Down wind faster than the wind

[mender]

Do you think NASA would donate ten minutes of wind tunnel time?

The largest wind tunnel in the world is at NASA's Ames Research Center. This subsonic tunnel, which can test planes with wing spans of up to 100 feet, is over 1,400 feet long and 180 feet high.

 

[John Freestone]

Think about this: how does a prop plane, motionless on the ground in still air, start to move forward? When the prop starts turning, isn't it "cutting through the same piece of air all the time"?

I don't think so, do you? When a plane is stationary on the ground in still air with its prop being driven, it has to have its chocks in place and there's a helluva wind being chucked through the prop. It's therefore not passing though the same bit of air, it's constantly sucking in new air. It is moving repeatedly through the same bit of air when it is moving forward wrt the ground in a following wind of the same speed wrt ground, I would have thought. At least, it is somewhere near that. If a prop of a plane as described, took off and got to the point where it is propelling the air backwards, but there were no drag, that would be equivalent to the 'perfect' piece of paper blowing in the wind, and the prop would spiral through the air (no wind wrt the ground). If it is held still in still wind and powered, it's pushing air through it. If it's moving forward but with a tail wind equal to its speed, it is caught in a standing wave, a still piece of air wrt it. It can now only slow the air down by reversing the air flow, pushing more air backwards. The question is whether it can do this propelled, i.e. driven by the wheels (in which case it is not extracting energy from the air directly, but giving energy up to the air), or whether it can somehow extract more energy directly from the air to use in accelerating beyond windspeed. All of this depends, I suppose, on whether this 'standing wave' eventuality happens at cart-going-windspeed, which in my head it seems it should. Does that make sense, or am I being stupid again?

It doesn't need to extract energy from the air around it. If we need more energy, we build a prop with the same pitch but a larger diameter.

You, like a prop, seem to be going round in circles, then. How does a machine ever go as fast as the wind powering it? It 'extracts energy' from the air. If that is wrong terminology, forgive me. It seems to be quite common. The essential problem, reduced again to simplicity, is wind is pushing this cart. Now, common understanding is that anything pushed along by the wind builds up a drag. In perfect situations (mathematical, unreal ones) the motive force of the wind equals the drag at terminal velocity, below or equal to windspeed. It seems sensible to apply the same truth to a machine with a prop. It maybe that I have misunderstood some reason why the prop is fundamentally different, but I would imagine that a bigger prop would attract more drag. This again seems to have the flaw of all previous arguments. It's like saying that if I can't pull myself into a levitation position with these bootstraps, I can build bigger bootstraps, and heave harder.

There's a fallacy that many people are falling into: they are equating "energy" with "speed", or thinking that for a given speed we need a certain fixed amount of energy. If we have a given wind speed, the amount of energy we can extract from it isn't fixed, it's a function of the size of the propellor.

Yes, but a bigger propeller needs more energy to race downwind on a cart and win!

Remember that we don't need a fixed amount of energy to "make the cart go at a particular speed". If the cart is going at a constant speed, we just need enough energy to overcome the forces of friction and drag tending to slow the cart down.

These sentences seem to say the same thing, but one is a negation. The 'fixed amount' of energy we need is the energy that is constantly overcoming the opposing forces, i.e. being dissipated as heat in friction, surely?

These forces aren't fixed either, nor are they tied to the speed.

That's not what I was just told a while back, nor is it what they say in my car manual or the survival schools - if I go faster in my car the aerodynamic drag increases non-linearly; if I don't twizzle my fire-starter fast enough, there won't be enough friction dissipated as heat to overcome its dissipation in the air and I can't light my fire to cook my hat and eat it.

If we want the machine to accelerate, we only need a bit more energy: nobody says it has to accelerate quickly, and it's very light anyway.

To levitate by pulling on my bootstraps, I only need to accelerate vertically off the ground very very slowly. How much should I weigh to succeed?

I'm not quite sure what you mean here. If the machine is only in contact with the fluid it's travelling through, it cannot extract any energy from it. A machine that is only in contact with the air can move through it if it has its own motor (plane or helicopter), or simply move with it if it doesn't (balloon). To extract energy from the wind, it's essential that the machine has contact both with the ground and with the air. Look again at my cotton reel example: it is essential that the reel has contact both with the surface of the table and with the strip of paper.

My fault you misunderstand this. My little imaginary invention does stay in contact with fixed surfaces and a fluid flow.

We're going to need a really long wind tunnel. In fact the exact equivalent to a test with the machine rolling downwind along a vast wind tunnel is the test with the machine in still air rolling on a treadmill, but for some reason many people don't believe this. Somebody said this before: a wind tunnel is nothing more or less than a treadmill for aeroplanes.

Ah yes, of course! That is an annoying complication, because, as you say, the treadmill 'equivalence' is another area for people to doubt. But I wonder if you can't, in fact extend a wind-tunnel into a longish pipe, scale the whole thing down...ah, is that what my gizmo is for after all?!

 

[ThinAirDesigns]

John, I would guess that your 'pipe racer' could be made to work just fine.

Now, you only have to build it, figure out how to demonstrate it, then sit back and listen to everyone tell you how bogus your test is and how it doesn't prove anything.

I support your quest fully.

JB

 

[John Freestone]

Prove it. Show where I am wrong.

I said that I believe you don't understand the math either. If you understood that sentence, you would not demand that I prove you wrong.You just jumped on me for not being bothered to approach the maths. I imagine it would take up even more of my time, and this is quite enough. If you just want to undermine my confidence here, let me know and I'll ignore you.

If you have 'the math' that you think you do, why not send that to NASA, have them check it, and we can all move on with our lives. There are 30 more brainteasers, at least. People who have done physics and maths all their lives are struggling. Please pick on someone else.

 

[John Freestone]

John, I would guess that your 'pipe racer' could be made to work just fine.

Now, you only have to build it, figure out how to demonstrate it, then sit back and listen to everyone tell you how bogus your test is and how it doesn't prove anything.

I support your quest fully.

JB

Thanks JB. I'll start by building a tread-pipe...

 

[Dan O.]

Let's try a little physics instruction in a virtual world far away from the everyday surroundings of space, time, gravity and the friction that bogs everything down.

In this sample world we have only kilograms of mass, joules of energy and velocity in meters per second. The basic rule that applies to this world is that if you have two masses of 1 kilogram each, you can expend one joule of energy to accelerate one of the masses by +1 meter per second and the other mass by -1 meter per second.

Your first task in this world is to derive how many joules are required to get the masses moving at +2 and -2 meters per second using only the basic rule provided.

(For extra credit, do +/- 3 meters per second)


By solving this simple problem, you will have everything you need to see where the energy needed to propel the cart faster than the wind comes from.

 

[Thabiguy]

When a plane is stationary on the ground in still air with its prop being driven, it has to have its chocks in place and there's a helluva wind being chucked through the prop. It's therefore not passing though the same bit of air, it's constantly sucking in new air. It is moving repeatedly through the same bit of air when it is moving forward wrt the ground in a following wind of the same speed wrt ground, I would have thought.

Think again about what you just said.

There's no difference between a stationary plane in still air, and a plane moving forward at the same speed as the wind. For the propeller, the situation is absolutely identical. In fact, if you were sitting on the wing and could only observe the air, there would be no way to tell which situation it is.

Think about it.

 

[humber]

I just read Charles Platt's "circle test". It would prove nothing more than the cart would start to move when a fan is placed behind the cart. The cart would then move forward, out of the influence of the fan (the power source) and then slow down. It is not in any way equivalent to a test in a wind. If evidence of a self-start in a wind is the object of this test, the result has already been provided.

This is a test doomed to failure of the actual question but seen as a legitimate test by those who think the cart is a fraud. A further indication of that flawed thinking was Platt's statement that if Goodman's cart isn't a hoax, a simple push should send it fleeing into the distance at an ever increasing rate in no wind. This is not an over-unity or even perpetual motion device.

Rejecting the evidence of wind speed as indicated by the streamer in the Goodman outdoor test then proposing that styrofoam pellets would be a better method of showing the relative wind speed strikes me as odd. I would suggest a test that utilizes outdoor conditions that I see fairly regularly but I'm positive that the results of that would be interpreted as needed to support whatever theory the dissenter ascribes to.

Therein lies the rub.

Mender, I will get back you concerning the treadmill.
Platt's test is quite useful because it eliminates the problems of suitable wind, replication, and scrutiny of the test and results. Anyway, as money is perhaps at stake, the test would need to be more controlled.

The fan is perhaps good enough for an informal test, but its output will depend upon the load and velocity. The fan could be replaced with a regulated source. It would be awkward to maintain the distance from the cart by hand, but a dolly or similar device, could solve that. Perhaps even regulate itself.
Measurement of the cart can be made against the reference source, either by external means, or on the cart itself. The beads would be only an indicator.

If the cart could achieve windspeed, in the manner claimed, Platt is right; by inference , the cart should continue to travel without the fan.

 

[huh34]

A shame I live in Germany and am not stinking rich: it would be really exciting to see a ride-on version.




I was imagining the lightest possible version, basically just the little cart scaled up somewhat. Of course it will be much more impressive if it actually transports a person. Do you already have an idea of what the ride-on version would look like?

I took the liberty of drafting the first version, without anyones permission. Pic attached, like it?

 

[humber]

.
<snip>
In this sample world we have only kilograms of mass, joules of energy...

In the same simple world, I can accelerate an oil tanker as I please.
Two airstreams, 10kmh, relative to each other. Reverse their velocities. Now the relative velocities have increased. What has happened?

 

[mender]

Platt's test is quite useful because it eliminates the problems of suitable wind, replication, and scrutiny of the test and results. Anyway, as money is perhaps at stake, the test would need to be more controlled.

Interesting. How does it address the faster than wind segment of the claim?

 

[humber]

Interesting. How does it address the faster than wind segment of the claim?

You are seeing the test as a failure because you presume the outcome.

 

[Dr. Trintignant]

A very interesting topic which, sadly, I have not been able to read in its entirety. Hopefully my thoughts haven't been repeated earlier.

Although I realized immediately that it was possible in principle to build a faster-than-wind machine, it wasn't clear that this particular machine would work until I thought about it some more. I think I have a simple argument why the machine works without resorting to anything tricky like vector analysis.

Let's assume we have a machine traveling exactly at the wind speed. We first ask how much work is being put into the vehicle over time. The definition of work is force times distance, so over some particular duration, the vehicle has traveled some distance, and the roadway has exerted force on the vehicle over that span. As such, the vehicle receives some finite, non-zero amount of work.

Based on conservation of energy, we know that the propeller can't perform any more work than that if there is no other energy source. So how much is it? The answer: exactly zero. Since the air is not moving relative to the vehicle, the propeller does not perform any work on the air, even with an arbitrarily large force. As such, it does not violate conservation to argue that the forward force from the propeller can greatly exceed the backwards force from the roadway--hence, the vehicle accelerates.

Of course, as soon as that happens, there is a net wind velocity, and the propeller now starts doing work. However, this amount is small at first, and the propeller can still exert a large force.

Given that the idea is plausible, let's look at it in more detail. Here's an equation for balancing work, taking efficiency into account:

[latex]
\begin{math}
\begin{subequations}
t = \mbox{time duration}\\
V_a = \mbox{air velocity}\\
V_g = \mbox{ground velocity}\\
F_p = \mbox{propeller force}\\
F_g = \mbox{ground force}\\
E = \mbox{system efficiency (number from 0 to 1)}\\
F_p V_a t = F_g V_g t E
\end{subequations}
\end{math}
[/latex]

Now, let's assume we're at steady state, with the vehicle neither accelerating nor decelerating, so that Fp = Fg. Also, if the wind velocity is Vw, then Va must be (Vg - Vw). Our new equation, after rearrangement and canceling, looks like this:

[latex]
\begin{math}
V_g - V_w = V_g E
\end{math}
[/latex]

Rearrange a bit more and we get this:

[latex]
\begin{math}
\frac{V_g}{V_w} = \frac{1}{1 - E}
\end{math}
[/latex]

The fraction Vg/Vw is just what multiple of wind velocity we can achieve, and here it's expressed in terms of system efficiency. As you can see, you don't need a particularly high efficiency to achieve noticeable gains: a 33% efficient system will get you 50% faster than wind velocity. A 67% efficient propeller system can go 3 times as fast as the wind.

So in conclusion, it's clear that the vehicles shown are not just plausible, but downright probable. A simple, inefficient mechanism is enough to demonstrate the effect.

- Dr. Trintignant

 

[Michael C]

Let's assume we have a machine traveling exactly at the wind speed. We first ask how much work is being put into the vehicle over time. The definition of work is force times distance, so over some particular duration, the vehicle has traveled some distance, and the roadway has exerted force on the vehicle over that span. As such, the vehicle receives some finite, non-zero amount of work.

Based on conservation of energy, we know that the propeller can't perform any more work than that if there is no other energy source. So how much is it? The answer: exactly zero. Since the air is not moving relative to the vehicle, the propeller does not perform any work on the air, even with an arbitrarily large force.

Stop right there! Look at the vehicle we're discussing. When it is travelling at exactly wind speed, the propeller is rotating. The propeller is certainly doing work against the air, just like an electric fan sitting in your living room.

In fact the propeller will go on doing work against the air until the vehicle has accelerated to a speed where the energy being extracted from the motion of the air relative to the ground is at a minimum.

 

[Dr. Trintignant]

Stop right there! Look at the vehicle we're discussing. When it is travelling at exactly wind speed, the propeller is rotating. The propeller is certainly doing work against the air, just like an electric fan sitting in your living room.

Just because the propeller is rotating, it doesn't necessarily mean that it is performing work. Just as holding a big rock in place may require great effort, but also requires zero work.

Your electric fan is only doing work because it is designed to move air, not to produce force. One could design a fan to generate the same force with arbitrarily little work, just by making it larger. Moving twice the air at half the velocity produces the same force but only requires half the work.

Or, imagine that instead of air, we have some very viscous fluid. The propeller can barely rotate, and therefore the fluid is hardly being accelerated at all, and so almost no work is being done. However, a large force can still be generated.

- Dr. Trintignant

 

[humber]

A very interesting topic which, sadly, I have not been able to read in its entirety. Hopefully my thoughts haven't been repeated earlier.

Although I realized immediately that it was possible in principle to build a faster-than-wind machine, it wasn't clear that this particular machine would work until I thought about it some more. I think I have a simple argument why the machine works without resorting to anything tricky like vector analysis.

Let's assume we have a machine traveling exactly at the wind speed.

That is largely what is assumed to be demonstrated by the treadmill. However, getting this zero relative velocity would be the problem. First it would be necessary to defy Newton, only to later invoke him in support.
If that were the case, it would already be possible, so it would then be a matter of explaining how this particular cart works.

 

[Dr. Trintignant]

That is largely what is assumed to be demonstrated by the treadmill. However, getting this zero relative velocity would be the problem.

Are you denying that a vehicle traveling at the same ground velocity as the wind will have a zero airspeed velocity? If so, you have greater problems than basic physics.

If that were the case, it would already be possible, so it would then be a matter of explaining how this particular cart works.

The exact details are irrelevant. I demonstrated why the propeller can exert a greater force than the roadway for a given amount of work, even considering a less-than-perfectly efficient system.

- Dr. Trintignant

 

[humber]

Are you denying that a vehicle traveling at the same ground velocity as the wind will have a zero airspeed velocity? If so, you have greater problems than basic physics.

If a vehicle is travelling at windspeed, then it could be argued that there is no relative velocity, but the cart first has to get there. Thanks for the tip.

The exact details are irrelevant. I demonstrated why the propeller can exert a greater force than the roadway for a given amount of work, even considering a less-than-perfectly efficient system.

- Dr. Trintignant

If you have truly constant force, then I suppose by some means it may be possible to accelerate away from it, otherwise that acceleration may simply reduce the driving force by the same amount.

 

[Dr. Trintignant]

If a vehicle is travelling at windspeed, then it could be argued that there is no relative velocity, but the cart first has to get there. Thanks for the tip.

I agree that I have not demonstrated that such a vehicle can accelerate to the wind speed; only beyond it (and then only to a point dictated by the efficiency). That is not a particular problem for me since we already know other ways to get to wind speed; an ordinary sail is one of them.

If you have truly constant force, then I suppose by some means it may be possible to accelerate away from it, otherwise that acceleration may simply reduce the driving force by the same amount.

The force is only constant for a given velocity, but yes, this produces an acceleration when at wind speed. As the vehicle velocity increases, the propeller force decreases until it balances with the force from the roadway, at which point the vehicle is at steady-state.

- Dr. Trintignant

 

[CyCrow]

Stop right there! Look at the vehicle we're discussing. When it is travelling at exactly wind speed, the propeller is rotating. The propeller is certainly doing work against the air, just like an electric fan sitting in your living room.

In fact the propeller will go on doing work against the air until the vehicle has accelerated to a speed where the energy being extracted from the motion of the air relative to the ground is at a minimum.

True, to get a force with no work, the air would have to be solid. As we generate thrust by accelerating air, the work the propeller does is transferred into kinetic energy in the air + losses.

The power needed to generate a given force depends on the mass of the affected air, which depends on the prop diameter and pitch, and the relative velocity of the air and the vehicle. If you do the math with the cart as described, you should end up with sensible numbers for thrust and efficiency.

// CyCrow