Down wind faster than the wind

[Modified]

The concept contained in the physics-bite you quoted is simple enough, but I fail to see why only the parameters of convenience are transferred to the new frame.

Only the belt, air, and vehicle matter to the vehicle's performance, and those three are the same relative to each other in both frames.

It is so selective in these parameters, such that it seems fit only for one peculiar situation; always moving at windspeed.

Because most treadmills are small. If you don't keep moving at air speed, you quickly find yourself off the belt. Fortunately air speed is at or near the speed of interest for these tests.

Given that the treadmill itself is a friction controlled force balance, I wonder how any of this can be related to the verification of the proposed design.

I expect that most treadmills have feedback to attempt constant speed under varying loads. In this case though, the vehicle is light enough that its effect on the treadmill is nearly negligible, not to mention that variations in belt speed would hurt performance rather than help.

The claim of windspeed seems not to be falsifiable. It is at windspeed, or not on the belt.
In practice, there will certainly be turbulence.

It will be the same on the belt. Of course any air currents may interact with the walls of the room, so you need a large enough room to simulate the open road. Putting the treadmill in a tight-fitting box might cause problems.

 

[Myriad]

I have the sudden image of some of the posters to this thread with bloody hands caught between bicycle chains and sprockets.

Hmm, good point. I meant to edit that line before posting, because although you can test this with a bicycle there are practical difficulties. For one thing, since most modern bicycles have derailleur chain tensioners on both sprockets, you can't really apply useful force to the lower part of the chain, even if you're not concerned about greasy -- or bloody -- fingers. (You can push against a pedal that's in the 6:00 position instead, but only for a short distance.) Also, because of the ratchet mechanism, you can only do it with backward movement of the bike; that is, the bike should be oriented with its handle bars to the left to match up with the downchain-crawler diagram.

Under those conditions, and in most gear shift settings, standing on the ground and pushing to the right on a pedal on your side of the bike, which is near its 6:00 (lowest) position -- which if you were riding on the bike would propel the bicycle forward, that is to the left -- instead moves the bicycle to the right, backwards, a greater distance than the distance you move the pedal.

It does work in the end, but as a way of clarifying matters, that demonstration is probably not very effective!

Respectfully,
Myriad

 

[Modified]

It would not be possible to transfer the force from the belt to the generator.
without motion. The increased load will increase the contact resistance, so the car will move back with the belt, and therefore accelerated.

It will be decelerated with respect to the belt, yes. The same as its counterpart will be decelerated with respect to the road.

The car is said to be equivalent when stationary.
Also, the amount of energy absorbed by the vehicle, if it were possible to connect the generator, cannot be said to be dependent upon the mass of the vehicle.

I'm not sure what you mean by "absorbed by the vehicle". If you mean "available to the generator", then of course it depends on the mass of the vehicle.

 

[humber]

It will be decelerated with respect to the belt, yes. The same as its counterpart will be decelerated with respect to the road.

I'm not sure what you mean by "absorbed by the vehicle". If you mean "available to the generator", then of course it depends on the mass of the vehicle.

If the behavior required of the vehicle to meet equivalency, is that it becomes that vehicle, then of course they will not be equivalent but the same.
This would mean that the kinetic energy must be excluded from the claim of equivalency.
To meet its requirement, the belt must be capable of supplying sufficient energy. The load taken from the belt, will simply be transferred by the wheels directly to the generator, independently of the mass of the car.

 

[Molinaro]

Hello all. I have spent the past 2 days reading this entire thread, and watching the videos. I would first like to outline my understanding of what is occuring with the device on a treadmill.

In this case, I would like to consider the situation where there is no slippage of the wheels on the treadmill. In this case the wheels turn, which turns the axle, which through a gearing mechanism turns the prop shaft and of course the prop. This generates a thrust by blowing air.

Since I've stated that their is no slippage of the wheels, this means that the thrust generated by the prop is currently smaller than the friction between the wheels and the treadmill.How do we change that relationship? We can overcome the friction in 2 ways.

We can speed up the treadmill so that the wheels are unable to stay in contact as the treadmill slides beneath. This will not assist in generating any forward motion of the device relative to the stationary ground on which the treadmill rests.

Or, we can change the gearing ratio so that the same speed of rotation of the wheels instead results in a faster rotation of the prop. This will create greater thrust and when large enough it will overcome the friction preventing the wheels from slipping. This slippage results in the movement of the entire device forward on the treadmill. That is forward with respect to the ground on which the treadmill rests.

This is what we see in the video. And the creator of the device mentions the need for such a minimum gear ratio for it to work.
Am I correct that this slippage occurs and the device is glidding forward on the treadmill?

With the device on solid ground and steady wind I would like to consider the case with no slippage and no acceleration and the device moving slower than the wind.

Here the wind is turning the prop at some constant speed based on the prop's current resistance to rotation and the force exerted on the prop by the wind. This will eventualy reach a point resulting in constant speed.

How do we change the situation without changing wind speed? We can do that by changing the gear ratio so that we get the wheels to spin faster with the same prop speed. The prop speed will again be constant with a constant wind and constant resistance to rotation. So again in this situation it's a matter of selecting the correct gearing. I think it's the importance of the gearing that is being completely missed by humber. They have seemed to have latched onto an insistance that no matter the gear ratio, the end resulting speed of the craft will always be slower than the wind.

If the gearing is too fast, it will cause the wheels to slip and that will slow the vehicle's acceleration. Note that slippage in this case acts opposite how slippage affects the vehicle on a treadmill. Is that a nit to pick or is it the expected symmetry?
Well that's where I'm at in my thinking. Tell me where I'm wrong because I like to learn

 

[Modified]

If the behavior required of the vehicle to meet equivalency, is that it becomes that vehicle, then of course they will not be equivalent but the same.
This would mean that the kinetic energy must be excluded from the claim of equivalency.

Not at all. In both cases the vehicles have the same kinetic energy with respect to their surfaces before, after, and at all times during stopping.

To meet its requirement, the belt must be capable of supplying sufficient energy.

The same is true of the earth. The earth is big enough for any acceleration to it caused by the car to have a negligible effect on its velocity. For the same to be true of the treadmill, we require a large and powerful treadmill, or a small car. If you can't get past that idea for a full-sized car, then just imagine the deck of a moving aircraft carrier in place of the treadmill belt.

The load taken from the belt, will simply be transferred by the wheels directly to the generator, independently of the mass of the car.

The car is being decelerated through the generator, so the mass does matter. If the car is very light, then with the same generator attached it will decelerate (with respect to the surface) much faster, just as with the car on the road.

 

[humber]

Not at all. In both cases the vehicles have the same kinetic energy with respect to their surfaces before, after, and at all times during stopping.

They do not have the same kinetic energy.

The same is true of the earth. The earth is big enough for any acceleration to it caused by the car to have a negligible effect on its velocity. For the same to be true of the treadmill, we require a large and powerful treadmill, or a small car. If you can't get past that idea for a full-sized car, then just imagine the deck of a moving aircraft carrier in place of the treadmill belt.

Yes. I meant the power of the belt is not an issue.

The car is being decelerated through the generator, so the mass does matter. If the car is very light, then with the same generator attached it will decelerate (with respect to the surface) much faster, just as with the car on the road.

The generator is but a brake for the belt motor, and that can be arbitrarily assigned, whereas the kinetic energy of the real vehicle is determined by its mass and velocity.

 

[sol invictus]

<snip>

Are you going to address Myriad's chain-driven device a few posts above? That's the cleanest I've seen yet, and it's obvious how it will move. Do you agree it will go down chain faster than chain-speed?

If so, why are you still arguing?

And if not, why not?

 

[Thabiguy]

Well that's where I'm at in my thinking. Tell me where I'm wrong because I like to learn

Okay then. (And that's a good attitude, btw!)

Since I've stated that their is no slippage of the wheels, this means that the thrust generated by the prop is currently smaller than the friction between the wheels and the treadmill.How do we change that relationship? We can overcome the friction in 2 ways.

The sliding friction between the wheels and the treadmill can be assumed to be very large indeed. And that's a good thing; why would you want to overcome it? You don't want the wheels to slip.

We can speed up the treadmill so that the wheels are unable to stay in contact as the treadmill slides beneath. This will not assist in generating any forward motion of the device relative to the stationary ground on which the treadmill rests.

This is not what you want to happen.

Or, we can change the gearing ratio so that the same speed of rotation of the wheels instead results in a faster rotation of the prop. This will create greater thrust and when large enough it will overcome the friction preventing the wheels from slipping. This slippage results in the movement of the entire device forward on the treadmill. That is forward with respect to the ground on which the treadmill rests.

Neither is this. The device doesn't get ahead by slipping forward. It gets ahead by means of its wheels rolling faster than the belt (wrt ground).

This is what we see in the video. And the creator of the device mentions the need for such a minimum gear ratio for it to work.
Am I correct that this slippage occurs and the device is glidding forward on the treadmill?

No, I'm afraid not. The device is moving ahead in the same way that a car does - by rolling its wheels, not by slipping.

With the device on solid ground and steady wind I would like to consider the case with no slippage and no acceleration and the device moving slower than the wind.

Here the wind is turning the prop at some constant speed based on the prop's current resistance to rotation and the force exerted on the prop by the wind. This will eventualy reach a point resulting in constant speed.

The speed at which the propeller is turning is proportional to the speed of the wheels turning. Always. Because it is connected to them. It's not a case of the wind turning a freely rotating propeller. The only way for the wind to turn the propeller is by making the device move forward. (Which is what the wind does.)

If the gearing is too fast, it will cause the wheels to slip and that will slow the vehicle's acceleration. Note that slippage in this case acts opposite how slippage affects the vehicle on a treadmill. Is that a nit to pick or is it the expected symmetry?

There's no slipping. At least there's not supposed to be any, as long as the device is built properly.

 

[Modified]

They do not have the same kinetic energy.

Yes they do. They both have the same velocity with respect to the surface before, during, and after stopping, and they both have the same mass, therefore they both have the same kinetic energy with respect to the surface before, during, and after stopping. You are still making the mistake of thinking that kinetic energy is not relative.

 

[Brian-M]

Damn... I was working on a simple animated image to provde a visual guide to how it works last night (and this morning , realized I did it all wrong), and now I find I've been beaten to it.

I'll post mine anyway, once I'm finished.

 

[Brian-M]

Brian-M,
There is no simple pushing and pulling. Change the people to a stream of small table-tennis balls.

OK, so you want to abandon the simple analogy that illustrates the point perfectly in favour of a much more complicated one?

Do you think that there will always be a "gap" in front of you, or that that some of the balls to the left and right will not get knocked into your path?
The balls from behind, do not "push" you. They momentarily strike you, and bounce off, transferring some of their momentum to you. (They now lose momentum, which they must pick up from somewhere else.)

If they momentarily strike you, transferring some of their momentum to you, then they have pushed you. If a ball in front to you is going slower than you it will do the same, pushing you back. I'm using 'push' instead of 'transfer of momentum from collision' for simplicity.

Note: The momentum which they must pick up from somewhere else comes from the other balls/air molecules. When you use a sail to accelerate in the wind, you are slowing down the wind, by a very, very, tiny amount.

You will be accelerated, little by little, and therefore must pass by others, but some will be much slower than you, (having lost almost all their momentum, in a previous collision) and you will lose some of yours if you hit them. Collision are not "equivalant" or you would not make progress at all.

Assuming this is random action...
If you are travelling in the same average direction the sideways collisions will cancel out exactly, with the same result as if there were no sideways collisions.

When you travelling at average velocity, all collisions will average out exactly, giving the same result as if there were no collisions.

When you are travelling at below average velocity, the collisions from behind will exceed the collisions from ahead, pushing you forward.

It requires no energy to travel at windspeed. A bubble or balloon can do it with ease. It always requires energy to travel at less than windspeed. If you are simply standing on the ground, you are using the earth's kinetic energy to travel at less than windspeed, reducing the earth's kinetic energy, relative to the wind.

If you want to see what I mean, take a look at something that describes the movement of electron through a conductor. The electron stream has an average velocity, but individual particles (charges) may take a longer path, or move backwards. They rattle through the wire as if in a pinball machine. This causes heat to be generated, through the wires "resistance".

You haven't got a clue what you're talking about. The heat from electrical current is caused by the energy exerted in order to pass through the material. Collisions from electrons bouncing off each-other do not produce waste energy. Electrons still do all this when passing through a superconducter, but no heat is generated because no energy is required to pass through the material.

I meant not on a treadmill, but in reality, assuming the person to be a simple mass?

Ok, if you don't want a treadmill, to set asside the differences in the relative velocity of the ground, we'll assume the person doesn't hit the ground, but remains stuck on the bonnet of the car.

Ok, let's say the person is on a skateboard and hits a car (or is hit by a car) at 60mph.

If a skateboarder is travelling at 60 mph and hits a stationary car, he is decellerated by 60 mph.

If a skateboarder is stationary, and is hit by a car travelling 60 mph, he is acellerated backwards to 60 mph.

Decellerating 60 mph is exactly the same thing as acellerating backwards to 60 mph.

The formula for kinetic energy is: E=1/2M(V^2)

In both cases the change of velocity is the same, and the velocity is the same (and the injuries are the same). The kinetic energy transferred is exactly the same in both cases.

There is no difference.

 

[Dan O.]

Here is a simple physics question from the YoyoDyne Propulsion Systems employment exam:



If the parachute is moving downwind at 3 smoots pre microfortnight, how fast and in what direction is the yo-yo moving?

 

[spork]

If the parachute is moving downwind at 3 smoots pre microfortnight, how fast and in what direction is the yo-yo moving?

Oooh - Oooh, I know this one. Can I answer?


Unfortunately, this question would weed out one hell of a lot of PhD's and other "experts"

 

[sol invictus]

Here is a simple physics question from the YoyoDyne Propulsion Systems employment exam:

[qimg]http://www.internationalskeptics.com/forums/imagehosting/thum_15144492613dfb1b89.jpg[/qimg]

If the parachute is moving downwind at 3 smoots pre microfortnight, how fast and in what direction is the yo-yo moving?

That's pure genius. This keeps getting better and better....

humber???

 

[spork]

ATTENTION SKEPTICS​

You should consider posting to the Mythbusters fan site to get them to do a segment on this and prove us all wrong:

http://forum.mythbustersfanclub.com/index.php/topic,12948.120.html

 

[ynot]

EUREKA! . . . and all that

I now believe that this type of vehicle can and does travel faster than the wind in the direction of the wind (no zigzagging required). What’s more important I believe I know and understand the why and how.

There are two important facts to consider that aren‘t intuitively apparent . . .

(1) There are two winds involved not one. The initial wind that moves the vehicle over the ground and a second wind created by the spinning propeller.

(2) The initial wind doesn’t push against any part of the vehicle (including the propeller) it pushes against the opposing propeller wind.

The total amount of potential energy available to move the vehicle is a combination of both winds as they are opposing each other. Of course it takes energy from the initial wind to create the propeller wind, but this loss is immediately replaced by a piece of “new” wind at full energy. The rolling resistance of the vehicle caused by the generation of the propeller wind means the vehicle will never reach the combined speed of both winds, but it can and will exceed the speed of the initial wind.

The treadmill demonstration proves this as it effectively recrates the conditions of the vehicle travelling at the speed of the initial wind. That the vehicle moves against the movement of the treadmill clearly demonstrates that the rolling resistance to create the propeller wind is less than the thrust of the propeller wind.

My slight disappointment at being previously wrong is more than compensated by the discovery of the truth. Thanks to those that were always right for your tolerance. Now I’m off to design a myriad faster than wind vehicles.

 

[Myriad]

EUREKA! . . . and all that

I now believe that this type of vehicle can and does travel faster than the wind in the direction of the wind (no zigzagging required). What’s more important I believe I know and understand the why and how.

There are two important facts to consider that aren‘t intuitively apparent . . .

(1) There are two winds involved not one. The initial wind that moves the vehicle over the ground and a second wind created by the spinning propeller.

(2) The initial wind doesn’t push against any part of the vehicle (including the propeller) it pushes against the opposing propeller wind.

The total amount of potential energy available to move the vehicle is a combination of both winds as they are opposing each other. Of course it takes energy from the initial wind to create the propeller wind, but this loss is immediately replaced by a piece of “new” wind at full energy. The rolling resistance of the vehicle caused by the generation of the propeller wind means the vehicle will never reach the combined speed of both winds, but it can and will exceed the speed of the initial wind.

Eureka indeed! Nice going, ynot.

The way I'd try to explain it intuitively (if I were starting from scratch with a new audience) is that the thrust the vehicle receives is the combination of two pressures.

The first is the differential pressure of the wind acting on the propeller (and on the rest of the vehicle, if the construction of the vehicle presents a significant wind shadow). The amount of this pressure is not directly affected by the propeller's pitch or motion; it would be essentially the same if we traced the propeller's shadow (cast by a light from directly behind) and cut out that shape from a thin stiff sheet and installed that in lieu of the propeller. It's also the same regardless of how fast the propeller is spinning or whether it's spinning at all. But it's very dependent on the air speed relative to the vehicle speed. It goes to zero when vehicle speed = wind speed and becomes negative when vehicle speed exceeds wind speed.

The second is the differential pressure generated by the spinning of the pitched propeller blades as they accelerate air backward. This is very dependent on how fast the propeller is rotating, increasing as the propeller spins faster. That in turn is dependent on the vehicle speed relative to the ground. But it is not very dependent at all on the vehicle speed relative to the wind speed. Prop planes can travel downwind much faster than the wind without their props losing much thrust.

So, it's graph time:

This graph is a freehand sketch to show the principle, not the result of quantitative analysis which in any case would depend on the details of the vehicle's construction.

The red line is the direct thrust from the wind; the blue line is the propeller thrust. The direct thrust, as explained above, decreases rapidly with increasing vehicle speed and becomes negative (drag) when the vehicle speed exceeds the wind speed. (Note that the red line is not very well drawn. It should be flatter where it crosses the wind speed line, more like the curve of half a parabola.)

The blue line represents propeller thrust which, in contrast with the red line, increases steadily with vehicle speed.

The pink line represents the minimum thrust needed to keep the vehicle moving at a given speed, to compensate for wheel friction and for the drag associated with the spinning of the propeller. It does not include air drag against the forward motion of the cart (including the propeller), because that is already accounted for as the negative-going values of Direct Wind Thrust (red line).

If the propeller were replaced by a flat disk or other structure of equal mass, angular momentum, and wind load area, but which could not provide any thrust, the vehicle would accelerate in a wind up to velocity A, where the red line crosses the pink line.

But with the propeller providing thrust, the total thrust is (roughly) shown by the green line. In this case the vehicle accelerates in the wind until it reaches velocity B, where the green line crosses the pink line. In this particular case velocity B is faster than the wind speed.

Respectfully,
Myriad

 

[Brian-M]

Here's my visual depiction... although it doesn't even come close to the simple elegance of the image Dan O presented, it does have the advantage of working in the same way as the DDWFTTW device in the OP does. (Only with a wheel pushing/pushed by a conveyor belt, instead of a propeller pushing/pushed by the wind.

Humber...

If you want to convince me the DDWFTTW device won't work, you will have to either show that the animated cart pictured won't move at twice the speed of the converyor belt in the same direction as the conveyor belt, or explain why using a propeller with the wind is different from using a wheel with a belt.

 

[spork]

The way I'd try to explain it intuitively...

There are just too many things here that do not represent physical reality. If this is just a way for you to model the behavior of the cart so it's easier to think about - that's fine I guess.

Prop planes can travel downwind much faster than the wind without their props losing much thrust.

A prop plane doesn't know a thing about whether it's traveling upwind, downwind, or crosswind. The thrust will be related to the throttle setting, altitude, air speed,... but not the wind direction.