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Oil psi drops the higher I climb

in 912iS Technical Questions 6 years ago

  • Re: Oil psi drops the higher I climb

    by » 6 years ago

    referred here from my post - same issue - Rotax 914UL - Also seeing drop from about 30PSI at 7500' to 20PSI at 11750' at which point I decided it was prudent to descend.
    Have reviewed other recent flights and same issue.
    Going to investigate the grounding but need to have a think about that - using an MGL iEFIS with iBox to read the info from the Rotax pressure probe.

    I wondered if it could be related to the orientation of the angle cut on the oil breather pipe which projects into the airflow - this pipe is not blocked but doesn't have a hole outside the airflow. The angle cut is oriented toward the airflow - can't find anywhere in the maintenance manual where it stipulates direction so probably not an issue?

    Link to Savvy Analysis of the flight in question 31 March 2019 Savvy
      
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  • Re: Oil psi drops the higher I climb

    by » 6 years ago

    I asked Rotax at the Sun & Fun tent about this problem today and they said to tee in a mechanical gauge and verify the pressures altitude increases. They have found that some pressure sensors have a port or small hole and also sense atmospheric pressure which can cause the sensor to output a false reading which will necessitate a sensor change to different type or manufacture.
      
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  • Re: Oil psi drops the higher I climb

    by » 6 years ago

    Regarding the propeller governor, if the governor is requiring more engine oil for coarse pitch at altitude, that might account for the drop in pressure. But if that was happening you could test it by setting fine pitch at altitude while adjusting the throttle to keep rpm constant. The oil pressure should increase back to normal values if this was the problem. I’m assuming the governor is standard single engine - increase propeller oil pressure = coarse pitch.

    Mike Blackburn’s 914 might be doing the same thing. If Changing prop pitch at constant rpm at altitude alters engine oil pressure then there is the problem.
    Thank you said by: Mike Blackburn
      
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  • Re: Oil psi drops the higher I climb

    by » 6 years ago

    A Prop Governor only uses Oil when the Pitch is moving from Fine to towards Coarse.
    It uses NO Oil when at a constant pitch.
    Oil is only going to be used when moving the BLACK or the BLUE knobs.
    - - -

    Look at the Savvy Analysis of the flight.
    Place the Oil Pressure on one Graph.
    Place the Altitude on the Other Graph.
    See the Correlation? Neither do I.

    Now plot the Oil Pressure against the Oil Temperature.
    The Oil Pressure is almost a perfect Mirror image of the Oil Temperature.
    Then you climb the Oil Gets Hot and the Pressure goes Down.
    When you back off on the Power to descend the Oil cools off and the pressure comes back up.
    It only appears to be related to the altitude.
    What you are seeing is the Pressure changing with the Oil Temperature and the Oil Pressure Regulator doing a poor job of keeping it constant.

    Try flying at pattern altitude for a half hour at Max Continuous Power.
    Don't climb, but keep the power set as if you were.
    Does the Oil Pressure drop as the Oil Heats up?

    Bill Hertzel
    Rotax 912is
    North Ridgeville, OH, USA
    Clicking the "Thank You" is Always Appreciated by Everyone.

    Thank you said by: Ken Ryan, Mike Blackburn
      
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  • Re: Oil psi drops the higher I climb

    by » 6 years ago

    Thanks @sunfish - in my case it’s not prop related as I have the Airmaster Electric CSU....

    I find it strange that Rotax say that some sensors have a port - I was under the impression that the sensor was a Rotax product included in the engine? I need to investigate....
      
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