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Sudden deep ventilation ?

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zfennell
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Re: Sudden deep ventilation ?

Postby zfennell » Sun Feb 22, 2015 1:37 pm

ronnie wrote:....

In this video at 1:24 there seem to be eddies left behind by the mast and air travels down them. Maybe if you had a foil design that could create eddies of a type that could be a path for air down to the rear wing? The sudden change of lift in the rear wing may tilt the front wing down?
Does it feel like you hit something? - or does it feel like the front wing didn't slow down but nose-dived?

No opinions regarding eddies, but the vid was fun.
Thanks

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gmb13
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Re: Sudden deep ventilation ?

Postby gmb13 » Sun Feb 22, 2015 6:28 pm

Johnny Rotten wrote:has anyone considered or experimented with gates on a hydrofoil?

A well placed/designed gate could

a) provide ventillation resistance.
b) limit twist and provide torsional stability.

at the expense of a minor amount of drag.

I'm sure the problem could be solved with VERY thorough design but it would likely require a VERY thorough knowledge of hydro/aerodynamics that is beyond the average hydrofoil maker and it appears to have a factor that is dependent on conditions as well.

Sometimes more "drivable" makes the race car faster.
I have actually tried gates as part of trying to solve the "glitching" problem.
They did not work. They actually caused more ventilation, basically causing the flow to separate at the gate when the gate touched the surface of the water and causing the ventilation bubble to travel down from there.

I tried adding another gate right above the Fuselage, but there is already to much drag from the ventilation that it has not effect on stopping the crash.

Like I explained in my first post. We solved this by stiffening up the mast. As soon as we did that, the "glitching" was gone, at all speeds. I have not had a Foil with "glitching" problems since. So I am sure you don't need a super knowledge of hydrodynamics to solve this problem, you just need to get your layups right to stop flexion and torsion in the mast.

--
Gunnar

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Re: Sudden deep ventilation ?

Postby revhed » Sun Feb 22, 2015 7:33 pm

Hello,
I will say that a super stiff, very low twist strut that has to sharp a L E and.,or to glossy, slick surface, nicks ,dings, will spinout, glitch, ventilate, cavitate, not sure which, but one sure, first hand experience, not to say a stiff, low twist strut is not ultra important.
Another case in point.
I once loaned my KBHF to a friend who is learning, he flew the board up and threw the lines and kite looped with the bar around and banging against the fuse and T E of the strut.
Small nick on the fuse just behind the strut to fuse connection.
Instant "spin out, glitch" at a certain speed, every time, until fixed that night.
Next day nothing.
Gunnar, I respectfully submit that strut flex, twist may be the major cause of this, but not at all the only one.
By the way my KBHFs glitch now once in a blue moon and I guess environmental causes?
R H

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Peter_Frank
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Re: Sudden deep ventilation ?

Postby Peter_Frank » Sun Feb 22, 2015 8:42 pm

Hi

I just had a session here in the "dusk" this sunday, with my buddy Martin.

Out perfectly full powered with an 11m2 wavekite.

Running about 23 knots both tacks at a course around 95-110 degrees on my strapless hydrofoil.

This time my goal before going out was to really look and feel what happens, and measure, to explain (particulary to you Bill as you love the "facts" in order to have something to really think further about....)

But what do you know - not one single time experiencing this "glitching" as I believe it is called (sudden loss of lift at high speed, not classic ventilation not a stall).

I believe, in my case at least (as it seems we have different cases as in the list), that Revhed is the one closest to the actual phenomenon, eventhough we still dont know WHAT it is, as he has written earlier:

It is condition/environment specific.

Well, the sea conditions to be exact :thumb:

It was not flat today, moderate chop waves, so could not see anything explaining WHY it should be better today.

But no glitching today whatsoever, even close to my "relaxed" top speed strapless at a broad reach.

This experience suddenly left out #5, 6, 7, 8, 9, 10, 11, 12, 13 :naughty:
And others say that #3 and 4 is not very likely.

Hmmm, closer to the cause now in my case, but still no idea apart from the sea/water conditions being the cause ?
Well, one COULD also claim, that it could be rider specific - meaning I could have had a different way of reacting than usually.
Its just, you dont really DO much when going on a broad reach full speed, so find it hard to believe...
In terms of being able to jibe/tack and carve and ride waves, YES, your balance and physical/mental state on a given day, whether you are "in the zone" or not, is the most important of course.

But going fast, when in control and at a stable depth ?
I dont believe rider input (how good you are on a given day) has much influence (when experienced), honestly.

Has anybody else got experiences lately ?

It seems the more we write about it here, the more aware we are and interesting what different riders experience and notice now :rollgrin:

8) PF

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Re: Sudden deep ventilation ?

Postby zfennell » Mon Feb 23, 2015 2:32 pm

thanks Peter.
I do enjoy the details.
I imagine its a character flaw.
similar to missing the 'forest for the trees'...but too late to change now.
regards,
-bill

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Re: Sudden deep ventilation ?

Postby Zeeko » Thu Feb 26, 2015 10:36 am

Hi Pete,

I had this trouble recently with a hydrofoil that touch some objects on the water.
I had very small scratchs on the mast and more important I noted very very small holes on the leading edge of the mast. It creates a lot of ventilation.
I just put some resin in the holes (holes were only 0.2mm) + sanding the mast with 2000g sand paper, and results was perfect.

Profile have a impact for the water flow but the surface state is one of the most important part of the ventilation.
Bad profil will ventilate all the time
Good profil with bad surface state will ventilate
good profile with clean surface will not ventilate
Also grass can be a troubles specially thin and long ones.

Ride easy
Nicolas


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