My buddy and I built a full-sized manned version of the vehicle (see the video of the "Blackbird" above) and established a world record for direct downwind speed of a wind powered vehicle. We've spoken at NASA, AIAA, Stanford, SJSU, and a number of other venues on this controversial topic, and won the Editor’s Choice award at the 2011 Maker Faire.
The question: Is it possible to make a wind powered vehicle that goes directly downwind, faster than the wind, steady-state?
Despite the relatively straight-forward analyses and explanations, we were assured even by professors of physics and aero that it simply can't be done. But after setting a world record with our full-sized cart, most of the naysayers have come around.
Now for about $40 in parts you can make your own working model and demonstrate this novelty for yourself.
Critical thinking! I originally conceived of the downwind cart when a friend asked me whether a sailboat could tack downwind and beat a free-floating balloon to a point directly downwind. I wasn't sure, so I did a quick vector analysis. Somewhat surprisingly the answer was "yes". Being a huge fan of brain-teasers, I asked myself how to make this little tidbit even more twisted. By making a wind powered vehicle that could beat the wind DIRECTLY downwind, I suspected it would go against most folks' intuition - and that proved truer than I would have ever guessed.
The reality is that one can analyze this with high school level physics and math. As engineers, we rely heavily on our intuition for problem solving, but we have to remember to use that intuition to guide us - NEVER to replace rigorous analysis (or worse yet - observed results). Much to my surprise there have been a fair number of professors of physics, NASA aerospace engineers, and aerodynamicists that assured me this could not work - even after it was demonstrated. These folks relied on their intuition when it was time to carefully consider the simple analysis.
The secondary learning objective is to remind us that we don't stop learning when we start teaching.
Note: I later learned that I was not the first to have conceived of such a cart. An engineer by the name of Andrew Bauer built one in the 1960's. He had learned of the concept from a student paper written some 20 years earlier. We've never found that student's name.
The reason we built the cart was to settle the long running debate across a number of internet forums. I was clearly not going to convince many people with my analyses. The matter is settled now to about the same extent the moon landing is settled. The difference being - you can build your own cart in an evening or two and prove it to yourself :)