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Thread: BBV Square Ducts

  1. #9
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    Default Re: BBV Square Ducts

    Quote Originally Posted by Ian Brooks
    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




    Or another way of looking at it would be:



    Getting over the hump at low speed..............static thrust would be much more important than velocity of thrust, right?


  2. #10
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    Default Re: BBV Square Ducts

    Yep... you need max thrust at about 8 mph to deal with hump, or lower speed to deal with long grass, slipways etc...



    Ian

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    Default Re: BBV Square Ducts

    If weight, cost and bulk were no longer concerns I can imagine a hovercraft equipped with centrifugal thrust fans to get over hump, propellers to cruise with at high speed, and if higher speeds were desired a bank of turbo jets, maybe even chemical rockets for short burst.



    Pretty Sci-Fi, but like I said, one would have to dispel many real life issues first.



    I think the old Soviet Union had some odd thruster combinations for their WIG's.



    EDIT

    A simple idea, variable thrust ratios of pressure verses velocity could be acheived by variable aspect conical nozzles.



    Example:

    http://www.answers.com/topic/jet-engine



    Variable Exhaust Nozzle, on the GE F404-400 low-bypass turbofan installed on a Boeing F-18


  4. #12
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    Default Re: BBV Square Ducts

    I also noticed differences in thrust by fitting the engine cover and changing the inlet profiles. With no change in RPM all I can surmise is that not only do you gain forward momentum of the craft by thrust out the back, action through the blades themselves but also the negative pressure in front of the duct acts on the bell mouth area to suck the craft forward. Why not?



    [img]index.php?t=getfile&id=1049&private=0[/img]



    So what’s in front of the duct needs to be taken in to account as well. Yes you could say that, what ever I did in front of the duct simply affected the mass airflow through it, but RPM seemed constant. More air would surely require more power to move it, Ok, there may have been less resistance, but I still think there is something to be gained there.



    [img]index.php?t=getfile&id=1050&private=0[/img]



    The air at max static chat was measured coming out the back at 75mph. Off the line with just 28hp the whole duct concept and design was accelerating the 300kg craft past quite a few of the tiny jet ducted ‘screaming’ 503’s and F3’s. It’s been fun getting to the first corner and seeing competitors looking back and being surprised that the cruising craft is still with them. After the first corner and after the 300kgs has had its effect on cornering ability or lack of it, well then it’s not such fun.



    [img]index.php?t=getfile&id=1051&private=0[/img]



    But I believe BBV are making a paper weight F35 with the same BDT for next year which should be very competative. No doubt they are looking into Variable Duct and Vectored Duct Nozzles to.



    Attached Images Attached Images

  5. #13
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    Default Re: BBV Square Ducts

    Quote Originally Posted by Burty
    No doubt they are looking into Variable Duct and Vectored Duct Nozzles to.

    I've wonder what the exact parasitic duct drag of an extra long duct which would act to straighten the swriling air flow would be.



    One then could place the rudders outside the airflow and not waste so much power in the turns.



    http://www.canosoarus.com/





    Polythene ducted ventilation in piggeries

    http://www.wisil.recumbents.com/wisil/nacaduct/naca-duct.htm












  6. #14
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    Default Re: BBV Square Ducts

    It is also true a duct generates a portion of its thrust exactly as you say - the airflow moving past the bellmouth has a lower static pressure than atmospheric so it does "suck" on the bellmouth somewhat. This effect is additional to the increased momentum exchange previously mentioned, and in an extreme example of a duct buried in a wing (pointing downwards for vertical takeoff) can double the total thrust - but only when the forward speed is almost zero. However, most practical ducts are limited in the inlet diameter and this static pressure effect is also limited.



    In a third effect, a prop or ducted fan at zero speed actually draws some air from behind the craft - this airflow actually reduces the net thrust by introducing a backwards thrust! A propeller typically has the tip washed out to zero incidence angle to help reduce this effect, and a duct reduces this effect by making the air move all the way around the lip of the duct first.



    The other thing you might do by "cleaning up" the profile at the bellmouth is reduce the turbulence, and this would also have a positive effect.



    All this gives some idea why ducts are so popular - in effect you get thrust for free. However, it turns out that, size for size, there isn't much in it, and a well designed propeller will match a well designed ducted fan in real world performance, partly because of the additional size, weight and drag of the duct assembly.



    In either case, it seems to be easy to get the installation wrong and end up with rather less thrust than you expected... in my case, my propller generates about 2/3 of the expected thrust, probably due a poor guard installation - expect to see some changes for next year!



    Ian

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    Default Re: BBV Square Ducts

    The air at max static chat was measured coming out the back at 75mph. Off the line with just 28hp the whole duct concept and design was accelerating the 300kg craft past quite a few of the tiny jet ducted ‘screaming’ 503’s and F3’s. It’s been fun getting to the first corner and seeing competitors looking back and being surprised that the cruising craft is still with them. After the first corner and after the 300kgs has had its effect on cornering ability or lack of it, well then it’s not such fun.



    [img]index.php?t=getfile&id=1051&private=0[/img]



    I agree you have mucho thrust getting airborne and rotated by approx 10 degrees coming of the water at Jakes due to being behind you was scary , then as you rightly point out 300kg dosn't help going up the hill at Jakes. I wouldn't say my F3 is screaming though, last time we looked i was running at about 6000, with my blades pitched well back



    Mart (148)



    [/quote]


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    Default Re: BBV Square Ducts

    FYI:

    Some interesting pictures from the 1930's on a similar topic.





    In 1932 Caproni financed and built this amazing ducted-fan flying barrel that was designed by the controversial Italian designer Luigi Stipa.

    http://www.fiddlersgreen.net/AC/aircraft/C...ying-barrel.php







    This is the 65% scale version of the Flying Barrel


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