REFLECTOR: Auto pilot challenges

Sat Nov 17 08:33:03 CST 2007

On the GRT EFIS A/P settings, I set all of the gain values to "1".  

The correct setting on the A/P is for "Vert Act" to be set to 2 or 3 in most instances.  One gets into the setup mode on the A/P by pushing and holding the right button I believe it is (if that doesn't work, then it is the left button :-) )--for the circular A/P...I'm not sure which button if you have the radio stack version.

After holding the right button and the menu items come up, push "next" until you see Vert Act pop up, then try it at 2 or 3.  No airplane will fly correctly at a setting of "0" and a large setting of  4 or above usually results in pretty aggressive altitude hunting.

Even with my old settings, the A/P flew the plane straight and level very well--it was just in the turns or turbulence that it started having Fundamentalist behavior.

Chuck Jensen 

-----Original Message-----
From: reflector-bounces at tvbf.org [mailto:reflector-bounces at tvbf.org]On Behalf Of Terry Miles
Sent: Saturday, November 17, 2007 9:07 AM
To: 'Velocity Aircraft Owners and Builders list'
Subject: Re: REFLECTOR: Auto pilot challenges

Hey Chuck,

Thanks for that post.    I have the same TT DigiFL II w/ a GRT and a 480.  I've got similar problems.  You are referring to the "gain" setting right?

Terry

  _____  

From: reflector-bounces at tvbf.org [mailto:reflector-bounces at tvbf.org] On Behalf Of Chuck Jensen
Sent: Saturday, November 17, 2007 6:56 AM
To: Velocity Aircraft Owners and Builders list
Subject: Re: REFLECTOR: Auto pilot challenges

Hi, Tim,

First, don't feel alone.  I had exactly the same symptom with the same A/P.  I struggled with it for an extended period of time. I sent the A/P to TT and they checked it out and even flew it in their RV and it performed as expected.  Like yours, when I went into a turn, whether it was with the heading bug or a turn initiated by the GPS NAV link, it would pitch up/down to the point that I would disconnect the A/P.  The A/P was equally disconcerting in turbulence including in forming cumulus when a person is IFR...just when it would be very nice to depend on the A/P.

With no luck in sorting out the problem, I had a trip to the Midwest planned so I set up a TruTrak appointment and flew into ASG (Springdale, AR).  John looked over the hookup to the elevator and Jim Younkin gave his blessing as well.  They put the unit on the bench and ran it through the tests which all seemed to be within specs.  While on the bench, they did several upgrades including the new lighted LCD with the light sensing eye--very nice readability improvement!  We reinstalled the A/P and I went out to fly it in the usual late morning thermals and forming cumulus.  The result was no real change.  Jim Younkin then installed it in his RV and we went out and flew it.  Naturally, it flew just fine, so we put the A/P back in the Velocity and Jim said he wanted to fly with me to see for himself.

As I changed heading with the SEL knob on the A/P and changed altitudes as we went through thermals and other light turbulence, Jim had me change the vertical activity setting on the A/P from 1 thru 7.  A setting of 2 or 3 seemed best.  All other settings remained conventional.  Jim, in his best troubleshooter/designer/engineering tradition, was focused on how the A/P behaved as it flew the plane, not how it behaved when it was interfaced with the CNX-480 GPS or GRT EFIS.  While I understand his scope of interest was the A/P, this is perhaps a bit of a myopic view since ePanels use instruments as a system, not as stand alone units.   As such, Jim was satisfied the A/P was set and functioning properly.  Of course, I never doubted the A/P was functioning properly, but that begs the question of why it developed a mind of its own when commanded to turn with the heading bug, but we did not investigate where such a problem, that being interface driving the A/P with the EFIS heading bug or GPS NAV signal, might lie.  

During the visit, all of the TT people were very helpful, friendly and determined to do everything they could to set things right and refused to accept any kind of payment since the A/Ps have a life time guarantee (or something to that effect), I did insist on contributing to their favorite charity, especially given that Jim bought lunch.

Out of this, I took the following action, 1) I use either the 2 or 3 vertical activity setting and, 2) I disconnected the static line from the A/P leaving the A/P open to the cabin instead of tied into the static line.  This keeps the noise in the static system from upsetting the A/P.  

At this point, I would say the A/P is behaving markedly better without the pitch up/down tendency in a turn.  I've yet to determine whether it is as rock solid as I would like in turbulence.  Velocitys, and all other canards, will always be a touch bumpier in turbulence since we set close to the canard instead of the center of gravity, so when the canard bounces, we will too.  Regarding the noisy static system, we speculatively attributed this to the static ports being located somewhat below and behind the canard, so it is possible that as turbulence hits the canard, a pressure wave passes over the static port, causing a bounce.  Previously, the static holes were located just under the pilots door but the embedded lines/fittings apparently developed a leak so the port below/behind the canard was selected.  I'm currently flying with the static line open to the cabin.  The static system is, not surprisingly, far less noisy and to date, I've not really seen any inaccuracies/discrepancies between the altimeter, the EFIS altitude and the GPS calculated altitude, so I'm wondering what the downside of operating in this mode is?

So Jim, I would check to make sure the vertical activity is set at a low number, but not 0 or even 1.  A high number will definitely get you a lot of ups-n-downs.  Let us know what you find.

Chuck Jensen 

-----Original Message-----
From: reflector-bounces at tvbf.org [mailto:reflector-bounces at tvbf.org]On Behalf Of TimRhod at aol.com
Sent: Friday, November 16, 2007 6:55 PM
To: Reflector at tvbf.org
Subject: REFLECTOR: Auto pilot challenges

Velocity fliers  I am Currently flying a Velocity XLRG with a new TruTrak Digifly 2 VSVG autopilot  It flies true to heading or track and altitude hold is good in straight  flight but when I turn the heading knob to change the heading as the plane banks it also starts to pitch up and down.  The longer the plane stays in the turn  the more severe the pitch oscillations become.  I've tried most combinations of settings on the trutrak and also disconnect from my static port to no avail. I have the C servo for pitch.  Anyone had and solved this problem ?     Thanks Tim Rhodenbaugh

  _____  

See what's new at AOL.com <http://www.aol.com?NCID=AOLCMP00300000001170>  and Make AOL Your  <http://www.aol.com/mksplash.adp?NCID=AOLCMP00300000001169> Homepage.

-------------- next part --------------
An HTML attachment was scrubbed...
URL: http://www.tvbf.org/mailman/private/reflector/attachments/20071117/cefb2ef8/attachment-0001.htm 