EFIS 1 /autopilot/trim

[Jim & Julie Wade]

This question may be for Greg! I am installing a dual screen EFIS 1 and BMA autopilot in my RV10. Is is possible for the EFIS 1 to control my Ray Allen Trim motor on the elevator trim?? Sure would be nice, not having to disengage autopilot to check trim.
Jim Wade
Mississippi

[billkennedy3]

Greg,

Me too. Why use a servo in the linkage if the appropriate control signal could be sent directly to a trim motor? Less weight, less complex, less money.

Bill

[Clay]

The trim servo is way too slow to react to be used by the autopilot as the primary control. There have been attempts to do this for an altitude hold in the past without much success.

As far as having the EFIS control the trim servo to keep the airplane in trim when the autopilot is being used.. I'd say it really isn't a problem to worry about enough in real life to add the relays, etc. required to make it work.

What I WOULD like to see, however, is a very small up, down, left, right arrow indicator on screen to tell you which way to bump the trim when the AP is in use. It can't be that hard... if the servo is constantly having to push "up" then the arrow would show up. Same thing for left/right. This would enable you to trim without turning the AP off to see if you're out of trim.

[Greg Richter]

Thanks, Clay. RIght on target. We are adding trim annunication in the next iteration of the AP.

[billkennedy3]

OK, so the trim motor is too slow. How about a little servo driving the trim tab instead of a MAC motor? I could probably dream up a way to control the servo from my trim button too.

Bill

[highspeed]

Greg,

Just a thought. I may be way off base here, but I thought I'd weigh in with a concept that may or may not even be feasible.

How about a "Counter-EMF" detection circuit that would measure the amount of resistance on the AP Servo Motor caused by an out-of-trim condition on the control surface in question. This circuit could then provide corrective input to the trim servos to keep the controls in-trim - at least to a degree. Obviously this correction would only be applied when sensed resistance is outside of an acceptable margin so that the AP servos aren't constantly battling the trim servos and vice-versa.

Undoubtedly, this would require a redesign of the AP controller and may be more expensive than would be feasible. But to many, this area of safe operation of an aircraft may be worth the added expense. It would also complicate the wiring since this would have to tie in with existing trim servo control wiring, but again the added effort may be worth the trouble.

This would provide a safety margin should the AP drop out for any reason and the aircraft would be in a relatively comfortable configuration without a sudden change in aircraft orientation caused by out-of-trim flight controls.

Again, just a thought...

Stephen "Highspeed" Kruse

[Clay]

I'll dispense one piece of free advice to those of you still building, from someone who has finished. KEEP IT AS SIMPLE AS POSSIBLE.

(of course, if you follow that advice, you might not install an EFIS either.. but that's another issue)

Believe me, EVERY LAST DOODAD that you add to your plane better be there for a darn good reason, or else you should enjoy maintenance better than flying.

Honestly, if Greg adds the indicator that tells you when you're out of trim, and you just take a look and trim appropirately after a power change, everthing will be fine, and you'll have one less thing to break or fiddle with. Adding the circuits, relays, wiring, etc. to link the AP with the trim is a LOT of added time, trouble, and complexity for very, very little benefit.

I don't know WHY there isn't a single autopilot out there, that I know of anyway, that uses a high-speed servo to move a trim tab instead a larger servo to move the whole control surface. I'll bet there's probably a good reason, though. Anyone?

[jon_scholl]

I'm with Clay on this one. There are way too many cool features greg is working to do this one, and the complexity is difficult to justify.

First rule of airplane building: If you think you want something extra on the plane, throw it in the air. If it comes down, leave it off.

Jon

[Greg Richter]

Originally Posted by billkennedy3

OK, so the trim motor is too slow. How about a little servo driving the trim tab instead of a MAC motor? I could probably dream up a way to control the servo from my trim button too.

Bill

That'll work great, Bill. Doesn't matter whether you drive the control surface or a trim tab that flies the control surface so long as the bandwidth to it is high enough.

I'm playing with a really, really WILD linear motor design. Looks like the wave of the future.

[Greg Richter]

Showing out-of-trim using loop following error is what we are working with. I really like to see trim and AP functions separated, but that's just me ...