I've got an heli where the motor starts cogging whenever I turn down the governor speed. I've got a swith and whenever I command a lower rpm it starts cogging. It happens rarely but does happen when I don't touch the rpm switch.
Symptoms are I loose rpm at the tail starts oscillating whith a frequency of approx 3hz. Same thing happens on the ground without main blades. It's violent enough to cut the screw holding the main shaft on my mini protos 450 within 15 seconds.
I've changed booth the esc, the motor and the tail servo, same thing. Could it be the fbl module (beastx) or the receiver? Now I'm thinking that i should run it without tail blades. And without any load. But I doubt it will do anything.
AFAIK,"cogging" is usually caused by the esc.
I had that problem recently with the motor in my AXN,and the only thing that cured it was a replacement esc.
I tried re-setting the old esc with the programming card,but no joy,so i think it,s kaput.
After a lot of experimenting I seem to have found the problem.
It is running in a mini protos. It got a lot better after releasing the tension of the belt between motor and main shaft pulley. Then after changing two ball bearings (guiding the belt) that were not in perfect shape (but not bad shape either), the problem went away.