Pitch problems - Digiflight IIVS

[DanBaier]

RV7A with Digiflight IIVS fed from a GNS 430W.

The problem is in altitude hold. It had been working exceptionally well (static lag was set to 1), and suddenly has begun porpoising very badly when altitude hold is selected. Various altitudes. No matter how smooth the air or how well trimmed, it starts down, then up, etc. with each cycle progressively worse before having to inactivate the altitude hold.

I've read about the static reservoir issue, but if this is the suspect, I wonder why it worked so well then stopped.

This is a standard steam gauge setup - split static ports join to single 1/4" line running up to the front - airspeed, altimeter, encoder, VSI, autopilot.

Help with troubleshooting this would be appreciated.

Dan

[PhantomPholly]

First thing to try is to run it disconnected from the static system (use cabin air). If the problem disappears, it is a static problem - period.

I have a small valve with a knob I can turn in flight to select "Normal" or "Alternate (cabin)" static source, which is a good idea anyway if you fly IFR (cheaper than breaking the glass in the altimeter).

If it IS the static source, you might consider running a small short dedicated static line to some place protected from the elements but open to static air. Using cabin air can result in a "surprise" (rapid pitch change) if you change your vent settings (e.g. open to closed or vice versa).

If it is "none of the above," check all the linkages. Did you perform the Service Bulletin to replace the screws?

[DanBaier]

Thanks for the note - I read about that approach and before I start down that path I was hoping Lucas would weigh in with why it worked and then stopped. Nothing in the aircraft changed.

Dan

[TrutrakTech]

Dan,

That's really the best place to start because it either confirms or eliminates one solution set. One other thing to double check is that you are powering up on the ground while stationary and not in flight. The gyros go through a centering procedure each time it's powered up, if you're in flight you can introduce drift/offset into the gyros.

[DanBaier]

Thanks for the reply. I have not been powering up stationary on the ground - completely forgot about that. Frequently, I've been powering up in flight. If I have created some drift / offset - can I fix or reset that?

It would be tempting to start there because that might fit the scenario a little better and it keeps me from having to crawl under the panel.

Thanks,

Dan

[TrutrakTech]

Thanks for the reply. I have not been powering up stationary on the ground - completely forgot about that. Frequently, I've been powering up in flight. If I have created some drift / offset - can I fix or reset that?

It would be tempting to start there because that might fit the scenario a little better and it keeps me from having to crawl under the panel.

Thanks,

Dan

That can absolutely be the cause of your problem. There are two ways to address it. One is to simply be sure to power up on the ground. They calibrate each time the AP powers up. The other is if you are already in flight, try to maintain as straight and level of flight as you can. With the AP disengaged (but powered up), press and hold the knob in for about 10 seconds. The display will read GYRO SET during this time. That will remove any offset that may have been introduced. Thanks!

[DanBaier]

Oops - found it. I'll try the gyro reset and post back. Thanks for the help.

By the way - this is a very nice autopilot - the GPS steering and the altitude hold really spoil you.

Dan

[TrutrakTech]

No problem! Let me know how it works for you. Thanks!

[DanBaier]

Tried the gyro reset with the airplane on the ground not moving. Also turned down activity a notch and turned up static lag to the max. Result was the pitch excursions are gone and the AP works fine with altitude hold.

That said, I can see where the static issue has to be dealt with and when I do the condition inspection in the fall, I'll plumb a dedicated line. I'm assuming that all I need is a constant pressure even if not ambient static pressure. (For example, inside the wing 2 or 3 ribs out.)

Thanks again for your help.

Dan

[TrutrakTech]

Glad to hear that it's good now.

Putting the static line in the wing probably won't make much difference over having it just vented to the cabin. It certainly won't negatively affect anything either though. Thanks!

[PhantomPholly]

Glad to hear that it's good now.

Putting the static line in the wing probably won't make much difference over having it just vented to the cabin. It certainly won't negatively affect anything either though. Thanks!

Not quite true - if your cabin is sealed pretty well and you suddenly open or close a vent, the resulting pitch change can be dramatic. The subsequent beating you get about the head and shoulders after such an excursion in weather from your passenger is also a down side...

:D

Wing root is a much better idea...