REFLECTOR: Auto pilot challenges

Sat Nov 17 09:10:09 CST 2007

Is anyone using the the TT autopilot with a different set up (other than GRT and the 480)?
Is the response the same?  
How about the Trio Avionics A/P in a Velocity?  Is the experience similar to that of the
TT autopilot?  Thanks

Regards,
Jorge Bujanda
Velocity XL FG 
Palmdale, CA
Website:  http://members.dslextreme.com/users/jbujanda/

"Make your best case... not the best case."
"Spare the noise... convince through silence."
                                                                         JB

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From: reflector-bounces at tvbf.org [mailto:reflector-bounces at tvbf.org] On Behalf Of Chuck
Jensen
Sent: Saturday, November 17, 2007 6:33 AM
To: Velocity Aircraft Owners and Builders list
Subject: Re: REFLECTOR: Auto pilot challenges

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

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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

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