REFLECTOR: Auto pilot challenges

Al Gietzen ALVentures at cox.net
Sun Nov 18 11:23:30 CST 2007


Subject: Re: REFLECTOR: Auto pilot challenges

 

Come on; Chuck, yesterday was my 'take it out on TruTrak' day. Now you want
me to justify it:-)? (Remember, I had just opened the bill for $1000 for the
servo I had already returned).

 

Interesting observations, but I'm curious...why do you care about the drag
on either the ailerons or elevator on the ground. 

I don't really.  When I first installed the servo, and felt the drag, it
seemed a little nonsensical to have made special effort to get everything
aligned and tight so there was very little drag, or play; and then connect a
servo that added a bunch of drag. 

 

.  When the A/P is disengaged in the air, I doubt that the the drag from the
servo is even noticeable compared to air forces on the elevator and
ailerons.

Before changing from the "C" to the "B" servo, I found that is was
definitely noticeable at pattern speeds; and especially for the small
corrections on very short final.  Now with the stronger roll trim springs
and "B" servo, I don't know. Of course the control forces required are now
higher, but I'm getting used to that. I'm sure partly it was having flown
for over 40 hours without the servo connected, and getting used to the very
light control forces.  I just gave me a better "feel" for the airplane at
low speeds.

 

As far as putting the aircraft into a roll and letting go of the controls
and expecting the plane to go back to level (and maybe I have my wires
crossed) but I thought in a roll, with no input to the controls, the plane
will stay in a roll, yes?  

OK; you must not have gotten the full Velocity sales pitch:-). Paraphrasing;
"At speeds above about 120 you put it into a bank and it stays there; but at
pattern speeds, if you get crooked, you let go of the stick at it returns to
straight and level.  That's why it's an incredibly stable IFR platform, etc.
etc."  (At least that's what I was told back in the mid 90's before I bought
my kit).  I believed it, and I found that it was true - well; sort of. Once
I connected the servo; at approach speeds the controls did not return to
neutral, and it wasn't true. 

The lateral control issues you cite sounds like either one of two things, 1)
you need to tweak the "lat act" activity a little or, 2) check to see if
your servo is hooked to the cable or directly to the ailerons. 

I'm expecting we can that figured out. It is true that I have the servo
connected to the bellcrank at the back of the keel.  When initially
installed, I had essentially zero 'play' between there and the ailerons (I
used full diameter bolts, etc.).  I reasoned that was the best environment
in which to install the servo, and that connecting it to one aileron simply
doubled the play to the other - which may not matter. I haven't checked the
play in the system since, and it could be a factor.

Thanks,

Al

 

 

 -----Original Message-----
From: reflector-bounces at tvbf.org [mailto:reflector-bounces at tvbf.org]On
Behalf Of Al Gietzen
Sent: Saturday, November 17, 2007 8:20 PM
To: 'Velocity Aircraft Owners and Builders list'
Subject: Re: REFLECTOR: Auto pilot challenges

Subtitle: Me and My TruTrak

 

Back in about 2000 I bought an S-Tec S-30. The servos were already installed
when in 2001 my project ran into about a 2-year delay.  When I got things
going again I re-designed the panel around the use of GRT EFIS.  The S-Tec
worked off analog input, and the EFIS systems are totally in the digital
age.  It would have cost many hundreds of additional bucks for the
converter, and meanwhile TruTrak came with the digital age AP at a very
reasonable price.  Decision: Sell S-Tec, buy Trutrak; have money in my
pocket. I went with single axis Pictorial Pilot as a starter.

 

The S-Tec servos have clutches that release entirely when the unit is not
active - zero drag.  When I got the TruTrak servo (C-version) I was sure
something was seriously wrong because the stepping motor had no clutch, and
had serious drag even when not engaged.  When installing my control system,
I had made sure there was very little drag in the aileron system; now I had
a servo that added serious drag, so bad that stock roll trim springs were
meaningless and could not overcome the drag.  OK, it turned out there was a
gear mesh problem, and TT replaced the servo.  Now the drag was maybe 20%
less but still many times the drag in the system without it connected.  I
was assured by TT that it wouldn't be a problem in flight conditions; and
whatever, they would make it right.

 

Fast forward - 2006.  I left the servo disconnected through the Phase 1
testing.  When I let go of the stick, controls returned to neutral.  That
was nice, particularly at pattern speeds.  I then connected the servo.
Major difference.  Push it into a roll at pattern speeds and let go; it
would not return to neutral control. Otherwise the AP seemed to work fine
coupled with the GRT EFIS.

 

I told TT that it was unacceptable.  They told me to put in stronger roll
trim springs, and they would replace the "C" servo with a "B" servo -
smaller and less drag. I found some suitable springs with roughly 3 times
the spring rate, and the "B" servo has less drag; so I'm not delighted but
it's OK.  I returned the "C" servo, and got the delivery confirmation about
10 days ago

 

Following their suggested procedure, I had the "activity" level set at 3 or
4.  In flying a flight plan I found that it seemed to track Ok, but near a
waypoint it wandered quite a lot.  A higher level setting seemed to help
that.  But I find now that it will slowly wander from the course; and then
make a fairly aggressive correction back toward the course, then a nice
transition back onto course; then repeat.  I will discuss with TT about what
action to take.

 

Yesterday I received a bill from TruTrak for $1000.00 pending return of the
defective unit.

 

The Trio system looks r-e-a-l-l-y good to me right now.  Their servo has a
clutch release.

 

YMMV,

 

Al

 

 

 

To: 'Velocity Aircraft Owners and Builders list'
Subject: Re: REFLECTOR: Auto pilot challenges

 

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

 

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.

 

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