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M210-RTK: Centre of Gravity validation
783 2 2017-10-30
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BvdM
lvl.4
Flight distance : 233950 ft
Netherlands
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Dear all,

There has been some talks going on recently regarding the COG (Centre of Gravity) of the M200 series aircraft, especially with respect to the use of single or dual gimbals on the one hand and the use of TB50 or TB55 batteries on the other. As demonstrated in the below video, the type of batteries to be used with a distinct camera setup have significant applications for how well the aircraft is and, more importantly, how much stress (or not) you will be putting on some of the motors.



Our CAA (Civil Aviation Authority) demands adequate and detailed pre-flight procedures, including validation of the mass/ balance/ COG, ideally following from the manufacturer's recommendations. Unfortunately, upon scanning the manual and other sources multiple times, I haven't come across any official recommendations, suggestions or procedures to do any of the above. Although a lot of information can be found on how different payload configurations affect the bird's flight time, I cannot seem to locate anything on how such configurations affect the aircraft's COG.

Furthermore, does DJI to the best of your knowledge prescribe a certain standard procedure to validate the bird's balance before every flight? Where should one ideally pick up the bird to allow for an adequate examination? At the center frame or at either ends of the propeller arms?

Also, how does one go about validating that the COG is within certain limits. In addition, where does DJI describe what limits are deemed acceptable?

Thanks in advance!
2017-10-30
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Ueli
lvl.3
Flight distance : 1696079 ft
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Switzerland
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Good analysis and video!
However, I don't see it so severe - but maybe I'm wrong.
A well balanced center of gravity on a multicopter is a good thing, but by far not as important as on a glider.
When I watch at it staticly, I can see whitch lift and whitch torque the motors have to bring. Lift is easyer, torque must be same for cw and ccw motors.
With TB55 batteries (total weight 5.6kg):
front motors each 14N (adequate of lifting 1.4kg)
back motors each 14N
With TB50 batteries (total weight 4.8kg):
front motors each 13.5N (estimated)
back motors each 1.05N
So, the stress for all motors is lower with the lighter batteries. Even the front motors don't have to work more. When the FCC can handle it, the system is stable with more reserve then with TB55s.
When looking at it from a dynamic point of view, the CG is not in the center in any case. To be centered, it must be at the spot where all force vectors get together, and that would be in the plane of rotors.

Another question is: can the TB50 handle the current for flying with such a heavy bird. Let's say they are rated at 10C, they can provide 42A x 2 = 84A. Two TB55 with 10C can provide 152A.

Am I wrong in any thoughts?

Would be really interesting, what DJI says to this topic.
2017-10-30
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BvdM
lvl.4
Flight distance : 233950 ft
Netherlands
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Ueli Posted at 2017-10-30 23:13
Good analysis and video!
However, I don't see it so severe - but maybe I'm wrong.
A well balanced center of gravity on a multicopter is a good thing, but by far not as important as on a glider.

Thanks for your analysis, Ueli! I am clearly not as much into this topic as you are, hence my question/ concerns, but your analysis makes somewhat sense.

As far as the capability of the TB50's to lift a dual-camera setup, I am not quiet sure. I certainly haven't found any official remarks, in the manual or on their website, that explicitely state to not use these batteries when operating two camera's at the same time. Then, on the other hand, their Specs page does explicitely mention that the MTOM of the M210-RTK remains unchanged when using either type of batteries, suggesting both should be sufficiently capable to lift the bird when its payload is maximized.

I too am eager to find out what DJI has to bring into this discussion. I just received a reply from their support department but the provided information was no use, at all.
2017-10-31
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