Merged DDWFTTW in a “Real” Wind

[69dodge]

Think of how a jet boat works. A jet of water coming out the back of the boat pushes on water to move the boat forward. A fluid pushing on a fluid.

I don't think that's how jet boats work. Isn't the exit nozzle above the water line? The boat pushes water backward out of the nozzle; this pushes the boat forward. It doesn't matter what happens later to the expelled water.

If the boat is going downstream it will go faster than if it would be in still water because the jet of water is pushing against water that is also pushing against it. I know there's also less forward friction on the boat when it travels downstream so if that’s an issue for you imagine a water jet powered hovercraft instead of a boat. Going upstream if the water was flowing at the same speed as the jet of water the hovercraft wouldn't travel upstream at all.

If the jet of expelled water flows backward at the same speed as the water in the river, the boat isn't accelerating the water backward at all. So the boat will not be pushed forward, I agree. But the reason is not that the water has nothing to push backward against after it leaves the nozzle; the reason is that the water doesn't push forward on the boat while it's inside the pump.

 

[RossFW]

Sorry but that makes absolutely no sense to me. Can you explain in detail how the speed of the ground can be more than the speed of the wind when the speed of the wind is directly relative to the speed of the ground.

No, the speed of the CART relative to the air is always less than the speed of the CART relative to the ground.

Even when it is above windspeed, it still has a tail-wind, in as far as an airspeed indicator would measure a lesser value than a spedometer attached to the wheels.

If the cat is doing 30 m/s in a 10 m/s talwind, it will have a relative airflow of 20m/s.

The fact that the wheels are being turned at 30m/s, while the prop only has to accelerate air that is doing 20m/s is the differential that supplies the energy.

 

[quarky]

I would think that the feasibility, or lack thereof, of a vacuum 'balloon' could be ascertained with computers. Perhaps I'll dig around and see if it has been worked on.
Not that I have much hope for it, but the advantage of such a lighter than air craft is that no expensive or flammable gasses would be needed. A valve and a vacuum pump would allow changes in altitude. Partial vacuums and membranes can be intriguing.

Simple home demo of the possibilities:
Using a large bowl, aprox. 1/3 full of water; cover tightly with plastic wrap, pushing the plastic down on the water. The bowl can than be flipped over, and the thin plastic will support the (relatively) enormous weight of the water. There is an odd architect that employed such technology in home sized structures.

 

[quarky]

There are ways to grasp the feasibility of ddwfttw that require no wheels and no propellers.

 

[mender]

Well, at least Windgrins had enough decency to say that in the recent examples I was "half right". The difference in the historical cases, is lack of any decency at all. NALSA data says so.
From their point of view, it's a marred top-speed record, but from the point of view of the original claim, an unabashed fraud.

See?

 

[A.T.]

The guy who started this whole topic-Spork, posed it as a physics/aerodynamics/sailing brainteaser and things that are along these lines seem just fine. Ask away!

At TR there is also a spin-off thread for other brainteasers:
http://talkrational.org/showthread.php?t=29316

 

[humber]

See?

Dying on your feet. A latter day Chauvin.

 

[humber]

Actually in regards to your claim about the vacuum sphere, I said Humber was "basically right".

The basically was added as a qualifier because it was jargon that did not add a lot of explanatory insight to his answer which I think is technically correct.

I also was pleasantly surprised, that even though it was a little difficult to read and extract the intended meaning, it was on the questioned topic, cogent, and technically correct and not self contradictory within the same sentence or paragraph.

A new high for the forum.

Congratulations Humber!

Wind Grins

You tossed in some nonsense to pad it. You are another of Spork's evangelists, and that is transparent.
TAD always relies on the kindness of strangers for his ideas and support.
Ruina' early notes are the primary source of this cart. It is all plagiarized, one way or the other and NASA's Steve Smith is the latest addition.

 

[ynot]

No, the speed of the CART relative to the air is always less than the speed of the CART relative to the ground.

I guess you mean when the cart is at or exceeding the speed of the wind. Obviously at the very start the cart doesn’t move at all (and then slowly) relative to the ground and it is in a moving wind.

Even when it is above windspeed, It still has a tail-wind, in as far as an airspeed indicator would measure a lesser value than a spedometer attached to the wheels.

When the cart is above wind speed it has no tailwind it has a headwind.

If the cat is doing 30 m/s in a 10 m/s talwind, it will have a relative airflow of 20m/s.
The fact that the wheels are being turned at 30m/s, while the prop only has to accelerate air that is doing 20m/s is the differential that supplies the energy.

A cart (or a cat travelling downwind at 30 m/s in a 10 m/s wind has a 20 m/s headwind.

The only thing that supplies energy to the cart is the driving wind. The transfer of energy from the driving wind to the cart when the cart is going faster than the driving wind is only possible if there is something on or interacting with the cart that is going slower than the driving wind for the driving wind to push against. That something is the propeller wind/thrust.

 

[ynot]

I don't think that's how jet boats work. Isn't the exit nozzle above the water line? The boat pushes water backward out of the nozzle; this pushes the boat forward. It doesn't matter what happens later to the expelled water.

If the jet of expelled water flows backward at the same speed as the water in the river, the boat isn't accelerating the water backward at all. So the boat will not be pushed forward, I agree. But the reason is not that the water has nothing to push backward against after it leaves the nozzle; the reason is that the water doesn't push forward on the boat while it's inside the pump.

True, water jets work in a “for every action there is a reaction” way but (as I understand it which could be wrong) they work more effectively when they also have something to thrust against. I think a rocket works more efficiently in an atmosphere than in space. In any event it’s a poor analogy so let’s say the hovercraft is driven by a paddlewheel and apply the same scenarios.

 

[ynot]

There are ways to grasp the feasibility of ddwfttw that require no wheels and no propellers.

If you mean with mathematics please remember this is attempt to create a non-technical, simple, plain English description for “Dummies”. If you mean something different please give an example.

 

[quarky]

A bit of a side-track, por favor?

I've played around a lot with small boats and novel propulsion systems. I've moved up this river here with a flagella drive system; a flexible rod with a flipper on the end; powered by a pair of legs; shoving back and forth; sitting side-ways in the boat; mimicking nature's best in our pathetic way. Which btw, was freaking fast! Compared to any paddling crap. And I've paddled for 50 years.

I've rigged aluminum canoes with small wood stoves. no joke. Cumberland River, near the Ky/Tn line. You may have seen me.
(no, there isn't anybody out there.)

So, as per jet propulsion on a boat:

What's the downside of simply blowing the steam out the boat's butt, so to speak?
Direct.
A flexible tube, venting the steam; under water; behind the boat; steer-able; by-passing the normal mechanisms of transfer and all their losses...

I'm pretty sure that I'm wrong about this implication, but I'd feel a lot better about it if someone that knows stuff would come in and explain my wrong-ness.

thanks

 

[quarky]

If you mean with mathematics please remember this is attempt to create a non-technical, simple, plain English description for “Dummies”. If you mean something different please give an example.

Sorry. No math implied.
In the other thread, I brought up a hypothetical ddwfttw scenario.
I didn't know it at the time, and am not surprised, that my hypothetical had been thought of before:

Picture a cart or a boat, with wheels or pontoons that are 1 mile apart.
Within this frame work, which constitutes the "cart" or "the boat", there is a sail, which is able to slide left and right, along a greasy rod, within the width of said really wide boat.

The overall craft could be going directly down wind, yet the moving sail would be tacking back and forth within the body of the craft.

To my primitive brain, a very significant aspect of the cart is the wheels...and the steering.
This is where the magic force goes. Think roller-skating. Side-ways shove.
Except, with the cart, making a straight line is analogous to shoving back and forth lightly.

I could be seriously wrong about this, and would welcome chastisement:

I believe that a ddwfttw craft could be made that had nothing to do with propellers; wheels; gears, etc.

I hope this is helpful, especially if I'm wrong, and smart kids show up to challenge the point.

 

[quarky]

I understand the sphere. And the circle. And ellipses. And forces.
Both Windgrins and humber can jump ahead here.
I'm not as dumb as I write.

 

[BillyJoe]

Originally Posted by RossFW
No, the speed of the CART relative to the air is always less than the speed of the CART relative to the ground.

I guess you mean when the cart is at or exceeding the speed of the wind. Obviously at the very start the cart doesn’t move at all (and then slowly) relative to the ground and it is in a moving wind.

Ross is correct, except that he means velocity (which has a vector) not speed (which does not)
When velocity of the cart relative to the ground = 0, the velocity of the cart relative to the wind = -10
So, in this example, Ross is correct:
The velocity of the cart relative to the wind < velocity of the cart relative to the ground.

When the cart is above wind speed it has no tailwind it has a headwind.

This is expressed more accurately as:
When the velocity of the cart relative to the wind becomes a positive value, the velocity of the wind relative to the cart becomes a negative value.

A cart travelling downwind at 30 m/s in a 10 m/s wind has a 20 m/s headwind.

This is expressed more accurately as:
When the cart has a velocity of +30 relative to the ground, it has a velocity of +20 relative to the wind.

 

[ynot]

[FONT=Verdana, sans-serif]Let's see how many things there are that we can all currently agree on. If anyone disagrees with anything below please say what and why . . . [/FONT]

1. [FONT=Verdana, sans-serif]The cart is ONLY powered by the wind.[/FONT]
2. [FONT=Verdana, sans-serif]Wind can't DIRECTLY transfer power to anything that is moving faster than the wind.[/FONT]
3. [FONT=Verdana, sans-serif]When the cart is moving faster than the wind there MUST be some part of the cart or something that interacts with the cart that is moving slower than the wind so the wind can transfer power to it.[/FONT]

 

[BillyJoe]

Quarky,

You are miserably wrong.

So wrong, in fact, that it is impossible to explain to you why.
Put it this way: if my five year old nephew provided me with an explanation of quantum physics as he understood it, I am not going to attempt to explain to him where he went wrong.

Yes, it's that bad.

 

[ynot]

Ross is correct, except that he means velocity (which has a vector) not speed (which does not)
When velocity of the cart relative to the ground = 0, the velocity of the cart relative to the wind = -10
So, in this example, Ross is correct:
The velocity of the cart relative to the wind < velocity of the cart relative to the ground.

This is expressed more accurately as:
When the velocity of the cart relative to the wind becomes a positive value, the velocity of the wind relative to the cart becomes a negative value.

This is expressed more accurately as:
When the cart has a velocity of +30 relative to the ground, it has a velocity of +20 relative to the wind.

Do "Dummies" know enough about velocity and vectors.

Okay, someome ask - "Well do you?"

 

[windgrins]

Noted!

I understand the sphere. And the circle. And ellipses. And forces.
Both Windgrins and humber can jump ahead here.
I'm not as dumb as I write.

WindGrins

 

[BillyJoe]

The cart is ONLY powered by the wind.

Not strictly correct.
It had to be given a push to get it started.[FONT=Verdana, sans-serif]

[/FONT]

Wind can't DIRECTLY transfer power to anything that is moving faster than the wind.

Correct. If the velocity of the cart relative to the wind is positive, the wind cannot directly cause the cart to increase its velocity. In fact the direct effect of the wind is to decrease the velocity of the cart[FONT=Verdana, sans-serif].

[/FONT]

[FONT=Verdana, sans-serif]When the cart is moving faster than the wind there must be some part of the cart or something that interacts with the cart that is moving slower than the wind.[/FONT]

Hmmm...
Certainly, If the velocity of the cart relative to the wind is positive, it cannot be the wind increasing the cart's velocity, so it must be something else.

In fact, as 69dodge explained earlier:
It is the road exerting a backward force on the wheels, which causes the wheels to turn, which cause the drive belt to move, which causes the propeller to turn, which exerts a backward force on the wind, which cause the wind to move faster backwards which causes the cart to move faster forwards.