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Does Autopilot placement matter - above or below CG?

I am running into a very strange issue and my only thought right now is that placing an autopilot below center of gravity versus above may make a difference. Could anyone comment on this?

In the past all of my single copters had autopilot placed above CG. This time I made a single copter with a very heavy battery placed high, so the autopilot ended up below center of gravity. No matter what I do, what the PID settings are I cannot eliminate huge oscillations and immediate crashes right after taking off (in Loiter, Alt-Hold or Stabilize).
What’s strange is that on a PID tunning stand it works fine. Just like my previous single copters.
The stand has a horizontal shaft that goes through the center of gravity of the copter. I push the top of the copter by hand or I apply flaps to get it off balance and it returns to the upright position right away without any oscillation. With some PID settings it may swing a bit past upright once, but that’s it. No oscillations whatsoever.
Unfortunately the moment it takes off, it starts swinging like crazy and quickly crashes. So far I cannot find any other explanation except that placing an autopilot below or above CG may make a huge difference. Please note - this time payload (battery) is really heavy.

Any thoughts?

One log below:
Single Copter Crash - small PIDs

Have you found anymore info on this? I’m building a coax copter with a swashplate and have found the same thing to be true and have to run a heavy payload very low


HI @Andrew_D can you pls tell what was thrust to weight ratio of the motor -propeller combination you have used for the flight.

More clearly, which motor and prop(max thrust it can give?) you used and weight of your airborne system, generally its a good practice to select motors which combined can give 2 times the weight of the system for better attitude control authority.

Unfortunately Ryan, I still have no answer to this problem. I’m not even sure whether the main cause is the placement of the autopilot.
I’m beginning to believe that this behavior may be caused by having CG and large mass close to the controlling surfaces (flaps). On a test rig there is no problem, but in free flight each time flaps try to upright the copter this also moves the CG (and autopilot) sideways, which affects the autopilot.
This would be very unfortunate because this forces the design to become long (tall) with CG far away from the flaps.

Perhaps there are some parameters that could “desensitize” autopilot from those side motions but I have no idea what those parameters are.
By saying “side movements” I do not mean that they are visible. They are very small but theoretically they are larger than those when CG and autopilot are far away from the flaps.

I’m speculating. The truth is that at this stage I’m lost.

Rajesh, I experiment with many different single copters - from small (800 g) to large (5 kg).
2:1 thrust ratio is nice if you can achieve it. For copters with payload this is not always practical.
The behavior I described in my original post is true for a copter that has the same thrust and weight as another one that runs fine, but it has much different configuration (CG and distribution of weight changed).
I tried an entire spectrum of PIDs with no luck.

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