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Pitch oscillation
« on: September 16, 2019, 10:26:08 pm »
I recently replaced some low power servos on a micro sized eVTOL experimental aircraft (with wings) on "elevons" which also control pitch with higher power/faster servos.
Now at certain midrange speeds and mostly level angles of attack, the plane starts a pitch oscillation which gets violent. I can lower power or lower AOA and it mostly goes away.
I've tried changing the ID values lower and it gets somewhat better, but not much - even at 0 its there. If I lower the P values too much I loose some of the overall pitch control I need.
I tried turning way down the max rate limit and max rate response from 220/deg/s 300 deg/s to 25 deg/s, but it doesn't seem to do anything.
Suggestions?

Re: Pitch oscillation
« Reply #1 on: September 17, 2019, 04:31:20 am »
Faster response should reduce oscillation, not increase it.  Is the servo power capable of handling the higher power requirements of the new servos?

Try this:  Start with original PIDs that worked and reduce the the P and I (or even P,I, and D; Advanced page) terms all by the same factor.  Say multiply them all by 0.7.  If it needs more, multiply by 0.7 again.
« Last Edit: September 17, 2019, 04:47:03 am by TheOtherCliff »

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Re: Pitch oscillation
« Reply #2 on: November 28, 2019, 09:19:43 pm »
FYI- adjusting the pitch gains helped, but the problem kept coming back. I put a keychain camera on the tail to watch what was going on more closely. It was quickly obvious the wings were fluttering! Turns out there was a weak point in the spar design, so the wings were oscillating up and down rapidly when a certain speed was exceeded or certain AOAs.
I rebuilt the wing spars with a MUCH bigger diameter, carbon fiber spar - problem gone.