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Thread: Trim Tab

  1. #1
    Wag2+2's Avatar
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    Trim Tab

    I have a Wag 2+2 that has a trim lever in the cabin with a 19 foot push/pull cable that runs to the trim tab. Doesn’t work very well/at all. Thinking about other options. Any one using a servo to run the trim? If so, how is that set up? Any other ideas? Thanks in advance.

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    Cub Builder's Avatar
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    Quote Originally Posted by Wag2+2 View Post
    I have a Wag 2+2 that has a trim lever in the cabin with a 19 foot push/pull cable that runs to the trim tab. Doesn’t work very well/at all. Thinking about other options. Any one using a servo to run the trim? If so, how is that set up? Any other ideas? Thanks in advance.
    I had the push/pull bowden cable fail on mine causing a few really exciting minutes flying with the trim tab in flutter. Not an installation I recommend. I replaced it with a Ray Allen trim servo installed in the bottom of the elevator. I welded a light 1/2" sq tube mounting frame into the elevator for the servo and the controls in the top of the sticks with a relay deck so the trim can be controlled from either stick. A couple of photos and written details of the installation and references a very similar installation I did on my other plane for more details. jeffsplanes.com/Cub/updates/N143W_updates.html

    By lack of effectiveness, are you referencing low speed trim? If you can't get enough nose up trim, you may have to adjust the incidence of your horizontal stab to make the trim more effective. The faster you fly, the more effective the trim, and vice versa. FWIW, mine worked pretty well with the bowden cable, and is really good with the longer stroke of the Ray Allen Servo. I did have to make a small adjustment to the horizontal stab to make the low speed trim more effective. Just for reference, mine is a Wag Aero Sport Trainer significantly modified to resemble a 160hp SuperCub Clone.


    -Cub Builder
    Last edited by Cub Builder; 12-30-2021 at 12:20 AM. Reason: site refused to allow web links.
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    RVBottomly's Avatar
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    Quote Originally Posted by Wag2+2 View Post
    I have a Wag 2+2 that has a trim lever in the cabin with a 19 foot push/pull cable that runs to the trim tab. Doesn’t work very well/at all. Thinking about other options. Any one using a servo to run the trim? If so, how is that set up? Any other ideas? Thanks in advance.
    Does the cable actually move the tab? I have heard of, but not seen, others who say it is adequate, but the pushing aspect of the cable is a weak point that can lead to flutter.

    I opted to put in a jack-screw like the cubs have. I don't think I'd try a servo to move the trim tab. If I were using the tab, I'd go with a "pull-pull" arrangement instead of push-pull. And I'd probably use a set of teflon lined cables going to a bellcrank and figure a way for finer adjustments.

    [edit to add: I saw cub builder posted while I was writing. He has some good ideas].
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    I did similar to Jeff but put a pushrod type into the baggage compartment actuated by same Ray Allen Servo. It is a sheathed pushrod I bought from Aircraft Spruce. It works very well and is a hollow rod instead of the solid wire in a sheath like the Bowden cable…which can fail in the manor Jeff mentioned.
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    Wag2+2's Avatar
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    Thanks for the feedback. I too experienced the flutter when the cable broke previously. Not pleasant. Now the cable just flexes midsection instead of pushing the trim tab. Been going without trim for a while. Now looking to tackle the problem.

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    cubdriver2's Avatar
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    Piper Vagabonds use 2 Bowden cables if I remember right. Worked fine but very sensitive, I think I could have looped it on trim only if I got it going fast enough

    Glenn
    "Optimism is going after Moby Dick in a rowboat and taking the tartar sauce with you!"
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    skywagon8a's Avatar
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    Flutter bothers me. Here we are in a trim thread and already 50% of the posters have experienced flutter. More than half a century ago I learned about pushing a trim tab against it's highest loads has a tendency to create flutter. IF you have a single wire or rod moving a trim tab it must always be pulling against the direction of the highest loads. IF you must use cables it is much better and safer to use two ... both pulling to move the tab. Similar to that used on the Aeronca. The air loads against a displaced tab can be extremely high. These loads if operating against a pushing wire or rod can cause the pushing materiel to buckle or break. Whenever you have a free floating tab, it will in turn move the surface to which it is attached. Then depending on the frequency of this motion it is possible for some component to fail. It could be the tab which fails, it could be the elevator which fails, it could be the stabilizer which fails, it could be the elevator control system which fails, any number of components could fail with fatal results. This failure can happen in a matter of a very few seconds. It can happen so quickly, no matter how much of an ace pilot you may be, you will be helpless to prevent disaster.
    Watch this video starting at 1:06. 2 seconds later the aileron is gone from the wing.


    One of my flutter experiences was a thin .080" aluminum tab flexing flutter which in turn buckled the push rod which in turn destroyed the elevator. At this point the flutter stopped and a safe return was made to the airport. The next flutter experience, the tab push rod broke, the tab fluttered, which fluttered the elevator, which in turn violently pitched the fuselage up and down. I was able to reduce speed to a minimum which reduced the oscillation frequency. The flutter continued until the wheels rolled on the ground. That airplane had a lead balance weight on a arm to balance the elevator. The arm was twisted like a pretzel and had been rubbing on the elevator cable. What would have happened had that arm broken the elevator cable? It was close. Had it moved to one side it could have. Would the parachute I was wearing saved my life? I am so thankful, I didn't have to test it.

    If you want to use tabs for trim, consider the separate tabs under the stabilizer on the early Taylorcraft. At least if they fail they will not flutter the elevators. Remember these are only for trim, not control. The trimming stabilizer on the Cubs works very well and has done so for a long time.
    Last edited by skywagon8a; 12-31-2021 at 06:09 AM.
    N1PA
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    mvivion's Avatar
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    There is an AD on the Beaver's trim system. I had one of the attachments break on a Beaver. Was all I could do, with my elbows braced against the seat back, holding onto the yoke with both hands, to moderate the pitch excursions. Finally got my right hand over to reduce throttle to idle, and managed to land on a nearby runway.

    Flutter is scary.

    MTV
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    behindpropellers's Avatar
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    A friend of mine had the trim cable break on his Bellanca 14-19-2. He could not hold onto the yoke from the oscillations. He ended up pulling the throttle and got it slow enough to control. After he got it home and inspected it, both main wing spars were cracked. The airplane was scrapped.
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  10. #10
    RVBottomly's Avatar
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    Quote Originally Posted by Wag2+2 View Post
    Thanks for the feedback. I too experienced the flutter when the cable broke previously. Not pleasant. Now the cable just flexes midsection instead of pushing the trim tab. Been going without trim for a while. Now looking to tackle the problem.
    Ugh! I also remember Tim, who has a 2+2 with a trim tab, once reported a failure and flutter. The push-pull design should be stricken from the plans, I think.
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  11. #11

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    https://www.aircraftspruce.com/catal...clickkey=66687
    here is what I used…then a threaded fork
    Click image for larger version. 

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  12. #12
    skywagon8a's Avatar
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    Quote Originally Posted by Dan Gervae View Post
    https://www.aircraftspruce.com/catal...clickkey=66687
    here is what I used…then a threaded fork
    Click image for larger version. 

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    That is much better. Those teflon lined controls are very smooth and friction free without slop. Still the trim tab horn acting against that clevis pin creates a wear point which allows a certain amount of undesirable slop. A better choice would be a ball end fitting which can be tightened without leaving any slop.
    Something like these: or https://www.aircraftspruce.com/search/search.php
    N1PA

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    Better yet, but much more expensive would be a Controlex or Teleflex ball bearing cable. Next to no friction, and no lost motion.


    Sent from my iPad using Tapatalk

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    mvivion's Avatar
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    Look at what Aviat had to do to certificate the new trim tab system on the Husky: Two, redundant telefax cables, big ones at that.

    MTV

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    Flutter is not your friend!


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