DJI E800 Tuned Propulsion System Fails to Reach Rated Thrust

While I am still waiting for DJI parts to continue flying, (have I mention DJI customer service sucks?) I don’t think that I have mentioned that their parts supply chain is also extremely poor. I thought that I would do some testing on the 620 ESC and 3510/350kV motor combination. I used the FrSky X8S receiver and their Taranus Plus transmitter to supply the servo Pulse Width signal directly to the ESC.

The motor responds to servo signals from 1145 micro-seconds to 2025 micro-seconds, which translates into motor RPM’s from 658 to 8670. At a Voltage of 24.87 this translates into 349.05 RPM/Volts which is approximately the rated value of 350kV, this was tested with the motor lightly loaded using a stubby prop that I fabricated from one of my broken DJI props that was destroyed by one of my brand new ESC’s that was defective from DJI ;-(

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I measured the RPM’s using a photodiode beneath the prop and a light source above it and measured the signal with my oscilloscope.

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Here is the signal at the max speed of 8760 RPM’s, note that the dips occur when the propeller shades the photodiode from the light source above it. Every other dip represents one full revolution:TEK00002However, when I redid the test with the full size DJI 13.5 inch prop the ESC motor combination only produced an RPM of 7500 and failed to reach its maximum RPM by over 1100 RPM’s. This translates into an RPM/Volt rating of only 307 kV. I used a fully charged battery with a capacity of 10,000 mAh. I took the measurements within one minute of operation.

By this time the battery voltage had dropped from its initial charge of 25.17 Volts to 24.43 Volts. This means that in the real world the motor specifications as advertised by DJI are worthless. DJI rates the RPM/Volt at 25 Volts rather than the half 15% capacity of 22.2 Volts, the point when warnings are sounded and the pilot should be thinking of landing very soon or risk an uncontrolled crash landing.  Now I am not sure of the standards in the RC community, if there are any, I can assure you that these are very deceptive and misleading marketing tactics.

I also measured the thrust by placing the motors on another rig with the motor weighted down on a scale and placed high enough that it was above any ground effects. I was only able to achieve just less than 1600 grams of thrust, well shy of the rated 2100g of thrust as advertised by DJI. This is most likely due to the fact that the motor fails to reach it’s maximum RPM by over 1100 RPM’s and it’s RPM/Volt rating when actually loaded down with the DJI propeller. I used a fully recharged battery and did the thrust test within ten seconds of operation.

This is also very shy of the 200% recommended maximum thrust rating  for its rated take off weight. I was deceived that its maximum thrust was well over the 200% rule of thumb. Actually there are several people who recommend 120% above this rating. Now with only a 1600g thrust I suppose that the 800 gram take off weight per motor barely fails to meet this minimum, and ONLY when the battery is fully charged 🙁

I also used this to test and calibrate the Attopilot current and Voltage sensor board. I found the board’s outputs actually comes extremely close to its rated values of 63.69 mV/Volt and 36.6 mV/A, the measured values were actually 63.66 mV/v and 36.22 mV/A. Remember that I overheated my board which may explain the extremely small discrepancy of the measured current.

Also if you look at the data, supplied in the spreadsheet link below, that the current compensation factor value has a much larger error in currents below three Amps but seems to level off at values above this. I did not test values greater than about fifteen amps so I would not expect laboratory grade signal from a current sense board that is rated for currents of 90A. And, like I’ve already said, this may be due to my ham handed soldering and repeated heatings of this board.

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Update, I received a new Attopilot current/sense board and the current factor is indeed much closer for currents above about 1/2 Amp. It seems that my previous board was indeed affected by excess heat but still remains fully functional.

Update 2015 06 7:

I tested four other motors that I have and found one of them made thrust, actually 2150 grams of thrust. The remaining three made between 1800 and 1950 grams of thrust.