View Full Version : equations for model hovercraft

davidhills - n/a

16-Nov-08, 07:20 PM

Good Morning

I wish to build a model Hovercraft and have bought a 12volt centrifugal fan with the following characteristics

Voltage 12volts

Current 15amps

Electrical power 180 watts

Outlet exhaust velocity 16.7meters per sec

Outlet exhaust area 0.01square meters

Inlet area 0.01 square meters

Inlet velocity 5 meters/sec

Volumetric flow rate (no load) 0.17 cubic meters per second

From this data I would like to be able to calculate the following variables

In a relationship

Gross weight (GW), skirt foot print area(SFA), ride hide (RH)

In the form

Gross weight(GW) in Newtons = Constant K(from above data) times function of SFA & RH

I.e. GW= K(Func(SFA,RH))

I appreciate that I have not expressed this using normal mathematical convention but hope there is enough information for some help in getting an answer.

Many Thanks all

David Hills

davidhills@riverford.co.uk

Ian Brooks - n/a

16-Nov-08, 10:24 PM

Hi

It doesn't work in quite that way... essentially, the cushion pressure is a fixed value determined by the weight and the footprint

Pc = f(GW,SFA)

and the ride height (assuming this is hovergap) is then a function of the flow rate and skirt perimeter (SP)

RH = f(VFR, SP)

You need to develop a function to describe the volumetric flow rate as a function of Pc and fan speed or power, and then you can get all you need. I haven't studied CF fans, so I can't help there.

Take a look at this, it describes a practical approach to the problem for real hovercraft, using axial fans. It relies on a fair amount of empirical data and then some numerical methods.

http://www.hovercraft.org.uk/downloads/files/block_0/estimat (http://www.hovercraft.org.uk/downloads/files/block_0/estimating_thrust_lift-D.pdf) ing_thrust_lift-D.pdf

Cheers

Ian

Paul Fitz - n/a

16-Nov-08, 11:24 PM

Hello David, unfortunately it is not quite as simple as preparing a formula in the way you would like it.

If you read the "Principles of hovercraft design..." available on the HCGB downloads page you will see the general principles and calculations explained. These refer to full size craft but are essentially the same.

Before you can proceed you require more information on your fan. You need either the static pressure curve, or a table of volume outputs at different pressures. You will also need to estimate the completed weight of the craft.

If the fan curve etc. is not available, you will need to experiment to find the fans lift potential. With this information you can calculate the craft size which the fan will support. Remember that low weight is crucial for a successful craft (full size or model) so choice of battery will probably be a major issue.

I would recommend that you read through a number of the model hovercraft websites to establish the maximum weight/area ratios which work with models, and the recommended skirt and hovergap heights to use.

These may be good starting points.

http://www.olshove.com/HoverHome/contents.html

http://www.model-hovercraft.com/calculator.html

http://myweb.tiscali.co.uk/rchovercraft/experimental%20corne (http://myweb.tiscali.co.uk/rchovercraft/experimental%20corner/Liftfantesting3.21.pdf) r/Liftfantesting3.21.pdf

HTH

Paul

davidhills - n/a

17-Nov-08, 10:21 AM

Hi all

Thanks for your replies, they are very helpful

OK now I want to deal with these two issues Ian Brooks has explained

1) You need either the static pressure curve,

Or a table of volume outputs at different pressures.

How can I measure this?.

Can I make my own static pressure measuring device? (Tube and water)

How can I "load up" the exhaust port in a controlled manor to generate the graphs i need?

Is there an easier way?

2) You will also need to estimate the completed weight of the craft.

answer 3 to 5 kg

Thanks all for your help

David Hills

profqwerty - n/a

17-Nov-08, 02:25 PM

You could take a large airtight container, put the fan blowing into it, then vary the size of the exit hole.

For the static pressure, do you have pressure measuring equipment? maybe a diaphragm with a known pressure on one side? or as you said tube with water but don't know how that would work with air..?

What about measuriong the velocity of the air coming out, would that give you the M(dot)?

Paul Fitz - n/a

17-Nov-08, 02:33 PM

You could make measuring equipment etc. but personally I wouldn't bother. It is not easy to measure very low pressures and volume accurately without expensive instruments. It will be easier just to test the fan with a practical experiment.

A very crude calculation using half the volume of the fan (0.17/2 = 0.085 M3/sec) and 5 Kg total mass, 1000 x 500 mm skirt and 3mm hovergap gives a volume flow of 0.08 m3/sec @ 98Pascal, which seems reasonable for your fan.

Obtain a plastic bag of approx 1000 x 500mm and a piece of plywood the same size. adjust the bag if necessary with staples and duct tape.

Attach your fan to one end of the bag and cut openings in the sides of the bag totalling 30 sq centimetres (This will represent a hovergap of) 1mm.

Place the Ply board on top of the bag so that it overhangs slightly and weight to 5Kg (including ply) using any known weights (eg. Bags of sugar)

Run the fan. If the bag inflates and lifts the 5Kg weight, the fan is ok at 0.03 cu.m/sec @ 98Pa Nominal.

open the holes to total 60 sq.cm. Try again if ok good for 0.05 @ 98 Pa nom.

open holes to total 90 sq.cm. Try again if ok good for 0.08 @ 98 Pa nom.

If the weight fails to lift on the first test you must either lower the weight or enlarge the "hull" and bag to reduce the required cushion pressure.

If you lift the weight with no problem you will probably have volume left over to use for thrust.

Do you have a manufacturer and model No. for your fan?

Paul

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