LibrePilot Forum
Users => Vehicles - Other => Topic started by: trust on September 16, 2019, 10:26:08 pm
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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?
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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.
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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.