Hello, I'm designing a hexa to carry a 1.5 kg load. I'm trying to create the absolute lightest configuration possible, so it's mostly made from CF. My question is around CG. My intuition tells me that for horizontal stability, the CG should be as low as possible. I would also think that the motion sensors would be most effective if they were as close as possible to the thrust plane. Can of any of you that have built or modified multicopters educate me on whether my assumptions are correct? My current diameter from arm-to-arm is 675mm.
Thanks,
Mike
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Everyone here is sorta on the right track, except that we're relying on automated fast control response to stabilize the craft. My experiments prove there is no way to ever get the response fast enough for manual piloting without either variable pitch or an inertial feedback system (both would be ideal, but the complexity of the former ought to give one pause). Of course, I am far from alone in this conclusion.
Therefore, the goal for a multicopter design is fast response, not necessarily inherent stability. Reducing rotational inertia of the entire airframe about the control axes is of paramount concern. A center of mass, (and as much of the mass as possible) nearest the pitch and roll axes intersection, or perhaps slightly below, is the ideal design target. Dr. Paul Pounds does a good job of providing the mathematical justification for this in the following paper (if you don't mind staring at a bit of calculus):
Pounds, et.al. - Modelling and Control of a Large Quadrotor Robot
I plan on using my copter for aerial video so to compensate for the weight hanging underneath, I plan on putting the battery on top. Basically keeping the CG as close to the center of the airframe as possible. I'm not sure if this is correct or not, but I will be testing this weekend. I'll try a few scenarios of CG and report back.