Down wind faster than the wind

[rehn]

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

What more do you want?

It is this video I am taking about:
http://www.youtube.com/watch?v=kWSan2CMgos
Note: is only the "self start" capability in steady wind I have trouble with.

 

[Thabiguy]

Sorry, but I can’t see how that answers my question. When cart speed == wind speed there is no energy input to the system from outside. Only some loss due to friction. The only energy available is in the forward momentum of the cart and some in the rotating wheels and prop.

What happens is that the vehicle keeps parts of itself moving forward slower than the vehicle does. This means that even at windspeed, parts of the vehicle will be below windspeed, and so the wind will still push them forward.

(Note that for this to work, the vehicle needs contact with the ground that is moving relative to air. Therefore, ultimately, it is the kinetic energy of the air wrt ground that drives the vehicle forward.)

What you say is that this momentum is used to –>accelerate<- the cart and the wheels and the prop. No… I don’t believe that it is possible; it is like lifting yourself in the hair.

You're right in that it's not possible, and that is indeed not how the vehicle operates. The momentum comes from the air; the air is slowed down (wrt ground) as it pushes the vehicle above windspeed (by means of pushing against its slower-moving parts; see above).

 

[Michael C]

When cart speed == wind speed there is no energy input to the system from outside.

If the cart is stationary with respect to the ground, air is moving past it. If the cart is stationary with respect to the air, the ground is moving past it. Somehow it's easy for people to accept that the cart is getting energy when it is stationary with respect to the ground, but find it harder to accept that it is still getting energy when it is stationary with respect to the air. Why is this?

In fact, whatever speed that cart is going at, there is energy input from the outside: it's coming from the difference in speed between the air and the ground. It has one moving part in contact with the ground (the wheels) and another moving part in contact with the air (the propeller). By connecting these two moving parts with a system of gears, it can exploit the energy gained from this movement.

Once more, the machine is not just a balloon in the wind. Imagine a very simple craft that is pushed by the wind: just a big sail perpendicular to the direction of the air flow, mounted on a chassis with free-rolling wheels. If there was absolutely no loss of energy due to friction, this vehicle would move at the speed of the wind, but not faster. In reality there will always be a little loss due to friction, so this vehicle will run just a bit slower than the wind. When it is running at nearly wind speed, the wheels will be turning: here is a source of energy. The simple craft does not exploit this energy at all; Spork's craft does.

 

[rehn]

What happens is that the vehicle keeps parts of itself moving forward slower than the vehicle does.

Eh ?? How can some parts of vehicle move slower than the vehicle. The upper rim of the wheels are moving faster. But slower ??

 

[Michael C]

Eh ?? How can some parts of vehicle move slower than the vehicle. The upper rim of the wheels are moving faster. But slower ??

You can think of a rotating propeller as forming a screw, screwing itself into the air. The thread of the screw is moving slower than the vehicle.

 

[Seymour Butz]

If the cart is stationary with respect to the ground, air is moving past it. If the cart is stationary with respect to the air, the ground is moving past it. Somehow it's easy for people to accept that the cart is getting energy when it is stationary with respect to the ground, but find it harder to accept that it is still getting energy when it is stationary with respect to the air. Why is this?

In fact, whatever speed that cart is going at, there is energy input from the outside: it's coming from the difference in speed between the air and the ground. It has one moving part in contact with the ground (the wheels) and another moving part in contact with the air (the propeller). By connecting these two moving parts with a system of gears, it can exploit the energy gained from this movement.

Once more, the machine is not just a balloon in the wind. Imagine a very simple craft that is pushed by the wind: just a big sail perpendicular to the direction of the air flow, mounted on a chassis with free-rolling wheels. If there was absolutely no loss of energy due to friction, this vehicle would move at the speed of the wind, but not faster. In reality there will always be a little loss due to friction, so this vehicle will run just a bit slower than the wind. When it is running at nearly wind speed, the wheels will be turning: here is a source of energy. The simple craft does not exploit this energy at all; Spork's craft does.

How does the cart, while at a standstill, extract energy from the wind when the prop is angled the wrong way? Or for that matter, how does it extract ANY energy from the air at any cart speed and/or wind speed? It seems that the prop would have to be configured as a turbine to do that. But it's configured as a thruster instead.

Or does the cart accelerate to near wind speed due to the wind drag on the frame only?

 

[spork]

I want a something better before I believe “self start” is possible in steady wind.

How would you feel about a video where I set mine in front of a fan in my garage? If that would do it I'll be happy to post it.

 

[spork]

Or does the cart accelerate to near wind speed due to the wind drag on the frame only?

It starts out using the prop as drag only. As the prop comes up to speed the flow over it becomes laminar and it begins to act as a prop. At speeds approaching and above wind speed the tip of the prop can be thought of exactly as the sail of an ice boat on a continuous 45 degree downwind tack.

 

[Myriad]

Eh ?? How can some parts of vehicle move slower than the vehicle. The upper rim of the wheels are moving faster. But slower ??

The point of the wheel that's touching the road is not moving at all (assuming the wheel is not skidding). Any point on a wheel below the axle of the wheel is moving slower than the vehicle. Here are a series of diagrams showing how that fact could be exploited:

I'll start with a simplified diagram of a cart on a wheel:

As each spoke of the wheel is pointing straight down, imagine that a sail is unfurled from the midpoint of that spoke. The diagram is a snapshot of one such spoke with its sail. Since the sail is only moving at 5 m/s (half the speed of the cart), there is a 5 m/s wind pushing forward on the sail. The force of the wind is transmitted via the spoke equally to the ground and to the cart resulting in accelerating the cart. With no other losses, the cart would continue to accelerate until it was moving at twice the speed of the wind.

The cart with the propeller driven by the wheels operates in a similar fashion. The pitch and gearing are set so the propeller if freely spinning (neither pushing forward or backward against the wind) when the cart is moving at some multiple faster than the wind. I think they used 1.7x in one example. When the cart is moving slower than this, there is a net force accelerating the cart forward. With other sources of drag, the cart will reach an equilibrium speed less than the 1.7x wind speed but still faster than the wind if the drag is small.

Here, we have the chain pulling the vehicle forward via the lower edge of the inner sprocket, but the vehicle is also pulling itself forward along the chain (that is, pushing backward against the chain).

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

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

After testing that device for some time, they found out that the parachutes got caught and entangled in the wheels all too often. The next prototype they considered therefore looked like this:

(coincidentally leaked by Spork in page #4 of this thread )

Now, the actual carts tested by ThinAirDesign and spork don't use the below-the-wheel-axle method the way all the above concepts do. In their cart, what's moving forward slower than the cart is the advance of the surface of the propeller blade, in the direction of the cart's movement, as the propeller sweeps past a given point in space.

Respectfully,
Myriad

 

[jsfisher]

How does the cart, while at a standstill, extract energy from the wind when the prop is angled the wrong way? Or for that matter, how does it extract ANY energy from the air at any cart speed and/or wind speed? It seems that the prop would have to be configured as a turbine to do that. But it's configured as a thruster instead.

My own simplified view:

When the wind is blowing on the stationary cart, two interactions are at work. The wind pushes against the propeller, but because the blades are angled, there is one force component that tends to push the cart forward and another component that tends to rotate the propeller.

These two force components are both transferred to the wheels. The cart moving forward alone would cause the wheels to move in one direction while the propeller rotation alone would case wheel rotation in the opposite direction.

The forward motion of the cart dominates because of the ratio of wheel advance to propeller advance. In effect, the wind blows the cart forward while the forward motion is able to fight the wind rotating the propeller the wrong way.

 

[rehn]

How would you feel about a video where I set mine in front of a fan in my garage? If that would do it I'll be happy to post it.

Yes please I would like to see that! But my main problem is the transition( do you have big garage ?

The point of the wheel that's touching the road is not moving at all (assuming the wheel is not skidding). Any point on a wheel below the axle of the wheel is moving slower than the vehicle. .

Of cause, my mistake. But I can’t see that as an explanation for the transition from below wind speed to above wind speed.

It starts out using the prop as drag only. As the prop comes up to speed the flow over it becomes laminar and it begins to act as a prop. At speeds approaching and above wind speed the tip of the prop can be thought of exactly as the sail of an ice boat on a continuous 45 degree downwind tack.

That make sense. ( I am a sailor and I used to hang-glide ) As I said before, I have no problem with the “above wind speed function”.

I would love to build a cart like this. Is it possible to get the spec of the prop and the gear ??

Thank you for your patience.

 

[spork]

Yes please I would like to see that! But my main problem is the transition( do you have big garage ?

OK. I'll take that video and post it (although you may have to remind me - I'm old and addled). As far as the transition from below wind speed to above wind speed, you can see that in the video in a bunch of places. We push it so it's coasting backward on the treadmill (below wind speed). It then accelerates to above wind speed again and moves to the front of the treadmill.

I would love to build a cart like this. Is it possible to get the spec of the prop and the gear ??

A very small number of people have been pressing me to make up a kit of parts. I think I can do that for about $30 at this point. But I don't want to do it unless I have at least 10 people that want a set. Please let me know if you would. In the meantime...

The gear ratio between the axle and prop shaft is 1:1.

JB's cart uses a tailrotor gearbox and torque-tube from a T-Rex 600 R/C
heli (H60133 "600 Metal Tail Torque Tube Unit" $61.99)

Mine will use a gear set from the T-Rex 500 R/C heli (H50096 Torque Tube
Front Drive Gear Set $10.99) and I will make the gearbox that holds those
gears using a mill.

The wheels are: GWS 4" Ultra-Light Wheel 102mm 6.3g ($2/set)
The prop is: GWS 14"x10" slow flyer plastic prop ($3 each)

The wheel size is relatively important as is the size and pitch of the prop.
These govern the advance ratio of the vehicle. That's what enables it to go
downwind faster than the wind. To make it go upwind, I'd simply buy a set
of the same style GWS wheels in a smaller diameter.

The axle is a 5mm carbon tube

The "frame" is a piece of soft aluminum tubing from your local hardware
store (Orchard Supply Hardware).

As I say, I plan to put together something more comprehensive. But this
thing is pretty darn simple, and the above info along with a picture should
be enough for most folks.

Notes:
- Make sure the prop spins clockwise from behind when the cart is pushed
forward.
- We've used rubber bands on our wheels for better traction - when needed.
It works fine on our treadmill without

 

[ynot]

Construction of my first test vehicle is well underway and when I find some suitable angled gears I will start testing.

Why the treadmill demonstration doesn’t fairly or credibly represent an outside wind scenario . . .

(1) Artificially stopping the cart moving back wards on the treadmill until the thrust of the propeller is sufficient to do this is essentially the same as artificially pushing the cart over the ground surface up to the speed of the wind in an outside test.

(2) Artificially stopping the cart moving forwards on the treadmill beyond when the propeller could do this is the same as artificially pushing the cart over the ground surface beyond the speed of the wind in an outside test.

How does artificially pushing the cart up to and beyond the speed of the wind prove that the energy of the wind alone can do this?

To fairly and credibly recreate an outside test on a treadmill the cart should be placed on the moving treadmill and not artificially restricted from moving along it backwards in any way. Unfortunately a very long moving treadmill surface would be required.

SPORK - I notice in a clip that you have built a large outside model (that you are testing on a treadmill for some reason). Have you tested this in an outside wind, and if so why haven’t you provided the results?

 

[Michael C]

Spork, you didn't reply to this message I posted yesterday. Would you be prepared to do a video with the idea I gave there?

 

[Seymour Butz]

It starts out using the prop as drag only. As the prop comes up to speed the flow over it becomes laminar and it begins to act as a prop. At speeds approaching and above wind speed the tip of the prop can be thought of exactly as the sail of an ice boat on a continuous 45 degree downwind tack.

Ok, I'm trying to throw my skepticism aside for the time being and get my head around this.

I can understand that the drag on the prop would overpower the angle on the prop to allow startup. I have no problem with that.

But what I think I understand from Myriad's post below yours is that the bottom point of the wheel is moving 1/2 cart speed. And so, the prop has a leverage advantage over the cart as a whole. that makes sense too.

What I think my misunderstanding has been is this: I've been equating the cart to a boat, where the leverage advantage of the wheels don't exist.

I admit that I still don't understand HOW this works, because no matter how I think of it, there still must be some torque input to the system. I see the leverage advantage that will minimize that power requirement, but it can never be zero, and I don't see where it's coming from.

Or is the fact that the prop is spinning irrelevant? I think this is what you're saying - the prop is NOT acting as a traditional prop, that is supplying aerodynamic thrust. If I read it right, like you say, the prop is nothing more than a sail that just happens to be moving, and NOT supplying thrust, but is in fact being "sucked" forward due to Bournelli's, much like a wing is "sucked" up to provide lift.

I should note that it's MUCH easier to conceptualize how a turbine powered car could move, since it would depend on the difference between the cart and the wind. I could easily see a turbine cart, with a rotatable mast, in a 10 mph breeze, going 20 mph into a headwind, 15 mph with a crosswind, but only 8 mph DDW.

This concept is much harder.

 

[Michael C]

Or is the fact that the prop is spinning irrelevant? I think this is what you're saying - the prop is NOT acting as a traditional prop, that is supplying aerodynamic thrust. If I read it right, like you say, the prop is nothing more than a sail that just happens to be moving, and NOT supplying thrust, but is in fact being "sucked" forward due to Bournelli's, much like a wing is "sucked" up to provide lift.

It is important that the prop spins: this is what is giving the extra thrust to get above wind speed. If the prop was just acting like a sail, the cart would never go above wind speed. The prop spins against the direction of the wind.

Bernoulli's (not Bournelli's!) principle is not important here.

 

[spork]

Construction of my first test vehicle is well underway and when I find some suitable angled gears I will start testing.

Have you looked for the parts I posted?

Why the treadmill demonstration doesn’t fairly or credibly represent an outside wind scenario . . .

(1) Artificially stopping the cart moving back wards on the treadmill until the thrust of the propeller is sufficient to do this is essentially the same as artificially pushing the cart over the ground surface up to the speed of the wind in an outside test...

The treadmill is useless for performing a self-start test. However it is perfectly valid for all other tests. Placing it on the treadmill motionless until the prop and wheels come up to speed is identical to starting it at exactly wind speed outdoors. From this point you can see it accelerate to greater than wind speed.

Pushing it so that it coasts backward on the treadmill is identical to starting it outdoors below wind speed. We can see that it accelerates to and beyond wind speed.

Pushing it so that it coasts forward is identical to starting it outdoors above wind speed. This would only tell us whether it could maintain greater than wind speed.

In all three cases it achieves and maintains greater than wind speed.

SPORK - I notice in a clip that you have built a large outside model (that you are testing on a treadmill for some reason). Have you tested this in an outside wind, and if so why haven’t you provided the results?

The big cart is a dog. It just barely advances on a perfectly level treadmill at 10 mph. We had originally planned to build a better prop, but the little ones perform so well that we aren't really excited on wasting the time and money on a prop for "the truck"

Spork, you didn't reply to this message I posted yesterday. Would you be prepared to do a video with the idea I gave there?

Michael, we have one video intended to address the steady-state question. We spent a long time balancing the cart as perfectly as we could and increasing the incline of the treadmill exactly to the point where the cart would "hover". We have an uncut video of the cart maintaining on the treadmill completely unassisted for nearly two minutes. Adding strings and weights would only give the critics more excuses for why it's not doing as we claim or additional ways we're scamming.

But what I think I understand from Myriad's post below yours is that the bottom point of the wheel is moving 1/2 cart speed. And so, the prop has a leverage advantage over the cart as a whole. that makes sense too.

The cart is designed with an "advance ratio" of less than 1.0. This means the theoretical distance the prop would advance through still air is less than the distance the wheels would roll with that same single rotation of the prop. This is a critical design parameter for a DDWFTTW vehicle of this sort.

I admit that I still don't understand HOW this works, because no matter how I think of it, there still must be some torque input to the system. I see the leverage advantage that will minimize that power requirement, but it can never be zero, and I don't see where it's coming from.

Here's one way of looking at it... imagine we built such a cart that was absolutely perfect - no frictional or drag losses. We could push it 10 mph on a flat surface and it would continue at that speed forever. In the real world we have a 10 knot tailwind on top of that. This tailwind can more than account for the internal losses and drags.

For me the best way to think of this vehicle really starts with the ice-boat on a 45 degree downwind tack. If you accept that the ice-boat can maintain a downwind VMG greater than windspeed on such a tack you're almost there. From there the only question is "how can we wind that downwind tack into a spiral to drive a vehicle straight downwind?" The way to do that is to stick one sail out to each side (i.e. the prop) and let them spiral one another downwind. The one thing left to do is provide the mechanism for constraining them to their 45 degree downwind path. That's exactly what the wheels and transmission do.

Or is the fact that the prop is spinning irrelevant?[/qoute]


Noooooooooo.

I think this is what you're saying - the prop is NOT acting as a traditional prop, that is supplying aerodynamic thrust. If I read it right, like you say, the prop is nothing more than a sail that just happens to be moving, and NOT supplying thrust, but is in fact being "sucked" forward due to Bournelli's, much like a wing is "sucked" up to provide lift.

The prop is providing thrust - in exactly the same way a sail or wing provides thrust - by being "sucked forward" as you say.

 

[Michael C]

(1) Artificially stopping the cart moving back wards on the treadmill until the thrust of the propeller is sufficient to do this is essentially the same as artificially pushing the cart over the ground surface up to the speed of the wind in an outside test.

In what video does this happen? In all the videos I've seen, the cart moves forwards on the treadmill straight away.

(2) Artificially stopping the cart moving forwards on the treadmill beyond when the propeller could do this is the same as artificially pushing the cart over the ground surface beyond the speed of the wind in an outside test.

No, stopping the cart moving forwards on the treadmill is equivalent to putting a brake on it so that it doesn't go faster than the wind in an outside test.

 

[rehn]

Some good info….

Thank you for the info. The prop and the wheels I know where to find ( I am into RC flying too ).
For the other stuff I will check around ( live in Sweden ).

If I read it right, like you say, the prop is nothing more than a sail that just happens to be moving, and NOT supplying thrust, but is in fact being "sucked" forward due to Bournelli's, much like a wing is "sucked" up to provide lift.

Oh..no don’t open that Bournelli can of worms too But…. Prop blades , sails ( and keels ) work like wings..always, ( as long they are not stalled )

 

[Michael C]

Michael, we have one video intended to address the steady-state question. We spent a long time balancing the cart as perfectly as we could and increasing the incline of the treadmill exactly to the point where the cart would "hover". We have an uncut video of the cart maintaining on the treadmill completely unassisted for nearly two minutes.

Is this one up on YouTube yet?

Adding strings and weights would only give the critics more excuses for why it's not doing as we claim or additional ways we're scamming.

Point taken.