Motor Layout: Center of Thrust, Mass and Gravity

[NineZeta]

{Hi, I posted the same thing on RCgroups and DIYdrones but i really didnt get an answer. I hope you guys can help me out..}

I was just doing some reading about asymmetrical multirotor layouts. From what I've understood, the motors are equidistant from the center of thrust. It's not the intersection of lines drawn diagonally from front to back motor.

So given that, would my diagram be a correct representation of a balanced hexacopter even if i did not place the motors on the corners of the hexagon? Is it also correct to assume that the center of thrust is the proper location of the FC so that all motors rotate at pretty much the same speed during a hover?

another thing is...will the center of thrust (yellow point in my diagram) serve as the CG as well. Should I balance my model so that if I held it under that point, it wont lean forward or backwards...Lastly...can anyone please explain what the center of mass is....and if its important to understand that in the context of building multirotors. I would really appreciate all the help i can get..thanks guys

 

[jipp]

i do not think you have to have the FC centered.. but you do have to get the center of gravity correct when you put your battery on.

chris

 

[NineZeta]

Looking at the drawing, if you remove the big black circle, the red lines and the green hexagon and just leave the drawing of the hexacopter frame (grey), it will look like the FC isn't centered. But in the complete diagram, isn't the CG located on the yellow circle?

 

[Craftydan]

9Z . . . not quite.

I think you've been suckered in by the wheelbase diagrams to assume the circle center is the center of thrust. simply not the case.

To illustrate, here's a perfectly horrible but flyable layout:



(Apologies for the abuse of MS Paint . . . it's all I have on hand where I'm at)

going by the "center of the circle method", this layout puts it's predicted CG between the front motors . . . with nothing north of those motors to balance the rear thrust. if I'd been a bit more sloppy with it, I could have moved the front rotors back a touch and they'd ALL be behind the CG -- BAD Mojo.

So if that's not the right way to estimate it, what is? that's why I drew the blue lines. Averages. Measure the length along the roll axis from the rear motors to each motor, then add them together (in the case of the hex, that's 4 distances, since the rear motors are already 0). divide by 6:


(L_m1+L_m2+...+L_m-last)/(number of motors) = distance to ideal CG from common measurement point.


since you've got the cad's, should be easy to measure it.

That being said, what you've drawn isn't impossibly far off -- it should still fly, but she might have some nasty tendencies, particularly when maneuvering with any speed.

 

[jipp]

i would think the center of gravity would be up further than where the yellow circle is. i could be wrong tho.
chris.

 

[NineZeta]

9Z . . . not quite.

I think you've been suckered in by the wheelbase diagrams to assume the circle center is the center of thrust. simply not the case.

To illustrate, here's a perfectly horrible but flyable layout:

View attachment 57674

(Apologies for the abuse of MS Paint . . . it's all I have on hand where I'm at)

going by the "center of the circle method", this layout puts it's predicted CG between the front motors . . . with nothing north of those motors to balance the rear thrust. if I'd been a bit more sloppy with it, I could have moved the front rotors back a touch and they'd ALL be behind the CG -- BAD Mojo.

So if that's not the right way to estimate it, what is? that's why I drew the blue lines. Averages. Measure the length along the roll axis from the rear motors to each motor, then add them together (in the case of the hex, that's 4 distances, since the rear motors are already 0). divide by 6:


(L_m1+L_m2+...+L_m-last)/(number of motors) = distance to ideal CG from common measurement point.


since you've got the cad's, should be easy to measure it.

That being said, what you've drawn isn't impossibly far off -- it should still fly, but she might have some nasty tendencies, particularly when maneuvering with any speed.

Dan, thanks for the reply... I think that's a fine layout! I really do! That cleared a lot of things up with the CG. Should I still worry about the center of thrust though? Anyway, I appreciate you taking time to use the power of MS paint...by the way, I dont have any CAD software. It's just Photoshop.

 

[Balu]

I did an article a while ago on this matter:

http://flitetest.com/articles/calculating-the-center-of-thrust-on-multirotors