Down wind faster than the wind

[Michael C]

Has anybody made a version of this vehicle that runs on water? I'm thinking that for testing purposes, it should be easier to find a large stretch of water than it is to find a large, completely flat area of open land.

There's an article here with sketches of two possible ways to make such a boat. The principle is the same, with a propeller in the water taking the place of the wheels. I imagine that the vessel would need to be a boat (catamaran or trimaran, maybe) that suffers from less drag than the simple monohull depicted in this article.

 

[ThinAirDesigns]

Has anybody made a version of this vehicle that runs on water? I'm thinking that for testing purposes, it should be easier to find a large stretch of water than it is to find a large, completely flat area of open land.

There's an article here with sketches of two possible ways to make such a boat. The principle is the same, with a propeller in the water taking the place of the wheels. I imagine that the vessel would need to be a boat (catamaran or trimaran, maybe) that suffers from less drag than the simple monohull depicted in this article.

Even in places where it is easier to find open water, it will be a difficult test to execute -- primarily because it will be much harder to overcome the friction of the hull compared to that of a land based vehicle.

Not impossible, just far more difficult.

JB

 

[JWideman]

-

You can balance out the drag and weight of both carts in all sorts of various ways - all you will have proven is that carts of equal weights and equal drags will tie in a downhill race: not something about which there is much debate.

Your questions about the coast test are a bit like a bunch of ice-boaters showing up on a big race day to find the winds calm. They say "let's push our boats around the lake and see which one performs the best" ... they clearly don't learn much. Someone says "Let's tilt the frozen lake up on an angle and then we can still race from one end to the other".

Ok, let's suppose they do just that: which boat will win the race? If they all weigh the same, the one with the least drag of course. Unfortunately, that won't tell you a much about who would have won the event if the winds had been blowing. It might be an interesting contest, but not related in any significantly valid way.

You can't effectively test a device designed to run on energy without said energy: A lightbulb test involves a difference in electrical charges. A hydro turbine test water moving from high to low pressure. A wind powered device test needs relative motion between surface and air.

Of course individual components can be tested outside of their composite environment but it takes someone who understands the entire device to start out with to understand how individual performance effects the whole.

JB

So basically, the prop will not be generating thrust as it rolls downhill. Not enough to overcome the drag, at any rate, right?
I feel kind of silly for not understanding what's happening before, as I understand how a boat's keel works when the boat is at an angle to the tailwind, and this is basically doing the same thing.

 

[spork]

So basically, the prop will not be generating thrust as it rolls downhill. Not enough to overcome the drag, at any rate, right?

In a no-wind situation the prop will be doing the opposite of creating thrust (or creating thrust in the wrong direction if you prefer to think of it that way). This is at the heart of what makes this thing go downwind faster than the wind. The prop pushes wind out the back of the cart, but it does it at a rate slower than the cart is rolling forward. With a tailwind, this works fine. Any amount of air it directs rearward as it coasts downwind at wind speed will theoretically cause it to go faster than that airmass in which it's submerged (in reality it takes a certain minimum to overcome frictional losses).

But in a no-wind situation you can see that the prop would be working against you. If the cart is rolling along at 10 mph, the prop is trying to let air "escape" at only 5 mph. This is precisely how a helicopter or glider descends slowly with no engine.

I feel kind of silly for not understanding what's happening before

It's only silly if you insist on an incorrect position despite any amount of evidence to the contrary. I think we all know someone like that.

 

[ThinAirDesigns]

We just finished our entry into the MythBusters video challenge.

http://www.youtube.com/watch?v=xHsXcHoJu-A

JB

 

[spork]

Wow - you can hear an echo in here now. I was expecting a remedial physics lesson from humber by now. Perhaps he's smartened up and realizes we're all beyond hope.

 

[Skeptical Greg]

I'm sure this has been posted more or less- but having read through the whole thread - I don't remember it being put as succinctly as this simple statement from RoofingGuy over at the Mythbusters forum ...---

" .... this cart can extract energy from the difference between the wind and the ground. It doesn't matter if the wind is moving and the ground is still, like you'd expect, or if you put it on a treadmill in no wind -- as long as there's a difference to extract, the cart will have propulsion. "

That's really all there is to it.

 

[ThinAirDesigns]

"Roofingguy" is the man.

JB

 

[humber]

Wow - you can hear an echo in here now. I was expecting a remedial physics lesson from humber by now. Perhaps he's smartened up and realizes we're all beyond hope.

Spork, I bet you can always hear an echo.

<snip>

" .... this cart can extract energy from the difference between the wind and the ground. It doesn't matter if the wind is moving and the ground is still, like you'd expect, or if you put it on a treadmill in no wind -- as long as there's a difference to extract, the cart will have propulsion. "

That's really all there is to it.

Diogenes, you are being misled.
Can you show me a wind-driven prop that does not "extract energy from the difference between the wind and the ground"

1. Even more succinctly than Roofingguy "this propeller extracts energy from the wind"

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?)

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?

 

[CyCrow]

Diogenes, you are being misled.
Can you show me a wind-driven prop that does not "extract energy from the difference between the wind and the ground"

An autogyro.

1. Even more succinctly than Roofingguy "this propeller extracts energy from the wind"

What is wind? Wind is motion of air relative to the surface. The fact that the earth is spinning and moving through space at a very high rate doesn't change the windspeed. (Of course wind is affected on the large scale by earth rotation and seasons.)

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?)

You still don't get the treadmill test. Windtunnels provide controlled moving air. The treadmill experiment shows that you can overcome 0 relative wind and accelerate past windspeed. To determine top speed, a combination treadmill and windtunnel would be needed.

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?

You still don't get it. The correct experiment is this:
Drive 10 mph, then hit the brakes.
Drive 10 mph on a 10 mph treadmill (with very high mass to absorb the momentum of the car), then hit the brakes.

You will not be able to tell the difference until you roll off the end of the treadmill.

// CyCrow

 

[Ivor the Engineer]

<snip>

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

Yes.

How about this experiment:

Get in your car which is free-wheeling on a treadmill moving at -10mph relative to the ground (i.e. the car stationary with respect to the ground), and hit the brakes.

Drive at +10mph relative to the ground and then hit the brakes.

Notice a difference?

No.

You are not doing yourself any favours by constantly demonstrating that you do not understand the equivalence of these two situations as far as an observer in the car is concerned.

 

[humber]

An autogyro.

AN autogyro, is not wind-driven. The driving force is gravity. Drag opposes this force. Next!

What is wind? Wind is motion of air relative to the surface. The fact that the earth is spinning and moving through space at a very high rate doesn't change the windspeed. (Of course wind is affected on the large scale by earth rotation and seasons.)


You still don't get the treadmill test. Windtunnels provide controlled moving air. The treadmill experiment shows that you can overcome 0 relative wind and accelerate past windspeed. To determine top speed, a combination treadmill and windtunnel would be needed.

This is false reasoning. It assumes the conclusion, to support the claim.

You still don't get it. The correct experiment is this:
Drive 10 mph, then hit the brakes.
Drive 10 mph on a 10 mph treadmill (with very high mass to absorb the momentum of the car), then hit the brakes.

You will not be able to tell the difference until you roll off the end of the treadmill.

// CyCrow

Yes.

How about this experiment:

Get in your car which is free-wheeling on a treadmill moving at -10mph relative to the ground (i.e. the car stationary with respect to the ground), and hit the brakes.

Drive at +10mph relative to the ground and then hit the brakes.

Notice a difference?

No.

You are not doing yourself any favours by constantly demonstrating that you do not understand the equivalence of these two situations as far as an observer in the car is concerned.

Try again. Perform the test as you have described using real cars. Drive another vehicle at the same speed, along with that containing the treadmill.
"Reach out" to the model on the treadmill and bring it to your "frame of reference" Now do you see a difference? Yes, the wind would pull the craft from your hand. Backwards, big time.

ETA:
I reiterate: To do any experiments, you must include the simulator. They are simulations of a simulation.

 

[Ivor the Engineer]

Humber,

Run my experiment with the windows blacked out on both cars.

Is it possible for an observer to know which car they are in when they hit the brakes?

 

[sol invictus]

(Why do manufacturers bother with those enormous and expensive
windtunnels, when they could do it this way? Pretty stupid, eh?)

You really don't get it, do you?

Something at fixed position on a treadmill is equivalent to something moving at the speed of the wind. That's not the situation you want to study if you are designing something to be aerodynamic or for powered flight, obviously. And something moving with the treadmill falls off the end quite quickly.

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?

In one case your car was moving relative to the ground, and in the other it wasn't. See the difference?

It's relative motion that matters, and only relative motion. Absolute motion is utterly meaningless. That has been understood by everyone else at least since Galileo.

 

[CyCrow]

Actually, an autogyro is usually powered by a motor that drives a propeller that gives it airspeed, which drives the rotors that provide lift.

If you want to extract energy, you need to take advantage of difference in windspeeds. One way to do this, although not with a prop, is dynamic soaring. http://en.wikipedia.org/wiki/Dynamic_soaring No ground involved, except as the drag source for the relative windspeed difference.

// CyCrow

 

[ThinAirDesigns]

AN autogyro, is not wind-driven. The driving force is gravity. Drag opposes this force. Next!

Try putting the autogyro in a vacuum and see how fast "gravity" spins the rotor -- you'll find it spins it at exactly 0rpm. Next!

George's inability follow simple logic used to be astonishing. It's simply become commonplace.

JB

 

[humber]

Humber,
Run my experiment with the windows blacked out on both cars.

Is it possible for an observer to know which car they are in when they hit the brakes?

Ivor,
Yes. All bodies within the braked vehicle will react to the loss of kinetic energy. They will therefore "slowdown" which is felt as forward or backward motion ( dependent upon arbitrary choice as to which is forward. )

You have not responded to my remark concerning the wind.

Put the treadmill in a vehicle. Drive downwind at windspeed. Open the window and put the toy outside. Now what happens?
Is seems that some would have you equate "inertial frames of reference" with "point of view". You have to travel with the treadmill (simulator) in each case!

You really don't get it, do you?
Something at fixed position on a treadmill is equivalent to something moving at the speed of the wind. That's not the situation you want to study if you are designing something to be aerodynamic or for powered flight, obviously. And something moving with the treadmill falls off the end quite quickly

In one case your car was moving relative to the ground, and in the other it wasn't. See the difference?

It's relative motion that matters, and only relative motion. Absolute motion is utterly meaningless. That has been understood by everyone else at least since Galileo.

No, your argument is not Galilean, but based upon pre-scientific notions. If this "treadmill system" could be idealized, the vehicle, when placed on the ground, would be immovable in the given direction.
The drag and motive forces, of any magnitude, would always oppose each other so as to prevent motion. This is clearly impossible to realise in practice. The treadmill is an imperfect model of that situation. Nothing more.

The real cart does move with the wind, but that is because its real model is not that of the treadmill. In the latter case, the "treadmill" is not part of "the system" but part of the vehicle itself.

ETA:
To anticipate an escape route. If this device exploits a putative "propeller tip effect", then this must apply to all existing propellers. Such effects have not been observed.

 

[humber]

So, now our winddriven device is in a vacuum?
Without air, the prop will not turn, there will be no drag. The auto gyro, will fall at a velocity determined by Newton's laws.
We should install a ticketing system, but, next!

 

[ThinAirDesigns]

humber:

AN autogyro, is not wind-driven.

humber:

Without air, the prop will not turn,

You absolutely realize how totally stupid those two statements of yours look when placed side by side don't you?

Of course you don't.

JB

 

[humber]

humber:


humber:


You absolutely realize how totally stupid those two statements of your look when placed side by side don't you?

Of course you don't.

JB

Appearances can be deceptive, don't you think?

The auto-gyro will obey Newton's laws in each case. Without drag, it falls faster.
Gravity pulls on the autogyro. The air passing over the propeller, turns it. In principle, this drag is is no different from that of a parachute