To be honest, by the time I get round to buying the plane, the Quantum will be back in stock and I'll get one of those. In the meantime, this is just curiosity as I like to understand my hobbies.

[Dave Hess]

I seem to remember about 30 years ago, when electric motors were just starting to become more popular, that they had 20, 30, 40 size electric motors that were supposed to be equivalent to 20, 30, 40 size IC motors, or was that just a coincidence and the number was the diameter of the rotor or something? Has that idea passed it's sell by date? I notice that OP's preferred electric motor was a Magnum 25 to replace a 25 size IC motor.

Edited By Dave Hess on 02/09/2018 09:24:48

[Ron Gray]

Taking the Easy Street as the example, do you want it to fly mega quick? Take the specification weight (the weight is the AUW), 45oz/16=2.8 say 3lbs then based on what sort of performance you want look at what motors will give you >100w/lb. so let’s assume that you want it to fly fairly quick, but not too quick, you would be looking at in the region of 150 - 200. With that in mind I would start by looking at motors that will actually fit, especially the length, after all, no point in having a motor that sticks out of the cowl! Looking at the quantum specs it is 42mm long (35mm dia) with a Kv of 1000. Looking at the 4-max website, George has got an equivalent motor for the Quantum, a 3540-1070 (35mm dia, 40mm length and 1070 Kv). Using the formula that BEB has detailed above, this gives on 3s 366W (3s x 3.7v x 33a) which when divided by the AUW gives you 120W/lb, on 4s it would be 162W/lb. the Quantum on the other hand on 4s gives you 246W/lb, the extra due to the fact that it is rated at 50a whereas the 4-max one is 33a. At that power with the Quantum the Easy Street is ballistic.

[Bob Cotsford]

I'm using a Propdrive 3542 from Hobbyking in mine iirc, running a 10*5 (must check!) prop with a 60A esc. I've not tried it on 2200 cells but I would imagine you would be looking at 5 minutes maximum, I run 3000 3S packs but I use a current sensor and have a warning set at 1800mAh consumption at which point I think about landing, this I would guess gives me about 7 minutes flying maximum.

Edited By Bob Cotsford on 02/09/2018 10:09:05

[Gordon Tarling]

I'd suggest that you take a look at Ecalc - it's an online calculator which costs peanuts to subscribe to. It's well supported and has matured into a very useful tool over the years. The restricted trial version is free to use and that will give you a taste of what it can do. The motors database doesn't have data for the 4-max motors and some others, but a very close equivalent can usually be found. I use it frequently and find it useful as well as fairly accurate.

[Dickw]

I would second Gordon's suggestion of motor calculator software. Ecalc is good as he says, but alternatives are Drive Calculator and MotoCalc.

I use MotoCalc mostly.

Dick

[kc]

The easy way around this is to see what works in another similar type of model of the same size& weight and just copy! But you need to keep the same kv, same number of cells and same size prop for it to work. And of course an ESC of sufficient amps.

The 4Max website includes a page of equivalent motors.

[Ron Gray]

If you look at the Easy Street model on the 4-max website George lists his equivalent motor for that ‘plane, takes out all of the guesswork. Just be aware that if you do go for the Quantum motor, it will be fast, even on 3s (185W/lb). My own Easy Street on 4s (Propdrive motor) eats up the sky and can soon disappear both vertically and horizontally!

[Nigel Heather]

Posted by Ron Gray on 02/09/2018 13:05:48:

If you look at the Easy Street model on the 4-max website George lists his equivalent motor for that ‘plane, takes out all of the guesswork. Just be aware that if you do go for the Quantum motor, it will be fast, even on 3s (185W/lb). My own Easy Street on 4s (Propdrive motor) eats up the sky and can soon disappear both vertically and horizontally!

Thanks.

Yes, I am expecting it to be fast even on 3S - that is why I’m not going 4S (and that I have a bunch of 3S batteries).

Cheers,

Nigel

[Keith Berriman]

Hi Nigel make sure you get the Quantum 2 25 Speed at 1250 kv for the Easy Street 2

I am flying mine with an under carriage on 3 s lipos with timer set at 5.5 mins and landing with 30% left in battery

the lipo is rated at 35 c discharge rate and all is cool on landing

The model is fast if needed and soon a spec on the horizon

[Peter Beeney]

If you do eventually decide to have a little dabble with the Easy Street II Nigel, there are some alternative motors which just might be worth having a quick butchers at such as this little gem, the Turnigy Aerodrive. This is the one that PatMc linked to first. Same price at 31 notes; size and weight too; 1 gram difference. This one has a kV of 1250, (I know, I know, but keep going!) 755 watts and a 5mm shaft. I particularly like the resistance though, at 22 milliohms that looks like good value for money. The total resistance of an electric motor power train is a bit of a Holy Grail, the lower it is the better it is!

This motor also has some good reviews, I reckon it will go very well on three cells. If you were to bolt a 10 x 3 APC on to start with with I’d roughly calculate a S & L speed of 33 - 35 mph on 3S. This is now handleable for sure, even a little on the docile side perhaps. The current consumption will be low as well, most likely below 20 amps so everything will stay cold and the battery will go on forever. Well, perhaps not ever but certainly a good few minutes. If you ever fancied winding it up a few shades you can just change the prop up one pitch at the time.

And indeed at the end of the day I’d consider that if you wanted to it’s still got all the potential to go all the way up to ballistic on steroids…

Happy landings…

PB