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I have been running my 912uls at 5600 cruise max. I am going on a 300 mile trip and set the prop to 5400 max cruise for some extra speed. Is that ok ?

  • Re: Setting the prop for crosscountry

    by » 12 months ago


    It’s ok if your initial power up on takeoff is close to 5200. If not try for 5450 or 5500.

    Ed


  • Re: Setting the prop for crosscountry

    by » 12 months ago


    Hi Fred

    I would not do that personally.  This tends to put a strain on your engine, particularly the crankcase, as it overloads the engine in the cruise range and can lead to some premature failures.  The reason Rotax had to change the crankcases back in 2006 was due to cracking which was created by deep heat cycles within the case.  These were mostly on aircraft with overloaded props.  

    The WOT position, if you have a constant speed prop, is 5800 but only for 5 mins.  Max continuous is a strange name but that is 5500 in the Rotax manual.  For cross country normally you want to be between max continuous and 75% power.  The best cruise for fuel economy will be nearer 75% and not WOT.  Remember that peak torque is 5200 RPM.  Your HP is low when you do not achieve the RPM so this is a factor.  The normal setting for pitch is look to get 5500 to say 5600 WOT when you are straight and level flight.  This allows a small bit for a slight nose down so you do not overspeed your engine.  Once you have that throttle back so you are running in the lean position of the carburetor, about 4800 to 5200 is best to save fuel.   I can't see the extra speed as being more important than saving your engine.

    here is a link to a very long article on setting pitch from the EAA circa 1986..it is still valid information.

    https://www.eaa.org/eaa/aircraft-building/builderresources/while-youre-building/building-articles/propellers-and-spinners/the-fixed-pitch-propeller-dilemma

    Last, I give you this comment from the FAA handbook in the propeller section: "

    An airplane with a fixed-pitch propeller, on the other hand, needs to accelerate down the runway to increase airspeed and aerodynamically unload the propeller so that rpm and horsepower can steadily build up to their maximum. With a constant-speed propeller, the tachometer reading should come up to within 40 rpm of the red line as soon as full power is applied and remain there for the entire takeoff. Excessive manifold pressure raises the cylinder combustion pressures, resulting in high stresses within the engine. Excessive pressure also produces high-engine temperatures. A combination of high manifold pressure and low rpm can induce damaging detonation.

    Keep safe, Cheers


  • Re: Setting the prop for crosscountry

    by » 12 months ago


    I guess I don't see the diiference between throttling back to 5300 with the prop set to WOT at 5400 vs 5600? With this setting it runs with less vibration and smoother?

    Like i said it is a temporary setting for a once in a while trip. It is an amphib so it works a harder no matter what  settings it is at. great engine.

     

    thanks for all that info. its very helpful and I will soak it in.


  • Re: Setting the prop for crosscountry

    by » 12 months ago


    hi Fred

    the last 3 lines are the key, you need to know what manifold pressure is doing as well as RPM.  Stress in the engine can cause damage.  Depending on the fuel you use you may also induce detonation, again damage to the engine.  Take a look at the power and torque curve readings for the engine.  Peak torque is at 5200 range and RPM above that range is all extra HP.  The pressure in the cylinder is at its highest at peak torque so is the stress.  RPM is not going to hurt your engine.  

    Cheers


  • Re: Setting the prop for crosscountry

    by » 12 months ago


    Like I said this engine runs smoother and seems to like the 5400 setting . Take off on wheels and high Density altitude was about 100 feet more than 5600 setting. 600 feet on amphibs seems pretty short for a normal TO. The cubs don't like to run with me but Im used to that. lol

    Isnt this one reason we have these adjustable props to adjust our speeds to our needs?

     

    Where can I find the proper Map for the 5400 rpm?


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