Down wind faster than the wind

[ThinAirDesigns]

humber:

At least 9/11 claims have some air of plausibility.

humber:

I make no connection with 9/11.

 

[CyCrow]

The braking vehicle is accelerating, but negatively. This is why I put "slowing down" within quotes. Most assume acceleration to only be "positive".
One represents the loss of kinetic energy, the other an increase.

I know the physical definition of acceleration, thank you. Remember that kinetic energy is always relative to a reference frame. (See my gun-on-a-plane example in a previous post) In the earth-static frame, the kinetic energy of the car is increased when you brake on a treadmill, as the car gains velocity. In the reference frame moving backwards at treadmill speed, kinetic energy is decreased as the car brakes, just like when rolling on the road in an earth-fixed reference frame.

A differential veloivity of zero, does not mean no drag. If fact, I can demonstrate this using your own "frames" idea.
Hold the vehicle within a tailwind. There is drag, as the wind flows over the static vehicle. Put that in a mental "van". Add say, 10mph, downwind to that. Add up the vectors. What do you get? The same drag as when static.

What? Please be more specific on the windspeed, van speed, and if we are inside or outside the van.

The "tip effect" is not of my doing, but an explanation given to support the idea that the vehicle in some way acts as if it were tacking the wind.

Close, but yet so far. The propeller gives more thrust than is lost through the wheels to spin it, because it is moving through the air at a greater rate than the vehicle is moving along the ground, and it exploits the resultant wind vectors to generate thrust. This only works when there is a relative difference in velocity of the air and the ground.

You are indeed confusing "inertial frames" with the observer's view.
At a stretch, the device may be though of as being in a separate inertial frame while accelerating, but no at any other time. The velocities are too low to even consider any other effects such as time dilation or mass increase.

Lift and so forth have nothing to do with the treadmill.

Inertial frames cannot accelerate. They have constant velocity. Who's confused?

Lift and so forth have nothing to do with the treadmill.

If you keep ignoring aerodynamic lift on the propeller, you will never understand how it works.

// CyCrow

 

[humber]

So any reaction to this;

In the van model turn the treadmill around. It fails to react. Turn off the drive motor, no visible difference (though prop no longer turns)
Stop the van, reverse the van, go beyond windspeed. The treadmill provides exactly the same information in all cases.

 

[CyCrow]

The sum of the lift and drag forces are always zero. Specific designs alter the ratio, but not the sum. Low drag items fall faster (a vacuum being the limiting case). Any prop that is driven by an external source such as a motor, is of course a different matter.

Um... No. Lift and drag are defined as the vector components of the aerodynamic force on an object, where drag is the force component opposite the velocity through the medium, while lift is the force component perpendicular to it.

// CyCrow

 

[humber]

I know the physical definition of acceleration, thank you. Remember that kinetic energy is always relative to a reference frame. (See my gun-on-a-plane example in a previous post) In the earth-static frame, the kinetic energy of the car is increased when you brake on a treadmill, as the car gains velocity. In the reference frame moving backwards at treadmill speed, kinetic energy is decreased as the car brakes, just like when rolling on the road in an earth-fixed reference frame.

Then you know that a = f/m at all velocities. The same force is required to accelerate an object no matter its velocity or "frame". The exception to this is of course can be made for objects approaching light-speed.

On a rolling road, the vehicle gains no kinetic energy. It is static.

What? Please be more specific on the windspeed, van speed, and if we are inside or outside the van.

Any variant you care to choose. You are retrofitting the test to suit the treadmill, which gives an illusion that it works.
These ideas consider only the relative velocities, but not the energy required to obtain them.

Close, but yet so far. The propeller gives more thrust than is lost through the wheels to spin it, because it is moving through the air at a greater rate than the vehicle is moving along the ground, and it exploits the resultant wind vectors to generate thrust. This only works when there is a relative difference in velocity of the air and the ground.

All energy comes from the wind. Claims of improved efficiency beyond that, requires over-energy.

Inertial frames cannot accelerate. They have constant velocity. Who's confused?

An example of an item in an "inertial frame" is given by the usual example of an elevator. Available in any primer on relativity. However, there are no inertial frames to consider in this case.

If you keep ignoring aerodynamic lift on the propeller, you will never understand how it works.

// CyCrow

You have already stated that the treadmill does not include aerodynamic effects.

 

[ThinAirDesigns]

So any reaction to this;

In the van model turn the treadmill around. It fails to react. Turn off the drive motor, no visible difference (though prop no longer turns)
Stop the van, reverse the van, go beyond windspeed. The treadmill provides exactly the same information in all cases.

To get a response, you must define words such as "it", "fails", "react", "visible", "windspeed" and "information".

Thanks

JB

 

[CyCrow]

To spork: Hope you get it on the mythbusters. Judging from the quality of most of the other videos, you should have a good chance.

Have you considered a variable pitch version? That should make it easier to optimize for different wind speeds, and might even be able to exceed twice the true windspeed. It would also be a very effective aerodynamic brake when you need to stop. A tail rotor assembly is a good candidate part.

// CyCrow

 

[humber]

Um... No. Lift and drag are defined as the vector components of the aerodynamic force on an object, where drag is the force component opposite the velocity through the medium, while lift is the force component perpendicular to it.

// CyCrow

All vectors sum to two. The motive and drag forces, which are always in opposition, and sum to zero. It is there in the above quote.

Lift is redirection of one of these forces, not an additional force.
To perform the calculation, you must compare orthogonal forces, with the like for drag.

I would still like an explanation please, as to why the treadmill gives the same information no matter what I do with it.

 

[humber]

To get a response, you must define words such as "it", "fails", "react", "visible", "windspeed" and "information".

Thanks

JB

As far as the observer in the van is concerned. The treadmill provides the same information
1. Van is stopped, in any wind, any direction
2. Van is moving with the wind, any wind, any direction
3. If the orientation of the treadmill within the van changes the, same result is obtained, save for the apparent direction.

Two such treadmills side by side, could contradict each other. (The treadmills do not react to change)

 

[Modified]

Energy is extracted from the wind passing over the prop. Indeed, the speed of the ground is irrelevant, but you think it is.

The DWFTTW vehicle is not a windmill, and does not extract energy in the same way that a windmill does.

 

[humber]

The DWFTTW vehicle is not a windmill, and does not extract energy in the same way that a windmill does.

If it's not, then why mention it? However, it does extract all of its energy from the wind. There are no over-unity windmills, and no over-unity propellers or carts.

 

[CyCrow]

On a rolling road, the vehicle gains no kinetic energy. It is static.

You forget the air.

All energy comes from the wind. Claims of improved efficiency beyond that, requires over-energy.

You seem to be under the misconception that moving faster than windspeed downwind requires over-unity.

An example of an item in an "inertial frame" is given by the usual example of an elevator. Available in any primer on relativity. However, there are no inertial frames to consider in this case.

That's the example of the inability to distinguish gravity from constant acceleration. At any rate, gravity is perpendicular to all the important vectors anyway, and only provides the needed traction between the wheels and the road/treadmill. In fact, adding mass to the vehicle has little impact on the flat-road top-speed performance.

You have already stated that the treadmill does not include aerodynamic effects.

You do know the difference between still air and a vacuum? A prop moving through still air has aerodynamic effects.

// CyCrow

 

[Ivor the Engineer]

<snip>

On a rolling road, the vehicle gains no kinetic energy. It is static.

<snip>

Tell that to the ant on the treadmill that just got hit by it!

 

[humber]

Tell that to the ant on the treadmill that just got hit by it!

A dynamo meter is a rolling road. The car gains on kinetic energy.

 

[humber]

Tell that to the ant on the treadmill that just got hit by it!

A dynamometer is a rolling road. The car gains no kinetic energy.

 

[CyCrow]

All vectors sum to two. The motive and drag forces, which are always in opposition, and sum to zero. It is there in the above quote.

The net forces on an object at constant velocity sum to zero. The vehicle on the treadmill accelerates against the apparent wind, so the net forward force is greater than the net backward forces. You don't seem to have a good grip on what the forces are, and keep assuming that lift-forces must always be matched by drag-forces of equal or greater magnitude.

Lift is redirection of one of these forces, not an additional force.
To perform the calculation, you must compare orthogonal forces, with the like for drag.

You do not understand aerodynamic lift. Start here: http://en.wikipedia.org/wiki/Lift_(force)

// CyCrow

 

[Modified]

If it's not, then why mention it?

You asked, otherwise I would not have.

 

[Modified]

A dynamometer is a rolling road. The car gains no kinetic energy.

You keep saying that, but it is meaningless. With respect to the surface of the dyno drums, the car gains a ton of kinetic energy. With respect to the surface of the earth, it does not. Kinetic energy is relative.

At steady speed, a car on a dyno behaves exactly like a car moving downwind at the speed of the wind on an open road with the non-driving wheels supported by a vehicle that is also moving at the same speed.

 

[Skeptical Greg]

However, it does extract all of its energy from the wind. ....

No it doesn't.

 

[Brian-M]

Energy is extracted from the wind passing over the prop. Indeed, the speed of the ground is irrelevant, but you think it is.

I thought the same thing too. I only came to understand how it works when I realized that it wasn't extracting energy from the wind. It's actually extracting energy from the ground through the wheels. The only function the air serves is as something for the device to push against to move forwards.

As long as the gearing is below 1, the device will be able to accelerate against the relative direction of the ground, buy pushing the air behind it.

The faster the air moves over the device, the higher the gearing needs to be in order to push the air back faster than it is already moving. If air speed
and ground speed were the same, the gearing would have to greater than 1, and the device would be trying to use more energy than it's extracting from the ground, and wouldn't work.

That's why it needs a tail-wind. Not as a source of power, but for something to push against.

2. The treadmill shows that propellers cannot extract energy from still wind.
(Why do manufacturers bother with those enormous and expensive
windtunnels, when they could do it this way? Pretty stupid, eh?)

Actually Spork's videos show that his device does accelerate in still wind, if the ground is moving (ie. on a treadmill). That's because the device isn't extracting energy from the propeller, but from the wheels. Watch this video again.

3. "The "principle of equivalence" says that there is no way to tell the difference between "inertial frames"

Get in you car and hit the brakes
Drive at 10mph, and then hit the brakes
Notice a difference?

The car's velocity in it's inertial frame is measured by it's speedometer. In order for your example to be valid, the speedometer would have to be the same in both situations. For example...

Using the car's speedometer to determine velocity:
Drive a car 10mph down a street and hit the brakes.
Drive a car 10mph on a giant treadmill (with a belt moving 10mph the other way, so the car is stationary relative to the ground) and hit the brakes.
There is no difference.

When you hit the breaks on the street, you are decelerating 10mph relative to the street. In this case the street is your frame of reference.

When you hit the breaks on the treadmill, you are decelerating relative to the 10mph relative to the belt. In this case the treadmill belt is the frame of reference.

As the device in question is pushing against the air to accelerate, I think it's logical to use the air as the frame of reference. If it's sitting in place on a treadmill in an enclosed room, it is traveling at 0mph relative to the air. If it is rolling down the street at wind-speed, it is traveling at 0mph relative to the air. The two scenarios are exactly equivalent.

This is an imaginary scenario. Even if there were no drag at windspeed ( zero differential velocity), then a little slower or a little faster would produce drag.
The treadmill ( though by no means a model), represents this knife-edge, of the balance forces of drag and drive obtained from the belt.

If it's moving 1mph up a treadmill, it has a 1mph headwind pushing it back.
If it's moving 1mph faster than the wind, down the road, it has a 1mph headwind pushing it back.

If the treadmill is moving it backwards at 1mph, it has a 1mph tailwind pushing it forward.
If it's moving 1mph slower than the wind, down the road, it has a 1mph tailwind pushing it forward.

The two situations are exactly equivalent.

But this is not even a "theoretical" point, because in order to reach it, the craft must first break the drag barrier, requiring over-unity.
The thought experiments are bizarre enough, but they simply gloss over the manner in which the craft might get to this supposed state.

Imagine a treadmill of infinite length. As the belt pushes the cart backwards, the wind resistance (ie. tailwind) slows the cart's backwards momentum (ie. adds forwards momentum), and the belt passing under the cart turns the wheels, which spins the propeller, which increases the cart's forwards momentum even more, so the belt turns the wheels even faster, which turns the propeller faster... and so on.

The faster it goes, the greater the forwards force provided by the propeller. This force is more than enough to overcome drag and propel it to wind speed and beyond.