Down wind faster than the wind

[spork]

Thanks Spork good info and advice. As I said I will be testing mine in an outside wind not on a treadmill (to start with anyway).

I would recommend getting it all checked out on a treadmill first. That way you'll know you have the drag minimized and the alignment all worked out. It's just a far more controlled way to experiment. Once you have that working (which shouldn't take much) you should take it outside. The really hot ticket would be to have the bubbles produced on the cart so you can see which way they go.

Do you think size is much of a factor for an outside wind test?

I think we all know size matters.

But to be honest, I think if you have a really good smooth surface and smooth wind you should be OK. The real problem will be the significant gradient near the ground. This little guy probably won't be outrunning bubbles that are a foot higher than it (is my guess).

Would yours work just as well in an outside wind and have you tested it this way?

We have run it outside - mostly to demonstrate that it self starts. It takes off like bat out of hell. Twice JB had to sprint to catch it before it hit a hard object. But... we haven't done any tests outside to see if it passed the wind speed. I'll see if we can do that tomorrow (if there's wind).

 

[ThinAirDesigns]

I am going to make my own model of this vehicle and want to make it as much as possible the same as the one in the video demonstration. I would like to know what the gear ratio is between the wheels to the propeller. I would appreciate any other design advice as well. I will test it in an outside wind and will use something like soap bubbles floating in the wind to establish what the actual air speed is at any given time. I don’t have much spare time at present so it may take a few weeks to get this done.

That's very cool ynot. I have great respect for those willing to get hands on.

Looking forward to your results. I'll help with any build advice I can give.

JB

 

[Brian-M]

Another physical demonstration of the simple principle of DDWFTTW.

http://www.youtube.com/watch?v=ZVSjA7Rhccs

Notice that the hand doesn't move as fast as the vehicle.

JB

Yay, you made a real version of my animated cart, and suspiciously quickly too. Did you already have one lying around?

 

[ThinAirDesigns]

Yay, you made a real version of my animated cart, and suspiciously quickly too. Did you already have one lying around?

T'wasn't me Brian -- I'm not ambitious enough to build something real. ;-) Someone left in the comments section of my Mythbusters video. Have no idea where they got the idea.

JB

 

[spork]

That's very cool ynot. I have great respect for those willing to get hands on.

Yeah - and the rules of the game clearly stipulate that you have to come back and post your results. Then we can welcome you to our club of scammers and hoaxers. Prepare to be insulted.

 

[Brian-M]

The effect is to minimise the force available to drive the cart forward, which is why it stays still. The progress up the belt I have already explained.

This is just too bizarre. In one sentence you claim it stays still, and the very next sentence you admit it progresses up the belt. Are you even aware that you're contradicting yourself?

And for your "explaination" as to why it progresses up the belt, it makes no sense at all. You claim that the stored energy in the prop pushes it forward slightly whenever it loses traction, which is indeed possible, but you forget that as soon as it loses traction the prop will also slow down, slowing down the wheels as well. As soon as it regains traction, the lower speed of the wheels would cause the treadmill to push it backwards to where it started from. Also, the prop is not 100% efficient, it's constantly wasting energy, meaning that the cart would progress backwards at a steady rate.

And if you watch the videos, the cart not only "progresses" up the treadmill, it accellerates up the treadmill. Your "explaination" completely fails to explain this, or anything else either.

When you are on the belt, you may say that you have a new frame of reference, but the all are the same, so I see what you see, literally.

Car on belt as usual, 10mph, 10 foot long belt. Get out of the car, you go backwards at Vbelt, until the end. Walk back to car.
How gar have you travelled, and what is your displacement?
20foot, 0foot. Car (same transit time) 20foot.
Infinite belt? No good, one way ticket.
Longer? Lower repetition rate.

Infinite belt.
The reason for this is the itself belt isn't the reference, we are simply choosing a reference that matches the belt.
Get out of a car while it's driving on the belt, and the car continues to drive away from you, exactly the same as it would if you got out of the care while it was driving along the road.
It may look to someone standing next to the belt that the belt is carrying you backwards while the car is just spinning it's wheels and going nowhere, but from your perspective there is no difference.

If you want to use a finite belt, then the real-world approximation would be if the road was vanishing behind you at a steady rate.

How can you be able to do that, and yet reach the wrong conclusion about the cart??? ( The prop and wheels are in series, you know)

You still don't get it. Having the drive-chain connected to both bottom wheels is just having a gear-ratio of 1/1, pushing on the same medium travelling at the same speed. The prop (or top wheel) has a gear ratio of 1/2, pushing on a different medium that's travelling at a different speed.

Can't you see the difference?

 

[spork]

This is just too bizarre. In one sentence you claim it stays still, and the very next sentence you admit it progresses up the belt. Are you even aware that you're contradicting yourself?

If I may be so bold... it's pretty bizarre that you're willing to engage humber in a discussion about physics. At the very best he simply doesn't have the capacity to follow a simple line of logic (which he demonstrates with each post) - and at worst, he's simply a troll with one hand on the keyboard and the other on his johnson as he intentionally tries to twist us all around with his truly bizarre "theories".

I applaud your patience and effort, but I suspect your energy would be better spent teaching your dog calculus.

 

[rehn]

We have run it outside - mostly to demonstrate that it self starts. It takes off like bat out of hell. Twice JB had to sprint to catch it before it hit a hard object.

Ops… This made me confused. I thought I understood how this worked and that the cart needed help to start. In both films it got help during the start. During (help)start the wheels are driving the prop, and then the prop are driving the wheels. As I understood the design I expect the cart to go into reverse if just left on the road with no help to start. What am I missing ?

 

[Michael C]

Ops… This made me confused. I thought I understood how this worked and that the chart needed help to start. In both films it got help during the start. During (help)start the wheels are driving the prop, and then the prop are driving the wheels. As I understood the design I expect the chart to go into reverse if just left on the road with no help to start. What am I missing ?

Have a look at the video Spork put together for Mythbusters:

Starting at 6:20 you can see the cart start on its own.

 

[Dan O.]

Ops… This made me confused. I thought I understood how this worked and that the cart needed help to start. In both films it got help during the start. During (help)start the wheels are driving the prop, and then the prop are driving the wheels. As I understood the design I expect the cart to go into reverse if just left on the road with no help to start. What am I missing ?

It all depends on the gearing and the pitch of the blade. The cart wheels rolling on the ground geared to the propeller shaft is mechanically equivalent to a propeller with the mass of the cart spinning on a threaded rod and that is mechanically equivalent to a fixed sail sliding on a track angled to the wind.

For a given angle of the track (gearing ratio) there is a pitch of the sail where the forces pushing the cart upwind and the forces pushing the cart downwind are equal. A steeper pitch of the sail (enough to overcome friction) will move the cart upwind. A shallower pitch of the sail will move the cart downwind.

A small push may still help. When the cart isn't moving, the cross section of the wind interacting with the blade of the prop is only the width of the prop blade. Once the cart is moving and the prop is spinning, the full arc of the prop is interacting with the wind.

 

[Michael C]

Spork, here's an idea for a test to put on video.

When you use the treadmill, there's a problem with lateral stability: if you don't keep intervening, the vehicle goes off to the right or left. You could stabilise it by attaching threads to the front and rear of it and tying weights to the ends of these threads. The threads would run over pulleys, thus:

Not only would this permit you to stabilise the vehicle, it would also permit you to do some different tests by trying different values for the weights. If you adjust the weights so that W1 is heavier than W2, but not heavy enough to prevent the cart moving forward when the treadmill is turned on, you could do a nice demonstration:

- With the power turned off, show that if you start with the cart at the front end of the treadmill, it will be pulled to the back end.
- Now, without touching the cart (so nobody can accuse you of deftly imparting some mysterious kinetic energy to it), start up the treadmill motor.
- See the plucky little machine advance against the treadmill belt, even when it has to pull a load.

 

[Brian-M]

Humber,

I think I've come up with a simpler explaination by converting my little animated cart to run up a treadmill by pulling a string.

The top wheel is pulling the string at half the speed the treadmill is turning the bottom wheels. Ie, as 2 feet of belt pass under the cart, it pulls itself along 1 foot of string.

Since work (or energy) is equal to force times distance, then the top wheel can pull the string at twice the force the treadmill is turning the wheels, using the same amount of energy. If 25% of the energy were lost due to inefficiencies, the top wheel would still be pulling the string at 1.5 times the force with which the treadmill is pushing on the cart.

Since it is pulling itself forward along the treadmill with more force than the treadmill is pushing it back, it advances up the treadmill.

The DDWFTTW device, when on a treadmill, is doing exactly the same thing.
Instead of using a wheel to pull on a string, it is using a propeller to "pull" on the air.

Can you understand it now?

 

[Brian-M]

If I may be so bold... it's pretty bizarre that you're willing to engage humber in a discussion about physics.

Yes, you may be so bold.

It was fun at first. Trying to explain things to him forced me to think more deeply on the subject, improving my own understanding.

Now talking with Humber is getting old. [/understatement]

I'm already planning to give up on him, but I thought it would be worth one last try, so I spent some time writing up a more detailed explaination of frames of reference for him, to see if he can finally "get it"...


Humber...

I'm going to make one last attempt at explaining the frames of reference thing to you. If you still don't get it after this, I just don't care any more. You're on your own.

There is no such thing as absolute velocity.
There is no such thing as zero velocity either.

Velocity only exists relative to something else.
When we measure velocity, we have to choose what velocity we want to call zero when measuring the velocities of other objects. Normally we choose to call the velocity of our local environment zero, for convenience. On the ground, we use the velocity of the surface of the earth. In a plane, we use the velocity of the plane. In a train, we use the velocity of the train.

But these points of zero velocity are entirely arbitrary. We could choose any zero-point we want, and the laws of physics would still work the same, the difference in velocities would remain the same, and the outcome of events would remain the same.

Whatever speed we choose to call zero, that is our frame of reference. (It should be noted that a frame of reference does not change even if the velocity of the object it was chosen to match does. This seems to be something you have trouble understanding.)

Let's say you're on a train travelling at 10 mph, and you throw a ball at 10 mph, and it hits someone in the head. Using the train as the frame of reference, the ball was moving at 10 mph, and the passenger's head was stationary. For someone outside the train, watching it go past, from their frame of reference the ball was stationary, and the passenger's head was moving at 10 mph.

Because there is no such thing as absolute velocity, it doesn't matter what frame of reference you are using. Everything works out the same. Whether you're using the ground, the train, or a plane flying overhead as your frame of reference, the relative velocity between the ball and head remains the same, and the outcome remains the same.

We choose our frame of reference purely for the sake of convenience and ease of understanding. What frame of reference we choose to use makes no difference to that is happening. All that matters is the mass and relative velocities of the objects and environment involved; these remain the same, regardless of which frame of reference we choose to use.

Sometimes, if we take two situations that seem completely different at first glance, and look at them from a different frame of reference, we can see that the situations are really the same.

For example, a skateboarder hitting a car vs. a car hitting a skateboarder. At first glance, you'd think that a car hitting a skateboarder at 60 mph would be different from a skateboarder hitting a car at 60 mph, because the car is a lot bigger and heavier, but let's compare these two situations from equivalent frames of reference...

Situation A:
A skateboarder is standing stationary to the road on his skateboard. A West-bound car is headed toward him at 60 mph. Because the skateboarder is moving at 0 mph relative to the road, we'll set our frame of reference to 0 mph relative to the road (reference A).

Situation B:
A skateboarder is travelling 60 mph East on his skateboard, heading toward a stationary car. Because the Skateboarder is moving at 60 mph East relative to the road, we'll set our frame of reference to 60 mph East, relative to the road (reference B).

In situation A from reference A, the skateboarder appears to be stationary and a car appears to be travelling West at 60 mph toward him.

In situation B from reference B, the skateboarder appears to be stationary and a car appears to be travelling West at 60 mph toward him.

In situation A from reference A, the skateboarder is struck by the car, which imparts exactly enough kinetic energy into him so that he is now travelling West at 60 mph.

In situation B from reference B, the skateboarder is struck by the car, which imparts exactly enough kinetic energy into him so that he is now travelling West at 60 mph.

You'll notice that in both situations, using a frame of reference equal to the skateboarder's initial velocity, the exact same thing happens, and the exact same amount of energy is imparted to the skateboarder from the car.

You'll also notice that in situation B, when the skateboarder struck the car, the frame of reference continued to move at the skateboarder's initial velocity. A frame of reference does not change velocity, even if the object it was originally based on does.

Newton's laws of motion are symmetrical. No matter what frame of reference you look at an event, the results are always the same.

Your idea that a car colliding with a person is somehow different from a person colliding with a car, or that a ball colliding with a house is somehow different from a house colliding with a ball, violates the symmetry of these laws.

Newton's laws of physics work the same no matter what frame of reference you are using, and identical situations produce identical results.

If a cart is moving downwind at wind speed in a 10 mph Westerly wind, using 10 mph West relative to the ground as your frame of reference: the air is motionless, the cart is motionless and the ground is moving 10 mph East.

If a cart is stationary on an indoor treadmill with a belt moving 10 mph East, using 0 mph relative to the ground as your frame of reference: the air is motionless, the cart is motionless and the ground is moving 10 mph East.

The situations are identical and the results will be identical

 

[ThinAirDesigns]

Nicely done Brian. Best wishes on your goal -- you're going to need it.

JB

 

[spork]

Yes, you may be so bold.

It was fun at first. Trying to explain things to him forced me to think more deeply on the subject, improving my own understanding.

Now talking with Humber is getting old.

Understood - completely. I've been through the same thing too many times. I should quit doing this altogether because as everyone can see I now have a hair trigger. I don't mind curiousity, or even ignorance, at all. But I just get ultra fed up with the folks that don't know how it works telling me either that it doesn't work or that I don't know how it works.

JB finds this entertaining and points out to me that as far as anyone else is concerned I'm just another yutz on the interweb - why should anyone take my word for it? I keep forgetting that people aren't aware that I know everything there is to know about everything (you can probably imagine the frustration ).

I agree that explaining it to people like Humber can have two positive effects. As JB points out, his answers to people like Humber aren't generally intended for Humber. They're intended for the other readers.

Also, you're right that there's nothing like trying to get a concept across to really hone your own understanding of the concept.

But I've done it too many times and have no patience left for the Humbers.

Good luck. If this brings him around you're a better man than me. I will truly be impressed.


By the way, I'd like to get all the believers together some time. We can all drink beer, recount stories of trying to convince the unwashed masses, and bang our heads on the table.

 

[rehn]

Have a look at the video Spork put together for Mythbusters:

Starting at 6:20 you can see the cart start on its own.

Thanks for the pointer. But I since there is no indicator for wind speed nor direction, it proves nothing.
I also have problem with the transition from the low speed situation where the relative wind is coming from behind, to the position when it comes from front. At some speed the cart must have the same speed as the wind. At that speed, from where comes the force needed accelerate the cart further? Maybe a wind gust at the right moment will help the cart past that speed but I think that is “cheating” .
I have been looking carefully at the “self start” video. Shorter version here:
http://www.youtube.com/watch?v=kWSan2CMgos
To me it looks like the prop at first is moving in one direction and after a while in the other direction. Is this an artifact from the video or is it real ?
If it is real, maybe the explanation for the self start is that they are using a gear that is spinning the wheels forward regardless of the direction of the prop.
But it still can’t explain where the force needed get past the point where the cart speed == wind speed.

Regards

 

[spork]

...

 

[Michael C]

At some speed the cart must have the same speed as the wind. At that speed, from where comes the force needed accelerate the cart further?

If it were a balloon, just pushed in the wind, it would indeed accelerate to the point where it it moving at the same speed as the wind, and no further. But the cart isn't just hanging in the air: at the point where the cart has attained the speed of the air, the wheels are being turned by the ground. The simple gearing (it's just 1 to 1 with no slippage) transmits the movement of the wheels to the propeller, providing the force needed to accelerate the cart further.

 

[rehn]

If it were a balloon, just pushed in the wind, it would indeed accelerate to the point where it moving at the same speed as the wind, and no further. But the cart isn't just hanging in the air: at the point where the cart has attained the speed of the air, the wheels are being turned by the ground. The simple gearing (it's just 1 to 1 with no slippage) transmits the movement of the wheels to the propeller, providing the force needed to accelerate the cart further.

Sorry, but I can’t see how that answers my question. When cart speed == wind speed there is no energy input to the system from outside. Only some loss due to friction. The only energy available is in the forward momentum of the cart and some in the rotating wheels and prop. What you say is that this momentum is used to –>accelerate<- the cart and the wheels and the prop. No… I don’t believe that it is possible; it is like lifting yourself in the hair.
And again. There is no wind indicator in the film, not even a claim( in the film ) that it is a downwind start. I want a something better before I believe “self start” is possible in steady wind.

 

[Molinaro]

Sorry, but I can’t see how that answers my question. When cart speed == wind speed there is no energy input to the system from outside. Only some loss due to friction. The only energy available is in the forward momentum of the cart and some in the rotating wheels and prop. What you say is that this momentum is used to –>accelerate<- the cart and the wheels and the prop. No… I don’t believe that it is possible; it is like lifting yourself in the hair.
And again. There is no wind indicator in the film, not even a claim( in the film ) that it is a downwind start. I want a something better before I believe “self start” is possible in steady wind.

What? It's called a Direct Downwind Faster Than The Wind vehicle! And before the one on the road starts moving it has a wind sock pointing forward before it starts moving.

What more do you want?