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tom kent
1-Dec-10, 05:57 PM
Hi all,

I'm a final year transport design student at Coventry University and for my final project I'm designing a large amphibious transport hovercraft intended to ship supplies to the remote mining sites of Northern Canada.

I understand that this forum is focussed on the recreational/sporting hovercraft and that the design considerations for a vehicle of the size I am proposing are a different kettle of fish compared to the size you are all used to. However, I thought I would throw this out there regardless to see if anyone could help.

I have a copy of "theory & design of air cushion craft" which seems to be a bible of information on the subject of ACV design, however - I am not an engineering student and whilst I have A-level physics under my belt, much of it goes right over my head! As I said, I am doing a design degree and although I have to prove the engineering principles of my concept, I donít have to specify exact calculations and figures.


A (very brief) initial specification of the vehicle I am proposing is as follows:
Length 20-25 m
Beam 8-10 m
Payload 60 Ė 80 tonnes
Unloaded weight 80-100 tonnes
Cruising speed 20-40 kts
Range 400-600 km


I have attached an image of a rough CAD packaging model I put together to illustrate my proposal.
5948
I know how to work out cushion pressure (F=M/A) and using a 22.5 m X 9 m vehicle with a All Up Weight (fully loaded) of 180 tonnes I came up with the figure of 888.8 kg/m2 or 8717 pascals.


I also (think) I managed to work out volume flow rate for this craft fully loaded at a hover height of 0.20 m Ė (I couldnít find any information regarding hover height for large ACV so I guestimated 20cm?!) and used a simple 22.5 x 9 m rectangle to estimate a length of 63 m for the hover gap, giving it an area of 12.6 m2.



Then to find the escape velocity of the air I used v= SQRT 2 x 8717 / 1.22 = 119.54 m/s
And then for the volume of air lost per sec I used 120 x 12.6 to get a figure of 1512 m3/s


What I donít know however, Is how to work out how big or fast my lift fans need to be to replace this kind of volume and maintain cushion pressure.


I also donít know how to work out how much power I need to move this thing at a speed of 20-40 knots as Iím not sure how to consider the friction of the hovercraftís skirt etc



Any help or pointers anyone could give would be much appreciated!


Thanks,


Tom

Paul Fitz
1-Dec-10, 11:22 PM
Hello Tom, Sounds like an interesting project, so long as you don't have to build it in your garage :o)

Nothing wrong with your maths, but you have omitted a couple of things which will affect the figures. The cushion pressure you give is correct but this is not necessarily the fan pressure as some pressure will be lost in ducting the output to the cushion and losses in the loop. You need to multiply the cushion pressure by another factor (Loop Ratio) to cover these losses, typically 1.05 - 1.25.

The air leaving the cushion is neither uniform or laminar. Another factor (Escape loss coefficient) is required to cover this resistance to flow. 0.6 - 0.7 is typical. This still assumes a good surface however, so yet another factor is required to compensate for differing conditions (Surface correction factor), which can typically change from 1.3 to 1.8 or higher dependent on the worst operating conditions.

Adding these to your calculations ( 1.2, 0.6 & 1.5) you would arrive at 1350 M3/s @ 10.460kPa
This would be the fan duty required, given the other criteria. If two lift fans were used, both must supply the full pressure and each at least half (more for safety) the maximum volume.

Your quoted cushion pressure (8700 Pa) is VERY high. The American LCAC Hovercraft carry about 60-75 tonnes and are 26 x 14 mtr giving a maximum cushion pressure of about 4.8 kPa.

The lift power can be calculated from first principles, assuming a fan efficiency, but this method is rarely accurate. Better to research a suitable backward curved fan(s) choose this to get as close as possible to the required fan duty and if necessary use the standard fan laws to extrapolate the results to achieve the actual power from the manufacturers fan duty chart.

The lift fans (assuming backward Curved Centrifugal) would typically rotate at 3000 to 4000 rpm Some lighter hovercraft have used Aluminium BCC fans at about 4800 rpm. (about 600 dia)

in simple terms the Craft speed is governed by forward thrust - (skirt drag, hull drag and dynamic drag). On larger craft additional drag is created by the cushion reaction over water as well (Wave Pumping). I think this is dealt with in Yun & Bliault's book.

PM me with your Email address and I will send you a Fan duty calculator (Excel) to play with, and anything else I can find on drag.

HTH.

tom kent
2-Dec-10, 12:48 PM
Thanks for that valuable information Paul, Just what I was looking for! Ive sent you a PM with my email address.

Many thanks,

Tom

Nicholas Flint
2-Dec-10, 04:05 PM
Dear Paul Fitz, Due to a singular mind set on centrifugals i have searched and only found steel VERY heavy fans. If you know of an aluminium cent fan supplier i would dearly love to contact them, even if i have to turn away from the silent lift holy grail AGAIN.
Nicholas Flint

Paul Fitz
2-Dec-10, 09:33 PM
Dear Nicholas Flint, (isn't this very formal :o) )

The only company that I new, that made bespoke Aluminium impellers was near Reading but went out of business about 6 years ago. If I were looking now, I would try B.O.B Stevenson in Derby or Howden group, who I think are in Glasgow. Expect to pay large amounts of money though. The biggest problem will be finding impellers with sufficient duty for a small craft. Typically for a 3-4 mtr craft the impeller would need to be about 600 dia. (measured at the inlet) making the overall impeller dia about 1200mm without the volute. you may be able to use a smaller impeller at much higher speed but this can itself lead to other problems of noise and dynamic balance.

Good luck.

Paul

Keith Oakley
2-Dec-10, 09:43 PM
Where do Griffon Hoverwork ( http://www.griffonhoverwork.com ) source their centrifugal fans from? A call to R&D director (and club member) John Gifford might be very fruitful.
Keith

Mustang Hovercraft
3-Dec-10, 12:30 AM
Yes - was going to suggest Griffon.

You could also try Air Vehicles:

http://www.airvehicles.co.uk/Fans&LiftSystems.html

The Slider from new Zealand is the smallest craft I know of that employs small composite constructed centrifugal lift fans. Their other craft also utilise larger fans of the same design as far as I'm aware:

http://www.hovercraft.co.nz

Update:

Just found this pic of one of the Sliders Lift Fans under power:

http://homepage.ntlworld.com/machineage/lift.jpg

Their Fan also makes a brief appearance here:


http://www.youtube.com/watch?v=GmECZzbjsCo

KipMac
3-Dec-10, 05:27 PM
Knickerless.
If you insist on being silly, try the Museum, as the Hoverhawks used a 24" Heba B backward curved aerofoil centrifugal. They might have an unloved one somEwhere
I think you will be suprised how large the scroll will need to be.
A properly designed axial can be just as quiet.

Dad. XXX

Paul Fitz
4-Dec-10, 07:50 PM
I may be wrong, but from the video, the impeller looks like a 315 dia pvc BC impeller made by Heinrich Brinkmann. If it is, it must be rotating at a much faster speed than recommended by the manufacturer (and faster than is safe in my opinion), which would make it "illegal" under UK, EHF and WHF regulations.

Listen to Dad, he knows best :0)

Paul

Paul Fitz
5-Dec-10, 12:38 PM
Nick,
PM sent