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915 and CSU

in 915iS Technical Questions 6 years ago

  • Re: 915 and CSU

    by » 6 years ago

    ...My original question was why does it have to have a CSU, what is the design reason, outside of Porsche's and Ferraris etc? Can I just bolt on a ground adjustable and still be within design standards? ...


    Let me be the Devils Advocate...

    It does Not "Have to Have" a CSU.
    It is Highly "Recommended" that it "Should" have one to make use of the engine's capabilities.

    You could bolt on a Ground Adjustable Prop and set it to max out in the climb.
    It will leap off the runway and climb like the proverbial Homesick Angel.
    Understand that as you gain altitude or speed and unload the prop, it will want to overspeed and it will be necessary to consciously and religiously reduce the throttle to avoid overspeed damage.

    You will essentially have a 135Hp 915is in the climb and a 100hp 912ULS everywhere else.

    You will not want to configure it as a cruise Prop because that will mean that it will be overpitched in the climb and you will never be able to use the full power of the engine just when you need it most as in trying to clear that obstacle right off the nose.

    If you understand and can accept these limitations, you are free to put on any reasonable prop you see fit.
    Make an informed decision. Unless you overspeed or overload the engine, nothing terrible is going to happen and it is not going to blow up just because it doesn't have a CSU. You will just have to be diligent in keeping it within normal operating parameters.

    You have gone as far as paying the extra expense to have the 915, you might as well complete the project properly with the CSU. If you do not want to incur the expense of the CSU it would make more sense to just downgrade to a 912 with a GA prop.

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

      
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  • Re: 915 and CSU

    by » 6 years ago

    I fully agree with Rob and Bill. Putting a ground adjustable prop on the 915 would be a sin and defeats the engine performance.

    But I'm sure some human will do it. :)

    Even though we can do some things doesn't always mean we should.

    Roger Lee
    LSRM-A & Rotax Instructor & Rotax IRC
    Tucson, AZ Ryan Airfield (KRYN)
    520-349-7056 Cell

      
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  • Re: 915 and CSU

    by » 6 years ago

    Let me preface this by saying I am currently building a Rans S-21 and I have already purchased a 915is and I will be using constant speed prop. I have a little experience in the subject since I have a Aerotreck/Eurofox with a with a 914 and a ground adjustable Whirlwind propeller. My home field UT 24 is 8141' MSL. The prop is adjusted to go to redline 5800 RPM on take off at UT24 but when taking off at Mesquite Nevada airport 1978' MSL the tachometer goes to about 5500 RPM so I am able to utilize most of the engines performance at both airports. So I think your analogies are a little extreme. BTW Trent Palmer placed second at the STOL Drags beaing a whole lot of Carbon Cubs, FX3s ect. with a Kitfox with a 915is with ground adjustable prop!
      
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  • Re: 915 and CSU

    by » 6 years ago

    That Rans S-21 is going to be sweet with the 915!

    In talking with Trent, he’s got his fixed pitch Luga prop set for about 5400 RPM and 45” of Manifold. The moment he puts the nose down he has to immediately throttle back to not over speed the prop. At 5500 he could only pull 33” of manifold. So he throttles back to around 5000 RPM at 26” of manifold and he’s only burning 4.6 GPH and still getting respectable cruise speeds. The MT CSU on the factory STi is getting around 110 MPH true and burning 5.5 GPH. I honestly don’t think the difference would have been all that drastic using a CSU in the STOL drags. It’s very impressive he was able to beat an FX-3 Cub!

    Anyways, for more numbers talk here is a nice video from Trent. https://youtu.be/HP2fAtf3p08
      
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