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Manual Trim Oddity

rmd

Active Member
I searched through the forums to see if this has been discussed and if it’s there, I failed to find it.

While prepping to land yesterday, I noticed that there seemed to be a dead spot in the manual elevator trim. Initially, as I turned the trim wheel, I felt the normal knob-turning resistance and decreasing stick force. Then there was a part of the knob turn that had no (or at least a lot less) resistance and the forces on the stick didn’t change. Turning slightly further and the operation returned to what has felt normal for the past 16 years.

My hangar-mate confirmed for me today that the elevator trim tab did stop moving at the point of no knob resistance and then began to move again when the resistance returned.

I should note that this was “nose-up” trim where this occurred after flap extension on downwind. Nose-down trim didn’t seem to have this “dead” spot.

Post-flight inspection revealed no signs of abnormalities with the tab, cable, or cable attachment points. I thought I'd check here prior to disassembling the trim cable from the trim knob mechanism to see if anyone else has seen this. If so, what was the corrective action?

Regards,
Rob
N706DR
RV-7A
 
Is this the green trim cable like on the RV-6?
My trim cable has a little play in it, which seemed to be common to the cables of the time. And why I added a spring on it to keep a little tension on it. As I recall I did that not long after first flight, as the trim would wander a bit. I don't remember a dead spot but perhaps in your case, at some point the play in the cable and the aerodynamic forces are in equilibrium until the slack gets taken up.

Just a guess, and the -7 may be completely different.
 
The feeling of a dead spot or more accurately slack in the long manual trim cable is normal. Eventually you will get used to adjusting just past the slack to get positive trim adjustment.
All models with manual trim have this same feel.
 
I would think that in a slow flight condition, with the nose up as in approach configuration, that the elevator trim tab would have a lot les effectiveness. Less down wash on the tail, and less airflow over the elevator & trim tab. This sounds normal to me.
 
Thanks for the replies.

It is the green cable, but I don't know what's on the -6. It wouldn't surprise me if they're the same or similar cable though.

I'd read some of the posts about the slack in the cable, but hadn't even associated them with this condition. Interesting thought that I'll have to cogitate some more since I don't recall feeling this in the knob before.

Is anyone aware of any failure or wear in the mechanism too which the cable attaches in the cockpit that would produce similar symptoms?

Thanks again,
Rob
N706DR
 
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