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Thread: petrol uses x-air

  1. #1
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    petrol uses x-air

    Hi, when you look at some statistics, the petrol uses for an xair standard is between 13 and 17 lph.
    My xair uses about 20 lph.
    What are other xair petrol consumptions and if better than mine, how can I improve the uses?
    Philip

  2. #2

    petrol uses x-air

    All depends on engine fitted weather and pilot, passenger weight and speed you are flying?
    My Xair hawk 912 burns approx 12 litres two up and 10 litres solo.

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    petrol uses x-air

    Hi, my XAir is Dacron covered and I have a 582 blue top using premix with normal BP unleaded. Two up just normal flying (take off, fly around, land) on average I get about 15lph, solo 12lph. However, if I'm doing circuit work or use full power alot this rises to 17lph-20lph two up. I find in cruise the 582 sips fuel but as soon as you go to full power it guzzles it. I tend to do all my planning based on 17lph to be safe.

    As William says the weather, weight and speed make this vary alot which is why I err on the side of caution with 17lph.

    M

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    petrol uses x-air

    You can download the Rotax 582 usersmanual here http://docusearch.flyrotax.com/files/pdf/d04495.pdf

    It shows a graph of fuel consumption with a minimum (normalised for power output, ie its in g/kWhr) at about 5500rpm. If you cruise at an engine speed significantly different to this then the fuel consumption is much greater - you can see that from the slope of the graph.

    So being careful to fly at the right rpm will make a big difference. The tautness of the fabric on the wings will make a difference too.

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    petrol uses x-air

    Thank you all for your input. I'll have another look at the tightness of the wings but I do fly at 5500rpm....
    Philip

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    petrol uses x-air

    Isn't best glide speed a good target?

    I found with the X-Air's cousin, the AX2000, to fly any faster than best glide with the HKS seemed to be flogging the engine. So I'm guessing about 43kn or 51 mph for best fuel consumption....... any pointers as to why this might be wrong?

  7. #7
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    petrol uses x-air

    Steve's right. The other way to look at it in practice is to aim for a target airspeed.

    5500rpm gives the best fuel consumption for the power output from the engine. But the power that is required to haul a draggy aeroplane through the air will be much greater as you try to make it fly faster than the airframe wants to go. So the actual fuel consumption in l/hr can be less at lower rpm than 5500rpm, if that results in flying at an airspeed that needs significantly less power to maintain level flight.

    I dont think I'm explaining this very well...

    Anyway the best glide speed represents the speed at which the lift/drag ratio is best and, as Steve says, is the speed at which the aircraft needs the minimum power to maintain level flight. Higher or lower speeds than this need more power.

    So what you really need is a graph for your aeroplane of power required for level flight vs speed and another graph for fuel consumption of the 582 (ie the one in the manual). By cross referencing the two you should be able to arrive at the speed and rpm setting that will give the best fuel consumption. The best compromise will be somewhere around best glide but not that exactly because of the way the engine performance varies with rpm as per the second graph.

    In practice this is all way too complex though because the shape of the first graph will depend on the individual airframe, how heavy the aircraft is and other factors like the propellor and gear box and the weather conditions (density altitude etc). So there is no first graph to use.

    Which leaves you back with trying things to see what happens. I'd suggest that you play with
    1. Flying just above best glide speed, and (which may turn out to be the same thing),
    2. Flying at 5250rpm (which I reckon is a good compromise for the Xair/582 combination, although as I said it depends on the prop as well). 5500rpm in practice is too much for the cruise - I mentioned this in my earlier post so that if you were looking at the graph in the Rotax manual you could see how steep the fuel consumption rise is when you operate away from the ideal, I didn't mean it was necessarily the best rpm to use in a real engine/gearbox/prop/airframe set up.

    So to simplify even further - you are probably trying to fly too fast and pushing the aeroplane hard to get just a few knots more speed than it really wants to go at. There's a good chapter in "Stick and Rudder" by Langeweische about the different speeds to use in an aeroplane for best performance that will explain all this more fully. And I always recommend that book to anybody who hasnt read it - its the best practical explanation of how an aeroplane flies from a pilots perspective that's ever been written.

    Phew, I need a lie down. But I hope that helps a bit.

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    petrol uses x-air

    Then there's the consideration, if travelling rather than just 'being up there', of lires per mile when heading into a wind. It's usually best to fly at more than minimum drag speed. Downwind, min drag speed delivers best mpg as well.

    Another (glider pilot's) way of keeping energy consumption down is to fly smoothly and calmly, and in balance. As one of my gliding instructors once said "every time you waggle those controls, you throw away height" - he used different words, but that was his meaning.

    Joan

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    petrol uses x-air

    I find mine cruises happiest at about 5250 RPM with an indicated airspeed of about 45Kn (CAS would be 52Kn).

    M

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    petrol uses x-air

    Can someone explain to me the drag speed. Has that something to do with the best glide speed?
    Or are we talking about the minimum rpm to keep the plane flying at level height?

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