The explanation of why this vehicle would travel downwind faster than the wind is simpler than Veritasium's mathematical explanation of how the Blackbird vehicle works. (explained below)
We all know that wind will push on an object moving downwind if the object is moving slower than the wind. For example, if the wind is blowing at 10 mph and an object is moving downwind at 6 mph, then the wind will push on that object and that object will speed up (in the absence of other forces).
The vehicle presented here has some fins that are designed to catch the wind. These fins move only about half as fast as the vehicle moves because of where the fins are positioned on the wheel axel. So when the vehicle moves 12 mph to the right, then the fins are moving 6 mph to the right.
So in a 10 mph wind the fins are being pushed downwind, because they are only going 6 mph. The car body is moving 12 mph, which is 2 mph faster than the wind. So IF the thrust developed by the wind blowing into the fins at 4 mph is greater than the drag of the body going through the air at 2 mph THEN the vehicle will maintain this speed and accelerate even faster.
The fins are shaped to catch wind, and the car body is shaped to have low drag, so these speeds seems feasible. One could imagine a more advanced design where the fins collapse when they are above the axel. This way the body of the vehicle could be much thinner. In this case there is no question that it would work because the drag of the body moving forward through the air would become negligibly small.
I believe this shows (without much math) that faster than wind vehicles are possible.
Notes:
(1) The idea that a part of the vehicle must move slower than the wind also applies to the Blackbird vehicle. The surface of the propeller moves backwards relative to the body of the vehicle, just like the fins move backwards relative to the vehicle presented here.
(2) If someone were to make this vehicle to test on a treadmill, then it seems like the fins should be straight rather than curved because they should be optimized for pushing air rather than catching air. I don't think this vehicle would work on a treadmill. I think the treadmill test is somehow harder than actually moving downwind. The Blackbird vehicle got to almost 3X the wind speed, but the treadmill vehicles barely worked. (I might be wrong.) This design is not nearly as efficient as the Blackbird vehicle.
(3) Thought Experiment: Imagine two very long gear racks laying side by side. One is slid lengthwise relative to the other by a strong motor. It only moves at 1 inch per second but the motor driving it can apply very high forces to keep the gear rack moving at 1 inch per second. It is clear that a vehicle could be designed which uses this available energy to move itself along the gear racks. This vehicle would not be limited to 1 inch per second. This example is analogous to a wind driven vehicle where one gear rack is the ground and the other is the wind. The roller device that Veritasium made is another example. There is no law of physics that limits the speed of the vehicle to the speed difference between the two gear racks (or to the speed difference between the wind and the ground). Here is a great video of exactly such a rack system -nice. https://www.youtube.com/watch?v=CbNCz0_iO7c
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