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-   -   "Faster than the wind," impossible they say but the Blackbird settles the debate (https://ecomodder.com/forum/showthread.php/faster-than-wind-impossible-they-say-but-blackbird-39464.html)

botsapper 06-11-2021 02:37 AM

"Faster than the wind," impossible they say but the Blackbird settles the debate
 
Counter-intuitive they say, that a wind-powered craft going straight downwind can go faster (no angled tacking/gybing) than the wind itself.
They built a three-wheeled land craft with a twin-bladed propeller/turbine, the Blackbird to test and prove their wind challenge. The bluff-body Blackbird first gets started by the wind from behind then the forward rolling wheels turn a geared sprocket that turns the propeller to push air rearward and provides added forward thrust! In their video you see the windsock showing the wind direction but the Blackbird's telltale streamer is waving backwards from an apparent headwind - it is moving faster the the wind.
Aerodynamicist Rick Cavallaro tweaked the Blackbird and achieved a top speed 2.86 times faster the the wind speed in 2010.

In 2012, they achieved a top speed 2.01 times faster the wind speed - headed UPWIND with a turbine rotating the opposite direction to transmit power to the wheels!

Blew my mind but it all made sense with the 'cylindrical Earth demonstration'


https://www.youtube.com/watch?v=jyQwgBAaBag&t=1213s

aerohead 06-11-2021 12:22 PM

over unity?
 
Quote:

Originally Posted by botsapper (Post 650114)
Counter-intuitive they say, that a wind-powered craft going straight downwind cannot go faster (no angled tacking/gybing) than the wind itself.
They built a three-wheeled land craft with a twin-bladed propeller/turbine, the Blackbird to test and prove their wind challenge. The bluff-body Blackbird first gets started by the wind from behind then the forward rolling wheels turn a geared sprocket that turns the propeller to push air rearward and provides added forward thrust! In their video you see the windsock showing the wind direction but the Blackbird's telltale streamer is waving backwards from an apparent headwind - it is moving faster the the wind.
Aerodynamicist Rick Cavallaro tweaked the Blackbird and achieved a top speed 2.86 times faster the the wind speed in 2010.

In 2012, they achieved a top speed 2.01 times faster the wind speed - headed UPWIND with a turbine rotating the opposite direction to transmit power to the wheels!

Blew my mind but it all made sense with the 'cylindrical Earth demonstration'


https://www.youtube.com/watch?v=jyQwgBAaBag&t=1213s

1) from the first sentence, once the craft achieves a velocity equal to the wind velocity, there can be no net force acting on the vehicle. Thrust = zero.
2) during the runup, the wheels, acting through a transmission, could be feeding power to an accelerating flywheel, storing kinetic energy, in excess of that necessary to overcome any aerodynamic or rolling-resistance road load.
3) once up to 'speed', and the blades re-configured for thrust, the flywheel could be tapped, to power the craft to some given velocity, momentarily, however, at some point, this energy would be exhausted, thrust would be lost, with deceleration.
------------------------------------------------------------------------------------
What is not said would be of much interest. Like, for how much time can they demonstrate this feat?
Upon opening the kimono, we may just discover some parlor trick, which has nothing to do with aerodynamics.

jakobnev 06-12-2021 02:41 AM

aerohead -

Watch from 9:15 to 9:45, inside the craft energy always travels from the wheels to the propeller, there is no wind turbine!

some_other_dave 06-13-2021 01:00 AM

It used to blow my mind that a sailboat could sail faster than the actual wind. Then I started wrapping my head around the fact that the sail in question was acting as an airfoil.

Once I realized that this video was actually serious, I figured it probably had something to do with that effect. And it does, indirectly, as the "cylindrical world" animation shows.

-soD

AeroMcAeroFace 06-13-2021 11:02 AM

Quote:

Originally Posted by AeroMcAeroFace (Post 638644)
Freebeard is right, it is possible. https://www.wired.com/2010/06/downwi...than-the-wind/ [/IMG]

I was planning on starting a "why don't cars have sails on them?" thread later this week, but the veritasium video delayed me.

I have been thinking about the downwind faster than the wind thing for years and still don't fully understand it, but I agree that the cylindrical earth thing makes the most sense.

I don't believe it is fake at all, but very limited in application, without gearing and a variable pitch/speed propeller it seems to need a narrow band of winds to be possible.

aerohead 06-16-2021 12:41 PM

9:15 to
 
Quote:

Originally Posted by jakobnev (Post 650213)
aerohead -

Watch from 9:15 to 9:45, inside the craft energy always travels from the wheels to the propeller, there is no wind turbine!

1) I'm going to give it a 'fail'
2) the tattle-tail is on the wrong end of the car
3) there's no actual instrumentation
4) we have no idea what the local free-air velocity is
5) we have no idea what the vehicle surface velocity is
--------------------------------------------------------------------------------------
6) the vehicle relies on aerodynamic drag to capture propulsive kinetic energy
7) there will be a mechanical loss of the propellor as a 'sail'
8) there will be a mechanical loss of the reversed vertical struts as a 'sail'
9) there will be a mechanical loss of the wheels, body, pilot, Go-Pro, and downwind excrescence as a 'sail'
10) there will be a mechanical loss of the drivetrain
11) there will be a mechanical loss of the propellor shaft
12) there will be a mechanical loss of the propellor.
--------------------------------------------------------------------------------------
13) I suspect that turbulence downwind of the vehicle structure is inducing a low pressure which impels the tattle-tail observed orientation, fooling the observer into believing that the vehicle has exceeded the local air speed ( you can see an example of turbulent wake-induced flow reversal in the video of ' Streamlined Toyota T-100 in the Wind Tunnel ) we video-graphed this specifically for this purpose.
14) proper instrumentation will reveal the facts.

AeroMcAeroFace 06-16-2021 01:15 PM

1 Attachment(s)
Quote:

Originally Posted by jakobnev (Post 650213)
aerohead -

Watch from 9:15 to 9:45, inside the craft energy always travels from the wheels to the propeller, there is no wind turbine!

sorry, that isn't true, the energy doesn't always flow from the wheels to the propeller, the energy can flow from the propeller to the wheels.

in the beginning, the car isn't moving, but has a tail wind, the rotor spins, which in turn drives the wheels. the car accelerates partly due to the drag of the vehicle, and partly due to the propeller driving the wheels.

Attachment 30860

at wind speed. so when there is no air resistance on the craft (essentially in still air in a reference frame), the wheels are driving the propeller. The propeller is driven through gearing faster and produces a small amount of thrust, This accelerates the craft slightly, at which point the craft is in a headwind, the propeller can then propel the craft upwind.

You would agree, that given the right gearing, a car could travel upwind?
you would agree, that given the right gearing, a car could travel downwind?
The hard bit to understand, is the changeover from the car going downwind to the car going upwind(or more technically downwind faster than the wind).

aerohead 06-16-2021 01:43 PM

energy
 
Quote:

Originally Posted by AeroMcAeroFace (Post 650475)
sorry, that isn't true, the energy doesn't always flow from the wheels to the propeller, the energy can flow from the propeller to the wheels.

in the beginning, the car isn't moving, but has a tail wind, the rotor spins, which in turn drives the wheels. the car accelerates partly due to the drag of the vehicle, and partly due to the propeller driving the wheels.

Attachment 30860

at wind speed. so when there is no air resistance on the craft (essentially in still air in a reference frame), the wheels are driving the propeller. The propeller is driven through gearing faster and produces a small amount of thrust, This accelerates the craft slightly, at which point the craft is in a headwind, the propeller can then propel the craft upwind.

You would agree, that given the right gearing, a car could travel upwind?
you would agree, that given the right gearing, a car could travel downwind?
The hard bit to understand, is the changeover from the car going downwind to the car going upwind(or more technically downwind faster than the wind).

1) we know that a 'propeller' is a lousy 'turbine.'
2) I didn't see a 'thrust-reverser' as found on a Lockheed C-130 Hercules, which allows the aircraft to back itself out of an airport terminal without a tug.
3) whatever energy is extracted from the 'lousy turbine' to accelerate and build kinetic, there are energy losses at every step of power transfer between blades and wheels, regardless of direction.
--------------------------------------------------------------------------------------
4) at wind speed, there is no net force acting anywhere on the vehicle, it's at terminal velocity.
5) if the tattle is 'WRONG', and the car is actually going slower than the actual airspeed ( which is what I believe), then it's just air drag propelling the car.
6) for the blades to be producing net thrust requires over-unity, which is impossible given the already stated mechanical efficiencies between blades and wheels.
-------------------------------------------------------------------------------------
A trip to Palo Alto, California, and NASA's largest wind tunnel would provide for a laboratory-quality test, where all parameters can be accounted for.
Until something like that, it's a 'stunt.' A parlor trick.

AeroMcAeroFace 06-16-2021 03:27 PM

Quote:

Originally Posted by aerohead (Post 650478)
4) at wind speed, there is no net force acting anywhere on the vehicle, it's at terminal velocity.

6) for the blades to be producing net thrust requires over-unity, which is impossible given the already stated mechanical efficiencies between blades and wheels.

at wind speed, the wings on the propeller are still moving, and like the sail on a tacking boat, still providing thrust.

It doesn't require over-unity, because essentially the energy is coming from the difference between air speed and ground speed.

aerohead 06-16-2021 04:05 PM

thrust
 
Quote:

Originally Posted by AeroMcAeroFace (Post 650486)
at wind speed, the wings on the propeller are still moving, and like the sail on a tacking boat, still providing thrust.

It doesn't require over-unity, because essentially the energy is coming from the difference between air speed and ground speed.

The Second Law of thermodynamics doesn't allow for the energy transferred from the blades to the air to exceed the energy coming from the wheels and inertial/ momentum energy transfer from the vehicle/passenger.
The energy at the blade-air interface must be lower than that at the road-to-tire interface due to energy loss in the tire, gears, wheel bearings, lower sprocket, chain, upper sprocket, jack-shaft bearings, then the blades themselves.
Even if the blades were 100% efficient, there wouldn't be enough energy.
And a look at Abbott and Von Doenhoff's book, Theory of Wing Sections will disabuse you of any notion about high efficiencies under high lift conditions.

some_other_dave 06-16-2021 07:11 PM

If I may take another tack here-- (see what I did there?)

Do you believe that a boat can sail faster than the wind speed? Your arguments would seem to say that no, they cannot.

If not, then you need to research sails, because there are boats that can do so.

If you believe that they can, then do you think it stands to reason that the same sort of phenomenon might be used to allow a vehicle to move downwind faster than the wind? Including a lot of losses along the way?

-soD

oil pan 4 06-16-2021 07:56 PM

The guy that built the machine says it can be done.
Aerohead says it can't done.
Who should we believe?

ennored 06-16-2021 10:41 PM

Quote:

Originally Posted by AeroMcAeroFace (Post 650486)
It doesn't require over-unity, because essentially the energy is coming from the difference between air speed and ground speed.

That's how I understand it. Traveling at say, 10 MPH, with a 7 MPH tailwind, you have 10 MPH of groundspeed to overcome a 3 MPH headwind.

jakobnev 06-18-2021 01:55 AM

Quote:

The energy at the blade-air interface must be lower than that at the road-to-tire interface due to energy loss in the tire, gears, wheel bearings, lower sprocket, chain, upper sprocket, jack-shaft bearings, then the blades themselves.
Sit down and enjoy this moment of enlightenment.

Power is force times velocity.

For the wheels it's simple: The velocity is just the difference in velocity of the wheels and the ground.

For the propeller it is just as simple: The velocity is just the difference in speed between the propeller and... (this is the facepalm moment!) the air!

The propeller doesn't give a hoot about ground speed!


Let's say we wanted to go 10m/s down a 5m/s wind and were transferring 100N of force.

We harvest 100N*10m/s = 1kW at the wheels.

The useful power of our propeller needs to be: 100N*5m/s = 500W.

Ergo (I love that word) we need a total system efficiency of 50%, including all losses, that is very very much not over unity, no magic needed!

Propeller efficiencies can be 82-92% according to the internet.

RedDevil 06-18-2021 06:23 AM

Oh crikey. We already had a thread on this.

Ah well. I'll repeat my explanation in concise form:
The propeller on the craft is slowly turning backwards.
Its blades form a surface that still moves forward with the craft, but less fast than the car itself.
The wind still exerts pressure on that surface, even if the rest of the car is moving faster than the wind.

The propeller is driven by the wheels, but as the gearing reduces the speed the forward pressure on the blades is larger than the backward force on the wheels through those gears needed to spin the propeller.
As all other sources of friction are pretty minimal, that's enough to make the craft accelerate.

The important point is that the propeller spins slower than the wind. If it spins faster than the wind then the car would move backwards, straight into the wind.

Quote:

Originally Posted by AeroMcAeroFace (Post 650475)
...
in the beginning, the car isn't moving, but has a tail wind, the rotor spins, which in turn drives the wheels. the car accelerates partly due to the drag of the vehicle, and partly due to the propeller driving the wheels.

That is exactly what is NOT happening. The rotor spins against the wind!
It does not drive the wheels at any moment. instead it exerts a force on the wheels fighting its motion (but losing), or rather the ground forces the wheels to turn which through the gearing drives the propeller to turn slowly against the wind.
It is just the wind blowing against the blades of the propeller which moves it forward, like a sail would.

If the blades were fixed it would act just like a sail.

Now we add a mechanism that very slowly turns the blades backward.
It would still be mainly a sail.

It will remain working like a sail until you set the ratio such that the propeller moves the air backwards as fast as the craft moves forward, in which case the forces cancel each other out and the craft wouldn't move at all.

AeroMcAeroFace 06-18-2021 08:40 AM

Quote:

"Faster than the wind," impossible they say but the Blackbird settles the debate
I think aerohead begs to differ on this.

Quote:

That is exactly what is NOT happening. The rotor spins against the wind!
It does not drive the wheels at any moment. instead it exerts a force on the wheels fighting its motion (but losing), or rather the ground forces the wheels to turn which through the gearing drives the propeller to turn slowly against the wind.
It is just the wind blowing against the blades of the propeller which moves it forward, like a sail would.
So you are saying that the car would accelerate faster if the blades were stationary and not connected to the wheels? At least in the beginning?

RedDevil 06-18-2021 09:29 AM

Quote:

Originally Posted by AeroMcAeroFace (Post 650618)
So you are saying that the car would accelerate faster if the blades were stationary and not connected to the wheels? At least in the beginning?

Yes! That's it!
Suppose the gearing is set up in such a way that the propeller blades push the air back at half the speed of the craft.
Then of all the force on the propeller trying to push it forward half gets lost by the gearing turning the blades against it. When the rolling resistance and other friction sources are less than that, there is still something left to push it forward.

Once you see it it really blows the mind.

When in motion the propeller blades wing effect kicks in and they become more effective, like as if the surface of the sail increases. So a stationary propeller is good to get rolling but you want it to turn immediately after.

The craft in the video has adjustable gearing. I guess in low wind you have to reduce the gearing as the other sources of friction are relatively large. And maybe you want oversized gearing to be able to brake or even back up against the wind.

AeroMcAeroFace 06-18-2021 09:51 AM

I am not saying you are wrong, but they have a variable pitch propeller and according to this paper it seems that it is preferable for thrust to be generated from the propeller to the wheels rather than parachuting up to speed.http://www.ijsrp.org/research-paper-...srp-p16135.pdf and the paper makes no mention of drag from the propeller pushing it up to speed.
Quote:

In 2006, Jack Goodman [8] explained that the correct gearing
of the propeller to the wheels of the car will speed up the car if
energy is available from a tailwind.

aerohead 06-18-2021 11:20 AM

sails
 
Quote:

Originally Posted by some_other_dave (Post 650513)
If I may take another tack here-- (see what I did there?)

Do you believe that a boat can sail faster than the wind speed? Your arguments would seem to say that no, they cannot.

If not, then you need to research sails, because there are boats that can do so.

If you believe that they can, then do you think it stands to reason that the same sort of phenomenon might be used to allow a vehicle to move downwind faster than the wind? Including a lot of losses along the way?

-soD

The only corollary which applies for 'with-the-wind' would be flying a spinnaker, and the sailboat fails to reach parity with the wind velocity.

aerohead 06-18-2021 11:26 AM

says
 
Quote:

Originally Posted by oil pan 4 (Post 650519)
The guy that built the machine says it can be done.
Aerohead says it can't done.
Who should we believe?

'Says' isn't equivalent to 'proves.'
The video provided no 'proof.'
It does provide 'illusion.'
It should never be about 'belief,' it should be about demonstrating scientific rigor.
Which wasn't.
Laboratory testing would tell the tale.

aerohead 06-18-2021 11:51 AM

10-mph from 7-mph
 
Quote:

Originally Posted by ennored (Post 650525)
That's how I understand it. Traveling at say, 10 MPH, with a 7 MPH tailwind, you have 10 MPH of groundspeed to overcome a 3 MPH headwind.

1) if the 'phenomena' of Rick Cavallero are of aerodynamic origin, then they are governed by the same principles as all aerodynamic phenomena.
--------------------------------------------------------------------------------------
If tested in a laboratory, I believe that you will find that:
A) the vehicle ground velocity never exceeded the wind speed.
B) at sub-critical Reynolds number, laminar boundary layer, all flow across the vehicle was separated.
C) the 'tattle' at the vehicle's nose was being pulled backwards into the low-pressure turbulent wake of the separated flow.
D) the backwards-facing tattle gave the 'appearance' of headwind, when in fact, that never existed.
-------------------------------------------------------------------------------------
Believing that a 'headwind' actually existed, Rick Cavallero's mind made the leap, concluding that he'd done the impossible, re-writing fluid mechanics and thermodynamics.
' Man believes what he wants to believe.'
--------------------------------------------------------------------------------------
If anyone wants to spend a little time studying Sir Issac Newton's second law, pertaining to force/ mass/ velocity/ time/ momentum/ inertia/ impulse, you'll see that this theory of Cavallero's was dead before it was ever mentioned.

some_other_dave 06-19-2021 01:44 AM

Quote:

Originally Posted by aerohead (Post 650625)
The only corollary which applies for 'with-the-wind' would be flying a spinnaker....

But we're not talking about sailing with the wind. And the propeller blades are not turning with the wind--when they turn, they are meeting it at an angle!

-soD

RedDevil 06-19-2021 06:25 AM

No amount of proof will ever convince anyone who does not understand the principle.
And those wo do understand the principle know it should work, and proof is just confirmation.
So please try to understand the principle. It is a real Aha! moment when you see it.

I'll try to explain with a stepwise approach.

Fact - You cannot blow a sail with the wind past wind speed.
But if you take a very long boat and you put the mast on a cart inside the boat and you push that cart towards the stern, then the boat would move faster than if the sail was fixed. The boat would move faster than the wind if you push hard enough.
I think you can agree that would work.

Instead of a sail we use a propeller. Instead of a boat we use a car.
If we motorize the propeller to push against the wind, of course we could move faster than the wind.
With our motorized cart, the force that drives the car forward is the forward pressure on the propeller that transfers through the axle bearings. All obvious, right?

Now comes the tricky part.
We set the motor speed such that the propeller turns against the wind but slightly slower that the wind speed.
The car will speed up beyond the wind speed, as the wind is still pushing against the propeller as its surface moves slower than the wind.
Even though the propeller forces the air backwards compared to the car, it still moves forward compared to the ground - but slower than the wind, while the car moves faster than the wind.
The wind still pushes against the propeller so the car still accelerates.

Because the car still accelerates in those conditions we might tap some of that energy to drive the propeller. The force needed to do so is less than the forward force on the propeller, as long as the propeller pushes the air backwards ar a slower speed than the car is moving.
We don't need the motor after all as long as the overall friction is less than the difference between the forward force on the propeller and the driving force turning it against the wind.

It is the gearing that makes it possible.
Likewise when you have a big sail and a system of pulleys to rile in the boom. If you let go of the rope the wind will catch the sail and the rope end will speed up way past the wind speed.
Here the sail is a propeller, the pulleys are the gearing and the rope end is the car.

As for proof, the wind vane on the car is all you need.
Of course the wind speed is lower close to the ground, but it is enduring quite a strong virtual headwind; much more than you'd expect.
There is a headwind even right in front of the propeller; in fact the headwind there should be even slightly stronger as the propeller slows the wind speed down, so it is a higher headwind relative to the car.

aerohead 06-23-2021 11:56 AM

talking about................
 
Quote:

Originally Posted by some_other_dave (Post 650711)
But we're not talking about sailing with the wind. And the propeller blades are not turning with the wind--when they turn, they are meeting it at an angle!

-soD

1) according to Newtonian physics, the wind from behind is supplying an 'impulse' of the force (F), over a period of time. (F delta-t)
2) F delta-t = ( delta- momentum ) [ delta-p ]
3) Momentum (p) = mass-times-velocity, or p= mv
4) impulse of the force (F) = delta-p/ delta-t
--------------------------------------------------------------------------------------
* the vehicle, with wind from behind, is a crude square-rigger.
* it's aerodynamic drag creates enough force to accelerate the vehicle, from rest V=zero, to some terminal velocity V2.
* while the vehicle is accelerating, it is building momentum as it's mass is being accelerated.
* under the force of the wind, the vehicle's wheel is transmitting power from the road/tire interface, through the wheel bearings, to the differential, to the lower sprocket, to the chain, to the upper sprocket, through the pillow block bearings, to the shaft, to the propeller, to the air, creating a counter-impulse.
* there's a specific, fixed amount of kinetic energy contained in the wind.
--------------------------------------------------------------------------------------
* If we presumed that the vehicle was a 'perfect' sail, and could extract all the available energy from the wind, we would still be losing energy at the tire/ ground interface/ within the tires themselves/ within the wheel bearings, within the differential gears/ at the lower sprocket-to-chain interface/ within every link pivot within the chain/ within the upper chain-to-sprocket interface/ within the two pillow-block bearings-to-jackshaft interface/ and finally at the propeller-to-air interface.
--------------------------------------------------------------------------------------
* Whatever amount of energy is delivered to the vehicle, that amount, less all the powertrain losses, is what is available at the propeller-to-atmosphere coupling.
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a) when the vehicle reaches the air speed, there is no delta-v
b) acceleration is now zero, v= constant, momentum = constant
c) whatever net-momentum exists in the vehicle, it must have a lower amount of net energy than that the surrounding airstream due to aerodynamic slippage, and rolling resistance on dirt ( very high! )
--------------------------------------------------------------------------------------
d) in order for the propeller to push the vehicle beyond the local air velocity, would require zero aerodynamic slippage, zero rolling resistance, zero powertrain losses, and a propeller wing section exhibiting a lift-to-drag ratio at over-unity, since it's being supplied less energy than is available from it's surroundings.
--------------------------------------------------------------------------------------
* All wing sections have a zero-lift angle of attack
* As soon as a wing begins to produce lift, it's also producing drag ( another drain on available energy )
* At maximum lift, all airfoils are producing maximum drag
* All thrusting airfoil sections are 'angled', so, an
'angled' propeller blade cannot be some new, unique quality of Rick Cavallero's system, not seen in other aerodynamic systems.
-------------------------------------------------------------------------------------
Somehow, the phenomenon of the backward's oriented tattle-tale, has convinced observers of a reality which cannot exist in our universe.

jakobnev 06-23-2021 02:00 PM

It seems involving wind and propellers obfuscates the principle too much, let me instead ask:

Can a cart pulled by a rope go faster than the rope?

(Nothing else powers the cart.)

aerohead 06-23-2021 02:41 PM

rope
 
Quote:

Originally Posted by jakobnev (Post 650982)
It seems involving wind and propellers obfuscates the principle too much, let me instead ask:

Can a cart pulled by a rope go faster than the rope?

(Nothing else powers the cart.)

1) Is the rope inelastic or elastic?
2) Are you measuring rope velocity at the rope/cart connection point, or the prime mover/rope connection point velocity?

jakobnev 06-23-2021 03:06 PM

1) The rope is perfectly inelastic.
2) All motion is relative to the road.

oil pan 4 06-23-2021 03:12 PM

1 build a wind car your self.
2 disprove the observation.
I'm inclined to believe the guy who built the the thing and did the thing.

aerohead 06-23-2021 03:33 PM

motion/ road
 
Quote:

Originally Posted by jakobnev (Post 650986)
1) The rope is perfectly inelastic.
2) All motion is relative to the road.

Would you please define the road, cart, and any topological or environmental considerations?

aerohead 06-23-2021 03:39 PM

disproved
 
Quote:

Originally Posted by oil pan 4 (Post 650987)
1 build a wind car your self.
2 disprove the observation.
I'm inclined to believe the guy who built the the thing and did the thing.

I've already disproved it.
Observation does not establish causality.
His remarkable claim requires the remarkable evidence to back it up.
He can submit his creation for third-party testing.
NASA Ames is the only laboratory large enough to test it.
In the meantime, it's 'cold fusion.'

jakobnev 06-23-2021 03:57 PM

Quote:

Would you please define the road, cart, and any topological or environmental considerations?
The road is a level flat surface, the cart is a small rigid body that has it's weight supported by four wheels mounted on two axles. There is small but positive rolling resistance, and small but positive air resistance. There are no magnetic nor electrostatic attractions nor repulsions. There is no magic nor telekinesis going on.

Our inertial frame of reference is the road, and there is no wind.

aerohead 06-23-2021 04:29 PM

faster than the rope?
 
Under the conditions stated, I would conclude that the cart velocity cannot exceed the tow-rope velocity.

ennored 06-23-2021 09:24 PM

So a water skier never travels faster than the boat that's pulling him?

jakobnev 06-24-2021 03:36 AM

Quote:

Under the conditions stated, I would conclude that the cart velocity cannot exceed the tow-rope velocity.
FAMOUS LAST WORDS!!

Prepare to have your mind expanded: (or blown if it was inflated to its limits)


https://www.youtube.com/watch?v=ek0D4Ehvoc8

The cart moves 93cm when the string moves 43cm, the cart is 2.16 times as fast as the string, exclamation point, QED, more exclamation points.

JulianEdgar 06-24-2021 04:07 AM

Quote:

Originally Posted by jakobnev (Post 651061)
FAMOUS LAST WORDS!!

Prepare to have your mind expanded: (or blown if it was inflated to its limits)


https://www.youtube.com/watch?v=ek0D4Ehvoc8

The cart moves 93cm when the string moves 43cm, the cart is 2.16 times as fast as the string, exclamation point, QED, more exclamation points.

I deliberately haven't entered this overall debate because if I don't know nothin', I say nothin', but wouldn't that just be a simple geared-up windless?

RedDevil 06-24-2021 11:53 AM

Ha, I actually tried that myself.
All you need is two wheels on an axle and a rope wound up on the axle.
Yes, it definitely works; it is not a trick.

On the cart the rope winds itself up on the axle.
It still gets pulled forward because the rope still moves forward (albeit slower than the cart).
The forward pull on the axle is stronger than the resistance it meets from the wheels, as the axle has a smaller diameter than the wheels so it rolls up the rope more slowly than the forward motion.

Likewise, the surface the blades of the propeller provide moves with the wind but more slowly than the car. Hence the wind is still able to blow against it.
And just like the axle winds up the rope slightly, the gears move the propeller against the wind slowly.

I did a step by step explanation in this post.
If you are still convinced this faster than the wind car thing is impossible or if you don't see how it works, please do go through those steps and tell me which of them sounds wrong.

some_other_dave 06-24-2021 07:20 PM

Quote:

Originally Posted by aerohead (Post 650966)
* the vehicle, with wind from behind, is a crude square-rigger.

No, it is not. You are starting from this assumption, and so you go wrong from the beginning.

It is not a square-rigger, it is much more like a modern sailboat with triangular sails. Which can and do sail faster than the wind speed. And the sails (the propeller blades) meet the wind at an angle.

-soD

aerohead 06-25-2021 10:15 AM

faster than the boat
 
Quote:

Originally Posted by ennored (Post 651054)
So a water skier never travels faster than the boat that's pulling him?

Not if they're just trailing straight behind the tow-boat, which would be the proper context with respect to Blackbird.

aerohead 06-25-2021 10:22 AM

video
 
Quote:

Originally Posted by jakobnev (Post 651061)
FAMOUS LAST WORDS!!

Prepare to have your mind expanded: (or blown if it was inflated to its limits)


https://www.youtube.com/watch?v=ek0D4Ehvoc8

The cart moves 93cm when the string moves 43cm, the cart is 2.16 times as fast as the string, exclamation point, QED, more exclamation points.

Thanks. I could see the video portion, but no audio.
Can you provide a transcript of what the presenter was saying?
------------------------------------------------------------------------------------
1) Did you notice that the pull-string shortens during the presentation.
2) The string is being wound around a spool within the 'cart'.
3) If there is a spring-powered clock-drive, or battery-powered- motor-spool within the 'cart', this power , added to the 'system', is what's responsible for the phenomena you are experiencing, as the string continuously winds itself onto the spool.
4) A parlor trick.
5) A visual distraction from the actual dynamics.

RedDevil 06-25-2021 10:32 AM

It is frustrating to see how you miss it. I really did not expect that.

You're so convinced it cannot work that you can't bring yourself to studying it in detail.

If you set the blades of the propeller flat, so the wheels make it spin but it does not push the air forward or backward compared to the car, do you agree that the wind will push the car forward, albeit not as fast as the wind?

Now if you alter the angle of the blades so that it pushes slightly against the wind, do you agree that would only increase the pressure on the blades leading to an even greater forward force, although a part of that is needed to overcome the resistance of moving the blades against the wind?

A sailboat cannot sail straight against the wind because its mast does not move relative to the boat. But propeller blades do move and that has the same effect as a sail boat tacking back and forth to end up in a position that is straight against the wind from the starting point.


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