Coaxial (x8) experts

[ebawho]

Hey guys, so I just got my x8 build put together and it flies fairly decently now with the default mk settings, but I noticed shorter than expected flight times. I looked at the current draw for the motors and they seem all over the place. The only pattern I see is that all the ccw spinning motors have a higher current draw. Wouldn't that make everything yaw ccw? Does anyone else flying mk coax set ups have any insight into this?


Here are the numbers


Avg. max
Motor 1 5.7 10
Motor 2 5.8 10.7
Motor 3 9.0 13.4
Motor 4 11.1 14.0
Motor 5 11.2 16.6
Motor 6 11.6 16.6
Motor 7 6.9 10.1
Motor 8 7.9 11.3


And here they are arranged as they are on the copter, I'm using the x8 setup from the mk wiki: Mikrokopter.de/ucwiki/mkm


Top motors on top


5.7......9.0
11.2....6.9


11.1.....5.8
7.9.......11.6

 

[gtranquilla]

From what I recall........Check out Freefly forums for your answers to this common issue on most longer motor arm MRs..... Mikrokopter resolved it by offering a shim kit but does not explain it at all. You need 3 degree shims for each motor (cw shimmed opposite to the ccw motors). In addition to increasing yaw rotational speed it also helps to tighten up yaw control so that your MR will fly on a straight heading without random yaw cw or ccw. It has something to do with gyroscopic forces when the MR attempts to change roll or pitch/nick direction apparently.

Hey guys, so I just got my x8 build put together and it flies fairly decently now with the default mk settings, but I noticed shorter than expected flight times. I looked at the current draw for the motors and they seem all over the place. The only pattern I see is that all the ccw spinning motors have a higher current draw. Wouldn't that make everything yaw ccw? Does anyone else flying mk coax set ups have any insight into this?


Here are the numbers


Avg. max
Motor 1 5.7 10
Motor 2 5.8 10.7
Motor 3 9.0 13.4
Motor 4 11.1 14.0
Motor 5 11.2 16.6
Motor 6 11.6 16.6
Motor 7 6.9 10.1
Motor 8 7.9 11.3


And here they are arranged as they are on the copter, I'm using the x8 setup from the mk wiki: Mikrokopter.de/ucwiki/mkm


Top motors on top


5.7......9.0
11.2....6.9


11.1.....5.8
7.9.......11.6

 

[ebawho]

Thanks for the response. The thing is that it flies nice and straight, if anything it seems to track straighter than it did as a flat setup. I posted over on the freely forums and I'll see what those guys have to say. I'm going to try a couple more flights tomorrow paying much closer attention to the yaw characteristics, and make sure everything is perfectly level (again)

 

[Lanzar]

Do you have the same propelers on all motors or not.
Cause the loss off power on bottom motors is a lot so you need to balance this with gains or propeler pitch. If you are lucky you will be able to balance the power. But this is totaly normal so i dont know why do you wory so much.
Based on testing with 4 motors and different position we realized that the loss off trust coax vs okto is 12% to 15%. And it does not matter and will never be less than this. Hope this helps

 

[ebawho]

Hi Lanzar, thanks for the reply. I've read a lot of your posts and it seems as though you are one of the most experienced people when it comes to coaxials. Im running the same props top and bottom (carbon 15x5.5) currently the MK mixer i am using does have a bit of a bias for power.

What do you find to be the best methodology for balancing everything out?

What I find odd is that the top and bottom motors seem to be consuming about equal power total, but all the CCW rotating props are consuming a lot more power on average, irregardless of wether they are on the top or bottom. I've attached a crude diagram to try and make it clearer.

View attachment 14673



I know there is a loss in efficiency, and to me the benefits of a coaxial copter outweigh the loss of efficiency. That being said, I still want to make it as efficient as possible.

 

[Lanzar]

I think that you have a problem that all motors are not aligned perfectly.
Please check that they are all aligned , small angle from 1 motor will distrurb the whole thing and will try to jaw the copter and this is propably why you have more power on ccw cause they are fighting to maitain the compas direction.
Hope this helps.

 

[ebawho]

I will recheck all the motors tomorrow with a more accurate level. I hope its something that simple! What have you found to be the best mikrokopter mixer setup for x8s?

Thanks!

 

[Bartman]

a good test for motor alignment is to hover at about twenty feet and then go immediately to max power. if the heli is built straight then it will zoom straight up, if anything is out of whack the heli will not go straight up and might yaw very noticeably. do this at your normal flying weight, not stripped down and lighter than normal.