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Why do Foils Howl?

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IainMacaskill
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Why do Foils Howl?

Postby IainMacaskill » Tue Sep 11, 2018 10:28 pm

Please can someone who knows tell me the real reason some foils Howl?

I've heard it's salt on the mast and other people have said it's because you're riding to slow but these answers sound a bit like "old wives tales".

There is a lad with a new Naish foil down where I ride and that thing Howls like a wolf, even when it's clean and he's going flat out, so is it just a dodgy mast or is there another reason he's howling?

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Re: Why do Foils Howl?

Postby Laughingman » Tue Sep 11, 2018 10:35 pm

I don't "know" the answer but my foil only howls slightly when I am in a stall type position... meaning I am going quite slow... probably too slow to keep flying... unless I increase my speed I lose lift rapidly

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Re: Why do Foils Howl?

Postby geokite » Tue Sep 11, 2018 11:24 pm

I don't know if this is the answer you are looking for, but this is what comes to mind.

All sound comes from vibrations, and lots of sounds come from one vibrational frequency matching another objects natural frequency (causing the second object to vibrate and amplify the sound). Water at the trailing edge of the mast or wings sometimes vibrates, and if the mast or wing vibrates at the same frequency that vibration (sound) will be amplified.

Sanding the trailing edge disrupts that vibrating water.

Check out 19:13s in the video at https://www.youtube.com/watch?v=ODs4eyQTz5Y The video shows what is happening at the trailing edge of the wing and/or mast, but with air (not water in our case).

Just starting foiling, got my 4th session in yesterday, and I like the sound my Lift mast makes. Reminds me of the ambient music I listen to.

Steve

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Re: Why do Foils Howl?

Postby Obe_1 » Wed Sep 12, 2018 4:07 am

The most probable cause of this would be from vortex induced vibration. The Karman vortex street in this link gives an explanation and animations of this.
https://en.wikipedia.org/wiki/K%C3%A1rm ... tex_street

When the vortice shedding frequency nears the natural frequency of the mast, wing or stabilizer, they will start to resonate or hum.
It's the audible frequencies that seem bother most people, but the reality is that there is some vibration going on at most every speed.
There is some advice about sanding the trailing edges of the mast or wings at specific angles to eliminate this hum. That may well change the vortice shredding frequency. It should certainly help at a some speeds, but really is only a bandage if this is truly considered a problem. On top of the fact you are sanding on what may be an expensive wing or foil. How many times do you have to get that right.
Its the mast or wings that are being driven into resonance and vibrating.
Structural or material changes would normally be required, but likely considered cost prohibitive. Isolation or some type of damping may be more suitable.
You may be able to try and isolate the mast with something as simple as a rubber mat or strip under the mast base. This could be a bit of a hit or miss as you wouldn't know what frequency your dealing with or the tuning of the rubber, so its effectiveness could be limited. The rubber should absorb some of the vibration.

Any decent CAD should able to get a reasonably close approximation of the frequencies and vibration modes of the foil and CFD simulations could provide vortex flow data. Ideally material layup/construction would be optimized to provide a "tuned" foil that isn't reactive to the vortex shedding. An isolation or vibration damping element could also be developed as an alternative.
Alternately the hydrofoil and board would be set up with a set of accelerometers and impact tested to determine what the natural resonant frequencies of the components are and then work on the design to push them outside of the audible and performance affected zones. Given the current state and pace of foil development, I am not sure the kite foil industry is able to warrant this type of activity, but maybe someday. :D
Sorry if I went too technical, just throwing in my opinion.

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Re: Why do Foils Howl?

Postby cor » Wed Sep 12, 2018 10:20 am

IainMacaskill wrote:
Tue Sep 11, 2018 10:28 pm
Please can someone who knows tell me the real reason some foils Howl?
Because they are happy! :D

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Re: Why do Foils Howl?

Postby Peter_Frank » Wed Sep 12, 2018 11:14 am

cor wrote:
Wed Sep 12, 2018 10:20 am
IainMacaskill wrote:
Tue Sep 11, 2018 10:28 pm
Please can someone who knows tell me the real reason some foils Howl?
Because they are happy! :D

Ha haa, yes they are happy :thumb:

There can be many reasons, obviously it is oscillations in the water at a frequency we can hear.

It is very often from the trailing edge, can be of ANY part of the foil, being too round or having an odd shape even if not round, or wing having the wrong shape at the last centimeters before the trailing edge.

But it can also be scratches on the wings or fuselage or mast, or the interconnections, or the screws.


Salt, I have never heard about that?

Usually the foils that "Howls" gets more and more aggressive (higher/louder) the faster you go.

But one thing CAN happen if you ride too slow but still foil: Low AR stable wings tends to vibrate or even hum very low frequency (deeper tones usually not possible to hear, just opposite to the high pitch howling at high speeds), because these wings get huge tip vortices (oscillations/vibrations) when semi stalled, which is actually good as it is a stall warning you can feel, BEFORE they stall, so you can still ride in this state for a while, and push forward or more kitepull so you avoid the stall - excellent :thumb:

Whereas superhigh AR narrow wings dont give you any warning, they simply drop you like a rock if you ride too slow, NO warning at all.

Regarding the classic howling though, it can be REALLY a PITA to find where it origins from - you might be lucky to find it quickly (and if others got the same on the very same foil and solved it you could use this as first approach), but as it can be the TE of the mast, the wing, the stabilizer(s), the fuselage - or the interconnections, or the surface, or the design/curves, or the holes/screws it can be pretty frustrating to find :roll:

8) PF

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Re: Why do Foils Howl?

Postby jkrug » Wed Sep 12, 2018 2:38 pm

my foil howled once. i went to shore and realized the mast was on backwards.

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Re: Why do Foils Howl?

Postby Mossy 757 » Wed Sep 12, 2018 5:44 pm

Howling is evidence that there is a surface that is generating significant, audible vibrations. This is likely due to imperfections in the control surfaces that are carrying the hydrodynamic load while riding.

The cure is to try to create smoother flow over your surfaces, so in some cases this means squaring off the trailing edges to a 1mm thick 90* angle using a sanding block, in other cases you might want to work on the mast, or both, but basically it's vibration occurring at the TE that's causing it.

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Re: Why do Foils Howl?

Postby TomW » Wed Sep 12, 2018 6:51 pm

I would spray the front wing with 3-4 layers of spray primer. Then wet sand with 400-800-1200. Do the trailing edge at the same time. I bet it stops.
Probably just as well to do stab at same time.
My Moses 590 had some very very small pin hole imperfections in the clear coat. It hummed a bit. Did the above and it dissappeared.

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Re: Why do Foils Howl?

Postby andrzejwodejszo » Thu Sep 13, 2018 3:40 pm

Image


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