Remember the debate about downwind vehicles

[spork]

We've had a couple of heated debates here about whether one could build a wind powered vehicle that would go directly downwind faster than the wind. So we built one...

First run: http://www.vimeo.com/10476453
Second run: http://www.vimeo.com/10477373
Last run: http://www.vimeo.com/10476216

 

[mender]

Hopefully, large scale physical evidence of DDWFTTW travel will help moderate some of the extreme anti invective and allow a calmer discussion of this fascinating phenomena.

 

[spork]

Hopefully, large scale physical evidence of DDWFTTW travel will help moderate some of the extreme anti invective and allow a calmer discussion of this fascinating phenomena.

Actually, the resulting discussion goes beyond calm - all way to non-existent.

 

[russ_watters]

...which then begs the question: where does the discussion go next?

 

[spork]

...which then begs the question: where does the discussion go next?

Ratification of an official DDWFTTW speed record (stay tuned), and then perhaps the overall land speed sailing record(?)

 

[rcgldr]

Perhaps the overall land speed sailing record(?)

DDWFTTW vehicles require a ground force that is backwards relative to the direction of travel (wrt ground) in order to provide the power to drive the prop. However the ground force on a sailcraft is perpendicular it's direction of travel (wrt ground).

Perhaps you mean the component of speed directly downwind? DDWFTTW vehicles require all of the ground force to be upwind, while sailcraft only require a fraction of the ground force to be upwind, depending on the sailcraft's heading. I'm pretty sure that ice boats will have a faster component of speed directly downwind, but land (sand) craft have more drag and I'm not sure.

 

[spork]

DDWFTTW vehicles require a ground force that is backwards relative to the direction of travel (wrt ground) in order to provide the power to drive the prop. However the ground force on a sailcraft is perpendicular it's direction of travel (wrt ground).

Perhaps you mean the component of speed directly downwind? DDWFTTW vehicles require all of the ground force to be upwind, while sailcraft only require a fraction of the ground force to be upwind, depending on the sailcraft's heading. I'm pretty sure that ice boats will have a faster component of speed directly downwind, but land (sand) craft have more drag and I'm not sure.

What you're saying is correct of course, but the numbers suggest that a DDWFTTW vehicle can take the existing overall land sailing speed record (not just the downwind component). Keep in mind the DDWFTTW vehicle gets a better break on aero drag too.

Greenbird is going after the ice record now. So I don't know what numbers they'll achieve. I'm pretty sure we're not interested in the ice record any time in the near future.

 

[rcgldr]

Greenbird is going after the ice record now.

Aren't they already in the 5x to 7x wind speed range? I recall something about 10 mph winds being ideal because the ice boats are designed with that in mind, achieving speeds of 50 mph to 70 mph? Beyond that and drag becomes an issue, so achieving the same ratio with a 15mph wind requires a different design (larger?), and the locations where they can use the ice boats are limited in size.

 

[spork]

Aren't they already in the 5x to 7x wind speed range? I recall something about 10 mph winds being ideal because the ice boats are designed with that in mind, achieving speeds of 50 mph to 70 mph? Beyond that and drag becomes an issue, so achieving the same ratio with a 15mph wind requires a different design (larger?), and the locations where they can use the ice boats are limited in size.

I haven't followed the Greenbird really. I believe they set the land record at 126 mph in winds that peaked at 47 mph. I believe they tried to outfit it for the ice, and found out they needed to basically start over from the ground (ice) up. But I haven't looked at the wind or grounds speeds for ice records.

 

[rcgldr]

I found an article posted at several websites. Apparently a Skeeter can achive 6x wind speed in light wind, less than the 10 mph I thought it was. In a chart of a 2 lap race, top speed was about 65 mph with 15 mph wind, but the goal there was to run a 2 lap course as opposed to a speed run.

The fastest wind powered devices are probably radio control gliders while dynamic soaring. At youtube there's a video showing a run of 375 mph, and later that day they recorded a speed of 392 mph, but didn't video the run, just the max speed on the radar gun.

http://www.youtube.com/watch?v=WaQB16ZaNI4&fmt=18

update - make that 399 mph as of 2009-12-22, same model, same location:

http://vimeo.com/8356047

 

[spork]

I found an article posted at several websites. Apparently a Skeeter can achive 6x wind speed in light wind, less than the 10 mph I thought it was.

Is that the NALSA article? I always thought it was more like 4x-5x

 

[rcgldr]

Is that the NALSA article?

Yes.

Skeeter ... It can sail about 6 times the wind speed in light winds and tops out a little over 60 mph.

nalsa_article.pdf

I think that the DDWFTTW carts are a bit more surprising and impressive that they could work at all; outrunning the wind that propels them.

 

[spork]

Interesting. I bet you found that in places that JB or I posted references to it. I never noticed that line. I've been saying 4X - 5X for a long time.

Interestingly, that article was written by Bob Dill. He's on the NALSA B.O.D. and was our contact and host at the Ivanpah event. We're working with him now to ratify a category to set a downwind record in.

 

[Subductionzon]

So where are all of the deniers to this topic? Now that spork has extremely strong video evidence of the cart going faster than the wind have they all run away? Check out the last run where the cart drives through a cloud of dust that is being blown by the wind. I think schroeder especially owes spork at least an "I'm sorry".

 

[DJM111188]

Spork, at your request, I have posted my questions below. Anyone with insight please feel free to chime in. Also I have now a better understanding of what the term perpetual motion means. At the time when I contacted you I hadn't taken into account that A)the wind is energy. But now I have revised the original question.

Hello I am not a frequenter of online forums but I recently was on the site and came across the "DWFTTW" machine and I felt the need to contact you. I have watched some of your videos on youtube. One of those was where you raced the turbine shaft vs prop shaft carts. I was wondering if you could clear some things up for me.

1) Are both carts using a 1:1 drive?
2) In the video you said the turbine shaft cart accelerated much quicker then the prop shaft cart correct? If this is true then would changing the gear ratio allow the turbine cart to have a much faster speed?
3) If your answer to number 2 is "Yes", then would the turbine cart be geared high enough to where it could keep up with the prop car? Or is the only reason a vehicle is able to travel faster down wind then the wind itself because of the wheels powering the prop?


I am just trying to learn so please take no offense to my inquiries. I would love to know how exactly this works. It seems to me that if 20mph of wind is applied to the turbine cart then it will never ever be able to reach a faster speed, even taking into account a gear ratio change, because when it would get to the 20mph threshold there would be no more force to turn the wheels. Am I correct in this assumption?

Also can you explain to me the end result in my hypothetical test? Say you had one of the carts that had a steering servo hooked up to the front wheels and you were able to find a stretch of 5 miles with absolutely flat and even surface. Let's say wind-speed could be controlled at exactly 20 mph. If you were to put the cart down and get it to travel the whole 5 miles directly downwind in a striaght line, is there any way to determine how fast it could actually go? More specifically would it have some magic number as a threshold top speed or would it continue to gain speed indefinitely as long as it had the room? If this description is too vague please notify and I will try to clear it up as best as possible.

Thanks for your time.

 

[spork]

Spork, at your request, I have posted my questions below.

Thanks. I'll do my best to answer your questions.

One of those was where you raced the turbine shaft vs prop shaft carts. I was wondering if you could clear some things up for me.

1) Are both carts using a 1:1 drive?
2) In the video you said the turbine shaft cart accelerated much quicker then the prop shaft cart correct? If this is true then would changing the gear ratio allow the turbine cart to have a much faster speed?
3) If your answer to number 2 is "Yes", then would the turbine cart be geared high enough to where it could keep up with the prop car? Or is the only reason a vehicle is able to travel faster down wind then the wind itself because of the wheels powering the prop?

1) Those particular carts don't use a 1:1 drive. I don't recall the gear ratio off-hand, but I think it was 13:16 in both cases. Of course the gear ratio is only part of the story. To get the "vehicle speed ratio" (speed through air divided by speed over ground) you also have to know the wheel diameter and prop pitch. The difference between the two carts isn't the gear ratio per-se, but the sign of the gear ratio. The gear ratio for the turbine cart is -13:16. So the turbine turns the opposite direction from the prop of the prop cart.

2) No, the turbine cart can never reach wind speed. It uses the tail wind to turn the turbine, which in turn turns the wheels. If it reached wind speed it would no longer feel any relative wind, and would thus have no torque at all on the turbine.

3) There are several approaches to traveling directly downwind faster than the wind. But all require that the vehicle exploit the energy of the wind relative to the surface - not relative to the vehicle. The BUFC does this by gearing the prop to the wheels such that the wheels do the work to turn the prop. A turbine cart can never do this.

I am just trying to learn so please take no offense to my inquiries.

On the contrary. This is what I'm here for. I enjoy these kind of questions.

I would love to know how exactly this works. It seems to me that if 20mph of wind is applied to the turbine cart then it will never ever be able to reach a faster speed, even taking into account a gear ratio change, because when it would get to the 20mph threshold there would be no more force to turn the wheels. Am I correct in this assumption?

You're exactly right.

Also can you explain to me the end result in my hypothetical test? Say you had one of the carts that had a steering servo hooked up to the front wheels and you were able to find a stretch of 5 miles with absolutely flat and even surface. Let's say wind-speed could be controlled at exactly 20 mph. If you were to put the cart down and get it to travel the whole 5 miles directly downwind in a striaght line, is there any way to determine how fast it could actually go? More specifically would it have some magic number as a threshold top speed or would it continue to gain speed indefinitely as long as it had the room? If this description is too vague please notify and I will try to clear it up as best as possible.

Not too vague at all. In fact it's quite well posed.

For a given VSR (vehicle speed ratio) the cart will have a theoretical top speed as a function of wind speed. For example, if the cart is trying to advance through the air at 1/2 the speed that it advances over the ground, it will never go faster than twice wind speed. As you change the prop-pitch and/or gear ratio such that the geometric VSR becomes closer and closer to 1.0, you'll get a higher theoretical multiple of wind speed - but at a price. To achieve more than about 4 times wind speed requires a cart with ludicrously efficient transmission, prop, etc. That being said, there is no theoretical bound to the vehicle's speed as a multiple of wind speed.

 

[ThinAirDesign]

I think that the DDWFTTW carts are a bit more surprising and impressive that they could work at all; ...

Then you will be *really* surprised when we take Greenbirds record of 126mph and smash it in a DDWFTTW vehicle.

JB

 

[rcgldr]

I saw the youtube discovery channel video and they missed the key point of the DDWFTTW carts, which is they rely on the difference between wind speed and ground speed, by using the wheels to drive a prop to generate an upwind thrust that only needs to be somewhat greater than the apparent headwind speed in order for the cart to work. Using an 2x example, with a 15 mph tailwind, and 30 mph cart speed, the prop thrust speed might only need to be 18 mph in order to compensate for all the opposing forces, only 60% of the ground speed, allowing the thrust to be 1.6 times as great as the opposing ground force if there were no losses.

 

[robinson]

This is fantastic!

 

[spork]

This is fantastic!

Thanks. We did some testing yesterday with the vehicle in its final configuration (finally!). We got to 2.85X wind speed. I think Pi is in sight.

 

[rcgldr]

When trying to optimize for speed, how do you choose between pitch and gearing? I'm assuming diameter is fixed since it's just one prop. The maximum speed is related to the overall reduction factor from ground speed to prop wash speed, but the forces involved are related to the prop thrust and opposing ground force used to drive the prop, and I assume that any force greater than the minimum required to overcome drag and rolling resistance increase losses in the drivetrain, but I don't know if those losses are linear.

Also assuming the prop doesn't have a one way clutch, how diffcult is it to slow the cart down via the brakes (how much of the momentum is in the prop)?

 

[spork]

When trying to optimize for speed, how do you choose between pitch and gearing? I'm assuming diameter is fixed since it's just one prop. The maximum speed is related to the overall reduction factor from ground speed to prop wash speed, but the forces involved are related to the prop thrust and opposing ground force used to drive the prop, and I assume that any force greater than the minimum required to overcome drag and rolling resistance increase losses in the drivetrain, but I don't know if those losses are linear.

Also assuming the prop doesn't have a one way clutch, how diffcult is it to slow the cart down via the brakes (how much of the momentum is in the prop)?

To make a very long story short, we do two things... we make research what kind of numbers we think we're capable of in terms of rolling resistance, prop efficiency, aero drag, etc. With those numbers we simulate the system and come up with the optimal parameters (pitch, gearing, etc.). From there we can vary the prop pitch on the fly to optimize a bit further.

If this were a high budget project with a real purpose, we'd re-optimize based on the real-world data, perhaps make a 2nd, 3rd, and 4th prop, change gear ratios, and even find the optimal parameters for any given wind speed.

As it happens, our initial parameter guesses were close enough to allow us to achieve better than 2.5X wind speed. So just adjusting prop pitch (which doesn't adjust TRUE prop pitch) is enough to satisfy us. When someone else breaks our record by a few percent, the next guys are going to have to do all that stuff to beat theirs - and we'll be long gone onto the next silly project.

Our prop does have a freewheel mechanism, but it's still incredibly effective in braking. It's the same as a helicopter in autorotation. No torque is put into the shaft, but it still slows the heli's descent drastically.

ETA: The momentum in the prop is a TINY fraction of the overall momentum in the cart. When we run the prop on the dyno, it slows down very rapidly when we stop powering it. It's trying to push a LOT of air.

 

[rcgldr]

rolling resistance, drag

Seems to me you could remove the prop, then get the cart up to speed using a vehicle tow, then measure the rate of decelertation to get an estimate of these factors.

The momentum in the prop is a TINY fraction of the overall momentum in the cart. When we run the prop on the dyno, it slows down very rapidly when we stop powering it. It's trying to push a LOT of air.

I was mostly wondering about the angular momentum versus the torque required to produce the thrust. Apparently the torque required to drive the prop and generate the thrust is relatively larger compared to the angular momentum of the prop. The prop must be relatively light for it's size.

helicopter auto-rotate

This is the opposite effect of what you'd want for braking in the DDWFTTW cart. Auto-rotation relies on a relatively high amount of angular momentum versus the opposing torque involved during the landing sequence under autorotation (prior to landing, negative pitch and/or forward speed during decent sequence is used to maintain rotor speed).

 

[spork]

Seems to me you could remove the prop, then get the cart up to speed using a vehicle tow, then measure the rate of decelertation to get an estimate of these factors.

Perhaps, but we've found it's a lot tougher to get accurate data than we'd hoped. Add to that the fact that some of the items are constant, some linear with speed, and some squared with speed, it might be pretty tough to tease out the various components. Then there's the dependency to changing losses when loaded (e.g. transmission). We've found that for the level we're playing for the moment, it's just as well to test for the actual results.

I was mostly wondering about the angular momentum versus the torque required to produce the thrust. Apparently the torque required to drive the prop and generate the thrust is relatively larger compared to the angular momentum of the prop. The prop must be relatively light for it's size.

I'm not sure how to compare angular momentum and torque other than to say the prop certainly doesn't freewheel for long - at least not when it has pitch. I would say it's definitely light for it's size - unlike the rest of the cart.

This is the opposite effect of what you'd want for braking in the DDWFTTW cart. Auto-rotation relies on a relatively high amount of angular momentum versus the opposing torque involved during the landing sequence under autorotation (prior to landing, negative pitch and/or forward speed during decent sequence is used to maintain rotor speed).

Auto rotation doesn't actually require any momentum at all - the final flare does however. In steady-state auto rotation the "gliding" blades slow your descent tremendously with no reliance on their mass. It'd be a bummer to land an auto without flaring, but you'd walk away (most likely).

 

[A.T.]

NALSA confirms 2.8 wind speed DDW

From http://www.nalsa.org/:

Get out your slide rules and physics textbooks...On July 2, 2010 on El Mirage Dry Lake, Blackbird sailed directly down wind at a speed of 27.7 mph in a 10 mph wind to set a first record for the ratio of Boat Speed to true wind speed of 2.8. BlackBird was designed and built by the Thin Air Designs team (Rick Cavallaro and John Borton) and sailed by Rick. Links to follow soon.

 

[spork]

 

[Curl]

Can anyone explain how this works? I've never heard about this before.

 

[rcgldr]

Can anyone explain how this works? I've never heard about this before.

The cart just like any sail craft, takes advantage of the fact that with a tailwind (or any wind) the difference between wind speed and ground speed for a sailcraft is constant, and independent of the sailcrafts speed. (If there's a 10 mph wind, then wind to ground speed is always 10 mph, regardless if a sailcraft is going 5 mph or 15 mph).

The cart is designed so that the wheels drive the prop, at some reduced ratio, so that relative to the cart, the prop thrust speed will be less than the ground speed. This effective reduction in gearing effect allows the opposing force from the ground driving the wheels, to be increased by the effective gearing ratio (minus losses) to allow a greater force from the prop thrust (but at a lower speed). This net greater force from prop thrust versus opposing wheel thrust allows the cart to accelerate until the net force is countered by opposing forces related to rolling resitance and aerodynamic drag. How much less the prop thrust speed needs to be depends on how effecient the entire cart is.

At start up, the prop just acts as a bluff body, a small sail, but it's enough to get the cart going. At some point, the prop wash starts to act as a bluff body, but one that is moving backwards away from the cart. This allows the cart to go faster than the tail wind, as long as the prop thrust direction relative to the ground is upwind.

The source of this power is the same as any wind powered device, the prop thrust slows down the tail wind, extracting the energy related to the change in momentum of the wind per unit time.

 

[kjl]

Can anyone explain how this works? I've never heard about this before.

Or, for a more visceral demonstration that may or may not give you that "aha" moment, see:



Pretend that the long toothed rod is "wind", and then the toothedRod->gear connection is basically just a 100% efficient prop. The vehicle moves faster than toothed rod in direction of travel.

Oh, and congratulations, Spork - I followed your crusade to not get shouted and laughed at with great interest :) 2.8X downwind is bad ***!

 

[Curl]

so what was the big debate all about?

 

[spork]

Oh, and congratulations, Spork - I followed your crusade to not get shouted and laughed at with great interest :) 2.8X downwind is bad ***!

Nicely done video. I got a little kick out of the notion that you think it's unfakable. You try and make something foolproof, and I can bring you a better fool *every* time.

so what was the big debate all about?

Physics, and why even some very highly educated people substitute their own faulty intuition and call it physics.

Critical thinking requires being skeptical but not closed-minded, and it requires using your intuition, but not in the place of rigorous math and physics. Learning requires not already knowing everything.

 

[Pythagorean]

Unfortunately, many people equate skepticism with intelligence.

 

[spork]

Unfortunately, many people equate skepticism with intelligence.

There's an element of truth to that though. The real problem is that many people mistake denialism for skepticism.

 

[sophiecentaur]

I want to understand this but my experience and intuition tell me it's wrong. I am not dismissing it out of hand and I would even be pleased to find it is genuine.
The movies are compelling but the explanations are not so compelling. There are statements like:
"But all require that the vehicle exploit the energy of the wind relative to the surface - not relative to the vehicle."
which worry me.
Energy is a scalar quantity and not 'relative' to anything so I can't see what is meant.
Some energy must be involved in turning the prop through the air and moving the cart against friction. If this is to come from the wind then there must be a Force X a Velocity to account for the energy taken from the moving air.

There seem to be two regions of operation. Firstly, at sub-wind speeds, the wind will turn the prop as you'd expect and also be pushing it forwards. Then, once above wind speed, where is the force coming from? Can there be any more 'forward' force if the prop is moving faster than the wind it is moving through?
A simplistic argument would say that the cart, moving faster than the air, would be pushing air ahead of it. That implies that the system just consists of a moving cylinder of air, pushing the cart. The cart wouldn't work in a simple tube - it would just end up going at wind speed.
However, there is a loophole argument, 'for' the system, which I haven't seen used in any of the above arguments. That is the fact that there is an almost infinite amount of moving air around the cart and prop. There is a lot of energy / momentum in this total body of moving air - much more than in the apparent cross section of the craft. I might suggest that the actual airflow through and around the prop gives it an effectively huge cross section. This is not unlike the way a thin wire dipole can gather a lot more RF energy than its thin cross section might suggest.
Have you (the builders / designers) ever considered a wind-tunnel type of experiment in which you put a smoke source at the centre of the prop or you could possibly drive the craft through a smoke plume and observed what is happening to the air around it? I suggest that a large 'cylinder' of air around the cart will be influenced and that this is where the energy is coming from. The prop will be producing a region of turbulence in which some of the slow moving air is, in fact, slowed down and some is sped up and it is that disturbance which produces the required Force times Velocity to power the cart at the high speeds.

Where did the design of the turbine come from? How much aerodynamic theory was involved and how much 'suck it and see'? I would be interested to know.

 

[cjl]

There seem to be two regions of operation. Firstly, at sub-wind speeds, the wind will turn the prop as you'd expect and also be pushing it forwards. Then, once above wind speed, where is the force coming from? Can there be any more 'forward' force if the prop is moving faster than the wind it is moving through?

You seem to misunderstand the method of operation. The prop is not acting like a wind turbine - it is never extracting energy from the air, so to speak. It is acting as a propeller, pulling the cart forwards against the air. The energy source is the wheels - they are rolling against the ground.

Basically, the wheels drive the propeller. This would not work in still air, as you could never gear it in a way which would result in a greater propeller thrust than the wheel drag. However, if the air is moving, the propeller pitch can be reduced such that the propwash is moving slower relative to the cart than the wheelspeed. This allows the propeller thrust to be greater than the wheel drag (assuming minimal frictional losses, so that almost all of the wheel drag is because of the power being extracted to drive the propeller). In still air, this would not move, since the propwash speed relative to the vehicle would be slower than the vehicle airspeed. If the air is moving relative to the ground though, thrust can be generated.

One of the easiest cases to visualize, at least for me, is the case where the vehicle is moving at the speed of the wind. The vehicle's airspeed will be zero, but its groundspeed will be the same as the speed of the wind. In this situation, any propeller will be producing thrust, regardless of its pitch, as the propwash will always be moving backwards (relative to the vehicle), while all of the surrounding air is stationary. All that is required for this to be a sustainable condition then is for the prop thrust to be equal to the wheel drag.

This is doable because the wheels can extract energy from the groundspeed, making the power generated by the wheels equal to the retarding force multiplied by the groundspeed. The power required for the propeller to counter this force is then simply the same force (assuming steady state) multiplied by the propeller's effective airspeed. If you consider this to be the speed of the propwash (a simplification to be sure, but it makes it easy to visualize), then the propeller requires less power to turn than is available from the wheels so long as the propwash speed is lower than the ground speed. With relatively small losses, this same balance works even if the cart is moving faster than the wind. The power required to turn the propeller will go up, as the propwash speed (relative to the cart) must always exceed the airspeed, but the groundspeed will always be higher than the airspeed, so there is always a potentially favorable energy balance.

A simplistic argument would say that the cart, moving faster than the air, would be pushing air ahead of it. That implies that the system just consists of a moving cylinder of air, pushing the cart. The cart wouldn't work in a simple tube - it would just end up going at wind speed.
However, there is a loophole argument, 'for' the system, which I haven't seen used in any of the above arguments. That is the fact that there is an almost infinite amount of moving air around the cart and prop. There is a lot of energy / momentum in this total body of moving air - much more than in the apparent cross section of the craft. I might suggest that the actual airflow through and around the prop gives it an effectively huge cross section. This is not unlike the way a thin wire dipole can gather a lot more RF energy than its thin cross section might suggest.
Have you (the builders / designers) ever considered a wind-tunnel type of experiment in which you put a smoke source at the centre of the prop or you could possibly drive the craft through a smoke plume and observed what is happening to the air around it? I suggest that a large 'cylinder' of air around the cart will be influenced and that this is where the energy is coming from. The prop will be producing a region of turbulence in which some of the slow moving air is, in fact, slowed down and some is sped up and it is that disturbance which produces the required Force times Velocity to power the cart at the high speeds.

Where did the design of the turbine come from? How much aerodynamic theory was involved and how much 'suck it and see'? I would be interested to know.

The airflow around it will be very much like the airflow around any propeller. As the cart moves, it will leave a cylinder of air behind it which is moving slower (relative to the ground) than the surrounding air. That energy gained from slowing the air down (again, in the ground frame) is what drives the cart. It's counterintuitive, but it works.

 

[Creator]

Nice work...now, how fast can it go AGAINST the wind??

...

 

[spork]

Nice work...now, how fast can it go AGAINST the wind??

...

To go against the wind efficiently we have to swap our prop out for a turbine. I expect we might build one over the winter and try it next season. From 20' away you won't notice any difference at all, except I expect it will go directly upwind faster than the wind.

 

[sophiecentaur]

"You seem to misunderstand the method of operation. The prop is not acting like a wind turbine - it is never extracting energy from the air, so to speak. It is acting as a propeller, pulling the cart forwards against the air. The energy source is the wheels - they are rolling against the ground. "

"Wheels" are not an energy source. Something must be pushing them along the ground, making them rotate, for power to be available. What is that 'something'? You need a Force X a Speed for power to be developed. Where does the Force come from? If the action of the prop were just like a conventional 'driven' propellor, you could do at least as well by driving the wheels - and that would be nonsensical, you must agree.

"The airflow around it will be very much like the airflow around any propeller. As the cart moves, it will leave a cylinder of air behind it which is moving slower (relative to the ground) than the surrounding air."

I think you would have to agree that, if your cart were inside a tunnel (tube) with the same CSA as the cart, then it wouldn't go any faster than the airflow in the tunnel (how could it do better than a well fitting piston). Therefore there must be another source of momentum to transfer to the cart to explain how it goes faster than the air flow. The cylinder to which you refer must be much larger than the cylinder immediately around the cart or where is the momentum / velocity difference / power coming from? You can't transfer momentum if there is no (positive) velocity difference so the system has to produce one in some way.
Just because you have a system that works (and very impressive it is too!) it doesn't necessarily mean that your explanation is correct. I can find nothing in your explanation that goes further than a 'by its own bootstraps' argument and that won't do, will it? I am trying to look at the system in more depth and to explain what's happening in terms of possible energy transfers.
I can't really accept that your use of the term "ground frame" is relevant. I feel that it could probably also work for an airship, using some sort of drogue, possibly.

[quotes fixed - russ]

 

[sophiecentaur]

You could resolve the matter once and for all by measuring the tension on both legs of the drive belt at the same time. The side with more tension in it would be the side that is transferring the power - i.e either from wheels to prop or prop to wheels.
That would be interesting, would it not? You could even infer it from still pictures of the thing at work. Just look at the relative angles / slack in each side of the belt.
You could prove me wrong or right. I dare you.

 

[cjl]

QUOTE "You seem to misunderstand the method of operation. The prop is not acting like a wind turbine - it is never extracting energy from the air, so to speak. It is acting as a propeller, pulling the cart forwards against the air. The energy source is the wheels - they are rolling against the ground. "

"Wheels" are not an energy source. Something must be pushing them along the ground, making them rotate, for power to be available. What is that 'something'? You need a Force X a Speed for power to be developed. Where does the Force come from? If the action of the prop were just like a conventional 'driven' propellor, you could do at least as well by driving the wheels - and that would be nonsensical, you must agree.

The force to drive the wheels comes from the propeller. The propeller is pushing the cart (and the wheels) forward along the ground. That's the force. The reason that the propeller can generate enough force to drive the wheels even despite losses is because of the speed differential between the ground and the air.

Effectively, the energy the wheels generate is equal to the force required to turn them multiplied by the distance they move along the ground (this is a simple statement of work). The energy the propeller puts into the air is equal to the force it exerts on the air multiplied by the distance it moves through the air (again, a simple statement of work). When the cart is in steady state operation, the force generated by the propeller and the force on the wheels is the same (ignoring friction losses), so there is enough energy to drive the prop so long as the distance that the prop moves through the air is shorter than the distance the wheels move along the ground.

This condition is satisfied so long as there is a tailwind (and the way I've worded it, it makes the most sense if you consider the cart already moving faster than the wind). If the cart is moving at twice the windspeed, then the wheels will actually generate twice as much energy as the prop would require (if it were 100% efficient), since the force is the same and the wheels move twice as far along the ground as the prop does through the air. If the cart is moving at triple the wind speed, then the wheels generate 150% of the energy the prop requires, again ignoring losses.

Note that this only works because the prop moves a shorter distance through the air than the wheels do along the ground. This is why your assertion about being able to do at least as well by driving the wheels is wrong.

QUOTE "The airflow around it will be very much like the airflow around any propeller. As the cart moves, it will leave a cylinder of air behind it which is moving slower (relative to the ground) than the surrounding air."
I think you would have to agree that, if your cart were inside a tunnel (tube) with the same CSA as the cart, then it wouldn't go any faster than the airflow in the tunnel (how could it do better than a well fitting piston).

Wrong. In a tunnel, the cart would do significantly better than a well fitting piston, and (perhaps more surprisingly), the cart would do better in a tunnel with a relatively close fit between the prop blade and the wall of the tunnel than it would in free air. This is because ducted propellers are more efficient than free ones.

Therefore there must be another source of momentum to transfer to the cart to explain how it goes faster than the air flow. The cylinder to which you refer must be much larger than the cylinder immediately around the cart or where is the momentum / velocity difference / power coming from? You can't transfer momentum if there is no (positive) velocity difference so the system has to produce one in some way.

The cylinder to which I refer will be roughly the size of the propeller, though airflow through a prop disk is quite complicated. You're overthinking things here - the airflow around the cart behaves exactly as if it were a simple, propeller driven cart with an electric (or other) motor driving the propeller. Relative to the cart, the air flows smoothly around it with some velocity, with the air moving faster behind the propeller due to the propwash. Do you agree that a propeller is capable of transferring (rearwards) momentum to air even if the propeller is moving forwards through the air? If so, then you agree to the basic principle that allows this cart to work.

Just because you have a system that works (and very impressive it is too!) it doesn't necessarily mean that your explanation is correct. I can find nothing in your explanation that goes further than a 'by its own bootstraps' argument and that won't do, will it? I am trying to look at the system in more depth and to explain what's happening in terms of possible energy transfers.

The energy transfers are fairly straightforward, so long as you understand that the propeller is doing work on the air and the wheels are having work done on them by the ground (and therefore, they have different effective velocities). Once you see that, everything else basically falls into place.

I can't really accept that your use of the term "ground frame" is relevant. I feel that it could probably also work for an airship, using some sort of drogue, possibly.

An airship could not go directly downwind faster than the wind unless it were dragging something along the ground (or in some other medium with a different velocity than the air). The whole reason the cart works is because the airspeed of the cart is slower than the groundspeed. Without it being in contact with both the air and the ground, it could not work.

Oh, and it also might help to visualize the opposite case. The cart as shown here is effectively extracting energy from its groundspeed and using it to drive the vehicle by pushing against the air. Many people find it easier to visualize a cart extracting energy from its airspeed, and using it to drive the vehicle by pushing against the ground. This would be a cart driving directly upwind powered by a wind turbine. With appropriate gearing, the turbine can extract more energy from the air than is required to drive the wheels, and the cart could go directly upwind. It's exactly the same principle, but easier to visualize.

In the case of the downwind cart, the correct "gearing" has to do with the prop pitch. With the correct prop pitch, the cart can generate more force than is required at the wheels to turn the propeller, since the propeller is acting as both the method of pushing against the air and the method of altering the force/velocity balance (like a gearbox would in a car driven by the wheels).

 

[cjl]

You could resolve the matter once and for all by measuring the tension on both legs of the drive belt at the same time. The side with more tension in it would be the side that is transferring the power - i.e either from wheels to prop or prop to wheels.
That would be interesting, would it not? You could even infer it from still pictures of the thing at work. Just look at the relative angles / slack in each side of the belt.
You could prove me wrong or right. I dare you.

There's no doubt at all that the cart is transferring energy from the wheels to the prop. The prop pitch is in the wrong direction for energy transfer in the other direction - the wind is actually trying to spin the prop in the opposite direction from the way it turns, but the torque from the wheels is larger than this windmilling effect.

 

[Curl]

LOL I find it hilarious that there is even a debate about this issue.

Its so easy to understand with about 14-15 seconds of thinking (with the imagination part of your brain involved).

Trying to apply your book formulas to this will get you confused. Instead, just try to simulate a car like this in your head. Your brain's physics simulator will say it works. Mine did.

 

[cjl]

LOL I find it hilarious that there is even a debate about this issue.

Its so easy to understand with about 14-15 seconds of thinking (with the imagination part of your brain involved).

Trying to apply your book formulas to this will get you confused. Instead, just try to simulate a car like this in your head. Your brain's physics simulator will say it works. Mine did.

I disagree.

It might be intuitive to some, but not to everyone (It seemed quite unintuitive to me until I worked through it in my head, but now it makes perfect sense).

 

[mender]

You could resolve the matter once and for all by measuring the tension on both legs of the drive belt at the same time. The side with more tension in it would be the side that is transferring the power - i.e either from wheels to prop or prop to wheels.
That would be interesting, would it not? You could even infer it from still pictures of the thing at work. Just look at the relative angles / slack in each side of the belt.
You could prove me wrong or right. I dare you.

http://www.nalsa.org/:

Look at the side of the chain that has tension (the left side as we face the cart). Then look at the propeller pitch. The wheels are driving the propeller, not the other way around. You are proven wrong; thanks for daring!

Now that it has been established that you are wrong, go back and reread the careful explanations that were provided. Ask questions about things you aren't clear on; likely that's the point that you're going wrong.

I suspect that you're having trouble with the wheels being the power source. While that is true in the frame of reference that has the cart stationary and the ground moving, most people can't let go of the ground-based reference frame and that confuses them until they understand how the cart works.

In the ground frame, the evidence of the cart's ability to harvest energy from the wind while moving faster than the wind is in the speed of the air after the cart passes. If the air is moving slower relative to the ground, the cart has extracted energy from the wind despite the fact that it is moving faster than the wind.

The wheels provide the link to the ground; the prop provides the link to the air. As long as the cart has both a ground reference and an air reference, it isn't limited to the speed of either and can, with varying gearing and sufficient wind (or ground) energy, travel slower than the wind with a large amount of force or travel faster than the wind with a small amount of force.

You can't transfer momentum if there is no (positive) velocity difference so the system has to produce one in some way.

As long as the wind is blowing, there is a positive velocity difference for the cart (system) to extract energy from. The velocity difference is between the air and the ground, commonly known as wind.

 

[Pythagorean]

There's an element of truth to that though. The real problem is that many people mistake denialism for skepticism.

It's semantics about the definition of skepticism, but I agree.

Congratulations, btw.

 

[russ_watters]

Impressive creation/effort, spork.

 

[russ_watters]

As someone who'se knee-jerk reaction was also skepticism (despite being a sailor!), maybe I can help:

You seem to misunderstand the method of operation. The prop is not acting like a wind turbine - it is never extracting energy from the air, so to speak. It is acting as a propeller, pulling the cart forwards against the air. The energy source is the wheels - they are rolling against the ground.

Both wind turbines and sailboats move faster than the true wind and when I learned to sail as a kid I was very surprised to learn that the top speed of a sailboat was not directly downwind but was at an angle roughly perpendicular to the wind. And I didn't at first understand how a sailboat could travel nearly upwind in seeming violation of the "direction" from which the energy was provided. But this concept has an added twist:

A wind turbine is fixed to the ground and a sailboat moves against the water (though big ones also have relative wind indicators). But a DWFTTW device finds a way to operate with the wind being the stationary frame of reference and the ground being the moving frame. This is made clear by the fact that most, iirc, have to be pushed to get them started. They start from "stationary" in the frame of reference of the wind.

Most people on these forums know the rules: they know that kinetic energy is frame of reference dependent. But having never seen a device that selects the wind as the stationary frame of reference, it is very tough to visualize and the knee-jerk reaction is to reject the concept.

...it may actually be easier to visualize if you consider the examples where these have been run stationary on a treadmill.

 

[spork]

If the action of the prop were just like a conventional 'driven' propellor, you could do at least as well by driving the wheels - and that would be nonsensical, you must agree.

I don't agree. The prop and the wheels are acting on two different media - which happen to have a relative velocity to one another. That is key. The energy needed by the prop to produce a specified thrust is LESS than the energy that can be harvested by the wheels when the cart is pushed with that thrust.

Just because you have a system that works (and very impressive it is too!) it doesn't necessarily mean that your explanation is correct.

True enough - but it happens that their explanations ARE correct. You'd do well to ask questions about them and try to understand them, rather than to explain why they must be wrong.

I can find nothing in your explanation that goes further than a 'by its own bootstraps' argument and that won't do, will it?

Then you're not looking hard enough. Their explanations are correct.

I am trying to look at the system in more depth

It seems more like you're trying to explain away the explanations that correctly predict the cart's behavior.

You could resolve the matter once and for all by measuring the tension on both legs of the drive belt at the same time. The side with more tension in it would be the side that is transferring the power - i.e either from wheels to prop or prop to wheels...
You could prove me wrong or right. I dare you.

Done.

Impressive creation/effort, spork.

Thanks.

most, iirc, have to be pushed to get them started.

Every one we built will self-start in a tail wind.

 

[sophiecentaur]

@spork
What an interesting thread!
Thanks for your patience. I now 'have it'.
The points that I hadn't taken from initial descriptions were:
1. Without the prop moving, there is still windage', which would push the cart froward anyway.
2. The prop direction, when the wheels are going forwards, is such as to push air backwards.

If the prop had zero pitch, then forward motion could make it rotate because of the forward force on the chassis and the backward force of the wheels on the ground. Introducing some pitch into the prop blades will cause some net backward force on the surrounding air and a resulting reaction force.
When you, so competently and fully, answered my point about the force times speed I realized the significance of the gearing (and, presumably, the pitch of the prop) which gives a power difference between prop and wheel. It is this power difference that provides the acceleration to enable acceleration at speeds faster than the wind. Clearly, the more efficient / aerodynamic the cart design, the faster the cart can go before losses account for all this power difference.

How many different turbines have you tried? I should imagine that the pitch / rotation rate could be chosen for a really optimum performance. I also take your point that it should be even better in a tube.

I still have an idea that an airship could work with, rather than a drogue, a second turbine with different pitch. Again you should be able to get a force times speed imbalance between the two turbines but there would be 'slippage' on the 'retarding' turbine which is not a problem when wheels are used. Possibly a lighter than air machine with a paddlewheel dipping into water could do the job and break yet another speed record.

As for the last comment in the last post (#48) - if the cart wouldn't start from rest, I can see that it should never work (apart from a small factor of static friction).

 

[spork]

How many different turbines have you tried?

None. We've only operated this as a downwind vehicle to date. Thus we've only ever used the one propeller. We're considering building a turbine to demonstrate that it can go UP wind faster than the wind. The prop we have has been operated fixed pitch and was later converted to variable pitch.

I still have an idea that an airship could work with, rather than a drogue, a second turbine with different pitch.

I think you mean a prop and a turbine. You must have one of each. AND... you must have them in different air-masses. So you could put one up high and the other down low for example.

Again you should be able to get a force times speed imbalance between the two turbines but there would be 'slippage' on the 'retarding' turbine which is not a problem when wheels are used.

But we still get "slippage" with our prop - right?

if the cart wouldn't start from rest, I can see that it should never work (apart from a small factor of static friction).

Not necessarily true. We can set the prop to a pitch such that the cart won't self-start, but it works great once underway. Keep in mind that the entire prop is stalled while at rest (or worse yet, producing significant lift in the wrong direction at the roots). As we begin to move, the tips become unstalled first. As we move faster, the unstalled region grows toward the roots.