Welcome Guest. Sign in or Signup

10 Answers

optimal altitude for fuel/distance efficiency for for piston engine aircraft?

Asked by: 19328 views , ,
Aerodynamics

As the altitude (OK, technically not the MSL but the pressure-altitude) increases the air get thinner and the power the engine can produce at a given RPM is lower... despite the carbs on my Rotax, or the manual fuel mixture control on other engines) trying to compensate and get the right mixture as the altitude changes.
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 at same indicated airspeed.
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, the sweet spot on altitude efficiency for jet engines is apparently about 35,000 feet(I assume the airlines carefully optimized for fuel efficiency and that's were they usually fly) or turboprops (if I got this right) I think it may be about 25,000 feet.

Any idea where it is for an LSA  powered by say a Rotax 912....or for that matter all(?) conventional piston engine aircraft? Is it , as might appear at first glance, "the higher the better?"

(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. )

Alex

Ace Any FAA Written Test!
Actual FAA Questions / Free Lifetime Updates
The best explanations in the business
Fast, efficient study.
Pass Your Checkride With Confidence!
FAA Practical Test prep that reflects actual checkrides.
Any checkride: Airplane, Helicopter, Glider, etc.
Written and maintained by actual pilot examiners and master CFIs.
The World's Most Trusted eLogbook
Be Organized, Current, Professional, and Safe.
Highly customizable - for student pilots through pros.
Free Transition Service for users of other eLogs.
Our sincere thanks to pilots such as yourself who support AskACFI while helping themselves by using the awesome PC, Mac, iPhone/iPad, and Android aviation apps of our sponsors.

10 Answers



  1. Mark Kolber on Dec 16, 2013

    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.

    +2 Votes Thumb up 2 Votes Thumb down 0 Votes



  2. Bob Watson on Dec 16, 2013

    The POH is the place to go, but when I’ve looked into this, it seemed like the sweet spot for non-turbo planes was somewhere between 8,000 and 10,000 feet. But as you start going up in altitude, the prevailing winds can play an increasing role in determining the best altitude for any particular flight (in terms of overall cross-country economy). Turbocharged and turbo-normalized planes are generally more efficient (e.g. in MPG) at a much higher altitude where they can get higher TAS, but then other limitations come into practical application of that info (e.g. oxygen system limitations and total flight distance) in addition to the even higher winds that you find at those altitudes.

    +1 Votes Thumb up 1 Votes Thumb down 0 Votes



  3. Wes Beard on Dec 17, 2013

    Alex,

    Open up Microsoft Excel or any other spreadsheet software. In column A, input all the altitudes for the airplane. In column B input the corresponding KTAS. In column C input the corresponding fuel flow for the given altitude. In column C, divide B by C. (i.e. =B1/C1 for row one). The resulting data in column is your nautical miles per gallon for each altitude.

    Create a chart where the altitudes (column A) are on the X axis and the NM/gal (column C) is the values for each altitude. It will become quite apparent which altitude is the “sweet spot” for that airplane.

    +3 Votes Thumb up 3 Votes Thumb down 0 Votes



  4. Alex Censor on Dec 17, 2013

    Hi Wes,
    Nicely done spreadsheet directions. Thanks .

    Alas, with the automatically altitude compensating carbs standard on the Rotax 912 series, as far as a know…and certainly not in my experimental’s POH…. there’s no data on fuel flow vs altitude.

    I’ll have to settle for Bob Watson’s useful ballpark of 8000 to 10,000 feet.
    Realistically what that translates to is “generally the higher the better up to 10,000 on a long flight.
    But of course there’s the issue of prevailing winds which if headwinds often swamps attitude efficiency gains. And the issue of what distance does the flight have to been for the altitude efficiency gain is sufficient to pay back more fuel that was lost by the prolonged climb TOO optimal altitude .

    0 Votes Thumb up 0 Votes Thumb down 0 Votes



  5. Wes Beard on Dec 17, 2013

    I had to google what altitude compensating carburetors were. They automatically lean out the fuel as the altitude increases and there is no need for a mixture knob (or lever).

    The thing to remember for all piston aircraft is this. There is a specific amount of fuel that is necessary for a given amount of air. If these two are the same all the way to altitude the power will remain constant and the fuel flow will remain constant.

    As the airplane climbs, air intake decreases and so does the automatically leaning carburetors. If you can maintain 75% power all the way up to altitude, fuel flow will be the same. If you can’t then fuel flow will start to decrease.

    For your original question, there should be a graph or chart relating %RPM to fuel flow. Take that information and transpose it on the previous graph with %RPM and altitude. From there, you can use the spreadsheet to calculate the “sweet spot”.

    Here is a interesting forum post on the subject.
    http://forums.flyer.co.uk/viewtopic.php?f=1&t=66834

    Take a look in your manuals for %RPM per altitude. In my Piper POH, it will give an RPM setting for each altitude to maintain 75%. There is a curved line up top where 75% power is unavailable to the engine due to a lack of air.

    +2 Votes Thumb up 2 Votes Thumb down 0 Votes



  6. Alex Censor on Dec 17, 2013

    Hi Wes,
    Thanks for the interesting link.
    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. 😉

    Some experimentals don’t have the sort of documentation you’re apparently used to seeing.

    My aircraft is an experimental LSA Skyranger kit built, and the kit seller never produced a POH. We had to write our own POH based both on a UK POH and our own experience, and neither ours or the UK one we cribbed from has the sort of %RPM vs altitude or altitude vs fuel tables or graphs you refer to. The Rotax manual I have does have a graph of RPM vs power output, and another graph of Fuel consumption per hour vs. RPM and one of manifold pressure vs. RPM.

    I’m happy for now to settle for it’s likely somewhere between 8000 and 10,000 as we rarely fly above 10,000 anyhow and other considerations that squeezing out a bit more fuel efficiency most of the time determine our altitude.
    After all we typically fly at between 2.75 and 4 gallons per hour, and even at the slow speeds we fly get pretty impressive miles per gallon compared to most of the GA aircraft we see. 🙂

    Alex

    0 Votes Thumb up 0 Votes Thumb down 0 Votes



  7. Brian on Dec 18, 2013

    “So you might not be able to get the IAS that mathematically translates to higher TAS?”

    This was not the question. The answer to the question is a simple one, for best fuel/distance (better understood as greatest efficiency) the answer is the highest altitude you can get out of your bird. Without oxygen that is 12,500. With oxygen go to the service ceiling. This is, of course, predicated on winds a loft.

    Do note that this may not be the fastest way to a destination. What it will give you is maximum fuel efficiency.

    0 Votes Thumb up 0 Votes Thumb down 0 Votes



  8. Alex Censor on Dec 18, 2013

    Hi Brian,

    Thanks for the straightforward answer.

    I know I did NOT ask this:
    Now that raises the question of where the break-even point comes.
    That is, ok, go as high as practical on a long flight.

    But if the flight is short….say 20 miles…will you lose more fuel and time climbing as high as you can than saved by being up high?

    Is the optimum something like “climb continuously for first half of flight (assuming you don,treachery max operation altitude before then) them begin a descent glide?”

    Alex

    0 Votes Thumb up 0 Votes Thumb down 0 Votes



  9. Brian on Dec 19, 2013

    “Now that raises the question of where the break-even point comes.”

    That question, though not specifically asked, has been specifically answered. There is no simple method to determine this point, that I’m aware of. You would have to use performance data and compare it to winds aloft as well as the mission for that day.

    The short answer is, it depends. I’m not sure what sort of performance charts you have available, but there are a couple rules of thumb you may find useful when determining the best altitude and power settings for a given day. TAS increases at roughly two percent per thousand feet and bhp decreases at roughly three percent per thousand feet.

    0 Votes Thumb up 0 Votes Thumb down 0 Votes



  10. Clark Hall on Mar 01, 2014

    This is not as technical as most of the other explanations. Let’s say you want to run 65% power. Climb until full throttle gives you 65%power and level off at that altitude. If using 60% do the same thing but the cruise altitude will be higher.

    +2 Votes Thumb up 2 Votes Thumb down 0 Votes


Answer Question

Our sincere thanks to all who contribute constructively to this forum in answering flight training questions. If you are a flight instructor or represent a flight school / FBO offering flight instruction, you are welcome to include links to your site and related contact information as it pertains to offering local flight instruction in a specific geographic area. Additionally, direct links to FAA and related official government sources of information are welcome. However we thank you for your understanding that links to other sites or text that may be construed as explicit or implicit advertising of other business, sites, or goods/services are not permitted even if such links nominally are relevant to the question asked.