REFLECTOR: Auto pilot challenges

Sun Nov 18 09:56:53 CST 2007

Hi, Al,

Interesting observations, but I'm curious...why do you care about the drag on either the ailerons or elevator on the ground.  Certainly, without the A/P hooked up, a near dragless feel is one indicator of things being in order.  On the ground, we could disconnect the fiberglass spring from out elevator and we would have virtually no drag, but flying might not be so much fun.   I would think the only drag you care about is in the air.  In the air, when the TT A/P is engaged, the manual force required to overcome the clutch in the A/P can be adjusted in the A/P.  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.

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?  Pitch oscillations are certainly different and are supposed to self-dampen, but I'm not sure that is true in the roll axis, so the plane may very well have been behaving properly.

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.  If the servo is hooked to the cable, I believe it is a common problem to have a little play in the cables even though they seem to be rigged right.  My aileron servo is hooked directly to the aileron and I haven't experienced the wandering you mention, but gosh only  knows, every installation is different.

Chuck

 -----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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