"Faster than the wind," impossible they say but the Blackbird settles the debate

[jakobnev]

1) the little model is not air driven.

To an observer riding on the conveyor belt it is.

How would you respond to someone claiming that wind tunnel tests are meaningless because the air is moving instead of the car?

[oil pan 4]

It appears more people are on the verge of reproducing this experiment.
A simple "I told you so" by years end should work.

[RedDevil]

I feel confident enough in understanding how it works that I can do predictions of what happens if you change things.

If you'd run the Blackbird backwards it would still run downwind faster than the wind.
Likewise, if you put the model backwards on the treadmill it would still run faster than the tread.
If you change the gearing on Blackbird so that the propeller moves air faster than the wheels run over the ground (instead of slower), then it would run straight against the wind instead of with it. It would do so even backwards.

[Xist]

Man! I thought that I felt sleepy before I opened this thread!

I saw some of these videos on YouTube, but I don't expect to view them.

I haven't watched much by Veritasium, although I will give him the benefit of the doubt, but since Aerohead mentioned scientific rigor.

Everything that I know about scientific rigor I learned from XYCD:

I found these along the way:

[oil pan 4]

Bookkeeping. Haha I like it.

[RedDevil]

Well, perpetuum mobiles pretend to work without a source of power. they create something out of nothing, like the 53th card in a deck.

The case this thread is about, and all of its examples, do have a power input: the speed difference between two media.

Some people struggle with that concept. Like the thread mill experiment: if there is no wind, nothing can move the car they think.

They fail to grasp that while there's no wind, the tread does move. The propeller can still create a force by turning. That force allows the wheels to drive the shaft that turns the propeller.

As for scientific rigor: notice how Aerohead refers to a lot what I've written but nothing about the equations and calculations, nothing about how the propeller moves the air backwards relative to the car but forwards relative to the ground.

Aha, the circle goes round, perpetuum mobile after all?
Not so fast.
The propeller does not have tu push the air away as fast as the thread runs!
It does move slower than the thread, so it requires less power than the wheels provide when they turn over a greater distance under the same force!

Just like the roller where the top wheel is turning against the movement of the plank, but slower than the wheels that touch the floor.
It is essential that the top roller goes slower than the floor rollers, just like it is essential that the propeller pushes the air away at a lower speed than the wheels turn.

But if that is too difficult to get and making jokes and insulting the people who do get how it works and accusing those who actually built these crafts is up your alley, well... we're done talking then.

[freebeard]

I thought that Permalink #84 was conclusive. Science is where you publish your result so anyone can try to reproduce it.


https://www.thingiverse.com/thing:4897423

Thingiverse was down for a day or two, but it's back up today.

[some_other_dave]

Originally Posted by aerohead

I have no skin in the game and am not convinced that Blackbird cannot do what has been purported. I would just like to see a fully-instrumented test, and all data and specifications.

So, either aerohead is guilty of moving the goalpost here, or we are all guilty of misunderstanding his objections to this in the first place.

Apparently, he just wants all of the numbers so he can run the equations himself, rather than saying that it can't be done.

I am not aware of anywhere that all of the numbers have been published, so I don't think he will be satisfied by anything that we can produce here. So there is no further point in participating in this.

-soD

[RedDevil]

The car should work for a whole variety of parameters, so he can take an educated guess at anything he wants to calculate about - just as long as everything meets the following criteria:

- The car moves downwind,
- The propeller pushes the air back more slowly than the car moves over the ground,
- The wind puts a positive force on the propeller and
- The car forces the wheels to turn over the ground, driving the propeller.

While the propeller pushes the air backwards compared to the car, it still moves forward compared to the ground, just slower than the wind.
Therefore the wind does put pressure on the propeller, thrusting the car forward.

When the car is not accelerating the force on the propeller and the wheels is roughly equal, but the wheels are moving over a greater distance than the air moves through the propeller, so less power is required to turn the propeller against the wind than the wheels provide.

Just pick some reasonable numbers for anything you want. It will work.

No need to wait until the make of the propeller, the viscosity of the oil in the bearings, the weight of the driver and the humidity of the air are determined and verified by an army of lawyers.
If you see how it works you'll understand none of that is necessary.

Keep it simple. Don't drag in all sorts of data that you really don't need.

[RedDevil]

Imagine a paddle boat in a narrow river.
On both sides of the river is a road.
The paddle boat has an extension with wheels on either side.
The wheels are on the road.
The wheels are geared to the paddle.
The paddle moves the water only half as fast as the wheels go over the road.

This paddleboat will go faster than the stream, just like Blackbird goes faster than the wind.
As the paddles move the water backwards compared to the boat, it does so less fast than the wheels move; so the scoops of the paddle actually move forward, even when they touch the water. Just less fast than the boat.

So the water flow in the river will push against the paddle scoops until the boat goes twice as fast as the water. It won't get just that fast if there is any friction, but it will go faster than the stream.
The boat would go as fast as the stream anyway if the wheels don't touch the road, so with the added push of the paddles for half the resistance on the wheels (as they cover twice the distance to power the paddle) it would go faster.

This is of course the same effect as the Blackbird. The boat could use a propeller instead of a paddle wheel for a more modern approach and an even closer resemblance