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Hi Marshall,
I did this same questioning before I bought the TruTrak and now after 182hrs. of flying my L4P I can truly say I love this autopilot.
I talked to several L4 flyers and flew one with the STEC 55 to OSH and back one year and this is what they all said. "I just turn it off in Turbulance and hand fly it." One person said that they flew all the time in Turb. and could never use the AP, in fact they had to land one time in route to rest up there hand and arm before continuing-saying it was very rough.
To be fair it just wasn't a Plug-in with the TruTrak and everything worked perfect. I think I had the seconded L4P to fly with this AP. I had the advantage of learning from the first one. So I had to move the Roll servo to a new location (from the right front control tube end to the right side linkage behind the Rt. Ft. seat) and we changed to the middle size Servo (there are 3 sizes) for more torque. This works fine now. I went though a period of time that I thought it was still not enough torque but found that my control system had TOO much friction, which was corrected.
The pitch servo was also moved to the rear bulkhead at that same time. Later we found that the push rod was not parallel to the control push rod and that caused a small rotation to occur when the servo made the smallest moves. With this corrected it is now perfect. In the control head you can adjust the Torque and Activity of each servo which is not possible with the STEC (I don't think).
This is the smoothest AP I have ever seen, i.e., when you have altitude select on you have to look at the Vertical speed to see it level off and hold altitude, you don't feel it. That is at 500/600 fpm. I have never had it do anything strange from an AP standpoint. The GPS couple will give you a surprise now and then and you will be saying "What the hell is it doing now?" which I understand is the most heard phrase in the airline cockpits.
One more thing. I recently added the third servo for Yaw. So far it is not perfect. It doesn't have enough torque and the activity needs to be on "0" to keep from wagging it's tail, but this is where Jim Younkin comes in. He is my hero and will come through with a fix. We just need to get together. It saves a lot of rudder trimming but at high speed and low speed I still have to help it with trim.
The way the Turn coordinator AP's work is, you have to have the airplane start a turn before the AP can put in an anti roll motion and that is too late in a turbulent situation. The TruTrak can be set to be so Active as to keep the wingtit from moving up or down the smallest amount. So much so that it could do accelerated ware on the control system, so you set it with less Activity.
I recently flew into Sedona, AZ in the hot afternoon, letting down from FL 250 and from 14K on down to 6.5K it was turbulent and I was indicating over 220 KIAS with the AP on all the way. It had us bumping around a lot to the point that I was ready to deploy the speedbrakes and slow down but the AP was doing a better job than I could have. It is so much better than my KFC 200 in my A36 that I have told others to put in a TruTrak and leave the old AP in for the certification but use the TruTrak.
They know how to mount everything in the L4's now and you won't have to go through what I did. The Yaw will get worked out but I'm not sure you need it especially if money is an issue.
Don't consider anything else,
Jim Hergert
N6XE, "An Sex Y" L4P
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