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

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

#211

Post by tofof »

rusk wrote: post #170
ladberg wrote: post #164

Earlier quoted context omitted.

The people-mover has to expend more energy to push you because you're applying a stronger drag force, so energy is conserved.

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 generators on skateboard wheels -- but you still, by drag, move at the wind speed, you're just stealing more energy from the wind/peoplemover now.

Then you use that energy (simply by a chain linkage) to spin the propeller, blowing air aft. The propeller pushes air backward, changing the speed of the air molecules in the region behind the vehicle from whatever they were moving at, e.g. 10 knots relative to the ground, to 0 relative to the ground. That energy pushes the propeller and with it the vehicle forward, by Newton's 3rd law. This is the second electric skateboard driving forward, in the frame of reference of the peoplemover.

So now you're moving faster than wind speed, in the direction of the wind, stealing more power from the airstream (peoplemover) than you could steal by the (original) drag of your craft alone. You got there by increasing your rolling resistance, which won't slow your craft until the rolling resistance exceeds the available force from the wind. In the analogy, this would be equivalent to jamming your brakes so hard against the floor that you stop the peoplemover rail from moving at all. You use the extra energy -- the amount being spent by the wind to counteract your increased rolling resistance -- to push yourself faster forward.

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

#213

Post by strainer »

ummonk wrote: post #202
twobitshifter wrote: post #136

Earlier quoted context omitted.

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 turbin…

I like your point about moving upwind, but think it rather relies on conservation of momentum as that insists the wheels must result in thrust in the opposite direction to the props thrust in order to change the momentum of the earth as much as the momentum of the atmosphere is changed. They must each be reduced, in order to transduce energy. If you have stored energy you can propel mass from rest, but forces can only be applied equal and opposite.

It is extremely enigmatic and curious that the engine can work to synergistically blow air into the atmosphere in one direction and resist the inertia of the earth in the other direction with the same force, without expending any energy reserves in the engine. Ive not seen a model or comment or article which fully illuminates the situation, although it is well demonstrated. Perhaps there is not language or a familiar concept which yet suffices. A sane and experienced physicist lost 10k bet on its plausibility.

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

#214

Post by lambdatronics »

dd36 wrote: post #23
sopooneo wrote: post #14

The treadmill demos made me think of another counterintuitive possibility that I wondered if anyone had pulled off: sailing a boat indirectly up a river in no (bank relative) wind. Turns out, it has been done: https://www.youtube.com/watch?v=q2il8Fagbyk

Can it be done with solar sails?

Not exactly, but you can do a crazy 'sun dive' maneuver: https://aapt.scitation.org/doi/full/10.1119/10.0002178

It takes advantage of two things:

1) The Oberth effect, where thrust is more effective the deeper you go into a gravity well

2) The thrust from a solar increases as you get closer to the Sun

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

#215

Post by aaron695 »

jasonwatkinspdx wrote: post #209
aaron695 wrote: post #68

Earlier quoted context omitted.

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, wh…

I think we just have different definitions. It's about the "Directly"

You need a propeller to go Directly Downwind Faster Than The Wind as far as I know. I think on water it has to be very large. I expect there are other ways other than a propeller but haven't seen them.

A half way definition (This isn't DDWFTTW to me either), I'd like to know if an America's Cup boat could beat a balloon? So it would be allowed off the 'race track' but it has to come back to meet/beat the balloon at the narrow finish line. So it would cover a much larger distance than the balloon and still beat it to the finish line. Is this possible in practice?

The Veritasium video doesn't actually answer this, it kinda goes around it.

[edit] If it's symmetrical which it should be, as long as they are going downwind faster than the balloon, they just reverse it at the halfway point and get back inline with the balloon but ahead.

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

#216

Post by IntrepidWorm »

dharmab wrote: post #97
jrs235 wrote: post #47

Earlier quoted context omitted.

Understandable that it doesn't take much wind for a vehicle well under a pound. But what about one that weighs well over 100 pounds?

My motorcycle with me on it can weight 700-800 pounds and has a much smaller cross sectional area than blackbird. And yet it still gets pushed around sometimes by crosswinds .

If I understand what you're saying though, you would be in motion on the bike as the crosswind pushes against you- it takes a lot less force to shift your heading while moving forwards than if you were being blown sideways while stationary.

Unless I'm misinterpreting your statement- perhaps you have a hobby of riding in hurricanes, which would be rad. :)

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

#217

Post by dharmab »

IntrepidWorm wrote: post #216
dharmab wrote: post #97

Earlier quoted context omitted.

My motorcycle with me on it can weight 700-800 pounds and has a much smaller cross sectional area than blackbird. And yet it still gets pushed around sometimes by crosswinds .

If I understand what you're saying though, you would be in motion on the bike as the crosswind pushes against you- it takes a lot less force to shift your heading while moving forwards than if you were being blown sideways while stationary. Unless I'm misinterpreting your statement- perhaps you have a hobby of riding in hurricanes, which would be rad. :)

I've definitely ridden through some storms that felt that way!

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

#218

Post by geocrasher »

The wind pushes the props because they are sails. Moving sails, but sails none the less. But instead of being windmills, they turn as if they were blowing into the wind. Their sheer sail area moves the car forward. The faster the car goes forward, the faster the blades turn.

Eventually the blades are churning the air in front of them, creating a cushion of air behind the vehicle. The wind is no longer pushing the blades, it is pushing the cushion of air created by the blades.

That air cushion is now part of the vehicle. As the speed increases, the cushion grows in size to the point which it collapses behind the vehicle. At that point, it's self sustaining and the vehicle keeps accelerating until all the forces involved reach equilibrium, which is around 2.8x the wind speed.

This all works because the blades are geared to the rear wheels in the opposite way everyone is expecting.

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

#220

Post by ummonk »

gibba999 wrote: post #179
whatsakandr wrote: post #106

Earlier quoted context omitted.

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.

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