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Thread: Optimising miles per litre

  1. #11
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    Bill Brooks has done quite a bit of long distance stuff on the HypeR, and with a 30 mph headwind it's worth his while flying flat out at about 90-something mph.

    OK, so he's 912S is that, but as soon as you start backing away from the ideal speed to fly, time in the air really goes up.

    With regard to the 582, in something like an X-Air Falcon, you have to be up at 5,300-ish to maintain height at best glide, about 51 knots. So it drinks fuel anyway.

    I could spreadsheet it, but the numbers may not win the argument...

  2. #12
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    This could be gibberish and I've got something else to do later so I haven't time to think it through, but thus far...

    I bunged the Rotax 503 data from the manual into a spreadsheet, but the fuel flow (litres per hour) curve counter-intuitively flattened out with increasing rpm. This is a result of the energy conversion efficiency improving with power demand (the grams per kilowatt hour vs rpm curve). Deciding that the curves must have been generated by load with a dynamometer, I put in a semi-arbitrary square law 'drag factor' based on rpm squared and a fiddle-factor constant which gave the fuel flow curve a more believable shape, rising faster with higher rpm. Then bingo! following FPW's post I changed the drag factor' to a cube law - this has resulted in a curve which sort-of matches a mix of empirical and anecdotal figures for a 503. See atached.
    Now all I need to do is fly experimentally and set up two curves (1-up and 2-up) of level flight speed at a set of rpms over the practical range for my aeroplane; and from that I can derive a pair of fuel flow per knot curves from which I can get as near as matters an answer to my original question... I think.

    503 graphs.jpg
    The pilot formerly posting as MadamBreakneck
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    and now a Tai Chi instructor

  3. #13
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    What's the vertical scale pls ?

  4. #14
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    Quote Originally Posted by mikehallam View Post
    What's the vertical scale pls ?
    Non-dimensional. Use description of line, such as mililitres per hour, or power in watts - a bit odd I know but its the way the numbers worked out for graphing in the same space

    At least it looks daft enough that it might inhibit the graph from wandering the net gaining some sort of unwarranted cred by repetition

    The lines Power (in Watts) and Fuel (in grams per kilowatt.hour) are from the Rotax charts. The rest are derived by M$magic assuming a constant petrol density of 0.72 grams per mililitre. The line Flight (in mililitres per hour) is derived from Flow (in mililitres per hour) multiplied by cube of RPM and an arbitrary constant to make the numbers look plausible... all the scientific rigour of a tabloid astrology column.

    Hope that helps.
    Last edited by Joan Walsh; 29th March 2018 at 15:48. Reason: slight clarification
    The pilot formerly posting as MadamBreakneck
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    and now a Tai Chi instructor

  5. #15
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    We'll, ok, but
    Aren't the figures/graphs of fuel consumption Vs rpm based on wide open throttle? I mean they are to help with selection of gear ratios and prop pitch design?
    In which case they don't help here.
    Maybe I've misunderstood, but I think it's even more complicated than we've said so far.
    For example, what happens at different weights? How do we account for that?
    Martin Watson
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  6. #16
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    Hi Martin,
    yes, you've identified one of the unknowns - Rotax don't tell us. I'd have thought for power and fuel consumption calculations, the figures would be most useful in steady state under flight conditions. That's the assumption I've made for this exercise.

    As for different weights for the purpose of this exercise I'm proposing a simple family of curves (only two in fact) namely pilot plus fuel only and pilot, passenger and fuel.

    My guess is that data will be sufficient to see if I'm barking up the right tree (or just barking).
    The pilot formerly posting as MadamBreakneck
    GR examiner and TST pilot.
    and now a Tai Chi instructor

  7. #17
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    A fuel rate calculator is easy enough to make, the sensor is about 25 and a bicycle computer can be calibrated very easily to record fuel burned and fuel flow rate. Just connect the rate sensor to the bike computer input. I built one after being embarrassed by an overly pessimistic fuel gauge. Since fitting it I have become much thriftier with fuel and have learned a lot about getting the best out of the engine.

  8. #18
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    FWIW.

    Today I looked up my own a/c consumption records of speed, RPM & MPG data taken over 7 years, below are my best actuals:-

    Sorry they're not for the idiosyncratic 2-stroke, but an 80 hp Rotax 912 in a Rans S6-116 (i.e. the cropped wing aeroplane version) one of genius Randy Schlitter's pop rivet and Ali tube creations.

    Operating at sensible rpm's between 4,500 & 5,200 consumption lies between some 13 & 17 litres/hour & yielded speeds over that rev. range of from 82 up to 106 mph. The higher speed realm was used only once flying from Sussex to Devon into a very strong headwind, or for gaining data.

    But for all the above theoretical 'V' squared and Prop RPM cubed powers it has none-the-less pretty consistently used MoGas at a rate of 29 mpg (full range 27 -30.6).

    I recall my 447 powered Rans S4 similarly gave 29 mpg at it's cruising 57 mph and my long gone DR1051 Jodel (105 h.p. Potez) the same at 122 mph.

    Consequently in practice, if you are happy with the noise increase and don't wish to molly-coddle the motor it's simply best for economy to use max reasonable power and reduce point to point time

    mike hallam.

  9. #19
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    .... all the scientific rigour of a tabloid astrology column.
    We Pisceans don't believe in that stuff, Joan.

  10. #20
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    Tom

    Mike - doing a bit of crude and ugly calculating (not fit for publication) using your Rans 912 figures* and translating from mpg to ml/mile, and then putting in a range of headwinds from zero to unlikely I get the following family of graphs which show a break-even between your two speeds at around 25mph headwind. Thus, below about 25mph headwind component you are lightly better off at your slower cruise speed and above 25mph you are better to cruise faster - and the greater the headwind beyond that, the greater your benefit for flying faster.

    So in practical terms in realistic headwinds - Paul Welsh's advice holds: no point flying faster unless you want to cut journey time. If as Steve U reports you want to cut into cracking headwinds, the you gain over the journey by flying faster.

    I suppose on the basis of Mike's two data points, we could consider a simple rule of thumb that it's best to keep your airspeed more than a certain multiple of the headwind; in Mike's case I'd suggest the multiple should be 3 (up to normal practical limits). Alternatively, he could say "keep your airspeed more than double your GPS ground speed"

    Now to do a similar exercise in the TST when we've finished servicing it.
    JW_Rans912.jpg

    PS * and some performance data from the Rotax 912UL Owners Manual (page 64)
    PPS. Just noticed caption error in graph - Y axis should be litres, not mililitres
    Last edited by Joan Walsh; 3rd April 2018 at 13:18. Reason: Credit(?) fuel consumption data from Rotax manual, and spelling Paul's name correctly
    The pilot formerly posting as MadamBreakneck
    GR examiner and TST pilot.
    and now a Tai Chi instructor

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