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Thread: Rudder trim tabs & alignment question

  1. #1
    Member Bart's Avatar
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    Dec 2015
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    Valdez, Alaska
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    408

    Rudder trim tabs & alignment question

    So last summer, when playing on my mini airboat, it would turn left on a dime, and if i let go of the stick, well i never wanted to go flying out of it, so i never did. But it had the hardest time turning right, there was times that it just wouldn't turn to the right at all, and when it did, it was after working the rudders back and forth a few times, trying to remember what all the variables where last summer. just to many what ifs.
    so when i set up the rudders last fall, they were perpendicular to the prop, at 20" center to center. with no toe in or toe out. ( if i remember correctly)

    a while back i was talking with a friend that built just a couple of the big air boat. and was telling him about the problem that i had turning to the right. and he recommended that I needed to install some 3" X 4" trim tabs on the trailing end of each rudder, and use them to adjust, so when or if i need to let go of the stick the boat goes straight.
    i never did think to ask at what height i should place them on the rudder tail. at the bottom, middle or top.
    after looking a little more, i see some ultra-light airplanes that also have fixed trim tabs, which are attached to the tail.

    then as for alignment goes, i see more comments are recommending to have about a 1" toe in,
    all i know is that when i set things back up this spring, this is one of the items that i need to address.

  2. #2
    Super Moderator Corky's Avatar
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    Jul 2014
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    Susquehanna river, Dauphin, Pa
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    Re: Rudder trim tabs & alignment question

    When I was doing my research for my build I had run across different theorys on how to keep the rudder stick centered while running supposedly having them dead centered might possibly create a flutter from them...How much HP it takes to get this condition is anyone's guess but you have more HP than some builds here so you might be onto something there....I've already added that 1 inch toe in to my rudders to keep them centered under power...Do I need it with an 084 ?? Who knows...
    It's my understanding the smaller, secondary trim tabs are used if there's still a problem with the stick centering itself under load...I'm not sure just how much it will help/correct with the torque steer effect some could be attributed to the steering geometry itself...

    Aluminum .120 14' X 76" hull
    Teledyne 4a084-4 engine
    Circle S 1.69:1 reduction
    67" Whirlwind "Razor X" prop



  3. #3
    Administrator Papee's Avatar
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    Jan 2008
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    Re: Rudder trim tabs & alignment question

    Yes the one inch tow does make a difference even at lower rpms. When just moving along at a slower speed and not on plane the tow will straighten the rudders on my boat. If I let go of the stick it will go straight.
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