Originally published 2021-07-05.
Yes, a wind-powered vehicle can travel directly downwind faster than the wind that drives it, and in 2021 that claim won Derek Muller, the physicist behind the YouTube channel Veritasium, a $10,000 bet. His opponent was Alexander Kusenko, a physics professor at the University of California, Los Angeles (UCLA), who argued that the effect must be an illusion caused by gusts. After Muller presented model experiments, data from a real vehicle and a mathematical analysis, Kusenko conceded and paid.
The episode became a rare public example of a scientific argument settled by evidence, with two celebrity witnesses and a cheque at the end. It also left a lot of viewers asking the obvious question: how can something pushed by the wind ever go faster than the wind?
The Blackbird and the original video
The vehicle at the centre of the dispute is Blackbird, an experimental cart built in 2010 by Rick Cavallaro, an aerodynamicist and engineer, together with John Borton, with backing from San Jose State University's aeronautics department and sponsorship from Google. It has a large propeller linked by a chain and gears to its wheels, and no engine, battery or flywheel.
On 2 July 2010, at El Mirage Dry Lake in California, Blackbird was measured travelling directly downwind at about 2.8 times the speed of the wind, a result recognised by the North American Land Sailing Association (NALSA). In 2012 a modified version went directly upwind at about 2.1 times wind speed.
In 2021 Muller published a Veritasium video in which he rode Blackbird across a dry lake bed and showed a streamer on the cart blowing forward, as if into a headwind, while the vehicle was heading downwind. The video went viral, and it attracted plenty of sceptics.
How the bet came about
Kusenko contacted Muller to say the video was wrong. In his view, the cart could not sustain a speed above wind speed while travelling straight downwind, and any apparent excess was the result of gusts or of energy stored up during a gust and released afterwards.
Rather than argue in comment sections, Muller proposed a wager of $10,000. The two signed a written agreement setting out what Muller would have to demonstrate, with Bill Nye and Neil deGrasse Tyson acting as witnesses. The terms meant the case would be judged on evidence rather than reputation, which is exactly what made it newsworthy.
The physics: how a cart outruns the wind
The key is that the vehicle sits between two things moving relative to each other: the air and the ground. Any time there is a difference in speed between them, there is energy that can be tapped. Sailboats already exploit this. A fast ice boat or racing yacht sailing at an angle to the wind can reach a downwind speed greater than the wind itself, even though sailing straight downwind on sail power alone cannot.
Blackbird does something similar without tacking. Once it is rolling faster than the wind, the wheels are turned by the ground and they in turn drive the propeller. The propeller pushes air backwards, slowing that air relative to the ground. Because the propeller pushes against a large mass of slow-moving air while the wheels take power from the fast-moving ground, the thrust gained is larger than the drag created at the wheels. The net result is a forward push, even above wind speed.
It looks like a free lunch, but it is not perpetual motion. The energy comes from the wind slowing down as the vehicle passes through it. If the air were perfectly still, the cart would gradually stop.
Muller described the moment of equal speeds this way: "When the speed of the car is exactly equal to the speed of the wind, it seems like the propeller can provide infinite force." In practice friction and other losses keep that force finite, but it explains why there is no barrier at wind speed.
The evidence that settled it
To answer Kusenko's objections, Muller worked with engineer and YouTuber Xyla Foxlin to build small models that could be tested on a treadmill and in controlled conditions, where gusts could not be blamed. He also reviewed data from the original Blackbird runs and wrote up a theoretical analysis of the forces involved.
Kusenko accepted the result and paid the full $10,000. Muller praised him for "changing his mind in light of the evidence presented", noting that this is hard to do during a public debate.
The story has one extra wrinkle. According to the Wikipedia article on Blackbird, Kusenko conceded on the terms of the bet but said he had still not seen proof that Blackbird itself had exceeded wind speed under the conditions he cared about. Physicists and engineers broadly accept the downwind-faster-than-the-wind principle, and NALSA certified the 2010 record, but the debate shows how carefully the conditions of any experiment need to be defined.
In the follow-up video above, Muller explains the bet, the models and the maths.
Common misconceptions about the Blackbird
"It's just riding a gust." Gusts can make a vehicle briefly faster than the average wind, but Blackbird sustained speeds of more than twice the wind, and treadmill models reproduce the effect with steady airflow.
"It breaks the laws of physics." It does not. Energy is conserved; the cart extracts it from the speed difference between air and ground, just as a wind turbine or a sail does.
"It needs a hidden motor or battery." Blackbird carries no engine, battery or flywheel; all of its power comes from the wind and the ground.
Why this story still matters
The Veritasium bet is often shared as a story of a YouTuber beating a professor, but the more useful lesson is about how disagreements should be resolved. Kusenko raised a reasonable objection, the claim was tested under agreed conditions, and the person who lost accepted the result. That is how science is supposed to work, whatever the credentials of the people involved.
Blackbird itself was later sold, and in 2023 its owner donated it to the Castle Air Museum in California, where it is being restored for display. If you get the chance to see it, look at the propeller and wheels together and remember that its secret is not the wind alone, but the ground beneath it.
Sources: Wikipedia: Blackbird (wind-powered vehicle); Veritasium on YouTube; models built with Xyla Foxlin. The story was first reported by Vice and Futurism.
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