Merged DDWFTTW in a “Real” Wind

[SOdhner]

Someone may have already linked this, but it is my favorite video on the subject so far:

http://www.youtube.com/watch?v=k-trDF8Yldc

There are follow-up videos as well, that make it even clearer for specific points.

 

[Subduction Zone]

Someone may have already linked this, but it is my favorite video on the subject so far:

http://www.youtube.com/watch?v=k-trDF8Yldc

There are follow-up videos as well, that make it even clearer for specific points.

That video was by Michael C. He seems to have a very large female fan club because of those videos.

 

[I Ratant]

Pulling the wheeled spool uphill.
Thread coming off the bottom of the spool.
The whole thing climbs up the thread because the thread is spooled -onto- the spool, shortening the distance from the zero point on the thread.. the washer... by the time the thing has moved 1/2 way up the inclined plane.
.
Thread coming off the top of the spool.
The thread gets...longer... puling thread off the spool, as the thing moves up the inclined plane. The second washer is 6 inches from the first, and on the bottom of the spool for the bottom-pull images.
It's seen wrapped up on top of the spool in the second image of the first two.
.
But how does any of this spooling stuff apply to the downwind situation?

 

[Myriad]

The famous hook can pull the reel uphill, with the hook facing down from above the spool, or facing up from below the spool.
The force required to pull the thing up is the same as with the thread, as is the force required to keep the reel in place on the inclined plane.

Actually no. The force required to pull the reel up with the wound thread (or hold it in place) is greater than the force required to pull it up (or hold it in place) with the hook. This is assuming all movement speeds are slow, so angular momentum can be disregarded. Using the wound thread, you apply more force (you have to pull harder) over a shorter distance (the reel moves faster than the thread so you only have to pull the thread a shorter distance than the reel moves). Using the hook, you apply less force but you have to pull it a greater distance, equal to the distance the reel moves. The work done on the reel ends up the same either way.

The relevance of this and the chain wheel thing to driving the propellor on the cart seems tenuous at best.

Here you go:

Here's the reel, being pulled to the right by the wound thread. This time a parachute in the wind, the one that's fully unfurled, is providing the pull. The wind is blowing left to right.

The reel works the same as in your own tests: it "winds up" the line as it rolls left to right. This means the parachute is being pulled closer to the reel as it moves. By the time it reaches the reel, the next parachute will be unfurling at the right end, to continue to provide pull.

Now imagine that the wind is pushing the parachute at a healthy fraction of the wind speed (say, 50% of wind speed). That means the whole thing has to be pretty efficient, the chutes have to be large (to generate a lot of force from the wind) while the contraption has to be light with minimum drag. That would be a difficult engineering challenge but there's no question it's possible in principle. The parachute cannot actually reach wind speed; moving downwind at 50% of the wind speed it's "feeling" only half the wind speed that it would if it were stationary. So the faster it goes the less wind it feels and the more drag it has to overcome to pull the cart, so it's probably not going to get much better. It -- keep in mind that by "it" we're talking about the parachute -- will certainly not be able to reach wind speed, let alone exceed it.

But, as you know from your experiments with the reel, if the parachute is pulling the line left to right at a certain speed relative to the ground, the reel is rolling faster than that. The particular reel pictured moves about three times as fast as the "thread" pulling it. Relative to the ground, the parachute is moving left to right at half wind speed, so the reel -- and therefore the cart frame -- is moving three times that speed, which is 1.5 times wind speed. In other words, downwind, faster than the wind.

Remember, the parachute is moving right to left at half wind speed, so it's moving left to right (toward the reel) relative to the cart.

Here's an improved model:

Not much different here; again, the unfurled chutes across the top are moving downwind (left to right) at less than wind speed, the cart is moving left to right faster than the parachutes and the wind, so the parachutes are moving right to left relative to the cart; they're being wound toward the top left wheel. This model is very much like the "Under the Ruler Faster Than The Ruler" in the video. The parachutes are equivalent to the top of the large black wheel. The wind is the ruler. The cart moves under the wind, faster than the wind, while the parachutes move slower than the wind allowing the wind to continue to push them.

The Blackbird works differently. Instead of parachutes moving counter to the cart's movement so as to continue to be pushed by the wind when the cart goes faster than the wind, the Blackbird has a propeller. The propeller surfaces' positions, along the axis of the cart's movement, sweep counter to the cart's movement so as to continue to be pushed by the wind when the cart goes faster than the wind.

Respectfully,
Myriad

 

[semper]

But how does any of this spooling stuff apply to the downwind situation?

It demonstrates the falsehood of the oft claimed "Law of Conservation of Velocity" and the associated "Law of not being able to outrun your Power Source". See here in a TV interview where Humber uses both these claims;
http://www.xtranormal.com/watch/11174070/

and wait for him to come back to espouse them in some way below.

 

[Llyricist]

It also demonstrates how the prop will turn against the induced torque from the wind at start up, through gearing.

 

[ridgerunner]

... Here's an improved model: ...

Hey that's very clever! +1

(.. imagining it climbing the treadmill incline now ...)

 

[A.T.]

But how does any of this spooling stuff apply to the downwind situation?

It shows that the air mass can push something, that moves in the same direction but faster than the airmass.

Note that this is just a schematic diagram. A real vehicle is completely immersed in the air, so a propeller is a much more efficient way to achieve the same effect.

 

[humber]

Humber now appears to have forgotten what an analogy is. Clue - an analogy to a thing is not the thing itself.

The wind is not a chain. It does, however, provide a driving force, in a particular direction and, with care, at an approximately constant velocity. A bit like a drive chain, in fact.

A cotton reel is not a land yacht. However it does helpfully provide a trivial demonstration of the principle that an object can be driven forwards faster than the thing which drives it.

And none are a sail cart. The point of the uphill argument was to check your analytical skills, and which video would work. So far, even when you do the experiment, you don't get the correct solution, so it's a bit unlikely that a video using that approach would work.
Push the reel up the incline with a pencil. Then do it with the thread.
Work against gravity is the same for both: w= m*g*h
So what can the difference be? The amount of string, nothing else.

 

[humber]

Remind me - was that before or after you posted "No little ones, it won't work on an incline"?

There are reels that will roll backwards. You forgot about the effect the incline has on the normal force.
I posted this diagram, and that should tell you why some roll up, and some don't.
ETA:
There are also effects from the type of thread and binding, but I have no idea of the reel you have, so I need to remove the variables by asking about them.
Moving an elephant is not the same as moving a gnat.

 

[humber]

Pulling the wheeled spool uphill.
Thread coming off the bottom of the spool.
The whole thing climbs up the thread because the thread is spooled -onto- the spool, shortening the distance from the zero point on the thread.. the washer... by the time the thing has moved 1/2 way up the inclined plane.
.
Thread coming off the top of the spool.
The thread gets...longer... puling thread off the spool, as the thing moves up the inclined plane. The second washer is 6 inches from the first, and on the bottom of the spool for the bottom-pull images.
It's seen wrapped up on top of the spool in the second image of the first two.
.
But how does any of this spooling stuff apply to the downwind situation?

Right, I Ratant. You do experiments, take measurements, and try other variants. Despite any vested interest in the cart, you do that, because you want to know. Carteers don't.

The video was complete, and with what I saw as proof, but it's going to take more than that to show it. Carteers are more interested in the "so there" of a video, than they are of its content. Spork often shoots himself in the foot that way, but Carteers never notice. It's all down to the sophistication of the audience.

I posted about the angle of the string, and how that influences the direction in which the reel may roll. Sometimes, the moments producing the rotation either way, are equal, so the reel slides. In practice, the spool will tend to wobble a bit about the finite contact point with the surface.
The incline changes that balance, and moves the contact point away from directly under the centre of the reel.

The twist is that the incline introduces a known amount of work.
The work against gravity is always the same, no matter how the reel is moved. The diameters change the force seen at the hand and at the surface, so are gears. Work = F*d, so doubling the force means that the same amount of work is done over half the distance,so, hey presto, the hand moves half of the un-geared case. It's a bit foolish to see that as the reel moving "twice as fast".
Power = work/time. If you do the same work in half the time, then you need twice the power. If you have that, you don't need the gears.

Gears "trade force for velocity" but there is always a corresponding change in the amount of work that can be done over the same time.

 

[Jack by the hedge]

...And none are a sail cart. The point of the uphill argument was to check your analytical skills...

Can you move the reel up the slope by backpedalling? You're certainly trying hard enough. "Ha ha - I only said it wouldn't work to test you, and you all fell for my trick". You really are five, aren't you?

Regarding the "none are a sail cart" comment, your failing memory lets you down again: you have conveniently forgotten that you have found no argument against the simple video illustrating the principle of a propeller acting as a sail. Want to give it a shot now?

 

[Jack by the hedge]

There are reels that will roll backwards.

There are reels which will roll forwards, and there are reels which will slip. There are no reels which will roll backwards in this configuration.

 

[Jack by the hedge]

Gears "trade force for velocity" but there is always a corresponding change in the amount of work that can be done over the same time.

Are you quite sure?

Work Done per unit Time is Power. I rather thought you declared previously that gears do not provide power.

Or, if you prefer:

P = WD / T
and
WD = F x D
so
P = F x D / T

Now
V = D / T
So
P = F x V

With gear ratio N:1, the force is divided by factor N and the velocity is multiplied by factor N

P = (F / N) x (V x N)
so
P = F x V as before.

 

[humber]

Can you move the reel up the slope by backpedalling? You're certainly trying hard enough. "Ha ha - I only said it wouldn't work to test you, and you all fell for my trick". You really are five, aren't you?

Regarding the "none are a sail cart" comment, your failing memory lets you down again: you have conveniently forgotten that you have found no argument against the simple video illustrating the principle of a propeller acting as a sail. Want to give it a shot now?

It was not a trick, you simply assumed certain conditions. When they fell into your lap, or you rejected the failures, you accepted the result as success.
And yes, you are right, one must treat Carteers as a naive audience.

Sails cannot go faster than the wind ddw, so what is your point?

 

[humber]

There are reels which will roll forwards, and there are reels which will slip. There are no reels which will roll backwards in this configuration.

So you think there can be no reels that will not roll backwards.
You don't need to think about how absurd that is. Take any cotton reel and place it on a table and pull the thread vertically. Good luck in trying to get it to roll up the thread.

 

[D'rok]

There are reels which will roll forwards, and there are reels which will slip. There are no reels which will roll backwards in this configuration.

So you think there can be no reels that will not roll backwards.
You don't need to think about how absurd that is. Take any cotton reel and place it on a table and pull the thread vertically. Good luck in trying to get it to roll up the thread.

See bolded above. Also, check your double negative. Apparently, you, like Pink, don't need no education. In English grammar at any rate.

 

[humber]

Are you quite sure?

Work Done per unit Time is Power. I rather thought you declared previously that gears do not provide power.

Or, if you prefer:

P = WD / T
and
WD = F x D
so
P = F x D / T

Now
V = D / T
So
P = F x V

With gear ratio N:1, the force is divided by factor N and the velocity is multiplied by factor N

P = (F / N) x (V x N)
so
P = F x V as before.

A 1kg load is lifted 1m over 1 second.
Work = m*g*h = 1kg*9.8N*1m = 9.8J
Power = 9.8J.1s = 9.8W
Average velocity assuming constant force = (Vf-Vi)/2 = 0.5ms-1

Use a 2:1 block and tackle.

At the pulling end, force is halved. But work is the same so the distance the rope is pulled is 2m.
Work = force* distance = (9.8/2)*2m = 9.8J
Do that in one second and the power is the same.
To reduce the power demand, the time must be increased to 2 seconds.

 

[humber]

See bolded above. Also, check your double negative. Apparently, you, like Pink, don't need no education. In English grammar at any rate.

And 90 degrees is just an extreme slope. It will roll backwards on any slope when the force of gravity is large enough, and that depends on the angle and the mass of the object. There is no fixed relationship between mass and friction. If scaled, you will find that out the hard way.

Again, it rolls forwards when the friction is high enough to support the angular CW torque. Nothing more than that.

 

[D'rok]

And 90 degrees is just an extreme slope. It will roll backwards on any slope when the force of gravity is large enough, and that depends on the angle and the mass of the object. There is no fixed relationship between mass and friction. If scaled, you will find that out the hard way.

Again, it rolls forwards when the friction is high enough to support the angular CW torque. Nothing more than that.

Can explain to us the internal contortions required to be completely wrong about something, argue vigourously from that mistaken position, and then switch to the contrary position and argue just as vigourously from that position without skipping a beat and while insisting that you were right all along?

Because that's some pretty impressive doublethink right there.

I guess your ultimate position is this: "Whatever anyone who accepts that the cart goes ddwfttw says is wrong, even when they're right, and whatever I say is right, even when I'm wrong." It's a flexible position, because, to be right, all you have to do is contradict the carteers and it is impossible to contradict yourself even when you do just that.