Trim Tab Limits

Discussion in 'Skybolt Miscellaneous' started by mjk51, Jan 22, 2018.

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  1. Jan 22, 2018 #1

    mjk51

    mjk51

    mjk51

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    I am sure this has been hashed over... but I really have a difficult time with the forum's search feature... What is the range of movement for the Skybolt elevator trim tab?? Degrees up and down from neutral??
     
  2. Jan 23, 2018 #2

    camwebb777

    camwebb777

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    This one I have been pondering. There does not appear to be a set limit in the plans. The amount of movement is controlled by lever (assuming cable control) so the only place you could put stops is on the lever in the cockpit. But it occurs to me that there doesn’t need to be a limit. At the end of the day it’s for trim so the limit would be that which is necessary for hands off flight. I can’t imagine that you would need more than say 30 degree deflection either way - max. I guess no limit is therefore acceptable, it is only trim.
     
  3. Jan 24, 2018 #3

    PittsDriver68

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    You would like enough trim available so that you could land the airplane if the control stick connection to the elevator failed. Sean Tucker can speak to this as he had to bailout when he discovered his elevator control broken and the trim could not adequately control the airplane.

    Best of luck,

    Wes
     
  4. Jan 24, 2018 #4

    Neil

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    You need to be careful here. There was an article back in the early 80s in Sport Aviation that involved a Skybolt, I think it may have been Dean Halls airplane, where two guys were out flying and the rear set pilot turned the flying over to the front seater and the airplane began an outside loop. I don't remember all the details but the whole point of the story was that when the rear pilot thought he had turned the flight over to the guy up front he inadvertently moved the trim to full noose down. The trim was powerful enough that the stick could not be pulled back. The airplane continued to do outside loops until it became obvious that no one was in control. At that point the rear seat pilot diagnosed the problem and recovered the airplane, but not before it had gone below the tree line. The whole point of the story was that it is possible to have a trim system with too much authority. You need stops!
     
  5. Jan 24, 2018 #5

    Dave Baxter

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    Neil there is a whole thread on this about three or four yrs ago, that I was unable to search? When I asked the same question no one seemed to have an answer? I was doing the trim system on my skybolt, I did make it positive stops with much difficulty, and I referenced that article with Dean Hall and David Ebershoff in colorado, if i remember the trim kept going from stop to stop full up and full down, when he pushed the release button detent, and at the last minute actually grabbed it and held it it neutral, making the recovery almost at ground level! a most terrifying experience. Dave

    The article is in the Aug 82 issue of sport aviation. Dave
     
  6. Jan 24, 2018 #6

    biplanebob

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    for a 2 seat Pitts tab limits from the TCDS #A8SO the trim tab travel with the elevator neutral are;

    for Pitts S2A, S2S, S2B, up 7degrees, down 19 degrees

    for Pitts S2C 25 degrees up, 25 degrees down

    the above info may be a starting point for rigging a Skybolt. :)


    Bob
     
  7. Jan 24, 2018 #7

    Neil

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    Thanks Dave. My recollection of the article is a little fuzzy but none the less trim limits are still the point. Everything need limits.
     
  8. Jan 24, 2018 #8

    PittsDriver68

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    The out of control trim issue is a function of friction in the trim control, not the limits. There needs to be resistance built in to the trim control to prevent the control surface from moving the cockpit control lever.

    You can accomplish that in multiple ways. An example is Cessna's elevator trim wheel in the -182's. There are serrations on the side of the wheel and a "clicker" that presses into those serrations. That is there to provide a resistance, or locking, function so that the trim surface on the elevator can not move the trim wheel next to the pilot seat.

    Most of our biplanes have a simple lever elevator trim control. The pivot bolt should be tight enough that the lever requires a little force for the pilot to move it. Otherwise the elevator trim surface can put force back through the linkage and change your lever position and the trim setting.

    Best of luck,

    Wes
     

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