Let’s take a close look at Oracle Team USA’s wing. This is the second wing they have built. The first was destroyed following their pitchpole / capsize in October 2012. Each team is allowed to build three wings (or, more accurately, six wing sections, since the rules require the wing to be built in two sections along its 40m length).
OTUSA’s wing has two “elements” – the forward element incorporates the “mast” structure and the trailing element functions like the flaps on an airplane wing. The flap is divided into four segments. By varying the angle of deflection of each segment you can “twist off” the upper part of the wing and depower it. You need to be able to depower, since the Protocol calls for racing in as little as 5 knots of breeze and as much as 33 (although the race committee can call off racing if they deem it unsafe). The angle and strength of the wind change from the surface of the water to the top of the wing, 40m (130 ft) higher – another reason to change the shape along the span of the wing.
Compare with Artemis’s wing, which has six segments in the trailing element.
And with Emirates Team New Zealand, like OTUSA, with a four segment flap.
Here is a closer look at Oracle’s wing.
From Wikipedia – diagram of slotted flap
The OTUSA wing has what is called a slotted flap. (Click the diagram to the right to read Wikipedia’s explanation of the multitude of aircraft wing flap configurations.) This wing has a tab on the leading element to help regulate airflow from the windward (high pressure) to the leeward (low pressure) surface. In the photos above and below, I’ve highlighted the tab in green. The tab is not actively controlled – when the wing is not cambered, the tab segments move freely. The tab is blown into position; it’s movement is limited by a simple string attached to the flap.
ETNZ does not appear to have a tab on their main element.
You can’t tell from my simple diagram on the photo above, but but the pivot point for the flap is forward of the tab. I’ll make a more detailed diagram in a future article. In that article, I’ll also go into more of the details of how the main (forward) element and the flap (trailing element) interact, and the subtle but important role of the tab. For now, suffice it to say that the wing functions most efficiently when the airflow around both elements stays attached to to surfaces, and the tab helps that to happen.