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Thread: Setting Whirlwind GA-RW3B pitch on 912ULS

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    Default Setting Whirlwind GA-RW3B pitch on 912ULS

    Am I going crazy or is the Whirlwind GA-RW3B propeller manual wrong? What pitch do you use with this combo? The manual clearly states in two places that for the 70" prop I have that the pitch should be set to 17 degrees. The problem is that when I use their setting (confirmed with two different digital levels), I cant even use full power on a 65mph climb without going over 5800 rpm.


    After that I did experiments at 19.5 degrees and 21 degrees.


    PERFORMANCE MEASURMENTS (with 22" tundra tires)
    21 degrees: 109mph @5200rpm. ROC 880fpm @ 1200lbs & 4000'
    19.5 degrees:104mph @5200rpm. ROC 950fpm @ 1200lbs & 4000'


    CLIMB RPM @ 65mph
    21 degrees: 5050rpm
    19.5 degrees: 5500rpm


    If it was just a performance tradeoff, I would choose the 21 degree pitch and take the faster cruise with only a small decrease in climb performance. Many people seem to say that its bad to "lug the engine" by having full power climb under 5300. Does anyone know if this is actually stated in Rotax documentation or know what is the reasoning for this recommendation?


    Thanks!

    Relevant pages from Whirlwind manual:
    setting-pitch2.jpgsetting-pitch.jpg

  2. #2
    Senior Member jrevens's Avatar
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    Default Re: Setting Whirlwind GA-RW3B pitch on 912ULS

    What is your static wide open throttle RPM? As stated, 17 deg. is just a starting point.
    John Evens
    Arvada, CO
    Kitfox SS7 N27JE

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    Senior Member Dave S's Avatar
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    Default Re: Setting Whirlwind GA-RW3B pitch on 912ULS

    Quote "If it was just a performance tradeoff, I would choose the 21 degree pitch and take the faster cruise with only a small decrease in climb performance. Many people seem to say that its bad to "lug the engine" by having full power climb under 5300. Does anyone know if this is actually stated in Rotax documentation or know what is the reasoning for this recommendation?"

    Yes, but 5300 is wrong, at least for the ULS

    Rotax states that at full throttle the engine must be operated at a minimum RPM of 5200. At least on the 912ULS - the other 912 versions may or may not be different. Reduced throttle settings not an issue.

    "lugging" is relative - it's the specific engine spec that counts which takes into account a number of items like compression ratio, spark timing, max density altitude, octane rating, etc.

    If a person operates full throttle below 5200 on the ULS they are tempting detonation.

    Until recently I had not actually seen an engine destroyed by detonation - not a Rotax but a honda auto conversion and that happened within the first 5 hours on the ground. The engine was aluminum compost after that.
    Dave S
    Kitfox 7 Trigear (Flying since 2009)
    912ULS Warp Drive

    St Paul, MN

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    Default Re: Setting Whirlwind GA-RW3B pitch on 912ULS

    Quote Originally Posted by Dave S View Post
    Rotax states that at full throttle the engine must be operated at a minimum RPM of 5200. At least on the 912ULS - the other 912 versions may or may not be different. Reduced throttle settings not an issue.
    Thanks Dave! I missed that in the Rotax manuals. 19.5 degree pitch which gives 5500 on a 65mph climb is the way I will go. Just need to go back in my notes and see what the corresponding static RPM is to insure its in the 5100-5300 range specified.

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    Default Re: Setting Whirlwind GA-RW3B pitch on 912ULS

    For those wondering where Dave S got the "minimum 5200rpm WOT" from, its in SL-912-016 under section 3.1.2 "Performance recommendations". Took me a bit of googling to find it

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    Default Re: Setting Whirlwind GA-RW3B pitch on 912ULS

    Quote Originally Posted by jrevens View Post
    What is your static wide open throttle RPM? As stated, 17 deg. is just a starting point.
    Hey John,

    Thanks! When I go to the hangar tomorrow I will check my notes and make sure the static RPM is in the 5100-5300rpm range specified with my current pitch setting of 19.5 degrees. I cant remember precisely as I did a lot of experiments.

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    Default Re: Setting Whirlwind GA-RW3B pitch on 912ULS

    IMO the Whirlwind recommendations do not make any sense and the Rotax requirement of keeping RPM above 5200rpm at wide open throttle upon climb is the important one to satisfy. Whirlwind says to set the WOT static rpm to 5100-5300 but if I set the static WOT to be anything less than 5200, I wont be able to satisfy the Rotax requirement. Here is what I found:


    17.0 degrees: WOT static cant be tested without exceeding 5800rpm
    19.4 degrees: 5450rpm WOT static, 5500rpm 65mph climb
    20.2 degrees: 5330rpm WOT static, 5340rpm 65mph climb
    21.0 degrees: 5050rpm WOT static, 5060rpm 65mph climb


    Again, no idea why they recommend 17 degree pitch as the starting requirement because it does not meet their own recommendation of WOT static 5100-5300rpm. I'd rather keep at least 100rpm over the 5200rpm minimum specified by Rotax so I will be using the 20.2 degree pitch setting.

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    Senior Member jrevens's Avatar
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    Default Re: Setting Whirlwind GA-RW3B pitch on 912ULS

    I think I might have seen something from Kitfox which recommended a higher initial setting for the angle. Of course you set it at whatever it takes. Whirlwind may even be aware that the recommendation is a little too fine and just haven't gotten around to changing it in their installation documentation. Wish it wasn't so, but that is not unusual for many companies, in my experience. Just out of curiosity, have you checked the accuracy of your tachometer?
    John Evens
    Arvada, CO
    Kitfox SS7 N27JE

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    Default Re: Setting Whirlwind GA-RW3B pitch on 912ULS

    Quote Originally Posted by jrevens View Post
    I think I might have seen something from Kitfox which recommended a higher initial setting for the angle. Of course you set it at whatever it takes. Whirlwind may even be aware that the recommendation is a little too fine and just haven't gotten around to changing it in their installation documentation. Wish it wasn't so, but that is not unusual for many companies, in my experience. Just out of curiosity, have you checked the accuracy of your tachometer?
    Hi John,

    I must have missed the Kitfox recommendation. As far as checking the accuracy of my tach, yes and no. I have both the Grand Rapids EIS2000 which gives the RPM in digital format as well as the analog AviaSport Rotax 912 tach and they both agree. Because they both trigger off the same signal provided by the engine, its not a true validation like as would be provided by a handheld laser RPM meter.

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