1982 421C Vibration/ rumble at cruise

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  • #83968

    Ever since we bought this 421 a little less than a year ago I have noticed a harmonic or rumble in the airframe at the ram recommended cruise setting of 32.5 and 1800. You can almost feel it more than you can hear it and when it reaches the top of the harmonic it is enought to see the vibration in the reflections off of the gauges on the instrument panel. If I bring the power back to an economy cruise setting that I use down low on shorter trips of 30 and 1700 it all but goes away. The plane has ram engines with less than 100 hours on them and 4 blade mt props. Maybe it is something in the airframe that gets going in a harmonic at 1800? Im stumped as to what it could be. Any ideas? Thanks.

    #97266

    Hi or Low settings on the heater or the A/C blowers can cause a vibration, which I have noticed seems to develop a harmonic at different power settings. The blowers on the Evap coils are especially noticeable in your butt if your seat is all the way down. The low setting is the worst on my 421.

    I would guess RAM mentioned the dampener on the starter adapter.

    A friend is chasing a similar issue on an ’82 C with RAM engines, so if you find anything else please pass it on.

    #97268
    rwelsh
    Participant

      If you have someone locally who dynamically balances props, I would have the props balanced. The MTs don’t have enough of a track record on 421s so I would suspect the props being out of balance or the power setting wasn’t explored when the props were approved for the 421C.

      #97272
      quote RWELSH:

      If you have someone locally who dynamically balances props, I would have the props balanced. The MTs don’t have enough of a track record on 421s so I would suspect the props being out of balance or the power setting wasn’t explored when the props were approved for the 421C.

      On that note, TCM has stated in facility tours that they have noticed an increase in accesory drives on MT equipped aircraft. Fyi.

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