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Some Foil theory

Created by RAL INN > 9 months ago, 12 Sep 2015
VIC, 2857 posts
12 Sep 2015 8:46AM
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For all you people maybe contemplating a DIY Foil and others who just like info.
this is a cut and paste from an old Kiteforum post
Happy reading

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:


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


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):

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


kiteboy dave
QLD, 6525 posts
14 Sep 2015 9:30AM
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Thanks for posting here Ral, that nugget really impressed me, made me much more inclined to choose a Zeeko foil after I read it, for the fact that a/ he clearly understands the theory behind design and b/ he took the time to explain it in his own words.

Bit of background this was posted in context of the argument over positive or negative lift rear wings (stabiliser). A lot of brands were trumpeting pos-pos as a breakthrough I believe. The Gong foil is widely considered by the frenchies as highly unstable at speed (despite vids appearing to the contrary)..

VIC, 2857 posts
17 Sep 2015 6:12PM
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Glad you liked it. I have known Nicolas since he started Zeeko and for sure he knows his stuff. Maybe the Phd helps


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"Some Foil theory" started by RAL INN