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kemo1988
9-Nov-10, 04:56 PM
Hi all

I am bulding a hovercraft model, and it is the first time for me to deal with finger skirts.
I saw a picture of the finger pattern and i dont know the design procedures, i tried to figure it out on my own but i couldnt.
I have a PDF which gives only a simple drawing with some angles and doesnt go through the exact steps.
can anyone please help me on how to get to the final pattern of the finger skirt

foster2958
9-Nov-10, 07:56 PM
Depending on the scale of your model the fingers are probably going to be tiny, so I don't envy you - sewing them will be a real challenge. Are you looking for a segmented skirt design, or is your model of a commercial hovercraft with a loop & finger skirt - like a tube around the perimeter with little fingers underneath?

For a segmented skirt there are design principles contained in the "Constructors Guide" which results in the traditional curved shape segment. There is a simpler segment design which is more square in appearance - see the attached design guide. If this is the sort of thing you want, it is pretty straightforward. If you need any help with this sort of design please get in touch through the PM system.

5916

Good luck with the model build.

Kevin

kemo1988
10-Nov-10, 10:07 AM
i am not buiding a small scale hovercraft, it is about 70 cm * 50 cm, and i am buiding this to only test the segmented skirt because i didnt deal with it befor and idont know any thing about it, and after i reach the proper design i will build a real one .
The PDF i ve got mentioned two types : extended and byconical, and you will see that the segment is not standing on its edge, there is a curve in the pattern to make the edge trimmed to reduce wear,
but it doeaent say how to get this pattern.

I dont know how to get the constructors guide.

thank you for your quick repond and waiting for you

foster2958
10-Nov-10, 03:45 PM
Your ratio of length to width is rather more square than most small hovercraft designs. Most practical single seat hovercraft are in the region of 3.0m long x 1.8m wide, so your model is about 1/4 scale - this means that segments would be about 35 to 38mm wide when scaled down.

The Constructors Guide is published by the HCGB and available for purchase through the on-line shop - it describes the design of the extended type segment in your diagrams. I can't say I've ever seen the bi-conical type of segment although it has similarities with the skirt design I attached earlier. The sketches you've been given are very indicative and do not reflect current practice. The lower skirt attachment point would normally be about 100mm up from the junction of the planing surface and the floor. The contact point of the skirt to the ground can be calculated when the weight of the craft and the dimensions of the craft are known, but as a rule of thumb I use 125mm on typical single seaters. Any skirt material to the right of line BC on the sketches is wasted - it does nothing, its not under tension when the skirt is inflated so the large curve shown on the developed drawing is unneccessary. All that extra contact area is just adding friction, a well designed skirt should sit up on its tips.

Hope this helps

Kevin

kemo1988
11-Nov-10, 09:38 PM
The lower skirt attachment point would normally be about 100mm up from the junction of the planing surface and the floor.

i am sorry i dont understand this one.

and one thing else, in the PDF you attached i tried to draw the main panel but i dont know angles AQP & QAS and their relation with the side view angles.
QP in the mail panel is equal to QP in the side view right, but i dont know AS in the main panel and PS in the main panel should be equal to PS in the side view right.

please could you help me with this.

foster2958
12-Nov-10, 08:45 AM
The lower skirt attachment point is point S on my section drawing - it is about 100mm from the bottom corner where the planing surface meets the floor.

The easiest way to develop the main panel is to draw everything full scale and measure the edges including the diagonals; AP and SQ, and the distance AQ. These will then allow you to draw up the main panel. If you measure all the lengths on the section and transfer these to the main panel drawing you shouldn't go wrong.

Once you've worked eveything out if you post your sketches/drawings on here I'll gladly check them for you.

Kevin

kemo1988
12-Nov-10, 05:49 PM
yes i will do this thank you

but you didnt tell me the angles AQP and QAS, it is a major obstacle for me now....

foster2958
13-Nov-10, 10:48 AM
The angles will depend on many factors, the width of the skirt segment, the hover height, the angle of your planing surfaces etc. If you follow the instructions, draw it up in section full size and then measure all the sides and diagonals you will have enough to draw up the pattern. You don't need to know the angles. Think of it as a series of triangles. With the length of all three sides known you can construct a triangle correctly every time.

Have a look at the attached drawings showing a section through the plenum/side view of the skirt and compare with the developed plan of the segment - this is taken from a design by the students on a course I'm running at the moment.

5930
5929

Kevin

kemo1988
16-Nov-10, 03:08 PM
Hi, i really appreciate your effort with me thank you very much.

i assumed a segment dimentions and tried to develope the pattern and i will scan it here very soon,
Here is the steps i followed:
* draw QQ
* draw AA
* draw QA
* transport the diagonals QS & AP
* transport QP to intersect with AP to get point P
* transport PS to intersect with QS to get point S
then i connect all the points togther to reach the final pattern, and i found that AS in the pattern is shorter by 3mm than the side view, IS this normal or what ?, except this all dimentions are correct.

kemo1988
16-Nov-10, 03:24 PM
i want to know some details about point R & S to reach a proper design:

First point R, from your experience what is the effect of moving point R right and left, i think it is has relation with the craft stability am i right ?

And the same for point S, i saw many pictures which the lower attachment point of the skirt is at the end of the plaining surface, what will happen if this point moves up and down on the plaining surface.

Kareem

foster2958
16-Nov-10, 04:34 PM
It's hard to say where your dimensions might have slipped without seeing your sketches, however it's very easy for a small mistake or mis-measurement to build up to a larger overall error. When constructing point S, I would tend to use the intersection of AS with QS rather than PS so that you aren't building on possible errors in the construction of point P.

Moving the contact point R closer or further from the centreline of the craft will affect the stability of the craft, but also the skirt pressure. Too far in and the craft tips over easily, too far out and the pressure drops unacceptably. This also has an effect on top speed, the lower the pressure the slower you can go before the skirt starts to collapse at the front due to the pressure of the air you are trying to push your way through.

Moving the skirt attachment point down the planing surface closer to the floor increases the chance of damaging the attachments in normal use, increases drag over grass and would also significantly reduce the dimension PQ - if you reduce this too much the air from the feed holes gets cut off and you will lose both air flow and skirt pressure.

Kevin

kemo1988
16-Nov-10, 05:39 PM
thanks kevin, i really own you, i will try to get point S as you said now.

give me an hour or two and i will attach my trial.

Kareem

kemo1988
17-Nov-10, 02:39 PM
ok, here is what i have done.

i think i found my mistake, QR and QA are not equal therefore ARA is not a straight line it is a curve,
and by doing this point S can be determined from any other point and it will be the same.

The model i built has 130 plaining surface and i assumed 3 cm hover hight and 6 cm segment width .

foster2958
17-Nov-10, 05:48 PM
You are correct, ARA is a slight curve. Based on your sketch I've drawn up the section and developed the plan of the segment. You will need to add tabs to this outline for fixing the top edge P1-P2 and at the bottom corners S.

5934
5933

Observations on your model:

Segments are very wide in comparison with hover height
The angle of the planing surface is very steep
Lower skirt attachment point is very low

Kevin

kemo1988
18-Nov-10, 06:30 PM
thanks kevin, your observations are correct, but as i mentioned befor this model is for testing the finger skirt only, and i am now working on cutting and sewing all the patterns and i will install it very soon and i will take some pics for you to see it.

OK, befor i post this thread i didnt know any thing about how to make the patterns, and there was a trial befor this one which i made the pattern like this...., and there was a massive air leak between the segments ( see attch )., so what do you think the cause of this, is it the pattern profile or what ???

foster2958
19-Nov-10, 08:48 AM
It is caused by the segment pattern. Because the pattern didn't make allowance for the shape of the inflated skirt - internal air pressure makes it adopt a semi-circular shape instead of a straight line - the effective width of the segment where it touches the ground is only about 64% of the design width. This leaves big gaps for the air to escape. You've also got large gaps between segments on the corners because the angle between segments is very large - it looks like one segment at 45 degrees between the front and the side of the model. You really want to use more facets to turn the corner, efectively making a curve.

Kevin

kemo1988
21-Nov-10, 06:12 PM
Do you have a plane and section of the extended type, because after this model i will build a real one with this type...

foster2958
22-Nov-10, 08:01 AM
No, sorry, just what you've seen.

Kevin

kemo1988
23-Nov-10, 04:54 PM
Now i am thinking of the body, i dont know where to begin, shall i need to know the length and width first or what, and what is angle and length of the plaining surface(PS) (normally), and also the hight of the hull.

For the feed holes, Is the diameter of each hole is half the segment width?, and doese all the holes have the same diameter...

For the body, Doese the CG of the hull needs to be at the exact center or shifted towards the driver .


Kareem

foster2958
25-Nov-10, 08:17 AM
The starting point has to be what is the craft to be used for, and how many people must it carry? The answers to these questions will begin to dictate the size and the height of the hull. Engine selection will also have an influence, as will the anticipated trip duration - long journeys = bigger fuel tanks therefore more weight to be accommodated and so on. Form follows function.

Kevin

kemo1988
27-Nov-10, 05:05 PM
The starting point has to be what is the craft to be used for, and how many people must it carry? The answers to these questions will begin to dictate the size and the height of the hull. Engine selection will also have an influence, as will the anticipated trip duration - long journeys = bigger fuel tanks therefore more weight to be accommodated and so on. Form follows function.

Kevin

ok, the craft is single seat integerated system and i am planing to use the craft for cruising at top speed may be 70 km/h, and there wont be any long journeys, the engine will be rotax 447 or 503(with gearbox) direct drived with a multi-wing fan not less than 90 cm dia., What is a craft size with this info ?, Is there a relation or ratio between the craft's dimensions and this information ?.

Kareem

Al
27-Nov-10, 09:00 PM
If you are serious about using the craft for cruising, seriously rethink your engine choice! Most modern cruising craft use 4 stroke engines these days.

foster2958
28-Nov-10, 10:40 AM
I'd reinforce the comment about 4 stroke engines. For a cruiser, a 2 stroke engine has a number of disadvantages - it tends to be louder and faster revving causing more perceived nuisance, has a much higher fuel consumption rate and is much more susceptible to electrical problems caused by water spray (especially in salt water conditions).

Design of a cruising craft is covered in the Club's Cruising Craft Construction Regulations - you'll need to be a member to access these:

http://www.hovercraft.org.uk/showthread.php?18948-Construction-regulations-for-cruising-craft

A typical single seat integrated craft would be around 3.0m long x 1.8m wide, possibly a bit longer for a cruiser, but if you make the footprint too big the skirt pressure will be too low for adequate performance. Maximum hover height is recommended to be not exceeding 1/8 craft width, planing surfaces should such that if one side or end of the skirt collapses, the planing surface makes a dihedral of 10-35 degrees with the ground, you also need to consider freeboard - 300mm minimum for a cruiser. With a cruiser you also need to give serious consideration to the amount and placement of more than adequate bouyancy.

The download section of the forum also provides a number of comprehensive design guides with plenty of complicated maths for the perennial insomniac. Seriously - good highly detailed stuff, but more than a little post like this can go into.

Kevin

kemo1988
28-Nov-10, 12:07 PM
if you make the footprint too big the skirt pressure will be too low for adequate performance. , .......planing surfaces should such that if one side or end of the skirt collapses, the planing surface makes a dihedral of 10-35 degrees with the ground,....... you also need to consider freeboard - 300mm minimum for a cruiser.


well the only available engine for me is rotax, so i may consider a racing machine, so can it be 3.0 * 1.8 or it will change.
i dont understand the three points in the quote, what do you mean by footprint and freeboard, these words are not familiar to me sorry, if you have any sketches i would be very grateful.

Kareem

foster2958
28-Nov-10, 02:50 PM
From Wikipedia:

Freeboard (nautical) (http://en.wikipedia.org/wiki/Freeboard_(nautical)), the height of a ship's deck above the water level.

Freeboard may also refer to:

In civil engineering, the amount of watertight surface between a given level of lake, sea or river water and the lowest possible entry point during flooding or large waves.
The difference between floating & flooding

Footprint is the area contained within the perimeter defined by the contact point between the skirt and the ground/water beneath. Skirt pressure is craft operating weight divided by footprint area, so the bigger the footprint, the lower the skirt pressure. Forward motion causes pressure upon the skirt, when this pressure exceeds the inner pressure of the skirt it collapses and you hit the ground with a bump at speed - not a good situation.

Dihedral is the angle between the planing surface and the ground, in this case when one side of the skirt collapses it must be min 10 degrees as shown in the image below.

5946

A racing craft has the same sort of design limits, but you don't expect it to be a lifesaver in the middle of the sea, so bouyancy and freeboard aren't such big issues. There is no reason why you cannot use a 2 stroke in a cruiser, but the reasons previously given should be considered. If you have to use a Rotax, make sure you use an inlet muffler on the carburettors and an additional after muffler on the exhaust system if you can, to keep noise to a minimum. Noise may not be an issue where you come from, but it is over here. Pay strict attention to waterproofing and protecting your electrics - you don't want to break down a mile from the shore.

Kevin

kemo1988
28-Nov-10, 04:49 PM
So you mean by freeboard the hight of the hull right, you said a minimum in cruising is 300 mm, so for racing would it be more or less.

now i understand the footprint part, what range of pressure should be maintained in the footprint area, i was paning to design for around 430-500 pa, because in the integerated system if you lower the static pressure of the fan, the more flowrate you get for thrust, but i dont actually get what can cause a pressure on the skirt to make it collapse, is it the drag force!

kareem

foster2958
28-Nov-10, 05:33 PM
Freeboard would be the difference in level between water level when floating and the lowest point from which the cockpit can be flooded - don't forget the bottom of the duct which will let water in from the plenum. You need to think about the action of waves and the amount of pitch and yaw (rocking & rolling); without enough freeboard your craft won't be able to withstand much motion from waves without filling with water.

430-500pa is a very low skirt pressure, a more typical reading would be in the region of 575-600pa or about 12lb/ft2 (58.6kg/m2). Your fully loaded craft at operating weight will be around 200kg or more. To acheive your proposed skirt pressure the finished craft including fuel and the driver will need to weigh less than 150kg - extremely unlikely for a first time design by an inexperienced hovercrafter, unlikely for almost everyone without using advanced composite techniques and ultra-light components. If you want this project to work, you need to be realistic in your estimates.

Dynamic pressure from forward motion will be the cause of skirt collapse as I described. Stick your hand out of a car window at 5kph and compare this with the pressure on your arm at 70kph - that pressure pushes on the front of the skirt when the craft is in motion. If the static pressure inside the skirt pushing back against it is too low the skirt will collapse.

Race craft tend to have less freeboard and less hover height than cruisers.

Kevin

Al
28-Nov-10, 05:53 PM
Nicely put Kevin.

Kareem, have you thought about building a craft from plans of a proven design rather than designing your own from scratch? It is the area below the skirt that is the critical bit. You could change the design above the skirt to put your own stamp on the craft. At least you will end up with a hovercraft that works as it should. Along the way you will gain experience of how hovercraft works and building methods and probably discover better ways of doing certain things.

I have built one hovercraft from plans, currently I am tidying up an old racing craft, but when that craft is finished I am hoping to build another large cruising craft. I would love to build a hovercraft of my own design, but at present I dont feel I have enough experience and prefer to build something that I know will work and not waste hundreds of hours and a shed load of money on something that may or may not work.

kemo1988
28-Nov-10, 06:26 PM
Nicely put Kevin.

Kareem, have you thought about building a craft from plans of a proven design rather than designing your own from scratch? It is the area below the skirt that is the critical bit. You could change the design above the skirt to put your own stamp on the craft. At least you will end up with a hovercraft that works as it should. Along the way you will gain experience of how hovercraft works and building methods and probably discover better ways of doing certain things.

I have built one hovercraft from plans, currently I am tidying up an old racing craft, but when that craft is finished I am hoping to build another large cruising craft. I would love to build a hovercraft of my own design, but at present I dont feel I have enough experience and prefer to build something that I know will work and not waste hundreds of hours and a shed load of money on something that may or may not work.
yes, actually i will buy the constructor guid from the club, but i am a mechanical power engineer and i with three of my friends trying to get our own design because it will be a very good achievment for us, and as you said it is all about experience i know, i dont expect a good performance it is my first trial. After a few trials you get your own design and starts to put your own ideas.

kemo1988
28-Nov-10, 06:28 PM
Freeboard would be the difference in level between water level when floating and the lowest point from which the cockpit can be flooded - don't forget the bottom of the duct which will let water in from the plenum.

i think i get it, look at the attch pic

foster2958
28-Nov-10, 06:56 PM
That's correct, lift the centreline of the duct so the bottom edge is about 300mm up - at or around the top edge of the cockpit and you will be less likely to be swamped when floating off-hover.

Kevin

kemo1988
3-Dec-10, 09:32 PM
OK, Now i assuumed the Base dimentions 80 cm * 194 cm and the hight of the hull 30 cm and with an angle of 150 PS it gives about 1.8 m * 2.9 m Deck dimentions (craft Dim.) , but i wanted to calculate the hight of the hull, so i did some bouyancy calculations based on the base dimension , the angle of PS and the designed weight (200KG) and gived 10 cm of the hull which will be under the water when switching off the engine, so based on the freeboard concept i need minimum 45 cm of hull hight and the plaining surface lenght will be 90 cm which is very very large i think and it will give a very large footprint resulting in very low skirt pressure, so there is something wrong here...

i am confused about two things : first i may did not understand the concept of freeboard very well, could you tell me the freeboard of the attached pic.
(the hight of the hull is 300 mm in the pic).
secondly, How you can calculate the footprint, is it the area of the base plus the area of each plaining surface or what, because when i do this it gives a very large area (8 m2)

Kareem

Al
4-Dec-10, 07:38 AM
How I understand it is, freeboard is the height above the water of the lowest point of entry that water can enter into the hull, when the craft is floating with no power.

Foot print is the skirt ground contact line when inflated.

I'm sure if I am wrong someone will correct me.

foster2958
4-Dec-10, 11:36 AM
As I mentioned before, the bottom edge of the duct should also be considered when looking at freeboard. Your duct is mounted very low. When floating the craft will start to ship water through the air holes filling the plenum, this will then flood the cockpit through the duct. See attached diagram for suggestions. The duct is also further forward than is common practice, it could move back 200-250mm. The duct is longer than most and could be shortened a bit - say 50mm. The rudders and any other control surfaces may overhang the edge of the craft. Having said this, there are models of craft where the duct is mounted further forwards such as some Vortex Storms and later Pintails. Except for the coments about the duct, the general configuration of the hull looks conventional - the length is about 3.4m according to the dimensions, not 2.9m. This would make it suitable for a 1-2 seater.

The footprint area is defined by the skirt and where it touches the ground - see the attached sketch.

5951

Hope this helps.

Kevin

kemo1988
5-Dec-10, 04:16 PM
Thank you kevin for the comments on the design.

In the mean time i am making some CG and Metacenter calculations, what is the recommended hight of the vertical CG from the base, if you have some recommendation about CG i would be very grateful..

Kareem

foster2958
6-Dec-10, 08:11 AM
Kareem

I've never seen a recommended height for CofG except that lower is better. Design is almost always a compromise between conflicting requirements.

You intend to use a Rotax 503 or 447 with gearbox, so you are limited to where the engine can be placed in relation to the centreline of the fan. The centre of mass due to the engine will be relatively high compared with a belt drive transmission, however many successful craft have been built this way.

Keep the centreline of the crankshaft below the centreline of the gearbox output shaft, and invert the engine so the spark plugs are at the bottom. This way the engine's centre of mass will be below the crankshaft. The gearbox can be fitted either way up, just make sure to swap over the drain plug and the vent.

Kevin

kemo1988
10-Dec-10, 05:21 PM
Hi Kevin, you said earlier that you are using a rule of thumb 125 mm for single seater, can this number increase because in my drawing i calculated the footprint according to 125 mm and it give 4.6 m2, and i am designing on 200 kg weight so it gives about 430 pa which is too low, so i need to decrease the footprint or increase the 125 mm, even if i increase the weight a little bit it is still very low, Can i make it at least 200 mm, you also said that this can be estimated according to the craft dimentions and weight, How?

one thing else, does all the feeding hole have the same diameter?

foster2958
10-Dec-10, 09:52 PM
Can you give us a dimensioned plan to look at? From the dimensions on your previous sketches I estimated your footprint area to be closer to 3.7m2 if the contact point is 125mm in from the edge of the hull, which gives a skirt pressure of 54kg/m2 or 530Pa. Reducing the contact area by 25mm all round would bring this up to 57kg/m2 or 570Pa - a reasonable result. With a second passenger this would increase to around 78kg/m2 or 770Pa.

Air holes can be all the same, the Challenger design in the Constructors Guide uses 75mm holes all round - you'll get almost as many opinions on this as there are hovercraft, however most people seem to accept that with an integrated craft you will use smaller holes in the rear chipbag segments and larger holes to the remainder. Some people increase the holes even more across the front. I wouldn't use holes larger than approximately 50% of the segment width for fear of weakening the hull - think of the perforations on a postage stamp - but you can elongate the holes to create ovals if needed. I use something like 50mm holes across the back and 75mm for the remainder for a small craft like this.

Kevin

kemo1988
11-Dec-10, 08:14 AM
ok, here is my plan, you see that the footprint is calculated using inventor

kemo1988
12-Dec-10, 05:50 PM
Hi Kevin, Did you see the plan, Is this area is correct or what? waiting your respond.

foster2958
13-Dec-10, 08:16 AM
Kareem

Your area appears correct for the dimensions given. Firstly, are you sure your estimated weight is correct? Have you estimated the weight of all the various elements, or just set a target? If you are confident that the weight is correct, then you need to look at the craft dimensions. Is this a single seater or a two seater? For a single seater the cockpit is generously wide, you could easily afford to lose one segment width - I've assumed from your dimensions that you've set the width at 150mm - this would immediately increase the skirt pressure to about 490Pa without effecting the skirt geometry. You could also reduce the length by 1 segment - pressure increases to 525Pa. Moving the skirt contact point in by a further 25mm to about 150mm (instead of 125mm) would further increase the pressure to around 550Pa.

I would be wary of bringing the contact point too far in as this will effect the lateral stability of the craft; the greater the difference between the craft width and the footprint width, the harder it will be to fly level - a bit like balancing on a ball - less stable in side winds and in corners.

Kevin

kemo1988
13-Dec-10, 01:05 PM
well, for the weight i assumed the hull with duct to be 50 kg (will be made of plywood and some fiberglass to reniforce), the engine with fuel 35 kg, the engine frame 10 kg, the fan 5 kg, rudders 5 kg, for one person 100 kg and i added another 10 kg for saftey this makes a 215 kg and it could be less ( for me i am 65 kg not 100 ).

if it is a single seat, what is the normal width of the cockpit (you mean by the width of cockpit is the width of the base right?).

Kareem

foster2958
13-Dec-10, 05:02 PM
Kareem

Some of your weights are very optimistic. Expect the hull and duct to be nearer to 75kg, a Rotax 503 weighs 40kg including carbs, gearbox and exhaust system (more if you add the intake and after silencers), a full 10 litre fuel tank (small for a cruiser) weighs 12kg+, a fan frame for an inverted rotax would be nearer 15kg, you've made no allowance for a duct guard - 7 to 8kg, skirt - 10kg including edge trim and fixings, handlebar and steering mechanism including morse cable - 5kg. Your all-up weight is likely to be much closer to 250kg than 200kg I'm afraid.

Cockpit width of 750 - 800mm is more than adequate.

Kevin

kemo1988
14-Dec-10, 11:32 PM
I have a problem now with the lower skirt attachment points, what i know is that the width of the skirt should be the same for the upper and lower attachments, but for the smaller plaining surface which has 3 & 5 segments it is impossible for the lower width to be the same as the upper, so i think the geometry of thease segments will differ, because if the lower width of the skirt is less than the upper, then the Perpendicular length on the plaining surface of the skirt (SR) will be larger than the upper part., Is this normal or what?

I put the lower attachments in the middle between each two feeding holes, but seems to be something wrong (please look at the attch).

Kareem

foster2958
15-Dec-10, 08:45 AM
You need to space the lower attachments equally across the reduced width, see the attachment:

5956

It's quite normal for the lower attachments to be closer together around the corners. Any slight differences in the length will be taken up with the cable tie, it only comes to a millimetre or two - insignificant really. There is no need to adjust the geometry of the corner segments.

Kevin

kemo1988
17-Dec-10, 12:41 PM
Any slight differences in the length will be taken up with the cable tie


I am sorry i dont know what a cable tie is, what it's function, Do you have a picture of it?

Winst
17-Dec-10, 12:51 PM
I am sorry i dont what a cable tie is, what it's function, Do you have a picture of it?
Have a look.
http://cgi.ebay.co.uk/100-x-large-370-x-4-8-Black-Cable-Ties-tie-wraps-zip-/250733637004?pt=UK_BOI_Electrical_Components_Suppl ies_ET&hash=item3a60e3ad8c

foster2958
17-Dec-10, 02:41 PM
Electrical cable ties are used to connect the skirt to the lower attachment point, see the attached picture:

5959

Hope this makes it clearer. The attachment point could be a loop of nylon rope or a p-clip riveted to the planing surface. The skirt will need a loop of webbing sewn to the bottom corners to make the attachment.

Kevin

kemo1988
19-Dec-10, 09:41 PM
Do you have the constructors guide, because mine is just arrived this morning and i want to ask you about something in the skirt design.

foster2958
20-Dec-10, 08:55 AM
Yes, I've got a copy - what's the query?

Kevin

kemo1988
20-Dec-10, 08:49 PM
on page 32, the pattern developed has a straight horizontal line at the bottom, Does this will make allowance for the shape of the inflated skirt to make a straight line and a curve?, if you remember my first pattern and you told me that this pattern didnt make any allowance to form a straight line and a curve, it is the same in the guide. I thought to make the same curve as i did befor when i told you that QR is less than QA and that what causes the curve, it is the same here as the inflated segment has half a circle which it's dia. is equal to the segment width as befor, right or what?...

foster2958
21-Dec-10, 09:44 AM
Look at the next page - ground contact seal, figures S3 and S4 - I've never made a skirt to this pattern so cannot comment on it's effectiveness. Personally I've never bothered to do this on any of the skirts I've designed or made. The very slight curve at this point on my pattern is not to increase ground contact, it's a result of simple geometry.

Having just spent a few minutes trying to understand the pattern and how it is constructed, I don't agree with how the cross section is translated into the developed pattern. This also treats the segment as a square sided box and ignores the effect of internal pressure making the material take up a curve. I believe this has the effect of making the bottom edge of the pattern too long by a factor of 2W-(Pi x W/2) where W is the segment width. How this effects the rest of the pattern I couldn't say.

Kevin

kemo1988
30-Dec-10, 11:04 AM
sorry i am very busy now in building the craft.

you are right , he treats the segment as square, XBX must equal to 0.5 * Pi * width not the width right?

you know one of the reasons i bought this guid is to get the design of the extended type and now there is a problem in the design, i thought to make it as befor and take the curved design only from the guid , can it work or what, or you have any other suggestions..

Kareem

foster2958
30-Dec-10, 11:21 AM
I don't have the time to try and draw it up at the moment, but you're part way there. XBX would need to be (Pi x W/2) - W.

I'll try to find some time to re-draw the diagrams from the Constructor's Guide as I think they should go and post them up in the next few days.

If you try to mix & match elements from the two skirt designs you're likely to end up with even more problems.

If anyone with more experience of designing this style of segment could share their expertise I'd be grateful.

Kevin

yamah
2-Jan-11, 04:30 AM
Hi Kareem,

1. good luck with your engineering graduation project, this group is more geared toward practical use and encouragement to use the knowledge here to fulfil practical applications,

2. The P20 Pointu Tornado plans you have are probably the best set of "free" off the internet plans you will ever get, but as Kevin pointed out earlier, the duct is too long and too low by today's standards and from a safety water ingress aspect. Changing those will give you a very nice craft, see the vids on youtube, note it requires a ~50hp lightweight engine to meet expectations. The skirt that is depicted on those plans works for that hovercraft.

3. The skirt on the tornado model hovercraft you have plans for is for use on the model only and makes numerous short cuts that do not necessarily translate to full size craft.

4. A Light Hovercraft is a "weight-shift", thrust, and rudder controlled vehicle, CG position is very much actively compensated by pilot position, pressure ratio of the fan is a function of what the craft requires in the way of fine pitch/coarse pitch to get off the ground and go forward. It could also be equated as pressure ratio/volume ratio.

5. My thoughts on the Constructors Guide Skirts are that the round ends of the skirt segments where they touch ground will soon become worn flat to the ground and provide a better ground seal than the gaps shown between each as shown in the photos. The Constructors Guide craft is designed to be easy to build and fly for beginners, it is not high tech by today's standards, appreciate it for what it is.

6. The 13hp engine and 80cm 5 blade fan you have are more suited to a UH 12 or a Sevtec Scout where it will adequately cover the power requirements. The integrated euro type hulls tend to require 25hp and up to achieve respectable performance.

7. Skirt design is something that quite a few here are passionate about and I hope you pass on the generous knowledge gained here to others in the future. Also, the various crafts mentioned pretty much have their own skirt type and interchangeability is not possible without sacrifices. Having said that, the best way for a young man to enjoy flying a home built hovercraft is to stick precisely to an existing plan of a craft that has the performance level that is suitable to the required application. But that does not necessarily satisfy the graduate project intimate knowledge of the processes that are happening, I hope that in building a craft, you will be able to see the parts to calculate the processes so you can make a highly discerned choice for your next craft build. To graduate, you need to know how to use the knowledge, not necessarily to have excelled in applying it, especially in a first build situation.

A publication you may find technical enough to appease the most pedantic tutor would be the "Light Hovercraft Design Handbook" compiled by the Department of Land Transport of the Landsborough University of Technology in 1969.

It may not hand you with the best skirt design for your own design hovercraft, that will come with experience and trial and error combined with the theory to crunch to smitherines to know why it works.

The object of building your own is to become master of the materials so you can REPLACE any part of the craft AT WILL and that will happen a lot as everyone here knows only too well after bumping into something. Hovercraft have to be built like aircraft rather than tanks if they are to fly well.

Kareem, what you have done by making this topic, is pointed out that there is a need for a newer design of lightweight sport hovercraft incorporating all the developments made since the moto-cyclone design, that is suitable for the first time home builder.

references, UH-12, Sevtec Scout, Integrated Euro type, P20 Tornado Pointu.

Plan sets that are available from their distributors, ASV Wedge/Superwedge/Viper. UH Hovercraft, Sevtec Hovercraft, HCGB Constructors Guide Moto-Cyclone, Tornado Pointu,

any more in to add guys ???



sorry i am very busy now in building the craft.

you are right , he treats the segment as square, XBX must equal to 0.5 * Pi * width not the width right?

you know one of the reasons i bought this guid is to get the design of the extended type and now there is a problem in the design, i thought to make it as befor and take the curved design only from the guid , can it work or what, or you have any other suggestions..

Kareem

foster2958
2-Jan-11, 04:17 PM
Hi Kareem & Happy New Year

I've drawn up a section & plan of a segment which matches your hull dimensions. I made it up as a paper model and it looks OK - see what you think.

5967
5966

The instructions in the Constructors Guide show the arcs which form the rounded top of the segment constructed on an equilateral triangle - 60 degree included angle - however this isn't possible on this layout; the arcs need to meet at point Z without overlapping, so I've reduced the included angle to 45 degrees. This has the effect of making the skirt slightly less bulbous. It may be a coincidence but the angle XZX is also 45 degrees; I don't believe in coincidences and suspect that this angle will be a limiting factor in any such design and am tempted to explore this further when I've got nothing better to do.

I'm not making any promises about it's efficiency as a skirt, but you're welcome to try it out if you wish. The PDF drawings are full size, so you could print the plan at A0 (do not use the print to fit setting) and use the drawing as a template. If you made up a couple of paper segments and offered them up to the hull you could see how it works. I'd suggest sewing webbing tabs to point S on the finished article - allow about 20mm past point S. This will allow a loop of cable tie about 2-3 fingers width between the tab and the attachment point. The edge S-X wants to be snug to the planing surface but not pulled tight as a bow-string.

I'd be interested to see any pictures of progress with the hull and of the skirt when it's done. Pictures or videos of your maiden voyage would be great too.

Good luck

Kevin

kemo1988
4-Jan-11, 08:36 AM
Hi Kevin, thank you very much for your effort, i really appreciate your help.

i said earlier that i almost finish the hull, but there were some problems, so here what i have done so far in the hull ( cant wait to hear your comments, and dont forget it is my first one), i will attach more with every progress.

Kareem

foster2958
4-Jan-11, 08:47 AM
Hi Kareem

Looks very tidy, you've made a lot of progress. Can I ask how thick is the plywood that you have used? It looks very substantial, we use 4mm generally and 6mm for the floor.

Kevin

kemo1988
4-Jan-11, 08:56 AM
Oh yes i realized that from the guid, but it arrived after i started, i was very concern about the weight so i used a 12 mm thick and i hope the hull exceeds 50 kg without the duct.

foster2958
4-Jan-11, 09:17 AM
My class is building a similar sized craft from 4 & 6mm ply and we forecast a weight of approximately 75kg - yours will be at least double that! 12mm ply weighs about 8kg/m2 - that's 24kg per sheet, and it takes roughly 6 sheets to build a hull. I'm afraid you've built a tank - it will be robust but performance will be severely compromised.

Kevin

kemo1988
4-Jan-11, 08:57 PM
Hi Kevin, i want to ask you about the protective coating for the plenum chamber from the inside when it is floaded by the water, Can i paint it with polyster resin only without putting any fibers?

foster2958
4-Jan-11, 09:57 PM
You could use polyester resin, but it's probably easier (and cheaper) to use a polyurethane varnish or a good oil based paint - something like an external aluminium wood primer which is suitable for hardwoods. You only need to seal the surface to prevent water ingress, not get a decorative finish. Make sure you allow any internal finishes to properly cure before pouring in any foam bouyancy and don't forget to seal the inside face of the top deck before fixing but mask off any contact faces so that your adhesive can bond with the wood fibres.

Kevin

gaz
5-Jan-11, 06:52 AM
I dont know if it was in your plans, but you should resin on a strip (maybe 2 at the front) of 50mm wide woven roving tape over both sides of EVERY joint. Just glueing them wont be enough.

kemo1988
9-Jan-11, 07:51 PM
More progress.....

atters
10-Jan-11, 07:04 AM
Listen to GAZ, you must put that resin strip in, mine fell apart at the seams and also sank because of this small detail. Are you going to fill the compartment with foam? if not cut a few drain holes in it so that it can empty once full, with no sealing of the joints like GAZ said it will leak.

Craft looks good, keep up the good work and get a proper duct and drive system, stick to the plans.

Enjoy

yamah
13-Jan-11, 10:52 AM
Hi Kareem,

I suggest you make the thrust duct so that it can easily be changed over to your next hull, whatever shape it is. This hull has about the equivalent of one person in extra plywood weight as you know and that means your next hull is closer on the horizon than it need be due to performance losses.

I am also thinking that the ground contact point on the skirts will be moved out to the max to part compensate for this.

Let us know how you are planning on powering and thrusting this craft and what your options are so you don't waste any more resources on it.

It's good to see your progress and to know you have the building skills to go forwards on the project.

kemo1988
14-Jan-11, 08:45 AM
thanks guys for the help, For the compartment i didnt pour in any foam, i sealed it very well with a thick coat of polyester resin from inside and outside so it can act like an air bag, i know that its reliability is low with general use, any crack could happen will allow water to fill the compartment.

For all the joints, i sealed it with fiber glass

For the thrust duct i made it fiber glass and i will release the moulding tomorow.

The current available engine and fan which i am going to test the hull and skirt with is 13 HP engine and a 710mm fan which i got from my old graduation project, and because the small engine and fan i will make for now the splitter plate takes half of the air ( this only to test the craft ).

For my plans of thrusting the craft, as i said, a rotax 447 or higher and a fan not less than 90cm dia. (i havent done any thrust calculation yet) which i havent got yet.

Kareem`

edenfeldt
14-Jan-11, 08:54 AM
Hi.

This is my first post on this forum. This drawing of the skirt is one of the best i have found. I have a BBV 2+2 with old skirts and have a role of fabric that i intend to make a full set of skirts from. I am not satisfied with the size of the skirt since i dont get enough lift height so i intend to make them higher. I can use one old skirt and just make a template of this but i would like to know more about how you made the drawing and how you made the calculation of hovering height. I am aiming for a hovering height of about 25 cm and today i have maybe 20 probably less.

Anny help i could get is appreciated.

Here is some pictures of my craft from last winter. The original 503 went up in smoke last april and i have now fitted a rotax 670 and a radiator i will send in some pictures of this as soon as i can.

http://3.bp.blogspot.com/_FKixw--5NTI/S5KWSk1YlyI/AAAAAAAAAX0/wWGLwn5vAqw/s1600/CSC_5163.JPG
http://4.bp.blogspot.com/_FKixw--5NTI/S5KTJcwyVgI/AAAAAAAAAXc/W25P-vX4cr8/s1600-h/CSC_5164.JPG
http://2.bp.blogspot.com/_FKixw--5NTI/S5KXGYzc4NI/AAAAAAAAAX8/JzH-QDYz9Jw/s1600-h/CSC_5161.JPG

Regards

Mats



Hi Kareem & Happy New Year

I've drawn up a section & plan of a segment which matches your hull dimensions. I made it up as a paper model and it looks OK - see what you think.

5967
5966

The instructions in the Constructors Guide show the arcs which form the rounded top of the segment constructed on an equilateral triangle - 60 degree included angle - however this isn't possible on this layout; the arcs need to meet at point Z without overlapping, so I've reduced the included angle to 45 degrees. This has the effect of making the skirt slightly less bulbous. It may be a coincidence but the angle XZX is also 45 degrees; I don't believe in coincidences and suspect that this angle will be a limiting factor in any such design and am tempted to explore this further when I've got nothing better to do.

I'm not making any promises about it's efficiency as a skirt, but you're welcome to try it out if you wish. The PDF drawings are full size, so you could print the plan at A0 (do not use the print to fit setting) and use the drawing as a template. If you made up a couple of paper segments and offered them up to the hull you could see how it works. I'd suggest sewing webbing tabs to point S on the finished article - allow about 20mm past point S. This will allow a loop of cable tie about 2-3 fingers width between the tab and the attachment point. The edge S-X wants to be snug to the planing surface but not pulled tight as a bow-string.

I'd be interested to see any pictures of progress with the hull and of the skirt when it's done. Pictures or videos of your maiden voyage would be great too.

Good luck

Kevin

kemo1988
14-Jan-11, 10:38 AM
i dont get enough lift height so i intend to make them higher.


Hi Mats

what do you mean by not get enough lift hight, Does the skirt is fully inflated with its whole hover hight or what ,any skirt design with a certain hover hight must be fully inflated if the pressure is well trapped, i think you are lossing pressure here.

Kareem

Jon Pert
14-Jan-11, 12:40 PM
Hi.

This is my first post on this forum. This drawing of the skirt is one of the best i have found. I have a BBV 2+2 with old skirts and have a role of fabric that i intend to make a full set of skirts from. I am not satisfied with the size of the skirt since i dont get enough lift height so i intend to make them higher. I can use one old skirt and just make a template of this but i would like to know more about how you made the drawing and how you made the calculation of hovering height. I am aiming for a hovering height of about 25 cm and today i have maybe 20 probably less.

Anny help i could get is appreciated.

Here is some pictures of my craft from last winter. The original 503 went up in smoke last april and i have now fitted a rotax 670 and a radiator i will send in some pictures of this as soon as i can.

http://3.bp.blogspot.com/_FKixw--5NTI/S5KWSk1YlyI/AAAAAAAAAX0/wWGLwn5vAqw/s1600/CSC_5163.JPG
http://4.bp.blogspot.com/_FKixw--5NTI/S5KTJcwyVgI/AAAAAAAAAXc/W25P-vX4cr8/s1600-h/CSC_5164.JPG
http://2.bp.blogspot.com/_FKixw--5NTI/S5KXGYzc4NI/AAAAAAAAAX8/JzH-QDYz9Jw/s1600-h/CSC_5161.JPG

Regards

Mats

Have you spoken to Magnus Ivanoff??

kemo1988
20-Jan-11, 03:51 PM
Hi Kevin

i just want you to see the progress

i have now completed the duct, the frame and the buttom of the hull is now well sealed and protected with two coat of polyster resin.

I need to know more about the extra millemeters you add to the segment width, will i stretch it to its full length or what, and the next one will it be right next to the previous or it will overlap the previous.

Kareem

foster2958
20-Jan-11, 04:01 PM
Just carefully & accurately mark out the skirt spacings on the edge of the hull and fit each segment up to the marks so they butt closely to the adjacent segment - fit all the segments to the top edge before starting to connect to the lower attachments and don't pull the cable ties too tight.

Kevin

kemo1988
27-Feb-11, 07:33 PM
Hi Kevin, it's been a long time scince my last post, i am sure you knew what happened here in egypt.

Any way finally i finshed the craft, the skirt i made is the first type(not the curved) and i will try the design in the guide and yours in a couple of segments. The craft works fine and like i said i am using for now a 13 hp engine and a 710mm fan to make my trials on various skirt designs, and according to multiwing optimiser the fan gives about 4m3/s, so i made the splitter takes half the air to get successful lift, and i will get later more powerful engine.(of course the splitter is too high and looks very bad)

I havent shoot any videos yet, but here is some pics, if you have any comment please tell me.
PS: NOT ME IN THE PIC.
Kareem

Winst
28-Feb-11, 07:10 AM
Looking good Kemo,
I would say your splitter plate in the duct is to high and I have to ask..............
is your hovercraft on a roof? How are you going to get it down?

foster2958
28-Feb-11, 07:38 AM
Hi Kareem

Good to hear from you again. You've made very good progress - the craft looks like it sits right up on the toes of the skirt so it should hover well. Speed will obviously be limited with so little air going to thrust, but as a working testbed for your skirt designs it's ideal. As you say, with a better engine in the future and modifications to the duct and splitter it should go reasonably well.

You must be fairly unique in Egypt, as I've never heard of anyone else in the hovercraft community in your region. What gave you the interest in hovercraft in the first place?

Kevin

kemo1988
28-Feb-11, 10:58 AM
How are you going to get it down?

By ropes....., OMG i know it seems scary but this is the fact.

kemo1988
28-Feb-11, 11:16 AM
What gave you the interest in hovercraft in the first place?


My graduation project was a Hovercraft under the category of Fluid Mechanics, and it was just an implementation of the hovercraft theory, we started by doing some small models using garbage bags as a skirt and powered by a leaf blower, we didnt have any backround to havercrafts and all we did was a sheet of wood with engine and fan fixed on it with lift and thrust calculations. After graduation me and two of my friends was very interested to this project and were not satisfied with the results, we wanted to build a real one with finger skirt, we started with the model which is in the begining of this thread, and now i have built a real one.

Kareem

Jon Pert
28-Feb-11, 11:33 AM
Hi Kareem,

Completely off topic but how is it in Egypt now??

We are having our honeymoon there at the beginning of April. One week cruise on the Red Sea and one week stay in a hotel in Sharm.

Anything we should be careful of?

datkins
28-Feb-11, 09:45 PM
Seems watching out for hover craft coming off the roof would be a good start :) I had wondered myself if it was a roof, good luck

Jon Pert
1-Mar-11, 05:16 PM
I wonder if I can get my craft over there in hand luggage... http://www.jonrb.com/emoticons/scratchchin.gif

kemo1988
9-Mar-11, 11:29 AM
Hi Kevin

The craft weights 72 kg with duct and seat, frame 9 kg, engine 35 kg, fan 3kg, total 119 kg, with driver 185 kg, and for a 3.9 footprint gives a 471.8 pa.(for 65kg driver), this varies to 521 pa (from 65kg to 85kg driver).

I have made a test on the beach in a very strong wind and it was very difficult to control the craft with the little thrust i have got, even the craft still steady in its place in the opposite direction of the wind.

The attached pic has the exact shape of the skirt when infalted, and you can see that triangle opening which gives a jet of air making the splash of water very huge even the sand on the beach.

Kareem

kemo1988
14-Mar-11, 07:44 PM
Hi Kevin

i am now thinking of changing the engine and fan, i have found an alternative for the rotax here in egypt which is the yamaha fzr 250 and it is rated to 45 hp @ 14500 rpm. what concerns me is the rpm, it is very high and the transmition ratio will be about 1:6, now i have a question what is the max rpm of 5Z multi wing blades (PPG) and what is the best fan to suit this engine.

Kareem

yamah
15-Mar-11, 12:36 AM
Good going on playing at the beach Kareem,

You can download the Multi-Wing Optimiser 7 software from http://www.multi-wing.net/downloads/optimiser/

Blade material (PAG) and hub diameter play a large part in blade loading which then determines fan rpm.

A quick look through came up 920/6-6/32.5*/PAG/5ZL @ 2800 rpm 16m3s 956pa 30kw 71%eff.

Hascon fans (Italian) are stiffer and give higher performance figures.

I would think the fzr250 has extra weight to carry in comparison with 2 strokes for the same output. Keep looking/saving for power plants is my suggestion while you get this craft running at it's best. The craft you have cannot really afford to get much heavier. I suggest taking some scales to the bike wreckers and get the weight for reference.

You may need a cowl and duct to cool a 'free-air' cooled 2 stroke, water cooled is better. If you can find a tzr250 or ~350LC it would be older but probably lighter and there are parts available from K&M to make them suitable for belt drives straight out the RHS of the motor which means the gearbox can be cut off completely.

yamah

foster2958
15-Mar-11, 09:16 AM
Hi Kareem

You do pay a weight penalty with 4-strokes, however you probably gain on price and availability, fuel economy, reliability and noise (depending on how well you silence it), you also get electric start.

The FZR 250 has been used to good effect before, and one is used regularly in the UK F3 race series. After 1991 they only put out 40HP so a 1988 model would be better. If you keep the gearbox intact and take power from the gearbox output shaft you can use the reduction ratios built into the engine. Ideally keep to the higher gears as this puts less stress on the gearbox. Primary reduction is 2.542 and 6th gear is 1.173 giving an overall reduction of 2.982, so at 14500rpm the output shaft is rotating at 4863rpm. You will then need to select pulley sizes to give (as described above) about 2800rpm at the fan.

Multiwing or Hascon fans should not be run with a tip speed of more than 168m/s, however for noise considerations keep it as low as possible - max. 135m/s and under 115m/s would be better. At the end of the day it's a balance between good lift, noise and good thrust.

With increased power you should be able to drop the splitter plate to about 200mm above the bottom of the duct, vastly increasing the volume of thrust air and improving speed and steering significantly.

Kevin

kemo1988
15-Mar-11, 09:18 AM
thanks Yamah, the craft's weight is not too much as you think it is only 72 kg without engine and frame, and the pressure will be around 500-520 Pa.

I have already got the optimiser, but the dealer for multi wing here doesnt have PAG only PPG and AL,
so how about this one: 1000/6-6/45/PPG/5Zl gives @ 2000 RPM & 500 Pa 20m3/s, 36 HP.

Kareem

yamah
15-Mar-11, 12:33 PM
Hi Kareem,

The other guys here might like to add to or qualify or confirm this, you lose around 400-500 pa just getting the lift air through the perimeter duct and perimeter holes BEFORE you get to the 500 or so pa required to support the hull so add those together and the pa needs to be up around 1000pa at the fan. That is probably only practically quantifiable by actual performance and pitch experimentation on a running craft. A bag skirt craft has far smaller losses.

They might like to qualify this one as well. The difference in performance with the Hascon fans over the Multi-Wing is great enough to warrant obtaining Hascon as an original purchase item. Conrad (Hascon Distributor in UK and member here) can work out the spec for a fee and deduct it from the Fan purchase price if you buy from him and you are assured of having the correct match.

The main difference with the Hascon blades is they are much stiffer and do not deform at high rpm as much as the Multi wing blades.

The five blade fan on a vertical shaft engine would make a nice lift fan for a slightly larger craft :D

yamah

kemo1988
15-Mar-11, 09:42 PM
now i am a little confused, Yamah said that the losses will be around 500 Pa which i think is high, the fan i select which is 1000/6-6/45 gives 20 m3/s @ 500 Pa @ 2000 if i add another 500 Pa the flowrate drops to 10 m3/s, but if add a 200 Pa losses gives 17 m3/s @ 700 Pa a resonable number, what makes this problem is the PPG material. Can the losses be from 400-500 Pa ? and where can i get 5Z PAG blades online?

I made lift calculations on 1 cm hovergap and a 1.8 safty factor which gave 4 m3/s (is this volume is high or low), and for a 17m3/s, deviding 4/17 gives 23.5% of the total air for lift, so 0.785(fan area)*0.235 gives a 18.5% of the fan area. Are these steps right or what, and what hight of spliter to achieve this percentage?

Kareem

kemo1988
17-Mar-11, 06:37 PM
Hi Kevin

I need to know if my calculations are correct, i think that 4m3/s is high or what, the fan is 1000mm dia. and as i said i need 18.5 % of the total air for lift ( idont know if it is right also or not ) and to achieve this i calculated a 23.5 cm splitter plate hight and i think it is high also, i think experience is better than calculations, Please need your Help..

Kareem

Paul Fitz
18-Mar-11, 12:06 AM
now i am a little confused, ........Kareem

I'm not sure where Yamaha acquired his information, it could be he is confusing Static pressure and Total pressure losses.

Static pressure losses from the fan to the cushion are relatively small (about 10%). Providing you use your calculator set to Static pressure, your calculations will be OK I recommend you stick with 6 blades.
From reading 'some' of your previous posts it appears that your main problem was putting too high a volume of air to the cushion. I suggest that you make the splitter plate adjustable if possible. 17-20 % area should be adequate but it depends on the type of surface being traversed.

If you PM me with an email address I can send you an Excel calculator to help calculate the required fan duty. 3-4 m3/sec is about right for your craft on most surfaces with a segmented skirt.

HTH
Paul Fitz

foster2958
18-Mar-11, 09:19 AM
Kareem

Take as much advice as you can from Paul and follow it - I've done so in the past when I've had problems and always been happy with the outcome.

An adjustable splitter is very useful. It allows you to achieve earlier lift and low speed hover when set high, and to optimise thrust at high fan speeds when set low. At high fan speeds you will have more air than needed for lift so a lower splitter height can be used, but if it was always set low you wouldn't get lift-off so easily and getting over hump from a static start on water would be that much harder. Your minimum speed over water would also be much higher. An adjustable splitter is the closest you'll get to changing gear in a hovercraft.

Kevin

kemo1988
18-Mar-11, 06:53 PM
Here is what i have been thinking, first a 4m3/s is needed so i made the splitter take this volume at max rpm of the fan thus the correct amount will be taken at the max not more, but what happen if the rpm is decreased, the pressure remains but less flowrate resulting in less hovergap..

Lets start from the begining, and with assuming linear relation between power and speed for the engine(which is very close to linear), and with optimiser and a 18.5% for lift.

1- @900 rpm, the fan is not able to produce a 600 Pa which means the craft still on the ground.

2- @1100 rpm, the fan starts to generate 600 Pa but with a flowrate of 0.6m3/s, so a 0.11m3/s is going to lift, but befor deciding that the fan will give these numbers we need to look at the power for both fan and engine, for the fan at this point it will take 8.6 HP and the engine will be running at 7975 rpm, so we need to check if the engine is developing more than 8.6 HP or not, assuming linear relation between power and speed the engine gives 24 HP, so the fan will give 0.6 m3/s @ 600 Pa, resulting in lifting the craft but there is no hovergap and the craft will not move.

In all other conditions the engine power exceeds the fan power which means that the fan will give this condition.

3- @1300 rpm, the fan gives 600 Pa and 4.8m3/s, so a 0.88m3/s for lift which is not enough to give hovergap, so the craft is still lifted but not moved.

4- @1500 rpm, the fan gives 600 Pa and 9.5m3/s. so a 1.75m3/s for lift which i think is enough to start a small hovergab and the craft starts to move slowly.

5- this continues untill 2000 rpm which the fan will give @ 600 Pa 20m3/s and a 3.7m3/s for lift.

So the craft start to lift @ 1100 rpm ( 7975 rpm for engine ), and start to move forward @ 1500 rpm ( 10875 rpm for engine ) and the craft take its full volume of lift and reach its top speed @ 2000 rpm ( 14500 rpm for engine ).

To complete this estimations and decide at what rpm the craft start to move we need to plot a relation between the drag force exerted by the skirt which is determined by the hovergap corresponding to rpms and the fan thrust corresponding to rpms.

As for a variable spliter, let me for now use a fixed one as i dont know how to make it, i will ask you at its time:D...

If there is any thing wrong in my thoughts Please tell Me.

Kareem

kemo1988
20-Mar-11, 09:07 PM
I want to know how to determine the length of the duct as it must carfully choosen to achieve best efficiency, it is my first hovercraft so i dont know the proper length, and also how many rudders to use with a 1m fan and its length

Kareem

Al
21-Mar-11, 06:10 AM
I don't know for certain, but I heard somewhere it is about 60% of the duct diametre. 1000 cm dia = 60 cm length.

I can't remember where I heard that, so don't take it as 100% reliable. I'm sure someone will either back this up or put me right

It's probably more to do with looking correct rather than function.

yamah
21-Mar-11, 10:50 AM
Hi Kareem,

Good to see you working it all out so thoroughly.

I'm very happy that those with greater knowledge were able to put the static losses between the fan and the ground right when there are so many variables and other experiences that get put around, hence the suggestion that it would be contributed to, Thank you Paul.

In response to your wanting the duct length correct for greatest efficiency, given that you are likely to end up with MW PPG material after knowing the difference MW PAG material can provide, you may find even greater benefits in selecting your fan. btw, I have the recommended duct length as being 40% of internal diameter with 4 degrees per side divergence to the rear. Fan 1/3 distance in from the front, inlet diameter 1.2 times internal diam. can't say where it came from, let the shooting begin.

Here is a quoted section from another Hovercraft forum where fan performance is in discussion.

From the USA Hoverally site, I sincerely hope they do not mind it appearing here and full credit is given to Hovermaster for taking the time to do this exercise and write it up. (see I've dodged responsibility for the data seeing as it will be shot down if it can be lol.) so long as it gets the discussion forthcoming all is good.

The test craft in this case is sized similarly to yours only it is a ~26hp on a good day Rotax 277 single cylinder.

"When comparing fans or props, the only way to do it properly is side my side testing on the same day, on the same craft, at the same location. This is very time consuming, so not many people do it.

Hence, we have a huge population of performance claims, but very little performance data.

Here is a summary of Hascon test data I've performed on stock SCAT hovercraft. Each test was performed on the same craft one after another with about 10 minutes between tests.

Scat 1 277 - Stock Multiwing setup with stators - 70lbs thrust
Scat 1 Rotax 277 - Stock Multiwing setup NO stators - 60lbs thrust
Scat 1 Rotax 277 - Hascon no stators - 90lbs thrust - 28.6% gain vs stock setup, 50% gain no stator vs no stator
Scat 1 Rotax 277 - Hascon setup with stators - 80lbs thrust

This proves what we all already know about stators. They work well when tuned for a specific fan/engine combo. But as soon as you change anything they can do more harm than good. What is interesting is the HUGE thrust difference between the apples to apples comparison of no stator MW vs no stator Hascon. I ran this test several times because as first, I was convinced I must have made a mistake somewhere. No mistake, the difference was that huge."

Paul Fitz
23-Mar-11, 10:53 PM
...Snip.....the fan gives 600 Pa and 9.5m3/s. so a 1.75m3/s for lift.........

To complete this estimations and decide at what rpm the craft start to move we need to plot a relation between the drag force exerted by the skirt which is determined by the hovergap corresponding to rpms and the fan thrust corresponding to rpms.

As for a variable spliter, let me for now use a fixed one as i dont know how to make it, i will ask you at its time:D...

If there is any thing wrong in my thoughts Please tell Me.

Kareem

I am not sure how you have arrived at the 18.5 % figure of 1.75. The power absorption in the lift area differs from that in the thrust area and so must be estimated seperately and added together.

When considering lift from an integrated fan you should use the manufacturers software with the fan static pressure set to the required fan duty static pressure (or cushion pressure +10% nominal) to calculate the whole fan volume at that pressure. the lift will be the 18.5% (or lift area %) of this figure (Also note the power absorption). HOWEVER........ you must then calculate the thrust for this fan at the SAME pitch but at 50-100 Pascals (nominal), (again noting the power absorption) deduct the lift area, add the power figures and check that the overall power required is available to you.

It is unlikely (and unnecessary) that you need to calculate the drag at low speed. Look at the article "Calculation of Drag...." on the Downloads Page. In that article is a typical drag curve for a small hovercraft. You will note that at 5 mph and below the skirt drag is virtually zero on a smooth surface. This changes depending upon the type of surface e.g Long course grass would be high drag. The skirt drag is dependent on the surface conditions not the hovergap unless the hovergap is too small because of insufficient volume flow from the cushion.

The hovergap should be part of the craft design criteria, used when originally designing the cushion performance. It is a common mistake to start a design with too small or too large a hovergap leading to the wrong fan selection. For a 3 mtr craft an average gap of 13mm is usually sufficient. When in flight the gap under the front skirt will be about 25mm and under the rear 0mm (given correct balance) to give sufficient drag for steerage. if more air is fed to the cushion the gap will increase, less air and it will reduce giving more drag and possibly (if too little air) a plough-in.

As the fan characteristic has to be sufficient to provide an occasional increase in pressure (eg. from the effect of gravity when 'jumping off of a bank' and landing) it is normal that the hovergap is more likely to increase than decrease if the correct fan is chosen.

Paul Fitz

Paul Fitz
23-Mar-11, 11:25 PM
This proves what we all already know about stators. They work well when tuned for a specific fan/engine combo. But as soon as you change anything they can do more harm than good."

I suspect that these figure are a little misleading. Stators should be designed to work with a specific fan, at a specific speed and pitch angle. They need to be of aerofoil section, and have twist to match the blades.

I believe that stock scat fans use the pitch pins so are not necessarily set up to absorb all the power at peek engine speed. Because the MW and Hascon blades have different stiffness, even if identical in shape they will absorb different amounts of power at the same engine/fan speed and pitch. If one fan absorbs more power than the other it would be necessary to de-pitch it comparatively to attain exactly the same engine/fan speed with the other. If such an adjustment was not made these figures would render a greater disparity, which I believe they do. As any Racer will tell you just 0.5 mm of difference in the position of the trailing edge of a fan can make an enormous difference in engine speed.

This apart, on a racing craft if designed well and installed well, stators can give a very useful increase in maximum thrust by recovering some thrust lost in swirl. On a cruising craft they are useful to stop the unenlightened who are wanting a breeze in their face, losing their nose when they stick their head in the fan ;o)

kemo1988
24-Mar-11, 08:14 PM
When considering lift from an integrated fan you should use the manufacturers software with the fan static pressure set to the required fan duty static pressure (or cushion pressure +10% nominal) to calculate the whole fan volume at that pressure. the lift will be the 18.5% (or lift area %) of this figure (Also note the power absorption). HOWEVER........ you must then calculate the thrust for this fan at the SAME pitch but at 50-100 Pascals (nominal), (again noting the power absorption) deduct the lift area, add the power figures and check that the overall power required is available to you.


you came to the point which exactly what i wanted to ask, and you made me more confused..

now the fan i selected according to my understanding is 1000/8-8/ppg @2000rpm

first the lift: as you said the fan now is set to 600 Pa @ 2000 rpm and it give 20 m3 and take 39.2 HP, this is the whole volume right, so 20*0.185 = 3.7 m3 for lift.

now to calculate the thrust : @ 100 Pa 2000 rpm the fan gives 23.8 m3 and take 34 HP,..... then what

kemo1988
24-Mar-11, 08:23 PM
When in flight the gap under the front skirt will be about 25mm and under the rear 0mm (given correct balance) to give sufficient drag for steerage. if more air is fed to the cushion the gap will increase, less air and it will reduce giving more drag and possibly (if too little air) a plough-in.

Paul Fitz

I dont actually understand how the front skirt will have 25mm hovergap and the rear 0mm, what i know is to make the craft steer you need to make for the craft at the front an axis to rotate around, and to do that you need more drag at the front than at the rear.

Paul Fitz
24-Mar-11, 10:01 PM
then what

Using Multi-wing data - set 1000/8-8/5Z using the AMCA B test method, Using your initial figures of 20m3/sec and 40HP, I get a working point between 40 deg and 45 degrees. For clarity I moved the working point to the 40 degree blade angle and get:-

1000/8-8/5Z/40 deg
19m3/sec @603Pa 40.1 HP 60% eff ---- 19x0.185= 3.51m3/sec Lift --- 40.1x0.185 = 7.4 HP

reset to 100Pa but stay on the same 40 degree curve.

23,2m3/sec @ 99.9Pa 34.6HP 55% eff ---- 23,2x0.815= 18.9m3/sec Thrust ---- 34.6x0.815= 35.6 HP

therefore total power absorption = 35.6 + 7.4 = 35.6HP

(0.815 being the % thrust area of the fan)

Figures will vary slightly between calculators and the results are somewhat crude as they do not take into account turbulence around the splitter plate. The distance between the splitter and the rear of the fan can also affect the result, as air 'bleeds' around the leading edge of the splitter. It is however the easiest method of obtaining a reasonable estimate of the overall power absorption for a given splitter position.

Paul Fitz

Paul Fitz
24-Mar-11, 10:24 PM
I dont actually understand how the front skirt will have 25mm hovergap and the rear 0mm, what i know is to make the craft steer you need to make for the craft at the front an axis to rotate around, and to do that you need more drag at the front than at the rear.

You are correct. However if the craft is completely clear of the surface with virtually no skirt or wave drag at speed it will be difficult to steer, as every little movement of the rudder would impart a turning force. Some drag is necessary. When the craft is trimmed it is normal to have more weight to the rear to counter the effect of thrust from the fan trying to push the nose down. This makes the craft run tail down giving the required drag. If a greater volume of air is fed to the cushion, the hovergap will increase. With practice the driver will automatically sense the craft trim and adjust it by moving his weight. In a turn the driver can move forward and sideways toward the turn to create drag on one front corner, allowing the drag to reduce at the rear and the rudder to assist in pushing the rear of the craft around the maximum point of skirt drag.

When you look at most craft travelling on water, you will see that the skirt starts to drag causing spray from the contact point about 2/3 down the length of the craft. Some of this spray is caused by direct skirt contact and some by air being forced under the rear side segments as the craft moves forward because the rear segments form a tighter seal to the water. This air forces water out from under the skirt contact point adding to the spray.

kemo1988
25-Mar-11, 12:36 PM
Thrust ---- 34.6x0.815= 35.6 HP

therefore total power absorption = 35.6 + 7.4 = 35.6HP

Paul Fitz

Thanks Paul, there was a couple of thoughts in my mind none of them was multiplying the power to the percentages. I think the thrust power is 34.6 * 0.815 = 28.2 HP and Total 7.4 + 28.2 = 35.6 HP.

I have the latest version of optimiser, i dont know where that difference came from, anyway, Now i have recalculated these numbers to just get the feeling:

Fan: 1000/40/PPG/5Z @ 2000 RPM, Pressure = 600 Pa, lift % = 18.5, thrust % = 81.5

For Lift: 20m3 @ 600 Pa for whole fan-------20 * 0.185 = 3.7m3 for lift------Power consumed for lift = 39.2 * 0.185 = 7.3 HP

For thrust: 23.8 @ 108 Pa for whole fan ------- 23.8*0.815 = 19.4m3 for thrust ----- Power consumed for theust = 34.4 * 0.815 = 28 HP

Now for air velocity, area lift = 0.145 m2, area thrust = 0.64 m2

For lift: 3.7/0.145 = 25.5 m/s giving a dynamic pressure of 390 Pa + 600 (static) = 990 Pa (Total).

For thrust: 19.4/0.64 = 30.3 m/s giving a dynamic Pressure of 550 Pa + 100 (static) = 650 Pa (Total).

For static thrust: 19.4 * 30.3 * 1.2 = 705 N = 158.8 Ibf

As you stated in the PDF the air velocity is lower for the lift area, but how the total pressure differs it is supposed to be constant along the fan.

For the drag force can i use the total drag curve in the PDF or it will differ to get the actual craft speed, and i didnt find the "Drag calculation" PDF if you have it please attach it.

Kareem

Paul Fitz
26-Mar-11, 10:03 AM
As you stated in the PDF the air velocity is lower for the lift area, but how the total pressure differs it is supposed to be constant along the fan.

For the drag force can i use the total drag curve in the PDF or it will differ to get the actual craft speed, and i didnt find the "Drag calculation" PDF if you have it please attach it.

Kareem

Total pressure (Pt) at the fan is usually considered to be constant across the disc. In fact this will not be true for a typical hovercraft fan due to the marked difference in performance at different parts of the blade.The total pressure within any part of a system will be dependent on the value of system losses up to that point (see downloads page - "Principles of hovercraft design"- section - pressures in a ducted fan system).

An integrated fan is an unusual and special case. The thrust part of it has a very low resistance to flow, but the lift section has a relatively high resistance to flow. So theoretically the area immediately behind the fan should have a constant Pt across the disc, but just within the entry to the area under the splitter Pt can be lost because of the different size and conditions of the duct at that point. Above the splitter Pt will be almost constant between the fan and the discharge, (a very slight loss due to wall friction losses). Each part of the fan should be considered as a different "system", as would two seperate fans.

I am not aware of a "Drag calculation PDF". There used to be an Excel Drag Calculator on the Old THCC website, which may still be available from Juergen Schoepf.

If any of our American cousins are listening in and still have contact with Juergen, perhaps they can help obtain this for re-publication on the HoA and HCGB websites.