Best rotax 912 rpm and propeller pitch for fuel efficiency and noise level

TrikePilot Social – Microlight & LSA Trike Flying Community Forums HELP! Best rotax 912 rpm and propeller pitch for fuel efficiency and noise level

3 replies, 2 voices Last updated by  Joe Hockman 2 months, 1 week ago
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  • #2323

    Clemens West
    Participant
    @clemenswest

    Hi guys, I hope that I can find some good advice here for my propeller pitch setting for the Rotax 912ULS (100HP).

    I fly a 2008 Tanarg 912ES (ExtraSilent) with an Arplast ground adjustable propeller (Arplast 158 GWAS 6275/3, approvel pitch setting is 24,5° at 593 mm or 75% Radius). The wing is a 13m² iXess, maximum trim speed (hands off) is around 90-100 kmh (55-60mph). Vne 120 kmh (75mph).

    I usually cruise at 4000rpm, I need to read the exact rpm at WOT (I think at climb is around 5300). I will write down the exact values at my next flight.

    Many say that a rotax 912 best cruise rpm is at 5100-5500 (for best engine durability and fuel consumption). But will a finer pitch and higher rpm not result in higher noise, and maybe more fuel consumption? I do not feel that the engine has a hard time pushing my trike at that speed.

    Does anyone have some data and experience with this?

    I live in the Netherlands (sea level), and do most of my trips at 1000-1500 feet (6500 max for water crossings).

    Thanks!

    Clemens

  • #2331

    Joe Hockman
    Participant
    @Joe Hockman

    Hi Clemens, I think you have started a great topic and posed some good questions. I would not claim to be an expert on the 912ULS since I do not own one. However your question is equally important for the 912UL which I do own. I don’t know who told you that best 912 cruise rpm for engine durability and fuel consumption is between 5100 – 5500. I do not agree with that. If you can cruise around below that range (and clearly you do since you stated you only need 4000 rpms) then fuel consumption will be lower and on average I would expect better durability. I attached a chart from my 912 manual that clearly shows fuel consumption positively correlated with engine rpm. Fuel consumption at 4000 is significantly lower than between 5100 and 5500. Please note that this chart applies to the 912 A/F/UL not the 912ULS where fuel consumption will be a little higher. If you only achieve 5300 rpms at WOT during climb out then I think your prop is pitched for cruise performance and lower fuel consumption at cruise. I suppose you could reduce the pitch slightly if you wanted so that you achieve a higher rpm at WOT (perhaps a bit better climb performance), say 5600 to 5700, but your fuel consumption on average and also at cruise will go up. I tend to associate higher prop speeds with higher prop noise so I would imagine that this would also increase prop noise. I guess from my perspective, if you really want low fuel consumption and lower prop noise then your current configuration may be very close to optimal. It would make no sense to me to recommend that you make a change unless you wish to do some experimentation to examine the impact of a pitch change.

  • #2350

    Clemens West
    Participant
    @clemenswest

    Thanks for your reply.

    I also wrote to AirCreation, and this is their answer:

    I don’t know who told you that the 912 need 5100 to 5500 rpm .. probably a Gyro pilot !!?
    4000 rpm is perfect .. = low noise & low fuel consumption ..
    There is absolutely no information from Rotax explaining that the 912 need 5100 – 5500 rpm..
    We have sold hundreds of Rotax 912 and we never had comments on this subject.

    The only good method to adjust the propeller pitch is to check rpm !! you might have 5300 to 5500 max rpm on ground
    Which will correspond to max 5800 rpm / in flight / full throttle / at maximum speed !

    All the other explanation are just bla bla !!

    Cruise speed – hands off – with trim at max speed must be around 105 Km/h

  • #2376

    Joe Hockman
    Participant
    @Joe Hockman

    Clemens thanks for your follow up post. The response you received from Air Creation is consistent with what I expected. Bottom line is your configuration with low cruise rpm is close to perfect. Fly safe.

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