View Full Version : Tuning up the GRT/TT Combination

12-15-2009, 11:01 AM
Last week I flew the initial tests of my new Tru Trak Digiflight VSGV coupled to my GRT EFIS, and found that everything worked, right out of the box! It?s hard to believe that I got all the wires going to the right places the first time, but I?ll take success when I can get it. This past weekend, I flew up to Dallas and back, which gave me a chance to play with the system a little bit, and I was extremely impressed ? twiddle the knobs on the EFIS for altitude and heading, and watch the airplane go. In fact, when I got to my destination, I remarked to a friend that it didn?t seem like I had been flying ? it seemed more like a simulator session!

One thing I did play with, however, is gain tuning. When I first engaged the pitch channel during testing (in a climb) I got some noticeable hunting behavior in GPSV mode (EFIS driving the autopilot). When I asked Mark at GRT about this, he was up front and said the gains in the EFIS were what I probably needed to adjust, to leave the autopilot alone. He also said he really didn?t have any suggestions on numbers to try, (since he hadn?t tuned the system, for an RV-8 before), except to slowly increase or decrease and then see what I get. That?s a reasonable and normal way to do it, so on the cross-country, I played around a little with the climb and altitude hold gains a bit. I was surprised that very small variations made a positive difference. Since it seemed a bit too lively, I reduced both gains from the default ?1.00? to ?0.95?, and the hunting behavior went away in both climb and hold. Now, the air was smooth as glass, so it?s probably not fair to say that I have them nailed ? just working better.

I?m curious if anyone else with the set-up in an -8 has numbers to share?


12-15-2009, 12:20 PM
I think I had to adjust only my lateral gain a bit for GPSS to stay right on track when the GPS source was the EIFS, other then that all works great with default settings. Although on one occasion, the plane was hunting seriously for the altitude to the point that I had to disengage it. Then I realized that I was setting it for too high of minimum speed during climb out and I was already at 15K altitude so with current power setting the plane could not sustain 150 mph and still climb at that altitude. So, it would lower the nose to gain speed and as soon as it gained the min speed it would try to climb again, thus the osculating nose up and down if that makes sense. So, be aware of power setting and minimum climb speed if those are set.

Martin Sutter
12-15-2009, 01:17 PM

I get noticeable pitch oscillation when the mode is set to GPSV and the airplane is out of trim. To fly smooth coupled approaches and missed approaches I trim the airplane first for the desired approach speed in level flight before I engage the AP. (I use 110kts). From then on I only manipulate the throttle to maintain that speed on the glide slope and on the climb-out to the missed approach point. This results in very smooth AP operation during the whole procedure.

Good luck,

Martin Sutter
building and flying RV's since 1988
Dual GRT, GNS430WAAS, TT Digiflight II VSGV

12-15-2009, 08:24 PM
I am flying an RV7a with GRT and TT VSGV and also had some oscillations which I corrected by changing the "Altitude Hold Gain" from 1.0 to .5. It made a huge improvement and still tracks the vertical guidance on approaches very well. Awesome equipment!!!

01-04-2010, 12:56 PM
Yesterday, Louise and I flew up to Brenham - her first chance to play with the new autopilot setup from the front seat, and my first chance to experience it with an aft CG. We discovered that the gains at 0.90 for both Altitude Hold and Climb were too high in this configuration - not surprising, since the pitch forces get considerably lighter with a passenger. There were significant overshoots and oscillations on level-off - took about 4 cycles to settle down.

I just got back from a little gain-setting flight solo, and dialed both of them down to 0.50, with very positive results. I was afraid of getting things too sluggish, but the oscillations were gone during both climb and level flight, and the response was about what you'd want in IFR flight. The gains might be a little more aggressive to make descents right - I set a - 1,000 fpm descent rate, and it never really got there in a 1500' altitude change - sneaking up to about -800. Next time up, I might try 0.60 for climb/descent.

Still thrilled with the system! Just tweaking it to be better (and wonder if I'll find a single set of gains that work with and without a passenger - tricky in a tandem!).