I see some old threads from Roger on this and I have set my prop as per his advice . I have a few other questions. I have a 912uls in a Kitfox

1. if you are going on a long cross country is it alright and advantages'  to increase the pitch to gain some speed? What would be the lowest WOT allowable?

2. When in cruise is it ok to run at max rpm (5500) for long periods of time or is there a cruise speed prescribed for the 912

 

  • Re: Cruise rpm and prop pitch

    by » 4 months ago


    5000 to 5500 is what the engine was intended to operate at.


  • Re: Cruise rpm and prop pitch

    by » 4 months ago


    Thank you.

    So it is alright to run it at 5500 for 3 hours straight?

    My green arc goes down to 4000 and what if you want to fly slow can you run it 4700 for example for a half hour ?

    I usually vary the rpm on longer flights.


  • Re: Cruise rpm and prop pitch

    by » 4 months ago


    Yes, you can operate at 5500 rpm continuously  according to the operating limitations. However the intake manifolds are tuned for smooth operation between 5000 and 5500 rpm cruising below that will increase vibration damaging/wearing the gear box components including increased wear on engine mounts, exhaust system, wiring harness and air frame components that are susceptible to vibration.


    Thank you said by: RotaxOwner Admin, Andrew Dunning

  • Re: Cruise rpm and prop pitch

    by » 4 months ago


    What follows are my opinion based on both personal experience and reading some on the WOT RPM relationship to prop pitch. Don't take them as gold:

    According to SOME of the Rotax respected gurus the optimal prop pitch should result in 5800 RPM at WOT in level flight. According to some others who should be credible (including the rep of one LSA manufacturer), including myself (and I do not classify myself as a respected Rotax Guru) a prop pitch that results in as low as 5600 or even 5500 RPM at WOT in level flight is still OK. Any prop pitch higher that would, for example, result in a WOT of 5400 RPM has some definite downsides.... sort of like running a car in too high gear with the pedal to the floor... lugging.. possible engine harm and loss of performance.

    One problem with any chosen target RPM for WOT in level flight is that the RPM you see will depend on how heavily the plane is loaded. Let's say you buy into the 5800 target and tweak the pitch to hit that perfectly. You do that with just the pilot and light fuel load. Next day you fly with a sumo wrestler in the copilot seat, full luggage allowed, and full fuel. At WOT throttle level you will see noticeably lower than 5800. So is your pitch then "wrong" for that load? And the air density that day can also change the RPM you will see at WOT.

    One advantage of setting pitch to, say, run at 5600 (rather than 5800) WOT in level flight relates to a rare "what if" scenario: If the throttle cable breaks or gets loose on the Bing carbs they go to WOT, and you're stuck flying WOT like it or not. At 5800 in level flight (to say nothing of what RPM it would hit if you have to descend) I'd be worried about how long the engine can handle that. Officially it should never be run at 5800 more than 5 minutes IIRR. Being stuck at 5600 would not be quite as worrisome... and if you turned of one mag switch it would drop to about 5500.

    That all said.... If I were going on a long cross country flight I WOULD set my pitch at the high end of acceptable, maybe to give me 5550 to 5600 RPM at WOT level flight. I get better cruise speed at most throttle settings that way and get a higher maximum sustained cruise speed than I do when pitch is set lower. (BTW: I have an IVO prop. It takes all of 5 minutes to change pitch on it with no protractor nonsense and most of that five minutes is removing the spinner.)

    At risk of stating the obvious, when you increase pitch you do trade-off/lose some climb ability and do lose a bit of short field take off ability. Lower pitch (assuming you don't go too low) better climb... higher pitch (assuming you don't go too high) better cruise, including lower fuel consumption.

    Once in experimenting with increased pitch went I went too high and immediately saw not just loss of climbout but even loss of cruise speed. Came right back down and backed it down.

    To have the best ideally we'd all want adjustable pitch in flight.

     


  • Re: Cruise rpm and prop pitch

    by » 4 months ago


    ".... cruising below [5000 - 5500] that will increase vibration damaging/wearing the gear box components including increased wear on engine mounts, exhaust system, wiring harness and air frame components that are susceptible to vibration..."

     

    Yeah....But you can easily tell by sound and feel if there's more or less vibration at any particular RPM. So that extra wear ain't necessarily so. And if could be less wear.

    And there are other RPMs that, depending on your particular engine and mounting, prop, etc, and resonances, and even what weight is in the plane, that may be just as smooth or smoother. I sometimes cruise at 4400 when lightly loaded and not in a hurry just sight seeing. It happens to be one of the sweet spots of RPM on mine. Saves wear, noise, and fuel. And after 1100 hours there's not a shred of evidence of gearbox wear... barely detectable _anything_ on gearbox magnet or filter dissections on oil changes. And of course everyone has to throttle down into very low RPM when landing, etc.


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