Downwind faster than the wind

[Steve100]

Hmmm, it seems to be going slower than windspeed.

Treadmill at 5mph
Wind at 5mph in treadmill frame
Cart at 6mph in treadmill frame

(made up numbers)

 

[A.T.]

Hmmm, it seems to be going slower than windspeed.

Scroll to 1:00, when he puts the propeller on it. And consider the speed of the air and of the cart from the frame of the surface.

 

[KitemanSA]

Hmmm, it seems to be going slower than windspeed.

Nope. When the prop is on the cart, the cart moves against the direction of the belt, i.e. faster than wind speed (belt speed).

 

[kwhilborn]

The inventor really did invent the Yellow first down lines in Football...

That's probably how he faked the video in the OP by using his special effect skills.

 

[KitemanSA]

The guy who invented this has a BETTER invention.

The yellow first down line in football. That's way better than some windmill on wheels. He even got an Emmy award for it.

So now all the players know where they have to run.

Yes I got the joke, but I wonder how football would change if there really was a yellow line on the field. Hmmm.

 

[kwhilborn]

@ Kiteman,
The fact that Emmy Award winner Rick Cavallaro built this Blackbird wind device was not a Joke.


http://en.wikipedia.org/wiki/Blackbird_(land_yacht)

The Blackbird was designed and built by Rick Cavallaro and John Borton of Sportvision in association with the San Jose State University aeronautics department.

http://www.fasterthanthewind.org/2009/10/rick-cavallaro.html

Rick has received 25 patents and an Emmy for his work developing computer graphic enhancements for sport broadcasts (such as the yellow first down line and the virtual strike zone).

I just thought the guy who invented this device we are talking about should be recognized for more important work. like FOOTBALL/BASEBALL.

That fact he invented these is not a joke.

There is no proof he did this though. Probably has a motor hidden in there.

 

[KitemanSA]

Didn't say it was. The "knowing where to run" was the joke.

I've seen these items by at leat 5 different people with different configurations. The physics is sound. There is no reason for it not to work. Suspecting a motor is sick.

 

[A.T.]

There is no proof he did this though.

The North American Land Sailing Association, who ratified both records, investigated the vehicle for cheats. Here is the account of one of the observers:
http://blueplanettimes.com/?p=3971

 

[iceaura]

At forward or downwind speeds greater than wind speed, the rotation force on the propeller shaft is counter to its driven direction - it begins to act as a rotation brake as well as a source of air resistance.

During directly downwind travel:

The aerodynamic torque at the rotor is always against the rotation.
The aerodynamic force at the rotor is always downwind or forward.

Nothing changes about those facts when the cart passes windspeed.

I continue to be confused about the role of the prop - it appears to be a couple of airfoils of fixed shape and orientation, capturing kinetic energy from the air by converting it to rotational acceleration of a prop shaft,

and appears to be presented as the sole source of the energy accelerating the craft?

When the cart passes windspeed, the airflow against the prop starts hitting the other side of the airfoil. This brakes the rotation of the prop shaft, rather than accelerating it as before, and also significantly increases the direct air resistance to continued forward motion of the craft overall.

As this rotational acceleration is the sole presented impetus for the downwind motion, converting it to a braking force would appear to be a factor in the continued acceleration of the craft downwind. It should, apparently, slow the thing down.

So what am I missing?

 

[A.T.]

I continue to be confused about the role of the prop - it appears to be a couple of airfoils of fixed shape and orientation, capturing kinetic energy from the air by converting it to rotational acceleration of a prop shaft,

No, that would be a turbine, not a propeller.

and appears to be presented as the sole source of the energy accelerating the craft?

No, the source of the energy is either the air or the ground, depending on the reference frame.

 

[iceaura]

No, that would be a turbine, not a propeller.

I'm sure you find the semantics fascinating, but they fail to enlighten. The question of what exactly was spinning the thing and pushing the craft is the main issue for those of us looking at the pictures.

and appears to be presented as the sole source of the energy accelerating the craft?
No, the source of the energy is either the air or the ground, depending on the reference frame.

Yeah, sure. The other guy straightened me out on what is going on, in the other thread, by quoting the designer. The main confusion (with me, anyway) came from the frequent references to sailboat and iceboat tacking - a different mechanism altogether.

 

[A.T.]

The question of what exactly was spinning the thing and pushing the craft

The torque from the wheels is spinning the rotor. The aerodynamic force on the rotor is pushing the craft.

The main confusion (with me, anyway) came from the frequent references to sailboat and iceboat tacking - a different mechanism altogether.

It's not that different. The individual propeller blades are like sails tacking on broad reach:

[video=youtube;UGRFb8yNtBo]http://www.youtube.com/watch?v=UGRFb8yNtBo[/video]

 

[Aqueous Id]

The torque from the wheels is spinning the rotor. The aerodynamic force on the rotor is pushing the craft.

There can be no torque from the wheels unless (a) the vehicle is going downhill or (b) the wind has stopped and the vehicle is coasting due to inertia. In both cases that torque is detrimental, since it presents a drag only.

 

[A.T.]

There can be no torque from the wheels unless (a) the vehicle is going downhill or (b) the wind has stopped and the vehicle is coasting due to inertia.

Or (c) the vehicle is pushed forward by the thrust of the propeller, which is the case.

 

[KitemanSA]

I really don't get this. Are there actually people here that don't get this simple mechanism or are they just being obtuse for kicks?

 

[Aqueous Id]

Or (c) the vehicle is pushed forward by the thrust of the propeller, which is the case.

There is no propeller, thus no torque, and no means of developing thrust. This is a wind turbine on wheels - the wheels can't develop torque because the potential difference between the driveshaft and the chassis is zero. There would only be torque if for some reason that were not true. But it has to be true because the mast is attached to the chassis, and any force imparted to the shaft is equally transmitted through the mast into the chassis. The "torque" developed in the driveshaft is coupled though the wheels into the chassis, which equalizes with an equal torque in the opposite direction coming from the mast, so the net torque on the wheels is zero.

 

[Aqueous Id]

Didn't say it was. The "knowing where to run" was the joke.

I've seen these items by at leat 5 different people with different configurations. The physics is sound.

What physics?

There is no reason for it not to work. Suspecting a motor is sick.

What's sick is the belief in violation of conservation of energy. It's not clear to me how many fans of this are in that camp, so I'm reserving judgment.

As long as every one understands that the power due to wind on a sail (or turbine) varies directly with the cube of the wind velocity we can avoid handing out barf bags. When the sail or turbine reaches wind velocity, there is no net wind velocity, and no more power. Obviously tacking a sail changes the meaning of this, since now we're talking about two different wind vectors.

 

[A.T.]

This is a wind turbine on wheels

Not in the DDWFTTW case. There it is a propeller on wheels.

When the sail or turbine reaches wind velocity, there is no net wind velocity, and no more power.

That's why you don't use a turbine for DDWFTTW, but a propeller.

Obviously tacking a sail changes the meaning of this

The propeller blades are tacking. They are not moving directly downwind.

[video=youtube;UGRFb8yNtBo]http://www.youtube.com/watch?v=UGRFb8yNtBo[/video]

Vectors for the boat on broad reach:

Vectors for a section of the propeller blade of a cart going DDWFTTW:

What physics?

High school physics.

http://www.aapt.org/physicsteam/2013/

Each year, AAPT and the American Institute of Physics (AIP) sponsor a competition for high school students to represent the United States at the 2013 International Physics Olympiad Competition.

Semi-final exam with solutions (page 11):
http://www.aapt.org/physicsteam/2013/upload/E3-1-7-solutions.pdf

 

[KitemanSA]

Aqueous ID seems to have missed the point the the blade turns the OTHER way than a turbine would. It turns due to the torque generated through the wheel by the wind force on the vehicle and the drag force from the ground. That torque is transmitted to the PROPELLOR (i.e. turns it the opposite way the wind would have). The propellor thrust adds to the wind force which makes MORE torque and more spin of the propellor, so MORE thrust and more torque... Eventually the wind is no longer pushing on the blades but is pushing on the air flowing backwards thru the blades. That airflow is always negative relative to the cart. And as long as it is negative relative to the wind, the interaction of the two flows will provide an outside thrust that can be used by the mechanism. The cart stops acceleration when the back flow becomes stationary relative to the wind. Simple physics.

 

[A.T.]

Aqueous ID seems to have missed the point the the blade turns the OTHER way than a turbine would.

This is demonstrated on a small model here:

With wheels off the ground it turns CCW, like a turbine:
http://www.youtube.com/watch?feature=player_detailpage&v=nudBjrOF3LE#t=240s

With wheels on the ground it turns CW, like a propeller:
http://www.youtube.com/watch?feature=player_detailpage&v=nudBjrOF3LE#t=360s