TruTrak Autopilot

[jnmeade]

I flew my CTSW for 3 hours in continuous light chop the other day. The TruTrak Digitrak II autopilot would occasionally respond to stronger lift or sink by diving as much as 700 fpm or climbing nearly as fast. Naturally, this played hob with the RPM and I'd sometimes have to pull the throttle to avoid overspeed. I was in a hurry and most of the trip was at 5450 RPM or so.

My question is whether there is a way to set the AP so that it doesn't hunt quite so hard and fast all the time. I'd like to set max VSI rates even at the expense of altitude hold.

If anyone has experience with the Dynon AP in continuous turbulence and especially if you have flown both in chop, I'd be glad to hear your impressions.

[Jack Tyler]

My suggestion would be to mine the Vans Air Force forum for relevant info, as many of those builders have chosen one or the other a/p brand, have lots of air time with those systems now, and some are quite technical in their analysis of performance. E.g. just searching 'trutrak autopilot' here's one of the first comments that surfaced:

"The Dynon autopilot I recently removed was very adequate after MUCH tweaking. I flew it IFR numerous times and would not hesitate to install one again. It kept the "pink" line centered all the way to the runway.
The GX pilot on the other hand is AMAZING. For example, with the Dynon, while flying a long cross country in formation I would not engage the autopilot if I was following a Tru Trak. It would constantly converge or diverge from the other aircraft. It was never enough to displace the cdi, and would not be noticed if you weren't flying in formation, but still it was annoying enough for me just to hand fly it. Yesterday, leaving Halls, TN following an airshow, Fellow G3X/ GX pilot Greg "Greese" Reese and I climbed to 11,500 and headed for Cincinnati ( LUK).
Greg let the autopilot fly up to altitude, while I hand flew his wing. Upon level off, and established in "route" position, I engaged the autopilot direct LUK. Greg was at full throttle and I required something less. I found a setting that was LOP, and from there on I only touched the mixture control to vary fore and aft position. The "TWIN" autopilots kept us both vertically and horizontally spaced identically. They did this for over 250 nm.
I installed the GX pilot out of the box, looking only at the wiring diagram, tested the servos, clipped a pitch reversal wire, and flew it. I still have not looked at the rest of the installation manual, nor have I adjusted any settings."

And while this specific report may not be for the model you are using, another option you have is to post in order to solicit the info you're looking for.

[ct4me]

...better yet, just go to the Tru Trak tech people directly. They are the absolute best, when it comes to support.... easily available, smart, well-spoken.

[jnmeade]

Tim,
I'll accept your suggestion to talk to TruTrak.

I continue to be appreciative of any pilot reports on how the two compare, as I'm on the edge as to which way to go in that regard.

[Mark Gregor]

Jim,

I have "heard" that the trutrak is much better than the dynon for controlling altitude but have no experience personally.
Turbulance is a subjective thing but 700ft is alot.

I have had good experience with trutrak since Tecnam switched from Trio. Heading is never more than a degree off and in smooth air it holds altitude within 5ft. I would say it could hold within 30-40ft in turbulant air but any worse than that i shut it off as we are jumping a hundered feet or more at a time and i dont think any auto pilot could figure that out.
Its a whole new world compared to the Trio.

Mark

[Dennis]

You had altitude hole engaged. If so, let's think about it. If you were hand flying the plane and you were caught in an updraft and wanted to maintain altitude, you'd push down on the yoke. The air speed would increase and the rpm would climb with a fixed pitch prop. Opposite for a down draft.

It sure seems to me that the autopilot was doing what it was supposed to do.

If you aren't on an assigned altitude and cloud conditions permit, you can ride the roller coaster up and down without altitude hold engaged.

Fly carefully.
Dennis

[jnmeade]

Dennis, please reread my OP. The question is whether one can change the settings on the TruTrak to respond differently than it is programmed to do.

We all know that if we set the AP for 5,500 feet and encounter a strong up or downdraft, it is possible that the AP can't compensate enough. We may still climb or descend from 5,500 even though we are at full pitch up and full throttle or full pitch down and full throttle. So, we expect the AP to try but not always succeed in holding altitude and it is up to us to deal with that according to the situation.

Since the AP was commanding radical pitch changes in response to some very aggressive weather, in my situation I'd have liked to be able to have the AP respond a bit slower and a bit less radically. To use your analogy, if I had been hand flying at 5,500 and the circumstances permitted, I might have let the plane ascend to 6,000 or descend to 5,000 in response to the wind by putting in some corrective inputs but not to the radical extent the AP was doing. That might mean I could let the thrust lever setting stay the same and vary the pitch. The AP can't vary the pitch, but it can respond slower and it can respond to a lesser degree that it is currently now set for.

AP must be able to do this or they'd chase every single little altitude excursion and the entire trip even in almost calm air would still be a series of up 10 down 20 up 15 down 5 at whatever the response rate was that the AP used. I'd be willing to bet they dampen the response rate within some parameters so that the AP responds slower and less radically to minor changes. That's all I'm looking for information on - how to put in new parameters.

My tractor has autosteer and you can put in a factor that tells it how fast to react to drifting off line.

[Merlinspop]

TruTrak's website does not have any documents referencing installation of a "DigiTrak II" but does have an installation guide for a "DigiFlight II." That document describes how to set the value for VRT ACTIVITY on pages 6 and 7.

Their intro to this section says, "The two activity adjustments (LAT ACTIVITY and VRT ACTIVITY) determine how briskly the autopilot responds to roll and
pitch disturbances. They can be adjusted, in flight, over a wide range; thus the autopilot can be tailored to adapt to any aircraft installation." Which makes me think that this is the adjustment you are looking for.

Document is found here:
http://www.trutrakflightsystems.com/doc ... anual1.pdf

Bruce

[drseti]

Since the AP was commanding radical pitch changes in response to some very aggressive weather,

Which all autopilots do, to varying degrees. My experience with TSO'd, STC'd autopilots in certified aircraft (S-Tec, Astronautics, Pathfinder, Century, etc.) has been that, in turbulence, I'm better off hand-flying the aircraft.

[jnmeade]

Bruce,
Just the info I was seeking. Thank you so much.

[Dennis]

I flew my CTSW for 3 hours in continuous light chop the other day. The TruTrak Digitrak II autopilot would occasionally respond to stronger lift or sink by diving as much as 700 fpm or climbing nearly as fast. Naturally, this played hob with the RPM and I'd sometimes have to pull the throttle to avoid overspeed. I was in a hurry and most of the trip was at 5450 RPM or so.

My question is whether there is a way to set the AP so that it doesn't hunt quite so hard and fast all the time. I'd like to set max VSI rates even at the expense of altitude hold.

If anyone has experience with the Dynon AP in continuous turbulence and especially if you have flown both in chop, I'd be glad to hear your impressions.

Today I had 0.7 hours experience with the TruTrak autopilot in my Skycatcher. I was at 3500 feet in continuous light to moderate turbulence due to thermals from farm fields below.

The A/P flew flawlessly in roll stabilized mode, heading mode, and GPS tracking mode. Altitude hold was a different story.

The A/P was over controlling all of the time. The "Trim Up" enunciator would blink momentarily in downdrafts and "Trim Down" in updrafts. The A/P had enough elevator authority such that the servos never slipped. But the ride was a roller coaster. I was able to maintain altitude better and with a smoother ride by hand flying the plane in pitch attitude with A/P handling GPS tracking mode.

In similar conditions in my C-172 with S-Tec system 30, the altitude hold was much smoother. I have concluded that altitude hold in the TruTrak is best used in relatively calm air.

Just curious if you were successful in tweaking your A/P. I have the Garmin 300 system for A/P control and there is no way to adjust the preset parameters.

Fly safely,
Dennis

[CharlieTango]

My AP works well flying on the horizontal but will result in a roller coaster ride on most days in the vertical. The same is true when I hand fly, I mostly hold a level attitude and don't even try to hold an altitude. Thats the nature of very light aircraft flying in the west.

[drseti]

Thats the nature of very light aircraft flying in the west.

And it's not all that different in the east!