View Full Version : Balancing Multiwing fan, 9-9 or 9-12?

ugglas - n/a
2-Nov-06, 07:43 AM

I am going to fit the fan to my craft soon, and now I wonder if anyone could answer some of these questions.

I know how to cut the fan blades to get the exact length and diameter but how do you guys get it in weight balance?

Someone one this forum wrote that one has to balance the fan every time the blade angles is adjusted, if so, why?

I also read somewhere in this forum that a 9-12 fan configuration is quieter than a 9-9, again if so, how much quieter?

And how much performance will I lose if I use a 9-12 instead of 9-9?

I will use a Subaru EA-81 as power source in a integrated craft with a 1000mm fan duct.

One reason I am thinking about the 12 blade hub is that a fan suplier in my area told me that the 9 blade hub would not take the power from the EA-81(70-80hp)

And as the craft will be a cruising craft it would be positive with lower noise levels.

hovercad - n/a
2-Nov-06, 10:40 AM

Im no expert.

What I did was once I knew what the length of the blade was tested it in Duct I then cut the rest to the same length.

I then weighed them all,found the lightest then made all the rest the same weight.


ugglas - n/a
2-Nov-06, 11:50 AM
Thanks for the answer Craig

I tought about something like that. That should also mean that it is no need to balance the fan every time the blades are adjusted.

But when the blades are balanced separately from the hub, should the hub also need to be balanced or are the casted hub halves already good enough?

And where to you take away material from the blade to not change the dimensions?

team black - n/a
2-Nov-06, 12:04 PM
It really doesn't matter that much provided you aren't after the last drop of perfection. Trim the blades to the same length and chuck them in. Provided you aren't wildly out either initially or after adjustment, there won't be a problem.

I used to leave blade cutting to inept teenagers and never had a fan that was discernably out of balance

Keith Oakley - n/a
2-Nov-06, 12:28 PM
On the noise issue provided the tip speed of the 9/9 fan is the same as the 9/12 noise level will be about the same. There's a small difference between the two in the pattern of frequencies it will produce but I wouldn't expect any significant overall noise reduction. Noise level is proportional to 5th power of tip speed so biggest way to reduce noise would be to reduce tip speed and keep obstructions like flow straightners more than a blade width away from the fan. But tip speed is a compromise with thrust and lift performance so I'll let others pick up on that.

Don83000 - n/a
2-Nov-06, 06:24 PM
One reason I am thinking about the 12 blade hub is that a fan suplier in my area told me that the 9 blade hub would not take the power from the EA-81(70-80hp)

In this case it may be worth you getting a hub from the UK as they will take your bhp with no problem I have been running 115 bhp through a 9/9 5Z Multi-Wing for 2 years with no problems and also ran a EA81 through the same hub prior to that.

jon_curtis - n/a
2-Nov-06, 08:30 PM
a good way to cut the blades, (and the balance will be pretty good as this is how multiwing do it)

is to setup a jig on a band saw , with the blades in the hub that can rotate and turn the fan thru the blade of the saw. the tips will be radius and with this method you can get very tight tip clearance!

Ian Brooks - n/a
3-Nov-06, 04:49 PM

The choice of fan and reduction ratio will have a huge impact on the performance and noise of your craft... I think you say somewhere that this is to be cruiser, so the choice of 9 blades is probably somewhere near - you might also want to consider 12 blades, depending on the cushion pressure. I you would like me to run some calcs to help you with your decision, then let me know and I'll give you a list of data you need to collect from your craft - with that I'll be able to estimate the lift and thrust performance of any given fan.



ugglas - n/a
4-Nov-06, 10:16 PM
Thanks very much for the replies everyone!

I think I will go with the 9-9 fan.

Good to know that it is good enough to cut the fan blades to get it balanced, would be much more complicated else.

But there is yet one thing I am wondering about, what tip speed should I aim at? As I am building a cruising craft low noise is preferable, which means lower tip speed. What tip speed should be enough for my craft, and still not loose too much thrust and lift performance?