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Hi, 

914 with Constant Speed prop

I wonder if anyone else has found this problem with constant speed props on a Rotax 914.

Using Rotax 914 manual recommended MP pressure and revs chart, this problem occurred with both an hydraulic and electric CS prop

1.       Take off; I find that on full fine prop pitch and take off power the engine is very smooth in the climb – fine, no problem.

2.       Transition to max continuous power;  i.e.  first reduce power to selected MP then increase prop pitch to matching recommended revs and the engine is still smooth.

3.       Transition to lower speed cruise; I then first reduce power to selected MP then increase prop pitch to matching recommended revs and the engine is still smooth.

4.       BUT if I increase the cruise speed;  i.e. first reduce prop pitch to selected higher revs and increase power  to matching MP, there is a small but definite vibration similar to being in too high gear in a car.    This vibration remains even at reduced or increased power settings, and only “resets” when landed and take off again.

  • Re: Rotax 914 with CS prop

    by » 5 months ago


    Nothing stands out in your description, but the devil is in the details.

    4.
      BUT if I increase the cruise speed; What Speed?

    i.e. first reduce prop pitch to selected higher revs  What RPMs?

    and increase power to matching MP,  What MAP?

    ... This vibration remains even at reduced or increased power settings, and only “resets” when landed and take off again.  Did you also Change the Prop (RPM) setting?  


    Bill Hertzel
    Rotax 912is
    North Ridgeville, OH, USA
    Bill.Hertzel@Yahoo.com
    Clicking the "Thank You" is Always Appreciated.


  • Re: Rotax 914 with CS prop

    by » 5 months ago


    What is the difference between steps 2 and 3 in your description?

    Moreover, reducing power does not increase MP necessarily. Increasing pitch does.

     


  • Re: Rotax 914 with CS prop

    by » 5 months ago


    Hi,  Below is a chart of Engine rpm, MP (inchesHg) from the Ritax manual & corerspnding level flight speed that has been determined for each setting.   It also shows fuel consumption (ltr/hr)

    So, I climb at 65 kts 115% power - fine.

          Reduce power to say 100% - 154 kts cruise - fine

          Reduce purther to say 65% for 130 kt cruise - fine

          Increase power to 100%, then vibration starts.

    The vibration is not severe.but slighltly unpleasant.


    %

    RPM

    MP

    V kts

    Fuel

    24

    2350

    19

    100

    4.9

    24

    2400

    19

    102

    5.0

    36

    3160

    22

    110

    6.7

    38

    3300

    23

    115

    7.0

    52

    4100

    27

    120

    11.9

    55

    4300

    28

    126

    12.5

    64

    4720

    29

    130

    17.2

    65

    4800

    29

    132

    17.5

    75

    5000

    31

    137

    20.4

    91

    5003

    32

    140

    25.0

    97

    5360

    34

    150

    26.8

    100

    5500

    35.4

    154

    27.5

    115

    5800

    39

     

    33.0

    Does that help?

  • Re: Rotax 914 with CS prop

    by » 5 months ago


    Hi,

    1. There is no difference between steps 2 & 3 - both reducing power.

    2. I understand, but the normal procedure is to adjust power first when reducing power, then rpm

         and prop revs first then power when increasing power - isn't it?

     

    Rgds


  • Re: Rotax 914 with CS prop

    by » 5 months ago


    re 2: correct, as far as I know.

    To increase power, first, increase rpm with the prop control, then increase MP with the throttle.

    To decrease power, first, reduce MP with the throttle and then decrease rpm with the prop control.

    I have 912ULS so the 


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