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Thread: Broken Flaperon Control Link

  1. #1
    Senior Member Av8r_Sed's Avatar
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    Default Broken Flaperon Control Link

    I'm a lucky guy. Yesterday I was just bolting in the seat belts as one of the last to-do's to get ready for the airworthiness inspection on my model III. I had just moved one of the flaperon control horns to get access to an attach point and noticed the stick wasn't moving with the flaperon any longer. After investigating, I found the threaded portion of the FRE-49 male rod end had snapped in the flaperon control linkage. I'd hate to think of what would have resulted if I had been flying at the time.

    I recently went through all the control linkages and didn't detect any issues. Has anybody else had issues with weakness in this component, or conditions that would stress it unduly?

    -- Paul S
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  2. #2
    Senior Member Dave S's Avatar
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    Default Re: Broken Flaperon Control Link

    Hi Paul,

    First of all - real glad you caught it - never turn down good luck. If it had broken in flight at least a clever Kitfox pilot should be able to grab one of the flaperon control rods going through the turtledeck area and prevent any excess roll issues.

    The main thing/condition that can stress a rod end at the point where this one broke is if the rod end hits it's limit of travel on the sphere between the rod end outer housing which rides on the sphere and some other component - that condition may result in torque/shear stress on the threaded part - considering the length of the control stick and the diameter of the threaded part - there is a lot of potential leverage on the threaded part if the part runs out of travel. Any rod end installation must allow for some free motion on the sphere in all possible limits of travel - if not the threaded part is extremely stressed. The rod end must never be what limits travel - it should be adjusted so the travel stops contact before the rod end runs out of movement. I am not saying that is what happened - but it is a potential issue with rod ends

    It is impossible to determine if that is what happened from an e-mail - but you may be able to inspect the parts carefully before disassembly and figure out if there may have been any travel limit issues with the joint. Get out a 9X or greater magnification hand lense if you have one and examine the broken surfaces in detail - a person can usually determine if the break was an old one which just finished cracking off or if there was any deformation of the metal that indicates rotational stress caused by the part running out of travel.

    Sincerely,

    Dave S
    KF 7 Trigear
    912ULS Warp

  3. #3
    Senior Member t j's Avatar
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    Default Re: Broken Flaperon Control Link

    I agree with Dave. There was some installation error that put the rod ends in a bind. It has happened before. Here's a link to those stories with pictures. http://forums.matronics.com/viewtopi...broken+rod+end

    I have seen another kitfox model II that had binding problems that couldn't be resolved. It had rod ends like in your picture. Here's a couple pictures of a model four. Note that the rod ends have a narrow shoulder around the balll. This allows it to rotate more without hitting any limits.

    The part numbers for these rod ends on the model 4 are 93001.000 for the male and 93002.000 for the female. I would call the factory and ask for an opinion and get the model 4 type rod ends. That clevis attachment on the model II aileron bell crank in your picture needs to rotate freely too.

    Each preflight I rotate the stick through the full limits of its travel while wiggling those rod ends with the other hand to make sure there is free play on them at all stick limits
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    Tom Jones
    Classic 4 builder

  4. #4
    Senior Member Mnflyer's Avatar
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    Default Re: Broken Flaperon Control Link

    Hi Av8r_Sed, that rod end broke on my model III also had approx 300 hrs on the plane, I also feel very very lucky I had done my run up checked the flt controls one last time and gave it full throttle just as the plane was about to lift off I noticed I had no flapron control pulled the throttle off and discovered the broken rod end.
    I blame myself about 20 hrs previously I tried taking any slack / looseness out of the control system and ended up tightening the clevis too much causing it to bind when the stick was moved to the limits to would then bend the threaded part of the rod end and it broke. I now make sure it is loose and free before each flt.
    GB
    Flying a HKS Kitfox III and a Champ

  5. #5
    Senior Member Av8r_Sed's Avatar
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    Default Re: Broken Flaperon Control Link

    Thanks for all the detailed feedback. After receiving a replacement part I'll take care to check for any binding at the limits of control stick travel.

    I'm curious though. I assembled the linkages per the Bottom View detail shown below. This resulted in the FRE-48 rod ends being reversed on the bottom of the control stick versus Tom's picture of his model IV linkage. I think this might explain the possible binding that led to the broken FRE-49 rod end. Can any model III folks chime in?

    -- Paul
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