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Thread: Optimising miles per litre

  1. #31
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    Thanks for those references Joan. So his rule of thumb is to speed up by a quarter of the headwind (based on a close approximation to the academically rigorous answer). That gives you a much simpler problem - all you need is to measure real fuel burn in your aircraft at various airspeeds and do the time/distances calcs for different headwinds.
    (I'm sorry to keep harping on about it, but I still think that the Rotax data is of little help to you because you need to know the power required curve for your aircraft for it to work - even just for guesstimating).
    Martin Watson
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  2. #32
    The way to calculate precisely is very similar to what Steve says - but rather than a speed/ sink polar what you need is a speed / MPG polar. Then move the axis up and down as Steve says to move the tangential point.

    The easiest way to produce this curve is to fit an accurate fuel computer - and get the fuel burn for a range of speeds.

    I used to do this when I was a keen competition pilot.

    But as Paul Welsh ( my long standing comp partner) says what it generally reveals for most microlights is that you donít want to speed up too much - unless the wind is very strong indeed.

    Indeed most of the time we cruise well above speed for best range, and if range is truly an issue you may find the calculations suggest you want to fly slower than Ďnormalí ( Joanís 45 knot floating excepted!)

  3. #33
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    Coincidentally there is a letter from Jeremy Harris in April's MF mag about specific fuel consumption and the power needed for level flight. This tends to support the idea that an empirical and pragmatic approach is simplest.

    (Incidentally I wonder if this the same Jeremy who used to post on here many moons ago?)
    Martin Watson
    Microlights in Norfolk
    Fixed Wing Instruction - Exams and GSTs - Revalidations
    07805 716407

  4. #34
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    Bound to be. He did the calculations and produced the graphs that demonstrated the surprisingly large amount of fuel saving that could be made by streamlining the struts on a MiniMax. I was gratified to find that my last hour in the air was made on 7.4 litres. I really am going to have to thrash it more because when the struts are streamlined, that will come down even more!

  5. #35
    Hi Joan, I've just found this thread and have enjoyed reading it. It demonstrates very well how rubbish FB is and how good a forum can be. Thank you for starting it.

    You mentioned Competition flying. As Paul and Paul have said Flying slow is the preferred tactic for a couple of reasons that don't relate to the performance of the plane/engine.

    We don't – usually – fly long straight legs. When you fly a curved track any calculation would be continually changing.

    The slower you go the easier navigation (map reading) is.

    We don't know what the headwind is because we're not carrying a GPS.

    Laurie (2)

  6. #36
    Well, my dearly-beloved took the TST up for a jaunt yesterday and came back with three approximate data points for airspeed vs rpm. I would have liked two more, one each above and below this range but given the thermic weather he restricted the speed range to 10knots above stall and below Va. These three do the job to show the principle though.

    I fed the numbers into my fuel flow vs rpm graph which I posted here [link] and the result is in the graph attached... and what do we see? Nothing other than what Paul W originally said, we're generally better to fly slower. Only at real really strong headwinds when we'd probably be tying the windsock down to prevent it blowing away would it be worth increasing airspeed to cover distance with limited fuel.

    The attached graph shows that according our data litres per ground-mile increases with airspeed up to about 30 knots headwind. At about 35knot headwind our TST would cover the same miles per litre at any of our chosen speeds. Above that(!) the optimum speed increases as the miles per litre reduces towards nothingness.

    Warning: these data are from one flight, in one aircraft, in thermic conditions so need to be taken with a large pinch of salt. I played with the achieved airspeed used in the graphs and even a one knot difference moved the optimum point for litres per nm, but I reckon the general principle matches Paul's prognosis: optimum airspeed for fuel burn per mile is slower than you'd think. I'd add that the greater the headwind, the lower the fuel benefit of flying slowly so increasing airspeed into a particularly strong headwind wind will at least reduce the journey time if that is an important consideration, otherwise fly slower and enjoy the time in the air.

    TST headwind fuel calc.jpg
    The pilot formerly posting as MadamBreakneck
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  7. #37
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    So at 35 Knot headwind, you use the same fuel at 45 as at 55 (so cane it,) but in still air, the extra ten Knots increases consumption by more than fifty per cent. That validates the fly slow theory. The results from the fuel computer in the MiniMax which has now been calibrated seem to suggest 7 litres an hour at 52 Knots 7.4 MPL and 12 at 63 or thereabouts.5.25 MPL. Broadly comparable. (447 and not quite as draggy as the Thruster.)
    24 MPG v 33. The car does three times the lower figure at 60 MPH. Fat lot I care!

    Still air in the Thruster 4.3 MPL at 55
    45 into ten Knots gives 5.6 MPL at what looks like it ought to be the same power setting.
    ? (goes off to scratch head.)
    Last edited by tomshep; 17th April 2018 at 16:21.

  8. #38
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    "Velly interesting ! "

    I think we four stroke flyers had better bow out of this zone as there's virtually no correlation between out sleek speedsters and your draggy bundles of plastic & cotton.

    As tomshepherd points out - and thus - I made a cross ref. using his thoughts to my findings with a 912 Rans S6-116.

    e.g At 80 kt into no head wind, petrol consumption in litres per n.mile = Z. Into 20 kt mpg it increases 100/80 = 25% to 1 1/4 Z - [or if only 10 kt + 12 1/2%.]
    Simply put, does a 20 kt increase in air speed cost more or less than 25% fuel. [Similarly 10kt and 12 1/2%]

    FWIW the Rans' mpg is pretty level with my WD ground adjustable two blade prop running at what I call sensible engine rpm's, i.e. 4,600 to 5,000, For me normal is 4,800. Too low and it's a regime that Rotax don't recommend, though they claim 5,500 continuous is O.K.
    For that higher rpm yet keeping speeds within the Rans useable form factor, could possibly mean fining off the prop. even more - which I feel may increase wear and be even noisier.

    mike hallam.

  9. #39
    Quote Originally Posted by tomshep View Post
    ... snip...

    ? (goes off to scratch head.)
    Tom, I suspect in your closing remark you have misunderstood. The speeds in the graph are IAS, not GS

    Quote Originally Posted by mikehallam View Post
    "Velly interesting ! "

    I think we four stroke flyers had better bow out of this zone as there's virtually no correlation between out sleek speedsters and your draggy bundles of plastic & cotton. ... Snip ...
    Two factors to consider there. First, I think the curves would be similar but stretched out over a higher range of airspeeds; and secondly, yes, from what I could tell from data I found, the 912 has a flatter fuel burn/rpm curve. From the two data points you gave earlier, I think the crossover point for you would be 25kt headwind; so above 25kt on the nose you'd save both fuel and time by flying faster.

    What I don't have is error factors for the figures (nor do I intend to bother working them out) so I can't be sure that the numbers are any good. The trend however seems clear - at the speeds I intend to fly, in the winds I intend to fly in, and in the aircraft I'll be bimbling in, I'll go further on limited fuel by flying slowly, just as Paul said near the start of this thread. At least now I think I understand a bit more why, and it is indeed counter-intuitive.

    Thanks everybody for an interesting discussion.
    What next?
    The pilot formerly posting as MadamBreakneck
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  10. #40
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    Ok Joan, I get it now. I've a little project I am working on which might be of interest...

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