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Help my indecision and confirm that I should build an x8

Discussion in 'Drone Builds and Kits' started by MKMaverick, Nov 10, 2016.

  1. MKMaverick

    MKMaverick Newly addicted...

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    May 8, 2014
    Posted by MKMaverick, Nov 10, 2016 #1
    Let me start by saying I flew an x8 years ago, then switched over to a flat 8 because the x just wasn't giving me enough flight time with the setup/payload configuration. The flat setup has been working for me until now, but I'll admit I've noticed a difference in wind resistance, handling in wind, etc. I'm now starting to build a new system for aerial LiDAR work, AUW will be around the 8KG mark.

    It will be a 6s octo with MK electronics stack (double quadroXL board), and I've all but decided on the dualsky xm7010 7.5 motors, mostly because the MK guys have successfully tested them with their boards.
    The frame I'll be using is 1200mm, so max 18" props in a flat setup. However...and here's where the indecision kicks in. I've been researching and evaluating the pros and cons of three different potential setups.

    1. Standard flat 8
    2. Coaxial X8
    3. Flat 8 with overlapping props (jumping them up to 20", which the motors like)

    The standard flat 8 seems to be the safe choice, but trades wind stability/resistance for better endurance.

    The coaxial would still allow me to swing the 20's, but give me better wind resistance and stability at the loss of some flight time.

    The overlapping prop setup would essentially give me the same wind stability/resistance as the "standard" flat, but some data and testing that I've read through suggests that there could actually be an increase in efficiency with a small overlap (which mine would be at 10%) combined with swinging a larger prop, which is inherently more efficient...this setup would hypothetically give me the longest flight times.

    But here's something I didn't really consider before...I live on the east coast of Canada, and it's windy here, pretty much all the time.
    So...if the flat octo is more affected by wind, and therefore burning more juice when flying in wind, wouldn't that offset any efficiency gain over a coax setup? But still give me a less stable platform in wind?

    If I'm right on that, which logically it seems like I am...then the only time the x8 would be ACTUALLY less efficient that the flat would be in a little to no wind situation, which is rare here.

    Like I said, it's been a long time since I was into the coax scene, but from what I've been reading all the discussions seem to be pretty much the same as they were back then....with no real conclusive data or testing on the "optimal" coax setup (ie, top or bottom motors spinning faster, top or bottom props larger/smaller diameter or more/less pitch)

    haha, so reading back through this it seems like I've decided on the x8....unless there is a compelling argument to not go that route based on what I've described.

    thanks
     
    bassmission likes this.
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  2. Jason S

    Jason S Member

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    Nov 11, 2011
    Japan
    Posted by Jason S, Nov 10, 2016 #2
    I love my x8 with microkopter its definitely my go to bird. I have a flat 8 that is my back-up bird it pretty much lives in the car!! lol
     
  3. MKMaverick

    MKMaverick Newly addicted...

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    May 8, 2014
    Posted by MKMaverick, Nov 14, 2016 #3
    Have you done any experimenting with different prop sizes and/or pitches, and different motor mixer settings? I've been searching around for data and user experience in coax setups and came across a study that offered a new point...they concluded that it doesn't matter if both motors are spinning the same or opposite direction in terms of efficiency, but that motor temps were lower when spinning in the same direction. Here is the document. https://fenix.tecnico.ulisboa.pt/downloadFile/563345090412782/Resumo.pdf This is contrary to everything I've seen so far for coax discussion...
    They also tested different prop sizes and pitch, basically concluding that the optimal setup (in terms of efficiency) is either same diameter props top and bottom, with higher pitch on the bottom, or larger prop on the bottom with pitch equal. They didn't appear to test a larger diameter lower prop WITH a higher pitch though.
     
    violetwolf likes this.
  4. Jason S

    Jason S Member

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    Nov 11, 2011
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    Posted by Jason S, Nov 14, 2016 #4
    I have played around with the motor mixer settings because my motor power was not even. But have not played with different props pitches etc. I also have read forums saying that its better to have a larger prop on the bottom with higher pitch. I have always just run same prop size and pitch top and bottom because everybody else was doing it. hahaha
     
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