Apparent Wind Angle & Congratulations Greenbird
In their latest News posting for April 1, 2009, the Wotrocket team point out that they’ve installed a “new Nexus instrument package…including a wind wand on the leading edge of the wing sail that gathers data which allows the co-pilot to trim to the apparent wind angle.” It’s worth mentioning here because trimming to apparent wind angle is vitally important to high speed sailing. It’s the one area in which, I think, most high speed sailing designs come a little short because sailboats are not conventionally trimmed to the apparent wind angle in the most efficient way. Let me explain using an aircraft analogy, as I often do.
An aircraft wing remains at a constant angle of attack–oscillations aside–as long as the pitch control surface (elevator, stabilator, elevon,canard, etc.) is kept at a constant deflection angle. It’s a direct consequence of aerodynamic behavior that this is the case. Sails, on the other hand, are trimmed in a way analogous to the way aircraft control surfaces are deflected, i.e. through a physical connection to the pilot. In a sailboat’s case, that physical connection is the main sheet.
So why is this difference so important? It’s important because all wings–and all high speed sailboats use wings instead of soft sails–have an angle of attack at which the lift to drag ratio is a maximum. And in a well designed high speed sailboat, the wingsail should be oriented at this angle of attack relative to the apparent wind in order to achieve maximum speed. That’s not to say that wingsails on all high speed designs WILL be at an angle of attack for maximum lift to drag ratio, but rather that it should be a design consideration for maximum speed.
It’s also worth noting that Greenbird, the new land speed record holder, uses a wingsail that is trimmed in just such a way as I’m explaining. It uses an elevator-like “pitch” control surface aft of the wingsail to control the wingsail’s angle of attack. Right on Greenbird! And congratulations on hitting 126+ mph!