Coaxial Boom Spacing/Mixer question... asymmetrical?

[multirotorxy8]

I am building an x8 setup, and I wanted to try and test a design with the front booms almost straight to the side, similar to an H setup, and the rear booms back at a large angle, so the setup is asymmetrical if looking at it from the side. Picture below.

I have it setup with the mixer settings from the mikrokopter wiki X8 seen here: http://gallery.mikrokopter.de/main.php/v/tech/OctoX_XL_Config.jpg.html?g2_imageViewsIndex=1

Testing it out, everything is spinning the correct way, and with enough throttle to test left/right/forward/reverse, everything responds correctly. However, when it lifts of the ground, it yaws hard counter clock-wise if looking at it from the top.

Any thoughts on what this might be? I have ensured the props are all balanced and aligned.

The rear booms give a little extra weight to the back of the copter, but its AUW is perhaps 2.5 kilos tops with landing gear (brushless gimbal with the bottom removed) so I don't think it will play too large a role in the yaw.


View attachment 14778

 

[FerdinandK]

I would mix CW and CCW on top and bottom plane. Also I would check once more the alignment of the motors should be less than 0,5° off. But if all that is done I strongly doubt that this will fly nicely with the COG so far off the geometrical center.

best regards
Ferdinand

 

[R_Lefebvre]

Yeah, looks like you've got all the top props turning clockwise, and all the bottom props turning CCW? That's not cool. Does mikrocopter use a top-bottom ratio?

 

[Bartman]

Yeah, looks like you've got all the top props turning clockwise, and all the bottom props turning CCW? That's not cool. Does mikrocopter use a top-bottom ratio?

i disagree with you guys on this one....been flying this configuration for years now and it's survived motor out episodes, stable as hell, etc...Hoverfly, MK, DJI fly just fine this way.

 

[Bartman]

as for the layout Eric, if you can get everything to balance front/back and side to side about the geometrical center of things, you can do it but then only if the flight control is as close to that point as possible.

i see what you're trying to do, i guess you're going for more room between the props with a 360 pan camera mount? the panning camera mount and the coaxial motors don't love each other, but i guess you've figured that out already.

bart

 

[multirotorxy8]

Yes, the xy8 I had from Bart had the same setup, cw tops, ccw bottoms, flew great.

Bart, yes this is for a 360 mount, not just for the extra prop clearance, but also a proof of concept for this asymmetrical design.

I see what you mean about the balance of the props, though I am wondering if this would manifest itself in the copter wanting to pitch forward or backwards rather than yaw? I am sure unbalance will affect the way it flies, but I am unsure of whether that is the issue that is causing this current issue. Any thoughts?

 

[FerdinandK]

@Bartman
Nobody said, that CW on top and CCW on bottom will not work, but if you mix the props on top and bottom it will just work better, give it a try.

@multirotorxy8
Probably you should once again start with the basic tests, all motors connected to the correct ports of the FC, mixer correctly setup? Motors are turning in the correct direction, all motors are of the same type? The correct props mounted on the correct position in the correct direction?
But still your COG will cause you severe problems.

best regards
Ferdinand

 

[multirotorxy8]

Ferdinand, thanks for the reply. After more tinkering, I am pretty positive the issue lays in the mixer setup and the cog. I don't believe this setup is unflyable, just need a better understanding of custom mixing in mikrokopter. I was directed to a great video on custom mixing for the multi wii that takes asymmetrical motor placements into consideration, and I understand the science behind it and how my setup would look in this programming language, unfortunately I am at an impasse in how to implement this concept in the MK Tools mixer. Any suggestions from the MK guys out there? I will keep digging on this end...

 

[multirotorxy8]

One additional note supporting the mixer setup... when it yaws, it was not around the center of the copter, but rather around a point at the end of one of the booms.

 

[FerdinandK]

There is no problem to fly such a copter, but your batteries will have to move (a lot) to the rear end of the copter.

 

[multirotorxy8]

Right now it is actually rear-heavy (the wide booms are the front) so I may have to devise them moving further forward to help the balance.

There is no problem to fly such a copter, but your batteries will have to move (a lot) to the rear end of the copter.

 

[FerdinandK]

There your COG should be (as well as your FC if it is not capable to compensate being off center)

best regards

Ferdinand

I am sorry to use your picture having not asked, if you do not like this please tell me and I will remove it.

 

[multirotorxy8]

No mate, I appreciate you taking the time to illustrate it! I am trying to figure out how to do this with the software to let the FC compensate. I never outright mentioned what I am flying here, it is:

Full MK Stack with the 2.2 upgrade to the FC and .90 firmware.

MK Tools motor mixer does have a way of adjusting for distance from the FC, but I can't fully wrap my head around it as your results must still equal zero, and if you state the front boom is, say +10 and the rear is -50, they won't come out to zero.

Anyway, I have just readjusted the mixer settings, so I am going to step out now and see if it has made a difference.

 

[Bartman]

@Bartman
Nobody said, that CW on top and CCW on bottom will not work, but if you mix the props on top and bottom it will just work better, give it a try.
.............

i'm skeptical. i'd read info on it if it's factual but until that time i think it's more hocus pocus. i've tried a lot of stuff that people pass around as facts, much of it doesn't make a difference and some of it makes a difference but at the expense of something else.

have links to real data on the concept?

 

[multirotorxy8]

After taking another look at the mixer and adjusting some settings, I took it out. While I am now able to get it off the ground without it yawing, it is incredibly unstable. An attempt to actually yaw gives a very uncertain response. It either doesn't yaw at all, or will quickly yaw. In general, it still needs a way to take the asymmetry into account within the motor mixer.

 

[Bartman]

After taking another look at the mixer and adjusting some settings, I took it out. While I am now able to get it off the ground without it yawing, it is incredibly unstable. An attempt to actually yaw gives a very uncertain response. It either doesn't yaw at all, or will quickly yaw. In general, it still needs a way to take the asymmetry into account within the motor mixer.

eric,
try this

measure the distance the front motors are in front of the center of the flight control stack and then measure the distance the rear motors are behind the center of the FC stack. Measure just the front/back distance, don't measure diagonally

the front distance should be smaller than the rear distance.

in the mixer table the front motors should be 8, 1, 2, and 3.....leave those numbers alone for pitch(nick)

divide the front distance by the rear distance, it should be less than 1

multiply the remaining motor numbers' (4, 5, 6, 7....the rear motors) mixer values by that number you just calculated

so the fronts will be 54 and the rears will be 54 times some number less than one.

does that make sense? i think that this will do what you're trying to do, leaving the FC where it is with the center of gravity behind it.

LEAVE ANY POSITIVE AND NEGATIVE NUMBERS POSITIVE OR NEGATIVE, ONLY CHANGE THE NUMBER VALUES

if this works we can try to get roll worked out next and this might resolve some of the yaw issues

 

[multirotorxy8]

eric,

if this works we can try to get roll worked out next and this might resolve some of the yaw issues

Thanks Bart, that is about where I am at, but unfortunately it leaves the nick total at less than zero and gives an error. That is where I am left scratching my head. If I adjust the number to represent the different distances from center, they won't equal zero due to the asymmetry.

Oddly enough, looking at the Octo-U, which has motors at distances that don't have equal opposing forces, they are all set to either + or -64

 

[Bartman]

Thanks Bart, that is about where I am at, but unfortunately it leaves the nick total at less than zero and gives an error. That is where I am left scratching my head. If I adjust the number to represent the different distances from center, they won't equal zero due to the asymmetry.

Oddly enough, looking at the Octo-U, which has motors at distances that don't have equal opposing forces, they are all set to either + or -64

64, that's what I meant, I said 54, it should have been 64

anyway, i didn't know that the positive and negative numbers had to total zero, is that what you're saying?

 

[multirotorxy8]

Yeah, that's the rub, if everything doesn't total zero in each column, it leaves it with an error. I haven't tried writing this anyway and seeing what happens. I tweaked it a bit earlier and it was an improvement, then I just upgraded to 2.0 firmware and took it out. It actually lifts off now and is considerably more stable, but now I don't have any yaw control, as the yaw figures need to be adjusted so that it rotates around its center rather than the center of the motors, which makes for some very shifty movement.

 

[R_Lefebvre]

Glad you did this this way. Some people draw an X between diagonal motors and say the CG needs to be at the center of the X, but that's not right. You did it the correct way.

As for the rest of it, I'm pretty sure he's always going to have a problem because the design is asymmetric, and he has all the props on a level going in the same direction.

On a Y-6, it's actually better if they are the same on a level. But on an H-8, I think not.

Though a lot of this depends on HOW the motor mixer works, and I don't know MK at all.

There your COG should be (as well as your FC if it is not capable to compensate being off center)

best regards

Ferdinand

I am sorry to use your picture having not asked, if you do not like this please tell me and I will remove it.