Docs: Plane: Add hint to solve 1Hz pitch oscillations

[menschel]

This patch adds a warning about a nasty side effect of Autotune.

I had the situation that I was through with tuning by documentation, then flew a longer distance straight and the plane started to oscillate over 5deg pitch and 1m altitude at 1Hz.
The cause was that Autotune did copy the FF value to the I-Gain which was >150% of the P-Gain.

I'm not sure if I got the cross-side link correctly. Please advice and I'll rework as necessary.

[tridge]

@menschel can you please upload a log that illustrates the issue?

[menschel]

Nope, I purged it along with 6 month worth of logs and FPV feed to make space again on the HD.

But I found the parameter file.
With oscillations.

AUTOTUNE_AXES,3
**AUTOTUNE_LEVEL,8**
AUTOTUNE_OPTIONS,0
PTCH_LIM_MAX_DEG,20
PTCH_LIM_MIN_DEG,-25
PTCH_RATE_D,0.0046223
PTCH_RATE_D_FF,0
**PTCH_RATE_FF,0.2132648**
PTCH_RATE_FLTD,10
PTCH_RATE_FLTE,0
PTCH_RATE_FLTT,5.305164
**PTCH_RATE_I,0.2132648**
PTCH_RATE_IMAX,0.4
PTCH_RATE_P,0.1438177
PTCH_RATE_PDMX,0
PTCH_RATE_SMAX,150
PTCH_TRIM_DEG,3

No oscillations, stable pitch even with 20km/h cross/head-wind, talking about a sub250g plane with 35km/h cruise speed.
**PTCH_RATE_I,0.11000000**

[menschel]

I believe I recognize the exact same issue in one of Keith's Videos. https://youtu.be/xRC3Ru-d6-Y?t=172 @keithluneau , you may want to cut PTCH_RATE_I in half on that plane. Cheers

[keithluneau]

I believe I recognize the exact same issue in one of Keith's Videos. https://youtu.be/xRC3Ru-d6-Y?t=172 @keithluneau , you may want to cut PTCH_RATE_I in half on that plane. Cheers

Thanks! I have always had a slight oscillation with this one but never did any more than the usual auto tune, made sure it was trimmed well, and tried shifting a CG a bit. I'll keep this in mind and lower PTCH_RATE_I to half as suggested.

[Hwurzburg]

@keithluneau if you do, please post a before and after log...Tridge wants to see this