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Thread: Trim indicator

  1. #1

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    Default Trim indicator

    My trim operation has occasionally been irregular, and even stopped working, a couple of times recently in flight. Once on the ground, it all seems fine. I did notice that the trim indicator pointer seems not to stick out of the slot properly, only partially. Is it possible that the indicator pointer can get jammed and keep the trim mechanism from operating correctly? Is the indicator cable strong enough to hold the trim mechanism from cranking down in back?

  2. #2
    Administrator Steve Pierce's Avatar
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    Default Re: Trim indicator

    I would bet that your jackscrew has old dried grease in it and is making it hard to turn thus slipping the cable when there is an air load in flight. The indicator is simply a wire with a spring on one end tied to the jackscrew yoke. The indicator is a rivet that rides in the slot on the trim plate. I slit a nylon washer and put it around the rivet shank to ride against the trim plate. Clean the jackscrew threads ad I bet the trim works normal in fl;ight.

  3. #3

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    Default Re: Trim indicator

    I just had the screw greased and the system works fine on the ground. But suppose the indicator got stuck. Could the indicator wire keep the trim mechanism in the rear from moving down?

    My indicator rivet is only partway through the trim plate, and seems to have a wire wrapped around it (above the trim plate) for some reason. Sometimes the rivet disappears above the slot. I wonder if it is catching and limiting the up-trim. Make sense?

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    Administrator Steve Pierce's Avatar
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    Default Re: Trim indicator

    Clean the old grease out of the jackscrew threads that is under the new grease or look for oil or grease on the trim cable or jackscrew pulley. The old grease gets hard and cause friction. The trim indicator wire has a spring on the cockpit end and flexes. It would break before it kept the jackscrew from moving.

  5. #5

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    Default Re: Trim indicator

    RGreenberg, read this again: "I slit a nylon washer and put it around the rivet shank to ride against the trim plate." Steve Pierce told me this little hot tip a couple years ago, or more. I had cleaned my jackscrew of old grease and re-greased it but my indicator rivet was worn to an oval shape and wearing a wider groove into the indicator plate so the indicator would get stuck and hang up during cranking. I could feel my jackscrew was binding prior to cleaning all the grease out of the grooves and re-greasing. After that it was smooth throughout the entire travel but the indicator was still snagging and hanging. I cut a small washer like piece of plastic out of an old soap bottle and put a slit in it like Steve said, and it fixed the problem. Later I made another one out of another old bottle of some polypropylene material - seems like we recycle about 10 or 15 bottles a week in my house - anyway, after 2.5 years my trim And indicator still cranks smooth. Steve says nylon, and that may be better material, but my poly bottle piece of refuse works great. The trim indicator head very rides smoothly in the slot now. And the large white dot is easy to spot with a very quick glance. Don't really have to look, just flash eyes north and you know where you are trimmed.

  6. #6

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    Default Re: Trim indicator

    I think Subsonic stated it more clearly than I can, but I would like to add that (most likely) why it sticks while flying but not on the ground is because of the air loads on the stabilizer while flying.

  7. #7
    JPerkins's Avatar
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    Default Re: Trim indicator

    Had that problem on a super cub. Trim cranked smooth as butter on the ground; got so stiff in the air, thought you were gonna break the handle. Turned out the tail plane wasn't square to the fuse and when loaded with the air pushing down on it it would bind on the jack screw.
    Figured this out on the ground with someone cranking the trim while pushing up and down on the elevator. Couple turns out on one bottom wire and in on the other resolved the issue.

    Sent from my SM-G930V using ShortWingPipers.Org mobile app
    Jason Perkins

  8. #8
    Administrator Steve Pierce's Avatar
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    Default Re: Trim indicator

    These are the nylon washers I use. They work great under cowl and inspection cover screws to keep from wearing through paint and aluminum.
    https://www.aircraftspruce.com/catal...lonwashers.php

  9. #9

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    Default Re: Trim indicator

    Just to help me be sure I understand the geometry, it would help to have the answer to this hypothetical question. If the indicator got totally stuck in its slot, and you forced the handle to turn in the trim up position, doesn't something have to give? Either the indicator wire must break, or the control cable slips in a pulley, or the handle breaks, or something. And which of these is most likely to fail first? (I don't think there is any spring attached to the indicator wire aft of the indicator pointer.)

  10. #10
    JPerkins's Avatar
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    Default Re: Trim indicator

    Indicator wire would break

    Sent from my SM-G930V using ShortWingPipers.Org mobile app
    Jason Perkins

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