B How Does Blackbird Leverage Wind Power to Move Faster Than the Wind?

[N1206]

Yeah, pretty much. The kite example is pretty good. Of course for people who are not willing to examine their beliefs, no argument does anything. The most remarkable aspect to me is just how anti-scientific some people get when the feel the need to defend science.

I guess a fairly general pattern is that a person realizes the wheel-propeller mechanism is supposed to do "something", but because they have a simple wind equation in mind they have already decided that no matter what, "it can't create additional power". This is the thought they use to convince their own mind to avoid thinking about it any further. It's a classic short-hand to avoid a rational analysis.

This makes them see anyone who implies otherwise as a "crank who obviously doesn't even understand basic physics". So even when it's pointed out it's just a leverage mechanism, that's never given any thought because it's "coming from a crank anyway".

The denial of various real-world demonstrations of the same principle is where it really gets hilarious, and tends to be where these people usually reveal their poor ability to reason about things. Almost without fail they will offer an explanation that would violate the known laws of physics.

And when their argument is reduced to some obvious absurdity, they usually stop responding to it (or lock a thread). It's not that they realize they were wrong. It's just that they realize they have no explanation - at least not yet - but they also are still convinced they are absolutely correct "somehow", and get back to the comfort of that initial reasoning allowing them to avoid thinking altogether.

I think intuition, and our education fail us. We think of this like collisions. 'I throw a ball at 20 m/s and hit a motionless object sitting on a frictionless surface, how fast does the object I hit move?' But if I throw a ball at 20 m/s at an object moving at 20 m/s, how much do I change its velocity? Well if they are both vectored the same, the ball never hits the object. But wind is NOT like throwing balls! It is NOT like throwing infinite balls of infinitesimal mass. And so we suffer a failure of imagination.

We have similar failures when someone thinks about lift on aerofoils and starts in on Bernoulli, and upward pressure on the wings and how is THAT supposed to work? But if you think about it as balance-of-forces, then all of a sudden things begin to make sense. A hovering helicopter is pulled down by a force equal to mass times g. It has to apply an equivalent force to a mass of air in order to hover. The aerofoil shape forces air to accelerate in a vectored way. THAT makes sense -- but it is not generally how lift is taught to people. And I think very, very few people actually get formal education on fluid flow--but think they can extrapolate from common sense and what they have been taught.

Fluid Flow.
A physicist is quoted as saying he'll have two questions for God. "Why relativity?" and "Why turbulence?" :)

 

[InkTide]

My intuition for this whole thing was "huh, they're taking advantage of the energy gradient between the motion of the wind and the motion of the ground to do work, neat" and I moved on.

Their demonstration on a treadmill is literally just reversing the direction of the gradient by moving the ground instead of the air. The physical equations to represent it didn't seem necessary to explain it - maybe the obsession is a result of paper representations superseding demonstrations and experiment in the education system.

 

[Baluncore]

My intuition for this whole thing was "huh, they're taking advantage of the energy gradient between the motion of the wind and the motion of the ground to do work, neat" and I moved on.

That is the essence of the thing. The differential velocity of two materials provides the source of energy. The various possible engineering solutions to energy extraction, and the propulsion reference, simply cloud the issue.

 

[Michael C]

My intuition for this whole thing was "huh, they're taking advantage of the energy gradient between the motion of the wind and the motion of the ground to do work, neat" and I moved on.

Their demonstration on a treadmill is literally just reversing the direction of the gradient by moving the ground instead of the air. The physical equations to represent it didn't seem necessary to explain it - maybe the obsession is a result of paper representations superseding demonstrations and experiment in the education system.

If you immediately see that the cart running on the treadmill is doing exactly the same thing as the cart moving along the ground in the wind, you are a whole leap ahead of many people who can't/won't accept this, equations or not.

 

[Mike S.]

This system is much simpler than a lot of people are making it out to be, including Derek.

a) Wind can press forward on any surface moving slower than the wind.
b) A surface moving at an angle to the wind is moving slower than it in one dimension. (that's why a sailboat can sail fast at an angle to the wind)
c) A surface sliding across the wind can move forward as fast as the point of the wind it is being pushed by - not the point of the surface. That's why a sailboat can tack back and forth fast enough to beat a balloon: because the forward part of the sail is replaced by the backward part of the sail (closer to the source of wind) as it moves.

Beating the wind in this boat, duplicating a boat tacking on a cylindrical Earth as in Derek's lovely video, requires a, b, and c.

Thinking of the propeller as a "fan moving the boat forward" is in my opinion true but confusing. Its point isn't to fan the air backward but to fan itself backward to meet the air. The air is still moving forward against the blade (referenced to the ground) as the vehicle moves forward. It is pushing the vehicle forward, and some of that forward momentum is siphoned to keep up the leverage that allows it to go forward.

It's not really any more confusing than the idea that humans can run forward, pushing against the ground even though they are moving forward, by using their feet. We're just not accustomed to the system.

 

[AnssiH]

Yeah, there seems to be many very intuitive ways to understand this. For me I just instantly started to think about two objects, like a rope moving inside a pipe, and thinking whether it would be possible to latch on to the rope, press a wheel against the pipe, and then gain leverage to move faster than the rope. The answer is very obviously yes.

But clearly different people have different kinds of intuitive sense, so what works for one person might not work for another. I think the biggest obstacle some people have must be that they simply try to convince themselves for reasons to not have to think about this further, and that escape is offered by the thought "no matter how you cook wind momentum, you can't turn it into faster speed", which as we know is patently false.

So that's why when you say the magic word "leverage", it does nothing because it sparks no motivation to think about the system.

On that topic, I attached a picture of yet another very simple way to think about the fact that no laws of physics are being violated. It's a long screw that is driven by a wheel. As soon as the screw is partially inside the wood beam, surely it's possible to push the wooden beam along, and have the wheel roll against a ground in such a way that the screw is moving faster than the wood beam.

It should be pretty obvious that that is analogous to gaining leverage from the momentum between two mediums. And it should be obvious that it must be physically possible for the screw to indeed push into the wood. And it should be obvious that propellers and screws are quite similar - there's just more slippage for the propeller. But that should not be a problem when we discuss about the theoretical possibility of moving faster than the medium you are "powered by" (in semantical sense).

 

[A.T.]

...because the forward part of the sail is replaced by the backward part of the sail (closer to the source of wind) as it moves.

Yes, the intersection of a streamline with the sail/blade is moving slower than the air along that streamline so the air can still push the sail/blade forward:

 

[Ranger Mike]

 

[Halc]

Hasn't this been covered already?

Yes, the thing is wind powered and 1) goes straight upwind, and 2) goes downwind faster than the wind.
This isn't a violation of any physics, however unintuitive it may seem.

I didn't watch the video, but I recognize the blackbird.

 

[Averagesupernova]

Hasn't this been covered already?

Yep. So has pulling on a bicycle pedal making the bike go in the opposite direction. Etc, etc...

 

[Halc]

Yep. So has pulling on a bicycle pedal making the bike go in the opposite direction.

Ah, but when it's going downwind the wheels do the energy input (exert drag) and the prop does the propulsion, sort of like the bike wheels making the pedals go instead of the other way around.

 

[berkeman]

Thread closed for Moderation...

Thread reopened while we search for the previous thread(s)...

 

[berkeman]

Hasn't this been covered already?

Multiple times, and I'd like to merge this new thread into the latest one, but I'm having trouble finding it. I seem to remember it was based on a good video about some bet that somebody won doing a test with a prototype. Anybody able to find it? Thanks.

 

[vela]

Multiple times, and I'd like to merge this new thread into the latest one, but I'm having trouble finding it. I seem to remember it was based on a good video about some bet that somebody won doing a test with a prototype. Anybody able to find it? Thanks.

Isn't that the video linked to in the OP of this thread?

 

[berkeman]

Isn't that the video linked to in the OP of this thread?

You think it was the same video? I'm not sure how to search on that. Plus, I seem to remember that the still picture for the video in the previous thread showed the vehicle sailing downwind on a desert flat of some kind, not faces...

 

[vela]

You think it was the same video? I'm not sure how to search on that. Plus, I seem to remember that the still picture for the video in the previous thread showed the vehicle sailing downwind on a desert flat of some kind, not faces...

The thread below?

https://www.physicsforums.com/threa...ckbird-exploring-wind-power-leverage.1009943/

 

[berkeman]

The thread below?

https://www.physicsforums.com/threa...ckbird-exploring-wind-power-leverage.1009943/

You are my hero! Thanks. Merging this new thread start by @Ranger Mike into that thread. Thanks vela.

 

[Halc]

You are my hero! Thanks. Merging this new thread start by @Ranger Mike into that thread. Thanks vela.

This topic is probably the better thread. But I found some older posts, but part of another general thread on wind powered land vehicles.
https://www.physicsforums.com/threa...ks-also-use-sails.1000632/page-2#post-6473245

I just did a search on "blackbird" "wind power" and all the good stuff came up

 

[A.T.]

Hasn't this been covered already?

Here the latest previous related discussion, which shows how counter-intuitive the basic mechanical principles behind it are to some, so it will come up again and again:
https://www.physicsforums.com/threa...king-physics-3rd-edition.1048870/post-6860418

 

[sophiecentaur]

To my experience, you can roughly divide people to two types of learners; those who tend to memorize things, and those who tend to understand things.

What do we mean by the word "understand"? Hard to say in the case of an on-going discipline like Science. To understand the present state of knowledge doesn't mean your picture will be the correct one in the future. It can only mean that you have arranged all the facts (at the time) in your mind in a way that's self-consistent and which allows you to extrapolate reliably and to predict accurately what will happen in new situations / experiments.

The depth to which the individual feels they can do that will vary. Many people feel that they understand something on the grounds of what they have merely memorised it but how many of us fully apply the criterion above before feeling we have understanding? (Some smart Alec on PF can come along and shatter that understanding so easily.)

Basic Maths is a field where I believe that we can justify saying that we 'understand' because we can repeat endless arithmetic problems and always get the right answer. But that's because it deals with axioms, I guess.