Mutual documentation of the undocumented yaw servo

[jaredyates]

I have not yet been able to get the autopilot working acceptably, and I think it is primarily because of the airplane having adverse yaw. This isn't an airplane that can be flown with aileron only, and when the autopilot tires to do that, it fails just as a live pilot would. I was about ready to give up and remove it all together when Dynon announced the new yaw servo option, so I installed the third servo before the documentation came out, thinking that like many Dynon things, it would be intuitive. And, that the documentation would be out shortly.

Both of those assumptions seem to have proven incorrect, so maybe we can help each other out and combine what we do know.

As of today, I have made a few calls to tech support and gleaned a few details each time, but my main problem is that the yaw servo keeps dropping offline. I'll just be flying along without having the autopilot on at all, and I'll get a "yaw servo offline" warning. This might happen two minutes into the flight, or it might happen 30 minutes into the flight. I've had lots of trouble calibrating the servo also. It will show up in the network fine, but then if the servo goes offline during the calibration, it disappears from the network for some unspecified period of time before I can try again. In the current configuration, it was able to stay online long enough to get through a calibration and test. TS suggested a bad connection, which did sound quite plausible, so I used a single length of Dynon autopilot wire bundle that was left over from installation. I put ends on it and ran it up and over the seats in the cabin to the back of the plane, and reproduced the same problems. I hooked the yaw servo up to the pitch servo wiring and attempted a 2-axis calibration, pretending that the yaw servo was the pitch servo, but then the pitch servo dropped offline.

So here's what I've been able to figure out so far about the yaw servo, please add in if you have been able to figure out any more, or anything to the contrary of the above.

The disconnect button must be wired separately from the pitch and roll, to a momentary pushbutton switch that connects the disconnect line to ground. Is one press supposed to revert it to YD, and a subsequent press supposed to turn it off? Or something like that? Or is it that YD comes alive automatically, and we have to push buttons to disable it? I'm not at all interested in having a YD function when the autopilot is off. I'm really only after two conditions in flight: autopilot on, and autopilot completely off. I have installed the extra push button as directed by the phone techs, and it does not seem that as of now we have any option for disabling YD in the software.

Not really sure what to do with the yaw servo settings. One thing that I have not been able to find in the printed documentation is the general relationship between sensitivity and gain. At Oshkosh one of the Dynon people said that gain was a multiplier for sensitivity. So a sensitivity of 20 on a gain of 2 was like having a sensitivity of 40 on a gain of 1, functionally speaking. That sounded really easy and intuitive, but then there are threads here that contradict that.

When is the "ball" supposed to turn green? Is it when the YD is active, or when it feels like it is doing a good job?

I've never gotten a "yaw slip" which makes me wonder if the software has the capability of annunciating a slipping yaw. One theory I have about the servo going offline is that Dynon did not write a "yaw slip" into the programming, but the servo is slipping, and eventually giving up. What in the code would cause the yaw servo to go offline? Is there a condition for slipping that would?

It seems to me that the next logical solution is to physically remove the yaw servo from the airplane and send it in, but it is installed back in the tail and that's a 1-hour miserable job for removal and for installation. If it would solve the problem I'd be glad to do it, but I'd be pretty disappointed if I went to all that trouble just to realize that it was actually a shortcoming in the software. This servo was one that came directly from Dynon in a swap, but has not been proven to be successful by me, so there is a chance that it is bad and they didn't catch it. This makes me even less enthusiastic about spending all of that time removing, shipping, waiting, and reinstalling. If their test regimen didn't find a problem before, and there is an intermittent internal problem, what's to say it will find it this time? Do they subject servos to a test regimen?

I see that some folks with long wire runs have found that the power wire was too small, which caused servo offline messages. My roll and pitch servos never go offline, so another option is for me to swap the servo from yaw to pitch to see if the length of the wire run (extra 8 feet or so) is causing a voltage drop that is making the servo go offline. But the yaw is a capstan and the pitch is an arm, and combining that with the inaccessibility of the servos, I'd be looking at half a day's work to make the swap. Is there a way to investigate the diagnostic data to rule out this possibility? Is there some sort of log file that would show the incoming voltage to that servo?

So to summarize, I have lots of possible troubleshooting steps, but don't know enough about the system to know if these are the only steps, or which is the best step to take. I welcome input about troubleshooting, and any tidbits that anyone else has been able to catch about the whole yaw situation.

 

[Raymo]

I don't believe the yaw damper in any plane is intended to deal with adverse yaw, though you might get it to work in some planes, with a lot of tweaking, where it is not so pronounced.

Beyond that, I'd suspect either a connector pin is not well seated or your theory that the wire run is such that it needs a larger gauge wire. There are charts on the internet that can help with this determination. Always err on the side of larger gauge when nearing the limit if the smaller gauge wire for its intended use.

The disconnect button should disable the servo. If it works like the standard servos, holding the button for 3+ seconds will re-enable it.

This is from another AP thread.

Sensitivity = Correction Accuracy
Gain = Aggressiveness

 

[jaredyates]

Thanks Ray. So I suppose that is a question that I didn't ask... is the yaw servo only intended to dampen yaw oscillations, or is it intended to add a third axis to the autopilot? I was assuming the latter, but perhaps that is not the case? At what point do those two functions cease to overlap? The wire run is fine per the published specs, but perhaps the servo needs something above and beyond the chart specs.

I feel like I'm getting closer to adding a couple thousand dollars to my wallet and removing half a dozen pounds from my airplane.

 

[Raymo]

It really depends on the airframe, IMO, and how fast the servo must react to adverse yaw. I'd expect it to work great in an RV with little adverse yaw but maybe not in a Luscombe, which requires roll and yaw at the same time to avoid it.

Hopefully the official response will come soon from Dynon but I wouldn't give up on the AP until you've done more tuning. Unfortunately, the documentation around the yaw damper is still in the works and likely competing with other business priorities.

 

[jaredyates]

I think you are right, here's hoping we'll hear from them soon!

 

[dynonsupport]

Dynon's yaw damper is designed to both stabilize yaw, but also to auto-coordinate the aircraft. In our experience with aircraft like an Europa with significant adverse yaw, it makes a tremendous difference in the autopilot's performance. It can also be independently engaged and used even when hand flying pitch and roll.

Technically, there is no tie between the yaw damper and the rest of the AP (other than activation/deactivation behavior). The YD is either on or off, and it has no idea what the AP is doing, but by centering the ball, the AP is much more effective as it flies roll.

 

[jaredyates]

That is good news, thank you. Any input on the sporadic "yaw servo offline" issue?

 

[jaredyates]

I'm wondering if the YD is coming on automatically every time the Skyview system is powered on... this might explain the yaw servo offline issue. If this is the case, can we please make it so that this is not the case, or give us a setting to make it so that the system does not automatically turn on?

And is there any hope for any dynon-provided documentation on this in the foreseeable future? Even in draft form?

 

[Dynon]

Generically, that means that the system isn't seeing the servo at all. You'd see it if you have the servo power externally switched, for example. But if that's not the case, most likely, there's a wiring issue with power, or less likely the data lines (if a wiring issue affected just one of the data pairs you'd get a standby network warning but no actual loss of function to let you know that there is a wiring issue). Or, it could possibly be an issue internal to the servo. Give our support team a call at 425-405-0433 for further help with this.

 

[dougnlina]

Some email feedback I got from Dynon when working on getting my Yaw Damper dialed in, as there is nothing in the autopilot tuning guide. 
BTW It may sound counter intuitive but I found when the damper was not performing well that turning sensitivity, gain, AY gain DOWN tended to solve issues better than increasing.

per Dynon...
>I am actually in the process of writing that information as we speak. We have gotten a little behind in some of these details, sorry about that.
>Below I have some preliminary information learned from our flight testing:
>1.       TORQUE
>This setting controls the amount of torque needed for the pilot to override the servo.
>
>To adjust, increase the setting value until the servo no longer slips during normal autopilot engaged flight.
>Test the adjustment by making sure that the servo can be overridden by the pilot.
>
>2.       SENSITIVITY
>This setting controls the size of the error the airplane can generate away from the yaw targets before a correction begins.
>
>To adjust, increase the setting value by units of 5 until the airplane behaves as if it is under constant yaw control. This is often described as "twitchy".
>Reduce the setting by small increments until the twitchy behavior goes away.
>A large value indicates high sensitivity, and a small value indicates low sensitivity
>
>1.       RUDDER GAIN
>This setting controls the rate of response the autopilot can provide to dampen turbulence driven oscillations about the yaw axis when the autopilot is not engaged:
>
>Increase gradually until the airplane acceptably stops wiggling its tail in turbulence
>
>2.       AY GAIN
>This setting controls the rate of response the autopilot can provide to overcome aileron induced adverse yaw when the autopilot is engaged:
>
>Increase gradually until the airplane acceptably centers the Slip/Skid indicator during autopilot turns to new heading or track.
>
>3.       RUDDER RATE
>This setting controls the rate of response the AP can provide in yaw axis to trim the rudder to center the slip/skid indicator during strait and level unaccelerated flight.
>
>If the airplane exhibits a small slip/skid deflection during cruise, increase this value so that the Slip/Skid indicator centers in approximately 30 seconds to one minute.
>
>Please let me know if the above steps work for you. Also fell free to ask me any questions.
>Best regards,

and..

The Yaw servo provides two functions: 1) Yaw Instability Damping, and 2) Adverse Yaw Compensation. Which of the following issues are you installing the yaw servo to address?

    Yaw Instability is the result of the vertical stabilizer not being large enough to provide adequate stability in yaw.
        This can manifests itself as a left to right yawing moments (tail-wag) when the airplane is upset by turbulence, or, a singular yawing moment generated by changes in power settings.
        The yaw servo can compensate for this by canceling the yaw moments when they occur, either autopilot engaged or disengaged.
        This tuning control for this condition is RUDDER GAIN
    Adverse Yaw is the result the downward deflecting aileron creating more drag than the upward deflecting aileron. This manifests itself as a yaw moment in the direction opposite of the intended turn.
        The result is large skids during turns causing the Slip/Skid Indicator (ball) to displace away from center.
        The yaw servo can compensate by applying rudder in the direction of the turn to center the Slip/Skid Indicator during autopilot driven turns.
        The tuning control for this condition is AY GAIN, such as during the roll in or roll out of a turn.

During constant state cruise, if the Slip/Skid Indicator is out of center, or very slow to center, increase the RUDDER RATE setting until the Slip/Skid Indicator settles in the centered position within 30 seconds to 1 minute.

The initial sensitivity should be set to 10. If the airplane acts like it is constantly being controlled by the autopilot in yaw, it is to sensitive. Reduce this value until it relaxes. If it seems to "loose", increase the sensitivity until it reaches the point of constantly being controlled by the autopilot, then reduce a small amount until it relaxes.

 

[kellym]

I really appreciate this discussion. Made two flights today. On both, when YD was engaged it initially seemed to do nothing, then after 30 sec-1 minute it started diverging, getting worse. Stepping on the rudder pedals and holding the ball centered seemed to calm it down, but as soon as feet of rudders it got worse. I entered the settings Gary posted 2 years ago. I have current 15.3.4 software. When I went into setup, just as when Gary and Myron found with theirs, the AY gain came up as 1.0, and I sure didn't want to experience what they did. So now I am trying to figure which value I need to increase or decrease.

 

[kellym]

It has been 2.5 years since the yaw damper was released and the software released. To date there are no published tuning recommendations, no in-flight tuning guide. Other than the guidance contained in this thread, there is nothing.
I know that at times Dynon has had difficulty having a technical writer to keep the manuals up to date, but it has been over two years since the relatively short in-flight tuning guide was updated, and does not include the yaw damper at all.
It would be extremely helpful if we could bet some guidance for the yaw damper.

 

[DBRV10]

Dial the gains down.....but you are best to get it unstable and then decrease gains and sensitivity until it is stable. Start bad, work backwards.

 

[dougnlina]

+1 on Davids recommendation, Dial the settings up until the performance is unsatisfactory then go backwards.
But also +1 Whats Up Dynon? how about an actual tuning guide? I am still not 100% convinced I have mine tuned as well as it could be...