Monday, March 18, 2013


[Disclaimer: This is re-post from another blog, now defunct. Unfortunately, I lost some rather excellent comments when I saved this copy.]


Here is an idea that 99 percent of people declare impossible. Ninety-nine percent of the rest can’t figure out how it’s done. The idea is known as DDWFTTW, or "Directy Downwind Faster Than The Wind".

It starts with a simple, but seemingly stupid question:
“Can a wind powered vehicle go downwind faster than the wind?” 
Being a logical person who has taken high school physics, the answer is simple. Impossible! It violates the laws of conservation of energy – i.e. it is a perpetual motion machine. Once the vehicle is going the same speed as the wind, the wind can no longer push it – so it obviously can’t work.

Back in 2001, a scientist named Rick Cavallaro answered the question "Yes", and published his speculation about such a vehicle on The Internets. He drew a complex diagram to illustrate his point.

Furious debates and name-calling ensued, (no, not the republican presidential race.) Prominent physicists, professors, and engineers all agreed it was impossible.  When the vehicle was finally built and it worked as designed, there was a brief hushed silence, quickly followed by embarrassed shouts of “Hoax!” 

Here it is working

How does it work? I can’t say I understand it well, but the important point is that it’s not at all like a sailboat. The wind pushes against the surfaces of the vehicle, which starts the wheels rolling, which turn gears, which turn the propeller. The propeller pushes against the wind; spinning opposite to the way a windmill spins. The wheels and propeller work together to push the vehicle. It doesn't violate the laws of physics because the pushing surface of the propeller is not going faster than the wind. 

Here is a good illustration of the principle, appropriately narrated by a German version of Mister Rogers.

Mister Rogers explains the physics of DDWFTTW

If you are interested, Wired Magazine does a great job of telling the story 

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