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915 and CSU

in 915iS Technical Questions 6 years ago
Hi there,

I am told that the new 915 must have a CSU. Can someone explain why that is necessary? I have also read that Rotax are bringing out a 915 that can handle fixed pitch, has anyone heard a rumor as to when it will be released?

If I have to go the CSU path, what is the lightest CSU available that is compatible?

I know that's a lot of questions, but I am struggling to find answers anywhere.

I am hoping to get the answers and make a decision on engines within the coming couple of days.

Thanks
Damien

  • Re: 915 and CSU

    by » 6 years ago

    The 915 does need a constant speed prop.
    When you have a engine making constant power (not constant manifold pressure) to 15,000ft you need to be able to harness/use that power.
    putting a fixed pitch prop on a 915 is like buying a Porsche 911 GT2, taking the wheels off, and mounting bicycle tires on it.
    MT makes a very light governor and prop.
      
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  • Re: 915 and CSU

    by » 6 years ago

    Hi Rob,

    I fully agree with you, but there is a guy in Florida working on an adapter to mount for a ground adjustable. I can't remember his name, but I bet you heard of him. Eric was talking about him a while back.

    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

    Maybe he is not a Porsche fan? Then how about a Ferrari analogie:
    You buy a ferrari, with an incredible engine, but the dealer tells you you can only have 1 gear!! (the rest of the gears will be removed from the transmision)
    Your choices:
    do you want 1st gear so you can start fast, but can't go on the highway?
    do you want 6th gear so you can go really fast on the highway but cannot go up a hill and must start very slow?
    Or do you want 3rd gear, a compromise but lousy at starting and going really fast?
    Do you want such a car?
    Or pay some more and get all the gears, and be really happy?
    That's what a constant speed prop will give you. It should definitely be mandatory on a turbocharged Altitude engine, even more so on the 915iS with its constant power output. (what do we term that? "Altitude+ engine"?)
      
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  • Re: 915 and CSU

    by » 6 years ago

    Or to use a Cub analogy. Maybe he doesn't particularly wan't to spend another 12k (AUS) on a CSU. Just kidding, I really appreciate the info.

    It is planned for a very draggy light weight STOL cub (that is used off airfield), so the benefits of a CSU are a little less apparent. It will provide better performance, but I suspect that the 141hp will do that on it's own (currently has a 912 ULS). Also the weight penalty of the CSU is worth considering. However, I would think you could offset this with less fuel as the Rotax when pulled back on power sips fuel. Also, the beauty of a turbo for us is performance remaining basically the same as density altitude increases. The CSU would be lucky to increase cruise air speed much as the airframe maxes out at about 80knots, to go faster takes lots of energy.

    Also, there is a point where the engine plus the CSU will cost approx 70,000 AUS, that's a lot of money for an LSA cub.

    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?

    Having said all of the above. The 915 would basically provide an aircraft that could land and takeoff in much the same distance and given how short this thing lands, it would be awesome.

    Maybe I need to track down "Eric" and see what this guy making an adapter is up to, is there any contact info?.
      
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  • Re: 915 and CSU

    by » 6 years ago

    Who says money can't buy happiness?
    No, I don't think it will be in the design standards. It would ruin the capabilities of a great engine.
    The engine makes constant power up to 15,000ft. The air is getting thinner as you increase altitude. You need the prop to match the engine power to the decreasing atmospheric pressure/increasing airspeed.
    So you will have to pitch a ground adj prop very coarse or it will overspeed, then you run the risk of detonation and you lose out on power.
    If you pitch it for your average airport elevation it will overspeed when you go to your nice high mountain lake/airport.
    Sure the c/s will cost more, but it will be worth it. (you can have performance, range, altitude capability and increased load... just the ticket for a better Cub)
      
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