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  1. #1
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    Default Why more or less

    From what i understand, on a intergrated craft the general rule of thumb is the more blades you have the better lift but less thrust you get ? if that is true then why is that the case?. My first thought is that the more blades you add the more air it can grab but wouldn't that increase the lift and thrust evenly?

    Just like removing blades decreases lift but increases thrust, i can visualise why it would decrease lift but why would it increase thrust ?



    If anyone can help me visualise the process a bit more that would be good

    thanks


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    Default Re: Why more or less

    Not sure on the lift thrust thing, all I know is it is like that from thrust tests, what I would liek to say is, Great video on your site, have you been on the lake yet?



    I found the falling off the gate part very funny, hope the misses is ok though.



    Keep it updated please. Like to know what is going on.

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    Default Re: Why more or less

    Quote Originally Posted by lazza
    From what I understand, on a intergrated craft the general rule of thumb is the more blades you have the better lift but less thrust you get ?
    YES.





    Static pressure "verses" velocity, the age old question.



    Static pressure = Lift



    Velocity = Thrust



    You need both in an intergrated craft...........more or less, getting the ratio right for your craft is an art. I think most craft are set up using rules of thumb and a shotgun.



    The engine's powerband has a lot to do with the loading of the fan or propeller. More experienced minds will have to talk about that.



    Sticking to gross generalizations and concepts here.






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    Default Re: Why more or less

    There is a fundamental difference between good lift and good thrust. For lift you need a large volume of air moving slowly, whereas for thrust, a smaller amount of air moving fast!



    In order to acheive a large volume of air flow, a slow fan with lots of blades is best, whereas for a smaller volume moving fast, you need a few blades spinning quickly.



    Therefore when setting up a fan to do both jobs, it has to be compromised between the two set-ups.



    Just a note on static pressure, the static pressure behind the fan is actually atmospheric (this can be shown by theory), the fan only generates dynamic pressure which is recovered to static pressure as the speed of the air flow drops throughout the lift air chambers.


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    Default Re: Why more or less

    Quote Originally Posted by Iceman
    the fan only generates dynamic pressure which is recovered to static pressure as the speed of the air flow drops throughout the lift air chambers.

    Iceman, you have me going back to an old thread to seek advise.



    Link:

    http://www.physicsforums.com/showthread.php?t=116358



    We will see what this turns up.



    Cheers, George/kach22i.


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    Default Re: Why more or less

    From Physics forum - "What is this thing called dynamic pressure anyway?"



    Download 'The principles of hovercraft design' from the downloads page on this website. You will find a description and definition of dynamic pressure, static pressure and total pressure.



    It also explains why your original question on that forum "In a strong head wind is it more desirable to have velocity or static pressure coming out of the tail of your thrust unit?", makes little sense. As David has suggested in his post, a thrust fan moves air to generate thrust. The static pressure within the duct is very low.



    The article "Calculation of thrust...." explains more about dynamic thrust and the momentum drag and thrust loss due to headwinds.

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    Default Re: Why more or less

    Quote Originally Posted by Paul Fitz


    Download 'The principles of hovercraft design' from the downloads page on this website. .......................The article "Calculation of thrust...."
    Thank you Paul, sometimes finding the right question does lead to the right answer. And sometimes you have to ask a few dumb questions before you can ask the right question.



    This is the download page for future reference:

    http://hovercraft.org.uk/hcgb.htm






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    Default Re: Why more or less

    Hi



    The work of the fan is a bit like sweeping water uphill... the hill represents the pressure rise, the brush represents the fan, and the amount of water you get up there represents thrust.



    It is the fan blades that 'sweep' air from the low to the high pressure regions, and the high pressure that can reverse the air back to the low pressure region. The fan blade can only 'sweep' when it passes by, but the pressure can reverse the flow at any time. So the more frequently you 'sweep' the air forwards, the higher pressure increase you can sustain before the reverse process takes over. For a given fan speed, then, more blades mean more frequent 'sweeping' and therefore higher pressure.



    However, each blade adds drag; this drag make the fan less efficient, that is, more power is used turning the fan and less 'sweeping' air.



    It turns out that thrust is generated by air movement, not pressure gain, and hence you can raise high thrust with little pressure rise. Low pressure rise means there is little to pursuade the air to go back through the fan. So it follows that you don't have to sweep too often, you just have to 'sweep big' (high pitch). Because you have less blades, less power is used up just moving blades and so more power is used doing 'big sweeps', hence you get greater thrust efficiency - or more thrust.



    Ian


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