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Zeeko revolutionise the hydrofoil for 2015

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Tinkersailor
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Re: Zeeko revolutionise the hydrofoil for 2015

Postby Tinkersailor » Mon Mar 02, 2015 3:29 pm

Zeeko wrote:Hi All,

There is some discusses about the positive lift stabilizator,
I tried with DWB to translate to english, sorry for the faults but the principle is here:

ZEEKO lift down stabilizer info


Negative profile for the stabilizer.

Hello everyone,
here is a short explanation of our choice negative profile for the stabilizer.

Indeed, as you know the stabilizer is a masterpiece in the behavior of a foil.

To show you the benefits of a negative lift profile for the stabilizer it is first necessary to study the interest of the stabilizer.
Aircraft or foil, can fly without stabilizers: we saw some video showing hydrofoil riders without stab. However, it is really difficult to ride without because it amounts to ride like the picture below:

Image



The first problems that appear during learning of hydrofoil are pitching problems.
Pitching is defined as below:

Image

Indeed, porpoising (yoyo up and down in foil) is a problem related to the mismanagement of the pitching from the rider.
This problem has two consequences:
1- a performance loss: a foil is highly efficient if it oscillates only slightly in pitch, once more he has to pitch oscillations and there is energy loss as increase drag
2- a total loss of control of the foil. When you are beginner, you create a lot of variations in term of pitch of the foil. This results is in large variations lift the foil.
How do we manage these impacts variations and thus lift: by moving our own center of gravity above the foil.

This pitching problem is a problem for beginners but also a problem for the experts. Indeed, we see it during competition, some riders with foil have a lack of pitch control of the foil (they explode in flight as we say, from personal experience in competition, when you are head to head it is not funny when one when you cannot control anymore the pitch of your hydrofoil because your reaction time is not quick enough to control the altitude of the hydrofoil).

So we have a problem which is a problem for beginners AND for experts.
Experts are not limited by the performance of their foil but mainly by the management pitching it (apart from a handful of riders on the planet).

What I see in the foil is that:
- Everyone does not want the same Yaw effect for a foil:
For this, we give the possibility for each rider to change his yaw effect by changing the size of the vertical rear fin of the foil in 2015.
- Everyone wants a controlled roll:
for that length of the mat is very important. Thus it is advantageous to change the length of the mat. Over the mat is longer and the management of the roll is important. This is not the only factor but it contributes greatly. If necessary I will return it.


But mostly everyone wants a stable foil pitch because it makes the predictable behavior for the foil (except of course those who want challenges).

How to limit the pitch of a foil without changing the front wing : using a stabilizer.

Now that the interest of the stabilizer is well established, then it must be asked what type of stabilizer interme of lift : Positive/ neutral / Negative .
I will not go into the Angle of attache (AoA) of the stabilizers because ultimately a stabilizer is taking into account their profile AND its angle of incidence and creates just very positive, a positive , neutral, negative very negative spoiler spoiler.

A positive stabilizer allows a stabilizer that creates lift up as the front wing of the foil (all fins lift the rider). So the center of Lift (taking in account the lift created by the front wing and the stabilizator) is between the front wing and stabilizer (see the diagram below (top left):

Image


This type of profile can increase the performance of a foil, but also has a great ability to change the balance of the foil when your ride change a lot the overall angle of Attack (lift up and down as beginners for example). Indeed, if the incidence angle increases foil (bottom left diagram), then lift of the stabilizer increases very quickly. One feels a big amount of lift made by the hydrofoil (for when the foil up, the center of gravity moove backward, and moved backward the center of lift of the foil).
To prevent this phenomenon without changing the setting, you must:
either ride as close to the surface of the water, where the water creates less pressure on the wings (more wings are deep in the water and more the water creates pressure on the wings and therefore increases the lift ). This can cause problems on choppy waters


Either you should moove frontward your center of gravity or push more strongly with the front leg to move frontward the center of gravity, it's an endless management.
On the other side as and as the speed increases, so it has more lift upwardly related stabilizer, it induces naturally that the foil lift down by a misalignment of the center of lift of the hydrofoil and point and total center of gravity, the latter now being too forward).
Also the faster we go, the more it is necessary to change the A0A of the stabilizer to make it less lift. The perfect world should be with a change of the AoA during the ride, it is what a moth foiler do but has an otherwise complicated system set up, it is not our case.


Another consequence of a positive lift stabilizer: for the same foil with a positive stabilizer, the center of lift the foil is backward. Therefore equivalent foil, the rider must be further back on his board. For making the test on my foil (I can ride my stabilizer positive of negative without problems), I have my feet back more than 10cm on my board with a hydrofoil with a positive lift stabilizer compare to a negative lift stabilizer. This is not a problem, but just a statement to be considered according to what is desired.


Thus, the positive profile on the stabilizer can be problematic, especially in terms of feeling as it leads to a constant control of the alignement of the Center of lift and the center of gravity. Faster I am and backward I need to be on the board.


How to remove this:
We have a second possibility: to create a negative lift stabilizer.
It is our choice. Why this choice?


Just because we are looking for a automtaic pitch control.
With negative lift stabilizer, the center of lift of the foil is frontward the front wing (see top right figure), but especially when the foil is with a positive AoA (the board lift up for example), then the stabilizer creates less lift as inversely to the front wing, the variation of the angle of incidence is negative (see bottom right picture). Therefore, it tempers the extra lift of the front wing. Indeed, he mooved back the center of lift of the foil at the time whenthe center of gravity also moving backwards, everything works together is harmony!


All the work of the designer is to create profiles that allow a constant alignment of the center of lift and center of gravity.

On the other hand, when the foil lift down, so naturally, the negative stabilizer creates more lift down, and therefore naturally helps lift up the nose of the foil, so take off the board.
At this property is added, the fact that changes of lift of a a negative lift stabilizator are much more linear (less violent) that variations positive lift stabilizator, because we are on incidence angles generally lower


Therefore, the question that may arise is finally to have the best of both worlds do not you need a neutral stabilizator?
According to all the tests I have done for over 6 years, a symmetrical stabilizer with the same angle of atatck as the front wing create same problem as a positive lift stabilizator. Therefore in general, and I think this is the case for everyone here, it puts a negative angle of a few degrees on the stabilizer relative to the front wing. Basically, when a front wing is stalled at 0 ° to the axis of the fuselage, while the stabilizer is rigged to -2 ° (this is an example).
But according to our tests in numerical wind tunnel, a negative lift profile at an angle of 0 ° can create the same lift a symmetrical wing stalled at -2 °. But mostly, this value is gained with a large reduction in drag, this is why asymmetric profiles are more efficient. For the beginner there is no big difference but for some expert it is a world apart.
Also the effect of the negative lift stabilizator has another great feature:


As we have seen its lift changes according to the angle of incidence, but also it allows to be used at all speeds without the changing the location of the center of gravity. That is to say that the behavior of a foil is the same high or low speed. That is why our foils are delivered without trimming of the stabilizator according to the speed required by the rider.
In waves or speed, the negative lift stabilizator does not need adjusting to fit, it adapts itself on both its intrinsic qualities.
Finally, depending on the speed of ride, the balance of front and back leg pressure is stable, and is not a function of the speed: the foil center o f lift is always aligned to the center of gravity, it makes the feel of a stable hydrofoil at any speed: at 30kt or 15knots speed you push the same on the front and the back leg (proportionally course).


I hope I was clear. For us our goal was to minimize pitching effect of the hydrofoil because it is a parameter that is harmful to the control of foil.
To have a perfect control of the pitch, you need a negative lift stabilizator.



Ride easy

Nicolas
Thanks for the post! I had to read this about 10 times before I started to see what you are getting at.

If I could paraphrase your conclusions to make sure I understand?

1) both a negative and a positive lift stabilizer will have the same static respons to changes in AOA brought on by movement of the C of G of the rider.

2) the dynamic response of the two systems is different.

3) the negative lift stabilizer system automatically moves the Center of lift of the entire system toward (but not all of the way to) the C of G resulting in a smaller shift from the rider to realign the lift and gravity vector sum to equilibrium. The net result being that although the system can never be fully automatic it is easier to ride because the dynamic response to C of G changes is less amplified and it gives the rider time to react....?

On another issue. Have you evaluated the flow at the rear stabilizer and how it is impacted by the down wash created by the front wing?

Cheers,

Tink

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Re: Zeeko revolutionise the hydrofoil for 2015

Postby Tinkersailor » Tue Mar 03, 2015 12:16 am

Bump.

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Re: Zeeko revolutionise the hydrofoil for 2015

Postby Zeeko » Tue Mar 03, 2015 10:44 am

Dear Tinkersailor,

Here my comments:
"1) both a negative and a positive lift stabilizer will have the same static respons to changes in AOA brought on by movement of the C of G of the rider."
Nicolas: In static yes, in dynamics no
But statics, never exists on the water.


2) the dynamic response of the two systems is different.
Nicolas: exactly and this is the most important, as we are never stable with water flow.

3) the negative lift stabilizer system automatically moves the Center of lift of the entire system toward (but not all of the way to) the C of G resulting in a smaller shift from the rider to realign the lift and gravity vector sum to equilibrium. The net result being that although the system can never be fully automatic it is easier to ride because the dynamic response to C of G changes is less amplified and it gives the rider time to react....?

Nicolas: yes, but this is not an automatic answer. For that all the job of the designer is to find a combo of backWing/front wing profile following the movement of the CG as precisely as possible. But not only because the movement of the CG can be the origin of the movement, but it can be also the consequence (fortunately, if not we cannot control the hydrofoil by ourself). The target is to find a system as stable as possible (in terms of dynamics) and not depending on the speed, the AoA: a predictable system.

On another issue. Have you evaluated the flow at the rear stabilizer and how it is impacted by the down wash created by the front wing?

Nicolas: this is a very good question,and the answer is quite complicated as results depends on several factors, there is not a simple answer. The flow at the rear stabilizer is impacted by the front wing design (profile, length of chord, anhedral etc...), the fuselage design, the mast design, and more important results can be totally different depending on the AoA studied.

Ride easy
Nicolas

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Re: Zeeko revolutionise the hydrofoil for 2015

Postby Tiago1973 » Tue Mar 03, 2015 10:39 pm

hi Zeeko - just out of curiosity, did you considered a canard configuration aiming at at the effect?
(but just w/ both surfaces providing positive lift)

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Re: Zeeko revolutionise the hydrofoil for 2015

Postby JS » Thu Mar 05, 2015 9:09 am

The extensive explanation above seems intuitive, but it's flawed.

That a foil board is more stable when the aft foil has negative camber and/or angle of inclination doesn't seem to be in dispute. The question seems to be why. So...

Referring to this first attached vector diagram which approximately represents two-dimensional foil forces during steady-state horizontal travel, with similar fore and aft foils of equal angle of inclination:
aaa WP_20150304_23_01_39_Pro.jpg
aaa WP_20150304_23_01_39_Pro.jpg (31.85 KiB) Viewed 2792 times
The resultant vector is within a few degrees aft of vertical (provided the foil assembly has a L/D ratio of about 20:1 or higher) and acts through both the CL (center of lift) and the CG (assuming the net kite force also acts through the center of gravity, thereby negating any pitching torque). Note also that because the foil board is traveling in equilibrium, the resultant vector of kite thrust and gravitational force is equally opposite.

Now here's the part that's not intuitive to visualize. Lift is only vertical when the foil board is traveling horizontally. When the board pitches up it ascends, and when it pitches down it descends. In either case it follows a sloping path through the water, and the instantaneous flow over the foil assembly is similarly sloped, as is the drag vector. And since lift is by definition perpendicular to flow, it also deviates from vertical, and the resultant vector deviates as well.

So, if the board in my sketch pitches up or down, the entire vector diagram pitches with it. The lift vector still passes through the board at exactly the same spot, and the resultant vector still passes through the CG. There is no pitching torque created by a sloped climb or descent, corrective or otherwise.

That said, here are three special conditions to consider, referring to this second diagram:
aaa WP_20150304_23_55_42_Pro.jpg
aaa WP_20150304_23_55_42_Pro.jpg (33.18 KiB) Viewed 2792 times
1. (edited twice, for clarity) Depth-dependent change in center of drag and L/D ratio: As I stated, the first diagram is an approximation. In reality, the CL and CD (center of drag) aren't at exactly the same location because only the horizontal foils contribute to lift whereas all submerged surfaces, including the vertical (mast) and fuselage, contribute to drag. So actually, a small torque results, inducing a slight downward pitching moment. Although the rider subconsciously counteracts this effect by shifting the CG ever so slightly, the deeper the board rides the greater the downward pitching torque will be, effectively creating a natural depth-compounding dynamic. And even if this wasn't so, the deeper the board rides the more mast is submerged, diminishing the L/D ratio of the submerged system and thereby causing the resultant vector to pass farther aft relative to the CG. Btw, on the sketch I've exaggerated the separation of CL, CD(shallow) and CD(deep), for emphasis.

2. Speed-dependent change in L/D ratio: The board will decelerate if it ascends, and it will accelerate if it descends, due to gravity. And as its speed changes, so too might its L/D ratio, which isn't constant across its range of speeds. If L/D increases, the resultant vector will act forward of the CG, inducing the board to pitch up. And if L/D decreases, the resultant vector will act aft of the CG, causing the board to pitch down. Unfortunately, there's no rule for which will either compound or correct pitch changes because it's a speed dependent dynamic. It will generally be corrective across lower speed ranges and compounding at much higher speeds. (Peter Frank, this could possibly be partly relevant to your high speed crash question, especially combined with my previous point, btw.)

3. Angle of attack (AoA) dependent change in CL, with "inverted" aft foil: If the aft foil is set with a negative camber and/or a lesser angle of inclination than the forward foil, then the CL will be forward of the forward foil at relatively high speeds (as has been correctly represented earlier in this thread). Then, if the AoA is increased (which is only possible by reducing speed, or alternatively by loading the foil more heavily by transitional "vertical" acceleration or by increased effective gravitational force caused by depowering or diving the kite), the CL will move aft. This dynamic is an effective stabilizing mechanism. Simply, if the board pitches up, it slows down, inducing it to pitch back down, and vice versa. The two sketches at the bottom of the second diagram roughly denote this, as has also been depicted previously in this thread.

I'm swamped with work. If I don't follow up on this thread any time soon, it's nothing personal.

Best regards,
James
Last edited by JS on Sat Mar 07, 2015 9:52 pm, edited 2 times in total.

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Re: Zeeko revolutionise the hydrofoil for 2015

Postby Zeeko » Thu Mar 05, 2015 2:26 pm

Dear JS,

Very interesting but I can't see the drawing,
Nevertheless, the control of the balance is quite complicated as you stated. I tried to make it easy to understand (for example, I located the CG at the same level as the wings, which of course it not here).
But as you know the control of the hydrofoil is very more complicated than I explained.
Also, going upwind or downwind will change the study, having the kite high in the wind window or down will create a different location and value of the center of gravity (and also movement of the CG).
The system is impacted by the front angle of the mast to the flow (creating also a lift itself).
Then there is a lot more components to study than a moth foil for example, where the rider try to be bvery steady in the of angle of the mast with the water flow (taking in account the front view of the moth foiler)
For Tiago the mecanism is the same (but opposite)

Regards
Nicolas

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Re: Zeeko revolutionise the hydrofoil for 2015

Postby JS » Thu Mar 05, 2015 7:55 pm

Thanks for the gracious reply Nicolas,

Your explanation really helped to get me thinking about the dynamics at play.

Best regards,
James

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Re: Zeeko revolutionise the hydrofoil for 2015

Postby Zeeko » Thu Apr 09, 2015 8:04 am

Dear

I simplify the posts updating that one.

Please find below a video of our new product,
ok next time I'll clean my windshield :D

Let me know if you have any questions!

Also, we are preparing a new hydrofoil with compatibles wings. It means that if you buy the feeride alloy foil you could keep the wings and upgrade to a carbon Mast and fuselage

It is made using alloy and it will reduce a lot the cost (public price will be 799€ including VAT at 20%). More news soons, the product will be released mif of May. This new foil is freeriding dedicated when the carbon one is all styles (from racing to waves)

Here a pic of the mast for the alloy hydrofoil.
The target is to use clear color to see it on the water, this is safer for the beginners.

Image


Ride easy
Nicolas

Last edited by Zeeko on Sat Apr 11, 2015 11:49 am, edited 1 time in total.

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Re: Zeeko revolutionise the hydrofoil for 2015

Postby ronnie » Thu Apr 09, 2015 11:54 am

[quote="Zeeko"]!

Also, we are preparing a new hydrofoil with compatibles wings. It means that if you buy the feeride steel foil you could keep the wings and upgrade to a carbon Mast and fuselage

It is made using alloy and it will reduce a lot the cost (public price will be 799€ including VAT at 20%). More news soons, the product will be released mif of May. This new foil is freeriding dedicated when the carbon one is all styles (from racing to waves)

Here a pic of the mast for the alloy hydrofoil.
The target is to use clear color to see it on the water, this is safer for the beginners.

Image

That foil looks like a much smaller version of the North foil. :D Looks safer though. :thumb:

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Re: Zeeko revolutionise the hydrofoil for 2015

Postby BWD » Thu Apr 09, 2015 2:18 pm

Interesting!
How thick is the aluminum mast?


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