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Has anyone had the opportunity to compare/contrast the behaviour of a 912 series Rotax engine with a 72" and 68" propellor, and maybe 70".  I have in mind the Warp-Drive 3-bladed type, but general observations are of interest.   The specific propellor had nickel leading edges and tapered tips.  This latter measure reduces the moment of inertia (MOI) - for the 72" type -  just below the upper limit of the Rotax engine.  I expect the 68" type to have considerably lower MOI.  I have heard that excess MOI is a cause of gearbox problems, and a rough manner of running at some speeds.

  • Re: Effect of prop diameter on 912 series engines

    by » 2 years ago


    Yes. Eight years ago a did a prop test with 14 different props. It took me months to do this study.  I had 2 blade, 3 blade, long, short, flexible and stiff props. I had 4 Flight Design CTSW planes all Mfg within months of each other. I was the only one setting prop pitch. I could control what flap settings we used and I could control other factors as well and we took off side by side. We flew at the same time in all temp and weather conditions and within 100' of each other. Prop Mfg's don't do a mass study like this and they may use different pilots, flaps and different times of day and different day to day conditions. Prop folks said it was the only study like it. We could compare climb and cruise speeds and throttle rpms.

    Just a quick overview. No matter what the prop people said the results were all dependent on the prop pitch. This is where I got the BEST BALANCED all around WOT rpm of 5600- 5650 for climb, cruise, fuel economy and engine temps. This should be setup for your average altitude flying. Someone in Florida that rarely goes over 2K feet in altitude will have a different pitch than someone who lives up in Colorado and always fly's at 10K. Yes sometimes we all fly higher or lower. Anything under 5500 WOT rpm in level flight is over pitched and there are no redeeming qualities being over pitched. Everything suffers and there is more stress on the engine. Some aircraft Mfg's thought it was a good idea to only get 5200 rpm at WOT in level flight. Bad idea and their failure to do real studies. 

    If you need a better climb prop because you're heavy or an Amphib or on short fields with obstructions to clear then 5800 may be better for you.

    Since the USA doesn't allow Light Sport and some others to have in flight adjustable or constant speed props most use either a fixed pitch or a ground adjustable prop. Then we need to pick a WELL BALANCED prop pitch for the engine's needs and our flight needs. Bottom line with all these props it was all rpm dependent and they all performed the same except the Warp Drive that suffered in climb vs the others. This proved out in multiple flights with the Warp. It was okay in level flight.

     

    This all said many are reporting better performance with the E-Prop which I have never tested, but people seem to swear by it for better performance..

    There really is no need to go from a 68" to a 72" especially with a heavy Warp Drive prop. A 68" prop from a different Mfg will out perform the 72" Warp anytime and it will be lighter and easier on the engine.


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


    Thank you said by: Sean Griffin

  • Re: Effect of prop diameter on 912 series engines

    by » 2 years ago


    Hi Roger,

    I understand you can not be all things to all Rotax drivers but you should know, when commenting on such topics as propeller design/capability, that here in the Antipodes, Rotax motivated aircraft can and are registered in GA as well as RAA (similar to your LS) . In both cases CS props and (not part of this conversation) retractable undercarriage are permitted and there is no practical  air speed limit (Rotax 9's can not usually reach controlled airspace speed restrictions) .


  • Re: Effect of prop diameter on 912 series engines

    by » 2 years ago


    Hi Sean,

    My comments do not include constant speed props or any type of variable pitch prop. Just pure fixed pitch and ground adjustable. Many places in the world gets to use some type of variable pitch prop on their light aircraft, but unfortunately we don't get to here in the USA. I see no reason why the FAA doesn't allow it, but I guess it's above my pay grade.


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


  • Re: Effect of prop diameter on 912 series engines

    by » 2 years ago


    Adam,

      Roger is as always 100% correct !

    The only other consideration is "prop clearance" depending on aircraft and use of rough strips etc, this may be an issue.

    For a lot of the STOL aircraft here in NZ we use the Aussie "Bolly prop" very strong, tolerant of river beds, mountains and beaches and good support.

     


  • Re: Effect of prop diameter on 912 series engines

    by » 2 years ago


    Hi Roger,

    I'm impressed by your experience regarding props. And I'm sure you are right regarding the rpm at WOT.

    Now I installed a brand new engine (the former engine had a bearing damage of the crankshaft) and i reduced the pitch of the propeller (the former owner setted too much pitch at the ground justable Neuform 2 blade). Now at WOT I have approx 5200 to 5400 rpm. Normally I reduce at cruising speed to 4900 to 5000 rpm (maybe 65 to 75% - for me the plane is fast enough at this setting). Why not less pitch? Maybe the Rotax Rev counter indicates too few rev but at approx 5100 rev the sound of the engine changes to higher frequences (no other changes, only the noise). It sounds to me like the engine is over-revving. I am too scared? Or can an exhaust resonance occour at this rpm. Shall i ignore this sound/resonace?

    I ask for your opinion - Greetings


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