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Thread: slapping sound at certain RPM?

  1. #1

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    slapping sound at certain RPM?

    I have a pa-18 with a 180 in it. This weekend I flew it and I noticed a slapping sound at certain RPM's (between 1600 and 1800). The sound is similar to a prop that is really digging in, but much more embelished. It is hard to explain. At these RPM's the plane really vibrates also. I have a borer prop on it and there is no range where I am not supposed to run it. I first noticed it on runup, but also later. It runs very smoothly at 2400 and 2500 cruising RPM's. Can anyone tell me what they think might be the problem? Engine or prop?

    Thanks,

    Sean

  2. #2
    behindpropellers's Avatar
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    Sean-

    Was it windy out? If you get the engine pointed into the wind the wrong way it will occasionally make some weird noises. Seems more like a shudder when it happens to me.

    Tim

  3. #3

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    Check prop bolt torque. It could be a couple of bolts not torqued correctly or under torqued letting the prop 'work' on the flange.

  4. #4

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    Tim,

    It actually was early morning and completely calm, but I know what you mean about the wind. I will check those torque settings. Thanks for the responses so far.

    Sean

  5. #5
    Aviator's Avatar
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    Wasn't on a cub, but I felt something similar in a 180. Turned out to be a spinner plate cracked all the way across. It seemed to right itself at hight RPM.

  6. #6

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    Had some ice in a spinner once did run rough .

  7. #7

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    slapping

    You beat me to it. Had the same thing with a 582 Rotax once. The only reason I knew what it was is I just changed the spinner. Chattered like crazey at 5800 and nowhere else. It was the backing plate hammering the hub. Made a thicker plate and it took out the resonance. Your spinner is telling you what needs to be done.
    Ron

  8. #8
    Lawn Dart's Avatar
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    I know this sounds too simple but, it’s quick, it's free, and you can check this one off the list in just a couple of minutes.
    Have someone that knows airplanes stand outside while you run up and give the plane and prop a good looking over. Is the prop tracking (all the time?), safety cable hitting something?, belly inspection cover flapping?, etc.

    There...now that you've eliminated that, move on to the wrenches.

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