Turning the Prop backward

Discussion in 'Flight Following' started by brien23, Apr 4, 2021.

  1. brien23

    brien23 Cleared for Takeoff

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    The age old question, can you turn the prop backward without damage to the vacuum pump. One of aviation’s old wives’ tales is that you’ll wreck your vacuum pump if you move the prop backward. Fortunately, that’s not true. While most vacuum pumps are unidirectional—they either turn clockwise or counterclockwise in service based on the direction of rotation of the accessory shaft on the engine (and have CC or CW as a suffice to their model number)—it takes more than a few turns in the wrong direction to cause damage.
     
  2. Shepherd

    Shepherd En-Route

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    Seriously? This is still a thing?
     
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  3. flyingron

    flyingron Touchdown! Greaser! PoA Supporter

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    It also takes enough speed to actually throw the vanes against the case which you're not going to get by hand turning.
     
  4. EdFred

    EdFred Taxi to Parking

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    You also need to have a vacuum pump.
     
  5. drjcustis

    drjcustis Pre-takeoff checklist

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    I knew a guy who had a mechanic that took care of a pilot in a backwoods airport that turned his prop the wrong direction and broke the vacuum pump and locked up the whole engine. So, there’s that. YMMV.
    ;)


    Sent from my iPad using Tapatalk
     
  6. Palmpilot

    Palmpilot Touchdown! Greaser! PoA Supporter

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    :confused2:

    So is this a possibility with certain vacuum pumps and not others?
     
  7. Dan Thomas

    Dan Thomas Final Approach

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    Look at this:

    [​IMG]

    See the angle of those vanes with respect to the rotational axis? They're not unidirectional and haven't been for a long time. The pump model numbers have CW or CCW in them to indicate the rotation, which also indicates the slant of those vanes.

    See how that vane at the 7 o'clock position pretty much fills the slot when retracted? That pump has few hours on it. When it wears to the point where it's short enough to cock and jam in the slot if the pump is turned backward, it will break the carbon rotor. If the rotor doesn't break, the plastic drive coupling will shear. The old straight-vane pumps will break their drives, too, not lock the engine.

    If you break it by turning the propeller backwards, it was long past its best-before date. Long past. Rapco and Tempest pumps all have vane wear inspection provisions top tell you when to replace that pump. Buy one and get it inspected when they tell you to and you will never have pump failures unless you fly 30 hours a year and let that plastic drive coupling get old. It has a six-year recommended replacement. I don't know why anyone would buy a pump that doesn't have the vane wear inspection port, but people do.

    Edit: that pump in the picture above turns clockwise as seen from the camera. This minimizes the cocking forces on the vanes during normal rotation, but also increases them if turned backward. And that prop can turn backward if the engine kicks back, or as the prop stops it sometimes rocks back and forth a bit, and when a mechanic checks compression or sets the mag timing he's moving the prop both ways. Like I say, if it breaks when you turned it backwards, it was shot anyway and one should be glad it failed on the ground and not in flight at night or in IMC.
     
    Last edited: Apr 4, 2021
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  8. Zeldman

    Zeldman Touchdown! Greaser!

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    Don't turn the prop backwards too fast or you might turn back time, and I think we all had enough of the year 2020.....
     
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  9. LesGawlik

    LesGawlik Pre-takeoff checklist

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    And that vacuum pump has to be a dry one.
     
  10. Lindberg

    Lindberg En-Route

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    It's a thing you post about when you're trying to start an argument.
     
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  11. 455 Bravo Uniform

    455 Bravo Uniform En-Route

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    No it’s not. :p
     
  12. JOhnH

    JOhnH Touchdown! Greaser!

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    In the cold weather starting section of my POH, one of the suggestions is to turn the prop a few times before starting. And it states that it SHOULD be turned backwards.
    I do have a dry vacuum pump.
     
  13. Shepherd

    Shepherd En-Route

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    Ahhhhh!. Let me get a beer and some chips and get comfortable.
     
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  14. Dana

    Dana Pattern Altitude

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    While we're at it, what about always leaving wooden props horizontal? :popcorn:
     
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  15. Matthew Rogers

    Matthew Rogers Line Up and Wait

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    I painted my aluminum prop with fake wood grain. Do I have to leave it horizontal too?
     
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  16. donjohnston

    donjohnston Cleared for Takeoff

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    What about the downwind turn with resulting stall?
     
  17. JOhnH

    JOhnH Touchdown! Greaser!

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    If you turn the prop backwards and it starts, will it fly backwards?
     
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  18. Timbeck2

    Timbeck2 Final Approach

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    Is that so its easier to smack yourself in the nugget?
     
  19. asicer

    asicer Final Approach

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    But then the Hobbs meter would also run backwards and you'd have to unlog your flights.
     
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  20. JOhnH

    JOhnH Touchdown! Greaser!

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    Fortunately, my plane doesn't have a Hobbs meter.

    But would that tach run backwards?
     
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  21. Tarheelpilot

    Tarheelpilot En-Route

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    It is a well known fact if you don’t all the sap runs to one end and the prop will be out of balance. If you leave it like that long enough the thrust will be asymmetric as well. Very bad.
     
  22. Doug Reid

    Doug Reid Line Up and Wait

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    If you have a hydraulic lock on a radial engine..do not turn the prop backwards...


    Most dry vacuum pumps will shear the plastic coupler if they lock up...only affects the vacuum pump from spinning...engine runs as normal.
     
  23. Dana

    Dana Pattern Altitude

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    And the FBO would have to pay YOU.
     
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  24. nauga

    nauga Administrator Management Council Member PoA Supporter

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    You must be very tall if a horizontal prop can hit a nugget.

    Nauga,
    too chicken to risk his nuggets
     
  25. NorthEast Ohio

    NorthEast Ohio Pre-Flight

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    Rotax...
    Not even once.
     
  26. Dan Thomas

    Dan Thomas Final Approach

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