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  • #16
    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
    R examiner and TST pilot.
    and now a Tai Chi instructor

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    • #17
      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.

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      • #18
        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.

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

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          • #20
            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, 13:18. Reason: Credit(?) fuel consumption data from Rotax manual, and spelling Paul's name correctly
            The pilot formerly posting as MadamBreakneck
            R examiner and TST pilot.
            and now a Tai Chi instructor

            Comment


            • #21
              If my airspeed were twice my groundspeed, I would be concerned that conditions would make control difficult and crosswind when landing would need to be carefully considered. I think I'd be turning around by then.

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              • #22
                Originally posted by tomshep View Post
                If my airspeed were twice my groundspeed, I would be concerned that conditions would make control difficult and crosswind when landing would need to be carefully considered. I think I'd be turning around by then.
                Why? I frequently demonstrate how windy it is aloft by getting the student to head into wind and reduce airspeed until we're almost stationary. Groundspeed 10 knots, airspeed 40 knots equals 30 knot headwind.
                The wind isn't that strong at ground level and it's coming from a noticeably different direction. It's a good lesson.
                Martin Watson
                Microlights in Norfolk
                Fixed Wing Instruction - Exams and GSTs - Revalidations
                07805 716407

                Comment


                • #23
                  Martin,
                  Thus Mike's aeroplane flying at 90kt airspeed into that 30 knot headwind would have a groundspeed of 60kt; and by my gibberish that Tom replied to he would be wanting to fly ever faster and faster... good spot Tom.

                  I should have thunk before sending, but the other (AS>3xHW for best mpg) still works based on Mike's Rans 912 I believe. Which on thinking about it would translate for optimum mpg to "keep your GPS ground speed more than two thirds your airspeed" which it usually would be unless your headwind is particularly stong and then and only then would he be better to burn more fuel to fly faster. All this within pilot, engine and airframe limits, of course.

                  I think!

                  Does that make more sense or have I really lost the plot?
                  Last edited by Joan Walsh; 31st March 2018, 14:20. Reason: Clearer that figures only apply to one particular aeroplane/engine combination
                  The pilot formerly posting as MadamBreakneck
                  R examiner and TST pilot.
                  and now a Tai Chi instructor

                  Comment


                  • #24
                    Well Joan,

                    You've introduced me to a thought provoking real life subject which I've enjoyed.

                    For my relatively short excursions of 3/4 to 3 hours duration I think I've now learnt that slow cruise "for economy" alone is hardly worth the trouble.

                    If one is sightseeing, perhaps appropriately slow & lowish may suit one's aims. Otherwise compared with the true operating costs, plus or minus 1 litre/hour is of low significance. Likewise if in a hurry and/or batting into a strong headwind with the aim of getting from A to B, the fuel use difference actually is seen to be a low element in one's concerns

                    Climbing to the height of a better tail wind is probably more rewarding for fuel mpg and ground speed, or when it's a headwind, reducing it by flying (legally) low.

                    Re. fuel cost, some folk pontificate over 'economy' jetting and fit 'Hacman' like mixture leaning devices - which activity appears to dominate their thoughts on how well their machine performs compared with others. The fun might surely be there in squeezing to a reduced apparent fuel usage, but I prefer to trust Rotax settings rather than running my own one off flying test bed.

                    Regards,

                    mike hallam

                    Comment


                    • #25
                      Looked up what Darrol Stinton in 'the design of the aeroplane' says about fuel consumption. It looks as if the outcome may be different between four stroke and two stroke engines.
                      He says that four strokes will have a specific fuel consumption of 0.5 lb/hour/BHP over a wide range of operating conditions. But that its much harder to get a two stroke to operate efficiently over a wide range of conditions and that specific fuel consumption will be higher, in the range 0.75 to 2.0. The Rotax manual shows for a 503 that the specific fuel consumption varies by at least 2:1 (1000 to 500 in some SI units, g/kWhr I think it is) over 3000rpm to 6000rpm.

                      So thrash a two stroke to get a few more knots of airspeed and you use a LOT more fuel.

                      Agree with Mike though that a few more spent on fuel is no big deal in the wider scheme of things.

                      It is however a fun thing to tinker with.
                      Martin Watson
                      Microlights in Norfolk
                      Fixed Wing Instruction - Exams and GSTs - Revalidations
                      07805 716407

                      Comment


                      • #26
                        Fuel burn is never a problem because there is always more money but range is very much an issue because a tank has a finite capacity.
                        Mogas is plentiful and readily available but not on the remarkably few airstrips that a 20 litre fuel tank can get you to with a 447.
                        My aircraft is currently getting a major aerodynamic cleanup because reducing drag is the best way to get a 2 stroke to burn less fuel (at 510g/KWh) Reduce the power required to maintain speed by four horsepower and that is two litres an hour. Streamlined struts, for example on a MiniMax usually get this sort of improvement when cruising at 60 MPH. A good fuel computer is the best way to set the aircraft up for the best range, which is, I think, the crux of this discussion.

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                        • #27
                          Thanks guys, I agree too that the fuel difference over practical flying speeds isn't worth the fret with a 4-stroke engine. It may well be a different matter with 2-strokes and I'll try it behind the 503 when I'm next up - I'll not bother doing long flights and measuring fuel, I'll just find speeds vs rpm and feed that into the graphs I derived from Rotax data.

                          I only wanted a rule of thumb and never expected to get this deep into an analysis, but hey as Mike said, it's been interesting. Another thing to consider would be comparison of the calculated speed to fly into a headwind vs best glide angle speed into same wind using the glide polar curve.

                          Tom's right too - if you want to get absolute best speed and fuel economy out of an aircraft, redesign it FAR103 and similar designs have an airspeed limit in their requirements so drag is desirable, but that doesn't apply in the UK and SSDR owners are free to tinker if they wish.

                          Meanwhile, back at the hangar;
                          Headwind Rule of Thumb part 1 - with a 4-stroke engine cruise speed doesn't significantly change mpg.
                          Headwind Rule of Thumb part 2 - with a 2-stroke engine... TBD
                          The pilot formerly posting as MadamBreakneck
                          R examiner and TST pilot.
                          and now a Tai Chi instructor

                          Comment


                          • #28
                            Originally posted by Joan Walsh View Post
                            I'll not bother doing long flights and measuring fuel, I'll just find speeds vs rpm and feed that into the graphs I derived from Rotax data.
                            I think I'm missing something still though, Joan. How do you know what power the engine is delivering at the combination of rpm and airspeed that you are flying at? Without that I don't see how you can calculate fuel consumption from the Rotax data.
                            Martin Watson
                            Microlights in Norfolk
                            Fixed Wing Instruction - Exams and GSTs - Revalidations
                            07805 716407

                            Comment


                            • #29
                              Originally posted by Martin Watson View Post
                              I think I'm missing something still though, Joan. How do you know what power the engine is delivering at the combination of rpm and airspeed that you are flying at? Without that I don't see how you can calculate fuel consumption from the Rotax data.
                              Martin,

                              I no longer have the 447 or the Rotax manual, but in my 4-stroke book (and its pdf is available above in one of Joan's attachments) they give several curves, WOT power but also the propeller power and below it one can see fuel used.

                              As my 912's Warp Drive ground adjustable pitch prop has been changed for seeing what it does at various times from 15 tip degrees through to 13 I can't say it has ever had the same loading as Rotax'. However from my real life consumption one can work back to their power curve and I've slightly adjusted (in pencil) alongside their curve to get my best estimates. In practise the finer end gives better take off (not surprising) but cruise is just as good and consumption very slightly better, so pulling a coarse prop isn't always a good thing. One thing, my figures above are mostly from a dip stick reading and tail level affects the accuracy combined with if one uses start engine to OFF (when it has miserly consumption) to my own take off to touch down plus half the taxy and warm up time - usually 3.5 minutes).

                              However as all you guys say the two-stoke is a different beast combined with its tuned exhaust system designed for a narrow optimum rpm range which BTW enables surprisingly high specific continuous power output (e.g. the 447 is something like 100 bhp per litre ~ a 1950's 'cammy Norton'.
                              The expanding and contracting 'silencer' cones are at lengths from the exhaust port to assist scavenging as well as blocking new charge loss via the open port all happening with pulses at the speed of sound for that temperature and gas composition. At something approaching WOT at the correct rpm I suppose it ought to be at its most efficient, perhaps some curves can be found to relate rpm, partial power & WOT power versus lb/hp or similar ?

                              mike hallam.

                              P.S. this forum wouldn't let me add a J-Peg of my own prop curve but it doesn't tell much more.

                              Comment


                              • #30
                                Dead right you're missing something Martin... what I think you're missing is that I'm not treating this with academic rigour, I'm experimenting and testing the result against intuition in an attempt to get a rule of thumb; thus:
                                • From Rotax data, I go from rpm v power to via fuel v power to fuel v rpm.
                                • I then do a simple fuel mass to volume conversion and get a fuel volume rate v rpm curve that I don't believe because it doesn't match experience.
                                • Then comes the cube law prestidigitation to deliver a fuel volume rate v rpm curve that more-or-less matches both experience and figures I found on various forums via Google (other search engines being available)...
                                • in other words I guessed, but I'm willing to run with it because neither my life nor my career depend on it - and it looks plausible for my purposes.

                                Having derived a curve which I'm prepared to play with for fuel flow vs rpm, I now intend to record - for my specific aircraft - a relationship of level flight airspeed v rpm which I can trace across to the plausible fuel flow v rpm curve to give me fuel flow v airspeed (which I'll do for one-up flight and 2-up). I can then translate this into curves for mpg vs airspeed over a range of headwinds, and voila! an answer to the question posed in my OP. My guess is that with the 2-stroke there is no fuel benefit in speeding up into a headwind, perhaps there'll even be a penalty. Either way it would be nice to have an inkling of its magnitude when deciding whether to plug on or give up and divert downwind.

                                I think what I'm trying to work out is simply: is it simply sufficient to fly at the speed implied by the standard methods using the gliding polar curve or does the engine fuel consumption v power curve distort that to give a significantly different best into wind cruise airspeed? At the moment I'm tempted by the Hallam/Uzochukwu formula "for fuel economy, cruise at the book best-glide-angle airspeed plus 50- 75% of estimated headwind component", bearing in mind the Welsh principle that we generally cruise faster than the fuel economy speed anyway so increasing speed further isn't necessary or even worthwhile.

                                Or if anybody want a truly rigorous approach, I found: http://www.nar-associates.com/techni...al-flying.html especially Range for a Piston-propeller Powered Aircraft and Wind Effects on Maximum Range
                                Last edited by Joan Walsh; 2nd April 2018, 17:35. Reason: Apologies to Paul Welsh for misspelling his name
                                The pilot formerly posting as MadamBreakneck
                                R examiner and TST pilot.
                                and now a Tai Chi instructor

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