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AP tuning question (Read 341 times)
Nov 10th, 2017 at 1:32pm

RVDan   Offline
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Dynon,
Since one of the distant past SW updates, I have had a slight but annoying porpoise in pitch during altitude hold. It is only really noticeable in smooth air.  The altitude deviation is very small and not an issue at all, it is just a ride factor.

I intend to do the AP tuning procedure this weekend from the start to see if I can improve things.  Is this porpoise the same as what is termed in the tuning guide as "twitchy"  To me twitchy is a sort of jerky response, whereas porpoising is a sinusoidal change, which is what I seem to have.  The period of the oscillation is about 1/sec.

On the other hand, in rough air, the AP will currently let the altitude drift 100 ft or less.  I don't find that unacceptable unless you tell me it is. 

The aircraft is an RV-6A.  Light or heavy doesn't seem to matter.  I intend to start the tuning with default values per the guide.
 
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Reply #1 - Nov 10th, 2017 at 4:22pm

jakej   Offline
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One troubleshooting method I use is to disconnect the static system (vented to cabin) to see if the static port design/location has an impact on a/pilot pitch performance.  I'd also tune the Alt Hold while the static is vented to cabin.

HTH Smiley
 
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Reply #2 - Nov 13th, 2017 at 11:17am

RVDan   Offline
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Flew this weekend and did the complete tuning process up to turbulence.  Will have to get the right conditions for that.  Had smooth air.
Fool tuning went fine.

Airspeed hold came in fine with a 1-2 kts overshoot in worst case, but typically 0 to 1 kts overshoot.  Gradually came back to set speed and held fine.
Pitch tuning was fine except that after being on altitude, I still get a bit of pitch poirpoise. It is observed as a very slight g change in the seat of the pants.  Command bars are steady, pitch change is  unobservable, and altitude change is unabservable.  Vertical speed does bounce back and forth between 0 and 50 ft at about the same rate as the porpoise.  Visually I can’t tell if one leads or lags the other.

I guess it could be a static system fluctuation, but one would think it could be filtered out by SW.


Suggestions?
 
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Reply #3 - Nov 13th, 2017 at 12:57pm

RayInGA   Offline
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You could try further reducing the pitch gain and/or sensitivity. My -7A is rock solid holding altitude after doing some cross country flights and tweaking it as I flew.
 

Ray Eaker
RV-7A flying since 27 Jan 2017
Dual Skyview 1000T with all available Dynon VFR goodies
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Reply #4 - Nov 13th, 2017 at 3:09pm

jakej   Offline
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SW utopia, it fixes every problem  Cheesy

Seriously though - changing gains etc could be the solution & worth a try.  I'd write down the existing settings to use them in case you decide to go the route of disconnecting the static line.
 
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Reply #5 - Nov 13th, 2017 at 6:12pm

Lawrence Doan   Offline
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Dan,

"Twitchy" is a fast movement of the stick; what you are describing doesn't sound twitchy.

Your airspeed hold performance sounds pretty good; what are your VSI GAIN and ALT GAIN set to?

If the AP is rounding out too late and going through the desired altitude, and your ALT GAIN and VSI gains are near the defaults, try increasing VSI GAIN slightly to start the round out sooner.   This will also help alt hold in rough air.

 
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Reply #6 - Nov 14th, 2017 at 10:49am

RVDan   Offline
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The AP is not rounding out late and it tracks altitude well, typically with a variation of less than 20 ft.

I will see how it does in rough air later.
 
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