Sailing downwind - faster than the wind?

DownWest said:
Comparisons with Oracle are useless.
Click to expand...

Imagine a large, floating board with a slot in the front edge (side to side). Imagine Oracle sailing on a beam reach in the slot. No problem. Now imagine Oracle bearing away: it pushes against the front side of the slot, which will cause the board to move forward (downwind). When Oracle gets to the end of the slot, it gybes back and contines to 'tack downwind'. Board contines to get 'pushed' downwind at VMG of Oracle. All true so far?

Replace Oracle/groove with just an aerofoil (which is the part doing doing the work) on rollers on a track, that auto gybe at the end of the track. Replace board on water with cart on low friction wheels. Should still work?

Improve this by raising the aerofoil track, and having another aerofoil pointing downwards, and going in th other direction. Should work even better. shouldn't it?

So you have two reciprocating aerofoils. Sounds a bit like a propellor.

So if Oracle can do 3 times windspeed VMG, This just leaves me with three questions:

- Why does the DDWFTTW device only JUST exceed wind speed?
- Why does DDWFTTW device have all the wheels/gears malarky?
- Did Oracle have a wind driven propeller underwater?

MD
 
Thanks for ruining my night. I have three scenario's for you to consider:

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1.

Assume a trolley has got near to wind speed but is powered by a huge sheet of plywood. A gust comes to overcome the friction and it is now at wind speed.

The machine has a generator on its front wheel. It is a perfect generator connected by a wire to a perfect fan. The idea is that you cut a hole in the plywood and put the fan in it. Now take a fraction of the power from the wheel and drive the fan.

Assumption: The fan will increase the speed. As its retarding force on the wheel is less than the driving force of the fan. The fan is obviously in "still air" as the machine is going the same speed as the wind. So any slow rotation will deliver some additional force.

The retarding force is "y". The fan force is (1+a)y. Where "a" is greater than zero.

Now we know we have a net force of (1+a)y - y = ay accelerating the machine.

So assume for a moment we fit another generator to the other front wheel. We generate enough power to retard it by a force "ay".
So at this point the trolley has no net force.

Now add the second fan driven by the "ay" power.
So now the new fan is producing (1+a)ay force which is a net force.

Repeat ad infinitum.

Now you have a trolley with an infinite force on it. Why stop at the sound barrier?

----

2.

The treadmill example is said to match the real world conditions of the wind and car going at the same speed. However, this is not the case. You will note that they need to hold the machine to keep it aligned. They are continually grabbing it or pushing it with a fork. At these points the machine turns into a simple gearbox and drives the wind in the room. Say the treadmill is going 10kts and the air past the prop is going 20kts

So now jump on the treadmill behind the car. Suddenly the car is racing away from you at 10kts. However the wind is actually coming towards you at 10kts (20 - 10). If the theory was correct the wind should be going away from you at the same speed as the car.

Since this is the same wind direction as the pushing prop produces, and the prop is seeing 20kts, it is acting at this point like a source of energy. It does not turn the prop but instead removes the need for too much more energy to be added by the prop to contribute to a forward motion.

Hence as long as you do not let it move far between "pokes" the machine has enough energy to go forward in the moving air stream

----

3.

Now the video of the road test with the RC buggy. First note the position of the wind sock. It is not behind the fan but on the side of the fan. Also note that at 0:19 seconds the fan is a "pusher". It also has a very strange enclosed turbine blade, but no enclosure.

The fan has the same rotation all the time, the odd effects are the video rate beating.

The reason for the wide, and flat, fan is to make it start as the windage has to be higher than the rotational force on the static fan otherwise it would go backwards!

When the car is slow the wind is being "pushed" aft but is actually still travelling forward through the prop w.r.t the car.

Now as it gets up to speed we rely on the wind sock to tell us it is going faster than the wind. At the point of no acceleration the wind pressure on the rear of the fan has to just overcome the friction. But at this point the flow through the centre of the prop could be backwards relative to the car. Making the centre a high pressure would lead to odd effects at the edges where they would now have enhanced forward air flow relative to the car.

The wind at the edges would be the sum of the natural wind (Still blowing forward) and the wind caused by the fan centre high pressure, so would be higher than normal. this higher wind would also revolve around the tips and cause the wind out to the side of the fan to become reversed. So when we see the wind sock reverse it is a local enhanced wind.

To be a fair experiment the wind sock should be out to the front of the vehicle many blade widths forward of the fan (or the fan enclosed in a true duct).


----
That is my 2p worth.
If I am wrong, do not tell me, as I need the sleep.
 
halfway said:
1.
Assume a trolley has got near to wind speed but is powered by a huge sheet of plywood. A gust comes to overcome the friction and it is now at wind speed.

The machine has a generator on its front wheel. It is a perfect generator connected by a wire to a perfect fan. The idea is that you cut a hole in the plywood and put the fan in it. Now take a fraction of the power from the wheel and drive the fan.
Now you have a trolley with an infinite force on it. Why stop at the sound barrier?
Click to expand...

You have also assumed that there is no drag from the plywood when the car is travelling faster than the wind (i.e. into the wind), and that there are no losses from either generator or fan.

Given your rather extreme assumptions, there is no need to stop at the sound barrier.
 
I made my remark obout odd statements because they were just that. To say that the wind has no energy is simply wrong. Air has a mass, as wind it has a velocity, ergo, it has energy. The ballon mentioned is accelerated up to the wind speed after take-off by this energy.

The Oracle bit: nobody here doubts that a slippery multihull with a very efficient aerofoil can tack down wind faster than the windspeed. Ice boats, with lower 'rolling friction' can do better, even with soft sails. But that is not what is in question about the OPs claims. (or rather the linked vids claims)
A

I am also waiting for the sonic boom, well, sort of.
 
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DownWest said:
The Oracle bit: nobody here doubts that a slippery multihull with a very efficient aerofoil can tack down wind faster than the windspeed. Ice boats, with lower 'rolling friction' can do better, even with soft sails. But that is not what is in question about the OPs claims. (or rather the linked vids claims).
Click to expand...

It is the same thing. If you accept you can design a boat to tack downwind faster than windspeed, then (as I tried to explain) it is just a matter of mechanics to convert the backward and forward tacking motion (with VMG > true wind speed) into a linear motion downwind. Use props (If you take the spiral path of the propeller and straighten it out you will see that it follows a path that would be called sailing on a broad reach), reciprocating aerofoils, whatever. Voila, a vehicle that goes DDWFTTW!

Once you accept that idea, (no sonic booms) then it is an implementation issue as to how you do it (wheels driving a prop in this case).

MD
 
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DownWest said:
I made my remark obout odd statements because they were just that. To say that the wind has no energy is simply wrong. Air has a mass, as wind it has a velocity, ergo, it has energy.
Click to expand...

Whether that energy is of any use depends on context.

Can you stand up in wind blowing at 100mph?

Can you stand up on a train moving at 100mph?
 
MD, I try to keep an open mind, but have yet to see the case proved. The transition from sailing on high apparent winds to dead down wind doesn't gell. It might be the odd analogies that people use. Boom= joke.
A
 
Hey Uber,
I missed the connection with trains, other than with boats in bath tubs. Still unconvinced, but also open to better argument.
A
I would like to see it proved, just can't.
A
 
4

For all the others to ponder


Now assuming a yacht sailing down wind. Turn to starboard at 45degrees and let the sail bag so it leaves the foil forwards. The boat will accelerate and the apparent wind will come around to the starboard bows. (Assuming you are in a racing yacht). The sail is trimmed all the time as you accelerate until you can not go any closer.

Assuming this is the method of going faster down wind than the wind. Say 10Knots of wind and the boat is doing 15kts to wind and 15 kts at right angles to the wind. So the boat is doing 45degrees to the wind. Just assume these nice number work.

Now the sail is giving a resultant force, mostly sideways, and a little bit towards the bows. But actually the real force is somewhere over to the port of the where the wind is coming from. So resolving in the direction of the wind and at right angles (instead of the boat course) you have a useful amount pushing forward.

So now lose the water and start rotating the sail around the central course wind line.

Gear it to the bottom of the sea so that the mid pressure point of the sail moves sideways (circular motion) at 15knts when the boat moves forward at 15knts. Hence the sail will turn in a cork screw equivalent to a 45 degree tack. Now duplicate the sail to make two opposite blades of a prop.

The force at right angle to the wind will have to be compensated by the gear box taking energy from a ground wheel. (rotational drag)

The force in line with the prop axis will give the required drive into the wind.


Now the assumption is that the force on the wheel required to compensate for the rotational drag is less than the force available from the axial drive component, if this is so, a rotating sail propeller will work.

Now how do you prove that?


The limiting case of 10kts wind and 10kts movement down wind and 10kts (mid blade point) prop rotational speed. The resultant wind would be 10kts at 90 degree to the prop axis. Therefore the "sail" would be mostly flat with the rotation. The lift would be mainly in line with the axis of rotation. So it looks possible.


So the concept is that the wheel on the floor only compensates for the drag from the apparent wind and the motive force is from the wind pushing the prop along. Sorry no diagrams.

Obviously there is a limiting case (ignoring friction) when the apparent wind gets too far forward and the prop stalls. That is the bit I was never happy about. It did not seem right that only friction limited the overspeed.
 
DownWest said:
I made my remark obout odd statements because they were just that. To say that the wind has no energy is simply wrong. Air has a mass, as wind it has a velocity, ergo, it has energy.
Click to expand...

No, the only energy wind has in itself is heat energy, and that is nothing to do with this. You presumably are talking about the energy of motion, and it only has this energy in relation to something else. You talk about velocity, but you really mean velocity between the air and the earth. Velocity is only created by the movement of one thing in relation to another. "Wind" is produced by the motion of the air against the earth (rather obvious really). This difference in velocity can be used for sailing, even when the cart, reaches the same speed as the wind - the velocity is still there, in the movement of the road beneath. How could this velocity difference go away? unless the wind stops blowing, it will always be there.
 
One last go to prove it is theoretically possible.

So imagine a prop blade with no angle of attack. Completely flat foil in the plane of the prop disk. The sort of prop disk that however fast you rotate does not provide any axial force. Now put it in the rig above so that it rotates at a mid blade speed equal to the forward ground motion of the vehicle. Therefore the point on the blade describes a 45 degree slope helix.

Drive the rig forward so that its speed matches the wind speed. The prop then rotates at a mid point rotational speed equal to the ground and wind speed.

Therefore the prop has an apparent flow over it exactly in line with the foils. Since the wind and the motion exactly cancel.

So assuming no friction the system is in a stable state.

Now turn the foils a tiny bit so the trailing edges are further back. The prop now has lift and drag.

Assume the ground wheel and the effective prop size are the same and connected. So the power, speed and more importantly the forces are all the same. So the drag of the prop is exactly converted to the motion retarding force on the ground wheel

Therefore if the lift from the foil due to the slight angle change is greater than the drag the machine will have a net forward force.

However as it increases speed the apparent wind angle on the prop gets worse until the lift vector is past 45degrees. At this point the drag and thus the force on the retarding wheel is bigger than the lift.

The under speed situation is handled as the flat prop would see an apparent wind from more astern. This would cause lift and contribute to the turning forces and supply the motive force. Hurrying it on to the point where it is at a stable speed.

This is a set up to explain it. Due to friction, leeway, like boats the theoretical angle will be no where near this maximum. If the wheel was a water generator and motor on the prop the operation would also work.

I still stand by the statements that the videos do not show the real process and are tricks. Even the toys with the ruler are not what happens. (The simple case of wind faster than vehicle is glossed over) The explanations about the wind being relative to the ground do not explain it as they seem to prove that there is no limit other than friction to the maximum speed. There clearly has to be a theoretical speed, that is a strict limit, otherwise the perpetual motion people would get all hot and bothered.

It is exactly like watching simple sailing while being strapped to the prop of a spitfire. Or better still, if the world was a cylinder, and we just sailed one tack down the helix from one end to the other.
 
Pierrome said:
No, the only energy wind has in itself is heat energy, and that is nothing to do with this.

OK, so why does the cart/boat/whatever not just warm up when the wind blows? Of course the wind has kinetic energy, that's why it moves things. Derived from the sun, where your heat bit comes in.
You made a statement a while back. "The wind has no energy". I just questioned that .
And velocity is relative. But I do not have to agree with your theories as to what happens after that.
A
Click to expand...
 
DownWest said:
Pierrome said:
No, the only energy wind has in itself is heat energy, and that is nothing to do with this.

OK, so why does the cart/boat/whatever not just warm up when the wind blows? Of course the wind has kinetic energy, that's why it moves things. Derived from the sun, where your heat bit comes in.
You made a statement a while back. "The wind has no energy". I just questioned that .
And velocity is relative. But I do not have to agree with your theories as to what happens after that.
A
Click to expand...

The "wind" or more correctly air, moves around the planet in response to heating effects derived, as you quite rightly say, from the sun, interesting, but not what this argument is about.

We are concerned with the air as it travels around in a variable fashion horizontally across the surface of the planet.
It moves things around because it is going at a different velocity than the things it moves.
If those things are going at the same velocity as the wind, it cannot move them, as on a windless day.
Or, when for instance, when the tide and the wind are in exactly the same direction and at the same speed (a sailing boat will not make any progress through the water) - in this case, the wind may be blowing, but because there is NO DIFFERENCE between the wind and the water, the wind cannot provide power. This is what I mean when I say it has no energy OF ITSELF, it only has energy when in simple terms, it puts pressure on other things.

Remember Newton and opposite equal reactions? Therefore, it is just as valid to say the "other things" (mentioned above) put pressure on the wind, as to say the wind puts pressure on them - its the same thing.

So you see, the "energy" is not in the wind, but in the DIFFERENCE between the wind velocity and anything else. We only say the wind has energy because we are usually standing fixed upon the earth and the wind moves around in relation to us. When, for instance, we are in a free floating balloon moving with the wind, it's the earth that is moving and not us.

That is what the doubters in this cannot grasp. They cannot see that although the wind has stopped blowing in relation to the cart, the energy is still there, because the difference in the velocity in the wind and the road remains the same. The cart simply harvests the "energy" from the moving road to make progress through the airmass.

The downwind cart has no way of "knowing" which medium is moving, the road or the air.
Click to expand...
 
mobeydick said:
It is the same thing. If you accept you can design a boat to tack downwind faster than windspeed, then (as I tried to explain) it is just a matter of mechanics to convert the backward and forward tacking motion (with VMG > true wind speed) into a linear motion downwind. Use props (If you take the spiral path of the propeller and straighten it out you will see that it follows a path that would be called sailing on a broad reach), reciprocating aerofoils, whatever. Voila, a vehicle that goes DDWFTTW!
Click to expand...

So what the antis are saying is that it is reasonable to do it with a reciprocating mechanism but if you suggest doing it with rotary mechanism you are a dangerous lunatic. Now who's being unreasonable?
 
This one passed unnoticed as a separate thread...

so I'll put it in here.

I have just come across an old thread which makes fascinating reading. It was a bit corrupted and appears to be incomplete as there was evidence of several hundred further posts but they were illegible.

It looks as though there was a bit of a glitch in the Y2K bug. The post is dated 16 Dec 03


Test flight tomorrow

Posted by Wilbur:
Well bro and I have just put the finishing touches to our flying machine and we'll be doing a test run tomorrow.

Posted by A:
That's old stuff, people have been doing it for years. What does it use, hydrogen or hot air?

Wilbur:
No it's not using lighter-than-air gas, it uses wings and a motor

B:
You mean it's heavier than air? Obviously that's impossible.

A:
There's something wrong here, April 1st is months away.

B:
Maybe he's planning on having the law of gravity repealed. Do Congress know?

Orville:
Why do you think it's impossible? Birds are heavier than air

C:
Yes but birds fly by flapping their wings. If your wings don't flap it's not the same. I still say it's impossible.

Wilbur:
Our wings work because the air flows faster over the top than the bottom and creates suction that lifts the wings.

C:
But the air starts at the front at the same time and it arrives at the back at the same time so how can there be a difference?

Wilbur:
The top surface is curved so the air has to flow further so it will flow faster.

C:
If the top is curved it will bunch the air up above the wing and push it down. Once the speed from the initial push dies away the machine will stop moving through the air.

A:
If you get off the ground with your giant catapult or whatever you're using, how can the engine push the machine along? You'll have to have wheels on a very long pole so your motor can push against the ground. Any fool can see an engine up in the air can't work without something to push against.
 
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SL,
I like that.

Though of course Lielenthal (sp) had proved gliders some years before. The wrights became famous for their research , a practical engine(and staying alive).
A
 
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