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As the altitude (OK, technically not the MSL but the pressure-altitude) increases the air get thinner aand 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 hand air resistance (drag) is less, and, for example, at 8500 feet an accurate indicated airspeed of 100mph results in a true airspeed 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 approximately it is for an aircraft powered by a Rotax 912?

(Let's for the moment ignore the practical complicating question/factor/issues rolleyes of on a given flight the extra fuel used to climb up to the optimal cruise altitude could offset the fuel saving from cruising at the optimal altitude, different winds aloft, need for oxygen, etc. And let's assume a fixed pitch propeller to keep things simple.)

A.

Note that this question was already posted in the Four-stroke technical questions section, but since we got no solid answers there, with the Moderator (Rodger) permission am posting here to see if anyone browsing here dares take on this slippery one. ;-)
  • Re: Optimal altitude for fuel efficiency for Rotax 912

    by » 11 years ago


    I got this interesting reply to a similar post I made at the BMAA (British Microlight Aircraft Association) forum.
    That, and other credible sources indicate, by the way, the generally the most efficient SPEED for flight is the same as best glide speed. Of course if you have a strong headwind all bets are off what the most fuel efficent speed is. For example if your best glide is 65 knots and you have a 65 knot headwind you will be flying very very ineffciently. :( :(

    ===================== BMAA POST ====================
    http://forums.bmaa.org/default.aspx?f=22&m=131081&p=1
    ====================================================


    If you haven't seen this already then av8n.com has the answers to everything!

    http://www.av8n.com/how/

    section 7.5.5

    http://www.av8n.com/how/htm/power.html#sec-more-energy

    Interestingly the energy required to travel any given distance at a particular Indicated Airspeed will be the same.

    So the fuel used at 90mph indicated at either 1000ft or 9000ft is the same.

    The difference is that at 9,000ft your TAS will be 100+mph. Your fuel burn rate will be higher but you get there faster so the same fuel (energy) is used.

    Thats all well and good in theory, but in practice the two biggest problems are going to be propeller efficiency and engine mixture.

    The bing CV carbs are only 50% altitude compensating by a quirk of bernoulli's equation so the higher you fly the richer (& less efficient) the engine will become.
    If you had manual mixture control and EGT probes & can fly high enough to have the engine at wide open throttle for 70% power then you could max the engine efficiency (no throttle pumping losses).

    Here's an example:

    lets say I want to fly 100 miles. The best fuel economy speed (given constant engine efficiency) will be best glide (lets say 65mph). Lets assume that gives a still air economy of 40 miles per gallons - so I'll use 2.5 gallons over 1 hour 32 mins flying at 1,000ft (1.63 Gal/hour - 7.5 litres an hour at 65mph, sounds reasonably possible to me.)

    Now lets move up to 20,000 ft and keep our IAS at 65 mph. Our TAS is now approx 90mph.
    We're still going to use 2.5 gallons over 100 mile distance (same drag force on aircraft, cos same IAS)

    Our flight time is now 1 hour 7 mins, and the fuel burn will be 2.25 Gal/hour

    There is no way at sea level you could possibly use 2.25 Gal/hour at an indicated 90mph, it'd be more like 3.5 Gal/hour.

    All said though, we're stuffed because if you look at our wind forecasts, the best thing to do is pick an altitude with the best tailwind/ least headwind depending on where you're going. At the speeds a Skyranger flies at, headwinds/tailwinds play a more important part in our fuel economy than height.

    Andy

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