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AC36: What's all the flap about "trailing edge flaps" — Sir Ben Ainslie and Mark Chisnell on how the AC75 will fly, via Yachting World

Friday, October 11, 2019


SAN FRANCISCO (#1224) — Sir Ben Ainslie (GBR) is the most decorated Olympic sailor of all time, and Team Principal of the British America’s Cup challenger. As you all know his INEOS Team UK is challenging for the 36th America’s Cup in New Zealand in 2021. Each month the UK's Yachting World is running an article with Sir Ben talking with fellow Brit sailor and celebrated mainstream author Mark Chisnell about the innovations and technology behind the new AC 75 foiling monohulls. Here's an excerpt from the October article released yesterday:


Trailing edge


The design rule for the AC50s in Bermuda also forbade the use of flaps, but this time around, they’re allowed. This is one of the biggest differences with the AC75 rule; it has ushered in a new era in foiling Cup-boats by specifically allowing flaps on the T-shaped foils.


“We will have direct control of the flap on the trailing edge of the T-foil,” explained Ainslie. “Moving the flap down will create more lift and more drag, but the lift is good for creating take-off. Moving the flap up to come more in line with the rest of the foil will reduce the lift and drag, which is then good for higher speed. It’s finding that trade-off really.”


The flap is a much more efficient way to control lift than moving the whole foil, and it’s no surprise that it’s been allowed for the AC75. Now we’re back to racing monohulls, which have a lot less stability at rest and slow speeds when the foils are not supplying any righting moment. The control systems will need to be more efficient to fly these boats compared to the AC50.


Read the full article on the Yachting World website here. And we will have more on "flaps" and what some say is, er, shaping up to be a contest between canoes and scows on today's TFE LIVE, as always at 1300 Pacific / 1600 Eastern / 2000 UTC on our Sailing Illustrated Facebook page


For those of you technically, or legally, inclined, and want the gory details, you can read the entire AC 75 Class Rule here. After a careful reading on the rule and checking with senior-serious people in several of the AC36 teams, our Dear Readers may be interested to know that adjustable trailing-edge flaps are only permitted on the main foils, not on the T-foil on the bottom of the rudder. The foil on the bottom of the rudder must be fixed (while racing) and flap-less. The "rake" of the rudder is, however, adjustable, and that in turn adjusts the angle of attack of the rudder's T-foil.


Photo at the top of this article is INEOS Team UK's AC 75 being launched at their team base in Portsmouth last week. You can see some detail of the foils, but not the trailing edge flaps. Indeed, teams have not released photos that detail their foil flaps, choosing to keep that aspect secret via careful choice of photo angle or, indeed, altering ("photoshopping") the images. The photo immediately above is the INEOS Team UK's modified Quant 28 test yacht "T5" with trailing-edge foil flaps clearly visible.


In this photo by Daniel Forster (not issued by the team) you can clearly see the grey trailing-edge flaps, and the bullet nose, on the port-side foil arm of DEFIANT, NYYC American Magic's first AC 75. The Class Rule limits the number of different trailing-edge flaps a team can use (two per side; a total of four at any one time) to 20  hence five different "sets" of foiling-edge flaps, not that all four being used at any one time have to be the same. Have a think about that.

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