In an autopilot, does trim sensing determine where you should put the trim to lessen the forces on the autopilot or does it sense when the trim is adjusted and correct for it before sensing the altitude deviation?
It is my understanding that trim sensing provides an indication that the autopilot is having to continuously work against an out-of-trim condition. It appears the Trio Altitude Hold will annunciate the need to adjust trim.
This is a nice feature, especially if it is included at no additional cost as it is in the Trio unit, but after flying the TruTrak AlTrak device for many hours I have not found trim sensing to be an essential feature. In my experience an RV will go significantly out of pitch trim only after two+ hours of flight. During most flights I find a need to change altitudes more often than that and the AlTrak and trim gets reset with each change. I always carefully trim the plane before engaging the AlTrak.
If the altitude hold is disengaged with the plane out of trim, there will be a sudden pitch excursion if you aren't holding the stick, but this motion will be quite small in most cases if the plane was trimmed properly before the altitude hold was engaged.
I don't see inclusion or exclusion of trim sensing as a deal breaker either way. If it is included, enjoy it; if not, don't worry about it.
Sam Buchanan (RV-6, 640 hrs)