Elevator trim tab question

[ronrapp]

Hi, I've purchased a flying RV-6 and noticed on a recent preflight that the brass-colored nut in this photos was loose. I'm not sure how tight it should be and what I should be checking for specifically with regard to this item on preflights. It doesn't look like there's any cotter pin or provision for safety wire, etc.

Also, if this nut comes loose in flight, what potential consequences would result?

I ask this because I've been fighting a weird problem with the autopilot (G3X Touch panel w/ Garmin servos) where it would fly perfectly for an hour and then suddenly just pitch up strongly for no apparent reason. It's definitely a pitch trim issue, as I can see the pitch trim moving nose-up during this event. I've been exchanging emails and G3X data files with the Garmin support folks for quite a while but haven't come to any satisfactory answer and I'm wondering if there might be a connection between the two.

Thanks,

--Ron

[WingsOnWheels]

The jam nut it a safety device to keep the cable from unscrewing itself from the clevis, so should not be loose.

Do you have electric trim? I see a cable like the manual trim. However, some builders put the servo on the aft deck with a cable to the tab. The CG changes as fuel burns, if you have manual trim, then the Garmin auto trim is not going to be doing anything. It sounds like you are getting an AP disconnect due to excessive stick forces. It is possible that the trim is drifting, or possible it is a configuration/setting issue in the G3X.

[lr172]

Jam nut should be tight, but if it gets loose, the clevice cannot spin and it seems improbable that the cable could rotate any meaningfull amount with the relatively tight radius bend that it has at the operator side, at least not over a short period of time.

That is I think a 32 TPI thread, so a loose jam nut can only allow about 10-15 thousands of an inch movement with a loose jam nut. No way that amount of movement is causing the issue you described. From the pic it looks like you have a vernier trim cable. How is it that you are seeing pitch trim changes with a manual trim setup? Or is that cable somehow terminating at a trim servo? Typical 6 has the servo mounted in the elevator with two clevices and a threaded rod and based upon the pic, that is not the case with your plane.

Larry

[ronrapp]

Originally Posted by lr172

That is I think a 32 TPI thread, so a loose jam nut can only allow about 10-15 thousands of an inch movement with a loose jam nut. No way that amount of movement is causing the issue you described. From the pic it looks like you have a vernier trim cable. How is it that you are seeing pitch trim changes with a manual trim setup? Or is that cable somehow terminating at a trim servo? Typical 6 has the servo mounted in the elevator with two clevices and a threaded rod and based upon the pic, that is not the case with your plane.

Larry

Hi Larry! It actually has electric trim, so I assume it ends at the Garmin pitch trim servo.

Perhaps the airplane was originally built with manual pitch trim and it was converted when the G3X system was installed.

--Ron

[lr172]

Originally Posted by ronrapp

Hi Larry! It actually has electric trim, so I assume it ends at the Garmin pitch trim servo.

Perhaps the airplane was originally built with manual pitch trim and it was converted when the G3X system was installed.

--Ron

The Garmin servo does NOT provide trim control directly. It can only send electricity to a dedicated trim servo motor to achieve trim changes. I would follow that cable all the way to the trim motor and see if there are any flaws at that end that could allow uncommanded movement of the trim tab. Not seeing anything at the elevator side that could allow enough movement. It would seem prudent to grab the trim tab and try to move it up and down and see how tight the linkage feels.

[ronrapp]

Originally Posted by WingsOnWheels

Do you have electric trim? I see a cable like the manual trim. However, some builders put the servo on the aft deck with a cable to the tab. The CG changes as fuel burns, if you have manual trim, then the Garmin auto trim is not going to be doing anything. It sounds like you are getting an AP disconnect due to excessive stick forces. It is possible that the trim is drifting, or possible it is a configuration/setting issue in the G3X.

Thanks for the reply! I do have electric trim.

I had the same thought as you: perhaps it's an out-of-trim condition and the autopilot is simply disconnecting when the control forces reach its limit. But the AP does not disconnect. It pitches up fairly aggressively and I can see the pitch trim moving, almost like a runaway. At this point I'll disconnect the autopilot manually, and then I hear the disconnect tone.

Also, to ensure out-of-trim condition is not the cause, I'll engage the AP only in level flight with the airplane perfectly trimmed and speed stabilized in smooth air.

I can't figure out any rhyme or reason as to why and when it will do this. Sometimes it will fly the plane perfectly for 10 minutes, 30 minutes, or more. Sometimes it will pitch up after just a few minutes.

As I said, it seems to have the Garmin experts stumped as well, but I'm still pursuing that avenue with them.

--Ron

[g3xpert]

Originally Posted by ronrapp

Thanks for the reply! I do have electric trim.

I had the same thought as you: perhaps it's an out-of-trim condition and the autopilot is simply disconnecting when the control forces reach its limit. But the AP does not disconnect. It pitches up fairly aggressively and I can see the pitch trim moving, almost like a runaway. At this point I'll disconnect the autopilot manually, and then I hear the disconnect tone.

Also, to ensure out-of-trim condition is not the cause, I'll engage the AP only in level flight with the airplane perfectly trimmed and speed stabilized in smooth air.

I can't figure out any rhyme or reason as to why and when it will do this. Sometimes it will fly the plane perfectly for 10 minutes, 30 minutes, or more. Sometimes it will pitch up after just a few minutes.

As I said, it seems to have the Garmin experts stumped as well, but I'm still pursuing that avenue with them.

--Ron

Hi Ron,

I think we will find the resolution to this one in the configuration/installation of the system. We have one log from your aircraft from a few months ago, with just a short period of autopilot engagement included. It is hard to draw a definitive conclusion from that flight but it does appear to be an issue with trim movement direction.

Please send us a current log and a configuration file from that flight, we should be able to sort it out from there.

To unload a configuration file, boot the system into configuration mode, select CONFIG, press MENU and select Export.

Thanks,

Justin

[ronrapp]

Originally Posted by g3xpert

Please send us a current log and a configuration file from that flight, we should be able to sort it out from there.

I'll do that this evening. Thanks!

[PilotjohnS]

If the jam nut is loose,I think it could cause the threads inside the clevis to wear. This could lead to a sloppy fit between the clevis and the cable. If it gets really bad, then the cable and clevis could slip a few threads.

I would check the cable and clevis threads for excessive wear. If any is found, then it might be prudent to replace both. JMHO