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Thread: petrol uses x-air

  1. #11
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    petrol uses x-air

    In theory they may be different speeds (let's not go there), but in practice in a draggy microlight they are the same speed. If you recall the U-shaped drag curve from your theory studies related to exercise 10a (slow flight) you'll remember that there is an airspeed at which the drag is minimum... to maintain level flight above or below that speed will require more power, ie higher revs and a higher fuel burn rate. As near as matters in practice, this is the same as your best glide speed. Thus for minimum litres per hour in cruise, fly at your best glide speed and you'll not be far wrong.

    However if you are trying to get somewhere as opposed to just 'being up there' for an hour, then wind has an effect. As a ridiculous example, if you are flying at 45knots into a 40knot wind you'll be burning your, say, 13 litres per hour but at a 5knot ground speed and about 2.5 litres per mile.
    Now if you were to increase your speed to 55knots and accept a 17 litres per hour consumption, then you'd be covering the ground at 15knots and making some progress at about 1.2 litres per mile.
    If however, you were going the opposite way with a 45 knot tailwind, at 40knots (13litres per hour in our example) you'd be doing 85knots at 13 litres per hour burning about 0.16 litres per mile.
    In this case increasing your speed to 55knot and 17litres per hour, you'll achieve 95knots at 0.18 litres per mile... so downwind your slower speed is more efficient whereas into wind increasing your speed can be more efficient.

    Short answer: for endurance (just staying up there) or going downwind fly at around best glide speed and for battling into wind fly faster than that. How much faster is another question with a complicated answer :-)

  2. #12
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    petrol uses x-air

    Again, thanks very much for all your replies.
    Hopefully the weather will improve soon so that I can try the new approach and see what works best for me.
    Philip

  3. #13
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    petrol uses x-air

    I've reduced my rpm to 5000 or 5100 and I it reduced the fuel consumption to 16 lph.
    That's quite a bit better than the 20 lph on 5500 rpm. It's early days, only one flight to Stoke on the Medway, but it's encouraging.
    Thank you all for your help
    Philip

  4. #14
    Diamond geezer Banned500 Club
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    petrol uses x-air

    Nice one Philip, that sounds a lot more like it

  5. #15

    petrol uses x-air

    Hi Philip.

    My X Air, with fairly new skins, "tight" wings and a 582 blue top seems happiest at about 5300 RPM. At those revs it returns around 13 LPH solo and 17 LPH two up. At same revs it will cruise at somewhere between 55-60 MPH depending on weather.

    Others with far more experience than me have pointed out that of course there is a big difference between being up there flying and actually travelling to get somewhere. That's the real bugbear for a draggy aircraft and results in some complicated "head hurting" working out of fuel/revs/speed trade offs.

    I also concur with the views expressed about "smooth" flying, gentle inputs, a nicely centered ball and thinking well ahead throttle position wise all help a lot.

  6. #16
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    petrol uses x-air

    Hi Flip,
    check prop pitch.....does the old girl leap of the ground on full power...see what revs you achieve in the climb..
    maybe 1/2 degree on the prop could make a big difference to cruise. but often you compromise the climb it flew well when me and Reg check flew it, but we do like a pie !!
    you are more waif like and maybe could allow for a better climb / cruise compromise
    pete

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