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Temperature of airbox, Rpm's

in 912iS Technical Questions 10 years ago
Hi everyone, I am new to this site and also to the 912is sport engine and I have learned so much on here from all you guys. Rotax dealers, engineers and manuals are ok but are limited in some of the sound practical knowledge and experience of you builder/pilots. Please excuse my ignorance at times but this computer controlled engine does my head in at times no matter how many times i read the manual. I am test flying an aircraft (Lsa) with a fixed pitch prop at a density altitude of between 2-3000ft and have a couple of questions: Q1 after a flight when the top cowl is taken off the black airbox with 'ROTAX' written on it is so hot you can fry an egg on it, you can not rest your hand on there. How hot should it be allowed to get? (prob a cowling issue) Q2 I have a static Rpm of 5300 but decreases down to 5000 rpm at lift off. Is this an intake issue that i somehow have a different value at 45kts? Q3 what do most of you guys use as a minimum static Rpm (fixed pitch) assuming sea level ISA conditions. The manual says 5800 for 5 mins etc, should we decrease the prop pitch to attain 5800? Thanks, no doubt i will have many more questions.

  • Re: Temperature of airbox, Rpm's

    by » 10 years ago

    Hi Phil,

    No such thing as a dumb question on thing dumb would be not asking questions. You're doing good. :)
    5300 static rpm is fairly high and probably gets you around 5800 rpm WOT in level flight. Better climb prop here.
    Static rpm can vary some between props and static only gets you in the ballpark. The best way is to fly it and go WOT in level flight at your average flight altitude. The best all around WOT rpm seems to be around 5600 - 5650 rpm. This should be set for your average altitude. Doesn't do any good if you set it for 1000' MSL, but always fly at 10k. If you get as high as 5800 then you have a better climb prop, but are giving up some cruise and fuel economy. Static around 4900-5000 is a fairly good place to start.
    Over pitching a prop has zero redeeming qualities.

    Seems like everyone else in the world gets to use an inflight adjustable prop except for the U.S. So we need to find a good balance between climb, cruise and fuel economy.

    Just don't put any eggs on the hot air box. I heard it makes a mess. :cheer:
    That is going to get fairly hot so don't worry.

    The change in rpm from static to take off has to do with prop loading and is normal.

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

    Thank you said by: Phill Hooker
      
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