View Full Version : blade adjustment

lazza - n/a
4-Mar-06, 06:56 PM
Hi all, I've always had my 5zl blades all set to 45 degrees so just for testing reasons i re adjusted them to the last notch in the multiwing hub which says it's 50 degrees and then took it for a spin around the local field but i personally didn't notice any difference to the way it lifted or performed. I'm sure i read in a earlier post that adjusting them even 1 or 2 degrees can make a large difference. At this stage i unfortunately don't have any strong tie down points at the rear of the craft to do a static thrust test but i am looking into that.

Should i of noticed a difference between 45 degrees and 50 degrees ?

My craft currently starts lifting at around 2500- 3000rpm and cruises around 4000 - 4500rpm with a maximum rev of around 6000 - 6200 rpm, what i was trying to achieve was to lower both those reading by 500 - 1000rpm. Any suggestions on the best approach would be appreciated. thanks

( for those that don't know, i have a 235 superwedge with a 9 blade multiwing fan and a subaru ea82 engine )

andycollins - n/a
4-Mar-06, 08:54 PM
Hi Lazza,

It sound like it's not the fan that's limiting you revs. Perhaps not enough blades? In my experience a few degrees can make a huge difference but I'd set mind up to consume the power and I only had 45BHP.

Remind me, how big is your duct? How many 5Zs do you have in there? What power does the engine kick out?

I don't have tables but With those details someone will chip in soon I'm sure.



Hovertrekker - n/a
4-Mar-06, 10:03 PM

I would expect to see a big difference if I did that on my craft. Even idle rpm would change. One other thing to note - remember to balance your fan after making adjustments. MW fans need balancing just like props. You might need a 12 blade fan to absorb more power if you're not seeing any difference (but if you're happy with it now why bother http://hovercraft.org.uk/images/icons/smiley_icons/icon_razz.gif )

srn4 - n/a
4-Mar-06, 11:31 PM
All of the calculations say that a 5Z 9 bladed multiwing hub with 9 blades in a 920mm duct should easily consume the rated power of that engine which is somewhere in the 90Hp region of space!...Unless my math is wrong, in which case I will cry!

lazza - n/a
5-Mar-06, 01:22 AM
My duct is meant to be 950mm, (but technically its 944mm), im using 9 blade hub with 9 * 5ZL multiwing blades and a Ea82 Subaru engine rated at approx 90Hp @ 5600RPM and the craft weighs in at a total of 400Kg with no passengers. The tip clearance ranges from 4mm to 6 mm depending on what position of the duct you measure it at. Gear ratio is 1.8:1 and it's big, its yellow, and attracts insects http://hovercraft.org.uk/images/icons/smiley_icons/icon_biggrin.gif

The craft does perform well as it is now but for personal preference and curiosity my reason for making blade adjustment was purely like i mention before , to lift earlier in the rev range and to cruise at a lower rev range, preferably without increasing the lift pressure that it currently has.

thanks again

Sean Atterbury - n/a
6-Mar-06, 04:12 AM
Maybe its your ratio, I have always been told to work on a 2.2 - 1 ratio, I think that would apply to any craft, thus the fan is turning to fast to be effective.

Guys correct me if I am wrong. Not too clued up about this. http://hovercraft.org.uk/images/icons/smiley_icons/icon_confused.gif

lazza - n/a
6-Mar-06, 05:29 AM
That could well be the case *shrug* i'm interested on other peoples opinion on if my ratio is wrong.

When i started to build my craft i got alot of facts and figures from a hovercraft manufacturer here in New zealand who has also built three superwedges for his personal pleasure with subaru's in them and he told me 1.8:1 was what i needed to use , and he even gave me the part numbers to the pulleys to buy, so being the novice i was/am who was i to doubt his figures http://hovercraft.org.uk/images/icons/smiley_icons/icon_smile.gif. Anyway it could well be wrong *shrug*, i'm currently trying to work out what my tip speed will be cause that will give me a indication if its right or wrong shouldn't it ?

So far i've come up with a figure of 165m/sec which is on the high side of the multi wing limits but once again i could be wrong in that too hehe, i'm still working it out http://hovercraft.org.uk/images/icons/smiley_icons/icon_smile.gif

Input gear turns 1.8 times to every 1 rev of the fan and the motor currently revs to a maximum ive seen of 6200rpm and duct is 950mm diameter, so if anyone wants to beat me to the calculation or even confirm my tip speed readings then feel free http://hovercraft.org.uk/images/icons/smiley_icons/icon_razz.gif

Jamie Lewendon - n/a
6-Mar-06, 12:02 PM
Someones calculations are wrong,

6200 rpm/60 = 103.33 rps = engine speed

103.33/1.8 = 57.4rps = fan speed

circumference = piD = PIx0.95 = 2.98m

Tip speed m/s = dist x rps = 2.98 x 57.4rps = 171m/s

Current HCGB and WHF regulations(I think)limit you to 167m/s for multiwing tipspeed if using the white plastic blades. Therefore you ar running the fan too fast in anyones eyes. From keith oakleys results, you should be aiming for sub 150m/s in a cruiser to decrease the amount of noise, therefore I think you need to look again at what ratio you run at. I can't recommend anything as I left all my figures at the workshop. However I'm sure someone will plug and play the figures for you.

To correctly match the fan to the engine is critical. The ratio will depend on blades, hub, duct size, engine power and revs. It will be different for every fan / engine combination. Example, my racing craft is over 4:1, off the top of my head you should be looking at at least 2:1. If no-one has given you an answer by the w/e, I will have the data back then.


Jamie Lewendon - n/a
6-Mar-06, 12:04 PM
Maybe its your ratio, I have always been told to work on a 2.2 - 1 ratio, I think that would apply to any craft, thus the fan is turning to fast to be effective.

Guys correct me if I am wrong. Not too clued up about this. http://hovercraft.org.uk/images/icons/smiley_icons/icon_confused.gif

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

See answer in message below.


Sean Atterbury - n/a
6-Mar-06, 12:23 PM
OK OK I am wrong, cut my head off will you... http://hovercraft.org.uk/images/icons/smiley_icons/icon_redface.gif

I love these guys, ready at any time to take you to the butchery. http://hovercraft.org.uk/images/icons/smiley_icons/icon_twisted.gif

Good to know that they care then hey? http://hovercraft.org.uk/images/icons/smiley_icons/icon_lol.gif

lazza - n/a
6-Mar-06, 05:27 PM
Thanks jamie, thats interesting. So i better not rev my craft past 5800rpm till i work out the correct ratio then hehe. Based on the calculations you've just provided i may end up looking for a ratio of 2.1:1 then.

6200 rpm/60 = 103.33 rps = engine speed

103.33/2.1 = 49.2rps = fan speed

circumference = piD = PIx0.95 = 2.98m

Tip speed m/s = dist x rps = 2.98 x 49.2rps = 146m/s

I'll hold off for confirmation on if i'm on the right track or not before changing the pulley. Thanks for the input. I'd actually be happy to change the pulleys because i currently have steel pulleys and would like to get some lighter aluminium pulleys if i can so it should work out nicely anyway http://hovercraft.org.uk/images/icons/smiley_icons/icon_biggrin.gif .

Btw i'm using the black 9 PPG blades with a 9 blade hub

jon_curtis - n/a
6-Mar-06, 06:15 PM
do you sniff a lot of glue in south africa then? cos to be frank you appear to be totally mental!!

Jamie Lewendon - n/a
6-Mar-06, 08:35 PM
"OK OK I am wrong, cut my head off will you...

I love these guys, ready at any time to take you to the butchery"

You set yourself up for that one. http://hovercraft.org.uk/images/icons/smiley_icons/icon_lol.gif


7-Mar-06, 01:03 AM
You might like to try a 7-7 Wing-Fan P6Z configuration. I had tried the 9-9 Z5 set-up, and came to the conclusion that they MW versions completely flatten out over 3500 rpm, (…I ruined an expensive racing engine that way to) Especially with the length you are running these at. The P6Z is a much stronger blade, and will well absorb the power at you current ratio. I would set them up to run at 40 degrees.

Unfortunately I have to limit my simulation to 110m/s tip speed, but then again, for cruising I think you want to keep it low anyway, and than have some extra power to spare when you need it.




Hope this helps.

Sean Atterbury - n/a
7-Mar-06, 04:24 AM
AAAAAAAAAAA JAAAAAAA officer the, there I mean noo there is aaaaa no glue here, snifffff http://hovercraft.org.uk/images/icons/smiley_icons/icon_redface.gif

But my ratio theory seems to be spot on.... http://hovercraft.org.uk/images/icons/smiley_icons/icon_smile.gif

I do however find myself in similar situations every day and thus you will find a lot more falling for a sucker situations in time... I however can sit back and laugh at myself, unlike many people I know, they just get frustrated and that does no one any good. http://hovercraft.org.uk/images/icons/smiley_icons/icon_rolleyes.gif

lazza - n/a
7-Mar-06, 05:15 PM
Thanks for the charts, unfortunatly i don't fully understand them but hopefully they will make more sense the more i look at them http://hovercraft.org.uk/images/icons/smiley_icons/icon_smile.gif

lazza - n/a
8-Mar-06, 04:42 AM
I'm just sitting here thinking to myself and was wondering... For the purpose of not having my blades self destruct due to exceeding the max tip speed i will certainly change my ratio sooner rather than later but... in my thinking if i make my fan spin slower yet the motor still pulls 6200rpm then wouldnt the craft effectively lift and crusie at a higher rev range than what it curently does ? (which kinda defeats what i was aiming for in the first place)

Maybe i'm visualising it all wrong *shrug* http://hovercraft.org.uk/images/icons/smiley_icons/icon_confused.gif

Sean Atterbury - n/a
8-Mar-06, 05:43 AM
No, don't think so, Infact it should be better, with the fan turning slower the blades can be opened up thus giving more thrust at the same engine revs, the fan will be more efficient and the engine does'nt have to work so hard.

By the way this is the second time Im typing this. Internet error.

I had the same problem with my craft and once I got the ratio sorted with the help of the folks here, man what a difference, the craft has never been better.

Now you can get better respons from the motor, it will rev to it's limit faster thus giving you powerful thrust when you need it. There is no real point in using a V8 if you only running on 4 cylinders is there?

I say, get the most gank for your buck.

Tune those blades to allow the motor to only rev to say 6 000 and or what ever you feel is the limit for you and there you go.

Now you have a craft that will respond much quicker to your demands and you have not worry about over reving or the fan speed, less troubled thaughts, more fun. More bang for your buck.

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

lazza - n/a
8-Mar-06, 08:06 AM
alrighty , sounds convincing to me. I'll go look into getting some different gears , which is a shame cause the last ones cost a fortune, but why have money if ya can't spend it http://hovercraft.org.uk/images/icons/smiley_icons/icon_smile.gif

The thing i keep thinking about is the fact that my blades are already at 50 degrees and the motor still revs to 6200, so i cant really "open them up" much further, but i don't want to exceed the tip speed anyway so i'll change the ratio and work it out from there

Hovertrekker - n/a
8-Mar-06, 12:47 PM

Talk to the folks you bought your plans/kit from before you spend more time and money. That will hopefully take the guess work out of it. Have you talked with Mike Nell from Nell fabrication in Australia? http://members.optusnet.com.au/~nell/index.htm (members.optusnet.com.au/~nell/index.htm) The Superwedge is not too far from a Viper in terms of overall design. He's a pretty good resource for figuring out problems like that.

jon_curtis - n/a
8-Mar-06, 05:24 PM
lazza dont listen to atters, he does not have any clue what he is talking about, if you saw his hovercraft you wouldnt listen to anything he says!

listen to the proper advice, from people that know! im no expert, but i do know this regards fans!

1) the faster a fan of given diameter spins the more hp it can asborb.

2) the more pitch on the blades of a fan of given diameter and given rotational speed, the more hp it will absorb.

4)the bigger the fan dia it can absorb more hp than a fan smaller, with less pitch set.

5) the more fan blades in a hub the more hp it will aborb. i.e a 9 of 9 will absorb more hp than a 6 of 9.

etc etc etc

if the engine can give out more hp than the fan can absorb the the engine CAN OVER REV. as from above the faster the fan spins the more hp it absorbs.

it sounds like you have a choice of fiddling the ratio, which would possibly put you over the tip speed and make the fan noisy, or stick with the ratio and put more blades in the hub, which in your case would mean a new hub? and more blades, this however can effect your lift and thrust dynamics, which starts to make the selection of the right fan complicated. i,e less blades generally means more thrust, but not as good lift, more blades means better lift, with not as good thrust! a balance for your craft and applications of flight needs to be sort! see if you can make sense of the tables as they will help in your choice!

hope this helps?



Paul Fitz - n/a
8-Mar-06, 09:32 PM
Lazza, If you read through the two articles on the dowloads page, "understanding and selecting lift fans" & "Calculation of thrust......." , you will quickly understand how to read the charts. Don't bother with the maths to start with, just look at the examples and graphs shown.

9-Mar-06, 03:26 AM
The Wing-Fan P6Z is a wider and stronger blade that will fit into your existing hub. According to my experience, they will absorb the additional horse power, and prevent excessive fan and engine speeds.

I thing using 9 P6Z blades will give excellent lift and payload at lower rpm’s, and most likely at your current ratio.

If speed is you preference, you can try a 7 blade hub? (btw, that’s what I now use, but inside a slightly smaller duct) If your feel you still need more lift, one option could be cut the 7's a bit further, get 2 additional blades, and make them fit your existing 9 blade hub? But keep in mind the additinal weight of the 9 vs. 7 blades.

In the US you can get one of these newer blades for about $25 dollars US.

lazza - n/a
9-Mar-06, 04:33 AM
Ok thanks, I'll go do some reading and research before i do anything else. thanks for your input everyone.

Sean Atterbury - n/a
9-Mar-06, 07:18 AM
DANGERMAN, You junior member, read more carefully, you just swopped over my words and made them your own. http://hovercraft.org.uk/images/icons/smiley_icons/icon_eek.gif

Anyway I agree with Wolfie, new blades, new ratio better thrust and lift, less danger man!!!! http://hovercraft.org.uk/images/icons/smiley_icons/icon_lol.gif

Get the punn... http://hovercraft.org.uk/images/icons/smiley_icons/icon_lol.gif

Me sinior member, you little shifffff. or running bull or flying bird, what ever you like realy. http://hovercraft.org.uk/images/icons/smiley_icons/icon_lol.gif

Nickinoue - n/a
9-Mar-06, 09:50 AM
Jon C?, does this mean u r back on the wallpath of the forum... http://hovercraft.org.uk/images/icons/smiley_icons/icon_biggrin.gif

BTW Atters i dont wanna take sides but u cant really judge peoples experience by how much they go on this forum http://hovercraft.org.uk/images/icons/smiley_icons/icon_smile.gif .


Sean Atterbury - n/a
9-Mar-06, 10:24 AM
True. & I know my craft look c**p but it works great and I have 5 years of trial and error behind me to get to where I am now, I could have baught an F1 brand new with the money I have spent over the time I have spent tinkering around. But if I had, then I would have learnt diddly squat. Now I get to pass on what I have learnt so that other's do not make the same mistakes as I did.

The next craft will not only go well but look great, as I practice with fibreglass I get better every time, so look forward a couple of years to the next one. http://hovercraft.org.uk/images/icons/smiley_icons/icon_smile.gif

9-Mar-06, 10:27 AM
I'm just sitting here thinking to myself and was wondering... For the purpose of not having my blades self destruct due to exceeding the max tip speed i will certainly change my ratio sooner rather than later but... in my thinking if i make my fan spin slower yet the motor still pulls 6200rpm then wouldnt the craft effectively lift and crusie at a higher rev range than what it curently does ? (which kinda defeats what i was aiming for in the first place)

Maybe i'm visualising it all wrong *shrug* http://hovercraft.org.uk/images/icons/smiley_icons/icon_confused.gif

You are on the right track with your thinking here! Maybe this visualization will help convince you to use better blades that can absorb the right amount of power?

Sean Atterbury - n/a
9-Mar-06, 10:36 AM
Been there, done that... http://hovercraft.org.uk/images/icons/smiley_icons/icon_cry.gif

nickyd - n/a
9-Mar-06, 10:58 AM

I think your best bet is to go to the links of this site and get in contact with one of the professional companies which offer fans. They should be able to help you select the correct fan for your application. Then you can spend your money once on the correct set up. I have recently gone the same route and found Conair Sport to be incredibly helpful and I am now confident that I will be spending my money on the right fan and pulleys for my application.

As you can tell from some of the above posts there is a big differing of opinions and you may not be able to tell who knows what they are talking about!

All the best


jon_curtis - n/a
9-Mar-06, 11:55 AM
atters you have learnt diddly squat, everytime you make a post, it becomes more and more evident you have not leartn anything and have no clue what so ever about the bull you are sprouting!

was it not only a few months ago, that you had a chain driving your fan? you have bent shafts, you have obviously smashed fans, your splitter plate is made of steel plate?? i would consider your craft to be dangerous! and you are probably endangering your self and others when you use it!

you come out with incredibly nutty remarks, you have sent me emails to my personal account (yes i have given you lots of advice in the past) that i would have considered personally offensive, but you thought were funny!

I think its about time you calm down a little and stop being such a nut case! how old are you atters? cos you act like your five years old?

its obvious to everyone you dont know what your talking about, but to the people that dont know this and listen to your advice, i think they need to be warned!!!

calm it down you nutter!

Sean Atterbury - n/a
9-Mar-06, 12:08 PM
OUCH http://hovercraft.org.uk/images/icons/smiley_icons/icon_eek.gif

Almost like a speech I got from my Dad once. http://hovercraft.org.uk/images/icons/smiley_icons/icon_rolleyes.gif

lazza - n/a
9-Mar-06, 04:36 PM
I have taken into consideration everyones posts and pushed to the side the posts that i believe are a little miss leading http://hovercraft.org.uk/images/icons/smiley_icons/icon_razz.gif . In my thinking i agree with the idea that if better blades were to abosrob my of my power then that would not only reduce my maximum revs and therefore keep me under the maximum tip speed but is also possibly going to achieve what i was aiming for in the start. The last thing i want to do is have my own personal pictures that look like those pictures wolfy posted http://hovercraft.org.uk/images/icons/smiley_icons/icon_smile.gif

As it works out, my duct which is classed as a 950mm duct is in fact 944mm and with a tip clearance of 4 - 6 mm means that my fan is actually 936mm in diameter. If i use 936mm instead of .95m in the tip speed calculator then the tip speed ends up as 168m/sec so i'm right on the limit rather than over it, and considering that is based on the maximum revs of 6200rpm which i don't usually allow it to get up to then untill i can work out for sure if i need better blades etc i feel alot safer as far as tip speed goes.

thanks again

lazza - n/a
31-Mar-06, 08:19 AM
Sorry to drag up my old post but i finally worked out the answer to my 'high rev' problem i was talking about in the above post's. Luckily before i went and spent money on throwing things at the craft to try and fix my problem i thought i better eliminate the basics first and so i got my rev counter checked for accuracy and it was 15% faster than what it should be http://hovercraft.org.uk/images/icons/smiley_icons/icon_biggrin.gif (Isn't it great when the simplest of things are the answer for a potentially frustrating problem )

Anyway as it works out, when the craft showed it was doing 6200rpm it was in fact doing 5270rpm which gives me a tip speed of 149.80m/s much better than the 168m/s i thought it was http://hovercraft.org.uk/images/icons/smiley_icons/icon_razz.gif so now all i need is another break in the weather so i can get back out and see the real revs in action.

Thanks for the help and suggestions

Happy man again



Sean Atterbury - n/a
31-Mar-06, 08:50 AM
Well Well Well http://hovercraft.org.uk/images/icons/smiley_icons/icon_biggrin.gif

That's some great news.

My friend rebuilt a 4X4 and it had a 4cylinder engine in, they they chucked a V6 in and just conected the thing up to the existing rev counter, now at 2000 proper revs the rev counter shows about 5 000, quite good to save the engine but not practical lettin it go past red line to cruise down the highway... http://hovercraft.org.uk/images/icons/smiley_icons/icon_lol.gif

You should see the faces of people that drive it http://hovercraft.org.uk/images/icons/smiley_icons/icon_lol.gif