### Pendulum rocket fallacy and the Quadcopter

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!

#### Replies

• Rockets with CG ahead of CP are dynamically stable. Hot air balloons are more stable with CG below CP. Of course, if you were to shoot a balloon with a gondola out of a (large) cannon, it would assume the dynamically stable configuration of gondola first (same as rocket dynamics).

Unless you are shooting quadcopters out of cannons, I think their low-speed flight dynamics resembles hot air balloons (and mars rovers) more than it does a rocket.

• I am planning number "9" build to return to this concept and maybe finish a 5 year old project (upside down heli) because I noticed while fying inverted, hovering was easy.

• I found this more stable under power and in ground effect but as soon as you lower the power you are fighting gravity and horizontal rotation (where as Gravity is helping you stay upright with a low CG).

I done this with the original pirates code and had the controller below the CP and the CG high, I abandond that line of build because of the amount of broken props I was producing while learning about P.I.D tuning and frame balancing  !

Would this not need higher powered motors the higher the CG ?

• Developer

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. :)

http://eprints.qut.edu.au/33767/1/33767.pdf
• Like this

?

• I'm trying to visualize this, but much like muscle memory, overcoming the common can be quite a trick.

Please forgive the broken rotor (which should help illustrate how I'm in a position to try what your saying out) So Obviously take what you see in this pic, remove the legs and then flip the motor 180 degrees right? If you take a closer look at the motor I'm using it has a prop shaft coming out the "bottom" so I can mount my rotor on either side....But from what I gather it would be best just to flip it because that would allow me more space to get payload above....Obviously there will have to be some sort of minimum landing skit below, but using small carbon rods it wouldn't take much.....30 grams? With the total weight of the quad 480grams (bare frame).

• 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)

• Developer

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?