Heavy Lift Motors and not only

Discussion in 'Cinestar 8' started by David Tivadze, Nov 23, 2014.

  1. David Tivadze

    David Tivadze Member

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    Hi,
    It is absolutely clear , that at some point most of us face with a problem of appropriate motor selection for our setup. I know, that there is particular web site for performing the appropriate calculation for this task. But I also sure, that many people just avoid to use it, trying to find some more easy ways.
    I would like the people, who are more familiar with this task then I, just say their point of view about the following.

    So, Let's say, that my AWU is about 7600 gr. It includes the Cinestar 8 itself, the DYS brashless gimbal with Panasonic GH4 and Tattu 16000 mAh 4c accu.

    Some internet resources says, that it is good idea to add 15-20% to this AWU for the final calculations. So, concerning that, the final "AWU" will be about 9100 gr.

    The adequate Total Thrust for this weight should be about 18200 gr ( double the AWU)

    So, each motor has to have 18200:8=2275 gr Thrust

    This is the target value for each Motor Thrust. Now we have to find the appropriate motor, which will have that Thrust with particular size of propeller (12,13,14,15….) and the particular power source, accu (4c… 6c).

    So, after that we look at the specification tables ( for example those ones, which has T-Motors), and try to find the motor with required Thrust, having in mind the propeller size and the working voltage ( 4c or 6c).

    Now, the question N1.

    Many specification tables have thrust values according to different % of stick movement, or generally, voltage.

    10%------- 780 gr
    20% ------ 960 gr
    30% ------ 1100 gr
    -------------
    ------------
    100% ------ 2000 gr

    Should my requested value of Thrust ( in my case 2275 gr) correspond to 100% ?
    Or may be it should be at 60%?


    And here is the conclusion. If we will see different motor manufacturers specification tables we could find, that there is absolutely no chance to build Heavy Lift Copter on 4C platform. I can not find a motor having Thrust 2300 gr even at 100% on 4C platform, not to say if this 2300 gr Thrust should correspond just to 60%.

    So, do you guys agree, that one have to forget about 4C accum if the target is Heavy Lift Copter?
    And do You agree with my logic of motor selection?
    Thank You for Your time. And sorry for my English :(

    David
     
  2. Andy Johnson-Laird

    Andy Johnson-Laird Administrator
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    Just as some points of clarification David:

    1. Lipo batteries with four cells are referred to as 4S (that's 4 cells wired in series). "C" refers to the battery capacity in amp hours and is used to refer to preferred charging rates (e.g. 10,000 mA hours is 10 A hours, therefore 1C is 10 Amps) or maximum discharge rates (e.g. 25C would be 25 x 10 = 250 Amps).

    2. If you download this document called "Rather Good Guide® to ESC, Motor, Propeller Research Results" (enter zero for the dollar amount -- you can find the link on this page: http://rathergoodguides.com/document-guides/esc-motor-and-propeller.html ) you will see that, as far as I can tell each manufacture rdetermines 100% throttle differently. I *suspect* that 100% to a manufacturer might mean whatever actual throttle setting corresponds to the motor dissipating the maximum continuous wattage -- and has nothing to do with the throttle stick or the width of the pulse width signal used to control the ESC.

    (I'm still working away on the ESC/Motor/Prop testing, but I seem to have developed an intense desire to make sure that the test rig is accurate so I'm on Mark III of the test rig -- but I'm working on it today so hopefully I will have results before year end.)

    3. Bear in mind, in addition to actually lifting the aircraft, you need some reserve power that can be used to provide differential thrust so that you can nick, roll, and yaw the aircraft. If you don't have that amount of thrust in reserve you will not be able to control the aircraft (nor with the flight controller) and so it will crash or lumber around the sky and damage your underwear as you try to control it. I'm not sure what the optimal amount of spare capacity might be, but I suspect you need to be able to hover at around 50% - 60% of the maximum rated power of all all motors. That gives you "headroom" for emergency climbs and full control authority.

    4. Check out the web site ecalc.ch -- it's a pretty good calculator -- search this forum for references to it and you will see some examples of how to use it as it has a rather daunting (for the beginner) interface.

    5. Yes, you will be better off with 6S Lipos -- and two off them, partially to have some backup, but partially because then they share the current load and it's kinder to the batteries.

    Hope this helps.
    Andy.
     
  3. David Tivadze

    David Tivadze Member

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    Thank You very much, Andy. I definitely will check out this document, sure it will be very useful.

    Could You, or anybody else here, give me an idea why it could be happened.

    Recently I fly with my cinestar 8. First time with the brashness gimbal and camera, so fully loaded. ( about 8 kg)
    My motors are QC3328, props are T-Motor 14x4.7, Tattu 16000mAh (4S).

    The flight lasts about 8 min and when I check the motors after landing I found unexplainable for me things: ALL CW motors were extremely hot, and ALL CCW motors were absolutely not even warm, simply cold.
    Do You ever face with this problem?

    Thank You

    David

    P.S. Is it possible to post video right here or just links to youtube and etc.?
     
  4. Andy Johnson-Laird

    Andy Johnson-Laird Administrator
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    David:
    If you look at the GPX files from your flights (presuming that you have the MK Navigation control board), you will probably find that all CW motors are drawing more current too. They are having to work harder to overcome some yaw introduced by the fact that not all the motors are absolutely vertical. When a CCW motor (in this specific case) is not absolutely vertical it introduces a horizontal force that wants to yaw the copter around -- the flight control detects this and increases the speed of the CW motors to compensate.

    If you fly the copter, go up to a height such that it is level with your eyes -- you should then be able to sight across all of the "disks" formed by the spinning propellers -- check that they are all in line with each other. The odd are that one or more will be canted over -- it's remarkably easy to see this when you have the benefit of the propeller "disks."

    More information here: http://forum.freeflysystems.com/index.php?threads/how-to-align-the-motors.312/#post-3082 with alternative techniques and tools for aligning the motors.

    Andy.
     
  5. Janne Hoglund

    Janne Hoglund Member

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    Did the same misstake once myself, your motors not aligned properly.
    Set every motor at the same angle, 0°, or to get better yaw power set every second motor + /- 3° tilt.

    http://wiki.mikrokopter.de/en/AngleAdapter

    regards.

    /Janne
     
  6. David Tivadze

    David Tivadze Member

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    I made that propeller "disc" check today. Not at the fly, but on the table at home. Seems they are all in line with each other. At least with my eyes I see that.
    Any other ideas :( why it could be happened. May be something happens with the ESC for the CW motors? :(

    I will try some of the technics described on Andy's link for motor alignment.

    Here comes the benefit of some Tarot frames, which just exclude the improper alignment of the motors by the design of the booms.

    David
     
  7. Janne Hoglund

    Janne Hoglund Member

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    Dont want to step on anyones feet but if you try using an app ¨Carpenter¨ and with your phone measure the angle of each motor you will get a much better result.
    I to used the the eyes motod before, but the app in my Iphone sure does a better job :)

    https://itunes.apple.com/se/app/ihandy-carpenter/id293621500?mt=8

    Regards.

    /janne
     
  8. David Tivadze

    David Tivadze Member

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    Janne, thank You very much , I will try this app today. Hope it will help.
    By the way, when we say, 0° alignment , what deviation could be considered as normal? I personally thought, that +/- 2° deviation from strictly vertical axis is ok, and will be compensate with FC without so huge efforts of other motors. But I had really extreme high temperature on CW motors, even if I could not find improper alignment by my eyes.

    David
     
  9. Janne Hoglund

    Janne Hoglund Member

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    From what I hear David you are making the very same misstake as I did.
    Download the app and measure. If everone of your motors are within +- 2° I´ll be surpriced.
     
  10. Andy Johnson-Laird

    Andy Johnson-Laird Administrator
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    You can certainly do that, but the real problem is that the entire frameset (booms and hubs) may be at an angle -- it sits rather loosely on the gimbal because of the vibration isolators.

    But the most likely cause still remains motor misalignment. Check a GPX file if you have one, David.

    Andy.
     
  11. David Tivadze

    David Tivadze Member

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    So, measurements done. Here is the deviation from the vertical direction

    1 ------- -0.3°
    2 ------- +0.5°
    3 ------- +1.2°
    4 ------- +0.2°
    5 ------- -1.0°
    6 ------- -0.1°
    7 ------- -0.4°
    8 ------- -1.3°

    Do You think, that such deviations could course this extremely hot temperature on CW motors?

    There is one more thing. Do not know if it could be the reason. For the central hub of my cinestar 8, I used not the original one from FF, but one clone from here
    http://www.aliexpress.com/snapshot/6005147902.html

    There was no holes for mounting the Power distribution board on this central hub, so I drill them myself. As a result of this fact, the ARROW on the PD board is not oriented strictly to the boom N1, but has a slight deviation may be 2 or 3° from this direction.
    When I was assembled the copter I asked here does this fact can somehow affect the behavior of the copter and had an answer, that this will not affect anything, as there is no directional sensitive elements on the PD board.

    Do not know, if this additional detail can help for the final diagnosis of my copter :)

    Thank You

    David

    P.S. Andy, I definitely have the GPX file, but have no idea what to check.

    P.P.S. The problem seems occurs when I fly with gimbal. Without gimbal, even after 25 minutes of non stop fly the motors are OK
     
  12. Gary Haynes

    Gary Haynes Administrator
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    David the deviation on the PD may be an issue. Motors will always work better if all are vertical. You can also check by simply putting the prop tips close together and checking alignment that way.

    Why not order a pair of 'real' Freefly hub plates?
     
  13. Dave King

    Dave King Well-Known Member

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    Earlier this summer I had a similiar experience where I leveled all the motors like I always due with a bubble level and I was getting the same thing you are experiencing with hot odd motors. I simply adjusted each motor ever so slightly and the issue went away. It only takes one motor to be slightly off to cause the problem.
     
  14. David Tivadze

    David Tivadze Member

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    Thanks, Gary , for Your response. I definitely will buy real FF hub plates , if You suppose, that deviation on the PD board may be an issue. Frankly. I do not understand why, because there is nothing on this board, except , lets say, 8 ESCs. I thought, that all direction dependent elements are on the FC board and NAVI board.
    But first I will try to align all motors "exactly" 0° vertical

    Regards
    David
     
  15. David Tivadze

    David Tivadze Member

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    Thanks Dave, I will correct them immediately.

    David
     
    Dave King likes this.
  16. Andy Johnson-Laird

    Andy Johnson-Laird Administrator
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    David:
    As far as the PD board is concerned, does it also offset the flight controller which is mounted to the PD board?

    As for GPX files, do a Google search for: MK_GPXTOOL -- it's a free display program for Windows that allows you to display GPX files from the microSD card on the Navigation Control board. The summary of a GPX file will show the minimum, average, and maximum currents and temperatures for the motors. It's very well worth investing the time to understand the information in the GPX file.

    Andy.
     
  17. David Tivadze

    David Tivadze Member

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    Andy,
    No, the Flight Controller is mounted exactly as it should be ,with arrow looking directly to the boom N1. For FC board, fortunately there were drilled holes in the hub normally.
    Thank You for GPX file advise. I will see it.

    David
     
  18. Gary Haynes

    Gary Haynes Administrator
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    David if the FC board is pointing forward then you should be ok. Usually the orientation of the two boards or the original stack were all symmetric.
     
  19. Jeff Field

    Jeff Field New Member

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    Hi Andy. I've had a look at your RGGuides and would like to complement you. This question relates to absolute throttle, so I decided to add it to this thread.

    In your ESC study, did you run into any of the "dynamic" features of the KDEXF-UAS55 ESC? I'm curious because we're seeing some odd behavior. Basically, the RPM as a function of absolute throttle changes from one state to another. There is a curve present at power up that looks very much like the one in your guide, then there is a "reclaibrated" curve after a flight-like abuse test on the bench. This wouldn't be so alarming if the difference was smaller, but the slopes and max throttle are so different, it would surely mess with the tune of whatever flight controller is used. My test setup is probably not as nice as yours, but many variables are controlled and recorded repeatably. It's not a difference in power, temperature, etc. The ESC simply changes this slope in flight. I looked around for anyone else pointing this out. I'll contact KDE as well. They seem pretty responsive.
     
  20. Andy Johnson-Laird

    Andy Johnson-Laird Administrator
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    Hi Jeff:
    Hmm. That's puzzling. I did not encounter anything similar to what you described. The data I show is the average of ten sweep tests and nine sensitivity tests -- so it's fairly similar to a 20 minute long flight by the time the test is complete -- and I ran those entire tests several times, but I do not see any "post flight" variation.

    I've spent the last few months upgrading my motor test stand and I'm now using laboratory grade load cells and data acquisition hardware. I've also been upgrading the post processing to improve the accuracy of the results -- and, better yet, I just got the Mikrokopter v3 BL-Ctrl's to work -- and finally, I've got the capability to test at 6S as well as 4S.

    If you could post (or email me) one of the curves you're getting it would make it easier to comment on what you're seeing.

    Andy.
     

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