Car Goes Downwind 2.5 Times Faster Than the Wind

[endlessplane]

As LFP said way back on the first speed of this thread, people have been sailing faster than the wind for thousands of years. The same aerodynamic principles are in play with Blackbird. The difference is the Blackbird developers have found an ingenious way to apply those principles so that they work when traveling dead downwind as opposed to across the wind.

Here is a very nice animation, that relates the Blackbird propeller going directly downwind, to sailing across the wind with a downwind VMG greater than windspeed (which landyachts, iceboats achieve easily). As you say, the aerodynamics at the airfoil is the same.

[youtube]UGRFb8yNtBo[/youtube]

 

[endlessplane]

Trace the path one step at a time from wherever the energy originates all the way to the outcome of accelerating the car.

This is the trap into which people fall that makes it a brainteaser. It is a mechanical feedback loop, not a linear cause and effect chain. You cannot analyze one step at a time, because things happen simultaneously. The amount of thrust at the propeller doesn't have just one cause, but depends on two things: Power from the wheels and relative headwind (which is reduced by the true tailwind). The later allows for more prop thrust than the wheels brake.


Trace the path one step at a time from wherever the energy originates all the way to the outcome of accelerating this toy car. It is the same energy flow scheme as the Blackbird, just with a ruler instead of the airmass:

[youtube]k-trDF8Yldc[/youtube]

 

[John in CR]

Endlessplane,

That's not how it works at all. A sailboat doesn't travel in the same direction as the wind faster than the wind, and sails aren't angled like that relative to the wind direction either. Once the blades are introduced, at least they have the correct angle, but the explanation is wacky. The wind doesn't act on them, they act on the air exactly like a propeller of a plane. Since they're acting on a moving mass of air, the friction of the wheels at the ground drives propeller faster.

Think about it. It's traveling faster than the wind, so the driver would feel wind on his face. How on earth is the wind going to push it from behind like a sailboat when you feel wind in your face?

 

[Chalo]

Think about it. It's traveling faster than the wind, so the driver would feel wind on his face. How on earth is the wind going to push it from behind like a sailboat when you feel wind in your face?

A spinnaker is a "pushed" sail such as you describe. A mainsail is a wing, and behaves like the one in the animation.

 

[endlessplane]

A sailboat doesn't travel in the same direction as the wind faster than the wind,

Sailcraft on land, ice and even on water can achieve a downwind velocity made good greater than windspeed, as the animation shows. See:
http://en.wikipedia.org/wiki/Sailing_faster_than_the_wind#Speed_made_good

She sailed 20 nautical miles (37 km; 23 mi) downwind in 1 hour 3 minutes, so her velocity made good downwind was about 2.5 times windspeed, consistent with being able to sail about 14 degrees off the apparent wind when sailing downwind.

and sails aren't angled like that relative to the wind direction either.

When sailing on a broad reach, they are angled like that.

The wind doesn't act on them, they act on the air exactly like a propeller of a plane.

Both is true. It's Newtons 3rd Law: The air exerts the same force on the blades, as the blades exert on the air. Locally at the airfoil the aerodynamics is the same, between a propeller blade and a sail on broad reach. See vector diagrams below.

How on earth is the wind going to push it from behind like a sailboat when you feel wind in your face?

The lift is not generated by the drivers face, but by the airfoils which see the air coming at an angle, not directly from the front.

Sailboat on broad reach with downwind VMG > windspeed:

Blade section of propeller cart, going directly downwind faster than the wind:

 

[John in CR]

You two obviously don't know anything about sailing and don't really understand how this machine works. While a sailing rig can move faster than the wind, they cannot move faster than the wind directly with the wind. A wind turbine's blades operate like sails. This vehicle's blades have the opposite angle of attack, because they form a propeller that provides thrust. The thrust is against a moving mass of air which propels the vehicle faster than it would with no wind. The difference in energy between the two is harvested by the wheels and transferred to the prop to try to create even more thrust

 

[endlessplane]

You two obviously don't know anything about sailing and don't really understand how this machine works.

Sure, you understand it better than the guy who designed, build and drove it. Here is an article by Rick Cavallaro:
http://www.seahorsemagazine.com/current-issue/84-rules-are-there-to-be-broken

First I imagined a high-performance sailboat on a broad reach with downwind VMG faster than the wind. But instead of gybing back and forth, I imagined it on a cylindrical planet following a continuous helical path along the axis of that imaginary planet. Then I imagined an identical boat directly opposed to the first one (page 40); the sails now form the blades of a propeller that is going directly downwind faster than the wind. The trick, of course, is that neither propeller blade is going directly downwind, but tie them together and they form a propeller whose centre of mass is going directly downwind.

Exactly what the animation shows.

While a sailing rig can move faster than the wind, they cannot move faster than the wind directly with the wind.

Efficient sailcraft can sustain a downwind VMG > windspeed. They can beat a free floating balloon to a point directly downwind, by tacking across the wind. That is all the animation shows, and it is perfectly correct.

A wind turbine's blades operate like sails.

Depending on the situation, a sail can act like a turbine blade, or like a propeller blade.

This vehicle's blades have the opposite angle of attack, because they form a propeller that provides thrust.

See the vector diagrams. It is the same angles, vectors, aerodynamics, physics. Once you understand those, you will realize that there is no contradiction between "rotor acting as a propeller" and "blades acting like sails on broad reach".

 

[John in CR]

Yes, silly me, I wasn't thinking about the lift a sail can get tacking to to reach a downwind point the fastest. You're going to need to explain to me when a sail provides thrust like a propeller, because I can't think of one.

I disagree with explaining how the vehicle works by starting with how a sailboat works, because all it does is cloud the issue since a sail primarily works like a wind turbine. With the vehicles prop blades turned to an angle of attack like a turbine, the wind turns the blades, which provide force to turn the wheels, and the vehicle will travel very slowly. It will work like that both traveling in the direction of the wind and will work even better in that manner traveling directly against the wind.

The magic doesn't happen until the angle of attack of the blades is changed to that of a propeller while traveling with the wind. Then the prop "screws" through the moving air providing thrust in the opposite direction of the wind and lift on the blades in the direction of travel. When the change in angle of attack is first made there is a surge forward, dragging the wheels which in turn makes the prop spin faster and create more thrust putting more force on the wheels to turn the prop even faster.

While boiling it down to the vectors and angles of wind and apparent wind and lift forces may be necessary to optimize the prop, it's no way to explain the machine, because it leads to the belief that the wind turns the prop. It doesn't, not directly, the wheels drive the prop. The prop drives the wheels only in wind turbine mode.

 

[endlessplane]

You're going to need to explain to me when a sail provides thrust like a propeller, because I can't think of one.

Well I'm not sure what you mean by "sail acts as a turbine blade" either, but here is the analogy that I'm familiar with: The boats movement component perpendicular to true wind corresponds to the tangential velocity of a rotor blade, resulting from the rotor's rotation.

When a boat is tacking upwind:
- the sail acts as a turbine blade, because it drives the movement perpendicular to true wind (which corresponds to rotation)
- the keel acts as a propeller blade, because it creates a force that has a directly upwind component (upwind thrust)

When a boat is tacking downwind with VMG > WS:
- the keel acts as a turbine blade, because it drives the movement perpendicular to true wind (which corresponds to rotation)
- the sail acts as a propeller blade, because it creates a force that has a directly downwind component (downwind thrust)

As you see the two situations are symmetrical. However the later is rather rare because only high-performance boats achieve it.

While boiling it down to the vectors and angles of wind and apparent wind and lift forces may be necessary to optimize the prop, it's no way to explain the machine, because it leads to the belief that the wind turns the prop.

In the prop-vector diagram you can draw the tangential component of the aerodynamic blade force, as a horizontal arrow to the right, which is opposed to the the tangential velocity component of the blade which points to the left (shown top left). So the aerodynamic torque is not turning the rotor (like on a turbine) but opposing the rotation (like on a propeller).

 

[Beachcruzer]

While boiling it down to the vectors and angles of wind and apparent wind and lift forces may be necessary to optimize the prop, it's no way to explain the machine, because it leads to the belief that the wind turns the prop. It doesn't, not directly, the wheels drive the prop. The prop drives the wheels only in wind turbine mode.

The wind doesn't directly turn the prop, but it provides all of the energy. It pushes the vehicle straight downwind, which causes the wheels to turn, which causes the prop to turn, which allows the prop to generate aerodynamic lift, which allows it to push the vehicle downwind faster than the wind.

The key concept is apparent wind, which means the actual airflow moving past the sail/prop/turbine/whatever you want to call it. The prop generates lift, or thrust, from the apparent wind acting on it. The biggest component of the prop's apparent wind comes from rotation, not the true wind. This is exactly how an airplane propeller works--it takes rotational force provided by the engine and converts it into aerodynamic lift, aka thrust. This machine replaces the airplane engine's rotational force with rotational force harvested from the wheels' movement over the ground. The energy comes from the (true) wind, but the speed comes from the way the wheels drive the prop, which drives the wheels, which drive the prop. That's the genius of this thing.

 

[endlessplane]

The wind doesn't directly turn the prop, but it provides all of the energy. It pushes the vehicle straight downwind, which causes the wheels to turn, which causes the prop to turn, which allows the prop to generate aerodynamic lift, which allows it to push the vehicle downwind faster than the wind.

Yes, the Blackbird never uses the turbine mode during downwind runs. Although it could, below windspeed. The Blackbird has a one way transmission with ratchets. The cart build by Andrew Bauer in he 60s could switch from turbine to propeller during a downwind run:

[youtube]yFPomTq_PRU[/youtube]

The biggest component of the prop's apparent wind comes from rotation, not the true wind.

In fact, at windspeed all of the prop-blade's apparent wind magnitude comes from rotation. However, the true wind is still crucial because it affects the direction of the apparent wind. Without the true wind component, the apparent wind would have a direction that makes it impossible to accelerate.

 

[John in CR]

Interesting. I didn't realize someone made it work first back in the '69.

I also didn't realize they were going to try to build one to travel up wind with a ground speed greater than actual wind speed. Going faster than the wind downwind was a lot simpler by comparison.

FWIW, I still insist that explanations of how the Blackbird work using sailboats in examples is a terrible way to go about it. Even the lift from the blades is an unnecessary component, because it would still work even with flat blades that had no aerodynamic lift such as on most ceiling fans. That throws apparent wind out the door too, especially when you think about the fact that with blades having no aerodynamic lift, the vehicle could still travel in 0 apparent wind. No sailboat works in zero apparent wind. Lift just makes the Blackbird work better.

John

 

[endlessplane]

Interesting. I didn't realize someone made it work first back in the '69.

Here is Bauer's article:
http://projects.m-qp-m.us/donkeypuss/wp-content/uploads/2009/06/Bauer-Faster-Than-The-Wind-The-Ancient-Interface.pdf
He claims to have achived ~1.2WS but it is a rough estimate based on a steamer. That's why they build the Blackbird and had NALSA confirm their records.

I also didn't realize they were going to try to build one to travel up wind with a ground speed greater than actual wind speed. Going faster than the wind downwind was a lot simpler by comparison.

They went 2.8WS directly downwind in 2010 and 2.1WS directly upwind in 2012. DUWFTTW is more difficult than DDWFTTW. The symmetry is actually between DDWFTTW and DUW(at any speed). However DDWFTTW is more difficult to accept for many people, because of the zero apparent wind at windspeed.

I still insist that explanations of how the Blackbird work using sailboats in examples is a terrible way to go about it.

To those familiar with the physics of sailing it is very intuitive. More importantly it is easier to accept that it can work in real life, because downwind VMGs > WS have been demonstrated for decades.

Even the lift from the blades is an unnecessary component, because it would still work even with flat blades that had no aerodynamic lift such as on most ceiling fans.

You seem to have a misconception about what "lift" means in aerodynamics. It is the aerodynamic force component perpendicular to apparent wind (see vector diagram). Flat plates like ceiling fan blades can have lift too.

with blades having no aerodynamic lift, the vehicle could still travel in 0 apparent wind

Not with a propeller axis that is parallel to true wind (and driving direction), like in the case of the Blackbird. See the vector diagram again, and set the blade lift force to zero. There is no way this would accelerate forward.

But if you rotate the rotor axis by 90° you get a paddle-wheel, that could theoretically achieve DDWFTTW with some fairing covering the top part. Practically it would be very difficult to achieve sufficient efficiency. Such paddle-wheels do not rely on lift.

[youtube]Ufk6HVWdSzE[/youtube]

 

[John in CR]

with blades having no aerodynamic lift, the vehicle could still travel in 0 apparent wind

Not with a propeller axis that is parallel to true wind (and driving direction), like in the case of the Blackbird. See the vector diagram again, and set the blade lift force to zero. There is no way this would accelerate forward

Just as I thought. You demonstrate that you understand the detail, but not really how the thing works. It doesn't work like a sailboat. On a sailboat the wind drives the sail just like a wind turbine. The Blackbird's functions with a different angle of attack exactly like a propeller on a plane. In the animation you posted before, the sail was at an impossible angle relative to actual wind. A sail cannot reach that angle no matter the speed, however that angle is correct for the angle of attack of the Blackbird's blades, because it is working like a propellor, not a sail nor a wind turbine. If you were riding on a platform behind the prop of the Blackbird when it is traveling at the same speed as the wind, you would feel the thrust of the propeller blowing air reward, because the wheels are driving the prop.

A flat bladed prop, like on most ceiling fans could be mounted on the Blackbird and it would work. It wouldn't be able to achieve much beyond wind speed, but it could get somewhat past it. That's because a flat bladed prop can provide thrust. It's just not nearly as efficient as a proper propeller.

 

[endlessplane]

On a sailboat the wind drives the sail just like a wind turbine.

Not always as I already explained:
http://endless-sphere.com/forums/viewtopic.php?p=710049#p710049

The Blackbird's functions with a different angle of attack exactly like a propeller on a plane.

Angle of attack alone says nothing about it being a propeller blade or a turbine blade.

In the animation you posted before, the sail was at an impossible angle relative to actual wind.

There is nothing impossible about it, as the vector diagram shows. Do you understand vectors?

See also this article (second vector diagram):
http://rightnice.blogspot.de/2010/08/racing-wind.html

And this one (last vector diagram):
http://www.phys.unsw.edu.au/~jw/sailing.html

A flat bladed prop, like on most ceiling fans could be mounted on the Blackbird and it would work.

Yes. And a flat board sail on a landyacht could achive a downwind VMG > windspeed.

 

[ThinAirDesigns]

John in CR, I am one of the two designers/builders of the Blackbird and unfortunately I only have a few moments currently but will add more later.

For the moment I just need to say that Endlessplane is absolutely correct on his sailing analogy/explanation. Everyone imagines a sailboat sail acting only as a turbine acts (slowing air relative to itself), but actually in many points of sail, a properly trimmed boat (even a small cheap one) will be utilizing the sail in a mode where it acts as a propeller (accelerating air relative to itself).

When setting the downwind record, the Blackbird was equipped with propeller blades and the rotor never acted as a turbine (not even below windspeed). These blades were acting EXACTLY the same aerodynamically as the sails on boat on a properly trimmed broad reach.

When setting the upwind record, the Blackbird was equipped with turbine blades and the rotor never acted as a propeller. These blades were acting EXACTLY the same aerodynamically as the sails on boat on a properly trimmed close reach.

Currently, you are just one of many, many thousands who insisted they were right but eventually learned the truth.

JB

 

[ThinAirDesigns]

And yes, I brought it back from the dead a second time without realizing it. LOL

JB

 

[doctorGONZO]

Ahhh, so, grasshopper....I can be a man dreaming that he is a butterfly, and I can leap up into the blowing wind, not moving my wings, and, without moving my dream wings, I can OUTRUN THE WIND 2.5!

Heavenly Bliss!

Go 'way, Isaac Newton.

Wake me up when my coffee is hot.

 

[Dauntless]

Where Issac Newton fails, reinforce with Francis Bacon.

Truth is so hard to tell, it sometimes needs fiction to make it plausible.
-Francis Bacon

And this thread wasn't dead, it was just sleeping. Maybe the rest will make it easier for everyone.

Silence is the sleep that nourishes wisdom.
-Francis Bacon

If you don't want to believe this, the way to prove it wrong is to build your own.

If a man will begin with certainties, he shall end in doubts, but if he will content to begin with doubts, he shall end in certainties.
-Francis Bacon

Read not to contradict and confute, nor to believe and take for granted... but to weigh and consider.
-Francis Bacon

Let's let the people arguing against it go first.

Nature, to be commanded, must be obeyed.
-Francis Bacon

Should just be one more thing for you to tinker with.

God hangs the greatest weights upon the smallest wires.
-Francis Bacon

 

[Hillhater]

Mmmm ! Arrhhhhh....Bacon
Homer (Simpson)

 

[doctorGONZO]

Anyone who lives for physics, and, as a starting point, believes that there is at least a little bit of truth in Newton Principia, must be able to explain how an air molecule can outrun a sail molecule and strike it and transfer momentum into it if the sail molecule is a priori moving 2.5 times as fast as the prevailing wind and its molecules.

All ears......

 

[doctorGONZO]

Let me splain a little bit more.

Spose I am messing round in your own tomato patch and you catch sight of me. Being a confessed coward, I take off running. Being a Fast Old Boy, I can run 25 MPH. You are really pissed at me getting your own tomatoes and you lose your temper and pick up your 44. You 44 has bad bullets and can shoot at only 10 MPH muzzle velocity.

For the math-challenged, 25 MPH is 2.5 times 10 MPH.

How is you bullet, going at 10 MPH, going to ever catch up to me, transfer momentum to my sad body, and throw me on my face on the ground?

No Way Jose.


Like El once said, slightly paraphrased," a little less conversation, a little more REAL EXPLANATION, please."

Reliance on Majicke is not compatible with Newton Principia.

 

[endlessplane]

explain how an air molecule can outrun a sail molecule and strike it and transfer momentum into it if the sail molecule is a priori moving 2.5 times as fast as the prevailing wind and its molecules.

That would be indeed impossible, if the two molecules where moving in the same direction, so that everything happens in one dimension only.

But here the sail/propeller molecules are not moving directly downwind. Their velocity vectors have a component perpendicular to the true wind, so it is a two dimensional problem and you have to use vectors to describe the collision. See the vector diagram I posted above, and this animation which visualizes the deflection of the air-molecules (strongly simplified) from different reference frames:

[youtube]63hvQABLOaE[/youtube]

Here the same thing for the propeller cart:

[youtube]FqJOVHHf6mQ[/youtube]

 

[itchynackers]

Doctor, please try to open your mind a bit beyond middle school and guns. Frankly, it is way beyond exhausting watching this conversation. Your refusal (or inability?) to even attempt to understand the facts leads me to the conclusion that you didn't graduate high school, let alone get a doctorate. If you are a doctor, please tell me what profession (so I know to avoid the whole specialty). Here is the ultimate truth for you...there are things that are true, but you do not understand (quantum mechanics, vectors, women.... It doesn't make them less true. I suspect, even if you hitched a ride on this downwind vehicle, that you still wouldn't believe it. Exhausting.

 

[TylerDurden]

The bullet is a very poor analogy. The car is not running away from the wind, it is pushing against it.


Endlessplane nailed it with feedback loop.

The wind pushes the car and the car pushes back at the wind... then the car goes faster and pushes harder... then the car goes faster and pushes harder... then the car goes faster and pushes harder...