Multirotor Design Theory? Shape / Mass / CG / Symmetry?

[blushoes]

Hi Guys

Any one with any inputs on what contributes to good stable multirotor design?
I am looking for stability for AP work, not acrobatics.

Some questions I have in mind

1) All things equal, what is effect of increasing wheelbase / diameter? Perhaps there may be some optimum ratio / relationship
of wheelbase vs weight vs thrust per arm?

2) X and Y axis symmetry... Traditional MRs are symmetrical about XY axis, but what about those that are not,
like the 'dead cat' style quadcopter frame where its only symmetrical in one axis? I am guessing this just means that the Roll and Pitch gains
in FC will be different as opposed to exactly the same values for a symmetrical design?

3) I am building an octo coaxial X4. Now that its taking shape, I realise that since it has double motors on each arm (and I chose to have
the ESCs on the arms near the motors too)... now it seems to have more of its mass spread to the arms vs, mass being in centralised in the hub area.
Any pros and cons to this?

4) Choice of MASS of props? Non scientific one-off try...found that heavier wooden props led to more wobbly yaw movement on a hexa I had
vs lighter carbon props.

5) I was reading somewhere where some people say that a more stable config is to have centre of mass/weight above centre of pressure
(COP i am guessing refers to the centre of where the thrust originates?), ie, Z axis CG and its effects.

Any other thoughts?

 

[kloner]

the larger they are the smoother they ride, cesna versus 747....... but it starts taking alot of power to make the big one go fast, and from what i've seen, they just can't go fast

the tbs style frames have a wider front than rear to get the props out of view and the arms reach bak like they do to leverage the battery which for forward flight you want the cg back a tad to keep it agile, but more importantly tilted up at 45 degrees forward you really need the cg to not shift as the craft rolls up on it's nose. i've flown exact same pitch values and never really noticed a difference

I personally would hae kept the esc's up in the center plate if for no other reason than to protect them

those tiger cf props are the bomb, there so light and easy to balance i'm never going back

only because it's hard to get cg up over a mr with a gimbal it's virtualy impossible to totaly counter cg with packs to the point it's top heavy........

from my experiance flying a little of everything, to do an x8 like you built takes a super stiff center plate and arm combo, esp a heavy lifter

i can take just about any size/config rig and make it smooth and stable. has to do with smooth motors, quality props and prop balance then gains are a relatively easy endeavor and the rig just works. Get any one of 8 motors/props not perfect and it's battle royal....... increase gains chasing mechanical problems and stuff gets weird. induce vibes into the flight controller from same said problem and it's game over.

 

[Electro 2]

There was a similar question a few back on the other forum, and the OP missunderstood my reply. My answer there would be the same here. All the things you mention are very true, and highly applicable to a *Passive* moving system, but this is not what a multirotor is. The dynamics of the moving system are precisely what the FC addresses. It forms an *Active* response to any undesirable non-linearities of the moving system. i.e. the system's desired behavior is reinforced and undesired behavior is actively surpressed. Any reasonable proportion of high polar moment, uncentered CG, excess weight, nonlinear motion in any axis, etc can all be dealt with given enough power. Peaking and tweaking these parameters to excessive precision is a wasted effort as the FC can defeat a fair amount of these issues, "on-the-hoof", so to speak, with relative ease. All the things "Kloner' mentions below are very true, however and may help your design.

 

[kloner]

I do a few different types of flying. one in short bursts with big heavy rigs, slow fly big heavy rigs or fly Fast Forward Flight. the different attitudes and the way the weight shifts around in either given rig is what makes them fly like them

in the case of the cat shape (tbs) when it's shifted up at 45 degrees forward and 90 degrees side to side, she is balanced waiting for input. starting out back heavy yields to top heavy because of the packs placement. This shape fff and holds a nice steady level line better than the other shapes. coaxial to me felt like brakes are on but it has extremely tight control. turned on a dime. the flats are still my favorite, there normal to me

just a naza and $20 motors

same place with alot of money rig and tried to get it down and fast, it's just nerve racking. she won't go near as fast

 

[gtranquilla]

To avoid overloading/overheating one or more of the Multirotor motors, it is best to have the static center of pressure and the center of gravity at the same point in x,y,z axes....I believe your item 5 question applies to rocketry and most other aircraft http://my.execpc.com/~culp/rockets/Barrowman.html Bear in mind that the MR IMU is capable of much compensation such that it can balance a long pole directly on the top without the pole falling off. see here.... http://www.ted.com/talks/<wbr>raffaello_d_andrea_the_<wbr>astounding_athletic_power_of_<wbr>quadcopters.html

Hi Guys

Any one with any inputs on what contributes to good stable multirotor design?
I am looking for stability for AP work, not acrobatics.

Some questions I have in mind

1) All things equal, what is effect of increasing wheelbase / diameter? Perhaps there may be some optimum ratio / relationship
of wheelbase vs weight vs thrust per arm?

2) X and Y axis symmetry... Traditional MRs are symmetrical about XY axis, but what about those that are not,
like the 'dead cat' style quadcopter frame where its only symmetrical in one axis? I am guessing this just means that the Roll and Pitch gains
in FC will be different as opposed to exactly the same values for a symmetrical design?

3) I am building an octo coaxial X4. Now that its taking shape, I realise that since it has double motors on each arm (and I chose to have
the ESCs on the arms near the motors too)... now it seems to have more of its mass spread to the arms vs, mass being in centralised in the hub area.
Any pros and cons to this?

4) Choice of MASS of props? Non scientific one-off try...found that heavier wooden props led to more wobbly yaw movement on a hexa I had
vs lighter carbon props.

5) I was reading somewhere where some people say that a more stable config is to have centre of mass/weight above centre of pressure
(COP i am guessing refers to the centre of where the thrust originates?), ie, Z axis CG and its effects.

Any other thoughts?

 

[blushoes]

To avoid overloading/overheating one or more of the Multirotor motors, it is best to have the static center of pressure and the center of gravity at the same point in x,y,z axes....I believe your item 5 question applies to rocketry and most other aircraft http://my.execpc.com/~culp/rockets/Barrowman.html Bear in mind that the MR IMU is capable of much compensation such that it can balance a long pole directly on the top without the pole falling off. see here.... http://www.ted.com/talks/<wbr>raffaello_d_andrea_the_<wbr>astounding_athletic_power_of_<wbr>quadcopters.html

OMG... That TED talk video is crazy!!! When will that kind of tech filter down to consumer

 

[DucktileMedia]

It's NOT larger is smoother! It's disc loading. If a 747 had wings that were too big proportionally for its size, it would have the same characteristics, in theory, as a fly with the wings of a pigeon. You dont want to be taking off with 30% throttle in manual mode. It's fine for flying but for filming you want to load your props enough that it doesnt react to small gusts. Generally spinning a smaller prop faster will yield better video. As for inherent stability, these multis are not designed for aerobatics so having a slightly lower CG is good. And with all the gimbal/camera being low anyways, thats a good thing.

 

[blushoes]

thanks for the inputs guys