Have i got somethinmg wrong or are these motors bad

[Rick Tee]

Bought a Weston UK Groove ID 6mm, this was probably a mistake, expected AUW 190g actual AUW 255g. Fitted it out with low cost gear from Giantcod and Budgetrc. Servos £2.30 each, synth RX £16, ztec 12A ESC £10, LiPo Loong-Max 800mah 20c £6.63, all works fine. But the motor is where it all went wrong. EMP C2028/20 1400kv 22g 90w (props tried: 8x3.8SF, 9x3.8SF, 9x7.5SF, pulled 5A) will not fly, bought EMP C2226/25 1800kv 69w 21g (prop 9x3.8SF, pulled 7A) the model flies but not enough power to go verticle and EMP C2222/31 2280kv 54w sounds like a bag of nails. Ordered EMP C2024/18 2400kv 80w 17g burnt out on a 7x5SF prop (pulled 9A), and C2226/18 2570kv 75w which i've not tried but i think like the 80w one it may be to high kv?

 

I'm new to electrics though i have been reading a bit and did check a lot of other shock fliers to compare motor recomendations with weight of model. Had the model not worked out 65g heavier then the claimed flying weight then the 70w motor would probably fly it, though i don't understand why the 90w motor produces less thrust than the 70w one. Are these EMP motors rubbish or am i missing some important info on how to select the correct motor? What motor could i buy which would fly the model without going to 3s LiPo or spending another £50?

 

I know i added about 10g by using a bigger LiPo but my motor and servos are lighter then those recomended by Weston.

[simon barr]

Hello Rick,

I am no expert on elecric flight either, but my limited knowledge tells me that you are going the wrong way with the kv ratings.  If you go for a lower kv, say 1000/1100, you will be able to turn a bigger prop which will produce more thrust.  The higher kv motors are for turning small props at high speed for fast outdoor models and ducted fans etc.

No doubt someone will come along now and put us both right...

Hope this helps,

Simon.

[Rick Tee]

Thanx Simon, i think your right, but there seems to be more to it. If you look at the motor recomended for the Click for example its 2320kv 55w, one of our members owns one and runs it with a 7x4 prop and 6A ESC, though this is a lighter model. Though as you say lower kv = bigger prop, yet the 90w motor would produced less thrust with a 9x7.5 then with the 9x3.8 and only pulled about .5A more (5.5A). Motor resistance is only slightly less in the 90w (.573ohms) then the 70w (.657 ohms, with the 9x3.8 pulls 7 - 7.5A). i realise the wattage ratings seem to be mainly based on 3s LiPo but power in does have a direct relationship with power out, the 90w has 5 less windings then the 70w and as its rated at a higher wattage i assume those windings are thicker wire hence it should draw more current relative to prop size. So the 90w should have produce more thrust on the same prop as the 70w unless the kv was too low (couldn't turn the prop fast enough to load the motor.) Either that or the info provided with these motors is inaccurate. (incidentally 2500kv ish is about mid range, I think (without checking) EDF use motors in the 3200 - 4200 kv range.)

[simon barr]

Oo err, your'e loosing me now...too technical for me...

All I can tell you is that I fly my indoor using a 1060kv motor turning a slowfly 9 x 4.7 prop with no problems...good for outdoors too, on a calm day.

[Big Bandit]

Rick,

 

I'd be inclined to contact Robotbirds for advice. I've flown a depron pitts of a slightly higher wieght to your model, on one of thier bell motors on an 8 x 4.3 prop. I did however use a slightly larger lipo (2S Kokam 1250). It may be that the one your 800mAh is sagging. have you tried the setup on a watt meter.

 

Cheers,

 

Chris.

[Rick Tee]

Thanx Chris. No i don't have a watt meter just checked the current, probably should have put a volt meter on it too. It probably is sagging with the 90w motor and 9x7.5 prop but unless i'm miles off with my guestimate 90w motor pulling 5A constant with a 9x3.8 prop or a 8x3.8 does not seem right (tested with 2 batteries, fully charged.) I suspect 3s LiPo would be the better way to go but this would cost more (need 2 as we only have an hour in the hall) then replacing the motor.

[Tim Mackey]

For your 90 watt motor to "get on step" at around say 75 - 80Watts, then on a 2s Lipo you need to prop it to pull about 11A.     80 / (2 x 3.5V =7 ) 7 = 11.4

Sometimes the data supplied with motors is next to useless - and alos rogue motors appear. I suggest you start froma different base.

Do you want fast RPM and top speed or high torque / quick accelaration? Once you have decided that, then you have "chosen your prop"

If its large and low pitch -  then choose alow Kv motor

If its small and high pitch - then choose a high Kv motor.

if ballistic flight performance style is required aim for at least 150 -200 watt per AUW lb.

Then buy a motor for that sort of wattage with the required kv.

It sounds to me like you really need a 3s pack for that mod / performance.

[Rick Tee]

Thanx Timbo. Coming to the same conclusion, once over 200g AUW there does seem to be little option but 3s for 3D performance, unfortunately i bought gear for the advertised AUW.

 

It's a 3D foam shockie so high torque / quick accelaration i guess, recomended; motor Weston Micro rex, prop 8x4.3, either 2s or 3s 450 - 600 mah LiPo. Though i very much doubt the 2s would pull this vertical with the recommended motor. I will return the motors and try calling BRC or RCM direct.

 

Couldn't find any info on the Weston Micro rex, Micro rex seem to be made by Flyware, assuming that's where it comes from then Micro rex is a range of motors, which one it is would be a guess and what info there is is in German.

[John Neale]

i use a Micro Rex on some of my shockies and find it a great little motor. However auw is the key to performance and 180g should be the top limit for good arobatic performance and prop hang with good pull out.

I use 350mah 2S 20C packs, 6amp esc with 8x4.3 prop. My experience with cheapo 20g motors is they don't do what they say on the label!.

800mah packs are far too heavy. Don't know exactly what current I draw, but I doubt it is more than 5 amps max as I expect 10 min flights from a full pack.

You may well have to go the 3S route with the weight your Groove is. However keep the pack capacity as low as you can. Certainly no more than 400mah

.

[Rick Tee]

Thanx John. Yep the Weston UK groove ID is a lemon, over weight and the structural design is poor, any moderately heavy landing or bump on the runway will result in the whole of the front end from mid wing forward being ripped off. If i'd had any experience of depron before this model arrived i would have taken one look and sent it straight back. Budget is over spent for leccy stuff so its back to IC for me.

 

Now who wants to buy a 'Groove ID rolls kinardly' (rolls down hills fine can hardly get up em), only been snapped in half once.

BTW John, which Micro rex? as i mentioned Micro rex is a range not a motor (3 motors i think.)

[John Neale]

The motor I have Rick is the  Micro rex220. It comes in 3 winds and I have the 1880 kv version.

I believe it is the standard motor that Weston uk supply. For indoor flying the 3mm depron groove is the best. I had one once and it flew quite well. However it did need quite a bit of carbon bracing as the wings were very flexible. The 6mm version is really for 3S and outdoor use.

Pity you didn't start with something else. You would have had much better siuccess 

[Rick Tee]

Yeah my bad on the 6mm thing, thought it would be a bit tougher DOH! Not sure what else I could have tried, as it would be my first experience of indoor flying i was looking for something low cost. The Click looks very nice and flies extreemly well at walking pace, but its kinda pricey. Probably get a sheet or 2 of depron an have a go at designing my own at least it couldn't be any worse than the Groove.