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Hornet - n/a
26-Oct-07, 12:25 AM
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?

team black - n/a
26-Oct-07, 07:39 AM
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.

Burtythepilot - n/a
5-Nov-07, 03:27 PM
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.



index.php?t=getfile&id=1046&private=0



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.



index.php?t=getfile&id=1047&private=0



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.



index.php?t=getfile&id=1048&private=0



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

kach22i - n/a
5-Nov-07, 05:16 PM
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 (www.globalsecurity.org/military/library/policy/army/accp/al0993/le2.htm)

http://www.globalsecurity.org/military/library/policy/army/accp/al0993/al099350050.gif

<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>

jon_curtis - n/a
5-Nov-07, 09:05 PM
http://www.aoxj32.dsl.pipex.com/NewFiles/HTWPhysics.html

Ian Brooks - n/a
5-Nov-07, 10:09 PM
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

kach22i - n/a
6-Nov-07, 01:00 PM
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 (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/aurora/assets/user_content/fi (www.hitechmarine.com.au/aurora/assets/user_content/files/65c44abbf8531d30ac6db6212237f083.pdf) 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?

Ian Brooks - n/a
6-Nov-07, 05:56 PM
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

kach22i - n/a
6-Nov-07, 08:52 PM
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?

Ian Brooks - n/a
6-Nov-07, 08:54 PM
Yep... you need max thrust at about 8 mph to deal with hump, or lower speed to deal with long grass, slipways etc...



Ian

kach22i - n/a
7-Nov-07, 02:15 PM
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

http://content.answers.com/main/content/wp/en/thumb/2/24/180px-Afterburner.jpg

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

Burtythepilot - n/a
7-Nov-07, 08:15 PM
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?



index.php?t=getfile&id=1049&private=0



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.



index.php?t=getfile&id=1050&private=0



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.



index.php?t=getfile&id=1051&private=0



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.

kach22i - n/a
7-Nov-07, 08:25 PM
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/

http://www.canosoarus.com/05UMAAV/UMAAV%20Images/Mar13-04%20Vectored%20thrust.JPG



Polythene ducted ventilation in piggeries

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

http://www2.dpi.qld.gov.au/images/3274.gif

http://www2.dpi.qld.gov.au/images/3275.gif

Ian Brooks - n/a
7-Nov-07, 09:31 PM
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

Mart366 - n/a
7-Nov-07, 09:41 PM
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.



index.php?t=getfile&id=1051&private=0



I agree you have mucho thrust http://hovercraft.org.uk/images/icons/smiley_icons/icon_smile.gif getting airborne and rotated by approx 10 degrees coming of the water at Jakes due to being behind you was scary http://hovercraft.org.uk/images/icons/smiley_icons/icon_eek.gif , 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]

kach22i - n/a
15-Nov-07, 03:48 PM
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 (www.fiddlersgreen.net/AC/aircraft/Caproni-Stipa/flying-barrel.php)

http://www.fiddlersgreen.net/AC/aircraft/Caproni-Stipa/IMAGES/Caproni-Flying-Barrel.jpg

http://www.fiddlersgreen.net/AC/aircraft/Caproni-Stipa/IMAGES/stipa-bw.jpg

http://www.fiddlersgreen.net/AC/aircraft/Caproni-Stipa/IMAGES/stipa-caproni-museum.jpg

This is the 65% scale version of the Flying Barrel

http://www.fiddlersgreen.net/AC/aircraft/Caproni-Stipa/IMAGES/Ducted-principle.jpg