Honest thrust measurements for Hextronic 5g 2000kv
Hey guys, did anyone measure the thrust of the Hextronic 5g 2000kv motor ?
I measured it by suspending it downwards from a scale and got these results:
-GWS EP-5030: 54.2g
-GWS EP-4540: 47.2g
-GWS EP-4025: 36.6g
using a 2s Nanotech 180mAh battery and a HobbyKing 6A ESC.
The measurements reported on the HobbyKing page are much better:
-GWS 5030: 76g
-GWS 4540: 65g
-GWS 4025: 53gr
I'm a bit suspicious of those results though.
Can anyone post their measurements here ?
I looked at that motor on HK. Since you've had it apart, how many poles, magnets, wire dia and turns, no load current and no load rpm at a measured voltage. I think you said its wound "y" and has 8 ohms res.
I would like to run the numbers and see how efficient it is.
I measured the current and power consumption with the same configuration as before:
-motor Hextronic 5g 2000kv
-ESC: HobbyKing 6A ESC: unknown settings
-battery: Nanotech 180mAh, 2s, 25-40C
-wattmeter: Turnigy 130A from HobbyKing
The measurements were peak power only, lasting only a few seconds.
The battery was freshly charged at the beginning but not recharged between measurements.
-GWS EP-5030: 1.72Ap, 13.3Wp, 7.72Vm
-GWS EP-4540: 1.82Ap, 14.0Wp, 7.67Vm
-GWS EP-4025: 1.35Ap, 10.5Wp, 7.79Vm
No load measurements were done with the motor mount on but without the plastic ring that holds the prop. in place:
Same config as above:
-0.34Ap, 2.7Wp, 8.12Vm, 8.20V at start, 8.18V at the end
The notation used are the same as reported by the wattmeter:
Ap = peak current in amps.
Wp = peak power in watts
Vm = the min. voltage in volts, not necessarily taken at the same time as Ap and Wp
Then I tried to measure the no load current using:
-3s 2200 old battery from an ArtTech SU-27 with 12.55V at the start
-ESC: HobbyKing 6A ESC unknown settings: 0.33Ap, 4.1Wp, 12.53Vm
-ESC: HobbyKing BlueSeries 30A ESC unknown settings: 0.40Ap, 5.0Wp, 12.52Vm
Problem: for both ESCs, at maximum throttle, the rpm was not constant but varying at about 1Hz, giving a "wailing" sound.
I'm not sure why that happened, maybe the rpm was too high for the ESCs ? Or maybe I need to change their settings ?
I'll discount these (the no load with 3s battery) results for now since they obviously are not correct.
I have ordered an rpm meter but it didn't arrive yet, so no rpm data for now.
I'll measure the rpm when it arrives.
For the other data you need see this thread:
Here is some relevant data from the first post:
The original configuration of this motor was a 12 pole stator with 16 magnets wound with 2 strands of 37 gauge with 17 turns (ABC wye)
What software are you using to compute the efficiency ?
Did some calculations with the online calculator at
-custom motor with Kv=2000, res=1.6Ohm, no-load: 0.34A at 7.94V
-LiPo: 150mA, 20/30C, 2s, res=0.15Ohm, voltage=3.7V
-controller: max 10A with res=0.015 Ohm
-propeller: custom, 0 deg., diam=5", pitch=3", blades=2, prop. const.=1.18, gear=1.0
-thrust=79.5g, current=1.53A, voltage=6.94, power=11.32, eff.=50.6%
For the same config. but pitch=1.5":
-thrust=54.1g, current=1.14A, voltage=7.06, power=8.44W, eff.=54.1%
The calculator seems to indicate a much higher static thrust than what I'm getting.
I did some calculation using similar data with this online calculator:
-custom motor with Kv=2000, res=1.6Ohm, current: 0.34A
-LiPo: 180mA, 2s, res=0.15Ohm, voltage=3.7V
-controller: Castle Phoenix 10A: res=0.013 Ohm
-propeller: custom, diam=5", pitch=3", blades=2, prop. const.=1.18, gear=1.0
-thrust=76.41g, current=1.637A, voltage=6.89V, power=11.28, eff.=45.2%
So the thrust is a little less than that of the previous calculator, but very close to it and very far from real life measurements.
Any ideas on why the big difference between the calculators and real life ?
The rpm is not needed to find the efficiency, nor is the true kv because you know the Io amp draw and Rint.
E = Efficiency = Po/Pin Io=no load amps Rint= internal resistance, should include esc fet resistance since they are in series with the motor leads.
Pin=Vin* Ain Po=( Vin-(Ain*Rint)) * (Ain-Io)
E = ( ( 1- (sq root of (Io * Rint / Vin )) )^2
Amps at max power = ( Vin + (Rint*Io)) / (2*Rint)
Io increases with rpm, its a curve. when you get your new meter graph kv and Io at various rpm. Graph the calculated amps, power and efficiency too.
Or, failing that, some ideas on where the difference comes from.
If there is nothing wrong with your motor, prop or scale then use your numbers, but the more accurate your measurements are the closer you can get to the truth.
Trying to understand or explain someone's calc program without knowing everything about it is a waste of time.
The equations I posted have been used long before brushless motors were available.
Not every motor is created equally. I've seen lots of variation in hextronic motors. In twin motor airplanes, I have to sometimes go through 6-7 motors to get two of them that run close to the same rpm. It does seem like the lower the KV and the larger the motor, the smaller the differences. The actual voltage supplied under load could be different too.
Thrust generated by a prop in flight is really hard to measure realistically. And we are often using props that, regardless of what the markings on them may say, often have measured pitches that are quite different from the markings.
Not sure if it is helpful to you or not but I like to go to the prop database at flybrushless.com and look up a prop by brand and model name or markings. Then I can see a graph of the static thrust obtained in test by various prop testers who voluntarily feed the motor and prop databases there.
That also leads me to a list of the motors that thrust data was obtained with and that is useful information. And if I'm contemplating a prop choice for a plane it gives me a potential thrust to weight ratio to consider and also a theoretical pitch speed.
I don't consider any of it to be empirical but it is certainly enough to get me in the ballpark as far as what might be good to try for props and motors.
Another thing I like about the data there is that the RPM readings for a given prop are all safe or practical to use numbers. They don't report readings that are in excess of safe RPM limits or that caused the motor temperatures to become excessive.
Either your motor is running significantly slower then it should, or your thrust measurement is inaccurate (perhaps a little of both?).
Plantoflap, Joe 1320, yomgui, jackerbes and Bruce,
Thanks a lot for the very useful information you provided.
Since the discrepancy between what others reported and what I measure is big, I need to take some additional steps to validate my results.
Once the rpm meter I ordered arrives, I will be able to get some additional data, i.e. the rpm at various voltages and thus reduce the number or possible causes.
As Bruce also suggested, I might need to validate the way I measure the thrust.
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