Downwind faster than the wind. Poll

[snowleopard]

Lets use the original vehicle and put a clutch in the drive shaft so that we can engage and disengage the prop. Lets assume the clutch is perfectly efficient so that it introduces no losses, it is simply a device to let us turn the drive to the prop on or off. We'll let the cart set off dead downwind with the prop disengaged and free-wheeling. It will accelarate under its windage until it reaches windspeed and is now travelling directly downwind at the speed of the wind.

This is the really crucial bit - we now engage the clutch to drive the prop, what happens to the cart?

One of those Aha! moments - let's run it through....

The vehicle stands there, prop disengaged, then you release it. Windage of the whole thing starts it rolling downwind and the prop starts to turn, driven only by the apparent wind flowing over the blades. They autorotate like a helicopter and create enough drag to accelerate the vehicle to a fraction of windspeed, in the region of 50-75%. The vehicle is now running at a steady speed and can go no faster. That is of course what the antis are arguing and I will agree, to approach 100% wind speed is very difficult and to exceed it is impossible. The driving force is the wind speed relative to the cart.

Now what happens when you drop the clutch? Well for a start there will be a squealing of tyres and crashing of gears as the prop REVERSES DIRECTION. Barring skidding, the vehicle will come to an abrupt halt then start to roll again with the prop turning the other way, this time providing drive rather than drag. Now it is linked to the wheels the source of the power is the differential in speed between ground and the wind and the constraint of apparent wind.

To check out the direction of rotation, take a look at the video of the free-running cart on the road, about 15 seconds in where they push it a second time. You can clearly see the prop rotating the opposite way to what you would expect if it were freewheeling.

 

[xxyyzz]

This will in part be counteracted by the thrust provided by the prop, but due to frictional losses in the system this will be less than the power input at the wheels and the net effect will be to slow the vehicle.

Well, first up we have "in part" why only in part?

Secondly, frictional losses are perhaps less than you think?

I think friction gets overstated because people are thinking of air drag on the cart, but there isn't any because the air is moving with the cart until the cart starts to go faster than the wind, then it gets air drag and reaches equilibrium.

If the cart slows to less than wind speed, then the air drag is pushing it forward.

 

[RAI]

You're wrong here and a little confused.
The propeller blades are not an analogue of the wing on BMW Oracle, just because propeller blades look a little like wings you have fallen into an obvious trap.
The wing on BMW Oracle is in fact an analogue of the road wheels on the downwind cart, the wing is the device taking the power from the wind and pushing the boat along via its keel or lateral resistance. This lateral resistance is the true analogue of the downwind cart's propeller.

I wrote a long answer and got timed out. So here's the short answer.

The yacht and the cart have the same issue, they must maximise their aerodynamic lift to drag ratio.

The yacht has shown it can do it. The cart appears to in the video. Perhaps this larger scale test with a carefully constructed propeller will do better.

 

[mikemonty]

I'm going to have one last try.
The cart being propelled down wind is using energy derived from the wind.
It can gain energy from no-where else.
That energy is ultimately derived by slowing some of the wind passing the cart and stealing ITS energy.

The basic premise of the faster than wind ... thing is that it is possible to sufficient energy out of the wind to go faster than it.

Now that might be possible if you had a fixed windmill and a very long extension cord attached to your electric powered cart.
But you haven't, the limit on the problem is that the windmill is fixed to the cart and that the motor is working as a generator to drive the windmill as a prop.
(or using a rubber band to the wheel in the original scenario - comes to the same thing)

So we have a loop here.
The wind drives the windmill and cart along producing energy which the windmill uses to drive the cart even faster.

Sounds OK, so where's the flaw?

If we assume the machine is 100% efficient and gets hold of ALL the energy available to it from the wind, which can only be the wind which hits directly on it (and I'm being generous here and allowing it to be assumed that all the energy from the wind within the circumference of the windmil/propellor is collected)...

the wind hitting it would come to a dead stop - no energy left - the carts got it all.

Now the machine has that energy and uses it to blow backward - (still at 100% efficiency), it still only has enough energy to move forward as fast as the original wind!

Once you factor in the real losses friction in the mechanics and in the air around the prop you will get nowhere near that efficiency and therefore nowhere near original windspeed.

It cannot magic up energy from elsewhere - though some will try to persuade you that it sucks energy out the ground - it can only get energy from the wind, it actually LOSES energy to the ground.

Others will suggest that the energy that you gain from the wind can be fed back at higher rates (power) but that cannot be sustained and you will run out of energy before your average speed reaches that of the original wind.

You will be diverted, red-herringed, obfuscated and deliberately misunderstood but there is only CONSERVATION OF ENERGY.

The whole thing is a crock of...

 

[Keen_Ed]

So please explain how BWM Oracle has a downwind vmg higher than windspeed?

 

[xxyyzz]

If we assume the machine is 100% efficient and gets hold of ALL the energy available to it from the wind, which can only be the wind which hits directly on it (and I'm being generous here and allowing it to be assumed that all the energy from the wind within the circumference of the windmil/propellor is collected).

This is the flaw, the energy is not in the wind, it's in the different speeds of the air and the ground. Even at wind-speed, the energy is still there, because the ground is moving.

You've seen the ruler video right?

You've seen how the vehicle moves faster than the ruler along the table?

 

[snowleopard]

I'm going to have one last try.
The cart being propelled down wind is using energy derived from the wind.
It can gain energy from no-where else.

No need to go any further than that. That is where your fallacy is. The energy is derived from the movement of the wind relative to the ground.

It's been said by me and others literally dozens of times during this thread.

 

[Ubergeekian]

The wind drives the windmill and cart along producing energy which the windmill uses to drive the cart even faster.

Sounds OK, so where's the flaw?

You are getting needlessly hung up on energy, and you're not accounting for it properly anyway.

Now the machine has that energy and uses it to blow backward - (still at 100% efficiency), it still only has enough energy to move forward as fast as the original wind!

You are once again confusing energy and force.

Let me try to put it very, very simply for you.

10 kt wind. Cart running along the ground at 15 kt. Not accelerating, so drag force at wheels exactly balances thrust force at prop. Call this force F. Rate of working by wheels on ground: F x 15. Rate of working by propeller on air: F x 5. Surplus: 10 F - available for overcoming friction, air resistance and so on.

 

[Keen_Ed]

No, but it's downwind vmg > windspeed.

Do you know what vmg actually stands for?

All I can say is that I really, really really hope you've never done any work that I've had to trust.

 

[ianj99]

So let's see what people think of the claims

Here is one of the videos of the experiment

What do people think of the demonstration?

Simple explanation:

Energy is being put into the system by the treadmill..

I

 

[RAI]

And the BMW analogy is mush. At no time was it going at more than wind speed directly downwind.

How did it cover the 20 nm downwind leg in just over 1 hour with only 6-8 knots of wind?

 

[xxyyzz]

Simple explanation:
Energy is being put into the system by the treadmill..

Yes, you will also note there is no wind.
Do you understand the scenario that is being demonstrated there?

Unfortunately you're arguing the energy position again, we know that the energy is available because the large racing yachts have done it.
To argue that it's not possible from a conservation of energy standpoint is to argue against an empirically demonstrated fact.

Yes, BMW Oracle etc use a different mechanism of action, but they nonetheless move something big significantly faster than the wind downwind whilst powered by the wind.

You can still argue that this particular device cannot capture that energy, but the energy is definitely there.

 

[mikemonty]

No need to go any further than that. That is where your fallacy is. The energy is derived from the movement of the wind relative to the ground.

It's been said by me and others literally dozens of times during this thread.

Saying it a dozen times does not make it right -
How does the cart gain energy from the movement of the wind relative to the ground?

Electrostatics?

The ground can gain energy from the movement of the wind relative to the ground - through friction.

The cart and the ground are only coupled by a set of wheels that - on the non-driven wheel(s) are parasitic losses to the cart - and on the driven one(s) is an even worse energy loss because the frictional forces are even greater on this one

 

[mikemonty]

So please explain how BWM Oracle has a downwind vmg higher than windspeed?

Because it works on a different principle from the DDWFTTW machine - the operative word being DEAD DOWNWIND

 

[snowleopard]

Simple explanation:

Energy is being put into the system by the treadmill..

I

Yes, obviously. The question is, how does the device turn that into forward motion faster than the belt?

 

[Keen_Ed]

Because it works on a different principle from the DDWFTTW machine - the operative word being DEAD DOWNWIND

Er ... it exploits the velocity difference between air and water. It's vmg exceeds windspeed. The fact that its foils (hard wing and daggerboards) are fixed, so that it has to gybe back and forth, rather than rotating - as a big air propellor connected to a water propeller makes no difference. In both cases, you end at the bottom mark ahead of any balloon that you released at the windward mark.

 

[mikemonty]

This is the flaw, the energy is not in the wind, it's in the different speeds of the air and the ground. Even at wind-speed, the energy is still there, because the ground is moving.

You've seen the ruler video right?

You've seen how the vehicle moves faster than the ruler along the table?

So what?
You presumably understand an epicyclic gearbox - that isn't much different.
Draw it on paper for yourself.
As you pull the ruler along the large wheel has to roll with it, driving the inner barrels of the spools but the outer rims of the spools want to roll further because their circumference is larger that the inner barrels, so they do! there is nothing stopping them.

No magic trick or transference of energy (much) just fecking gearing!

 

[snowleopard]

Saying it a dozen times does not make it right -
How does the cart gain energy from the movement of the wind relative to the ground?

Look at the 'under the ruler' video. That shows a vehicle in contact with 2 surfaces that move relative to each other, producing motion faster than the surfaces. It uses gearing to do that.

When you replace the upper wheel and ruler by air and a prop, it works in just the same way. Just as the wheel rolls forward along the ruler the prop moves forward through the air.

 

[DownWest]

I think we should leave out the windmill driven boat going upwind. It works, but at a fraction of the windspeed, because of the losses in the system.Windmill/prop losses, drivetrain losses and water prop efficiency.Plus the hull friction.
And leave out Oracle too, tacking downwind is not what is under discussion.
A

 

[hrchivers]

A vote for 'doesn't demonstrate what they think' ...

These are cute toys, but sadly the treadmill doesn't prove anything. The operator is simply putting energy into the system by holding it still on the treadmill; when released the prop has enough stored enery to overcome the friction and other losses between wheel and prop(which are minimised in construction) and drive it forward for a short time.

Note that there is some energy driven into the prop by the wheels, the stored energy only has to overcome losses, so in a well constructed model the stored energy doesn't have to be all that big to drive it a short distance.