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Speed vs Batt Life.

Discussion in 'Pro/Adv Discussion' started by Deep6, Jan 23, 2016.

  1. Deep6

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    Does anyone have any info/experience on what is the optimum/most economical speed vs batt life for a P3P? For example at 90 in my car I am burning more fuel per mile than at 60...

    For instance, if I were to plan a mission on Litchi mission planner, say at 30 mph, I would expect a shorter flight time than at 20 mph, however it would take longer to complete the mission at 20 mph. There must be a sweet spot in there somewhere for max distance vs batt life...

    Thx!
     
  2. Wibble

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    Interesting post would be very useful to know!
    Anyone???
     
  3. msinger

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    This would be very hard to calculate and/or to reproduce later since the wind speeds are always variable.
     
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  4. Gforce98

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    I have an interesting experience on this and have been wondering the same thing myself. Keeping in mind the changing wind variable this is what I noticed.

    After flying out with a tail wind to about 18k feet away as I initiated RTH I noticed it would require more battery to get home than I had remaining looking at the meter on the DJI go app for RTH.
    The gap between batt life and getting home was about 1:30-2 min with return home "cruising" speed of about 20mph With that said, and panicking a bit I give it full power on the right stick to come back faster. After about a minute and doing around 26mph I noticed the gap from battery remaining to needed for getting home was increasing and now closer to 2:30-3 min. So I release the right stick and let it " cruise" home and again I see the gap shrink to where it eventually closes and matches up for a close call but safe home landing.


    Sent from my iPhone using PhantomPilots mobile app
     
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  5. fordruid

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    Much like driving a car, each persons mileage is different (YMMV). Keep a log of your flights and the details and you will soon find the 'sweet spot' for you. But flying styles are different so what works for me probably wouldn't work for you. Heck, I'm 58, my left turn signal has been on for 3 years.
     
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  6. donutlou

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    It should be able to be calculated given static values for wind speed and altitude and direction change. Using zero then adjusting for your local conditions.


    Sent from my iPad using PhantomPilots mobile app
     
  7. Deep6

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    Yeah I fully realize that conditions will change results, but that's what I was fishing for, was a control condition where at 0 mph wind and a forward flight at x speed it will be more efficient than at y speed....


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  8. RCFantom

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    I have often wondered the same thing, even if your way out over a lake, and you need to get it back... Quick or Slow????
     
  9. WMTI

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    I wonder if DJI didn't already calculate the optimal power setting for 0 wind conditions and uses this for RTH which might explain why it seems less than full throttle?

    With GPS measuring ground speed, it could be further optimized for a head or tail wind. Maybe in a future update they will do this. Then with a tail wind and in RTH mode it could throttle back and enjoy the free assist and arrive home with the max possible battery life remaining.
     
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  10. Air Ontario

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    With your current configuration, see what the speed is at cruise(75%) and zero wind.

    Figure your time and speed to climb to altitude.

    Plot desired heading. Plot wind speed, direction and its affect on ground speed and heading.(ie how much of that 75% cruise ground speed do I really have against a 20 mph headwind?)

    Then you should have an accurate heading and groundspeed at a cruise speed with the wind you have at that time from that heading. Plot your flight and waypoints, timing, etc.

    After launching, time with watch between a few distinguishable landmarks on GPS map. That will tell you if your planning was on the money or better/worse. Then you can adjust mission speed, length etc. based on battery left.
     
  11. alokbhargava

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    I'm lost with your suggestions:) Never mind.

    Can you workout these details and give us the numbers?

    Let's figure out DJI has worked out 20plus minutes of flying time at what conditions? That would give us a hint at the optimal conditions and the battery life.
     
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  12. neven

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    There is no optimal speed, there is optimal throttle % which will cover most distance from single battery charge. From my expirience that is around 60%.


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  13. joeruby

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    ~~~~~~~~~~~~~~~~~~~~~~~~
    I think I can figure this out with some more work and more analysis.
    During the flight:
    1) Speed is a function of time. s(t)
    2) Distance traveled d(t) is the integral of the speed s(t) with respect to time t. (I can't type integral math symbols here)
    3) Battery current is a function of time i(t)
    4) Amp hours used out of the battery is the integral of current i(t) with respect to time t.
    5) The capacity of the P3P battery is a constant = 4.48 Amp-Hours
    6) We should plot speed s(t) on the vertical axis, and current i(t) on the horizontal axis.
    7) There is a max speed S, and there is a max current I. Plot this point on our graph.
    8) If you draw a straight line from the origin (0,0) to S,I then any combination (speed, current) along this line will give the same maximum distance by the time the battery capacity is consumed.
    9) Note that in a hover the speed s(t) = 0 , but the current i(t) is not zero. Plot this point on our graph.
    10) Now draw a dotted and curved line from the hover point to the maximum point S,I Should the dotted line be drawn concave up or concave down? My experience with physics tells me to expect the dotted line will be concave down, and thus the dotted line will go above the straight line. When the raw data of s(t) and i(t) is measured and plotted, we will know for sure if it is concave down.
    11) The optimum speed for longest distance will be where the vertical (speed) difference between the dotted line and the straight line reaches a maximum. Once we have the curves plotted we can calculate this maximum.

    Note that we need to find a way to measure the battery current i(t) during the flight and save it with the rest of the data for each flight.

    Maybe DJI will do that for us and save i(t) in the txt, csv (excel) file.

    This is my first thoughts on the problem after thinking about how to solve it for about an hour.

    Maybe this will get some of you guys/gals thinking about this problem too.

    Keep the shinny side up,

    Joe
    KC7GHT
     
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  14. gegetrane

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    I truly expect 20mph to be very close to the sweet spot in low to moderate winds. Maybe 1min of flight more or less with a 5mph variation ? Anyway, good to know but I try to leave 25-30% battery life on landing. And very sure full throttle is bad ratio.
     
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  15. Mark The Droner

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    Yes.

    This discussion was in another thread a while back. My assumption is the engineers at DJI already figured this out and wrote the optimum speed into the RTH algorithm.
     
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  16. happydays

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    I would tend to agree with this train of thought.
     
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  17. bubbers

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    I would increase speed returning with a headwind to get most distance. For example if you were returning into a 20 mph headwind at 30 mph your speed over the ground would be 10 mph. If you increased to 35 mph your ground speed would increase by 50% to 15 mph. Our airliners use this formula in their on board computers to compute optimum long range cruise speed using wind component.
     
  18. N017RW

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    Problem is battery drain is non-linear over most of it's duration.

    Did I miss that correction???
     
  19. olof Ekbergh

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    One thing to bear in mind is that wind changes drastically with altitude both in direction and velocity. In general it is faster higher up. So flying into a headwind in general will be faster ground speed at lower altitude, and flying down wind will be faster at higher altitude.

    This is one reason that flying in Atti is so nice, you can instantly figure out the wind direction and speed by just drifting for a bit.

    My guess as to most efficient airspeed is that DJI figured that out as stated in posts above. And it seems to be around 15-18 mph. A helicopter will always be more efficient at a certain speed where the transitional lift balances with the aerodynamic drag. Hovering is not efficient at all.
     
  20. bubbers

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    True, as is fuel burn per hr increases with higher speed.
    My example of 10 mph vs 15 mph by increasing speed if you had 6 minutes of battery left you would go 1 mile. By increasing speed you may lose up to 20 % battery life so would have 4.8 min at 15 speed so would go 1.2 miles. Someday that might keep you dry. Also usually lower altitudes have less wind so altitude is important.