VSGV porpoising in a turn

[scard]

Here is a partial cross post from the GRT EFIS group.

..."One activity that seems to set it
off consistently is if the plane is being driven by the GPS and it turns a 90
degree turn as it turns from the base leg of an RNAV approach onto the final
course. Sometimes, as little as 20-30 degrees turn will trigger the initial dip
and then the porpoise is on..."

Chuck,
I have exactly the same symptom of the porpoising when rolling into a gps commanded turn. However, it seems to me that this is a TT VSGV autopilot thing more than an efis issue as mine does the same thing when getting its commands directly from the GNS430W. Something about a change in vertical lift component in the turn and change in trim needed? I find that I can dampen it out almost fully by applying just the right amount of back pressure with the a/p coupled. Definitely very annoying. I wonder if TruTrak could provide some insight. I think I'll cross post to their new board.

Cheers,
Scott

[TrutrakTech]

Here is a partial cross post from the GRT EFIS group.

..."One activity that seems to set it
off consistently is if the plane is being driven by the GPS and it turns a 90
degree turn as it turns from the base leg of an RNAV approach onto the final
course. Sometimes, as little as 20-30 degrees turn will trigger the initial dip
and then the porpoise is on..."

Chuck,
I have exactly the same symptom of the porpoising when rolling into a gps commanded turn. However, it seems to me that this is a TT VSGV autopilot thing more than an efis issue as mine does the same thing when getting its commands directly from the GNS430W. Something about a change in vertical lift component in the turn and change in trim needed? I find that I can dampen it out almost fully by applying just the right amount of back pressure with the a/p coupled. Definitely very annoying. I wonder if TruTrak could provide some insight. I think I'll cross post to their new board.

Cheers,
Scott

Scott,

The first thing to try is increase your static lag setting from 0 (default) to 2. Most of the time, this works. If it doesn't stop it completely, try venting the AP static port to the cabin. If that takes care of it, try installing a dedicated static port for the AP. Thanks!

[PhantomPholly]

Scott,

The first thing to try is increase your static lag setting from 0 (default) to 2. Most of the time, this works. If it doesn't stop it completely, try venting the AP static port to the cabin. If that takes care of it, try installing a dedicated static port for the AP. Thanks!

Yep, definitely flies smoother on cabin static.

Suggest disconnecting from the static system, as the autopilot only needs it for "relative altitude." That worked very well in my Lancair 235, which was extremely pitch-sensitive.

[deene]

I wonder if any slip/skid during any turns might be biasing the static ports generating small pressure transients which the AP reacts too.

Using cabin pressure as a static source would minimize this.

Just a working theory...

Deene Ogden
RV8 N299AD, ready to fly...

[TrutrakTech]

I wonder if any slip/skid during any turns might be biasing the static ports generating small pressure transients which the AP reacts too.

Using cabin pressure as a static source would minimize this.

Just a working theory...

Deene Ogden
RV8 N299AD, ready to fly...

This is a definite possibility with static ports on opposite sides of the aircraft. It doesn't take much of a variance to disturb the AP, as it's quite sensitive.

[PhantomPholly]

Ooops - I was reminded in another forum of a rather nasty side-effect of cabin air I had forgotten.

If you change your cabin vents (open, close, or simply change) while on AP using cabin static, be prepared for a violent pitch change!

Caveat Emptor...