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Sudden deep ventilation ?

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Scuba
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Re: Sudden deep ventilation ?

Postby Scuba » Fri Feb 20, 2015 11:54 am

I found this same problem with my first carbon hydrofoil.
I put a post on here, and was answered.
The foil was to shiny and the water wasn't attaching its self to the foil mast wings etc.
It was shown on a pic with ventalation the length of the mast.
Also the leading edge to sharp, it needs to be blunt like a windsurfer fin.

I rubbed the leading edge down so it was blunt, and rubbed the whole mast, fuse & wings with 320 then tried to sail, I was blown away the problem had gone.
Hope this helps


viewtopic.php?f=196&t=2386520

Scuba

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Re: Sudden deep ventilation ?

Postby gmb13 » Fri Feb 20, 2015 12:48 pm

Hi Everyone,

I encountered this problem when we were testing different constructions and profiles for the mast.

I initially thought it had something to do with the wings, but no matter what wings we used, the problem kept occurring. Another problem was that it was not easy to reproduce the problem.

Until I started filming with the Go Pro. Then the problem became quite clear. It was a combination of too much twist in the mast and the profile being too sensitive to this twist.

When the twist of the mast deformed the profile at the surface of the water, we would get an aggressive stall. The bubble did not even need to make it to the Wings, the whole Foil would loose traction and you would crash. And Peter_Frank described it well. At high speed it feels like some has ripped the carpet away from under you.

We initially solved this problem by stiffening the mast, but have also changed to a less sensitive profile.

Here is the only video I can find that is on my Computer from that time.



Look closely at the mast and watch it in 720p



--
Gunnar

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Re: Sudden deep ventilation ?

Postby IanNJ » Fri Feb 20, 2015 1:22 pm

It amazes me that these things are that sensative...
So basically if you ride the foil slow there are no problems...but trying to go fast it dumps.. I thought this was my skill. Im on the LF foil... I found there is a good mellow crusing speed that i am comfortable and so is the foil. I thought my crashes were because i still suck at real high speeds

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Re: Sudden deep ventilation ?

Postby gmb13 » Fri Feb 20, 2015 1:27 pm

IanNJ wrote:It amazes me that these things are that sensative...
So basically if you ride the foil slow there are no problems...but trying to go fast it dumps.. I thought this was my skill. Im on the LF foil... I found there is a good mellow crusing speed that i am comfortable and so is the foil. I thought my crashes were because i still suck at real high speeds
You are overgeneralising. Only foils that are badly designed or untested should have this problem. A good foil with become quite stable at speed and should not have this types of dropouts.

If you are used to your LF foil now and want to go faster, there are a few things you can modify to make your LF work at high angles and speeds. You can PM me if you are interested.

--
Gunnar

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Re: Sudden deep ventilation ?

Postby zfennell » Fri Feb 20, 2015 5:41 pm

Gunner is great, gunner is good, let us thank him for… what he says makes sense.
Plus, he has supporting evidence regarding the cause.
But, I’m not sure if the specific ‘ailment’ has been identified.

I do like Scuba’s assessment that it is also related to foil properties. Either nicks in the surface or an excessively sharp leading edge. ..Particularly, if you combine that with gunners concern over twisting of both strut and wings.

Cavitation is a possibility, but that would be accompanied by significant noise and vibration would be readily detectible if damaging enough to cause a crash, imo.

Laminar separation is one guess. Sharp leading edges, smooth profiles , shiny surfaces and low Reynolds number are good conditions to support laminar boundary layers. However, laminar BL are sensitive to laminar separation. This could lead to complete loss of lift on top surface, similar to stalling.
As Scuba noted, rounding the LE, makes the foils less sensitive to AOA. sanding the surface causes early transition to a turbulent BL which is much more resistant to separation of the boundary layer ( a legit analogy to the reason for dimpled golf balls)
I'm sure Peter can recall many stories of airplanes when laminar flow wing profiles were initially implemented. Despite the low drag, many fell out of the sky at inconvenient times.
Turbulent boundary layer separation (stall) still happens, but usually a higher AOA that the 'sweet spot' for foils

All pretty interesting.
I would like to ask for more details if possible.
What was the ‘fast’ speed (or range of speeds where this happened)?
What was heading (upwind, downwind etc)?
Any particular maneuver involved?
Heel Angle?
Wing profile (eppler, naca, DIY, etc)?
Strut profile (eppler, naca, DIY, etc)?
Wing size?
Surface condition of foils?
Any comparative values for stiffness of wing or strut?
Wings on top of fuselage?
were Top of Wings well faired with fuselage (possible interference effects)?
Wing loading (weight/area)?
Can your description of ‘loss of lift’ also be described a significant increase in drag?
Just prior to event, did you feel you had ability to alter AOA and jump or dive at will?
lastly, are you still convinced that ventilation from the air/water interface is not happening?

...and all this time I thought crashing was my fault.
-bill

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Re: Sudden deep ventilation ?

Postby revhed » Fri Feb 20, 2015 7:27 pm

P F,
Thanks for great question, needed to go thru all my old notes as I knew I saved some info on this.
Just some ideas of possible causes, most already mentioned.
To sharp L E, mostly on strut, but wings also.
Strut twist, poor profile shape, or workmanship.
Nicks, dings dried on stuff anywhere on the T bar again more so on strut L E.
And this is most interesting I think.
A super gloss, shinny paint applied, instant glitch action, where before painting almost never.
There is also maybe some valid info here,
viewtopic.php?f=196&t=2384533&p=837568&hilit=+glitch#p837568
my post
In closing.
Thankfully I don`t get this much, but from time to time in debreifing the session we have noted that many experience glitching on the same day with different KBHFs. Environmental issues?
R H
It sure would be nice to know and understand exactly what is happening to help my sore neck sometimes!
ZF? others?

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Re: Sudden deep ventilation ?

Postby Scuba » Sat Feb 21, 2015 12:01 am

Check out this link that revhed posted last September

http://www.boardlady.com/fast.htm

I think that if the water isn't attatching to the mast air will instead.combined with a to sharp leading edges mast wings etc.

My situation traveling at 20 knots the foil would lift 100mm and fall to the side with very little chance of recovery, which is a sign of ventalation.
It would happen on any reach at the same speed.
I followed what Surf coast said and bingo the problem went.
I had no noise, no vibration, no nicks in wings.
Check out the link to surfcoast in my previous post, it's well worth a look.

Scuba
Last edited by Scuba on Sat Feb 21, 2015 12:07 am, edited 1 time in total.

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Re: Sudden deep ventilation ?

Postby Kite Rider » Sat Feb 21, 2015 12:05 am

Is it possible you are experiencing what this guy is talking about (supercavitating foil section), happening when the pressure on the top of the wing is so low that evaporation then cavitation occurs? I know the guy talks about this phenomenon occurring at around 50 knots, but maybe some particular wing design can create it at lower speed.


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Re: Sudden deep ventilation ?

Postby revhed » Sat Feb 21, 2015 9:55 am

SURF COAST POSTED as scuba noted;

"The leading edge has been made more elliptical.

The best mast leading edge shape to use as an example are found on good sailboard slalom or racing fins, low drag & more importantly the water flow re attaches quickly when ventilated under extreme conditions."

I can not agree more with this!
My windsurf fin is a much used reference tool.
Thanks S C and Scuba.

So,
A strut with as little twist as possible
Well shaped L E, not to sharp or round
surface texture not to glossy
Profile shape respected
No nicks or dings
AND?
R H
Last edited by revhed on Sat Feb 21, 2015 11:02 am, edited 1 time in total.

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Re: Sudden deep ventilation ?

Postby Andokite » Sat Feb 21, 2015 10:20 am

I love this post, so many wise feedbacks from Gunnar, Peter Frank, Revhed, etc,...

Let me add a little video that imho sums up all that has been said before.

Just one more thing: using CFD sotware or building a "soft" aluminium front wing, you will notice that the resulting shape of the wing under load is a "seagull" shape. If you counteract this forces´ distribution by designing and building a front wing ready to absorb these forces (this is not new, it's being applied to everything in this world from bridges to buildings to airplanes), you'll come out with a very stable wing that will maintain perfect shape under heavy loads (very powered riding, extreme angles up and downwind) offering a very stable and fast riding.

Please check how close to the water surface this wing is traveling without the slightest sign of ventilation or lack of stability

https://www.youtube.com/watch?v=MmlagYZxvq0


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