Down wind faster than the wind

[sol invictus]

No, the forces of drag created by the prop, equal the drive, to produce zero velocity. If idealised, that would apply to the real vehicle. It could not move.

Huh?

The real vehicle has a drag that is proportional to the velocity. At low speeds it is low, but as the velocity rises, so does the drag. When the drag equals the drive,that is the terminal velocity of the device.

No, real vehicles do not have a drag that's proportional to velocity. Drag generally goes as velocity squared. But you're correct about terminal velocity.

If you still think the treadmill represents performance at windspeed, drive at windspeed, release it, and see what happens. It will not run along side as suggested.

These guys have made many videos to demonstrate that it does do that - or rather that it accelerates. And the treadmill experiments prove it even more cleanly.

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

I don't know what you're talking about.

For the last time, the treadmill - so long as you don't fall off the end or interact with the edges - is completely identical to a steady wind. Fixed position on the treadmill is moving at wind speed, moving with the treadmill is moving at ground speed (i.e. at rest with respect to the ground). It's an utterly trivial consequence of relativity, one which even high school physics students should understand.

 

[CyCrow]

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.

Beware of the details of the vector math here. Your description almost sounds like it would accelerate indefinately. As the speed increases relative to wind speed, the neccessary blade angle to produce lift goes up until the force on the airfoil doesn't have the needed forward component. This depends on the lift-to-drag of the propeller. It's similar to the highest angle a boat is able to sail relative to the wind. The best iceboats can do 8-10 times the true windspeed. I'm not sure what the practical limit is for a this one.

// CyCrow

 

[Brian-M]

Beware of the details of the vector math here. Your description almost sounds like it would accelerate indefinately.

It sounds like it, but you can add up an infinite series of numbers which never pass a fixed limit.

In this case, the theoretical upper speed is wind/(1-ratio).
(where ratio is < 1.) I hope I got that right.

So...

A ratio of 1/2 = Twice wind speed.
A ratio of 2/3 = Three times wind speed.
A ratio of 3/4 = Four times wind speed.
A ratio of 1/1 = Infinite. (And no power, so it won't work.)

Top speed increases exponentially with gear ratio, and power decreases exponentially. As long as the gearing is kept low enough to overcome power loss from inefficiencies/drag/friction, it should accelerate to close to the top speed determined by it's gearing.

 

[spork]

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.

Fingers crossed. I can hardly imagine how this wouldn't make a great segment. It's not exactly obvious, although it sure as hell seems so to people on both sides.

Have you considered a variable pitch version?

We have for the full-sized version. But the problem with variable pitch (beyond the added complexity) is that it only has "true pitch" at one predetermined setting.

Top speed increases exponentially with gear ratio, and power decreases exponentially. As long as the gearing is kept low enough to overcome power loss from inefficiencies/drag/friction, it should accelerate to close to the top speed determined by it's gearing.

Right on the money - but you already knew that.

 

[Ivor the Engineer]

<snip>

For the last time, the treadmill - so long as you don't fall off the end or interact with the edges - is completely identical to a steady wind. Fixed position on the treadmill is moving at wind speed, moving with the treadmill is moving at ground speed (i.e. at rest with respect to the ground). It's an utterly trivial consequence of relativity, one which even high school physics students should understand.

Has anyone considered that perhaps Humber lives in a parallel universe* and he is simply accurately relaying what happens there?



*I propose we call this place "The Humberverse", in honour of the first of its citizens to make contact.

 

[spork]

Has anyone considered that perhaps Humber lives in a parallel universe* and he is simply accurately relaying what happens there?

Maybe it's time I come clean with you guys and admit humber is me.



Nope - can't do it. But that'd be great!

 

[Cuddles]

All vectors sum to two.

OK, I have a monkey sitting on the ground. A force of 10N is applied to it in one direction. Another force of 3N is applied in the opposite direction. What do the force vectors sum to?

If you say "two", I suggest attending some remedial physics and maths classes.

Edit: Actually, I didn't realise at first just how stupid the next sentence is as well.

The motive and drag forces, which are always in opposition, and sum to zero.

Just, wow. Motive and drag forces sum to zero? That must be why nothing ever manages to move anywhere.

 

[technoextreme]

This is wrong. A belt in still air is an excellent simulator if what you're simulating is a road with a steady tailwind of equal and opposite velocity to the belt.

Sure, a belt will entrain a thin boundary layer of air to move along with it. Similarly, a road in a steady wind will produce a boundary layer where the wind is slowed down. The effect relative to both the solid surface and the air mass is the same.

Respectfully,
Myriad

Actually, he is right. Your ignoring the fact that the air always blows back. In one frame of reference the windspeeds add. In the other frame of reference the windspeeds subtract. Does that mean the cart doesn't work or that you can't build a cart that does go upspeed? No but the physics is more complicated than just the cart on the treadmill.

A ratio of 1/2 = Twice wind speed.
A ratio of 2/3 = Three times wind speed.
A ratio of 3/4 = Four times wind speed.
A ratio of 1/1 = Infinite. (And no power, so it won't work.)

Top speed increases exponentially with gear ratio, and power decreases exponentially. As long as the gearing is kept low enough to overcome power loss from inefficiencies/drag/friction, it should accelerate to close to the top speed determined by it's gearing.

Errr.... Is that in relation to the prop vesrus the wheel or the wheel versus the prop. I think you might want that backwards because you want the prop rotating twice as fast as the tailwind right?

 

[humber]

double post

 

[humber]

OK, I have a monkey sitting on the ground. A force of 10N is applied to it in one direction. Another force of 3N is applied in the opposite direction. What do the force vectors sum to?

If you say "two", I suggest attending some remedial physics and maths classes.

Edit: Actually, I didn't realise at first just how stupid the next sentence is as well.

Just, wow. Motive and drag forces sum to zero? That must be why nothing ever manages to move anywhere.

The monkey would move. The cart on a treadmill is static. The forces are in balance. That's the point, which you missed. Poor behavior for a moderator. You should know that it is difficult to be 100% accurate when arguing against woo.

[/QUOTE]

You don't understand the last remark? A little later, you will see its meaning, but for now, I will allow you to gloat.

ETA:
Actually, Cuddles, while you are waiting, why not try and answer the "riddle of the treadmill"

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)

ETA:
I did not understand your "spoiler". Literal again. "all vectors sum to two" (vectors), not 2.

 

[humber]

Huh?

It think that it is obvious what I mean, but I will later post an explanation with diagrams.

No, real vehicles do not have a drag that's proportional to velocity. Drag generally goes as velocity squared. But you're correct about terminal velocity.

Yes, I know, but proportional does necessarily mean "linearly proportional".
It is in fact a complex relationship, that may be a function.

These guys have made many videos to demonstrate that it does do that - or rather that it accelerates. And the treadmill experiments prove it even more cleanly.

The videos are evidence that they can change the pixels on my monitor. It is the principles that are in question.

For the last time, the treadmill - so long as you don't fall off the end or interact with the edges - is completely identical to a steady wind. Fixed position on the treadmill is moving at wind speed, moving with the treadmill is moving at ground speed (i.e. at rest with respect to the ground). It's an utterly trivial consequence of relativity, one which even high school physics students should understand.

No, it is not. Please show me the "high school" textbook that supports your assertion.

 

[humber]

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.

A car on a dyno, is a test of the engine's power. The vehicle remains static.
The dyno itself is a load for the engine, usually a hydro-dynamic brake; a
dissipator. The object in fact, is to minimize any energy storage.

 

[technoextreme]

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

Actually, all propellers will act like a windmill when the windspeed is great enough. The technical term is windmilling.

 

[spork]

The videos are evidence that they can change the pixels on my monitor.

EXCELLENT!!! I nominate this for quote of the week. I'm pretty sure you're the first one to come out into the light and claim we're faking it.

No, it is not. Please show me the "high school" textbook that supports your assertion.

I'd be happy to. But I'll have to read it to you - otherwise I worry you'll try and eat it.

 

[spork]

Actually, all propellers will act like a windmill when the windspeed is great enough. The technical term is windmilling.

Beauty! So I'm flying along in my C172 at 80% power. I climb into the jetstream (yeah my C172 is badass) - just how fast do the winds have to be for my prop to start windmilling?

 

[humber]

You forget the air.

A dyno stores energy in the air ?

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

No misconception. That is a definite denial of your claim. Your device, must be over unity to achieve windspeed, let alone exceed it.

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.

True, or at least for the purposes of discussion. It means then, that your frames are velocity only. OK, but not inertial frames.
The original remark concerned an autogyro that is in free-fall in still air.
Additional effects, such as gliding, require energy. However, the gravitational component remains the same. The work done to hoist it to altitude, is dissipated as drag, or stored as kinetic energy released upon impact.
It should be noted, that claims for aerodynamic effects for the cart, may be a two-edged sword. Lift that causes loss of contact with the road, may serve to invalidate your hypothesis re the gearing.

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

// CyCrow

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.

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

// CyCrow

It is not that I expect all motive forces and drag to be equal, but that in stasis, is it so. "Opposite and equal reaction" refers to acceleration.
The treadmill is such a case, and indeed, the forces are equal, by its very design.

No need. Drop an autogyro in a vacuum. Observe. Add some gas. Observe.
The energy lost to the gas is drag, because it retards motion, but you may call it lift. It is the difference in the kinetic energy at impact. The "lift" that you appear to mean, requires additional energy.

 

[humber]

EXCELLENT!!! I nominate this for quote of the week. I'm pretty sure you're the first one to come out into the light and claim we're faking it.

No, they are not worth the bother of faking and if it were the case, a knowledgeable hoaxer would do a much better job. They are simply inadequate.

I'd be happy to. But I'll have to read it to you - otherwise I worry you'll try and eat it.

Ooohh, sharp. But I would like to see the book that supports your assertion.

 

[Modified]

A car on a dyno, is a test of the engine's power. The vehicle remains static.

You keep saying that but it is meaningless. The vehicle remains static with respect to the earth, but not with respect to the dyno drum surfaces.

The dyno itself is a load for the engine, usually a hydro-dynamic brake; a
dissipator. The object in fact, is to minimize any energy storage.

The dyno load is pulling against the restraints through the vehicle. The behavior of the vehicle at steady state is no different than a car moving downwind at wind speed with non-driving wheels supported by a vehicle moving at the same speed and some mechanism pulling back against the restraints.

 

[humber]

You keep saying that but it is meaningless. The vehicle remains static with respect to the earth, but not with respect to the dyno drum surfaces.

The dyno load is pulling against the restraints through the vehicle. The behavior of the vehicle at steady state is no different than a car moving downwind at wind speed with non-driving wheels supported by a vehicle moving at the same speed and some mechanism pulling back against the restraints.

Please define "steady state".

Are you definitely claiming that if I drive or tow a vehicle at windspeed, that is the same as being on a dyno at that same speed? (In general I mean, noise and perhaps some turbulence excepted)

As an aside, it is also possible to construct a dyno that produces no lateral force and has no drums

 

[Thabiguy]

I think you might want that backwards because you want the prop rotating twice as fast as the tailwind right?

No, he wants it slower than the tailwind. So that the air blown by the propeller still moves forward wrt ground.