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Tuesday, May 21, 2013

Moving Pics

Another video of Paradox starting to properly exploit new stable foiling setups.
Stability is heavily influenced by the relative lift contribution of rudder winglets and main foils.
We now set up the rudders to provide some upward lift when the boat is at neutral trim.
If driving force from the sail increases, causing the bow to dip, rudder lift decreases as the rudder winglet AoA approaches neutral.
If trimming moment should keep increasing (this would only happen if a gust an order of magnitude greater than the average wind speed is encountered), rudder winglet AoA would become negative, pulling the sterns down.
When the setup is correct, crew weight can be placed surprisingly far forward. This is more efficient as it puts more mass over the main foils, reducing the burden on the rudder winglets which are smaller and so have to work harder to support a given weight.
Even though speeds are significantly higher than in displacement sailing, the feeling of losing the bow 'down the mine' disappears completely.
If anything the instinct to shed power must be reprogrammed as the limit is much, much further away. Easing sheet in a panic just causes ride height to momentarily increase and then settle again.
We are now confident that this mode is significantly faster in a straight line at least in winds over 10 knots.
The next question that must be answered is whether it is faster around the course when tacking and jibing are considered...

video

Saturday, May 18, 2013

Pilotcam

A short video from a recent testing session.
Our understanding of the settings necessary for sustained stable foilborne sailing is steadily improving.

It is worth re-iterating the definition of stable flight with reference to the feedback loop that arises when external forces upset pitch angle and ride height.
A stable setup settles on an attitude and altitude without input from the crew.

We announced that Paradox could foil in a stable mode only when we were sure we had proven that it could.

Stability comes at some cost and we are open about the uncertainty regarding whether the benefits outweigh the costs. I believe we are close to finding an answer and I will describe our findings in detail in later posts.

Recent experiments by other manufacturers have shown that an unstable setup can be 'tamed': a well practiced skipper on a small boat can anticipate departures from the desired attitude and altitude given certain provisos, and make corrections, akin to balancing a ball on top of an inverted salad bowl.
In a racing context the conditions when this becomes unmanageable may not occur very often so overall an unstable setup can be competitive.
Think of it as riding a unicycle instead of a tricycle. Obviously humans are capable of learning to ride unicycles so the question becomes one of costs vs. benefits.
Exploiting an unstable platform is a muscle memory skill that can be learned 'by feel' with practice and is arguably more 'natural' once mastered than the mechanical and intellectual skill of adjusting foils for optimum trim.

In any case, our conclusions will reflect what we learned in testing. We will adopt the fastest configuration for getting around a course. Since we have been learning from our testing it will be different from the original setup.


A few notes on this video:

The discontinuities in the editing correspond to where Tom backed off to make adjustments using a control system that we would like to keep to ourselves for now.
The 'flight' was reliably uninterrupted for the whole run. The boat was safe and controllable throughout.

Looking at the wake carefully you can see the occasional disturbance due to ventilation of the surface piercing foil. This is an issue inherent in highly loaded surface piercing foils and we are experimenting with ways to mitigate it.
Fences are an obvious solution but have practical drawbacks as the foils must be retracted through the bottom bearing.
More promising options are leading edge discontinuities (cuffs) and boundary layer turbulators...
Better still is an optimised foil section with ideal pressure dstribution to prevent ventilation.