The key thing to understand this is that it isn’t using a turbine blown by the wind, but rather a propeller that blows the air backwards, with the propeller being driven by the wheels connected to the ground.
A wind-powered vehicle that can travel twice as fast as the wind itself
201–210 of 263 posts
Re: A wind-powered vehicle that can travel twice as fast as the wind itself
#202> The secret to Blackbird, Cavallaro explained, is that once the wind gets the vehicle going, its wheels start to turn the propeller blades — they're connected to the blades by a chain. As the vehicle speeds up, its wheels turn the propeller faster and faster. The propeller blades, in turn, act as a fan, pushing more air behind the land yacht and thrusting it forward. That's a terrible explanation - no wonder people…
It’s really not that complicated - wind speed is not the same as energy. The propellor is a device that pushes against the air to move forward, it takes energy from the wind, produces a force, and the force produces momentum in the vehicle. We know that a stationary turbine can continually take power from the wind. When that wind energy is stored in a battery it can power your car much faster than the wind. This just…
The more important point is conservation of energy. Since the ground is moving backwards faster than the air, you can generate more thrust against the air for the same amount of power than the amount of backwards force exerted by the ground on your wheels (since power = force * velocity). For the same reason, if traveling upwind rather than downwind, you’d need it to use the opposite mode of operation - have a turbine driving the wheels rather than wheels driving a propeller.
Conservation of momentum is pretty irrelevant, since you have a giant immovable sink/source of momentum known as the Earth to push against
Re: A wind-powered vehicle that can travel twice as fast as the wind itself
#203Earlier quoted context omitted.
The way this clicked for me (it took some thought!) was to see it as a lever, or at least an analogue to one. Imagine a lever with the fulcrum on one end, attached to the ground. Wind pushes on the middle, and the car is at the other end of the lever. So wind exerts more force with less speed, and the car moves forward with more speed but less force. That's basically what the linkage and gear ratio between the wheels…
I think this is a good intuition about it, and Xyla Foxlin goes into this a bit in her video explaining the build for Veritasium's video. https://youtu.be/VUgajGv4Aok?t=432 The essential 'gear' ratio is between forward motion of the vehicle and the propeller pitch (essentially how far the prop would screw through a tub of jello for a given amount of rotation). The vehicle has to move *faster* on the ground than the p…
Yes exactly. And if you were driving the other way into the wind, you’d flip the lever and use a turbine to extract energy from the wind and push against the ground.
Re: A wind-powered vehicle that can travel twice as fast as the wind itself
#204> The secret to Blackbird, Cavallaro explained, is that once the wind gets the vehicle going, its wheels start to turn the propeller blades — they're connected to the blades by a chain. As the vehicle speeds up, its wheels turn the propeller faster and faster. The propeller blades, in turn, act as a fan, pushing more air behind the land yacht and thrusting it forward. That's a terrible explanation - no wonder people…
I really would like a simple explanation of how that is possible. AIUI, when speed is stable, the wheels turning provide power to the propeller. But if the propeller pushes the car, which in turn makes the wheels turn faster making the car faster than the wind itself, what makes it different from perpetual motion machines? I would love a diagram showing how the power flows in this system.
It’s not perpetual motion because it’s extracting energy from the fact that the wind is moving relative to the ground. Without wind moving relative to the ground it wouldn’t work.
Re: A wind-powered vehicle that can travel twice as fast as the wind itself
#205Earlier quoted context omitted.
I really would like a simple explanation of how that is possible. AIUI, when speed is stable, the wheels turning provide power to the propeller. But if the propeller pushes the car, which in turn makes the wheels turn faster making the car faster than the wind itself, what makes it different from perpetual motion machines? I would love a diagram showing how the power flows in this system.
Here's a simple explanation: Let's assume that the windspeed is 10 km/hr. We all agree that the wind can push this vehicle so that it is now going 10 km/hr downwind, right? From the vehicle's point of view, the wind is now 0 km/hr. But its wheels are turning, which turns the propeller, which behaves like a fan, pushing the vehicle forward in the still air, so that it is now traveling faster than the wind. This isn't…
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Re: A wind-powered vehicle that can travel twice as fast as the wind itself
#206Earlier quoted context omitted.
I really would like a simple explanation of how that is possible. AIUI, when speed is stable, the wheels turning provide power to the propeller. But if the propeller pushes the car, which in turn makes the wheels turn faster making the car faster than the wind itself, what makes it different from perpetual motion machines? I would love a diagram showing how the power flows in this system.
Here's a simple explanation: Let's assume that the windspeed is 10 km/hr. We all agree that the wind can push this vehicle so that it is now going 10 km/hr downwind, right? From the vehicle's point of view, the wind is now 0 km/hr. But its wheels are turning, which turns the propeller, which behaves like a fan, pushing the vehicle forward in the still air, so that it is now traveling faster than the wind. This isn't…
There is no theoretical limit on the speed of the vehicle though - just a practical one based on limits to propeller efficiency and the fact that drag for the craft keeps going up.
Re: A wind-powered vehicle that can travel twice as fast as the wind itself
#207Earlier quoted context omitted.
Where the power comes from is easy: the wind gets slowed down. Of course the nature of wind is such that you could never measure this, but the point is that when the propellor pushes against the air, that air's velocity relative to the ground is decreased while the vehicle's is increased. The vehicle leaves behind it an imperceptible region of less-energetic air and takes that energy with it.
Now I think I'm beginning to understand... At first, wind propels the vehicle, moving it forward and turning its wheels. Its wheels turn the propeller which slows down the air, even the air in front of it. The energy lost by the air is now the energy that moves the vehicle! Indeed! There is no requirement that the vehicle must be slower than the wind! I think understood it!!!
Yeah. Wind pushes the propeller (and attached vehicle) forward, and as the vehicle starts traveling forward, the wheels turn the propeller generating extra thrust (at the expense of the wheels pushing backwards by a smaller amount against the ground), allowing the vehicle to continue to accelerate past the wind speed.
Re: A wind-powered vehicle that can travel twice as fast as the wind itself
#208The thing I wonder about stuff like this is that a phenomenon we see in one domain will typically transfer to another domain. For example, the 'hydraulic analogy' [1] for electronic circuits is pretty damn good at least on a macroscopic level. I wonder if there are applications of this phenomenon in other types of physical systems (including electronics).
Acoustics relies on an analogy to electronic circuit components.
Re: A wind-powered vehicle that can travel twice as fast as the wind itself
#209Sailing faster than the apparent wind[0] has been going on for a while, especially on v.high performance land and water yachts. [1] The physics involved still twist my brain, but there's nothing terribly new here. [0] https://en.wikipedia.org/wiki/Apparent_wind [1] https://en.wikipedia.org/wiki/High-performance_sailing
This is not what they are talking about. It's always step one to get past to understand what's going on here. I don't believe DDWFTTW has been achieved on water.
You are mistaken. It has been achieved, and the fastest race boats today do it routinely. This is one of the reasons you don't see spinnakers on boats like the America's Cup AC72s, AC50s, F50s, huge open class trimarans, etc.
I'm not sure the first boat to achieve it, but it wouldn't be surprising if it was done during the Clipper era. Those boats held 24 hour records from 1860 something all thee way to the 1980s, where modern racing yachts finally eclipsed them.
Re: A wind-powered vehicle that can travel twice as fast as the wind itself
#210> Any sailor worth their salt can tell you that a boat can travel faster than the wind by cutting zigzag patterns; that's called tacking. This is a pedantic point, but when you're cutting zigzag patterns downwind it's called jibing, not tacking. I guess there are high-performance boats that can go faster than the wind upwind, and that are so fast that they can perform downwind tacking (since apparent wind stays ahead…
Sport kites can also be great as a sort of demonstration of what a sail's crosswind potential could look like if it didn't have to lug around a vehicle, technically both downwind and upwind since they're essentially tracking a spherical concave shape centered directly downwind of the pilot. Speedkites in particular push the sail speed vs wind speed ratio to ridiculous limits. For scale, this kite is 190cm across.