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Cruise rpm in 912 motor

in 912 / 914 Technical Questions 9 years ago
At full rpm in my RV12 with a 912 motor I would get about 5400 rpm. What is the appropriate rpm for cruise. At present I set about 5200 and usually cruise about 115 knts.

  • Re: Cruise rpm in 912 motor

    by » 9 years ago

    Hi Ian,

    5400 rpm at WOT is a tad too low (not terrible). It should be a minimum of 5500 rpm @ WOT at your average altitude, but a sweet spot is around 5600-5650 WOT at your average cruise altitude.
    5400 rpm @ WOT in level flight will cost you climb, fuel economy, cruise speed at a given rpm and higher temps. It is just a tad overpitched. Not bad, but could be a bit better to help your overall performance since you have a ground adjustable prop.

    Try flattening the pitch by about 1/2 degree and see if that puts you in the 5600-5650 area and then try it for a few flights. If for any reason you don't like it you can put it back. To this day I have seen 100% of the people through the shop and around the country go from 5200-5400 and reset to 5600 or so and and stayed there.

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

      
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  • Re: Cruise rpm in 912 motor

    by » 9 years ago

    Yes that is where the pitch was set originally. We changed to where we are to get a better cruise speed which I am happy with. It doesn't detract much on take of performance. I just want to know how many revs below maximum should you cruise at to be best for the motor.
      
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  • Re: Cruise rpm in 912 motor

    by » 9 years ago

    Hi Ian,

    The rpm cruise and every other rpm setting is determined by the WOT rpm pitch setting. Depending on the year of your engine over pitching could crack your crankcase. Your 5400 rpm isn't as bad as someone down around 5100-5200 WOT. The engines around mid 2006 and earlier are more susceptible. Over pitching cause undue stress on the engine and parts.

    This answer kind of boils down to personal preference for many. It's not so much a perfect cruise rpm, but one with a range that may work best for your aircraft.
    To answer your question I think the average owner cruises between 5000-5300 rpm depending on their aircraft. You shouldn't be below 5000 for all your cruise time and are better off above it up to 5500 rpm. Seems like most owners I personally know are 5100-5300 rpm in cruise.

    If another RV12 had his prop pitch set to get 5650 rpm WOT he would out perform you in climb, reduced rpm compared to yours in cruise, better fuel economy and a higher top speed.

    I have spent years in testing many props for different prop Mfg's and was able to test more rpm and prop combos than I ever want to do again.

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

    Thank you said by: Andrew Dunning
      
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  • Re: Cruise rpm in 912 motor

    by » 9 years ago


    ... If another RV12 had his prop pitch set to get 5650 rpm WOT he would out perform you in climb, reduced rpm compared to yours in cruise, better fuel economy and a higher top speed. ...


    I can understand all of those except the last point. How can a lower pitch, to achieve the higher WOT rpm, get a higher top speed with a lower rpm? Seems like the lower pitch would require a higher rpm just to equal the cruise speed of the higher pitched prop and it seems like a higher top speed would require an rpm beyond redline. You've done the testing, though, and I haven't so maybe I just misunderstand?
      
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  • Re: Cruise rpm in 912 motor

    by » 9 years ago

    Hi Tom,
    "How can a lower pitch, to achieve the higher WOT rpm, get a higher top speed with a lower rpm?"

    Not quite right. Top end max speed is at WOT. If I have a pitch set to get a WOT of 5650 versus one set to get 5400 WOT in level flight I will run faster at a reduced throttle setting (let's say 5200 rpm) because my prop can now turn much easier and doesn't take as much power and fuel to turn it. The engine and prop marriage is now being more efficient. So someone with a prop WOT setting of only 5400 will use more throttle to achieve let's say 110 knots than the guy with the prop pitch set for 5650 rpm WOT.
    So his 5200 rpm cruise speed will not be the same if he had a prop set to get 5650 and cruised at 5200.
    My guess here is he would run 5300 rpm (WOT of 5400) to keep up with himself at 5200 rpm (WOT at 5600).

    It is all about torque, HP and making the prop do the most work with the least effort. All props and engines are designed to run best at certain rpms. You need to match up torque, HP and prop (length and prop design) to achieve the best flight characteristics for your specific aircraft. There are variables here and let's just talk basics.
    Ideally it would be nice to have an in flight adjustable prop for the light Rotax driven aircraft here in the US.
    Since we don't and most of us have ground adjustable props we need to find that balance between climb and cruise and last is fuel economy. The key word here is balance. Not too flat and not too coarse.

    It all has to do with the prop efficiency at any given rpm with a prop pitch we can't change in flight. If you set it too course all phases of the flight envelope suffer including stress on the engine and higher engine temps. Set it too flat and you have a good climb prop up to a point (it can be too flat) and you're spinning your wheels (so to speak) for cruise efficiency and higher fuel usage. If you made it too flat then even climb would suffer and the engine might over rev. In this case you're wasting your HP and torque and prop performance if you want cruise performance too out of this prop setting.
    Set the prop too course where the engine can't develope its best torque and HP and you suffer in all flight characteristics. HP and torque are finite and you must stay in its best and most efficient operational range. HP and torque is a marriage and you shouldn't have too much out of balance from one or the other. The engine has a design operating rpm range and so does the prop. Going too far in either direction out of the best rpm range and prop pitch makes some flight characteristics suffer. Remember balance is the key here.

    We should ask ourselves this question: At what prop pitch will I achieve my best balanced aircraft performance? What pitch is best for a balanced flight characteristic of climb, cruise, fuel economy, engine temps and least stress on the engine components?

    Here's a good short story to demonstrate.
    I had to pick up a plane and fly it back to my home base. I flew over in my plane a Flight Design CT. The one I picked up was the same. My plane's prop pitch was set to achieve 5600 rpm at WOT in level flight. The other plane was set at 5200 rpm at WOT. We took off together. My plane was faster on the ground, it climbed a whole lot faster and left me in the dust. I had the other pilot throttle back. When I finally caught up I was running 5200 rpm WOT and burning 6.1 gph. My plane was running at 4600 rpm and burning 4.2 gph. Not to mention my temps were higher than in my own plane. My own plane would hit 120 knots at WOT in level flight and his was approximately 105 knots. The difference here is my prop pitch setting was more efficient at all rpms than the one set too coarse and I was able to better utilise my engine torque and rpm to the prop. If the plane I was flying had a prop pitch set to get let's say 5800 rpm WOT in level flight then I would have had to run at 5300-5350 in the other plane if my own plane was running at 5200 rpm in level flight. The prop at this point would have done well for climb, but would have suffered for efficiency in level cruise flight. Been there and done that too many times over the years.
    During this person's annual I reset the pitch to get right around 5600-5650, same as my plane. On the way back to his home field we took off together and flew at the same rpms. I was able to throttle back on the other owners plane and save the engine in stress, fuel economy and engine temps. The prop was now unloaded and it was able to breath and work easier because it could develope its best torque and HP for that particular prop and rpm point. When it is set this way you will fly faster at a reduced fuel usage over being too coarse. I have a hundred stories just like this one.

    If you have an engine that only gets 5200 rpm WOT in level flight and reset the pitch to 5600-5650 you'll think someone supercharged your engine. I have a Gobosh story, but I'll save it.

    5400 rpm WOT in level flight has been demonstrated over 25 years not to be an efficient place to be. It isn't faster than the same plane set at 5600-5650 and it will start down the runway slower, climb slower, use more fuel and have less speed at the top end (WOT in level flight) and even when just cruising around 5200 rpm.



    Some need a slightly better climb prop because they want to get in and out of short fields or maybe they have floats.


    Bottom line:
    Over pitching has no good flight characteristics and can cause damage to engines from over stressing components and higher engine temps. Some over pitching is certainly worse than others. Some SLSA MFG's years ago thought over pitching was a good idea to slow a plane down, but the consequences were far worse to the engine. I think most (unfortunately not all) are onboard now days for at least 5500.





    Sorry for being so long winded. I just couldn't find a few words that worked. :side:

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

      
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