Vertical CoG

Discussion in 'Cinestar Misc' started by Chris Fox, Nov 15, 2013.

  1. Chris Fox

    Chris Fox Active Member

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    I have been doing some reading on flight stability and performance, and I know quite a few of us spend a bit of time balancing our rigs, particularly for lateral and longitudinal balance to get the CoG central to the vertical centre line of the copter.

    The following article discusses among other things the effect the vertical CoG has in relation to flight stability http://eprints.qut.edu.au/33767/1/33767.pdf and on other some google researching there are two schools of thought out there rocket stability where the CoG above the rockets (we can assume this would be analogous to the rotor disk) which also is demonstrated in the athletic power of quads with the balancing act of the water glass and pole on the quad. The other school of thought would be the pendulum effect of having the vertical CoG below the rotor disk, which would tend to keep the copter right side up, when static, but when in flight requires larger control inputs from the flight controller to manage attitude (I suspect this would also play a part in the dynamics of toilet bowling, or how much work is required to correct it), and requires the system to do more work for any corrections, particularly in maintaining position.

    Given that general consensus is for the longitudinal and lateral CoG position to be over the vertical centre line of the copter, does anyone have any guidance on an acceptable vertical CoG range for best performance.

    We don't fly planes/helicopters without a doing a weight and balance check, just wondering about the copter ....
     
  2. Chris Fox

    Chris Fox Active Member

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    "Although the MK is not very sensitive to CG shifts in both height and width, it is easier to fly the model with the CG a bit below center vertically and well cantered horizontally" from MK Wiki
     
  3. Chris Fox

    Chris Fox Active Member

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  4. Steve Maller

    Steve Maller UAV Grief Counselor

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    I, for one, believe that my copter is likely a bit top-heavy, as I fly dual 8000 batteries and a GH3 beneath, as opposed to the 5DM3 that I used to use. I wonder if my CoG is off, and whether I should maybe mount the batteries lower. This will be difficult, of course, because of the way the Cinestar's battery shelf works.

    I use the booms to balance the copter laterally by suspending the copter from boom 1 & 5, then from boom 3 & 7. But what should be the vertical CoG? The rotor disc? The Flight Controller? I'm curious now...

    Thanks for starting this important discussion, Chris.
     
  5. Steve Maller

    Steve Maller UAV Grief Counselor

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    "It was found that as the rotor speed increases, the system displays chaotic semi-stable behaviour that would make untethered flight impossible." (ref: http://eprints.qut.edu.au/33767/1/33767.pdf)

    I sometimes describe my behaviour in a similar manner. My copter’s, too. :eek:
     

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