Originally Posted by Endlesslag
I think I'm going to limit my throttle endpoint to 85%...above that it gets too squirrely, and I have a habit of maxing the throttle out in a trouble situation, and of course that causes a crash if I'm down low, and a mad scramble to get back under control if I'm up high.
Today I suddenly had pretty much exactly your issue.... massive instability on full throttle punches and less than usual stability on rapid descents.
First of all, the issue is definitely NOT the control board. The issue is that the four props do not produce identical thrust for identical power input. Assuming that all four motor channels reach 100% throttle when you command full throttle, in still air the mQX should climb straight up from a standing start on the ground OR a full throttle stab from level flight. Mine used to but today it did not. It had a PRONOUNCED forward tilt at or near full throttle.
So I started to attempt to separate out the possible causes (wiring, motor, prop, excess friction).
What I found was:
1. Swapping motors front to rear had next to no effect. So it did not appear to be a motor issue per se.
2. Replacing the motor on the weakest corner (the one the mQX tilted towards at full throttle) with a new one had next to no effect. So it still did not look like a motor issue.
3. The gear mesh on the right front (the corner that seemed the weakest) was tight enough to bind. There was gunk in the gears which I cleaned. But the mesh was still tight. I tried rotating the motor in the housing, and this helped a bit. I left it in the loosest orientation (which seems pretty much OK), but this needs further diagnosis.
4. The right front motor frame was VERY loose on the boom. The problem was the boom, which had just been replaced... the old one was nice and tight. Cracked, but tight. This allowed the motor assembly to vibrate at full throttle. Fixed with a drop of hardened CA on the boom, but didn't fix the unbalanced thrust.
5. So assuming the motors were delivering the same power to all four shafts, that left it up to props. I don't have an optical tach to check the props in the air, but by hovering at eye level, I could sight across the front/rear pair of props and see by the interference pattern whether they were turning at the same RPM. They were NOT. Since all four props were developing the same thrust (the heli was hovering level), the fact that the front and rear props on each side were not at the same speed indicated that they were not the same pitch. The props can get bent, and due to their shape, when they get bent, this changes the pitch. Since the rear props were developing more thrust than the front props (remember, the mQX tilted forward on full throttle climbs), I gently bent each rear prop to de-pitch it. After several iterations, I got to a point where the mQX climbed nearly vertically AND the props were turning at the same RPM in a level hover.
Once this was done, outdoor flight verified that the mQX was once again stable.
So the bottom line is that all four props must generate the same thrust at full throttle or the mQX will be unstable. This applies to partial throttle operation as well, since the gyro makes corrections based on the assumption that all four props will generate the same thrust at the same power setting. If they do not, it unbalances the system and it is less stable after a perturbation from turbulence or maneuvers. And there are several issues that can cause unequal thrust.
Long winded, I know.... sorry. But necessary if the mQX is suddenly unstable.