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Why have upward winglets on the new 09 Foiltech front wing?

There are several benefits of having the upward winglets on the Foiltech, but the main reason they exist will take some history and background information. Some of this is proprietary information that was learned during the development of the 09 Foiltech front wings, but the fanatic foiling consumers eat this stuff up…then some of them become future competitors. J

The main reason the Foiltech has upward winglets is to help eliminate wingtip cavitation.

Just a note: A lot of this information was observed using underwater cameras in a clear, salt water lake with bright sunlight, so you could see what the Foiltech front wings (and some other brands) were actually doing below the surface.

Just another note: In this article, I only reference the high-end, game changing equipment of the time that relates to the winglet discussion (2001 LEX, Foiltech B3F), but the concepts can be applied to the other commonly used foils.

Cavitation

What is wingtip cavitation? First, what is cavitation? Basicially, cavitation is like air pockets or air bubbles that form on the wing. These air pockets hang on the wing and disrupt the control of the wing, because it changes the pressures and water flow on the wing itself. Cavitation causes wing to fail to fly/lift, either partially or completely.

What are the effects of cavitation on the sit-down hydrofoil?

Suck-down, glitches, board stuck to the water… all of these are commonly known terms from the effect of cavitation. This assumes that you do not have some physical object stuck to the wing or t-bar to interrupt the flow of water, i.e. changing the flow pressures on some surface of the foil assembly.

This is the basic idea of cavitation, but you need to know more to really get a good understanding of why the 09 Foiltech has upward winglets…

Why Wingtip Cavitation?

First, let’s look back at some of the history with the front wing designs of the 2001 Sky Ski LEX style front wing, which had downward winglets. These were good riding foils, but it wasn’t until the 2005 tests of the Foiltech B3F front wing (wingletless) that the game really started to change. This describes what was learned about wingletless designs versus the standard downward winglets…

2 or 3 board-taps after landing, then the foil re-flys – The LEX style wings did not clean up after most landings like the B3F. It was very noticeable. On the LEX, you had a bit of time to wait before the wing cleaned up and started flying again, but the B3F would clean up and start flying so fast that you had to change your riding style. Obviously, the B3F was a bigger wing and designed differently, but test were also done on the same exact Sky Ski wing shapes, with and without winglets. It was noticeable that wingletless wings cleaned up on landings and re-gained flight better than the downward winglets.

Winglets create drag – This is the opposite of what the aerodynamics people tell you, but we are doing different stuff than the aerodynamics researchers. The drag is most noticeable when you cut out into the flats on the foil. The B3F was a bigger wing, and thinner, but test were also done on the same exact Sky Ski wing shapes, with and without winglets. It was noticeable that the downward winglets caused added drag.

Squaring up the foil – At the time, it was noticed that there was a difference when squaring up the foil in the flats without winglets. It was a difference, but not really labeled bad or good in the beginning. The nice part about getting rid of the downward winglets was on the landing. Without winglets, you could land with the tip pointing toward away from center and not get snapped back into place pointing forward. Like on a 270 spin landing, with the downward winglets, it can sometimes be a bit forceful on the landing when the winglets help snap you forward again. Without winglets, the landing was much more forgiving and slower to point you forward.

With that information, winglet-less wings were the future of foiling. Again, this is just trying to focus on the winglet/less facts, not the other design changes that started the front wing revolution.

So everything was great about wingletless wings, right? Well, no, but it took some time to learn some of the downfalls of the wingletless wings.

What are some of the weird things that wingletless wings do?

Here is some of the feedback we started hearing from people riding the new Foiltech B3F and Double Bat (DeathBlade)…

· “Sometimes the wing will do a second jump when I land and I know how to land it right most of the time, but sometime there isn’t much I can do to stop it from popping out of the water.”

· “Sometimes when I cut really hard into the flats, it just falls over (potholes)…like I hit a leaf or something…but I’m pretty darn sure I didn’t hit a leaf.”

· “Sometimes when I do a swoop combo, it will shudder during the transition. It shoots me to the left or right, which makes the second invert hard to control.”

· “Gainer to spin combo is so freaking hard to do on this damn wing!” (yeah, yeah, this was one of my big complaints)

So why sometimes? It can’t be just mysteriously random. Why were swoops so nice and smooth one time, but then twitch out the next time? Most of the time the wingletless wings ride like butter, but then all of a sudden, whoa, act weird. Was it the front wing every time something went weird? No…but that’s where the underwater study began!

Why do wingletless wings do weird things…sometimes?

The main difference between the wing riding like it should and when it acts weird is… ready?

Board-tap

Yeah, not that exciting, but what does it really mean? Well, you already know that cavitation is what causes the foil to behave poorly, so the “board-tap” is what the sit-down hydrofoil has been using as the main defense against cavitation for all of these years. Board-tap is when you land the foil and smack the board down onto the water when you land. Many of these weird problems on the wingletless wings happened when the rider didn’t get a very good board tap to clear the cavitation…and it was repeatable in some situations…sometimes the cavitation would still clear without the board tap, but when it didn’t, that’s when those “weird” situations occurred.

Just a small sidebar, think about the problem that the sit-down hydrofoil has that no other aero/hydro contraption has…

Sit-down foils (wings) have to change from the air to the water very frequently. This density change from the air to the water is a very big change for fluid dynamics and yet, we expect the foil to clean up right away and fly for us every time. I guess you could see this like the problem of breaking through the sound barrier or high-speed atmosphere re-entry – these analogies aren’t that good, but I’m just trying to make a point. Who else, besides sit-down hydrofoilers, expect a wing to transition from air to water and expect sub-1 second, perfect response for the next agile, aerial trick that the rider wants to perform?

What actually happens with board-tap?

The wing enters the water fully cavitated with bubbles/pockets boiling everywhere. When the board smacks the water, the foil gets a big jolt. This jolt clears the cavitation off the wing…the water starts flowing properly over all of the surfaces (constant pressures are regained)…and the foil has lift…it is flying through the water properly again.

So now, with the cavitation and the board-tap knowledge, let’s look back at the 2001 LEX designs and talk about suck-down…

Most of the time (and assume no foreign debris on the foil), when your board is stuck to the water after a landing, and you have to lean back REALLY far to jump it or run the foil into the propwash to get it to fly again…IT IS THE T-BAR STRUT that is fully cavitating. The front wing is usually only a contributing factor in some of these cases, and this assumes the foil is generally a decent riding foil without some major flaw (design or physical flow). So here is what is going on (generally speaking)...

When the 2001 LEX downward winglet wing landed, the cavitation on this front wing stuck on the bottom of the wing, near the spot where the winglets were bent downward. With the cavitation hanging on the bottom of the wing, there is less pressure flowing on the bottom of the wing, so the foil drops down. This causes the board to sometimes tap-tap-tap, and then finally clears the cavitation to fly normal again. If the board-tap was somewhat more violent than normal, i.e. cavitation not clearing or a bunch of cavitation, then that increased the possibility of strut ventilation.

Ventilation is when air from the water surface slips down the strut and forms the air pockets/bubbles on each side of the strut, which then hangs and boils off the strut’s trailing edge. If the wing isn’t clearing and one of those board taps causes ventilation down the strut, you have total suck-down…board stuck to the water. The t-bar can ventilate in other ways, like riding near the lip of the wake, or poor board design, but let’s get back to finishing the winglet story…

So, most people noticed that with wingletless wings, they did not get the total suck-down that they used to get on the downward winglet LEX foils…even in cold water, which generally causes more suck-down/glitch issues with foils. This was great, but why?

One reason, but not the complete reason for less suck-down, is the way the Foiltech front wings cavitate on landing. The cavitation on the Foiltech wing goes over the top of the wingtips, and not just on the bottom of the wing. Sure, these new wingletless wings clean up cavitation better than the old LEX winglet wings, but cavitation is still there. When one of these wings cavitates over the top of the wing, the pressure on the topside of the wing is less… so what is the wing going to do? It is going to lift upward, instead of pull downward like in the case of the LEX wing. So the Foiltech wingletless wing’s cavitation is mainly over the top of the wingtips and pushes up, instead of down? Ahh… now some things started to make sense…

Wingtip Cavitation – Some Answers…

· When the Foiltech front wing cavitates, it has a tendency to push the board up and off the surface of the water, so it lowers the chances of repeated board-taps that can cause strut ventilation issues. (as previously mentioned, not the only reason the FT wings/foils tend to suck-down less than the previous LEX setups)

· Even when you change your body position on landing, expecting these wings to cleanup faster, sometimes it is going to relaunch…because the wingtips are cavitating and pushing the rider up, instead of pulling the board down to the surface like LEX cavitation.

A note on the relaunch after landing issue: There are several factors that can cause relaunch… bad body position on landing, front wing positioned too far forward, rider seated too far back for that particular setup, front wing size to rider weight ration, etc… but wingtip cavitation is a contributor to some of the relaunch issues.

So what do the upward winglets actually do?

First and foremost, they are there to help calm down the effects of wingtip cavitation and help rip the cavitation off the wing sooner, when compared to the wing without winglets. That is priority number one for the upward winglets on the 09 Foiltech front wing. These upward winglets do not make the wing any more efficient; they don’t create more lift; they don’t make better use of the surface area…which are all good reasons that a jetliner would have upward winglets.

· The primary purpose of the upward winglets on the 09 Foiltech front wing is to rip apart (split apart) wingtip cavitation on re-entry into the water, and help stabilize the direction of the wing when asymmetrical wingtip cavitation occurs.

Asymmetrical wingtip cavitation? Remember some of the issues described above with wingletless wings like glitching when cutting hard, twitching during swoop combos, and gainer-spin? Sometimes the cavitation occurs over one wingtip and not the other (or rips away faster from one side and not the other), because we often have the wing angled, rather than flat. When this occurs, the winglets help calm the twitching effect of the wingtip caviation. Without these winglets, the effect of assymetrical wingtip cavitation is much more pronounced.

So let us summarize the purpose and benefits of the 09 Foiltech front wing upward winglets…

Landings

The upward winglets help rip away wingtip cavitation on landings. After reading this entire article, it should now make sense, but basically, the upward winglets help make good landings smoother and bad landings easier to pull out. It is that simple. J

If you manly throw a single trick, smack the board, sometimes multiple board smacks, (sometimes death-gripping the handle), when riding away, then you may be clearing the cavitation on any wing you are using. With this information, you can start paying more attention to your landings…the board taps (how forceful)…how many taps before re-flight, and how much control in general you have with your landings. It will make you a better foiler no matter what wing you are using. It will also allow you to make better choices in your own equipment evaluation and setup.

The Swoops

Swoop combo transitions are hard and if you don’t get a controlled board-tap during the transition, you are risking the wingtip cav twitch. If the wingtips are cavitation during the transition, it makes it tougher to get the board tap anyway, so the upward winglets help lessen the effect of the cavitation during the transition. It helps you get a clean board-tap and helps rips the cavitation away with the board-tap. If you get asymmetrical wingtip cav (cavitation on one side of the wing and not the other), the winglets help make that situation much less of a mess! J …especially on the gainer-spin!

Sinker combos…who cares? Just kidding, sinker combo usually have a nice, controlled board-tap during the transition to clean up the wing cavitation for the next take-off. So the upward winglets are mainly to help benefit the swoop combos, rather than the sinker combos.

The Square-Ups

Near the beginning of this article, you might remember discussion about winglet versus wingletless square-up in the flats. Basically, that is the transition from cutting hard into the flats on an angle away from the boat, to exiting the water going with the boat…like on a gainer or a spin. Going back to these winglets made that transition in the flats easier, because the wing helps to square up the take-off in a consistent manner. Without winglets, it take a bit more focus to make sure you are consistent with squaring up, otherwise you can take-off with too much line or not enough (over-shoot the square up or under-shoot the square up)…this issue mainly deals with spins.

Also the “short” landings that square you up (snap your around forward) are not like the old downward winglet wings. These winglets are smaller and point upward, so you do not have the semi-violent twist effect as the old downward winglet wings.

Drag

This isn’t a benefit, but a small downfall of the upward winglets. On cuts in the flats, you may notice that it doesn’t feel as smooth or loose as the wingletless wing you were just riding. Yep, but the difference is not large enough to make you get tired any quicker with these upward winglets versus wingletless. It is only noticeable for a small amount of time and then forgotten….a non-issue.

To summarize this story…

Above are the reasons and benefits of having upward winglets on the 09 Foiltech front wing. I tried to stay focused on just the winglets and not the benefits of the other design aspects of the wing. Also, winglets are not the holy grail of stopping all cavitation, glitching or suckdown. They definitely help calm the negative effects of wingtip cavitation and help you gain more control of the foil.

Now, if anyone tells me that so-and-so wing doesn’t have a cavitation problem or this brand of wing doesn’t have cavitation, my response will be “Your an idiot.” (correct, “Your”)… Not because I really think the person is actually an idiot, but really I mean (to a designer) that you should continue looking for ways the wing fails so you can make it better… or (to a rider) that you should pay attention to the way the foil fails, because it will make you a better rider by avoid those risky situations.

Most of today’s high end wings are very nice rides and can be difficult to determine which one is “the best.” It takes a lot of time testing different equipment back-to-back, years of experience in riding different waters, and years of riding to learn the tricks that are required to put the wings through the test. So the moral of this story is to go out and progress your riding…keep it fun and interesting.

• To be an efficient learner, relating all mistakes back to the basic elements will increase your skills faster by choosing good corrective actions, therefore crashing less.
  
• To be a good instructor, you can relate the mistakes that the rider makes back to the basic elements, and then help to create corrective actions for the rider for their next attempt. 
  
• To be an innovator, you can build a new trick based on the basic elements, prior to attempting this trick, and then determine its potential only after a few attempts.