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Can anyone tell me what is the limiting factor of a 912ULS engine with a fixed pitch prop?

The book says you can only run the engine at 5800 rpm for 5 minutes but can run it at 5600 all day.

Can someone please tell me the parameters of this. Is the actual limit the 5800 rpm or is it the max power of 100hp that is the limit?

Scenario is

I pitch my prop for 5600 rpm at WOT with a particular propeller. Can I run that at 5600 rpm as it is not rotating at 5800 or because I am using the complete 100hp can I only operate the engine for a max continuous for 5mins at that setting with that prop?

Assuming say ops at 2000 ft at whatever MAP is the max you can get with the conditions

Mark

 

  • Re: Engine limiting factors

    by » 6 weeks ago


    My "book" says max continuous RPM is 5500...


  • Re: Engine limiting factors

    by » 6 weeks ago


    The Continuous RPM limit is 5500rpm NOT 5600prm.
    - - -
    100 hp is not just pushing in WOT, it is a product of RPM and Torque which is a product of MAP.
    The engine only makes 100hp at 5800rpm and 27.5"Hg MAP. (Standard Sea Level Conditions.)

    At 5500rpm you can not make 100 Hp due to the lack of RPMs and are limited to 27"Hg MAP. (90Hp.)
    If you are above ~1000MSL you will not be able to get 27" and WOT at 5500rpm will be fine all day long.


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


  • Re: Engine limiting factors

    by » 6 weeks ago


    Ok so if I pitch my prop for 5500 rpm at WOT that will produce the maximium thrust possible and I can run at that if necessary all day long....not that I would I would still cruise around 5000 or 5200 rpm. To pitch your prop for 5800 rpm WOT is basically a waste of time then as you can only really use it fully at 5500 rpm so then you are not getting the best performance from your system. I take it this is the analogy to what you are saying


  • Re: Engine limiting factors

    by » 6 weeks ago


    I believe you have it correct.....except...in theory having 5800 rpm on take off and climb can be very useful for short fields and you only need a few seconds at that power so the 5 min limitation is fine. I have done this for STOL work and you definitely get better performance.

    Your "use case" is important is it STOL performance you want or cruise performance the settings are different..

    Having said that this is not possible with a fixed pitch prop so if you search Bill has many times explained the best way to pitch a fixed prop take his advice...

    You need to understand the difference in setting WOT rpm static on the ground and in the air at cruise

     


    Thank you said by: Bill Hertzel

  • Re: Engine limiting factors

    by » 6 weeks ago


    Hi Mark,

     

    The better configuration to use all the possible output of your engine is to adjust your configuration to 5800 rpm WOT.

    You will use this on takeoff for a maximum of 5 minutes and then adjust to 5500 rpm that is the maximum continuos power and you can fly all day whitough restrictions.


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