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Thread: blade adjustment

  1. #1
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    Default blade adjustment

    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 )




  2. #2
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    Default Re: blade adjustment

    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.





    Cheers.



    Andy.




  3. #3
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    Default Re: blade adjustment

    Larry,

    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 )

  4. #4
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    Default Re: blade adjustment

    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!

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    Default Re: blade adjustment

    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



    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


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    Default Re: blade adjustment

    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.

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    Default Re: blade adjustment

    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 . 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



    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




  8. #8
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    Default Re: blade adjustment

    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

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