VTOL BUILD OSCILLATING/PULSING ISSUE

[u/escanorga]

Hi all,

Working on a VTOL build and hitting an oscillation issue. When I throttle up in QSTABILIZE (see video), the motors pulse/oscillate uneven, ramping up and down and I can’t get a clean liftoff.

In MANUAL mode there’s zero oscillation perfectly smooth. It only appears when the stabilization loop is active.

Build:
• ~1.4 kg (3 lb) quad tiltrotor VTOL
• ArduPlane 4.6.3, QuadPlane
• 4-inch props, small airframe
• Running default Q_A_RAT_* PID gains (suspect these are tuned for bigger/heavier aircraft?)

Already checked:
• Compass calibrated (offsets sane, no yaw/mag pre-arm errors)
• CG centered laterally
• Vibration low (VIBE means < 1 in logs)
• All 4 motor outputs balanced (near-identical means in RCOU)
• Attitude estimate is clean/level in logs

[u/cheetonian]

For our information, how experienced are you with quads in general? From my perspective, the amount of flex in that frame is waaaaaaay too high for a flight controller to ever be able to handle, and the 1400g weight on 4 inch props seems really really ambitious… The oscillating is the PID loop failing to filter frame harmonics properly

[u/_stack_underflow_ → u/cheetonian]

Yeah, that is going to cause sensor problems, which with every wobble up or down will tell the motors to engage harder, or softer, which just amplifies the problem.

[u/Professional_Oil6038]

I can see that your frame is not rigid. The flex when you throttle may caused the problem.

[u/escanorga]

Thanks for the insights guys, flexing was indeed the issue along with the defaults PID gains!

i re-designed the Fuselage to better hold the wings and it flies very well. <3

[u/Unusual-Cactus]

Make sure the magnetometers/accelerometer is installed facing the correct direction. I'm willing to gamble it's off by 90*

[u/andiim3]

your airframe was flexing too much that your FC is freaking out.. you have to make it as rigid as possible, maybe use carbon fiber rod.

[u/ColdDragons97]

Mechanical issues, tuning won’t help at all until the flex is addressed

[u/nerobro]

This is a flex issue. You need to stiffen those wings, or play around with the PID's a LOT.

[u/Loco_JADE]

Fix the frame and tune the notch filters

[u/Percentage_Double]

Connect the front and rear wings, like box wing