aerodynamicsclassical-mechanicsforces
Recently a group set a record for sailing a wind-powered land vehicle directly down wind, and a speed faster than wind speed. Wikipedia has a page talking about it, but it doesn't explain exactly how it works.
I can imagine some configurations, but I don't get how a system could be set up to push forward with a tailwind (when the cart is moving slower than the wind) and push forward with a head wind (when the cart is moving faster than the wind).
Could someone help explain how this is possible?
The fastest point of sail depends on the boat (both its hull shape and its sail plan), the wind strength, and the sea state. In general, a beam reach is not the fastest point of sail.
For instance, in very light wind some boats will go fastest on a close reach due to the increased apparent wind from going toward the wind. For boats that sail faster than the wind, the limiting factor for speed is how close they can sail to the apparent wind; when they are going faster than the wind speed the apparent wind is always forward of the beam. A broader angle to the true wind allows them to go faster before their sails are sheeted all the way in, so a broad reach is the fastest.
Check out the polar diagrams on this site:
https://76trombones.wordpress.com/2009/10/17/polar-diagrams-vmg/
90 degrees to the true wind is not usually the fastest.
As for why some angles are faster than others, that's a bit complicated and beyond the scope of this answer. Suffice it to say that as you sail closer to the wind the component of lift in the direction of travel is less, plus there's increased drag. So you slow down as you approach a close-hauled course. As you bear away to a run, the sails eventually become stalled and less efficient. Somewhere in between a close-hauled course and a run you'll find the angle of maximum speed, which is sometimes approximately 90 degrees to the true wind but not always.
BTW, I should add that the term "beam reach" doesn't have a precise definition. Some sources say a beam reach is 90 degrees to the true wind, others say its 90 degrees to the apparent wind. Most sources introduce the points of sail before the concept of apparent wind and elide over the difference entirely. For boats that only go a fraction of the wind speed (i.e. the vast majority of them) its not that important a distinction. In the answer above, I've used the true wind definition, but even if you choose the apparent wind definition a beam reach is not always fastest. Boats that go faster than the wind have their sails trimmed to close hauled regardless of their point of sail.
Yes this does work, but the blades work opposite of, say a windmill which is turned by the wind. Initially the wind blows on the blades, which act more like a sail, to begin pushing the craft forward. This causes the wheels to rotate, the wheels are connected to the blades to turn them to push air back against the wind. this creates a higher pressure on the blades than a flat sail would get as it approached wind speed. This higher pressure can push it past the wind speed.
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It does sound counterintuitive. The actual device used gears attached to a wind turbine. Even though the turbine was moving faster than the wind, the thrust (IIRC) was such as to slow down the wind field. So energy was being extracted from the wind field to turn wheels via the right gear ratio. In any case energy is removed from the wind field so no laws of physics were violated. Traveling at angles to the wind is easier to visualize. In fact wind turbine blades travel at right angles to the wind at multiples of the wind speed. But that is different. This was a really clever device, and unless shown how it works (I don't remember the details) most physicists think "no way". I suspect it might have to be pushed to faster than wind speed to get it going. Without wheels attached to the solid ground it wouldn't work, its basically increasing the wind field coupling to the solid earth.