Sailing downwind faster than the wind: resolved?

[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

 

[ThinAirDesign]

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.

When you have a video of your cardboard box advancing against the motion of a treadmill in a still air room, using only said motion as power, send me the link and we'll chat. I'll be looking forward to it.

JB

 

[yoavraz]

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.



Most here have -- you still have not.

JB

Forget external. Real wind only by the propeller. You can call any imaginary object "wind," but it exists only if air really flows. With the treadmill DWFTTW is an empty name.

 

[mender]

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.

Wind: the cart needs a source of power to work. It uses the wind as that source. The amount of power available from the wind is measured by the wind speed. That always assumes (and rightly so) that the ground is stationary.

Yoavraz, let's say we want to check a wind turbine to see how much power it can harness from a 10 mph wind. We mount the wind turbine on a trailer and drive around until we find a place that is experiencing a 10 mph wind. We stop, point the wind turbine into the wind and start taking our measurements. Everything is good, we get our measurements before the wind goes calm. We pack up but forget to secure the wind turbine. As we get to 10 mph in the still air, we notice that the wind turbine is turning. For the fun of it, we again hook up our test equipment and find that when the wind turbine is being towed through the air at 10 mph, it acts exactly the same as when it is sitting still in a 10 mph wind. Therefore, from now on we decide that instead of wasting time looking for the right wind, we generate our own wind by moving the ground reference at 10 mph.

The wind turbine doesn't know the difference. When the air is still and the ground is moving, the wind turbine behaves exactly the same way as when the ground is still and the air is moving. Those are equivalent frames of reference, and both are perfectly satisfactory for testing. According to the wind turbine and the output that is measured, the air is flowing past the wind turbine. To the wind turbine, that is wind. To the cart, air flowing past is also wind and generates the same results. You can call it fake wind (most would call it relative wind which is the correct term - look it up) but the results are the same as a wind moving across the ground.

The treadmill allows us to "tow" the cart along the ground at 10 mph. If we had a really long treadmill that was moving at 10 mph, we could step onto the treadmill surface holding our cart, set it down facing "downwind" and let it go.

What are the possible outcomes of this?
1: The cart doesn't move.
2: The cart moves and reaches a speed of less than 10 mph in reference to the treadmill surface
3: The cart moves and reaches 10 mph in reference to the treadmill surface.
4: The cart moves and reaches a speed above 10 mph in reference to the treadmill surface

Since we only have a short treadmill (I hope that you now accept that the treadmill is a valid substitute for a wind - if not read the frames of reference again and ask questions), we can only test a short portion of the full scale outdoor test. That segment is with the cart at 10 mph, with only a few feet either way to indicate a trend. If we place the cart on the treadmill surface moving at 10 mph, the outcomes that I listed above would look like this:

1: The cart would quickly move to the back of the treadmill when it is released
2: The cart would slowly move to the back of the treadmill when released (may be hard to see the difference between this and #1 but in either case this would be a failure)
3: The cart would stay in position on the treadmill (this would be downwind at wind speed)
4: The cart would move forward on the treadmill ( this would indicate moving directly downwind faster than the wind)

 

[ThinAirDesign]

Forget external. Real wind only by the propeller. You can call any imaginary object "wind," but it exists only if air really flows. With the treadmill DWFTTW is an empty name.

And therein lies the crux of your misunderstanding. There is nothing "imaginary" about this wind. To any object on that treadmill, including the device, there is *exactly* the same wind in that room as out on the street in a tailwind.

I know it seems impossible, but not only is it possible, it's the law.

JB

 

[ThinAirDesign]

On a treadmill the treadmill provides the energy. No wind exists.

With a "conclusion first" position, facts to fit will follow.

JB

 

[swerdna]

I don’t see that a treadmill is comparable with a wind as an energy source in this experiment. A treadmill moving at 10mph is always moving at 10mph relative to the vehicle and the vehicle always receives a constant 10mph worth of energy. The vehicle traveling at 5mph with a 10kmph wind is only receiving 5mph of wind energy. The faster the vehicle travels with the wind the less energy the vehicle receives from the wind. If the vehicle reaches the speed of the wind it receives no energy from the wind at all.

If you think the vehicle on a treadmill is the same as a vehicle in a tailwind, put a flag on the treadmill vehicle and see if it flutters in the wind.

 

[swerdna]

As I understand it the claim is that the propeller is gaining it‘s energy from the rolling wheels as the vehicle moves over the ground. And that the movement of the vehicle over the ground gets it‘s energy from a combination of the thrust created by the propeller and the speed of the wind against that thrust. When the vehicle reaches the speed of the wind however only the thrust of the propeller remains as an energy source. How can the thrust of the propeller alone provide extra energy to the vehicle to move faster over the ground than the wind into a headwind?

 

[cesiumfrog]

What a long thread.

However, with a sailboat it is different, and with some side-wind component most land sailboats can go faster than the wind. Never in 180 wind though.

Think about what you are saying. First your sailboat tacks, goes at right angles to the wind, and builds its speed up to be much faster than the wind. Then it collapses its sail, and turns 90 degrees to coast parallel with the wind, but retains its speed and so will be overtaking the wind (provided that friction is small and this cycle is repeated, the average velocity of the sailboat can be larger than and in the same direction as the wind).

Perhaps we should poll whether anyone still thinks such a contraption isn't possible.

 

[ThinAirDesign]

What a long thread.

Think about what you are saying. First your sailboat tacks, goes at right angles to the wind, and builds its speed up to be much faster than the wind. Then it collapses its sail, and turns 90 degrees to coast parallel with the wind, but retains its speed and so will be overtaking the wind (provided that friction is small and this cycle is repeated, the average velocity of the sailboat can be larger than and in the same direction as the wind).

Perhaps we should poll whether anyone still thinks such a contraption isn't possible.

Your above scenario may or may not be possible, but it is not necessary for a sailboat to ever change directions to reach VMG of greater than 1.0.

Also, the above doesn't meet the requirements of our claim as it strays from DDW.

JB

 

[mender]

I have just realized that without treadmill, i.e., outdoor, you need the wind as an energy source. Wind is the only possible one on horizontal surface. On a treadmill the treadmill provides the energy. No wind exists. Vehicle speed depends on treadmill speed.

Right! Now you're got it! In the outdoor test, the wind provides the difference in speed between the air and the ground reference. In the room, the treadmill provides the difference in speed between the air and the ground reference. In Frame one the outside wind is the power source; in Frame two, the treadmill is the power source.

This brings us again to the question of faster than the wind. At wind speed the relative wind on the vehicle is 0, and no wind force exists to transfer energy. Above wind speed the wind force is backwards, which slows the vehicle (energy is being lost with no renewal) and drives it back to "slower than the wind." Thus it only temporarily can go faster than the wind, and so on, possibly in cycles. This with DWFTTW.

Wrong! Sorry! You're making another classic mistake. You're assuming that the cart outruns its power source. It doesn't; it is immersed in the air that is moving along the ground at 10 mph. All the cart needs to do is interact with that mass of air that is moving and it can always get energy from it. When the energy it takes from the air is the same as the total drag of the vehicle, it reaches a stable speed and stays there until something changes.

This works the same way as overdrive in a car. The propeller is geared to run faster than the ground speed. On Jack Goodman's cart, the ratio was 1.75:1. If the cart had no friction, the cart would always move 75% faster than the wind. Because of friction, it doesn't quite get there. In a 9 mph wind, the cart moved at about 13 mph for a ratio of 1.4:1. At slower wind speeds, there isn't as much energy to be harnessed so it can't achieve that high a ratio. At 4 mph it runs at 1:1 or 4 mph.

 

[Subductionzon]

Here when Thin Air Design and spork developed their cart I thought all controversy would disappear. I was definitely an anti before and have come to learn the errors of my ways. The treadmill to me was an obvious proof of concept. As I have said before make the treadmill arbitrarily large and you cannot tell the difference between being on a large treadmill and being in an open field. Now it is understandable to look at the cart an say "No way" but people here should understand frame of reference and how the treadmill perfectly emulates a tailwind.

 

[yoavraz]

With a "conclusion first" position, facts to fit will follow.

JB

Again: When the vehicle is stationary, in a room with stationary air, no air moves around the vehicle, no wind is felt by the vehicle, no force of wind is applied to the vehicle, and no wind energy is transferred to the vehicle. The vehicle gets its energy from the turning wheels, that are turning because of friction with the treadmill belt and the pulling propeller. The belt applies force on the wheels and transfers energy. The vehicle keeps its stationary position due to the thrust generated by the propeller.

I hope you understand this in spite of your sarcasm.

 

[mender]

I see we are posting past each other. It seems to me that threads get out of control when this happens. Let's slow things back down and let everyone catch up.

Yoavraz, you are correct, in Frame two the treadmill is providing the energy to turn the wheels and power the prop. Please read my last post and comment on that so I can see where you're at.

 

[ThinAirDesign]

Again: When the vehicle is stationary, in a room with stationary air, no air moves around the vehicle

And this is your critical mistake. There is still plenty of wind apparent to the propsail as it is spinning.

no wind is felt by the vehicle,

No wind is felt by the *chassis of the cart* - there is plenty of wind felt by the propsail as it is spinning.

Put a fly on the tip of the prop and try to tell it there isn't any wind.

no force of wind is applied to the vehicle, and no wind energy is transferred to the vehicle.

The apparent wind of the propsail generates more than enough thrust to move the vehicle forward relative to the wind. This is demonstrated in both indoor and outdoor videos.

I hope you understand this in spite of your sarcasm.

There is no sarcasm whatsoever -- the mistake is yours and it's a common one. There is an entire posse of knowledgeable folks here attempting to demonstrate to you the error of your position, but you have "concluded" it is not possible and thus refuse to listen.

You continue to say there is "no wind" and yet relative motion between the air and the rolling surface exists in fact, and can be measured, documented and exploited.

JB

 

[cesiumfrog]

Your above scenario may [..] not be possible

Why?

[..and] doesn't meet the requirements of our claim as it strays from DDW.

The blades of the contraption's fan also stray from DDW.

 

[ThinAirDesign]

If you think the vehicle on a treadmill is the same as a vehicle in a tailwind, put a flag on the treadmill vehicle and see if it flutters in the wind.

How about we put a flag on the chassis *both* vehicles ... the one on the treadmill, and the identical on out on the street. We'll place the streamers on a small shaft far enough above and ahead of the propsail so as not to be influenced by it.

Let's cover the street one first:

10mph tailwind
Wheels rolling 10mph.
Chassis speed = 0 relative to surrounding air and thus the streamer hangs limp.

Next let's go to the treadmill in still air room:

10mph treadmill speed
Wheels rolling 10mph.
Chassis speed = 0 relative to surrounding air and thus the streamer hangs limp.

So now, for emphasis, I will repeat your above statement:

If you think the vehicle on a treadmill is the same as a vehicle in a tailwind, put a flag on the treadmill vehicle and see if it flutters in the wind

As you can see from the demonstration that you requested, *both* streamers show the *exact same behavior* and thus according to your own presented logic, a vehicle on a treadmill is the same as a vehicle in a tailwind.

It was an excellent test to suggest BTW and if you think carefully about the results it will help you understand why the treadmill test and the street test are the same.

JB

 

[ThinAirDesign]

The blades of the contraption's fan also stray from DDW.

Of course they do -- that is the secret to it's success after all.

If you plot the course of the indiviual parts of an airplane, ship, car etc, they go in all sorts of different directions ... doesn't mean that the plane, car and ship can't be going straight south.

The pistons in a Porsche after all go back and forth with a regular East/West component even while the driver, chassis and CG of the Porsche drive directly South on the Interstate. No one argues over the which direction the car is going.

As with the Porsche, both the Chassis and the CG of the cart are going DDW.

JB

PS: I suppose anyone can define the point that defines the direction a vehicle is going any number of ways. I believe a quite logical point to pick is the CG. It's hard to argue that if the vehicles center of mass is moving south, that the vehicle isn't.

 

[ThinAirDesign]

Me:

Your above scenario may [..] not be possible

Why?

Because the outcome depends on data not in evidence.

You say things like "provided that the friction is small", but you don't say how small.

You also say the cycle is to be repeated -- how long does it take to "collapse the sail" and then set it again?

May or may not be possible.

JB

 

[russ_watters]

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.

That isn't what is being claimed at all. Please read and respond to one at a time, the description of the parts of the scenario I described. Your error is in there somewhere, but I think the basic problem is you are not working through the problem - you are going with an instinct in a situation where instincts often fail.

Please forget all these hypothetical’s and address the argument I made earlier about the floor being the common reference frame.

There is no need for a "common reference frame".

 

[russ_watters]

There is no need for a "common reference frame".

The only way out of this is to look at a diagram and I asked you to draw one, but you haven't. So I have. Please excuse the crudeness.

We're going to take this slow. The cart is sitting on some surface, which is moving past the cart at 6mph as measured by a speedometer mounted on the cart. The air is moving past the cart at 1mph as measured by a wind speed indicator mounted on the cart.

Is the surface the cart is sitting on a treadmill or outside on a road in a 5mph wind? How can you tell?

Put another way: Is it possible for the sensors on the cart to measure those conditions on either or both scenarios?