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SoutherlyBuster

Naish Foil Board

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Final sanding done. Paint job done. Now just have to wait for the paint to harden, then try it out in the surf. Might have to experiment with different angles of attack of the rear wing by way of shims, will see how it goes first.

Here it is, out with old and in with the new:

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Sea trials are complete, took the board out into the surf with the home made rear wing today. Took some shims along just in case the zero angle of incidence needed adjusting. The lower angle worked to my favour, lifting later but at speed needing to be less forward on the board, so was more managable to control pitch at higher speeds. Gave a nice stable flight. There was a hum though, will need to investgate possible causes. And the rear wing did not break, ya hoo.

After the sea trials:

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(No fairing added between wing and fuselage, might be the cause the the hum.

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Counterbores applied to the wood were a neat flush fit, but when the glass was applied, the precision was lost, so this might also be the cause of the hum by way of turbulence. All final brush strokes of paint applied in the stream wise direction, to avoid roughness in the wrong direction.

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Highly approximate, but I remembered the frequency of the hum, then when I came back to my car, by memory I sang the same hum and used the stroboscope on my mobile phone to measure the frequency. 146 Hz, rather low frequency.

Foil thickness distribution was "eye" balled, so maybe there are some parts of it causing too negative a pressure, leading to cavitation which also might be the reason for the hum. The wing feels reasonably stiff so would not have thought the hum was due to the wing being too flexible.

 

Oh and pumping the board works better now, used this once standing with kite to get the board quicker up to speed.

Edited by SoutherlyBuster

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Some googling, reading and applying some engineering common sense ....

There appears to be a lot of “wive’s tales” and snake oil surrounding the source of the humming, but also a lot of good information out there. From my reading it appears the shape of the trailing edge is the most important aspect to reduce or eliminate the humming. As the two flow streams from the top and bottom sides of the wing come together at the trailing edge, where the vorticies peel off is the important bit. If they peel off consistently on either the top, middle or bottom then no vibraton; if they peel off alternatively between the top and bottom surfaces, this causes a vibration and this is what the hum is. From a Yatching design book, there are some guideliness to reduce or even eliminate this vibration, see below for extracts and source of the book:

20110417Principles%20of%20yacht%20design

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What I have is a rounded squared off trailing edge — not a favourable shape. So what I need is some thing like option 5 or 8. The fibre glass is wrapped around a shape like option 1, so as not to expose the wood underneath the fibre glass protective layer, I need to add some fairing to the trailing edge to make it one of the more favourable shapes, but also ensure I don’t make it too sharp to avoid cutting myself.

 

Thanks @Kamikuza for the suggestion, it lead me to find the above information. By the way your name appeared in some of the searches :) . Looks like a lot of other foilers experience the same problem. Also one suggestion was to apply some tape to one side of the trailing edge to stop the fluctuatin of where the vortices peel off, might be a quick way to see if this is where the vibration problems come from.

 

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That's interesting. I wonder if it matters that the article is talking about vertical surfaces rather than horizontal, and surfaces that are much further apart than kite foils are...

5 or 8 is going to lead to cuts when you kick the foil under water. I think I still have scars from kicking a stab last year.

I think my name pops up because I've asked questions and commented on my experiences, rather than offering anything useful :D Searching images for "axis kite foil" used to turn up my photo in first spot, but now for me it's the end of the second row :(

This topic is interesting to me because I've ridden two different foils of one brand, and one has whistled like a steam train, the other has been completely silent. We've got a guy with an Alpine foil that whistles so loudly you can hear him literally hundreds of meters away. He's swapped the wings and it still whistles. Another guy with the same foil is silent. Why?

Is the whistling related to vibrations, or is that something else? Both my Axis foils are smooth as silk through the water, but the kite foil would squeak softly if I loaded it up in a carve, and the S-series has a gentle whistle when running flat out. Neither are as bad as the Alpine, which is bloody loud.

So I think it comes mostly from the stab, or interaction between stab and water flow around mast and stab.

 

Is it vibration you're feeling too, or just the sound? With the tape residue on the other wing, it was feeling like I was driving over shingle rather than any noise...

 

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I applied some tape to the trailing edge starting on the top side level out extending beyond the trailing edge by about 2mm fold over and stick to the bottom surface of the foil. Humming gone (which I could hear and feel).Screenshot_20181117-201031.thumb.png.180c76eb21e4489a6653d3fc912ec9b6.png

The tape is very sticky, I use it for reinforcing my foam gliders, sticks like sh@#$/ $# on a blanket. Did not come off during the light wind session 6.6 to 9 knot wind with 18m FlySurfer Lotus.

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17 hours ago, Kamikuza said:

That's interesting. I wonder if it matters that the article is talking about vertical surfaces rather than horizontal, and surfaces that are much further apart than kite foils are...

5 or 8 is going to lead to cuts when you kick the foil under water. I think I still have scars from kicking a stab last year.

Is the whistling related to vibrations, or is that something else? Both my Axis foils are smooth as silk through the water, but the kite foil would squeak softly if I loaded it up in a carve, and the S-series has a gentle whistle when running flat out. Neither are as bad as the Alpine, which is bloody loud.

 

Vertical or horizontal makes no difference as gravity makes no difference here on the formation of the vorticies, and it is the vorticy formation alternating between either side of the foil that causes the vibration.

Surfaces being further apart, yes agree. I thought about this too, there must be some sizing effect going on. If you zoom in close enough there will always be a blunt end of the trailing edge. I suspect the characteristic length would be the boundary layer thickness. I expect the above article was an A to B comparison, same foil size, just alter the taper angle. So at some point a blunt end would make no noticable difference.

Agree some of these sharp edges look nasty for rider safety. Reckon the blunt hard trailing edge plus the relatively soft tape I used is a good solution, just a mater of find tape that lasts long enough. Helicopters use a special tape for the leading edge for abrasion resistence ... hmmm.

10 hours ago, Kamikuza said:

That's very interesting too. You stuck the tape to itself past the trailing edge? Basically just longer and thinner trailing edge, eh. So are you going to thin out your trailing out to a point? Or just slim it down some more?

Yes the tape was double backed onto itself to creat a thinner trailing edge. At the moment I am thinking, don’t mess with the foil, just use tape, main motovation is rider safety against cuts. The tape lasted well for one session, will see how long it lasts. Longer term solutions would be tapes with heat activated glue applied to the resin coating.

I suspect if the foil wing and mast were made of a very heavy highly damped material with zero vibration transmision you would not hear the wistling. Apart from the very low frequency vibrations that one would definitely feel through your feet, the high pitched frequnecies may be more noticable depending on the stiffness and mass of the mast and board, if the whistling frequency excites one of the natural frequencies of the mast and board, then it would be clearly audible, like a sounding board of a piano.

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I was thinking more differences in water pressure at different depths, and how they might travel.

My foil's TE are noticably squared, the front thicker than the rear.

Tape would be good for free ride but I doubt the racers would countenance it :D

I think the relative stiffness of the fuselage matters too -- wouldn't stiffer transmit more "sound"?

"Rumbles" are the turbulence in the water I think, and the trim difference between wing and stab feel similar but more rearward to me...

So where is the whistling actually coming from?

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14 hours ago, Kamikuza said:

I was thinking more differences in water pressure at different depths, and how they might travel.

So where is the whistling actually coming from?

Depth, ah right the p=density x depth x g. For the mast can be important but I doubt important for whistling.

Where is the whistling coming from? Well in my opinion, it is not just from one thing. Think of it like a guitar or violin, the sounds come from the vibrating string (our vortex sheet coming off the foilig trailing edge), the vibrations then get transfered to something which amplifies (the wing to mast to the board) the sound, then eventually to something that creates air pressure waves so we can hear it. If the vibrations were solely confined to the water, I doubt we would hear it. To amplify the vibrations, you need to tune the natural frequency of your strucutre to match the forced vibrations, hence why same foils with different mast/board some sing, some don’t. Ever pick a guitar and notice the sound rings nicely and others just sound dull? It is all about getting that resonnance right in the structure of the body and for guitars and violins the air space since it has it’s own set of natural frequencies.

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Is a whistle the same as a hum though? We're using them interchangeably but I think they're unique.

The Alpine is a definite hum by the character of the sound, and the DA would just start humming at a certain speed, like a switch was thrown.

My whistle grows in volume with board speed, and probably speed into the wind, I'd have to concentrate. There are mounting holes in the plate, I wonder if they're the source. It's really almost non-existent, very quiet -- unlike the DA or the Alpine.

Now I think of it too, the Alpine is a Tuttle mount on a Spots board...

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