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Thread: lift calculation

  1. #1
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    Default lift calculation

    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




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    Default Re: lift calculation

    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.....

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    Default Re: lift calculation

    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.

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    Default Re: lift calculation

    Hey Sean - your spelling! It's awesome. What happened? Sorry, I could not resist

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    Default Re: lift calculation

    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




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    Default Re: lift calculation

    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.






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    Default Re: lift calculation



    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.


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    Default Re: lift calculation

    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 )

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