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A wind-powered vehicle that can travel twice as fast as the wind itself

businessinsider.com

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Re: A wind-powered vehicle that can travel twice as fast as the wind itself

#151

Post by cfallin »

michaelt wrote: post #13

> 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…

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 and propeller do; it's the counterintuitive power flow that makes it so confusing...

Re: A wind-powered vehicle that can travel twice as fast as the wind itself

#152

Post by jjeaff »

cblconfederate wrote: post #147
6510 wrote: post #146

Earlier quoted context omitted.

the blades move sideways

if it sails away from the wind, the lift is pushing it sideways, not forward

And if you turn a sail sideways and put it on a spindle with several other sails, you have what looks a whole lot like a propeller.

So instead of sailing sideways so that your air foil sail can move sideways through the wind, the Blackbird harnesses the power of sales moving sideways through the wind while pushing the craft directly into the wind.

Re: A wind-powered vehicle that can travel twice as fast as the wind itself

#154

Post by rusk »

thehappypm wrote: post #150

I think I have an analogy that actually explains how this works. Imagine one of those people-mover walkways like they have at the airport. Standing next to it on a skateboard, you grab the handrail. Even though the skateboard has energy loss to friction, you go exactly as fast as the handrail. Now imagine you put a little electric generator in the wheels of the skateboard. Now, not only are you moving at the speed of…

This apparently violates conservation of energy as you’re claiming to take energy from one source, put it through some apparatus (i.e two skateboards) to produce more energy without putting anything additional in. Can you account for this?

Re: A wind-powered vehicle that can travel twice as fast as the wind itself

#156

Post by haberman »

> 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 of the mast), but this is the exception, not the rule.

Re: A wind-powered vehicle that can travel twice as fast as the wind itself

#158

Post by notJim »

Someone1234 wrote: post #2

The most surprising thing here is that someone on Twitter made a bet, lost the bet, and then actually paid said bet. Playing devil's advocate here a little: The whole machine is very counter-intuitive, and I can see how I myself may disbelieve it if not for the irrefutable demonstration(s)[0]. [0] https://www.youtube.com/watch?v=yCsgoLc_fzI

How does a professor have $10,000 sitting around for a bet like this?? I am a nicely-paid tech worker, and $10,000 wouldn't ruin me, but it would be a major setback for that year.

Re: A wind-powered vehicle that can travel twice as fast as the wind itself

#159

Post by jcims »

cfallin wrote: post #151
michaelt wrote: post #13

> 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…

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 prop does through the air for this to work at all, and it sounds like it should be roughly twice as fast for it to work on something that is reasonably simple to engineer. So if your prop pitch is 5", your wheel size and gearing should allow the vehicle to move forward approximately 10" for each rotation of the prop.

(Note that her video does not explain 'why' this is the case, just that it is a phenomenon that the Blackbird inventors had determined/discovered and is an essential design criteria).

Re: A wind-powered vehicle that can travel twice as fast as the wind itself

#160

Post by jcims »

Derek from Veritasium asked Xyla Foxlin to build a functional model, and she goes through the trials and tribulations of that in quite a bit of detail in her video here:

https://youtu.be/VUgajGv4Aok

There's a key aspect discussed in 7:10 relating to the ratio of the propeller pitch to the vehicle forward motion that is necessary for this to work.

(Xyla's channel is a great follow overall btw)

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