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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

#171

Post by mypetgoat »

jjeaff wrote: post #152
cblconfederate wrote: post #147

Earlier quoted context omitted.

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.

Exactly. The article does not understand this at all.

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

#172

Post by jcims »

haberman wrote: post #156

> 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…

Side note, this is my favorite mode of exhibiting pedantry. Let the conversation flow while still correcting terminology. Thanks for the info.

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

#173

Post by rusk »

jcims wrote: post #169

The 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).

This seems to be highly domain specific though. It’s reading to me like an example of what you can do when you have highly advanced knowledge of a domain to the point where you can do stuff that looks like magic, specifically because it apparently violates generalised principles established across domains.

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

#174

Post by sopooneo »

haberman wrote: post #156

> 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…

But what about once you are going downwind faster than the wind, and thus experience an apparent headwind? I'm not a sailor, but I thought I'd seen people using the term "tacking downwind" to describe that.

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

#175

Post by [deleted] »

rusk wrote: post #154
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?

[deleted]

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

#176

Post by jasonwatkinspdx »

haberman wrote: post #156

> 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…

State of the art in crazy expensive racing yachts (think America's Cup) is limited by cavitation on the foils to a bit over 50 knots. Ice boats are setting records over 120 knots. If someone figures out a magic foil, that gap may close, which is a thought that blows my mind.

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

#177

Post by jasonwatkinspdx »

jfoutz wrote: post #119
mumblemumble wrote: post #50

Earlier quoted context omitted.

Asking what is spinning what is maybe framing it in a way that impedes understanding. A quick summary of the basic forces involved are: The wind pushes the propeller, the propeller pushes the wheels, the wheels push the ground, the ground pushes the wheels, the wheels push the propeller, and the propeller pushes the wind. Or perhaps it's better to say that everything is dynamically interacting with everything else. I…

Sailboats are one I struggled with, especially sailing into the wind. I think my intuition there came from a boat going the same direction as the wind, say, north at 1 meter/sec. now the boat can't go faster than 1 m/sec, but if you turn 45 degrees to the east you can travel sqrt(2) m/sec, because you're still going north at 1/ms and the boat gets pushed along to the east as well. the closer you get to perpendicular…

A boat absolutely can sail downwind faster than the wind speed. The crazy cutting edge boats like you see in America's Cup or the giant trimarans that vie for various world records do this somewhat routinely.

The key to understanding this is: sails are devices that work like wings, via lift, not via drag (exception, spinnakers and such). A sailboat has two wings: the sail, and the keel (or centerboard). The boat is extracting energy via how the lift vector of the sail projects onto the vector the keel keeps the boat tracking along. As you sail downwind the apparent direction of the wind begins to rotate forward in response to the boat's forward momentum. With extreme performance boats, this process can continue to the point where the boat is experiencing a forward apparent wind while sailing downwind.

This is all pretty counter intuitive but valid physics. Speaking for myself, the most easy way to get an intuition for it all is to rent a windsurfer for an afternoon. It'll all make sense after that.

People have tried sailboats with propeller rigs like this cart, but they end up being more trouble than they're worth. The current best anyone has figured out is solid wingsails, which have anywhere from 3x to 7x the lift drag ratio of traditional cloth sails. Hence their use in the crazy billionaire bragging contest races.

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

#178

Post by tshaddox »

nichohel wrote: post #78
Jedd wrote: post #54

Sailing 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

No, the issue in question here is sustained sailing faster than the wind directly downwind, which is not what water yachts and sailboats are doing. The connection (belt, gear) between the wheels and the propeller is necessary to do the former.

Yeah. Two boats could sail faster than the wind such that their center of mass is moving directly downwind faster than the wind.

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

#179

Post by gibba999 »

whatsakandr wrote: post #106
gibba999 wrote: post #26

Earlier quoted context omitted.

It's not at all counterintuitive. It's just poorly explained. Energy is 1/2mv^2, and momentum (related to force) is mv. Anytime you have different speeds, you can arbitrage. If I am pushing against still air, I can generate nearly-infinite force for arbitrarily little energy. That's why you use wings instead of pointing jet engines downwards. You push a lot of air by a little bit (lots of mv, not a lot of 1/2mv^2, si…

I think you're getting at it, but at the limit, you throw out the air, which Derek did in the follow up. Once you have two surfaces moving relative to each other, energy can be extracted. Doesn't matter how fast you're going relative to the two surfaces.

I actually find the limit to be somewhat less insightful.

It sorta shows that it's possible, but it doesn't really show why it works in this case.

1. Air isn't solid.

2. There aren't two rollers.

The analogy is a little bit distant.

I just need to find time to make mine more eloquent.

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

#180

Post by tunesmith »

pkulak wrote: post #139
munchler wrote: post #130

Earlier quoted context omitted.

Right. This mechanism doesn’t actually require wind. You get the same effect by placing the vehicle on a treadmill in a windless room: It will move forward on the treadmill.

Well, that's the same as having wind.

Yeah I think that if you put a fan in front of the treadmill, the vehicle would stop. Or is it behind the treadmill? In front, I think.
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