By Mark Frauenfelder
Rick Cavallaro demonstrates his treadmill wind cart.
In February 2007, MAKE’s project editor, Paul Spinrad, emailed me a link to a YouTube video shot by a man named Jack Goodman. The video opens with a woman in a pink shirt and blue shorts standing in the middle of an asphalt road somewhere in Florida. She’s holding onto a 2-foot-high, three-wheeled, unmanned cart with a large propeller mounted on the back. There’s no motor on the cart, but the propeller is connected to the back wheels with pulleys and a belt.
A man’s voice offscreen says, “OK, April 13. Wind light and variable, about 5 or 6 knots.” A small mast with a windsock attached to the cart shows that the wind is blowing from the back to the front of the cart — in other words, the cart is pointed downwind. The man instructs the woman to “give it a shove,” and she pushes it lightly down the road. It rolls for several feet, making a clicking sound, then slows down and comes to a rest.
“Oh,” says the unseen man. “I had the brake on again. Give it a push.” The woman pushes it again and the cart takes off down the road, this time picking up considerable speed.
The video camera continues to tape the cart as it rolls down the road. The camera operator is on a bicycle, shooting the video while pedaling along the rural road. After a moment, the windsock stops pointing downwind and changes direction. It’s now pointing toward the back of the cart. This means the cart is traveling downwind faster than the wind.
For the next three minutes, the cart rolls down the road. The man says, “I’m going 10 miles an hour, the wind is about 6 knots.” The cart races ahead, and the man says, “Up to 13 miles an hour.” A minute or so later he says, “Brake on.” The vehicle stops and the video abruptly ends.
Jack Goodman’s three-minute video has been the subject of an intense, hotly-contested speculation ever since it was uploaded to YouTube on November 30, 2006. In an article for Catalyst: Journal of the Amateur Yacht Research Society, Goodman explained that he built his curious cart to settle an ancient debate among sailing enthusiasts: Is it possible to for a wind-powered vehicle to travel directly downwind faster than the wind?
The intuitive answer to this question is “of course not.” Imagine tossing a balloon into a steady breeze. It will go along at the speed of the wind (or slightly less, due to drag) but it’s inconceivable that it could go faster than the wind. How could it? If it were to go faster than the wind, it would be outrunning its source of power and move into a headwind, which would slow it down.
Think of a sailboat moving downwind. Once it gains enough speed to be moving at the speed of the wind, the sail will go slack, because the wind speed relative to the boat is zero. With no wind in the sails, how in the world could the sailboat go any faster? To claim that it could go faster than the wind is the same as claiming it could move forward with no wind at all!
People immediately began attacking Goodman’s video, saying it was a fake. The video doesn’t have a clear shot of the road ahead, so many commenters accused Goodman of towing the cart behind a car or bike with a piece of fishing line. Some said the cart was moving downhill; others said Goodman was deluding himself — the windsock changed direction because of propwash, not because it was moving faster than the wind.
Defenders of Goodman’s video said that detractors weren’t thinking about the problem properly. It wasn’t the propeller that was driving the wheels — it was the wheels that were making the propeller spin, and this critical bit of information made all the difference. The energy, said the defenders, came from the net motion of the air over the ground, not the net motion of the air past the vehicle. Unlike a sail that goes slack when a boat moves directly downwind, the propeller will continue to turn, because the wheels are driving it.
As Goodman wrote in his article, “The key to understanding DWFTTW [Downwind Faster Than The Wind], is that the wheels are turning the propeller and the the propeller need only produce enough lift in still air to overcome the forces required to turn it.”
The naysayers weren’t buying it. One critic said “Yeah, every time I sit in a calm, wind-free room, the floor rushes by underneath me too.”
After having digested more than 100 comments on YouTube and other online forums on the matter, I didn’t know what to think. Some of the pro-DWFTTW arguments were quite persuasive, and they’d almost convince me that Goodman was right. But then I’d read a convincing counterargument from a naysayer that would make me doubt again.
Paul, the MAKE editor who sent me the video, thought it plausible. He said, “It’s taken me a while to put my brain around how this works — for me, the key point is that the propeller is a propeller, not a wind vane, and when the cart is rolling, the wheels are powering the propeller, not the other way around. With the right gearing, the propeller will always push backwards against the air, whether or not the air is moving forwards or backwards relative to the cart. The tailwind and the propeller action combine to make the wheels spin fast enough to keep the whole system rolling faster than the wind. Definitely counterintuitive, or a hoax I’ve fallen for.” He too, was on the fence, but leaning over on the side of the believers.
If Goodman wasn’t perpetrating a hoax, I had a lot of admiration for him for actually building something and trying it out. It was a great example of a DIY science experiment. I showed the video to my friend, Charles Platt, who’s written many excellent how-to articles for MAKE. His solid knowledge of math, physics, and electronics, coupled with decades of hands-on experience making models, medical equipment prototypes, games, and electronic circuits has made him one of the magazine’s star contributors.
Charles watched the video and emailed me back saying he thought the video was bogus. The reason the cart can’t work, said Charles, is that as the cart goes faster, the wind force on the cart is diminished. “If it’s a 10mph wind and the cart gets up to 8mph there is now only a 2mph push behind it. Really it should slow down, but, no, it accelerates! When it is running equal with the wind, it is like a sailing ship ‘becalmed.’ No net force from air behind or in front. Yet still it runs faster!”
Even though Charles’ experience-informed intuition told him the cart couldn’t work, he decided to build his own cart. He constructed a tabletop model of the cart out of wood and attached model airplane wheels. He built a propeller and connected it to the rear axle with a rubber belt. Making sure the moving parts were thoroughly lubricated with WD-40, Charles placed the cart on his workbench, put a 15-inch fan behind it, and turned the fan on.
“The cart barely moved in response to the fan,” explained Charles in his write-up about the experiment for MAKE (Volume 11, page 58).
He then used a massive 38-inch warehouse ventilation fan and ran it at its highest speed. “[T]he cart ignored it and did nothing.” After building a shroud to channel the wind onto the cart, he observed a forward motion of 2 inches per second. At least it was moving, but the wind propelling it was traveling at 30 feet per second.
Charles’ conclusion was that the cart didn’t work, and that the Goodman video was highly suspicious. He wrote, “Perhaps Jack Goodman has some clever explanation for this. Perhaps I didn’t build my version exactly the same way that he built his. Perhaps you should build your own, just to make sure. Building a prototype that doesn’t work is always educational, provided, of course, that you are willing to face facts and admit that it doesn’t work.”
I agreed with Charles’ conclusion, figuring Goodman had pulled an excellent prank on his yachtie friends, and that now that his cart had been debunked, the story was over.
But shortly after the article ran, a number of believers started chiming in on the Make: Online forums, claiming that Platt was wrong, and Goodman was right. Charles patiently answered each comment. He defended his article and his cart, and even offered one believer, who used the screen name pelesl, “$1,000 cash of my own money if he can prove that [a cart of Charles' design] will start by running slower than a constant wind, will accelerate to equal the wind speed, and will then accelerate further to exceed the speed of the wind, without any outside interference, on a flat surface,” just as Goodman’s appeared to do in the video.
Soon, people began posting YouTube videos of their homemade wind carts. Instead of running them outside in the wind, though, they placed the carts on exercise treadmills. The videos showed the carts, their propellers spinning furiously, move in the opposite direction of the treadmill belt. This was impressive, but was it the same as testing a cart on stationary ground with wind pushing it? The DDWFTTW proponents argued that the two scenarios — “moving ground and still air” versus “still ground and moving air” — were equivalent for the purposes of proving DDWFTTW, but I wasn’t sure. (The phenomenon is also called Directly Downwind Faster Than The Wind, to eliminate the possibility of tacking or angling into the wind.)
Rick Cavallaro (known on message boards as both spork and spork33) was the most impassioned defender of wind carts. He said that the treadmill videos (many of which he shot and uploaded himself) proved Charles wrong and that he should pay the $1,000 to pelesl. Charles responded that he’d said the test had to be conducted with wind, not a treadmill. Cavallaro raised the stakes, writing: “I will do exactly what you suggest both indoors and out. But it won’t be an open challenge. It will be a bet. Your $10K against my $100K. But we’ll both put the funds in a joint escrow account to be awarded to the winner. I will then pay pelesl from my winnings.”
I wondered what a physics professor would make of all this, so I emailed Dr. Paul J. Camp, a professor in the physics department at Spelman College in Atlanta, Ga. Camp had helped me with other physics brain-teasers in the past and I appreciated his clear explanations. I asked him to take a look at Goodman’s video, as well as Charles Platt’s article, and the treadmill cart videos. “What do you say?” I asked.
“Impossible,” replied Camp. “Would violate conservation of momentum and conservation of energy … In fact, we can state this in pretty bare terms — for a car moving downwind at wind speed to go a little faster is physically indistinguishable from a car at rest on the ground in stationary air to suddenly leap into motion. What can be done in one inertial frame can be done in any other inertial frame with the same physical circumstances.”
Camp’s explanation, which was the same as Platt’s, made sense to me. But I couldn’t stop thinking about those wind cart enthusiasts’ videos, which clearly showed the carts moving up the spinning belts of treadmills.
I contacted Rick Cavallaro. He’s the chief scientist for a Silicon Valley company called Sportvision Inc., which creates systems that overlay computer-generated visual effects onto televised sporting events. He has a B.S. in aerospace engineering from Georgia Tech and an M.S. in dynamics and controls from UCLA.
Cavallaro told me he’d started making kits for wind cart enthusiasts to put together and try out on their own. I PayPaled him $40 and, a few days later, a FedEx tube arrived with all the parts I needed to make my own cart: three plastic wheels, a pre-bent aluminum tube, a plastic propeller, and some gears. It only took a few minutes to assemble the cart, which looked quite sleek.
A DWFTTW model that can be built from plans found here
There wasn’t a lot of wind that day, so I used an electric fan and tried the cart indoors, in a long hallway. I turned the fan on full blast and held the cart, off the floor, in front of it. The propeller spun counterclockwise, making the wheels spin backward.
Then I set the cart on the floor, with the fan pointed behind it. It started moving forward and the propeller spun clockwise. This meant that the wheels were driving the propeller, as the Goodman defenders claimed it would, and not the other way around, as many detractors insisted that it would. So the cart proponents were correct on that count.
The cart picked up speed, but then slowed and bumped into the wall at the end of the hallway. This, of course, was not a good environment to conduct the test, but at least I learned that the cart could indeed move in a tailwind.
While I waited for a day in Los Angeles with a steady wind so I could take the cart outside, I checked out various message boards, and found that the debate was raging even more than before. The doubters said the treadmill was not an accurate simulation of a cart in the wind, while the believers insisted it was. At times, the debates would get highly emotional, with name-calling and slurs. (My favorite: “I am more than physically capable of taking that little cart and shoving it up your ass, sideways.”)
One of the doubters, whose screen name was swerdna123, wrote that only a wind tunnel demonstration would satisfy him. But there were no takers. Apparently, it was too difficult for anyone to rise to the challenge of making one. So swerdna123 took it upon himself. His intention, like Charles Platt’s, was to show the believers that the cart wouldn’t go downwind faster than the wind.
Swerdna123’s design was clever — a circular wind tunnel, made from a cardboard drum that had a rotating spindle with a motor that drove wind-making flaps around in a circle. He designed and built a tiny propeller cart to roll around the drum on a circular track. After he built the setup, he posted several videos to YouTube. The cart had a stiff wire coming off it, like a radio antenna, with a small paper flag. This small paper flag was to indicate the direction of the wind speed relative to the cart, just like the wind sock on Goodman’s much larger cart.
As the rotating flaps picked up speed and created wind the cart started moving. At first, the flag pointed in the direction of the cart’s rotation, meaning it was traveling slower than the wind inside the setup. After a few seconds, the flag hung straight down, meaning the car was traveling at exactly the speed of the wind. Then, to my surprise (and delight) the flag began to tilt in the opposite direction of the motion, and the cart was rolling faster than the flaps. It really was outrunning the wind!
I contacted swerdna123 and learned that his real name is Tony Andrews. He’s a 58-year-old website developer and self-described “backyard inventor” from Christchurch, New Zealand. (He’s also a former New Zealand and World silver medal finalist Monopoly champion.) Like a lot of other wind cart skeptics, Andrews didn’t think the cart-on-treadmill demonstrations were adequate to prove the DDWFTTW theory. In addition, he was annoyed by the nasty tempers of the treadmill crowd: “Their main purpose seemed to be to insult and incite argument.”
Andrews wanted to see a wind cart move from the wind, not from a treadmill. And since no one else was stepping up to the challenge, he had to take it on himself. “Although I was very busy with other things at the time,” he said, “I couldn’t resist building and testing my own DDWFTTW equipment.” He said he possesses “natural mechanical abilities,” and has “invented and built many weird and wonderful mechanical things like rotary motors/pumps, constant velocity transmission systems, toys, games, and puzzles.”
Andrews said he has several ideas on how to improve the wind cart test, like a better wind tunnel and a camera that moves with the cart so it would be easier to compare its speed with those of the wind vanes, but that he probably won’t get around to building them, as his interests have shifted. Besides, he said he felt that his experiment shows that continuous DDWFTTW travel is possible.
How does the cart do it? I asked him. His theory is that the cart uses “some of the force of the wind to continuously push back against the wind.”
I felt confident that Andrews had settled the matter. But to make sure, I sent links of Andrews’ videos to Paul Camp, the physicist at Spelman College. He immediately began poking holes in Andrews’ test apparatus. Camp’s main beef was that the fan blades that created the wind were so close to the cart itself, that the cart was being subjected to turbulent air flow.
The viscosity of air, Camp explained, produces velocity gradients when air flows past an edge. “The behavior would also be very peculiar next to the fan blades in a linear wind tunnel. In that case, to see the effect of wind alone, you get far enough from the fan blades for viscosity to damp out turbulent flow. In this case, it is impossible to get that far away. What we would really like to see is smoke trails in his wind tunnel to see how the fluid is actually flowing.”
Once again, I’d been lured over to one side of the fence, only to be pulled back to the middle. I started wondering what Jack Goodman thought about this particular DIY science movement he had kick-started. Apart from one or two comments early on, he’d been quiet for the last two years.
In August 2009 I got Goodman on the phone and learned that he was a retired consultant/inventor living in Maryland. As a member of the Amateur Yacht Research Society, he’d been interested in the concept of DDWFTTW for a number of years. The members of the society frequently argued about whether or not DDWFTTW was possible. Goodman didn’t see how it was possible, but he had a nice model shop in his basement with a lathe, milling machine, and other shop tools, so he decided he’d try to settle the matter once and for all by making a wind cart.
Goodman initially tested his cart on a treadmill, and found that it could move in the opposite direction of the treadmill, which encouraged him. But he wanted to try it out in the wind. The wind and terrain in Maryland weren’t right to test it, so he brought it with him to his winter home in Florida and scouted around until he found a level road with wind conditions suitable for the experiment he wanted to run. He waited a “long time” for the wind to blow in the right direction before he was able to conduct the experiment, which he videotaped and posted online.
Goodman said he never expected his video to cause such an uproar (he didn’t post it to YouTube; someone else from the Amateur Yacht Research Society did that) and that he has gotten a tremendous kick out of reading the thousands of posts about it. He even gets a chuckle out of being called a charlatan and a fake.
I was even more convinced that DDWFTTW was real when I learned that Mark Drela, a world-renowned aerodynamicist at MIT, says it’s possible. Drela is the designer and maker of a variety of amazing human-powered, propeller-driven vehicles. In 1998, Drela designed a human-powered airplane called the Daedalus, which set a world record when it island-hopped more than 72 miles from Crete to Santorini.
Drela is also the developer of a computational aerodynamic analysis system called XFOIL, which allows aircraft designs to be tested before they’re built. An article in MIT’s Aero-Astro magazine reports that Drela has “engineered aircraft for Boeing, the wing for the Predator UAV, the keel of America’s Cup yachts, and experimental aircraft used by NASA. In addition to XFOIL, he has written design programs for rotorcraft, machinery blading, and axisymmetric bodies such as zeppelins.”
In other words, Drela knows his stuff. And so what does his analysis conclude?
“Although DDWFTTW seems like it violates physics, it really does not,” Drela explained to me via email. “The various analyses show this, and the cart experiments on YouTube are definitive proof. In my view, the most closely controlled and unambiguous DDWFTTW demo is the cart climbing up the tilted treadmill. The main problem is then convincing some people that this is equivalent to DDWFFTW. But whoever tries to argue against that equivalence is really arguing against Galilean relativity, which is unassailable. So that secondary argument is a complete waste of time.”
So Goodman, Andrews, and Cavallaro have been vindicated. DDWFTTW is possible.
But even with Drela’s analysis floating around the web forums, the debate won’t die. Last year, Cavallaro and his colleagues demonstrated the wind cart to the aeronautics and mechanical engineering departments at San Jose State University and Stanford. After those presentations, Cavallaro said, a UC Berkeley Ph.D. student contacted him about demonstrating the cart at Berkeley. She asked her professor, Dan Kammen (an Obama advisor and Nobel Peace Prize winner) about having Cavallaro present the wind cart to his students.
“He decided DDWFTTW is not possible,” said Cavallaro. “So it does fool some pretty sharp people.”
In July 2010, Rick informed me that he and his colleagues had taken a downwind cart they’d built onto the El Mirage Dry Lake Bed in southern California and had achieved a speed 2.8 times faster than the wind. They carried out the test in front of authorized representatives of the North American Land Sailing Association (NALSA). You can read Rick’s article about it here.
Mark Frauenfelder is editor-in-chief of MAKE.