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hovercrafter
17-Mar-11, 05:51 PM
Greetings,

I am building a hovercraft as my final year engineering project.
However, due to the limited resources where I live, I cannot quite easily find the needed fans for lift.

My calculations, using the extreme load case, returned that I need to maintain, at most, a pressure of 0.14 PSI (or 952.46 Pa) under the craft, and a lift air volume of 70.25 ft3/sec (or 1.989 m3/s, about 4200 CFM).

I have found that in the local market, there are some S&P fans. My question is can such fans be used for lift??
I am really worried that they might not!

Here is one of the fans I thought might fit the task : http://global.solerpalau.com/product.jsp?PRODUCTID=181&CATEGORYID=51
(catalog : http://global.solerpalau.com/docs/catalogo_general/en_208_209_tbt_fid2909.pdf)

If not this one, will any of their products (mentioned here : http://global.solerpalau.com) work with me?

Thank you guys, I really need your professional help :D

kemo1988
17-Mar-11, 07:16 PM
These fans doesnt produce your estimated pressure, the max pressure of the 2/450 fan is 570 Pa and it gives about 1.7 m3/s, and one thing else is that this fans powered by electric motors and if you are thinking to take only the impeller you will need to get its performance chart to select your engine according to your operating point.

If you want to get fans for hovercraft, the most common is Multiwing and Hascon, it is only impeller and each of the two companies have their own selection software, search google for Multiwing and Hasconwing to get their websites, the most common blade type is 4Z & 5Z (airfoil section).

If you need any help just ask, my gradution project was also Hovercraft.
Good Luck

Kareem

hovercrafter
17-Mar-11, 08:00 PM
Greetings Kemo1988,

Thank you for your quick reply,

I had already downloaded the softwares and experimented with them in order to get a better feel in reading the curves and determining the operation point.

However, as I had stated in the first post, my main problem is the lack of these types of fans in the local market in the country I live in.

I was wondering if I could replace the engine, in order to apply the fan laws (http://www.tpub.com/content/NAVFAC/mo114_3/mo114_30031.htm) in such a way that I could modify the rpm (perhaps double it), thus getting double the volumetric flow rate and four times the static pressure, but requiring eight times more horsepower.

I also am kind of lost concerning this kind of analysis since I'm really not sure how these formulas can really be used for hovercrafts and practical application.

Thank you for your kind help! :D

kemo1988
17-Mar-11, 08:36 PM
First befor thinking to increase the rpm of the fan you need to know its maximum rpm, from the PDF the fan runs at 2800 rpm and for a fan this is a very high number which is difficult to increase more than that and if so the efficiency will be very low in addition to the sound, the reson for that is these fans are mostly used for ventelation and HVAC which have much lower static pressure and it will give very bad performance in operating conditions like hovercraft.

You can order multiwing fans online (Hub and Blades) and assemble them your self, First of all are sure your estimated pressure and flowrate are correct.

Paul Fitz
17-Mar-11, 11:30 PM
Greetings Kemo1988,

I was wondering if I could replace the engine, in order to apply the fan laws (http://www.tpub.com/content/NAVFAC/mo114_3/mo114_30031.htm) in such a way that I could modify the rpm (perhaps double it), thus getting double the volumetric flow rate and four times the static pressure, but requiring eight times more horsepower.

I also am kind of lost concerning this kind of analysis since I'm really not sure how these formulas can really be used for hovercrafts and practical application.

Thank you for your kind help! :D

Hovercrafter, you must be very careful when using the fan laws to adjust performance figures.
From the performance chart the 450 fan appears to have a stable working point at 500 Pa.
To achieve your 950 Pa it would appear that you use k=1.38 (from root(950/500) as the base multiplier giving new speed of 3887 rpm and volume of about 3.8m3/sec

This looks quite good, but note that the fan chart does not state State "Static pressure", only Pascals. It should therefore be assumed that this is total pressure. for a 0.45 Mtr dia fan this volume would give a velocity discharge of about 24 m/sec and a Velocity pressure of about 340 Pa which must be subtracted from the 950 target i.e 640 Pa Static. Not so good now is it? It is essential to work from a 'good' fan chart.

Apart from this, the fan you show has an impeller made from cast aluminium. This is a material which is prone to cracking under stress and is therefore NOT SUITABLE for hovercraft use.
I would follow Kemo's advice and seek one of the suggested fans (Wingfan is another) and import a hub and blades. 450 or 500 dia are the most popular sizes for lift fans but more craft detail required to confirm the best option. - What country are you in?

You do not give details of the size of craft being built. The target cushion pressure of 950Pa and volume of 1.99 m3/sec appear unlikely for a practical working craft. Perhaps you could give us more information.

HTH
Paul Fitz

hovercrafter
18-Mar-11, 09:12 AM
Hey there Paul,

You have a very valid point concerning the cast aluminum's tendency to crack under stress.
Note that just above the curve, the legend says "Pe = Static pressure in mmWG and Pa.".

Now, as for the dimensions, I have a 2.44 x 1.22 m wooden plate. I was thinking cutting out some edges and such, and therefore assumed a total area of about 2.44 m2.
I have also assumed that the total weight on the craft would be about 230kg (including passenger, motors and fans, and all wooden materials needed) ... I don't know if it's an overestimation or an underestimation really.

Then I used the following xls file to facilitate the calculations (http://sites.google.com/site/kearnsbryan/HovercraftDesignToolsrev3-20-08.xls) (after first converting the units, of course)

And that is where I got the values I mentioned in my first post.

Now, I'm open for any suggestions/criticism as I really am a novice in this field and I'm still exploring the difficulties I could face in the design process.

Looking forward to reading your opinions and analysis.

Cheers :D

Al
18-Mar-11, 09:38 AM
Hovercraft are usually bigger than 2.4mtr X 1.2mtr. Single seaters are usually around 3mtrs X 1.8 mtrs. By the time everything is fitted, such as fan & fan & fan duct, engine , steering, fuel tank etc. it will be surprising how nuch space this lot takes up. Also the extra length will help with C of G.
2 & 3 seaters are obviously bigger. What sort of craft are you thinking of making (cruising or racing) Cruising craft are used in open water and have a higher freeboard and more floatation built in. How many people are you hoping to carry?

hovercrafter
18-Mar-11, 09:54 AM
I actually am hoping to make a small hovercraft that is capable of lifting one passenger. I don't want to use it for racing as it is just my Mechanical Engineering final-year-project.

kemo1988
18-Mar-11, 07:39 PM
You really need to think about your wieght, it is too high for the area, you know that axial fans is mainly for low static pressure high flowrate compared to centrifugal fans, and like i said the fan can gaive 900 Pa but with very low efficiency and you are working on your engineering graduation project, so if you are an engineer you may want to power your project with the highest possible efficiency and bad efficiency will be taken on you during your project discussion. Mine was also a sheet of wood with the same dimensions but it was an integerated system and it was very simple with no plenum chamber as the lift air was pushing directly to the ground, the overall wieght without driver was 80 Kg and a 150 Kg with driver giving a 625 Pa.

Kareem

Paul Fitz
21-Mar-11, 11:46 PM
Hey there Paul,
Snip.... Now, as for the dimensions, I have a 2.44 x 1.22 m wooden plate. I was thinking cutting out some edges and such, and therefore assumed a total area of about 2.44 m2.
I have also assumed that the total weight on the craft would be about 230kg (including passenger, motors and fans, and all wooden materials needed) ... I don't know if it's an overestimation or an underestimation really........ Looking forward to reading your opinions and analysis.
Cheers :D

OK You asked for it :o)

I wouldn't go 'snipping' any more off. The larger the footprint the lower the cushion pressure; the lower the cushion pressure the better.

As the craft is very narrow it will almost certainly require a bag skirt for stability. A segmented skirt is unlikely to recover when the craft starts to tip sideways. To obtain some stability the bag will need to be at a relatively high pressure relative to the cushion. I would suggest a minimum of 1.15:1 ratio.
Using this loop ratio, and 1.44 x 1.22 as the contact point area (in practice it is usually slightly smaller than the craft), an 8mm hover-gap and 230Kg total mass I obtain a minimum fan duty of 1.6M3/sec @ 871 Pascals static pressure (18.2 LBf/ft2) The cushion pressure would be 757 Pa (15.76 Lbf/ft2). This is high for a small craft but usable on very smooth surfaces (hence 8mm hovergap).

AS you pointed out the error of my ways, and I now recognise the fact that the fan chart shows Static pressure. (my excuse.. Either failing eyesight with the onset of old age or stupidity) :o)

This should bring the fan-speed down to about 3720rpm and the volume produced to approx 3.5 m3/sec @ 871Pa. Assuming that this is only a student thesis project and not intended to be used as a working hovercraft, you could (with no other choice) use the 450 dia aluminium fan, but I would recommend that (a) it is belt driven (b) You use the original steel case (c) you use a heavy steel mesh guard over the upper side of the fan (2.5-3mm wire 1 cm mesh) covered with 2 layers of fine steel mesh (.5mm wire 1-2mm mesh) to protect the driver in case of impeller failure. Broken aluminium forms very sharp edged sherds. Special care would need to be taken in mounting the impeller and maintaining the tip clearance. The mesh would lower the volume flow and possibly increase the resistance (a pressure loss) so the belt drive option will gie the possibility of tuning the fan speed if necessary.

I must re-iterate however that an alternative fan type with plastic blades as suggested is a much better and safer option.

The down-side of this, is that all this metal is likely to increase the overall weight and you may therefore have to re-work the calculations.

Increasing the size and area of the craft will reduce the required pressure and fan speed.

HTH regards

Paul Fitz

hovercrafter
22-Mar-11, 03:59 PM
Wow Paul, that was a pretty great explanation.

I will definitely consider expanding the area (in order to considerably decrease the required pressure).

I was considering ordering online the plastic fans you mentioned and have them shipped to my country.
Do you think I should order the multiwing fans from hovercraft . com? And what do you think of there direct drive wooden lift fans?

Thank you :D

Paul Fitz
23-Mar-11, 09:46 PM
....snip...Do you think I should order the multiwing fans from hovercraft . com? And what do you think of there direct drive wooden lift fans? Thank you :D

I have no experience of the wooden lift-fans, they are not generally used in the UK.
Wait until you have all the weights sizes etc firmly decided and are able to calculate your required fan duty and then select the best fan using manufacturers software. Some fans give very poor efficiencies at particular duties. it often takes several experimental selections to find the most efficient type. This can often be the fixed pitch fans types but if chosen, they should be able to cater for a higher duty than required in case of error etc.

Once you have chosen the fan, email the suppliers and ask for quotations to supply including delivery costs. Different makes of similar fans are available from the UK, France, Germany and the USA.

Paul Fitz