A warning to eCalc users

Posted by DG on June 4, 2015 at 9:28am

eCalc estimates for throttle @hover have grossly changed in recent versions at least for my KDE 2814XF-515 motors.

According to eCalc throttle @hover should be 65%. In reality it averages ~45% with the exact same configuration as shown below. When first considering building this quadcopter, eCalc was close to reality when using 13x5.5 APC props and 12000 mah battery. I clearly remember plugging the numbers in and being satisfied my motor/prop/weight combination would work and were well below 50%. 

I have contacted eCalc and their position at first was I must not be interpreting the flight data correctly. This is nonsense, as the flight logs clearly show Throut and Thrin both firmly below 50% @hover in stab mode. If eCalc data were correct, the copter would barely have enough power after takeoff. The pwm agrees.

eCalc says current draw is 6.07A * 4= 24.28A total. Reality is ~20A. On my Y6 eCalc says 36A. Reality is ~26. 

I don't know how many scenarios these errors apply to. This is just a cautionary note to double check eCalc before using it.

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Replies

  • Antonie Kruger June 9, 2015 at 7:40am

    Ouch...I've just built a Talon 820 hex with 6 KDE3510's and KDE ESC's. I ran the calc around a month ago. I did maiden today...around 3 minutes checking that all seems sound. I'll put her through her paces over the weekend an compare to my original ecalc sheet. My design current draw has changes as well and the mixed flight time has reduced by 30%. Cant wait for the weekend.

    • DG > Antonie Kruger June 9, 2015 at 11:50am

      what is your actual current draw at hover compared to eCalc?

      I have Frsky Lipo sensor that gives me total and per cell v on the Taranis. It is much better than the PM which as voltage drops the error increases telling me the total is say 14.5 when the Lipo sensor says 15. Battery failsafe wants to land the copter, but I know there's a lot more flight time left. I'm to the point of disabling the PM and use just the Frsky Lipo sensor, but then if there's a problem (flyway cough cough), it should have battery FS functioning. I wonder if there's a way to integrate the more accurate Lipo sensor into the FC. 

  • letiidronletii June 7, 2015 at 2:10am

    I checked this morning and it seems to be repaird. Now we have two values for throttle - log and linear. Log throttle values are the ones we have before the update.

    • K V > letiidronletii June 8, 2015 at 6:53am

      Values are better than a couple of days ago but still not what i used to see a few weeks ago.

    • DG > K V June 8, 2015 at 10:22am
      Agreed, the numbers are still quite a bit off compared to when I originally did it.
    • K V > DG June 9, 2015 at 2:58am

      Also, I forgot to mention this but ecalc doesn't take the weight of the propellers in the calculations. If you are using sub 12" props, its not a big deal but 13-18" CF props add about 50gms each and when you go to 26-29" CF props, each adds about 100gms.They really have to add this to their calcs.

    • DG > K V June 9, 2015 at 11:37am

      I think you're expected to take everything into account when entering the frame weight (not including drive). For example, my quad is 390g, but I estimated the total at 590g (FC, GPS etc.) before eCalc does its thing.  It worked out quite well when eCalc was giving good throttle @hover estimates. Their current estimates were off though.

  • DB June 6, 2015 at 7:09am

    Have you tried this tool?  

    http://controls.ae.gatech.edu/dbershad/EMSTAirTimeCalculator.html

    I ran your data and got the following results.  Seems to match better with what you've posted.

    Battery C required 2.00 (C) Component weights: motors 628.61 (g, total)
    Throttle / collective required 42.70 (%) ESCs 85.30 (g, total)
    Drive current required 20.56 (A) Batteries (drive) 1742.20 (g, total)
    Motor electric power / motor 82.02 (W) Payload 0.00 (g, total)
    Motor mechanical power / motor 59.06 (W) Structural 635.17 (g, total)
    Efficiency: specific thrust / motor 8.36 (g/W) Wiring 135.85 (g, total)
    Battery degredation coef V 100.00 (%) Avionics 50.00 (g, total)
    Battery degredation coef C 100.00 (%) Propellers 49.22 (g, total)
    Vehicle pitch angle 0.00 (deg) GTOW 2716.99 (g, total)
    Propeller speed 3254.59 (RPM)
    Motor speed 3254.59 (RPM)
    Tip speed 60.60 (m/s)
    Tip Mach number 0.18 (Mach)
    Motor torque / motor 0.08 (Nm)
    Disc loading 6.84 (kg/m^2)
    Power loading 33.44 (kg/kW)
    Max governor 0.00 (%)
    Blade pitch 4.70 (deg,")
    Mixed flight time 18.24 (min) Hover time 30.39 (min)
    Thrust/weight @ 80%, 100% throttle 3.54 5.53 (-)
    Specific thrust @ 80%, 100% 2.18 2.20 (g/W)
    Current / motor @ 80%, 100% 17.26 26.80 (A)
    Motor electric power @ 80%, 100% 275.53 427.73 (W)

    • Sensitivity analysis

    Hover time sensitivity to KV, dt/dKV -0.06 (min/KV) -5.71 (min/100KV)
    Hover time sensitivity to diameter, dt/dD -2.00 (min/in) -2.00 (min/1in)
    Hover time sensitivity to pitch, dt/dP -11.21 (min/in) -2.80 (min/0.25in)
    Hover time sensitivity to GTOW, dt/dm -0.01 (min/g) -0.99 (min/99.9998g)
    Hover time sensitivity to capacity, dt/dC 0.00 (min/mAh) 1.96 (min/1000mAh)
    Hover time sensitivity to altitude, dt/dAlt -0.00 (min/m) -1.95 (min/1000m)

    • Maximum range analysis

    Airspeed (m/s) Pitch (deg) Throttle/collective (%) Drive current (A) Flight time (min) Range (km)
    0.00 0.00 41.90 19.84 31.46 0.00
    1.50 10.10 42.70 20.35 30.69 2.76
    3.00 15.60 43.90 20.99 29.79 5.36
    4.50 20.10 45.20 22.01 28.43 7.68
    6.00 24.10 47.20 23.87 26.27 9.46
    7.50 27.60 50.50 27.64 22.75 10.24
    9.00 30.60 54.30 32.29 19.52 10.54
    10.50 33.10 58.60 37.80 16.71 10.53
    12.00 35.10 63.20 42.89 14.75 10.62
    13.50 37.60 68.50 51.43 12.32 9.98
    15.00 39.10 73.80 58.28 10.89 9.80
    • DG > DB June 7, 2015 at 1:45pm

      Now that eCalc is supposedly fixed, I'll compare the two.

    • DB > DG June 12, 2015 at 1:07pm

      Any luck with the comparison?

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