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As the altitude (OK, technically not the MSL but the pressure-altitude) increases the air get thinner at the power the engine can produce at a given RPM is lower... despite the carbs trying to compensate and get the right mixture as the altitude changes... and I'll have a lower indicated airspeed at the same throttle setting than at a lower altitude.

On other other hand air resistance (drag) is less, and, for example, at 8500 feet an accurate indicated airspeed of 100mph results in a true airspeed of of about 113mph.
So I will be traveling further in an hour of fuel consumption than at a lower altitude.

So is there an approximate altitude where the gain of true airspeed due to altitude with a Rotax 912 gets you the most miles per gallon?

For example, it's known the sweet spot on altitude efficiency for jet engines is about 35,000 feet, for turboprops (if I got this right) it's about 25,000 feet.
Any idea where it is for an aircraft powered by a Rotax 912?

A.
  • Re: Optimal ALTITUDE for Rotax performance and economy

    by » 11 years ago


    Hi Al,

    Good question. I'm looking forward to seeing some answers.

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


    Thank you said by: Al C

  • Re: Optimal ALTITUDE for Rotax performance and economy

    by » 11 years ago


    Well the best answer I got (not here, where other than Rodger, no responses) so far is from askacfi.com

    Here it is FWIW:
    Not too helpful for Rotax flyers who generally don't get such a chart he refers to:
    ---------------------------------------

    One thing you can't ignore is, as altitude increases, power available decreases. So you might not be able to get the IAS that mathematically translates to higher TAS? For example, you are not going to get 110 KIAS in a 172 at 10,000 msl. The engine just can't produce the needed power at that altitude.

    The "sweet spot" for optimum altitude efficiency (speed vs fuel use, without adjustment for the climb or wind) is going to be found in certified pistons in the performance chart. Without drilling down too far, the "sweet spot" is going to be the altitude that the cruise performance chart shows you the best TAS when compared with the fuel flow.

    As I recall, some LSAs have that in the chart, some don't.

    You can see all comments on this post here:
    http://www.askacfi.com/15750/optimal-altitude-for-fueldistance-efficiency-for-for-piston-engine-aircraft.htm#comments

  • Re: Optimal ALTITUDE for Rotax performance and economy

    by » 11 years ago


    Given that we go no substantive replies here in 4-stroke technical, would you, Rodger, mind if I cross posted to the General section?

    Alex

  • Re: Optimal ALTITUDE for Rotax performance and economy

    by » 11 years ago


    Hi Al,

    Go for it.

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


  • Re: Optimal ALTITUDE for Rotax performance and economy

    by » 11 years ago


    OK, I'll cross post and see what we come up with there.
    By the way, here's an interesting discussion about a similar Rotax vs Altitude question:

    http://forums.flyer.co.uk/viewtopic.php?f=1&t=66834

    I feel a little better seeing that many, including those with more expertise than me even specifically for the Rotax 912, were unable to really give a straightforward answer to my question. ;-)

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