From: veritasium
The concept of a wind-powered vehicle traveling directly faster than the wind itself has long been a subject of debate in physics, often appearing counterintuitive and seemingly violating the law of conservation of energy [00:00:03]. To settle this “brain teaser” [00:01:02], a unique propeller craft known as Blackbird was constructed [00:00:00].
The Counterintuitive Premise
A common understanding regarding wind-powered vehicles is that a sailboat traveling directly downwind can only match the speed of the wind, never exceed it [00:01:15]. This is because once the boat reaches the same speed as the wind, there is no longer any apparent wind to push the sail [00:01:19], effectively leaving the sail in still air from the boat’s frame of reference [00:01:46]. Therefore, a sailboat can only match, not exceed, the wind’s speed when going straight downwind, even without drag [00:01:57].
The claim for Blackbird, however, is that it can go straight downwind faster than the air that is pushing it [00:02:10].
Blackbird: The Experimental Vehicle
Blackbird is a one-of-a-kind propeller craft, invented by Rick and currently owned by Neil [00:00:54]. It was built to definitively prove the downwind faster than the wind hypothesis [00:00:00]. The vehicle’s design incorporates wheels geared to a propeller [00:11:27].
How Blackbird Differs from a Sailboat
Unlike a sailboat, which uses a sail as a wing to generate lift when traveling at an angle to the wind [00:06:53], Blackbird’s propeller functions as a fan [00:11:38]. The wind pushes the vehicle from behind, causing the wheels to turn [00:11:14]. These wheels are geared to spin the propeller in the opposite direction the wind is pushing it, effectively pushing air backward to propel the craft forward [00:11:25].
Early Development and Skepticism
The idea originated as a “brain teaser” [00:07:53]. Rick initially posted the concept on online forums, expecting initial disbelief followed by acceptance [00:08:39]. However, he was “more wrong” than he could have imagined [00:08:49]. Many found the concept “stupid” and “clearly fake,” arguing that once wind speed was reached, there would be no power to accelerate further [00:09:08].
To counter this skepticism, a small model was built and tested on a treadmill [00:09:16]. This model successfully demonstrated the principle, climbing the treadmill [00:09:24]. Despite this, online skeptics and even physics professors argued that a treadmill was not a valid analogy for the real world [00:09:33]. Accusations ranged from hidden magnets, wires, to out-of-view fans [00:09:48]. This persistent disbelief ultimately led to the decision to build the full-scale Blackbird vehicle [00:09:58].
The Demonstration Runs
The Blackbird was brought to El Mirage dry lakebed in California to perform the demonstration [00:00:44]. A telltale string on the front of Blackbird was used to indicate apparent wind direction:
- Pointing straight out: Wind is coming from behind [00:05:15].
- Dangling down: Vehicle is moving at the same speed as the wind (effectively in still air) [00:05:20].
- Pointing straight back at the driver: Vehicle is going faster than the wind (experiencing an apparent headwind) [00:05:47].
Several attempts were made by Derek Muller (Veritasium) to pilot Blackbird.
- First Attempt: The vehicle reached 5-6 miles per hour, with the telltale dangling, indicating it was at wind speed but couldn’t accelerate past it due to insufficient wind [00:06:06].
- Second Attempt: The vehicle reached 8-9 miles per hour before the chain came off the propeller [00:10:36].
- Third Attempt (Rick in control): Rick piloted the vehicle in stronger wind, and it did not shake excessively, suggesting it was safe for Derek to try again [00:14:36].
Successful Demonstration
On the final attempt, as the sun set, Derek successfully drove Blackbird directly downwind faster than the wind [00:15:42]. The telltale on the front of Blackbird was observed pointing straight back at him, indicating an apparent headwind, while a windsock on the ground confirmed the true wind was still coming from behind [00:16:32]. Blackbird previously achieved a record speed of 2.8 times the wind speed [00:19:50].
The Physics Explained
The core of Blackbird’s operation lies in its propeller acting as a fan, rather than a windmill [00:17:56]. When the vehicle reaches wind speed, there is no relative air motion over the cart [00:17:36]. However, the propeller, being geared to the wheels, continues to spin. As the vehicle moves forward, the wheels drive the propeller to push air backward [00:11:25].
The energy to power the fan comes from the wind itself [00:19:05]. Even when the vehicle is moving at or faster than wind speed, the propeller can still push air molecules backwards relative to the ground [00:18:51]. This action slows down the wind directly behind the propeller [00:19:02]. For example, if the surrounding wind is 10 km/h, the air directly behind the propeller might be slowed to 8 km/h [00:19:14]. This reduction in the wind’s kinetic energy behind the propeller is precisely the energy transferred to the vehicle, accelerating it forward [00:19:23].
In essence, Blackbird extracts kinetic energy from the moving mass of air (the tailwind) and converts it into the kinetic energy of the vehicle itself, enabling it to accelerate beyond the wind’s speed [00:19:41]. The demonstrations prove that a vehicle can indeed travel directly faster than the wind [00:20:05].