# 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!

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### Replies to This Discussion

http://diydrones.com/profiles/blogs/open-source-g10-quad-frame    somthing like this with all ars up?

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.

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)

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 would change things from a pull to a push.. the APM2 deal with that?

And this may sound like a really stupid question, but in your analogy using a single string holding a payload would of course require more input to balance.....but were using more then one string here no?

I just keep picturing this attempt at the Star Wars skidder bike that has 2 props beneath it (like a hovercraft) and hovercraft just slide all over place.

As long as the thrust from the motor is still down, I don't think the APM2 will have any problems.

Like this

?

Standard issue ArduCopter.

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. :)

Ok, I have a couple of the Turnigy HAL frames on the way..... The skids are made of carbon fiber and VERY small....  Within the dome I am going to put the APM2 on the top of a stack within the protective bubble at dead center, but as close to the top as will fit.(Below it will be power distribution, at the very bottom the battery...but all still on top of the quad within the bubble) Im guessing that because the higher the APM2 (and ergo it's sensors) is mounted, (Like a lever) it will feel the quad leaning and be able to correct for it far sooner than it would be if mounted below. (Does that sound right?) and using the motors and mounts I showed you before only flipped 180 Degrees. And the Direction of the motors reversed.

What do you think?

Why would flipping the APM board over be better? I would of though flipping motors and props over and leaving APM like it is? Read my ealier reply and how switching props would allow you to not change rototion of motor with APM change of program. In reality the rotation changed with flipping of motor so swapping prop with prop on next arm would do the trick. Of course flip all props on hubs to face upward in the real world.

With motors moved to bottom of arms facing down instead of the normal facing up. Instead of reversing the motors, would it not be easier to switch propellors on a nieghboring arm, with the props flipped over so the top of prop would be up to the world.  This would reverse Yaw, so you would have to also reverse your yaw control in Mission Planner. No rewiring (for those with soldered wires) just moving props to next arm over (also flipping prop to face up correctly) and reversing Yaw setting.

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