REFLECTOR: Auto pilot challenges

Terry Miles terrence_miles at hotmail.com
Sat Nov 17 08:07:07 CST 2007


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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