A wind-powered vehicle that can travel twice as fast as the wind itself • Hacker News Board

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

#231

Post by cblconfederate » Tue, Aug 10, 2021, 10:08 AM UTC

jasonwatkinspdx wrote: post #182

cblconfederate wrote: post #143

Earlier quoted context omitted.

Sailboats can run faster than the wind because of lift (which is not involved here), but not when running away from the wind

Propellers are lift devices. The lift vector is parallel to the axis of rotation. It all works the same downwind, and the crazy fast boats like the AC72s do in fact sail faster than the wind when running away from the wind.

it's confusing to talk about sailboats and this cart, because this doesn't use lift from the wind to move itself forward, it generates some lift with its propeller

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

#232

Post by cblconfederate » Tue, Aug 10, 2021, 10:17 AM UTC

marcodiego wrote: post #61

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…

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.

The wind that pushes the cart can provide more push than can be harnessed by the cart passively (F=Pressure * area). To increase this force you can either increase the area of the cart facing the wind, or increase the pressure. The propeller harnesses part of the wind's energy to increase that pressure thus increasing the total force acting on the cart but the work done on the cart (F*displacement) is always limited by the energy that can be provided by the wind.

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

#233

Post by cblconfederate » Tue, Aug 10, 2021, 10:21 AM UTC

robbrown451 wrote: post #212

I think this explains it conceptually best: https://www.youtube.com/watch?v=k-trDF8Yldc I'm surprised people have such trouble with this.

in this case though, it's not the ground that provides the motion but the wind. The mental equivalent of this with wind is not straightforward

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

#234

Post by cblconfederate » Tue, Aug 10, 2021, 10:25 AM UTC

pengaru wrote: post #105

The propeller is being used a forward motion compensator. At a standstill, the wind just acts on the entire vehicle including the stationary propeller, and sets the whole thing in motion. Without the propeller, going directly downwind would be limited by the speed of the wind acting directly on the vehicle which otherwise lacks any relevant moving parts for the wind to act on. By adding the propeller, driven by the f…

I wonder how one would go about making such machine that would work in zero gravity. Without friction, something else would have to convert part of the wind force to propeller torque. Maybe small turbines could be added that would spin according to apparent wind (forward or backward) and spin the propeller to generate additional lift.

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

#235

Post by cblconfederate » Tue, Aug 10, 2021, 11:05 AM UTC

aaron695 wrote: post #224

jasonwatkinspdx wrote: post #223

Earlier quoted context omitted.

I know what you meant. You are mistaken. Look at the 180 degree data on the polars for AC72 or Bank Populaire. They're more efficient at a broad reach, but they can do it a few knots faster than the wind.

Why does the Blackbird use a propeller, why not a simple high-performance land sail boat like the Greenbird, a design that fits with the original idea from the sailing community? There is nothing online that says a sail boat can go DDWFTTW. DDWFTTW is at least a 4 decades old term in the sailing community looking at magazines. Here's a design from 1985 using a propeller https://issuu.com/latitude38/docs/latitude3894a…

the sails on those sailboats act like a wing even downwind . That's only usable with racing boats which use light foils fly over water instead of heavy keels , but yes they can move faster than the wind . Curiously i can't find a good explanation of their design.

The Blackbird cannot use a land sail to go faster because it needs something to act like the keel on a sailboat and provide a counterforce. The propeller is an elaborate way to provide that force

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

#236

Post by gibba999 » Tue, Aug 10, 2021, 11:06 AM UTC

ummonk wrote: post #220

gibba999 wrote: post #179

Earlier quoted context omitted.

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.

A propeller in air is fundamentally just like a roller on the ground, but with some efficiency loss.

I understand that. However, that's not something I find easy to explain to a someone who doesn't understand that. The analogy is a little distant.

For example, "some efficiency loss" is complex and a little bit inaccurate. It's not just efficiency loss:

- In the mechanical analogy, I can extract infinite power from a board moving at a given speed. There is no upper bound on the force the board can apply to me.

- In the case of a fan, there is an upper bound on the power which can be extracted given a specific air speed, even given perfect efficiency.

Now, it's possible to correctly argue that as the fan size increases, I can make a closer and closer to this analogue. However:

- Once you start making arguments like that, cognitive load increases, and beginners start to drop off. You need to keep more and more steps in your head.

- And more often than not, an imprecise argument simply reinforces or introduces some misconception, rather than giving an honest understanding.

I think a force*distance argument (which is probably a clearer way of stating the argument I was making above) has the virtues of:

1. Being 100% precise

2. Being compact enough someone who understands force, power, and energy can keep the whole argument in their head

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

#237

Post by hend_daszweiter » Tue, Aug 10, 2021, 11:10 AM UTC

chongli wrote: post #72

slowhand09 wrote: post #66

Earlier quoted context omitted.

Its actually the correct explanation tho. This article sucked by the way. It said "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." Any sailor worth their salt knows that is not true at all. This practically destroys any credibility on this topic by this author.

What's not true about it? According to Wikipedia [1] there are a bunch of speed records for sailing faster than the wind by tacking downwind. [1] https://en.wikipedia.org/wiki/High-performance_sailing

tacking is up wind by definition.

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

#238

Post by aaron695 » Tue, Aug 10, 2021, 12:11 PM UTC

cblconfederate wrote: post #235

aaron695 wrote: post #224

Earlier quoted context omitted.

Why does the Blackbird use a propeller, why not a simple high-performance land sail boat like the Greenbird, a design that fits with the original idea from the sailing community? There is nothing online that says a sail boat can go DDWFTTW. DDWFTTW is at least a 4 decades old term in the sailing community looking at magazines. Here's a design from 1985 using a propeller https://issuu.com/latitude38/docs/latitude3894a…

the sails on those sailboats act like a wing even downwind . That's only usable with racing boats which use light foils fly over water instead of heavy keels , but yes they can move faster than the wind . Curiously i can't find a good explanation of their design. The Blackbird cannot use a land sail to go faster because it needs something to act like the keel on a sailboat and provide a counterforce. The propeller is…

You are agreeing current sail boats can go DDWFTTW which is directly downwind not a degree either way, faster than the wind, for a indefinite period of time? Tacking is not allowed, since that's not directly downwind.

This should also mean going directly upwind is practically possible, not a degree either way, directly upwind for a indefinite period of time with sails.

Do you have a link for this?

Here's a link to a working directly upwind rotary sail boat model - https://www.youtube.com/watch?v=j1bR5hb8RCQ

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

#239

Post by koonsolo » Tue, Aug 10, 2021, 12:26 PM UTC

twobitshifter wrote: post #136

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…

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…

This does not explain the most obvious issue (or at least my most obvious issue): when going as fast as the wind, there is no wind anymore. The answer: at that point, your wheels are turning, and so you can get energy from your turning wheels, but not directly from the wind.

This is how I explain it in my head:

1. When going slower than the wind, the wind pushes you forward. Propeller at this point is useless.

2. When going as fast as the wind, the wheels will drive a propeller that pushes wind back (same as an airplane)

3. When going faster than the wind, all your energy will come from the turning wheels.

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

#240

Post by thehappypm » Tue, Aug 10, 2021, 12:55 PM UTC

tofof wrote: post #211

rusk wrote: post #170

Earlier quoted context omitted.

Okay so in this case the people mover is drawing the additional energy. Additional energy is being provided from somewhere i.e a diesel generator somewhere up stream has had to increase her fuel consumption to maintain the power on the network.

Right. The people mover is analogous to the wind. You can get to move at the windspeed through drag alone, at which point you no longer move relative to the air. The air is being robbed of only the fraction of energy that you're losing to drag (rolling resistance, as there is no air resistance now). This is you riding along with the peoplemover on the skateboard. But then increase the rolling resistance - the generat…

To add on — the power you’re stealing from the ground is the power you can use for propulsion. So before you switch on the handrail skateboard, you have some power, P, available from the generator. You can use that P to power an air conditioner or a light bulb, or to crawl down the rail. The fact that you’ll then use that power for more propulsion is not relevant — Newton’s laws are linear so you can just add up the two scenarios and they don’t have any non-linear interactions.

The power input, P, is constant—you’re not going to magically get free energy from the skateboard system. With a constant power source, so your velocity increases but in a sqrt(x) fashion, with diminishing returns. With real-world constraints like friction you’d hit an equilibrium.