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Strange Elevator Trim Behavior

jherzog

jherzog

I'm New Here
Hello,

I have an RV9A that uses a Ray Allen actuator and relay for elevator trim. There are two sets of up/down switches wired in parallel to the relay -- one on the stick, the other on the panel. The behavior I'm encountering is the same for both switches, so the issue does not appear to be with either of those.

What's happening is, about half of the time, trying to add down trim will not cause movement unless I first use a bit of up trim. That seems to free something up, and down trim begins working again.

Has anyone encountered something like this?

Thanks in advance,

Jordan
 
Have you tried disconnecting it at the trim tab itself and using a 9 volt battery to watch it and see if it's binding? Does it happen only airborne when under air pressure or does it happen on the ground too? You said it's intermittent, is there a particular set of circumstances you can induce to make it happen? If it's not binding, are you certain it's not an electrical connection where the signals from the sticks meet up and go to the trim tab? Maybe you have a recessed pin or a cold solder joint along the way?
 
It wasn't apparent in your initial post, but what are you using to determine the movement of the trim tab? Are you looking at the bug on the display or the actual trim tab?

If it's the former, then I would verify that display with the actual movement of the tab -- it's conceivable that the position sensing mechanism is providing an erroneous reading.
If it's the later, verify the actuation of the motor (it makes a noise when activated) - if it's not continuous with the constant activation of the switch, then look at the wiring connections to/from the relay deck/relays, verify the relays also (voltmeter on the outputs); it could also be a faulty servo.
 
squeakersc063 said:
Have you tried disconnecting it at the trim tab itself and using a 9 volt battery to watch it and see if it's binding? Does it happen only airborne when under air pressure or does it happen on the ground too? You said it's intermittent, is there a particular set of circumstances you can induce to make it happen? If it's not binding, are you certain it's not an electrical connection where the signals from the sticks meet up and go to the trim tab? Maybe you have a recessed pin or a cold solder joint along the way?
Click to expand...
Thanks for the response. It happens on the ground as well as in the air. I will try using a battery as you suggest to see if it is binding.
 
bjdecker said:
It wasn't apparent in your initial post, but what are you using to determine the movement of the trim tab? Are you looking at the bug on the display or the actual trim tab?

If it's the former, then I would verify that display with the actual movement of the tab -- it's conceivable that the position sensing mechanism is providing an erroneous reading.
If it's the later, verify the actuation of the motor (it makes a noise when activated) - if it's not continuous with the constant activation of the switch, then look at the wiring connections to/from the relay deck/relays, verify the relays also (voltmeter on the outputs); it could also be a faulty servo.
Click to expand...
Thanks for the response. I have confirmed a couple of ways that it is not a panel indicator issue but an actual failure of the trim tab to move. When flying, I can see by the lack of change in control forces that the trim tab is not moving. Sitting in the hangar with the engine off, I can hear that the actuator motor is not turning on.

I will start with a voltmeter at the relay, and see if the downward trim movement signal is still being emitted from the relay when the trim fails to move in the downward direction.
 
The issue is likely with the servo it self.
They sometimes develop a sticking point in their travel range and if it is common with a position that you trim to it will not start in one direction but will in the other.
Once it moves from the sticking point, it is able to run past it in the direction that it originally wouldn't move.
 
Scott may have identified a possible issue, however do not forget that a stuck relay contact (internal to your relay board assy.) could also be the cause. Both legs of the servo motor get switched And if 1 leg contact pair is stuck, say in UP.... then only UP will work until the contact becomes unstuck (possibly by cycling the relay to UP). One test might be to tap on the relay(s) next time it become unworkable while on the ground - if the relays are accessible. Just another possibility.
 
I had a similar issue. In my case it was a relay deck with bad (getting stuck) contacts. I replaced it with a REL-2. The newer REL-2 is bigger than REL-1. Hopefully it will last longer.
 

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rvbuilder2002 said:
The issue is likely with the servo it self.
They sometimes develop a sticking point in their travel range and if it is common with a position that you trim to it will not start in one direction but will in the other.
Once it moves from the sticking point, it is able to run past it in the direction that it originally wouldn't move.
Click to expand...
Thank you Scott. If my issue is the servo being at a sticking point, would I be expected to hear the servo motor "fighting" against being stuck? Currently, I hear nothing when the trim won't move in the down direction.
 
jherzog said:
Thank you Scott. If my issue is the servo being at a sticking point, would I be expected to hear the servo motor "fighting" against being stuck? Currently, I hear nothing when the trim won't move in the down direction.
Click to expand...
I have never noticed a detectable noise.... perhaps if it was listened to with a stethoscope?
You could confirm by having someone activate the switch while you check for voltage at the servo.
If there is voltage but it doesn't run, then the servo is bound up for that direction.
If the relay you are using is the REL-1, that is another potential cause of your problem.
 
rvbuilder2002 said:
I have never noticed a detectable noise.... perhaps if it was listened to with a stethoscope?
You could confirm by having someone activate the switch while you check for voltage at the servo.
If there is voltage but it doesn't run, then the servo is bound up for that direction.
If the relay you are using is the REL-1, that is another potential cause of your problem.
Click to expand...
Thanks again Scott. Yes, it is the REL-1. Are those prone to failure in this mode? I do have a REL-2 to substitute if necessary.
Jordan
 
Update: I began by confirming the issue remained. I removed and reattached the access plate in the horizontal stab that the servo was attached to, to check for any obvious frayed or loose wiring (none). After reattaching the servo and confirming the issue remained, I removed the fiberglass fairing around the vertical stab to gain access to the servo power wires. I attached a voltmeter to see whether power is coming from the relay when the trim refuses to move -- and -- then could not repeat the issue! No matter how hard I tried. Frustrating. Capped everything up and confirmed the elevator trim worked in both directions (many, many times) without issue.

I'm thinking that repeatedly running the trim back and forth so often in testing somehow freed up, at least in the short term, an issue with the relay or actuator. The issue will no doubt return, and everyone's advice will I'm sure prove helpful when that occurs.
 
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