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Keith Oakley - n/a
16-Feb-06, 09:48 AM
When I published here late last year a draft formula for calculating fan noise on multiwing style fans I didn't include a term for the effect of blade angle or pitch. The logic was that most folk operate with blade angles in the 35-45 degree range and I didn't have any data on the blade angles used on the particular craft I'd sampled. Conrad has suggested that blade angle might be major component of why these fans are apparently noisier than ultralight aircraft props, which typically run at around 20 degrees.



I've conducted some tests using my Kohler engined F25 running a 3 bladed fan at different angles to explore this. The results from the tests show there is about an 8db increase in noise from the same m/w fan at a given tip speed when blade angle is increased from 20 to 40 degrees and a further 3 db increase from 40 to 45 degrees. The tests were conducted over tip speeds from 60 to 110 m/sec, which is lower than the 120-150 m/sec used by most craft so its possible the difference is more marked at higher tip speeds.



Of course the bhp absorbed by the fan at different angles but at a given tip speed varies significantly - the fan is doing more work at higher angles but perhaps thats a clue to the lower noise of the prop. Each foot length of the prop blade is doing less work, causing less noise but the overall longer length of the prop blade and often it's higher tip speed means it does as much or more work overall.



The lower pitch at the tip of a prop compared to a fan may also be a major component. The pressure on one side of a airfoil blade is greater than on the other and the result is a spanwise flow of air around the tip from the high pressure to the low pressure zone. This turbulence causes noise. The duct wall on the fan should help to contain this flow, in much the same way as the winglets used on some aircraft, but maybe the high tip clearances needed for practical reasons are less than fully effective.



On another tack I reported before that from the Blackditch sample some craft seemed to have unexplained high levels of fan harmonics whilst others were much lower. My craft seems to be mostly in the latter group with total fan harmonic noise 5-6db lower than the fan blade passing frequency (bpf) noise, even though the flow straightners are close to the fan. However when using the 45 degree angle at max rpm (just 3120rpm because of the loading) the fan harmonics rise dramatically so that the total harmonic noise is 1 db higher than the bpf. This is probably due to cavitation, which Conrad tells me may go away if the craft was allowed to move forward.



Other byproducts of these tests are that its confirmed that my Kohler F25 even with a poor exhaust is significantly quieter than the fan. The fan guard does not appear to cause noise. I can see no increase in either overall dba or broadband (indicating more turbulence) noise with or without the guard.



A much larger and wider ranging series of tests is needed to see if there is a sweet spot offering minimum noise with the same or better propulsion efficiency.



More to come in my AGM lecture. Comments welcomed.

jar2 - n/a
16-Feb-06, 09:20 PM
Excellent work Keith.



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.. but perhaps thats a clue to the lower noise of the prop. Each foot length of the prop blade is doing less work, causing less noise but the overall longer length of the prop blade and often it's higher tip speed means it does as much or more work overall.
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My basic experiments with the Scout prop and fan (both the same diameter) didn't show this. The prop did more work (and generated less noise) than the fan at a slightly higher tip speed both with the same power input. The fan pitch and tip speed were optimized for maxumum thrust as was the prop.



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The lower pitch at the tip of a prop compared to a fan may also be a major component.
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A 5Z fan blade has a very similar tip angle to a prop (about 3-10degrees) at the most common pitch settings.



The only really obvious difference I can see between a fan blade and a prop blade is the width (I think chord is the 'proper' name). They both have similar aerofoil sections, blade twist, blade angle (prop blade angle is usually measured at about 70% diameter whereas fan blades are set at the root), etc. As fans are designed specifically to shift air efficiently (and quietly) at slow speed (much slower than hovercraft) it could be that the wider blade just doesn't work properly at higher speeds and the noise is an indication of this.



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The duct wall on the fan should help to contain this flow, in much the same way as the winglets used on some aircraft, but maybe the high tip clearances needed for practical reasons are less than fully effective.
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I suspect that you probably need very close clearances to reduce tip turbulance. A duct is, after all, just an aerofoil section wing folded around in a circle. As the angle of attck of the 'wing' duct is fixed, there have to be a limited range of airflow conditions under which it isn't in either the stalled or high drag state.



Any chance of a copy of your presentation for us people who won't be at the AGM (after you've presented it of course!)?

Ian Brooks - n/a
16-Feb-06, 09:49 PM
Hi



Did you measure the thrust? If not, I could estimate it given a few details. It would be interesting to plot noise against thrust & power for a range of blade angles. Let me know if you want me to work the estimates.



Ian

Keith Oakley - n/a
17-Feb-06, 07:26 AM
Ian

Thanks - no the craft was on a trailer and with just 3 blades instead of the normal 6 and the splitter plate removed to eliminate it as a source of noise it was a bit tricky to measure thrust! I'm keen to see some kind of power output v noise plots. I'll send you the data via email.

Clearly from Johns comments I should stick to reporting data not speculating on aerodynamics conclusions!

Nick Long - n/a
17-Feb-06, 11:01 AM
No you keep at it Keith.



I would not be surprised at all to find that blade angle of attack is a key parameter. But it has to be the actual angle of attack with the airflow, which is a bit more difficult to work out, but is a measure of "how hard the blade is working".



Nick

Paul Fitz - n/a
17-Feb-06, 10:06 PM
Nick is correct on this one Keith. Most ducted fan systems (industrial) are designed to move air at the lowest speed possible conducive to providing the required volume of air. The main systems which deviate from this, are extraction systems where a high design velocity is chosen to ensure entrainment of the contaminant within the airflow (smoke, dust, wood-chip etc). The MW type fans are designed for use in the former type of system (they will not handle anything heavier than smoke very well). As such, the twist in the blade and the cord is designed about an "average" system, having a slower fan, than we use. The result is an overloaded tip and poor efficiency over a relatively large part of the blade length. As the efficiency drops, the noise level climbs with increasing power absorption.



With a correctly designed fan blade for hovercraft travelling at a maximum speeds of perhaps 40mph, It is likely that the efficiency could match that of proprietry propellers which are mainly designed for cruising speeds in excess of 80mph. Add to that the shielding effect of the duct and the lower fan speeds that could be used with a high efficiency blade, I suspect that the noise levels could fall below that of current prop craft.



The tip clearance problems can be solved - look at the Australian craft!! it just requires better engineering of the duct assy. than most craft use in the UK. The problem regarding the angle of attack with respect to the incoming air will of course always remain, because of the typical sideways attitude of a hovercraft and the limited length of affective duct which is practical.



Please don't stop offering opinions, speculative or otherwise, they drive the ideas and suggestions from others.

hover t - n/a
18-Feb-06, 12:54 AM
Hi all

Interesting reading, If someone develops a purpose built

hovercraft fan (quiet + efficient) I will be looking to replace

my MW setup as I find it unsatisfactory



Would it not be in the clubs interest to fund the development

of such a blade?

If the club was sole supplyer of the blades i am certain

development costs would be more than covered.



does FUD forum stand for

Fan

Under

Development

forum http://hovercraft.org.uk/images/icons/smiley_icons/icon_biggrin.gif

TrevB

Keith Oakley - n/a
18-Feb-06, 07:30 AM
Thanks for the support guys. I think the question of funding is cart before horse. Setting up moulds is expensive (5-10k?) but in the scheme of things affordable either by the club or other bodies but the real issue is to identify a design with some certainty before anyone invests real money. Thats were we need to get a better understanding and preferably reliably computer model a design first. And sorry thats beyond me!

BIGRACING - n/a
19-Feb-06, 03:37 AM
We all know about the twist in the fan tip at the extreme speeds we are running these fans at. As mentioned earlier, these fan are primarily designed to move a lot of air at low speeds.



In using some of the manufacture analysis tools, I find that the static pressure is generally a bit higher at lower angles. (…which is more important for integrated crafts) The theoretical sound analysis doesn’t seem to differ much with the change of the blade angles. And even though horse power absorption is not necessarily accurate, the numbers do give us some sort of efficiency indication and optimization trends.

index.php?t=getfile&id=59&private=0

index.php?t=getfile&id=60&private=0

index.php?t=getfile&id=61&private=0

On a personal note, I find the Wing-Fan product to be a bit superior over the Multi-Wing product, This is mainly due to the higher glass content in the PAG blades, which make these a much stiffer blade. I do not have any personal experience with the Hascom fans, but I’m told their class content is even higher than the Wing-Fan’s, (…keeping in mind that to much glass mixed with the plastic will make the blade more brittle.)



I only mention this, as I believe a stiffer blade will have less twist, and therefore cut the air with more efficiency. Hence, they tend to have better sound properties. After having done many tests, with different angles and configurations, on both Z4 and Z5 blades, I’m happy to report that I’m very pleased with the use of the much stronger Z6 blades. Using them in a 7-7 configuration, I get more air at less rpm. Hence, less sound.

BIGRACING - n/a
19-Feb-06, 04:00 AM
FYI, here are some expected numbers at about 50 yards away:

index.php?t=getfile&id=62&private=0

Paul Fitz - n/a
19-Feb-06, 07:00 PM
Hello Raoul,

the graphical outputs you show here are interesting. Although I know you are using the figures in this instant for demonstration purposes, I have to say they are classic examples of the miss-use of the manufacturers tools and demonstrate the reasons why the manufacturers often are loath to supply them to non-trade customers. http://hovercraft.org.uk/images/icons/smiley_icons/icon_surprised.gif)



Taking the first example - 900/6-12/5Z/35deg/3500rpm

the example shows a static pressure of 1370 Pascals. This could not be achieved unless the fan were to be used for lift only and the air severely restricted on a heavy craft. Typically the fan static pressure on a craft such as yours will be of the order of 500-700 Pascals laden although it may climb momentarily to c.1000 when landing from a 'jump' etc.



When used for pure thrust, the only pressure this fan will need to overcome is that of the duct and any obstructions + its own discharge loss. Consequently the efficiency will be lower than shown (about 53%), as the working point will be much lower on the curve.



This particular calculator also provides figures to the ANSI / AMCA A Rating which does not realistically mimic a hovercraft thrust fan, hence the volume and velocity predicted will be considerably higher than actual and the efficiency will normally be shown as much higher than achievable. The theoretical thrust obtained therefore much higher than possible.



The last point is concerning the noise levels. The human ear detects Sound pressure levels, consequently the output you need to use is the SPL(A) rating which you do in your second post. These give a more realistic output of about 83 dB(A). @ 25mtrs. (A higher figure than you show because of the above and also you calculated it at 50m.) This is for the fan in ideal conditions. In practice more noise will be generated by the obstructions (which the calculator does not account for) taking the level to about 85dB(A) or possibly more. A 3dB increase represents an effective doubling of the perceived noise level. I believe that Keith's readings for similar fans were of the order of 93dB(A) although I am not sure which rating or filter he used.



With regard to the change of pitch, calculate these again but for the same power absoption (you will need to change the speed) http://hovercraft.org.uk/images/icons/smiley_icons/icon_surprised.gif)

BIGRACING - n/a
20-Feb-06, 03:51 AM
Hey Fitzy,

I figured I hear from you on this. You're absolutely correct; this demonstration is for educational purposes only. Also note, the specific manufacture is not mentioned in these pictures! Interestingly enough, now that there was a split between the two major manufactures, even they are bickering about the different testing methods used in the development of this type of software. (…for more details, see enclosed disclaimer note)



The real point I was trying to make was that I think Keith is not far off with his actual testing results. When I first looked at the numbers produced by the software, I found it very interesting to note that the different angle measurements seem to match the sound testing results described in Keith’s original write-up. I also purposely did not want to display the graph results, as the main topic here is related to sound.



Yes, I use the AMCA B: (free inlet - ducted outlet) testing method. But most importantly, you and I know that the listed results in these theoretical calculation need to be adjusted in order to make more sense for our particular application usage. I do however strongly believe that the software can be useful showing certain trends in comparing the various available fan configurations and their inherent sound properties. Considering the blade twist at high speeds and some of the other variables, including needed power, being non-stationary, and some of the other variables, I would also think we both agree that the calculated "efficiency" can be used as an indication for a particular fan setup!



I find another good use of the software are the mechanical load predictions. Together with industrial supply engineering manuals, they helped me in find an appropriate shaft and belt size, as well as bearing and pillar block setups.



Anyhow, here are some of the numbers you had asked for, and at a slightly lower rpm. The interesting part is that even in this case there consistently seems to be a theoretical 3dB differents between the various blade angles.



index.php?t=getfile&id=65&private=0

Paul Fitz - n/a
20-Feb-06, 09:37 PM
Interesting disclaimer, I hadn't noticed that before http://hovercraft.org.uk/images/icons/smiley_icons/icon_surprised.gif)

Great place to knock the competition.

Keith Oakley - n/a
20-Feb-06, 09:44 PM
Paul

Could you re-iterate the lessons we could learn from Owen's Australian craft?

Keith

BIGRACING - n/a
21-Feb-06, 03:47 AM
Maybe this picture will help?

index.php?t=getfile&id=66&private=0

The bottom line, there is no production blade out there that is specifically made for hovercraft applications, and this is about as close as you can get! It's still a bit experimental. It has been used by people in several races, but I haven’t seen any specific write-ups about it. In any case, I believe this blade shape is one of the optimal ones I've seen out there yet.



I'll let Paul elaborate more on this.

Sean Atterbury - n/a
21-Feb-06, 06:41 AM
WOW, that's nice http://hovercraft.org.uk/images/icons/smiley_icons/icon_eek.gif



We should all strive to achieve this. http://hovercraft.org.uk/images/icons/smiley_icons/icon_biggrin.gif

Hovertrekker - n/a
21-Feb-06, 01:44 PM
Hi Raoul,

Here's a couple shots of Owens latest version. An 8 bladed version of the fireblade hooked up to a 140 bhp Rotax.

index.php?t=getfile&id=71&private=0index.php?t=getfile&id=72&private=0

Paul Fitz - n/a
21-Feb-06, 06:49 PM
Quote "I'll let Paul elaborate more on this."



I wish I could, but I've seen no info on these blades.



Keith, I think your question relates to my statement in a previous post, ref: tip clearance. The point is that the Revtec design uses an integrated duct/fan-drive system where the fan and drive is rigidly mounted within the duct and the whole assembly is then a/v mounted onto the craft. This allows the tip clearance to be kept to a minimum (c. 3mm typ). This does not prevent breakage in the event of a severe shunt, but it appears to work quite well in normal use, and without doubt will improve the fan performance.

Keith Oakley - n/a
21-Feb-06, 07:12 PM
Thanks all - I see that Owens craft seems to have stators rather than flow straightners. Does anyone have any info on those blades? - preferably with noise and general performance data.

Hovertrekker - n/a
21-Feb-06, 10:11 PM
I thought flow straightners and stators were essentially the same thing ??! Owen's have an airfoil profile and are made of carbon/epoxy and hollow. Sorry I don't have any technical data on noise and performance. Ironically, Owen will tell you that they are not really neccessary and just add drag, but in the case of that particular duct they are also needed to support the fan hub/tail spinner

Paul Fitz - n/a
21-Feb-06, 10:47 PM
Quote "I thought flow straightners and stators were essentially the same thing ??"



Flow straighteners are stators, but stators are not necessarily flow straightners. A stator is a fixed blade upwind or downwind of the fan. A radial stator is a straight flat (or aerofoil) section which radiates out from the hub. To be an effective 'flow straightener it would have to very long (i.e. about 1x the fan dia.) Flow straighteners are aerodynamically designed vanes, which fit into the duct and divert the flow. See JB's article on the downloads page.

Hovertrekker - n/a
22-Feb-06, 12:42 PM
Paul,

Can you give me an example of any hovercraft out there that have stators which would not be considered as flow straighteners?

Sean Atterbury - n/a
23-Feb-06, 04:35 AM
so that will meen I have? http://hovercraft.org.uk/images/icons/smiley_icons/icon_confused.gif

Paul Fitz - n/a
25-Feb-06, 11:39 AM
Sorry Hovertrekker, I missed your question.

Off hand I could not say which of the commercially available craft use plain stators, it is not something that they advertise. Many of the European racers omit flow straighteners. Some of them use simple radial bracing in the duct. It might be a stretch of the immagination to call these stators however.

Michel - n/a
2-Mar-06, 10:12 PM
That topic is surely ZE one interesting all of us,

We' ll never promote hovercraft with such a noise.

I cannot provide diagrams and technical results.

My experience is : as an ex- pilot, I have followed all research about noise reduction on blades and turbo- engines.

Great lines:

slow down blade edge speed.

If possible, be sure that number of blades is not the same than static flow blades inside your fan duct ( to avoid one prop blade is always in front of one stator blade )

try to break the regularity in air flow.

On my craft , I' ve tried several settings , best is :

On a hub of 12, I have 9 blades in 3, free hole, 3, free hole, 3, free hole.

Curved blades are the future of propellers.

Thick and long ducts are quieter.

4- stroke engines are quieter.

Better put cover on engines.

Unfortunately, most of tricks good for noise reduction are bad for efficiency.

We must find many agreements.........

Hovertrekker - n/a
2-Mar-06, 10:24 PM
Michel,

I also sometimes use a 9/12 blade/hub configuration in my Revtech Rocket (37.5 degrees pitch). Have you tried 8 blades? If so, did you notice much difference?

Michel - n/a
3-Mar-06, 05:41 AM
Hi Dave,

In fact , no, the 8 on 12 is the only one I did' nt try.

12/12 good cushion low max speed

6/12 soft cushion good max speed

9/12 good cushion, good max speed and less noise.

On my small hovercraft, I have only a 25 HP Kohler, and my range of possibilities is limited by power.

Must try.

See ya.

Michel

BIGRACING - n/a
3-Mar-06, 05:55 AM
Let me know if you gentleman would like to try a 9-9 configuration. The main advantage is the much lighter and smaller hub. On the 25hp Kohler you can use it with just 3 blades and get some more lenght out of them.

I still have two fan sets fore sale. More details can be found here:

9-9 fan set for sale (www.hoverclubofamerica.org/forum/index.php?showtopic=306)

Michel - n/a
3-Mar-06, 06:09 AM
Hi Wolfy,

Could be nice but,

as I told previously, an irregular positioning of blades reduces noise.

A 9/9 setting is better in theory, to improve thrust with same power, but noise is our goal for cruising.

So, no thanks for me.

Michel

Hovertrekker - n/a
3-Mar-06, 12:45 PM
Let me know if you gentleman would like to try a 9-9 configuration. The main advantage is the much lighter and smaller hub. On the 25hp Kohler you can use it with just 3 blades and get some more lenght out of them.

I still have two fan sets fore sale. More details can be found here:

9-9 fan set for sale (www.hoverclubofamerica.org/forum/index.php?showtopic=306)




Raoul,

Can you run your software for me to tell me what the ideal starting pitch angles should be for the following setup?:

900mm duct, 12 blade hub, 83bhp@ 8000 engine rpm for 6,8,9 and 12 blades (5ZL) combinations.



Thanks,

Hovertrekker - n/a
3-Mar-06, 12:48 PM
Hi Dave,

In fact , no, the 8 on 12 is the only one I did' nt try.

12/12 good cushion low max speed

6/12 soft cushion good max speed

9/12 good cushion, good max speed and less noise.

On my small hovercraft, I have only a 25 HP Kohler, and my range of possibilities is limited by power.

Must try.

See ya.

Michel




Michel,

Do you remember what pitch angle you used for the 6/12 and 9/12?

BIGRACING - n/a
4-Mar-06, 03:15 AM
I need the approximate fan speed rpm? If I'm to assume you like to run at close to maximum tip speed at this diameter, 168m/s, it would be close to 3565 rpm. This would imply your current ratio is assumed to be about 2.244 : 1? Also, it is not necessarily max engine rpm, but rather the rpm that max hp is being produced thats important. In most performance curves this peak point is slightly lower than max rpm?



Here are some numbers at 3200 rpm with the Z5's.

index.php?t=getfile&id=117&private=0



Also, check out this article from the download section.

estimating thrust and lift performance (www.hovercraft.org.uk/download/estimating%20thrust%20and%20lift%20performance%20R ev%20D.pdf)





Based on my personal experience, I’d like to mention the Wing-Fan P6Z blades (…not the MW versions!) I can recommend them for any craft that has more than 75hp. I’d like to believe that a 5-5 setup would work great in your light weight Revtec machine. The wider blade would give you good lift performance, and without compromising the trust output. The P6Z 5-5 is lighter in weight (see numbers below), the 12-6 weights in at 3.91kg (~8.6 pounds), The P6Z's are much stronger blades for output, (longer and wider.) At just 2334 rpm and 110m/s tip speed, here are some of the numbers:

index.php?t=getfile&id=118&private=0

index.php?t=getfile&id=119&private=0

Paul Fitz - n/a
4-Mar-06, 12:37 PM
This particular fan would be a very poor choice for an integrated craft, because the fan curve is recursive. I.e it curves back on itself. This creates a serious problem in the event that the cushion pressure rises momentarily to perhaps 800Pa as it is likely to do if the craft jumps from a bank onto water. The working point will move to the position shown by the blue dot. As the cushion recovers, the working point will move down the curve to the normal 500Pa But...... the volume flow will remain at 29000 m3Hr not the previous 47000m3Hr (shown by the two red spots). The cushion will be unstable. It is important to avoid fan curves of this shape, and this is often a problem when choosing to operate with too steep a pitch, (depending on the choice of fan).index.php?t=getfile&id=120&private=0index.php?t=getfile&id=120&private=0

Hovertrekker - n/a
4-Mar-06, 01:15 PM
Raoul,

Thanks for all the fan info. I'm still trying to sort it all out! I'll get back with you after I can think about the data.

I believe my ratio is 2.56:1 so that would give 3125 fan rpm @ 8000 engine rpm. I would be a little concerned about disk density with only 5 blades, but as you say they're wider than the 5ZL. Also since the hub is smaller I'd have to rework the tail spinner and flow straightners = lots of expensive rework http://hovercraft.org.uk/images/icons/smiley_icons/icon_eek.gif Something to ponder anyway.

Hovertrekker - n/a
4-Mar-06, 01:22 PM
This particular fan would be a very poor choice for an integrated craft, because the fan curve is recursive. I.e it curves back on itself. This creates a serious problem in the event that the cushion pressure rises momentarily to perhaps 800Pa as it is likely to do if the craft jumps from a bank onto water. The working point will move to the position shown by the blue dot. As the cushion recovers, the working point will move down the curve to the normal 500Pa But...... the volume flow will remain at 29000 m3Hr not the previous 47000m3Hr (shown by the two red spots). The cushion will be unstable. It is important to avoid fan curves of this shape, and this is often a problem when choosing to operate with too steep a pitch, (depending on the choice of fan).index.php?t=getfile&id=120&private=0index.php?t=getfile&id=120&private=0




Paul,

You're refering to the 5/5 setup right? Interesting stuff. Kent Gano runs a 5/5 fireblade in his Rocket, but I know he says that you need to run flat out to get good lift. Of course he's set up purely for racing anyway.

BIGRACING - n/a
4-Mar-06, 07:32 PM
Hi Paul, you have a good point. Considering that this theoretical data is only at a moderate 110 m/s tip speed, which is btw just about right when it comes to cruising applications, my practical racing experience is as follows. My craft holds enough back pressure that it takes quite a bit of a time delay for the cushion to collapse during temporary obstacles or jumps. In addition I found 3 main ways how I can make up for the cushion recovery you are talking about.



1.) A larger splitter area to create good cushion; my splitter height measured 9" below the plate, inside a 36" duct.

2.) Throttle control; I have a very responsive and torque 4 stroke engine, which revs up nicely between 6000 and 8000 rpm under cushion, and up to 11000 when put in neutral. Yes, I still have the gearbox attached, and use 2 different gear settings between racing and cruising.

3.) Pressure skirts; closing off the bottom or the skirts middle section keeps the them inflated during times when one side or the other is lifted higher from above the ground. It also drastically avoids the front plough in problem. Granted, my craft is a bit on the heavy side, and does not rise into the air as much as some of the other lighter crafts. Together with a lower profile and a slightly larger foot print, I still have smoother landings with the much stronger 7-7 P6Z's than I did when I tried the weak MW flex 9-9 Z5 configuration.



But back to the noise topic, after the initial going into the water part, the bigger and stronger blades allow me to keep the engine and fan revs down, and still cruise along at a good pace and speed over smooth river water surfaces in an integrated craft. I can cruise along just fine at around 5500 engine rpm, keep the tip speed low, and therefore cruise at a much reduced noise level. If I encounter ruff water, or even strong rapids, (which by nature make some noise to begin with) I simply crank up the volume a bit for the brief period of time it takes me to maneuver past these natural obstacles.

Paul Fitz - n/a
4-Mar-06, 09:28 PM
Quote (Paul,You're refering to the 5/5 setup right?)



No. I am referring to that particular fan. If you have 150Hp installed you could run the 5 blade at 3500 rpm and not have this problem, because the working point would be in the low part of the curve and could rise to more than 1000Pa without becoming recursive. (I don’t think I would want to put that much power through five blade though http://hovercraft.org.uk/images/icons/smiley_icons/icon_surprised.gif) )

BIGRACING - n/a
5-Mar-06, 07:07 PM
Quote (Paul,You're refering to the 5/5 setup right?)



No. I am referring to that particular fan. If you have 150Hp installed you could run the 5 blade at 3500 rpm and not have this problem, because the working point would be in the low part of the curve and could rise to more than 1000Pa without becoming recursive. (I don’t think I would want to put that much power through five blade though http://hovercraft.org.uk/images/icons/smiley_icons/icon_surprised.gif) )




Actually Paul, in the mean time I’ve upgraded to about 175hp. Using the P6Z 7-7’s is not a problem at all. Even when stressed tested in overspend conditions, these much stiffer blades have no flex that I can notice. I don’t think Hovertrekker has anything to worry about using a P6Z 5-5 configuration with 83PS.

However, I can't say the same for the Z5's. I think you have a very valid concern when trying to use a lot of power on the Z5’s. I’ve seen the MW versions flatten out completely in overspend conditions, and yes, all you get then is a very in efficient fan with nothing but a lot of noise.



As for comparing Z5’ 6-12 and 9-12 at various blade angles, see if this makes any sence to you Hovertrekker:

index.php?t=getfile&id=123&private=0

I think particularly the K7 line coresponce with the above description provided by Speedymartin

Michel - n/a
5-Mar-06, 08:57 PM
Hello Dave,

I ' m sorry being unable to say you what angle I ' ve used.

I' m not well trained on these hubs, I do my setting so as my engine never exceed 3700 rpm, that means prop turns at 2450-2500 rpm. I must check.

At my best cruising speed, engine is at 3200 rpm, and consumption is about under 4 liters/hour for 28 kt.

Above 3300 rpm, torque decreases, and Kohler is supposed never exceed 3700 rpm safely.

take care.

Michel

Paul Fitz - n/a
6-Mar-06, 08:53 PM
Hovertrekker will be OK with 5 Blades with 83 Hp. Actually Raoul, I was referring to Kent's craft, if he is feeding the power from a Honda Fireblade into 5 Blades that is more power per blade than you are using!!....... Scary http://hovercraft.org.uk/images/icons/smiley_icons/icon_surprised.gif)



Interestingly there appear to be some members within the HCGB who are vehemently against the use of the Wing-Fan blades, justifying the reason as 'they haven't been tested yet'. I am not aware of any body that wants to spend money on testing them, and suspect that they will eventually slip past a scrutineer and be tested in situ. Not that I would promote such a stratergy of course.



Now Gents pleeeese take this to another thread, with all the large images in this one, it takes forever to download the thread through a 56K connection. :0)

Hovertrekker - n/a
6-Mar-06, 09:48 PM
Paul,

Slight correction - Kent's craft has the same engine as mine (Yamaha VMax 500). 56K? http://hovercraft.org.uk/images/icons/smiley_icons/icon_eek.gif Ouch!

BIGRACING - n/a
7-Mar-06, 01:20 AM
This particular fan would be a very poor choice for an integrated craft, because the fan curve is recursive. I.e it curves back on itself. This creates a serious problem in the event that the cushion pressure rises momentarily to perhaps 800Pa as it is likely to do if the craft jumps from a bank onto water. The working point will move to the position shown by the blue dot. As the cushion recovers, the working point will move down the curve to the normal 500Pa But...... the volume flow will remain at 29000 m3Hr not the previous 47000m3Hr (shown by the two red spots). The cushion will be unstable. It is important to avoid fan curves of this shape, and this is often a problem when choosing to operate with too steep a pitch, (depending on the choice of fan)

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Paul, I just like to emphasize it one more time, you are absolutely correct when you say to avoid fan curves of this shape. Besides the mentioned cushion issue, fans tent to also have some unwanted vibration in between the two red doted areas you pointed out. Just like the resulting cushion pressure, fans have the tendency to get a bit unstable when the curve isn't smooth enough, and yes, this type of vibration and the resulting inefficiency, causes unnecessary noise.