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  1. #1
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    Default BBV Square Ducts

    Was looking at some pics on the hovercraft pic & video site and noticed that the BBV Cruising craft have a duct that squares off at the rear. Can anyone tell me if there is an advantage to this or is simply done to make mounting rudders etc easier?

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    Default Re: BBV Square Ducts

    It's not uncommon on cruising craft; it does make mounting rudders and elevators far easier. If you look at the back end of a Vortex or Osprey, you'll see an array of elevators which all but shut the duct down, meaning it is possible to approach static lift.

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    Default Re: BBV Square Ducts

    Nothing is ever done at BBV without a damn good reason. I’ve learnt the many advantages of their BBV square duct and I’ll list them below.



    The 110cm BBV duct was apparently designed with cruising at mind and therefore using four stroke relatively low powered engines with efficiency, noise levels and strength as major criteria.



    [img]index.php?t=getfile&id=1046&private=0[/img]



    High levels of efficiency are created by increasing the area of the duct outlet compared to the 110cm inlet diameter. This is achieved by diverging the duct from a 110cm diameter circle and carefully changing the outlet shape to a 110cm rounded square.



    This divergence slows the air down and increases the duct outlet pressure, therefore this increased pressure seemingly increases the force supplied to the surrounding atmospheric air creating more thrust for the craft. Airflow as a result of the size of the duct is increased, the blades are pitched up but slowed down reducing noise levels and with the aerodynamic shape of the duct inlet optimized, figures of 6.8 pounds of thrust per horse power are achieved. The guys there call it BDT... Big Duct Technology. I call it other things...



    Straighteners were fitted and tried but only created marginally more thrust compared to the greatly increased noise levels.



    Having such a large duct unsupported internally by straighteners you would think would create a weak duct, but changing the duct shape from a circle to a square creates geometric strengths. The circle doesn't want to conform or flex like the square, neither does the square outlet want to deform like the circular inlet, creating an unbelievably strong duct for its weight.



    [img]index.php?t=getfile&id=1047&private=0[/img]



    After much research of previously proven design ideas, the final BBV compromised duct design was bonded on to the hull of a BBV F35. This created the quietest cruising craft built to date (in the UK anyway) with 75db's at full throttle and 220lbs of thrust.

    In its first year, even though it was never designed or built to race, weighing some 300kg, in the hands of a complete novice, this BBV research craft showed its strength and reliability to gain more points and win more races than many other far more powerful craft and if there had been a European Novice championship, it would have won it.



    [img]index.php?t=getfile&id=1048&private=0[/img]



    One final advantage of the BBV square duct outlet is that it makes it dead easy to mount the rudders.

    Attached Images Attached Images

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    Default Re: BBV Square Ducts

    I thought that in a "thrust only" situation a propeller, fan or jet engine thrust duct should be always be converent to increase the velocity and lower the pressure. And that the resistance to forward movement as presented by the body of the craft does provide all the resistance you would need to create a pressure differential which is the basis for how fans and propellers work.



    Pressure is the result of resistance, there is air resistance unless you are in outer space, right?



    Somehow I think the mass of the object in motion comes into this, but I'm not sure how.



    Evidence against divergent duct:

    http://www.globalsecurity.org/military/lib.../al0993/le2.htm



    <table border="0" align="center" width="90%" cellpadding="3" cellspacing="1"><tr><td class="SmallText">Quote:</td></tr><tr><td class="quote">
    Turboshaft engines used in helicopters do not develop thrust by use of the exhaust duct. If thrust were developed by the engine exhaust gas, it would be impossible to maintain a stationary hover; therefore, helicopters use divergent ducts. These ducts reduce gas velocity and dissipate any thrust remaining in the exhaust gases. On fixed wing aircraft, the exhaust duct may be the convergent type, which accelerates the remaining gases to produce thrust which adds additional shaft horsepower to the engine rating. The combined thrust and shaft horsepower is called equivalent shaft horsepower (ESHP).
    </td></tr></table>

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    Default Re: BBV Square Ducts

    Hello



    The generation of thrust is due to momentum change and has nothing to do with pressure as such... and following this through leads to an understanding of why, for static thrust, a larger prop/fan is better.



    Momentum change actually causes thrust, and is found from mass x change in speed.



    However, it takes power to perform the change in speed of the air, and this power is found from 1/2 x mass x (change in speed)^2.



    So thrust is proportional to mass x velocity and it takes power to cause this, which is proportional to mass x velocity squared



    Following through with the sums, due to the squared term, you will see that, for a given amount of power, to get more momentum change you need more mass flowing through a smaller velocity change. This is what you get from a larger propeller - more area = lower velocity = more thrust per HP.



    A ducted fan works in a similar way - due to the duct, it behaves more like a larger open prop and creates more thrust when compared with a same-size open prop.



    A divergent duct works in a similar way - if the duct works properly, the flow slows down towards the exit, behaving like a larger prop. This is great, thrust for free, but there is a fly in the ointment. For this to work, the divergence has to be so gradual (<8 degs) that the duct becomes very long. Otherwise you just get turbulence, which is air going nowhere and this is bad!



    This all hangs together for static thrust - for "high speed", you need high exit velocity but this doesn't affect hovercraft which are "low speed"!!!



    Ian




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    Default Re: BBV Square Ducts

    Quote Originally Posted by Ian Brooks


    This all hangs together for static thrust - for "high speed", you need high exit velocity but this doesn't affect hovercraft which are "low speed"!!!
    I've always wondered about that, thanks.



    This is what I typically find when Googling this topic (assuming water is similar to air).



    HOW PROPELLERS WORK !

    http://www.baypropeller.com/how.html

    <table border="0" align="center" width="90%" cellpadding="3" cellspacing="1"><tr><td class="SmallText">Quote&#58;</td></tr><tr><td class="quote">
    and this water jet action of pulling water in and pushing it out adds momentum or acceleration to the water which results in a force which we call thrust.
    </td></tr></table>



    One Page pdf (How Propellers Work):

    http://www.hitechmarine.com.au/auror...ser_content/fi les/65c44abbf8531d30ac6db6212237f083.pdf



    From this, first we get a push/pull effect from the spinning propeller/fan acting as a wing, then we have momentum added to the air (or water).............the change in momentum is called thrust.



    The shortcut I took was taking out the middle part of momentum, but I knew mass had to be in there somehow and that was the missing part.

    ............................................



    Question:

    In theory (as applied to hovercraft) is the most effective propelling device one which has the lowest velocity and highest net pressure?





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    Default Re: BBV Square Ducts

    Hi



    The most effective propelling device for static thrust would have the lowest velocity increase and the largest mass flow, this hypothetical device would have the largest static thrust, which would unfortunately not be too much use as the thrust would diminish quickly once the craft got underway.



    Ian

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