Sailing downwind - faster than the wind?

DownWest: the Wrights made themselves famous by locking the machine up in a garage for 1 year while they touted their idea around. They where marketing chaps. The "Engineers" doing the same thing just spent their days in the fields bouncing around and killing themselves.

Guess who got remembered..
 
halfway said:
DownWest: the Wrights made themselves famous by locking the machine up in a garage for 1 year while they touted their idea around. They where marketing chaps. The "Engineers" doing the same thing just spent their days in the fields bouncing around and killing themselves.

Guess who got remembered..
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The Wrights were brilliant engineers who made two important breakthroughs.

First of all, the realised that the tables of wing performance produced by Lilienthal were wrong - lethally wrong - and did the first ever proper extensive set of wind tunnel tests to produce their own.

Second, and more important - they were the first people to realise that in aircraft deisgn there has to be a payoff between stability and controllability. Previous designers like Lilienthal and Pilcher had gone for stability and had produced stable aircraft. Unfortunately this meant that when they started crashing they did so stably and couldn't be stopped from crashing. Otto Lilienthal RIP. Perchy Pilcher RIP.

Wright aircraft were much less stable and much more controllable. That's why they won, and how they earned their place in the pantheon of great engineers.
 
Just accept that this theory is possible for 2 minutes.


Imagine a world that is a cylinder. There are two islands on the end caps of the cylinder. "Windward" and "Leeward". We sail to the islands at either end by spiral tracks down the cylinder. Either up wind or down wind we sail. In the upwind case we actually go through the wind and make progress against it. Try telling that to an early Greek sailor. In the down wind case we travel faster than the wind.


Now make a new world that is a donut. The wind always comes from one side. Now we sail in a helix around the donut. On the up wind side we take energy out of the wind and go into it. Amazing. Down wind we take energy out even when we are above the wind speed. More amazing.



Now in the real world build a great tower and put a cross bar across the top. Spin the crossbar in a horizontal plane. Now on the ends fit these faster than wind type props with pitch control (similar to a helicopter cyclic pitch).

The prop blades do no more than the yacht in the donut world.

So you should be able to rotate the crossbar so that the tip speed is higher than the down wind speed and still get power out of the shaft down the tower.

The rotational (tacking) energy (the wheel on the ground in the car example) would come from a pinion at the tower top driving a shaft out, along the crossbar, to the props.



To my knowledge, I do not think anyone has ever made such a wind mill. otherwise a quick addition of a load cell would have proved this theory. As it would provide drive on the downwind leg, while faster than the local wind. It would mean the generator would be on the ground and the blade system would be self limiting in speed. Too fast and the apparent angle would stall the props reducing the drive. Once both where stalled the windage on the two would be equal and no more speed increase.


(Having 3 arms would be more realistic for the starting conditions. As unlike the yachts in the donut world they could not beam reach. Note in the donut world to beam reach would mean swapping sail sides at every half revolution)


So who wants to build this new windmill?
 
halfway said:
Now in the real world build a great tower and put a cross bar across the top. Spin the crossbar in a horizontal plane. Now on the ends fit these faster than wind type props with pitch control (similar to a helicopter cyclic pitch).
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In your donut world, the land yachts interact between the wind and the surface of the world. Lift them off the ground and they just drift off downwind.

When you put the props up on a tower, what do they press against? If you put them lower so they could touch the ground it would work - briefly. The snag is that though the upwind and downwind machines look the same they are geared differently so they can't convert from upwind to downwind mode just by turning them round.

However - here is an experiment where a downwind device is mounted on a turntable. It not only manages to cancel out the rotation of the turntable but exceeds it.

But of course if you just want to produce power from the wind using something on a vertical axis, just look up at your anemometer.
 
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snowleopard: "what do they press against"

Just like the car, they would extract the energy needed from a wheel running on the fixed ground. In this case a pinion wheel on the fixed central tower gear.

Yes, upwind and down wind are different mechanism. But in the end we use the same piece of cloth on our yachts to make both possible. As I said the blade angles would be controlled by the same method used in helicopters that changes the blade angle as the head rotates. Otherwise helicopters would never go forward, only up and down.

So on the down wind and upwind side the blades would be different angles to the prop hubs. The change would obviously be progressive during the times the prop was side on to the wind and doing nothing.
 
halfway said:
Yes, upwind and down wind are different mechanism. But in the end we use the same piece of cloth on our yachts to make both possible. As I said the blade angles would be controlled...
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Note though that an upwind cart uses a gearing of 10 or 20:1 whereas a downwind cart is more like 1:1

However, let me suggest another method of doing the job. If you pin your sails in tight, put the helm down and lock it, most boats will sail indefinitely round in circles, tacking and gybing once each turn. If you put a sailing rig on your rotating beam it would do the same.

Hmm. Where are we going with this?
 
snowleopard: "Note though that an upwind cart uses a gearing of 10 or 20:1 whereas a downwind cart is more like 1:1"

Only because the carts are mostly tricks and are not using the correct props. They would both be 1:1 geared to get the equivalent of a 45 degree tack/gybe angle.

In the case of upwind the blade cord lines would be nearly inline with the axial rotation line.

In the case of the down wind the blade cord lines would be nearly in line with the prop plane.


The most dramatic difference between gybing and tacking at high speed is that in gybing you can not see where you are going. But you can see the boat next to you in the race. Where as in tacking you can not see the boat next to you but have a clear sight of where you are going.

Hence the prop would have to alter the pitch significantly.


"Hmm. Where are we going with this? " It is called a "thought experiment" if you can find a situation or machine that exercises every aspect of a theory it might show up any holes. For example your machine would gybe and tack, but would mine? I think it would be a more progressive movement.
 
relative speeds, and is the prop turning the wheels or vice versa

Apologies for posting the 'under the ruler' video again but it really illustrates two important points.

Link (again, skip the first 1:40)

Question 1: Where is the cart getting its power from: (a)The ruler, (b)the ground or (c)both?

Question 2: Is (a)the top wheel driving the bottom ones or (b)vice versa - or are they (c)driving each other?

If you answered (a) to either question, what would be the effect of lifting the cart off the ground so it could slide freely. How would the cart move then?

For those who can't see the video, this is what I'm talking about:
ruler.jpg
 
The answer to the power source, is the muscle in the arm of the person. But since it is not accelerating the fiction is the only loss.

You would need to make the ruler very slippery to show the acceleration case. Which is what everyone thinks of as the car passed the wind speed. Just accelerating the ruler is not the same. The car would have to slip on the ruler as it accelerated.

However it does show the limit to speed for each wind speed. This is determined by the gearing.

So the car can not just keep extracting air energy and keep accelerating until it succumbs to friction.

It is the gearing that sets the limit. Which is much lower. With the car and prop, the gearing is the angle of attach of the prop. So with a flat prop you would never get to wind speed due to losses. As the prop pitch is raised, the limit would be above the wind speed. But at a point the aerofoil chosen would not make any more gain by increasing the attack angle. This would be the final limit of the system and has nothing really to do with friction.

If there were multiple top wheels you could choose initially to run the ruler on a small wheel and then jump to the bigger ones. That would be the pitch change.

The size of the largest wheel would be the in built limit, exactly like the aerofoil. So no speeds faster would be possible.

So to go back to the example earlier if you reach the limit (assuming no friction) then putting up two props will not help. Halving the prop size will not help. The only way it would help would be when it was running significantly below "design" speed due to friction. In this case doubling would get you closer to the design speed. But not above the limit.

Aerofoil design for a given wind speed will limit the top speed.
 
The answer to the power source, is the muscle in the arm of the person. But since it is not accelerating the fiction is the only loss. Ultimately it is muscle providing the power but it is transmitted to the car by the combination of the two srufaces moving relative to each other. One surface alone will not do it.

You would need to make the ruler very slippery to show the acceleration case. Which is what everyone thinks of as the car passed the wind speed. Just accelerating the ruler is not the same. The car would have to slip on the ruler as it accelerated.If you really wanted to simulate the acceleration you could coat the ruler with treacle! I think though that the steady-state faster than the wind condition is what really matters.

However it does show the limit to speed for each wind speed. This is determined by the gearing.Yes, the pitch of the prop and the gear ratio between prop and wheels

So the car can not just keep extracting air energy and keep accelerating until it succumbs to friction.Agreed, the 'accelerating to mach 1' argument is nonsense

It is the gearing that sets the limit. Which is much lower. With the car and prop, the gearing is the angle of attach of the prop. So with a flat prop you would never get to wind speed due to losses. As the prop pitch is raised, the limit would be above the wind speed. But at a point the aerofoil chosen would not make any more gain by increasing the attack angle. This would be the final limit of the system and has nothing really to do with friction.Yes, the gearing sets an upper limit. Drag, friction and prop losses mean the real top speed will be lower. If the losses aren't kept down it will top out well below wind speed, hence all the super-light construction.

If there were multiple top wheels you could choose initially to run the ruler on a small wheel and then jump to the bigger ones. That would be the pitch change.

The size of the largest wheel would be the in built limit, exactly like the aerofoil. So no speeds faster would be possible.The ratio of the sizes of the upper wheel and the two diameters of hte lower rollers (in the video the lower rollers were old-style cotton reels with the spindle about 1/2 the diameter of the flange)

So to go back to the example earlier if you reach the limit (assuming no friction) then putting up two props will not help. Halving the prop size will not help. The only way it would help would be when it was running significantly below "design" speed due to friction. In this case doubling would get you closer to the design speed. But not above the limit.Agreed, the only thing that helps is optimizing the prop efficiency by good choice of pitch, diameter, blade form and gearing.

Aerofoil design for a given wind speed will limit the top speed.
 
"hence all the super-light construction"

Is this necessary? Even if it is a tank, it still should work. Might take longer.

Inertia is good! I would go for aerodynamics and the bearings. Weight does not matter unless it affects the bearings. We are not expecting it to go up hill.

"Agreed, the 'accelerating to mach 1' argument is nonsense"

So do you agree, that since we now have an upper limit, the wind energy is used relative to the car. In fact it is relative to the blade on the prop in the car. The prop's rotational speed is 1:1 with the car's velocity. So the wind's energy it can impart to the prop is directly related to the car velocity.

The wind energy is provided to the prop through the air movement w.r.t. the complex mechanical moving reference of the road, car and prop drive arrangement.

Just in the same way the power of a jet washer is transferred to the dirt on a raised sail by the water velocity w.r.t. the complex elastic properties of the surrounding material and momentum of the lose boom. (Which reminds me I have work to do)

In the end it is just a little atom of air that hits one side of the prop, and one of its friends missed out hitting the other side, resulting in a net force forward. How that atom became so lucky was that it had an approach angle relative to the blade that made it more likely to make a contact. And the friends around it bumped into it more.
 
Hmm the same debate has been going on on the Boatdesign forum for over a year now. I wonder if we can beat that.

MD
 
halfway said:
"hence all the super-light construction"

Is this necessary? Even if it is a tank, it still should work. Might take longer.
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Agreed a heavier vehicle will take longer to get up to speed but once a steady state is achieved the mass becomes irrelevant. But - the heavier the construction, the higher the level of friction to be overcome . A 1 kg push will get a pram going pretty well but applied to a car it won't move it at all.

halfway said:
We are not expecting it to go up hill.
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Though in fact in one of the demos it does - by tilting the treadmill!
 
RAI said:
Instructions to build your own copy are here

http://www.rtfa.net/tag/dwfttw
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It may be a tad pedantic, but the instructions call it a 'self-propelled' cart. Surely it's 'wind-propelled' as without the wind it wouldn't work.

Also, shoot me down if i missed it, but where in the video is it proved that the cart is going faster than the wind? Where are the wind & speed measuring devices? Surely they're not relying just on a crude tell-tale, that is in turbulent air from the prop?

Edit: ** hell i didn't realise there was a 2nd page, this was supposed to follow on from #200!
 
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flaming said:
Sailing Anarchy's thread is over 3600 posts long.

I gave up caring about this a looooong time ago.
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I had my two pennys worth on this subject a while ago on this but since have been reading all the entries with interest.

The one thing that strikes me is that there are several supporters of this subject that argue incredibly vociferously for their case. Sorry guys but it almost seems like fanaticism, you seem so upset that peeps can't see the (what to you) is obvious that you have to keep restating your case ad nauseam.

Lets all cool it, drink some wine and catch a few rays.

Don't worry be happy
 
Well we are not all fanatics. I initially thought that this concept was a load of rubbish, and that conservation of energy would prevented it from working. I have now learned, thanks to this thread, that it is eminently possible to arrive downwind faster than the wind (Oracle + waterskier- great analogy), which opened up my mind somewhat. I'm not completely convinced that the device works as shown, but I'm willing to give it the benefit of the doubt because I believe that it is possible to build a machine that can do this.
 
quote from Ubergeekian "There's a lot of fuzzy thinking going on about these two sets of work". Remember you said it!
In the video of the model, the moving road is turning the wheels and the wheels are turning the propeller. In fact they go to some trouble to show that there is no wind. All that is happening is that the energy transmitted from the wheels to the propellor is giving thrust which is just about balancing the frictional forces at the wheels.
In the "road " model the wind turns the propeller and this transmits drive to the wheels.
In your confusion you have picked out the best of both worlds and devised a machine where the wind drives the propeller which drives the wheels which drives the vehicle which causes wind which drives the propellor which drives the wheels...ad infinitum.
Its a nonsensical self-perpetuating argument which I've seen in any number of "perpetual motion" arguments.... The cause gives the effect and then the effect gives the cause........
 
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