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Arry - n/a
29-Mar-06, 09:31 PM
Hi there,



I'm trying to design the lift system on my hovercraft. Using 'Homebuilt Hovercraft' I am told to calculate the duct size and then choose a fan using volume flow rate and pressure difference from a table. I have done this but my duct size is smaller than the fan size. Do I just need to calculate the fan size i need from the volume flow/pressure difference and then build a duct around it to increase efficency or am I doing something totally wrong.

This book is from the late sixties and I appreciate much of the theroy still applies but I guess things have changed over the years.



Many thanks



Arry

Michel - n/a
29-Mar-06, 10:36 PM
I' m not a dude in this matter , computing is always the same, but tendency in forty years is for bigger ducts. Air flow and skirt pressure is generally tuned by changing number of blades and pitch After choosing duct and engine power.

Look at all hovercrafts the same size than your, if they work well, do the same.....

Sean Atterbury - n/a
30-Mar-06, 05:16 AM
Maybe Dangerman can step in here, he knows a lot about calculations and not doing it via trial and error like most of us. For my craft a 1/3 lift was specified, but I am using less than that and it works fine, and the type of skirt you are planning on using could also play a role in your calculations.

Hovertrekker - n/a
30-Mar-06, 01:23 PM
Hey Sean - your spelling! It's awesome. What happened? Sorry, I could not resist http://hovercraft.org.uk/images/icons/smiley_icons/icon_razz.gif

Ian Brooks - n/a
30-Mar-06, 06:38 PM
Hi



Is it integrated or separate lift fan?



I can help with calculations for the main types (twin engine, shaft drive lift fan (integrated power) and integrated (integrated air)) but I will need some details.



Starting with:



type

proposed fan type (I can advise no. blades & size)

engine power curve or max power @ revs

craft all-up weight (inc driver, fuel etc)

skirt type

craft plan area (lift area)

skirt perimeter (ie the hover gap length)



The twin engine is the easiest to design, followed by shaft drive then integrated.



Ian

Arry - n/a
30-Mar-06, 07:12 PM
Thanks for the time everyone.



I am going to have a integrated power source (snowmobile engine probably)powering to fans. The lift fan will be fitted so the air is being sucked in perpendicular to the freestream.



I was going to use a type W fan from http://www.multi-wing.co.uk/wseries/index.htm



I was going to increase the rpm using p. fitzgeralds method in the download section. craft plan area is 102.68 ft^2. cushion perimeter is 40.33 ft. craft all up weight is estimated at 2250lbs. the craft is going to be 22ft by 7.5ft. I know the craft weight is huge compared to the size but it needs to be armoured and carry a weapons system which make it very heavy. ther volume flow and pressure difference i am getting seem ok if a little high. i plan to use a segmented finger skirt.



The engine power curve i dont have. I have calculated the HP required to be 32HP for lift. do i mulitply this by two to get total HP required and then choose an engine.



As you can see the only knowledge I have of hovercraft is from two books from the late 60's and P. Fitzgeralds stuff so i might be on completely the wrong path.



Thanks for all your time again



Arry



p.s.



Ian, if it helps i could email you my spreadsheet with my calculations if you dont mind but I don't want to put you out.

Sean Atterbury - n/a
31-Mar-06, 06:57 AM
http://hovercraft.org.uk/images/icons/smiley_icons/icon_lol.gif

ITS FINE, I KNOW MY SPELLING IS, WELL, NOT OF THE BEST.



I AM MAYBE ONE OF THOSE PEOPLE WITH THAT DESIESE THAT TOM CRUISE HAS, ???????



DO NOT QUITE KNOW WHAT IT IS THOUGH.



WELL AT LEAST I CAN LAUGH AT IT MYSELF SOMETIMES.

http://hovercraft.org.uk/images/icons/smiley_icons/icon_lol.gif

Paul Fitz - n/a
1-Apr-06, 12:16 PM
With a craft of this size and weight, you would need to be careful of the lift blade loadings. I would suggest something like a 750/9-9/4Z or 5Z rotating at not more than 3000 rpm. The absorbed power would be about 26-28Hp. You could use a 7 blade fan, but the 9 has the advantage of removing 3 blades and increasing the speed to get you out of trouble in case of breakage. You have not stated the type of terrain the craft will operate on and so I have estimated the worst.



The Thrust fan and its power will depend on the required craft speed and power available.



What is the intended use for this craft? Is it a commercial venture?



...and the name is FitzPatrick http://hovercraft.org.uk/images/icons/smiley_icons/icon_surprised.gif)

Arry - n/a
1-Apr-06, 12:26 PM
Hi,



Thanks for the replies guys.



Mr FitzPatrick I apologise! I can't believe you live in Ilford, because so do I (Newbury Park)! Your notes have been very useful to me in understand how the ACV works.

This is my third year project. I am an Air Transport Engineering student but decided to jump in at the deep end and choose something very different to the usual dissertations.

What I have found has been very interesting but has given me problems because what I am really looking for is marks! There just isn't enough time in the day at the moment.

I've had to put the actual design on the back burner and start typing up. I only have one week left and I intend to do an intensive design starting on monday. I did have an integrated design ready before but my lecturer decide to tell me that it was unacceptable about two weeks ago, which had left me frantically trying to redesign something new.



Anyway enough of me crying, back to work!



Arry

tonyfoord - n/a
13-Jun-06, 04:37 PM
"Using 'Homebuilt Hovercraft' "



I keep seeing references to this book! Can anyone point me to where to get hold of a copy / photocopy / scannned version of it please?



Tony