Well, I don't know if this will be understood very well because it appears to be correct but my experience with rocket's tells me otherwise. I have not gotten around to doing this myself but working with rocket stabilization all the time keeps me from building a quad like this.
But! Because the quads can be built this way very easily I'd like to explain WHY I think you guy's build your quads UPSIDE DOWN! :)
The CG is supposed to be over the CP! In other words if you put all the weight ON TOP of the rotors these craft SHOULD BE MORE STABLE!
I've had a hard time proving to even some so called rocket scientist (Like the ARCA GLXP Team) you cant put the CG below the CP and get stable flight without a lot of control input. It's just harder!
Imagine a seal balancing a ball on it's noze. The amount of correction needed is very small. Now with a ball hanging from a string the amount of correction needed is much greater to balance the ball on a point.
I think the reason we still put the CG below the CP is because it looks right and helicopters pretty much have to work that way but quads DON'T!
So how about trying my theory out? :)
If you notice the Curiosity Mars Rover for example the rocket engines are BELOW the CG like they should be.
Quad rotors will be more stable with the CG on TOP!
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Hi, I came across this paper when looking for control designs for Quads.
http://eprints.qut.edu.au/33767/1/33767.pdf
This paper says "the system exhibits an unstable oscillation when the CoG is below the rotor, pure divergence when it is above the rotor, and neutral stability when coincident with the rotor."
My understanding is that the best case is when the rotors line up with the CG but it is also where the rotor position error changes the characteristics of the system the most.
So it looks like your gut is right. :)
Like this
?
this is interesting. i think that yes initially your corrections would be very small. however the problem would be when some great outside force tipped the copter too far over and it would flip. the example in my head is that of balancing an 8ft 2x4 in your hand. it's easy to control while within a few degrees of vertical... but along comes a gust of wind that knocks your balance out of kilter, all of a sudden you're going to have your hands full keeping that 2x4 from crashing to the ground
(admittedly this would be hard without some sort of engine failure)
hmm..ok. Maybe we do this because it keeps the props higher off the ground so they are less likely to be broken? I have certainly seen people put the motors on the bottom of the arms.
The best way would be to try it both ways and then, if you're using an APM2, compare the Roll-in vs Roll an Pitch-In vs pitch in the dataflash's ATT message.
http://diydrones.com/profiles/blogs/open-source-g10-quad-frame somthing like this with all ars up?
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