How Does DDWFTTW Work and What Are Its Key Principles?

[jduffy77]

Or we simply have energy transferring from the water to the submarine.
Sails work above water. There's no reason why an adapted sail couldn't do the same thing for a submarine. Sails can even work in space. Nothing about air is special or magical.

Agreed and one consistent feature of all sails is they can not drive something faster than the media which is pushing them without leverage.

 

[spork]

Or we simply have energy transferring from the water to the submarine.

Which means the submarine could approach, or even reach, the speed of the water - but nothing more.

Sails work above water.

With sails you have two choices.
- You can stick a sail up in the air and let the air push you. In this way you can approach the speed of the air.
- You can stick a sail up in the air, and stick another wing down in the water. Now you're in business. You can exploit the energy available at that interface. That's because the water has energy relative to the air - and vice versa.

In your submarine example, you're just sitting still under water. The water has no energy relative to itself.

 

[A.T.]

Or we simply have energy transferring from the water to the submarine. Sails work above water.

Sail craft use two foils (sail & keel) in two media (air & water).

There's no reason why an adapted sail couldn't do the same thing for a submarine.

A submarine has only contact to one medium.

 

[rcgldr]

Or we simply have energy transferring from the water to the submarine. Sails work above water. There's no reason why an adapted sail couldn't do the same thing for a submarine.

An example of a DSFFTTS (down stream faster than the stream) water vehicle is a Brennan Torpedo. The wires could be attached to non-moving posts instead of take up reels, and the torpedo released downstream so it travels DDSFTTS. In this case, comparing to DDWFTTW cart, the wires are the "ground" and the water is the "air".

http://en.wikipedia.org/wiki/Brennan_torpedo

 

[kmarinas86]

If all that is so, I still wonder how you get the "slingshot effect" to work with only two bodies.

I mean, if you had an ensemble of such slingshot effects at the particle level, you are basically allowed to increase the velocity difference.

This happens with a probe and a planet.

You slow down the orbit of the planet and speed up the probe. That results in a transfer of energy.

But there is no third medium to interact with in this case (assuming no ether).

This can happen multiple times, still without a third medium, and still using the same two objects.

The orbital "center" could be the center of mass of those two bodies (I would guess that this arrangement would not be stable and would disintegrate or collapse). In any case, there is potential energy involved... so.

At microscopic level, you still have this going on between attracting particles.

I guess you only invoke the third object when you try to explain the velocity difference in the first place. Or maybe in place of the third object, you can consider already present rotations (which only brings the problem back even further).

 

[jduffy77]

An example of a DSFFTTS (down stream faster than the stream) water vehicle is a Brennan Torpedo. The wires could be attached to non-moving posts instead of take up reels, and the torpedo released downstream so it travels DSFTTS. In this case, comparing to DDWFTTW cart, the wires are the "ground" and the water is the "air".

http://en.wikipedia.org/wiki/Brennan_torpedo

Yes, with the most important point, (for kmarinas) being that the wires are necessary.

 

[rcgldr]

"slingshot effect" ... You slow down the orbit of the planet and speed up the probe. That results in a transfer of energy. This can happen multiple times, still without a third medium, and still using the same two objects.

There would need to be a another planet (or sun) for the probe to change it's path to result in the probe re-intercepting the original planet at a later time. Otherwise it's just a 2 body system, and depending on the intial state, it either ends up with both objects in elliptical orbits, or if escape velocity is achieved or exceeded, with both objects moving away from each other.

 

[spork]

If all that is so, I still wonder how you get the "slingshot effect" to work with only two bodies.
...
You slow down the orbit of the planet and speed up the probe. That results in a transfer of energy.

Correct.

But there is no third medium to interact with in this case (assuming no ether).

But there is something else - there is the coordinate system in which you're measuring all velocities. You talk about the planet slowing down and the satellite speeding up. So you're clearly not using either of them as the basis of your coordinate system. You're presumably using the Earth or Sun as the basis of the coordinate system in which the satellite increases it's velocity.

Both energy and velocity are entirely frame dependent. Depending on the frame chosen, we could say the satellite sped up and got energy from the planet, or that it slowed down and gave energy to the planet.

This can happen multiple times, still without a third medium, and still using the same two objects.

I'm not so sure about that. Perhaps it could do so if that planet was orbiting another body at exactly the right period so that satellite intercepts its orbit at the right time and place on subsequent passes, but that does involve a third body.

 

[periboob]

May a noob post a simple thought experiment that may convince a few doubters?

Think: spokes of a tall wheel, half way down each spoke is a sail that folds up on the reverse. Sail moves at Vw, hub goes 2*Vw

Fits in a tweet. I humbly suggest that this may convince a greater percentage of the public than equations of torque, propellers and fluid dynamics.

 

[A.T.]

Think: spokes of a tall wheel, half way down each spoke is a sail that folds up on the reverse. Sail moves at Vw, hub goes 2*Vw

https://www.youtube.com/watch?v=Ufk6HVWdSzE

 

[spork]

Think: spokes of a tall wheel, half way down each spoke is a sail that folds up on the reverse. Sail moves at Vw, hub goes 2*Vw

Agreed. I've used that and similar examples many times. I encourage people to grab a spoke on their bike below the hub and push it forward - or pull a yo-yo along a table with the string exiting below the hub.

I humbly suggest that this may convince a greater percentage of the public than equations of torque, propellers and fluid dynamics.

I fear that you drastically underestimate the challenge of convincing those that prefer not to be convinced.

 

[periboob]

...
I fear that you drastically underestimate the challenge of convincing those that prefer not to be convinced.

I am rarely anymore surprised by the stubbornness and stupidity of my fellow beings. In younger days I was equally guilty, though I think I have out grown most of it. It just seems that the solution to this cute little problem, which has little practical value, and no money on the table, is particular difficult to see.

But I am told, if you only communicate with people who agree with you, you learn nothing.

 

[spork]

It just seems that the solution to this cute little problem, which has little practical value, and no money on the table, is particular difficult to see.

I certainly didn't think it up (and it turns out I was not the first to think it up in any case) for any practical value. But it turns out that it could potentially have a good deal of practical value. You can harvest a great deal more wind energy with a moving propeller or turbine than you can with a stationary setup.

 

[GiTS]

Ddwfttw

Direct Down Wind Faster Than The Wind.

First, I am not refuting that it is true. I've seen the videos and believe.
Second, this is about the treadmill test with no apparent wind.
Third, I am looking for an explanation of how this works from someone who understands DDWFTTW.

Link to video


As I understand it, the propeller gets energy from the wheels and departs this energy to the air to produce thrust. What I don't understand is, doesn't the energy have to come from the wheels in the form of drag? So If the propeller was geared up more and more to generate more and more energy, then the effect should be higher and higher drag on the wheels.

So if I wanted to calculate the drag on the wheels at any given moment it would be equal to the thrust of the propeller right?

I have seen lots of diagrams depicting velocities but nothing depicting energy. I just want to understand this and I figured by now people have a good grasp on what's happening.

Thank you,
GiTS

 

[willem2]

The energy you can get from the wheels can't be highter than the energy you can put into the propellor, because of conservation of energy, but this isn't true of the forces on the wheels and on the propellor, because Power is force * speed.

Power gained from the wheels is F_wheel * v_road, and power put into the propellor F_prop * v_air.

because v_road = v_air + v_wind > v_air (with a tail wind). It's possible to have F_wheel * v_road <= F_prop * v_air, together with F_wheel > F_prop.

 

[A.T.]

So if I wanted to calculate the drag on the wheels at any given moment it would be equal to the thrust of the propeller right?

No. Thrust and drag are forces. There is no such thing as "Conservation of Force" in physics. Every lever can output more force, than you put in. But at a lower speed than the input force.

Here, the thrust is greater than the wheel drag, because it is applied at a lower speed. The air moves slower relative to the cart, than the ground.

I have seen lots of diagrams depicting velocities but nothing depicting energy. I just want to understand this and I figured by now people have a good grasp on what's happening.

Energy balance here is best done in terms of power, which is energy / time (instantaneous energy transmission rate). You will find the power analysis in this papers:

http://www.aapt.org/physicsteam/2013/upload/E3-1-7-solutions.pdf (page 11)

http://orbit.dtu.dk/fedora/objects/orbit:55484/datastreams/file_3748519/content (section 2.3)

http://www.boatdesign.net/forums/attachments/propulsion/28167d1231128492-ddwfttw-directly-downwind-faster-than-wind-ddw2.pdf

http://projects.m-qp-m.us/donkeypus...aster-Than-The-Wind-The-Ancient-Interface.pdf

 

[GiTS]

I came across this youtube video.


It explains what you both are explaining. The key to the contraption working seems to be the angle of attack of the prop blades creates for force in the forward direction than the downward direction. If the angle of attack was steeper, the vehicle would go backwards into the wind.

 

[rcgldr]

Second, this is about the treadmill test with no apparent wind.

All "wind powered" devices rely on a difference in speed between the air and the ground. In the case of an observer on the ground watching the treadmill and the ddwfttw mini-cart, the treadmill surface is moving at some speed, while the ambient air is not moving. In this case the ultimate source of energy is the motor driving the treadmill belt, which in turn drives the wheels, which in turn drive the propelller at a greater force but at a lower (air) speed. The induced wash aft of the propeller will be less than the speed of the treadmill, so as mentioned in post #2, the output force is greater, but the output power is less than the input power due to the reduced output speed.

If the observer were moving at the same speed as the upper surface of the treadmill (perhaps a very long treadmill), or if this was done outdoors with an actual tailwind, then to this observer, the treadmill or Earth's surface would not be moving (relative to the observer), but there would be a tailwind (relative to the observer). In this case, the source of the power is the slowing down of the relative wind (relative to the observer).

 

[rcgldr]

I came across this youtube video.


It explains what you both are explaining. The key to the contraption working seems to be the angle of attack of the prop blades creates for force in the forward direction than the downward direction. If the angle of attack was steeper, the vehicle would go backwards into the wind.

If the effective prop speed (effective pitch x angular velocity) is less than the ground speed, it's a downwind cart. If the effective prop speed is greater than the ground speed, it's an upwind cart. The closer the speed ratio is to 1.0, the faster the cart, if the cart is efficient enough.

If the effective prop speed is negative, it's a directly downwind slower than the wind cart, where the prop is acting as a turbine driving the wheels as opposed to a directly downwind faster than the wind cart, where the wheels drive the propeller.

If the effective prop speed is greater than twice the wind speed, it's a directly upwind slower than the wind cart. For an upwind cart, the propeller always acts as a turbine and drives the wheels.

Not mentioned in the video is that given the same overall efficiency, the upwind cart moves 1x wind speed (relative to ground) slower than the downwind cart. If the downwind cart can move at 3x wind speed, the upwind cart can move at 2x wind speed. This ignores efficiency issues such as the reduced energy related to rolling resistance times a slower ground speed for the upwind cart.

 

[A.T.]

The key to the contraption working seems to be the angle of attack of the prop blades creates for force in the forward direction than the downward direction. If the angle of attack was steeper, the vehicle would go backwards into the wind.

Yes, it becomes an upwind turbine. And if you make the pitch negative it becomes a downwind turbine. But to go downwind faster than the wind, you must set it in between, as a downwind propeller. All the cases are shown here:

http://oi54.tinypic.com/2gv0kew.jpg

You can see the cart as a gearbox, working between two large objects in relative motion. Here some analogies:

https://www.youtube.com/watch?v=E7vcQcIaWSQ

https://www.youtube.com/watch?v=k-trDF8Yldc

https://www.youtube.com/watch?v=pw_B2MnMqZs

And here how the gear ratio A affects the velocity ratio V/W. To go DDWFTTW you must set 0 < A < 1

For the wind-cart at the bottom the pitch of the rotor P is part of the gear ratio. Unlike the rigid models the wind-cart will never achieve exactly the speed dictated by the gear ratio, because the rotor has slippage. But as efficiency is increased it can get arbitrarily close to that limit.

 

[GiTS]

Assuming there are no losses to friction, what's the maximum velocity ratio (wind-cart : wind)?

 

[rcgldr]

Assuming there are no losses to friction, what's the maximum velocity ratio (wind-cart : wind)?

The maximum velocity ratio depends on the efficiency of the cart. If there are no losses, then the maximum velocity is infinite.

ws = wind speed
gs = ground speed
ps = prop speed (the speed of the air flow through the prop)

gs = 1 / (1 - (ps/gs))

ps/gs  gs

   -1  1/2 ws
 -1/2  2/3 ws
    0    1 ws
  1/2    2 ws
  2/3    3 ws
  3/4    4 ws
  7/8    8 ws
    1    ∞
  9/8   -8 ws
  5/4   -4 ws
  4/3   -3 ws
  3/2   -2 ws
  2/1   -1 ws
  5/2 -2/3 ws
  3/1 -1/2 ws

 

[CWatters]

I came across this youtube video.


It explains what you both are explaining. The key to the contraption working seems to be the angle of attack of the prop blades ...

Indeed. Prop pitch (eg on an aircraft) is analogous to gear ratio. Fine pitch prop = fast acceleration/low top speed. Coarse pitch prop = low acceleration/high tops speed.

The magic is all in getting the right effective pitch for the air prop vs the water prop or wheel gearing.

 

[A.T.]

Assuming there are no losses to friction, what's the maximum velocity ratio (wind-cart : wind)?

What is the maximum gear ratio a loss less gearbox can have? There is no hard limit on the ratio. The only hard limit is the speed of light on the actual speed. But long before that becomes an issue in the real world, efficiency and material strength will limit it.

 

[rcgldr]

Indeed. Prop pitch (eg on an aircraft) is analogous to gear ratio. Fine pitch prop = fast acceleration/low top speed. Coarse pitch prop = low acceleration/high tops speed. The magic is all in getting the right effective pitch for the air prop vs the water prop or wheel gearing.

Prop pitch has reduced effect at relative low speeds (the prop become inefficient due to excessive relative angle of attack which is affected by aircraft speed relative to the air). Simple thrust calculators for static thrust (the thrust produced when an aircraft is not moving relative to the air) don't even bother inputing pitch as a parameter.

 

[A.T.]

I'm assuming it uses the momentum of the rotors to get across the dead zone of no wind speed.

No stored momentum is used. The rotor is turned by the wheels with fixed transmission ratio. It cannot slow down if the wheels are accelerating.

The acceleration is due to the wheels. It doesn't matter which direction the wind is blowing, it's(the wind) just used to move the object.

The wheels are braking, while the rotor provides the forward thrust. Turning the rotor with the relative headwind to turn the wheels would accelerate the true wind. instead of slowing it down to extract energy

 

[Suraj M]

No, the wheels are braking, while the rotor provides the forward thrust. Turning the rotor with the relative headwind to turn the wheels would accelerate the true wind. instead of slowing it down to extract energy

I'm sorry but i don't have the patience to read 235 posts to understand it, could you please tell me how the object moves in the video in the other thread! And i don't understand what's going on here, any link or video that would explain the total working? , thank you.

 

[A.T.]

any link or video that would explain the total working?

It's like this spool, just with the airmass instead of the paper strip:



It terms of sailing, it's like circular tacking:

 

[Suraj M]

I understood the 1st video, but the 2nd doesn't say why it goes faster than the wind.

 

[A.T.]

I understood the 1st video,

That's the key, because it is a more general explanation of the concept.

but the 2nd doesn't say why it goes faster than the wind.

The 2nd one is for sailors, who already understand how a sailboat works. The situation shown there is explained here: