Sailing downwind faster than the wind: resolved?

[russ_watters]

schroder, ThinAirDesign is right. This is a problem of reference frames and due to the well established principle of relativity, the treadmill scenario is exactly identical to running downwind in a steady wind.

The first time I saw the video of the device on the treadmill, I got the scenario twisted around in my head. Then I drew myself a diagram and figured it out. That's what I suggest you do. Draw a diagram for each scenario and see if you can detect any difference in the motion vectors for wind and surface relative to the craft. The only surface that matters is the surface that is spinning the wheels and the only wind that matters is the one that is spinning the propeller or being pushed by the propeller.

In the previous thread, we used the following values:
Cart on treadmill:
-5mph, tredmill relative to ground
-1mph, cart relative to treadmill
-6mph, wheels spinning
-1mph, wind being generated by the propeller

Cart on ground:
-6mph, cart relative to ground
-6mph, wheels spinning
-5mph, wind speed
-1mph, wind being generated by the propeller

If the vectors all add up and you find no math errors, then the scenarios must be identical.

 

[russ_watters]

All experiments I have seen on videos do not prove the 180 possibility, to my opinion.
It is impossible in such conditions to keep the wind at 180 all the time. Even small fluctuations generate deviation from 180, which makes it equivalent to tacking at <180.

Certainly, the wind could not be constant, however a propeller isn't a sail and the cart has no control system to adjust for wind speed variation. So the only thing that variations in the wind speed or direction can do is make the cart not work. They can't make it work better.

 

[ThinAirDesign]

You understand it perfectly Russ.

Thanks

JB

 

[yoavraz]

OOOPS. I knew there was a disconnect, but until now I have not realized we are talking about different experiments. I understand you have previous context. I apparently was thinking about a different forum from the one that was mentioned (I see this subject is popular...). At least we agree about the outdoor experiment description, I hope. Though I can guess now the scenario, I do not want to guess anymore.

Mender, Could you kindly refer me to your treadmill experiment description, or describe it here, or send me a private message. Also pls point me to the related video. Thnx

 

[russ_watters]

Here's the other video: http://www.youtube.com/watch?v=aJpdWHFqHm0&feature=related

 

[mender]

yoavraz, it sounds like something went awry. The Jack Goodman video which was taken under actual wind conditions indeed will be subject to the vagaries of the shifting wind; however as Russ stated, for this device anything off directly downwind will serve to decrease the cart's performance, not enhance it. That's the one that Russ linked.

The treadmill test is as shown by TAD, and consists of a treadmill in an enclosed room. The speed of the treadmill is set at 10 mph, which simulates an outdoor test by providing a speed difference between the air in the room (still ) and the ground (10 mph).

I'm going to continue this tomorrow, I'm not quite up to snuff tonight. Please feel free to pose any questions or clarify what you think is relevant regarding a treadmill test regime. My intent is to build a non-prop cart device and I would like to make sure I'm not leaving anything out of my test program.

 

[schroder]

schroder, ThinAirDesign is right. This is a problem of reference frames and due to the well established principle of relativity, the treadmill scenario is exactly identical to running downwind in a steady wind.


If the vectors all add up and you find no math errors, then the scenarios must be identical.

Sorry to tell you this Russ, but it is you who are wrong about this. Just calm down and think about it rationally. Imagine two frames. We will keep the treadmill in both, but in the first one the tread is stationary with respect to the floor, and the air is moving from the back at 10 mph. You have to consider the floor and the tread together as the stationary reference which remain stationary throughout. All that is moving initially is the wind from the back. The wind now blows the cart forward, and we will assume 100% efficiency, so the cart now moves forward at 10 mph with respect to both the tread and the floor. I think we can all agree with what is happening here.
Scenario number two: The cart is sitting on the tread, the air and the floor are both stationary and will remain stationary throughout. The only thing that is moving initially is the tread trying to drag it backwards against the stationary air and the equally stationary floor. There is no difference between this scenario and scenario one. The cart must advance against the stationary reference of air and floor and it must do so at the tread velocity. We have all seen the cart advance against the tread and against the floor, but it has never reached the velocity of the tread (wind).I know that this is difficult at first to see, but once you do see it, it explains everything! Until or unless the cart achieves tread velocity, it is not going DDWFTTW or DDTFTTT where T is for Tread. You Must tie the floor to the air because it is otherwise impossible to gauge the velocity against the still air! They are both stationary so it is the most convenient reference. Once you break through and clarify this in your own mind, all will be clear and the mystery of these carts is solved. NOTHING goes DDWFTTW there are NO exceptions!

 

[swerdna]

Why I think traveling directly downwind faster than the wind is impossible . . .

The only energy sources involved are the wind and the kinetic energy of the vehicle.

The kinetic energy of a vehicle alone can’t be geared up to make the vehicle travel faster (that would be perpetual motion/free energy).

Whatever help the vehicle gets from a tailwind is completely lost when the vehicle reaches the speed of the wind.

At the speed of the wind the vehicle only has it’s own kinetic energy to accelerate faster than the wind (impossible)

When the vehicle travels faster than the speed of the wind it loses the advantage of a tailwind and gains the disadvantage of a headwind.

 

[Hurkyl]

Sorry to tell you this Russ, but it is you who are wrong about this. Just calm down and think about it rationally. Imagine two frames. We will keep the treadmill in both, but in the first one the tread is stationary with respect to the floor, and the air is moving from the back at 10 mph. You have to consider the floor and the tread together as the stationary reference which remain stationary throughout. All that is moving initially is the wind from the back. The wind now blows the cart forward, and we will assume 100% efficiency, so the cart now moves forward at 10 mph with respect to both the tread and the floor. I think we can all agree with what is happening here.

The equivalent scenario to this is the following:
The treadmill is running at 10mph, and the cart is moving backwards at 10 mph along with it, and there is no wind. Air resistance slows the cart, and because of our 100% efficiency assumption, the cart slows to a stop, and thus moving 10 mph forwards relative to the treadmill.

 

[schroder]

The equivalent scenario to this is the following:
The treadmill is running at 10mph, and the cart is moving backwards at 10 mph along with it, and there is no wind. Air resistance slows the cart, and because of our 100% efficiency assumption, the cart slows to a stop, and thus moving 10 mph forwards relative to the treadmill.

Relative to the treadmill yes, but Not relative to the stationary air and the floor. You are using the wrong reference for your measurement.

 

[schroder]

Why I think traveling directly downwind faster than the wind is impossible . . .

The only energy sources involved are the wind and the kinetic energy of the vehicle.

The kinetic energy of a vehicle alone can’t be geared up to make the vehicle travel faster (that would be perpetual motion/free energy).

Whatever help the vehicle gets from a tailwind is completely lost when the vehicle reaches the speed of the wind.

At the speed of the wind the vehicle only has it’s own kinetic energy to accelerate faster than the wind (impossible)

When the vehicle travels faster than the speed of the wind it loses the advantage of a tailwind and gains the disadvantage of a headwind.

You are right. We need more voices of reason here.

 

[Hurkyl]

Relative to the treadmill yes, but Not relative to the stationary air and the floor. You are using the wrong reference for your measurement.

I used the reference I said I used. In both scenarios, the cart is:
1. Stationary with respect to the air
2. Moving 10mph forward with respect to the treadmill

 

[schroder]

I used the reference I said I used. In both scenarios, the cart is:
1. Stationary with respect to the air
2. Moving 10mph forward with respect to the treadmill

It does not matter what the relative velocity is of the cart with respect to the treadmill. All the treadmill is doing is replacing the force of the wind, which would normally be coming from the back, with another equivalent force to turn the propeller, simulating an equivalent wind force. The treadmill just happens to be oriented to be coming from the front. You can have it running perpendicular to the direction of the cart, with a right angle adapter. What the cart must do, whether it is being driven by the wind at its back, or an equivalent force, is advance against the stationary frame. In the case of the wind, the stationary frame is the surface of the tread (not running) which is stationary with the floor. In the case of the tread providing power, the stationary frame is the air (not moving) which is stationary with respect to the floor. In both cases, the stationary frame is stationary with respect to the floor, so the floor is common to both frames of reference. The cart must advance, at the tread velocity, with respect to the floor, in order to be going at tread (wind) velocity. Moving against the tread, or standing still on the tread, does not establish that is moving at tread (wind) velocity. You have to open your eyes and your mind a bit to see this, but once you do, you will slap yourself in the head for not seeing it sooner! It is the only explanation that makes any sense!

 

[wouterd]

If you believe a sailing craft can go DWFTTW VMG-wise (which I believe), then you could think of a craft consisting of two of these devices going in opposite directions and(virtually) connected to each-other which we call one device... and that consolidated device will technically go DDWFTTW! Isn't that exactly what these rotor/propellor blades on these devices ar doing? The blades on itself don't go DDW anyway.

 

[russ_watters]

It does not matter what the relative velocity is of the cart with respect to the treadmill.

Yes it does: the treadmill is spinning the wheels of the cart.

You really need to draw a diagram of what is happening. You're simply missing it.

 

[russ_watters]

Perhaps another approach: Setting the scenario up one piece at a time, with assumptions, then removing the assumptions.

First, assume no air and no friction except the static and sliding friction between the wheels and treadmill. You place the cart on the treadmill and the wheels spin up to speed due to that friction. When you remove your hand, what happens? Nothing. Via Newton's first law, the wheels and propeller keep spinning because there is nothing to "slow" them down and the cart stays where it is on the treadmill.

Add the internal friction (but not wind) back in and what happens? The cart slowly moves toward the back of the treadmill and falls off because the internal firction slows the wheels.

Remove the internal friction again, but this time add the air. Since the propeller is and the air around it is stationary wrt the cart, the propeller generates thrust. It also generates drag as it moves through the air, which would tend to slow the wheels of the cart. So if the thrust is greater than the drag, the cart will move forward. If the thrust is less than the drag, it will move backwards.

Now add the friction internal to the cart back in. Similar answer to the above: if the thrust is greater than the drag and internal friction, the cart moves forwards. If less, the cart moves backwards.

So what we have determined is that what would cause this device to not work is inefficiency: build a device with low friction and an efficient propeller and it should work.

 

[M Grandin]

Some appear having difficulties realizing this vehicle will work as claimed. That may in some extent depend on associations to "perpetuum mobile" and aquainted things, repelling rational thoughts in this case.

To make it more easy realizing this kind of vehicle is at least theoretically feasable, imagine this possibility: You have a wind generator standing on wheels, equipped with energy ackumulator and engine to run the wind-generator on its wheels. Let the wind
generator stand still ackumulating wind energy a moment - and then let it swiftly drive away on ackumulated energy downwards the wind (although any direction would do). The same process repeatedly. You realize there is no theoretical limit of how far or how swift this motorized windgenerator could run on ackumulated energy at each step. Average speed could be for instance 10 times speed of wind. So theoretically there is nothing preventing that kind of vehicle if engageing ground is permitted.

 

[rcgldr]

Why I think traveling directly downwind faster than the wind is impossible . . . The only energy sources involved are the wind and the kinetic energy of the vehicle.

The only energy source is the wind, any change in kinetic energy of the vehicle is due to work done by the wind.

When the vehicle travels faster than the speed of the wind it loses the advantage of a tailwind and gains the disadvantage of a headwind.

Except in this case the propeller generates a modest amount of thrust at a small amount of speed against the tail wind, allowing the cart to go faster than the wind. The cart is taking advantage of the difference in speed between the air and the ground.

This next quote (2nd on this thread) explains the power situation, force output (thrust) is greater than force input, but the relative speed of the output force against the air is much slower than the relative speed of the input force of the wheels against the ground (or treadmill in these experiements), and the required power output for DDWFTTW is less than the power input.

One way to explain why these carts work is to note that the power input is equal to the force at the driving wheels times the forwards speed of the car relative to the ground. After losses, the power output is equal to the thrust times the relative air flow through the prop, which is much slower than the ground speed. Via gearing, prop diameter, and prop pitch, the torque at the wheels is multiplied so that the prop generates more thrust than the force from the wheels, but at a much lower speed, so that power output remains well below power input. As long as the difference between wind speed and ground speed is large enough, (and perhaps not too large), the cart can go downwind faster than the wind, depending on the ratio of power output versus power input (efficiency factor), and the ratio of air flow speed through the prop versus ground speed.

This video, of the first "mini-cart", was made on a longer treadmill, the last 3 runs are pretty good, and you get some sense of a modest amount of acceleration. The only issue here is that the cart starts near a wall, where "ground" effects could be an issue, but the prop thrust and wash are relatively small, the cart appears to accelerate or maintain speed.

http://www.youtube.com/watch?v=MfZt19F-OA4&fmt=18

This is a video of the improved second "mini-cart". Traction is an issue on the outdoor runs. On the treadmill, note that the cart is tapped so it's moving backwards relative to the non-moving air, slowing both the wheels and the propeller, while increasing the relative tail wind. The cart responds by accelerating forwards, until it's moving forwards relative to the still air:

http://www.youtube.com/watch?v=kWSan2CMgos&fmt=18

Another video of the second "mini-cart". At about 1 minute into the video, the treadmill is angled upwards enough to allow the cart to hold it's position for relatively long periods of time.

http://www.youtube.com/watch?v=7xL8gRJ5F6k&fmt=18

 

[schroder]

So what we have determined is that what would cause this device to not work is inefficiency: build a device with low friction and an efficient propeller and it should work.

No Russ. What you are in effect saying is that the work done by the propeller is more than the work done by the wheels which are powering the propeller. You and I both know that is impossible. Your basic instinct was right when you rejected this thing, but now you have convinced yourself otherwise, unfortunately for you and for the credibility of this forum Please forget all these hypothetical’s and address the argument I made earlier about the floor being the common reference frame. Also forget about the tread working “against” the cart and or the cart advancing “against” the tread; that is immaterial. The tread is just a source to power the wheels and it can be designed to run in any direction relative to the cart. All that matters is the cart’s progress against the floor, whether it is powered by the wind, the tread or a laser light beam!

 

[schroder]

You want an experiment to prove this? Gear up the cart so that it runs forward when sitting on a treadmill that runs in the same direction, the tread is running from behind the cart. That is a very simple reversal of gear and/or propeller pitch. The tread advancing from behind gives a truer representation of the wind blowing from behind. Hold your hand on the cart until it fully winds up and let it go. Tell me if it outruns the tread. That’s the challenge. I know that it will not but you all have a go at it and be sure to let me know how it works out!

 

[rcgldr]

... work done by the propeller is more than the work done by the wheels which are powering the propeller.

Not work, but force. The force at the propeller is greater than the force from the wheels, but the prop applies this force to air moving at much slower speed than the cart's speed relative to the ground. Assume the wind speed is 15 ft / sec (a bit over 10mph). Assume cart is moving directly downwind at 18 ft / sec, 3 ft/sec faster than the wind, and the relative wind through the prop is 5 ft / sec (2 ft / sec in addition to it's relative wind speed due to induced wash). Assume force from the wheels is 1/2 lb, and thrust at the prop is 1 lb. In a span of 1 second, the work done at the ground is 1/2 lb x 1 sec x 18 ft / sec = 9 lb ft. The work done by the prop is 1 lb x 1 sec x 5 ft / sec = 5 lb ft. Assuming the cart isn't accelerating, the losses are 4 lb ft, or about 44.5%. If efficiency is >= 55.5%, then the cart works with these parameters.

 

[yoavraz]

OOOPS. I knew there was a disconnect, but until now I have not realized we are talking about different experiments. I understand you have previous context. I apparently was thinking about a different forum from the one that was mentioned (I see this subject is popular...). At least we agree about the outdoor experiment description, I hope. Though I can guess now the scenario, I do not want to guess anymore.

Mender, Could you kindly refer me to your treadmill experiment description, or describe it here, or send me a private message. Also pls point me to the related video. Thnx

Thanks for video and info. See also the following treadmill DWFTTW video:
http://uk.youtube.com/watch?v=dgHBDESd38M&feature=related
that I picked in a search.

First of all, a substantial difference exists between a sailboat and a DWFTTW. I was completely in sailboats, and the headline "sailing downwind" completely misled me. Also the outdoor experiment misled me, though I was puzzled by the flag behavior (relative wind) a little. DWFTTW is definitely NOT SAILING. I stand behind all I said regarding a sailboat, also with wind turbine, but it is completely irrelevant to DWFTTW!

Second, what striked me the most, is the fact that a same machine architecture can be either a SAILBOAT, or DWFTTW, depending on parameters: The two major parameters that define the type are gear, TRANSMISSION RATIO and PROPELLER STEP.

In a SAILBOAT the wind pushes the vehicle. The wind (propeller) turns the wheels. The propeller step is low, the propeller is very high-speed (many revolutions per second), while the connected wheels are relatively very slow.
This behaves with all the characteristics of a sailboat, including the all the effects and physics with relative wind (apparent wind), which is the major factor (the propulsion source). Here all that I said earlier with the 180 direction applies. Here movement will start spontaneously at a certain minimum wind speed.

In a DWFTTW it is almost the opposite: The wheels turn the propeller, which generates relatively strong wind, thrust. The wheels turn fast, and the propeller relatively medium-slow. Now the propeller has a relatively large step to create max thrust. Here apparent wind does not play any major role (or any at all on the treadmill, if the air is really still - see below). Thus deviation from 180 wind does not matter much, only to the extent that its component from behind (180) pushes a little the vehicle. Here Gravitation and inertia play a major role. A little downhill makes a major contribution to wheel speed, and thus to propeller thrust. Inertia overcomes temporary disturbances that negatively affect wheel speed. I'm not sure at all about (relative) wind effect, though it may contribute a (little) force to overcome a little drag/friction. However, with a treadmill, when the vehicle is stationary, no relative wind exists (unless a substantial moving air boundary layer - wind - is generated close the the moving treadmill surface, but this in opposite direction!). Here you need a push (or downhill, or already running treadmill relatively to vehicle) to start movement.

In-between SAILBOAT and DWFTTW, I guess it has hybrid properties, depending on the two major parameters. I guess that in very strong winds it goes to a sailboat, and in relatively slow winds it will tend to be DWFTTW. I guess that in many parameter ranges it will not work at all because of conflict of forces.

All this brings us to the last question, the main subject of the discussion here, I understand: Is treadmill experiment equivalent to outdoor?

Now I'm not sure, and cannot tell by the videos without measurements. In the treadmill relative wind does not exist, and thus cannot play a role. This is the main difference with outdoor, where wind exists. Secondly, I'm not sure about the the slop. Uphill, or downhill, or horizontal. I do not know about the conditions in the experiments since a very small angle can make a big difference in tilting the equilibrium. I think I saw something about uphill on treadmill, but here the question is for how long. Inertia may sustain it a little, but then a little push is needed again. To try it you need guides/rails to keep the vehicle on the treadmill for long enough time.

 

[schroder]

Assume the wind speed is 15 ft / sec (a bit over 10mph). Assume cart is moving directly downwind at 18 ft / sec, 3 ft/sec faster than the wind, and the relative wind through the prop is 5 ft / sec (2 ft / sec in addition to it's relative wind speed due to induced wash).

What? Your argument consists of asking me to assume that the cart is already going faster than the wind? Are you serious?

 

[ThinAirDesign]

Thanks for video and info. See also the following treadmill In the treadmill relative wind does not exist, and thus cannot play a roll.

How someone can see a treadmill belt moving at 10mph in a still air room and say "relative wind does not exist" is simply amazing.

Actually, nothiing should amaze me anymore with this device -- It causes people to say some of the most mind bending things.

JB

 

[rcgldr]

Your argument consists of asking me to assume that the cart is already going faster than the wind?

Yes, assume the cart is already going faster than the wind. Assume something pushes the cart up to the desired speed then releases the cart. The question is what happens after the cart is then released to run on it's own with just the wind power. My point was that power output can be much less than power input, even when traveling faster than the wind. Look at the math again. I chose an example where prop thrust was double the wheel force, but for the real carts, it's probably less, maybe 3 to 2 or so.

 

[yoavraz]

How someone can see a treadmill belt moving at 10mph in a still air room and say "relative wind does not exist" is simply amazing.

Actually, nothiing should amaze me anymore with this device -- It causes people to say some of the most mind bending things.

JB

Very simple: If the AIR DOES NOT MOVE, there is NO WIND!

Look up definitions of "wind"

 

[rcgldr]

Is treadmill experiment equivalent to outdoor? In the treadmill relative wind does not exist.

The treadmill is equivalent. You don't need relative wind speed, or a relative ground speed, as these depend on a frame of reference. What is needed is a difference between wind speed and ground speed, independent of a frame of reference. For outdoors, you have +10 mph wind speed and 0 mph ground speed. For the treadmill, you have 0 mph wind speed and -10 mph ground speed. Both cases are equivalent. With a 10mph and a DDWFTTW cart, regardless of the frame of reference, if it's going DDWFTTW, then the cart's speed relative to the ground is a bit greater than +10mph, and the cart's speed relative to the air is a bit greater than 0 mph.

Relative to a ground (or treadmill) based observer, the wind moves at +10mph and the cart slightly faster. Relative to the air, the ground (or treadmill) moves at -10mph, while the cart moves forwards slowly. Relative to the cart, the ground moves a bit faster than -10mph backwards, and the air moves backwards at slow speed.

 

[ThinAirDesign]

Very simple: If the AIR DOES NOT MOVE, there is NO WIND!

Tell that to the fly on the hood of your car.

JB

 

[mender]

No Russ. What you are in effect saying is that the work done by the propeller is more than the work done by the wheels which are powering the propeller. You and I both know that is impossible.

Shroder, you are making a very common mistake. You have decided that the results that you are seeing are impossible and have come up with an explanation that allows you to accept that the cart is moving forward on the treadmill, yet conforms to what you think is happening. You are letting your decision/conclusion alter your interpretation of the data and conditions.

Things you need to understand to get past this: the test is valid, and the results are conclusive. The cart is demonstrating DDWFTTW when it moves forward on the treadmill. It is also demonstrating that it is harnessing energy from the difference in speed between the ground and the air - what most people understand as wind. It is also demonstrating a surplus when it is holding station against a 4.4 degree incline. It is not perpetual motion. It is not a hoax. It is non-intuitive, meaning that you have to work through the scenario and find out what is really happening, rather than depending on your gut reaction. It is all there to be found and calculated, a balance of forces that is straightforward (pun intended) once you go through the complete exercise.

Once you accept this and work through it (which may take some time off by yourself) you'll see the error in your "equivalent" frame of reference description.

Yoavraz, I looked up the definitions of wind, wind power, etc., to make sure I am dutifully following what you are saying. I'm hoping that you will do the same - actually I'm sure you will without me needing to coax you.

If what you are saying is true, my car won't feel any air resistance as it speeds up. After all, the air isn't moving, just the car. And I won't feel any "wind" if I stick my hand out the window at 60 mph because again there is no wind because the air isn't moving relative to the ground.

 

[yoavraz]

It is also demonstrating a surplus when it is holding station against a 4.4 degree incline.

Yoavraz, I looked up the definitions of wind, wind power, etc., to make sure I am dutifully following what you are saying. I'm hoping that you will do the same - actually I'm sure you will without me needing to coax you.

If what you are saying is true, my car won't feel any air resistance as it speeds up. After all, the air isn't moving, just the car. And I won't feel any "wind" if I stick my hand out the window at 60 mph because again there is no wind because the air isn't moving relative to the ground.

kjgilkgihghi

 

[yoavraz]

It is also demonstrating a surplus when it is holding station against a 4.4 degree incline.

Yoavraz, I looked up the definitions of wind, wind power, etc., to make sure I am dutifully following what you are saying. I'm hoping that you will do the same - actually I'm sure you will without me needing to coax you.

If what you are saying is true, my car won't feel any air resistance as it speeds up. After all, the air isn't moving, just the car. And I won't feel any "wind" if I stick my hand out the window at 60 mph because again there is no wind because the air isn't moving relative to the ground.

That is great, and actually I can now see how when having the initial momentum, with inertia an effective DWFTTW can sustain enough thrust to maintain it moving. No wind exists when the vehicle is stationary relatively to room. (Of course, the propeller makes wind backwards, but no external wind.)

As I wrote, only some possible wind close to the belt may exist due to boundary layer effect (the belt moves the close air layers; I remember quite strong wind from a conveyor belt), but in opposite direction! Thus it can only slow the vehicle a little, but apparently not stop it.Still I'm not sure if back-wind is essentially needed for the outdoor vehicle. It may provide the "last push" needed to overcome the drag for this particular vehicle, but in principle back wind looks secondary. For sure no wind exists indoor on treadmill.

----------------------
Added: Without the treadmill you must have wind, at least a little, to supply energy after the energy in first push of the vehicle is dissipated by drag and friction.

 

[ThinAirDesign]

You want an experiment to prove this? Gear up the cart so that it runs forward when sitting on a treadmill that runs in the same direction, the tread is running from behind the cart. That is a very simple reversal of gear and/or propeller pitch. The tread advancing from behind gives a truer representation of the wind blowing from behind. Hold your hand on the cart until it fully winds up and let it go. Tell me if it outruns the tread. That’s the challenge. I know that it will not but you all have a go at it and be sure to let me know how it works out!

Schroder is asking us to place the device on a belt in such a way that it advances across the belt in the same direction that the belt is traveling through the room. He wants to know if the cart can advance up the belt in this configuration -- he wants the device to 'beat the belt' or "outrun the tread" to the other end of the room.


In other words, he is asking if a wind powered device can move into the wind, powered by the wind.

Think about it -- he's asking the wheels to turn 1mph (beating the belt) into a 10mph relative headwind. Forget the treadmill -- just take the device out into the street with a 10mph wind and see if it advances down the street at 1mph against the headwind.

Proving any particular device can/cannot advance against a headwind in no way demonstrates that a device can/cannot go DDWFTTW.

(For what it's worth, we don't claim that our device will drive against a headwind powered only by that wind (though it's easy with regearing to make it do so). However the issue of whether a device can be built that will do so has been demonstrated so many countless times that it is of no interest to us.)

JB

 

[rcgldr]

In the treadmill relative wind does not exist, and thus cannot play a role.

In the treadmill case, a relative wind does exist, it's 0mph. It plays a role because the treadmill is not operating in a vacuum. The propeller is accelerating the still air backwards, and this generates forwards thrust. The treadmill surface moves at relative speed of -10mph. The cart works because it's forward speed relative to the air is much less than it's forwards speed relative to the treadmill, allowing the speed at the wheels to be geared down while at the same time increasing the torque applied to the propeller, so the propeller generates more force but at a lesser still speed than the wheels.

This is the main difference with outdoor, where wind exists.

Well outdoors the relative wind would be +10mph, and the ground's relative speed would be 0 mph. From the cart's frame of reference, the treadmill case and the outdoor case are the same (ignoring gusts).

I'm not sure about the the slope. Uphill, or downhill, or horizontal. I think I saw something about uphill on treadmill, but here the question is for how long.

It's uphill in this video, near equilibrium:

http://www.youtube.com/watch?v=dgHBDESd38M&fmt=18

Also uphill in the second part of this video:

http://www.youtube.com/watch?v=7xL8gRJ5F6k&fmt=18

I think someone mentioned this was 4.4 degrees uphill?

 

[mender]

The original question of whether a treadmill test is valid has introduced some new objections. This leads me to believe that yet another method of testing may be beneficial in developing a device. The treadmill test only shows the results of the total system, not each loss along the way nor the total amount of power that is being harnessed to counter the drag.

Shroder's assertion that the treadmill only serves to drive the wheels is correct. However, his reasoning that the treadmill can be oriented in any direction other than directly in line with the cart's path is not. That would change the direction of the force acting against the wheels, which is what the propeller has to overcome in order to work. A way to separately measure the two opposing forces should help when developing another design. For this discussion I'll use a propeller as the air interface.

I need a simple method of measuring how much resistance the air has to a moving propeller. What I propose to do is spin the wheels of the cart at the equivalent of 10 mph and measure how much force against the air is generated (lift in a forward direction), then measure how much force (or drag) is required to generate that lift force. From that, I can figure out the lift to drag ratio for the entire mechanism. Next, spin the prop by itself at the same rpm, measure the two forces and figure out the lift to drag ratio for just the prop. That will tell me how efficient my drive mechanism is and also how efficient my prop is. Finally, measure the rolling resistance of the cart with the drive mechanism disengaged so I know how efficient the vehicle is.

From doing this, I should be able to determine where my efforts can best be directed. After doing that, it should be a relatively simple matter of testing my other non-propeller air interface and fine-tuning its efficiency in isolation.

Is this reasonable? Can this be achieved with fairly common equipment? Or am I going to miss something?

 

[rcgldr]

Total amount of power.

If the specs for the propeller are known, that could be used. An electric motor with a known loss factor could be used to drive the propeller at the same rpm, and a watt meter used to determine the input power to the motor. I don't know if it's possible to drive the wheel axle with a motor, but if so, then again the input power to the motor required to drive the wheels which in turn drive the prop to a specific rpm could be measured. All the stuff I mentioned here should be available at a hobby shop or via hobby online store, although I don't know the cost as you need a watt meter, motor, motor controller and a power supply and/or rechargable batteries.

 

[ThinAirDesign]

Mender, I've only quickly reviewed your test request so bear with me.

Here is something I could do for you as it's something I've wondered about myself:

Test: 1
On a level treadmill we could nose the device up against a gram scale and show net thrust. we could do this at several speed.

Test 2:
On a level treadmill we remove the prop from it's shaft (leave the shaft and associated bearing drag) and "pull test" against the same scale. Again, this can be done as specific and associated to the above speed points.

Does this give you what you want? All? Some? Of no use?

Thanks

JB

 

[Subductionzon]

I am new to this forum, but on another I violently (well not quite) disagreed with spork on the veracity of the original video. In fact I can still see my comment on youtube.

Here are some points that will help clear things up, I hope. First off the treadmill tests are perfect proof of concept. If you don't think so imagine a treadmill arbitrarily large. If you are on a large enough treadmill you could not tell the difference between it moving 10 mph, and as a result you feeling a 10 mph apparent wind, or standing still on an open field and being hit by a 10 mph wind. There is no point in referring to the ground underneath the treadmill. Second, the wheels of the cart are coupled directly to the propeller. The propeller does not freewheel, so the cart's energy is coming from the difference between the speed of the wind and the speed of the ground. To those who think that what they are trying to do is approaching the idea of a perpetual motion machine you have to realize that with a air speed of zero (real and not apparent) the cart could not advance at all since the difference between ground speed and air speed is zero. It is a subtle difference but a very important one. If you think of the wind blowing on the cart you will be misled.

 

[rcgldr]

Test: 1
On a level treadmill we could nose the device up against a gram scale and show net thrust. we could do this at several speed.

Test 2:
On a level treadmill we remove the prop from it's shaft (leave the shaft and associated bearing drag) and "pull test" against the same scale. Again, this can be done as specific and associated to the above speed points.

Not enough information. One issue is that prop pitch increases both thrust and drag on the wheels, so the no prop pull test doesn't help much.

Replace the gearbox with a motor and drive the prop at the same rpm, and then measure the thrust with the gram scale. Then you'd know the thrust, and this reading minus the test 1 gram scale reading would give you the overall drag factors on the cart.

 

[mender]

Mender, I've only quickly reviewed your test request so bear with me.

Here is something I could do for you as it's something I've wondered about myself:

Test: 1
On a level treadmill we could nose the device up against a gram scale and show net thrust. we could do this at several speed.

Test 2:
On a level treadmill we remove the prop from it's shaft (leave the shaft and associated bearing drag) and "pull test" against the same scale. Again, this can be done as specific and associated to the above speed points.

Does this give you what you want? All? Some? Of no use?

Thanks

JB

Yes, JB, "all" will be very helpful! The first test should give me the L/D ratio of your cart and the second the rolling resistance of the cart and drivetrain.

Can you also tell me the gearing that you're using and repeat the prop specs? And measure the amount of force/resistance the prop generates at the same mph intervals? This may be asking a lot but ...?

With that data, we should be able to plot several curves and predict velocities - maybe even increase the L/D of your cart, although that may be hard to do!

 

[mender]

Jeff's suggestion echos my request for the force the prop generates at the 10 mph rate. However, if you can add the extra data points we can graph that and be able to provide numbers to prove what is happening. Sometimes the numbers are believed more than the eyes.

 

[rcgldr]

Well one alternative is to visit web sites that calculate thrust versus rpm for given prop diameter and pitch, then average those, since I don't trust any of them (the AMA one ignores the pitch aspect).

If the prop pitch is excessive, then it's adding both thrust and drag factors into the cart, and could be the dominant drag factor on the wheels. This is independent of L/D of the prop, and if the L/D is high, than it's just adding stress to the drive train, which could indirectly increase the drag factor.

The only way to know if the pitch is excessive is to try larger driving wheels to slow down the prop and see if the results improve or degrade.

 

[mender]

How about this site and calculator:

http://members.jcom.home.ne.jp/4223215501/staticthrust.htm

It sounds like I need to get a power supply, one or two decent size RC electric motors and a few props to evaluate and baseline, then compare my non-prop design against that baseline and develop as needed.

 

[schroder]

Those are all excellent tests you are proposing. I predict you will find without fail that the drive power necessary to turn the wheels will be significantly more than the available output thrust at the propeller. In cannot come out any other way. I am disappointed that no one can answer my question about the validity of using the floor as a common reference for both frames where frame one is a cart sitting on a stationary tread and being pushed from the rear by a wind; and frame two where the cart is now being driven by a moving tread in stationary air. The floor is stationary at all times with the tread in frame one and with the air in frame two. The floor is a valid reference in both cases. In frame one, the cart advances relative to the floor at close to wind speed. In frame two, the cart is also advancing relative to the floor, but at very much less than the tread speed. For some reason which you cannot clarify or justify, you use the moving tread as your reference in that frame, which is clearly not a demonstration of anything. I can place a cardboard box on a treadmill and run the tread fast enough so the air makes the box advance against the tread. What does that prove? The cart must advance relative to the floor in both frames as they are equivalent. Again, for a physics forum I am amazed and disappointed that I need to defend conservative physics principles such as Nothing powered by the wind can go faster than the wind going directly downwind. You really need to get sorted out on this issue if you are to avoid becoming the laughing stock of the physics world.

 

[ThinAirDesign]

Again, for a physics forum I am amazed and disappointed that I need to defend conservative physics principles such as Nothing powered by the wind can go faster than the wind going directly downwind.

You are confusing established principles of physics with opinion -- yours.

There are no principles of physics stating that the above is impossible and I have the device on my desk which upon demand (and even polite request) can demonstrate the fallacy of such a position.

JB

 

[yoavraz]

Again, for a physics forum I am amazed and disappointed that I need to defend conservative physics principles such as Nothing powered by the wind can go faster than the wind going directly downwind. You really need to get sorted out on this issue if you are to avoid becoming the laughing stock of the physics world.

Forget about the wind. The only place you have a (external) wind in a DWFTTW on a treadmill is in the name DWFTTW (the W's)... I thought we have cleared this.

Thus you have nothing to defend here. It boils down to a little energy from the treadmill that transforms in a quite sophisticated way to thrust and movement in the opposite direction. No law of Physics is broken...

Actually, after I wrote the last paragraph above I have realized that without the treadmill you must have wind, at least little, to supply energy after the energy in first push of the vehicle is dissipated by drag and friction.

 

[mheslep]

...Nothing powered by the wind can go faster than the wind going directly downwind. ...

That is not an apt description of what is happening with this cart. The propeller surface is not going directly 'downwind', downwind being the direction of the cart, it is moving across the direction of the wind. In that case it is just another mechanical device like a sail boat traveling across the wind that converts energy in the wind stream via mechanical means to the kinetic energy of the platform. As any sailor knows a sail boat traveling across the wind can travel faster than the prevailing wind speed over the surface of the water. Regards perpetual motion, physics only states that the device must convert somewhat less of the energy it captures from the wind stream to the overall kinetic energy of the device. Focusing on the 'faster than wind' aspect is fraught with missteps.

To further demonstrate how the wind speed/ cart speed coupling is a red herring: the propeller cart could be equipped with a small electric generator/motor and a battery to store energy. The cart is fixed for a time in wind speed X and charges the battery as a wind turbine. Later, the cart can take off under motor power and travel many times the wind speed X in whatever direction.

 

[mender]

Frame one: the cart is sitting on the stationary tread. It has a wind blowing from behind at 10 mph. The movement of the air relative to the ground is what we call wind. Therefore the cart feels that relative movement as wind. Just to make sure, the cart is tethered to hold it in position.

Extend this: tow the cart forward at 10 mph in the 10 mph wind. The cart no longer feels a relative wind even though the wind relative to the ground is still there. Correct? At that point the cart is moving at the same speed as the air.

Frame two: to duplicate the conditions in Frame one, a speed difference between the air and the ground needs to somehow be achieved so that the cart can feel a 10 mph wind from behind, just like Frame one. One way to do that would be to tow the cart backwards at 10 mph in still air. But that raises a problem. In Frame one, when the cart feels the 10 mph wind from behind, the wheels which are part of the drive system are stationary. In order for the conditions to match Frame one, the wheels need to stay stationary as the cart is towed backwards. Easy enough: we attach a long flat surface that can slide along the ground to the same tow line and pull both backwards at the same time. Now we have matched the conditions of Frame one exactly. The air flow relative to the cart is 10 mph from behind, The ground speed relative to the cart is zero. The cart is held in position.

An observer standing on the ground beside the cart in Frame one would feel the same 10 mph wind from behind as the cart. An observer standing on the surface being towed backwards with the cart in Frame two would also feel that wind. An observer in Frame two standing on the ground beside the surface being towed would not feel the same wind that the cart feels; therefore the ground reference in Frame two is invalid. The only valid reference in Frame two is the moving surface.

Shroder, your question about using the floor as the reference for the second case has been answered by several people. The answer is no. You are failing to accept that answer. Your reason for not accepting the answer is based on your belief that "Nothing powered by the wind can go faster than the wind going directly downwind."

I appreciate your recognition of the validity of the proposed tests:

"Those are all excellent tests you are proposing. I predict you will find without fail that the drive power necessary to turn the wheels will be significantly more than the available output thrust at the propeller. In cannot come out any other way."

I think I understand why you have this belief. You see the thrust generated by the propeller as being, at best, the same as the drag needed to spin it. Since a propeller is essentially a wing, a quick look at typical lift to drag ratios for wings and propellers should help you get past this. Consider the thrust as lift.

 

[Subductionzon]

I am disappointed that no one can answer my question about the validity of using the floor as a common reference for both frames where frame one is a cart sitting on a stationary tread and being pushed from the rear by a wind; and frame two where the cart is now being driven by a moving tread in stationary air. ... . You really need to get sorted out on this issue if you are to avoid becoming the laughing stock of the physics world.

Why would you want to use the floor as a point of reference. Look at it from the cart's point of view. On an long treadmill if the cart was stationary (we will give it some imaginary locked brakes for this) it will feel a ten mph tail wind. That is what spork et al were trying to simulate by running their belt. There is no point to using the floor as a frame of reference. So when the cart outruns the treadmill, that is when it advances forward on the treadmill as was evident on the videos then it is clearly outrunning its perceived tail wind. Not only did they outrun the wind, they also outran it even when the treadmill was sloping up. Please note the cart derives its power from the difference between the speed of the wind and the ground (or in the case of the treadmill the perceived wind) so if the treadmill is not moving or if the wind is not blowing the cart will not move. No wind equals no power.

Speaking of points of reference I don't know if this has been asked before but what could go faster: A boat sailing down a stream moving ten mph with no wind, or a boat sailing down the same ten mph river with a ten mph tail wind? If you can answer this correctly you can see why the cart on the treadmill is the same as the cart sailing directly downwind faster than the wind.

 

[schroder]

You are confusing established principles of physics with opinion -- yours.

There are no principles of physics stating that the above is impossible and I have the device on my desk which upon demand (and even polite request) can demonstrate the fallacy of such a position.

JB

Does it look like a cardboard box? I have one of those also and it will advance against the tread. Maybe I will put in youtube.

 

[ThinAirDesign]

Forget about the wind. The only place you have a (external) wind in a DWFTTW on a treadmill is in the name DWFTTW (the W's)...

Nowhere will you find the parenthetical "(external)" in our claim with all it's implications. This is apparently a requirement of yours, though definitely not ours.

Wind it merely relative motion between surface and air. IFOR physics principles dictate that one cannot actually determine which is moving and which is not for all motion is relative.

A sailor in the middle of the ocean could see a nice breeze across his derigged boat and say "ah ha! -- the wind finally arrived", but in truth perhaps it is not the air moving at all, but a current of the ocean pushing his hull. He nor the boat is none the wiser and his sailing day moves forward the same way in either case -- he can use this wind to reach his next waypoint.

The cart on the treadmill is truly going DDWFTTW -- you're just confused by the frame of reference you happen to be standing in. It's a common mistake -- people have a hard time realizing that physics isn't about 'them' and their particular frame.

... I thought we have cleared this.

Most here have -- you still have not.

JB