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Where to place flight control

Freeshot74

Junior Member
#1
Hello! I'm working on developing my third multicopter. The first was a classic X configuration.
The second was an H-configuration. Both flew very well but had shortcomings when it comes to flying with camera and other things.

Now I have come to the conclusion that I should build one with the configuration of the spider-X, aka dead cat. (Hate the expression!)

My thoughts regarding spider x is the following:
Where are you placing the flight controller?
I have read on some forums but have not found a clear answer.

Princip.png

The blue circle indicates the COG (Center Of Gravity) and where to put the flight controller.
Where do You think it should be and Why?
I'll throw this question to discuss.

/Tomas, Sweden, Jämtland
 
#2
The way I see it, the controller should be in the middle at an equal distance from all four motors as I would have thought you would run into handling issues where the motors further away from the controller will be moving greater angles than the closer ones and the controller would be unable to compensate?
 

Craftydan

Hostage Taker of Quads
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#3
Thomas,

All moton through the copter acts levereging around the center of gravity. That means all rotations of the frame center there. your Gyros are *best* located at the center of gravity, so they get the cleanest read of the angular motion. farther out in any axis and you'll get a moment-arm deamplification of both the signal and any noise -- part of the angular motion will be coverted into lateral motion and will be invisible ot the gyro.

CoG and natural CoL (if all motors were at the same throttle, where is the lifting force centered) do not have to be in the same place, but to keep the loading balanced on the 4 rotors, it's best they are co-located. In flight, the contorl board will have to balance the motor speeds to make them colocated (for stability), but the closer they are, the easier job the board has to account for it.

BTW, Welcome to the forum!
 
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Freeshot74

Junior Member
#4
Thomas,

All moton through the copter acts levereging around the center of gravity. That means all rotations of the frame center there. your Gyros are *best* located at the center of gravity, so they get the cleanest read of the angular motion. farther out in any axis and you'll get a moment-arm deamplification of both the signal and any noise -- part of the angular motion will be coverted into lateral motion and will be invisible ot the gyro.

CoG and natural CoL (if all motors were at the same throttle, where is the lifting force centered) do not have to be in the same place, but to keep the loading balanced on the 4 rotors, it's best they are co-located. In flight, the contorl board will have to balance the motor speeds to make them colocated (for stability), but the closer they are, the easier job the board has to account for it.

BTW, Welcome to the forum!
Thanks Dan!
I'm not shure what You mean.

You write "your Gyros are *best* located at the center of gravity".
I'm not sure where I should place COG. In my picture I draw three figures. Which do you prefer?
I think you mean Fig 1 but I'm not sure.
 

Craftydan

Hostage Taker of Quads
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#5
first paragraph is the "ideal and why" for the sensors placement -- cleanest signal at CG

Second paragraph is "ideal and why" for the airframe's CG -- centroid of the motors, since the CB will have to level out the motors anyways, it's best to make it easy on the CB and put it there naturally.

the second answer completes the first.

One caveat: the centroid of the motors is *NOT* the center of a circle all the motors rest on (not fig 2), but instead a weighted averge in X and Y of the motor's positions -- it's really the point where the force vectors actign at that point balance out any torque.

We assume all motors being equal, the weighting is all 1, so the math becomes easy. Also the roll-symetry between the motors makes the pitch position averge trivial -- it's the centerline of the craft.

Still with me?

So if you pick the rear boom line as your 0 along your roll axis the rear motors both contribute weight to the averge, but not distance, so if the length to the front booms is L:

CG position forward of the rear motors = (0+0+L+L)/4 = 2L/4 or 1/2 L.

Figure 1 is the most correct, but it depends on equality in motor balance -- if your tail motors would be half the strength but co-located (like in a y4) The true balance point would be farther forward of the rear motor, becaues the total weight contribution on the bottom woudn't be 4 -- it would be 3 (.5 +.5+1+1), and CG would be 2/3 L, just like a tricopter.
 

Craftydan

Hostage Taker of Quads
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#7
Oh, sure, throw me to the wolves ;)

(not to say you're a wolf, Thomas . . . I'm sure you're a nice guy )

Thomas, I'm fairly certian I've got my math right . . . but I'm not perfect. If I've lost you on a point or you've got a counter-example, we can go over it a little more carefuly.
 

Freeshot74

Junior Member
#9
Thanks for the help. I like people who know their stuff and want to share their knowledge.
I think i got it now. Back to the drawingboard and move the FC.

/Tomas
 

Craftydan

Hostage Taker of Quads
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#10
This thread needs more drawings. :)
You haven't seen my drawings -- my wife is the artist in the family ;)

Thanks for the help. I like people who know their stuff and want to share their knowledge.
My pleasure Thomas.

We've got a good group of people here who are willing to do just that, and you're always welcome to hang out with us, if for no other reason but to shoot the breeze :)

Good luck in your layout -- shoot us a picture or two once you've made some progress!