The team that proved it’s possible to travel downwind faster than the wind has done it again, this time modifying their cart to go upwind at more than twice the speed of the circulating air.
Last time around, the Blackbird cart raced downwind at 2.86 times the speed of the wind. Earlier this month, Rick Cavallaro and the Blackbird team braved 104 degree heat at the New Jerusalem airport in Tracy, California, clocking in a top speed 2.01 times faster than the wind speed when headed upwind – which could end up being a new record.
It’s an impressive feat, but not as controversial as the downwind run. Where the prospect of traveling downwind faster than the wind once inspired thousands of internet arguments and heated debates in physics classrooms, an upwind sail just isn’t as provocative. In fact, there’s already a racing series in the Netherlands devoted to upwind land surfing.
“For some folks, the idea that it can advance directly into the wind at all has been counter-intuitive,” said Cavallaro, an aerodynamicist, kitesurfer and paraglider. While it may seem like a wind-powered vehicle heading directly into the wind could end up traveling faster and faster in an endless feedback loop, that isn’t the case. “There’s at least an element of truth to this, but as with the downwind cart, frictional losses still win out at a certain speed,” Cavallaro said.
The principle behind the upwind-configured Blackbird should be familiar to anyone with knowledge of sailing, except the Blackbird prefers runways and dry riverbeds. It uses two large “sails” – turbine blades – that spin around a common axis, moving forward as the cart sails into the wind and moving cross-wind as the blades turn around the axis.
“This combination of upwind and cross-wind motion is identical to that of a sail on a boat on an upwind tack,” Cavallaro said. “Where the sailboat has a keel to constrain the motion of the sail in the correct direction, we have a transmission and wheels that perform the same job.”
Cavallaro made most of the modifications to the Blackbird on his own, with the occasional help of some kitesurfing friends. First, he created blades with a different pitch. Configured for a downwind run, the Blackbird’s wheels were set up to turn the propeller, which in turn moved the vehicle forward. He had to reverse the setup for an upwind run, where turbine blades power the vehicle. We imagine that he listened to Christopher Cross’ “Sailing” and “Ride Like the Wind” on repeat while working.
Aside from the blade pitch, Cavallaro also had to take into account the Blackbird’s unique design: To keep the turbine’s torque from flipping the vehicle over, one of its axles is longer than the other, so the Blackbird’s chain drive had to be reconfigured in order to fit the asymmetrical axle setup.
Now that he has upwind and downwind runs under his belt, Cavallaro is looking forward to advances that other aerodynamic enthusiasts make. “I would like to see both our upwind and downwind records broken – regularly,” he said.
Photo: Rick Cavallaro
