Downwind faster than the wind. Poll

[mono]

....a direct-drive windmill powered boat that can sail directly against the wind (you can see it here http://www.youtube.com/watch?v=NNbNNSDljGI ) which is a similar thing but the "other way round" -

I believe you when you say it's possible (to go directly to windward) but I can't agree that it's the same thing but the other way round. The faster you go to windward, the greater the power of the wind - quite the opposite of the experiment referred to here.

 

[Keen_Ed]

Out of interest.Have those who don't believe that DDWFTTW is possible ever sailed: A performance beach catamaran or a true skiff - Int 14, RS800 or 18' skiff?

 

[halfway]

Pierrome: "BMW Oracle the wing is the COLLECTOR of energy from the wind and the boat is "driven" by it's keel, and on the cart the propeller is the driving force through the air and the wheels are the COLLECTORS or energy from the road. "

Sorry, but the keel on Oracle is slowing the forward (windward) speed of the vessel. If you took the keel off the boat, it would side slip to the finish line and not have any ability to do the zig zag course and make the amazing speeds. The keel is stopping the windward progress.

So the wheels on the cart are the same thing, they are resisting the vehicle just bumbling down the course slower than the wind.

Now for the prop. Do not think of it as a jet engine compressor. It is a rotating sail. The energy supplied to it is just to rotate it, so that it sees the correct apparent wind. It is the apparent wind that is the driving force just like the Oracle sail.

Wind is not behind you, as many have implied, and you can not run away from wind. It is an atom of gas that has a net velocity that you catch up to. When you catch it up, and your wind catcher has a perpendicular velocity (gybing or rotating), it therefore has an apparent angle.

If your catcher is an aerofoil. You can create lift from this apparent angle. If more lift is made forward than perpendicular you have a net force forward. Therefore you accelerate.

----

Mono: You must not like all the other explanation so how about this one:

There is one special prop. It is one with nearly zero pitch. (Very flat) Just enough pitch to create the minimal attack angle and compensate for the drag of the aerofoil.

If you where to put this special prop on a motor it would not make any forward force. It would just heat the air. Because as soon as the thing rotated, and the air flow started to move, the apparent angle would destroy the attack angle and the aerofoil would not work. This is while at a fixed position.

(When in a fixed position, the air will start to rotate around the outside of the prop because it creates a high pressure on the "blow" side which tries to get back to the "suck" side. This is the reason why I do not like the treadmill as a demonstration. The air is not going the same speed as the vehicle. It is an invalid assumption)

So this special prop seems to be as useful as a chocolate fire guard.

However, if you rotate it slowly and advance it at the speed of the wind, the prop sees no wind along its axis, and gets a perfectly perpendicular apparent wind. In this case the "chocolate" prop does create lift. So at the exact speed of the wind we still have acceleration.

Obviously you would advance the prop pitch to actually progress to faster than the wind due to the losses in the system, but the principle of still pulling at the point where the vehicle travels at the wind speed is clearly shown.


----

Edit: One last bit to add is that the lift produced by a nearly "flat" prop is about 6 degrees back from the forward axis. So divide this into a force inline with the axis and another perpendicular to the axis. The wheels transfer to forward resistive force the equivalence to the perpendicular force. The vehicle is propelled forward by the force in the direction of the axis.

So as the lift angle is say 6 degrees the net force is forward.

The key is that it can not work with pitch angles anywhere near 45 degrees. Because at this point the lift vector makes more retarding force than propelling force. This is the limit to the multiplication factor possible.

 

[Pierrome]

All the time the cart is on the treadmill and - more importantly, being held there - it is going to carry on developing power through the wheels to drive the propellor at sufficient speed to make the cart travel backwards.

I think anyone can see it is not being held there. It's just being pushed back to prevent it running off the top of the treadmill. And the cart doesn't travel backwards it travels forwards - did you see the right video?

The point is - the air is stationary. The road is moving. The cart wants to advance through the air using the power from the road. To prove it even further they inclined the treadmill so that the cart would have to go uphill (check the bubble at the start), but that doesn't stop it, it still wants to go uphill through the air.

Air stationary, moving road, is exactly what happens when an "outdoor" cart - like the one in the other video (I hope you've seen it) reaches windspeed. Both carts behave in the same way and power though the stationary airmass driven by their props. The "outdoor" one is then effectively going downwind faster than the wind.

 

[mono]

I think anyone can see it is not being held there. It's just being pushed back to prevent it running off the top of the treadmill. And the cart doesn't travel backwards it travels forwards - did you see the right video?

No, I can't let you get away with that. "Held there"...or..."pushed back"...same meat different gravy, as an old friend used to say. Just semantics - as is questioning the use of the word "backwards".

The point is - the air is stationary. The road is moving. The cart wants to advance through the air using the power from the road. To prove it even further they inclined the treadmill so that the cart would have to go uphill (check the bubble at the start), but that doesn't stop it, it still wants to go uphill through the air.

Now you're talking! "The cart wants to advance through the air using the power from the road". I couldn't agree more. I saw it went uphill...I saw the bubble...and yes, it might be a surprise but I also saw the video...several times.

Air stationary, moving road, is exactly what happens when an "outdoor" cart - like the one in the other video (I hope you've seen it) reaches windspeed. Both carts behave in the same way and power though the stationary airmass driven by their props. The "outdoor" one is then effectively going downwind faster than the wind.

Yes - I saw the other video and my original comments stand.

 

[mono]

Mono: You must not like all the other explanation so how about this one:

There is one special prop......etc.

Now that explanation I like...I'd probably love it even more if I understood how it might explain how a cart that accelerates downwind (at exactly 90 degrees to,say, a 12 knot wind) until it reaches, say, 15 knots isn't travelling into a headwind of 3 knots. In other words, a cart that is moving at a speed greater than the wind wouldn't be going downwind.

Nobody has yet answered that question.

 

[nmeyrick]

Air stationary, moving road, is exactly what happens when an "outdoor" cart - like the one in the other video (I hope you've seen it) reaches windspeed. Both carts behave in the same way and power though the stationary airmass driven by their props. The "outdoor" one is then effectively going downwind faster than the wind.

Guys this really is an excellent brain teaser, and very difficult to get your head round. I've read plenty of people saying that its very simple, but if that were the case it wouldn't have sustained so many thousands of posts on fora across the internet. Much as I tried to put this down I've been fascinated and have read quite a lot on it the last few days - and the more you read the more confusing it gets. It really does seem to be like one of those optical illusions where something looks completely different depending on how you look at it - is it a duck or a rabbit? I've even found myself cringing when reading the same old obvious objections over again - "oh no, the fool just fell into the trap of thinking that the prop is a turbine and powers the wheels!"

Just for the record I'm pretty agnostic about whether or not DDWFTTW is possible in concept. I have no problem with turbine powered vehicles travelling into the wind, or even in aeroplanes taking off from treadmills. But I'm 90% certain that this model, as described, cannot do what it claims. The 10% is because I'm no great physicist and it is possible that there's some amazing effect going on that nobody has explained, but given the number of (often contradictory) explanations I've read you would think someone had mentioned it.

But as others have mentioned the explanation doesn't seem to be doing a great job of convincing people, as shown by the fact that more than 70% of people who have taken the poll do not believe it works. And the one bit I am pretty confident is that the treadmill test is indeed completely useless - not for the reasons most people have said, but because you're measuring the wrong thing.

The fact that the vehicle moves from left to right on the treadmill tells you nothing at all - absolutely nothing - about the its velocity over the belt of the treadmill. Counterintuitive, sure, but if you think about it for a while you'll see that its absolutely true.

 

[halfway]

Mono: I do not know where the 90 degrees came from in your answer, but ignoring that, I will try to answer the faster than wind bit. Please draw it.


1. OK, 12kts wind coming from the South and going North and a vehicle travelling North at 15kts. So the apparent wind is 3kts head wind.

2. Now make a MAST travel ACROSS the width of the car at 15kts towards the East. OK just go with me on this. I know it will not take long to get across the width.


3. That mast is "making" a wind due to its motion of 15kts from the East.
This wind is due to the mast rushing into the poor molecules, or whatever, of air.


4. Now add the head wind, to this, and you get a movement of the apparent wind for the mast by an angle arctan(3/15) = 11 degrees forward. So that means the apparent wind comes FROM 79 degrees True.


5. Now place a sail on that mast so that its clew is further aft (South) than the mast. Make the angle about 75 degrees True for the sail chord. Clew is to the South West of the mast.

6. The apparent wind will hit this sail and make lift. The lift force will be pointing in the North West quadrant. Assume 10 degrees due to drag losses. So lift is 90 degrees to the sail plus 10 degrees drag loss. So the lift is about 75 - 90 - 10 = -25 Degrees = 335 True.


7. Now that lift has a component in the line of travel (North). And a component perpendicular to the travel (West). The North being bigger than the West.


8. The West one needs to be supplied with a force to counter it. So gear a wheel off the road surface at 1:1 gearing (but with a 90 twist in the transmission belt or gearbox). Now the two forces are experienced by the vehicle in line with each other.


9. The wheel is trying to slow down the vehicle to counter the West force. The North lift component is trying to make the vehicle go forward.

10. Since North component is bigger, the vehicle accelerates forward.

11. If you can accept so far. The leap to making the mast rotate instead of slide is not too hard, as long as you assume a short sail at the top of a long mast and that the sail moves at exactly road speed but in a circle. The force to rotate it comes from the wheel at a 1:1 overall ratio.


(Note that the net acceleration force is the DIFFERENCE of the two forces. So the internal forces, especially through the gearbox are much higher than the actual motive force. The aerofoil characteristics determine the maximum speed multiplier and the concept. But the friction in the gearbox determines the practicality.)

I hope that helps. The hardest part is the concept of wind being ahead of you, when it is coming from behind. You walk into the wind ahead, and due to your motion actually catch up with those energetic atoms. The faster you go the more energetic atoms you encounter! I blame all the childhood books that have "Mister Wind" blowing and pushing things around. Mister Wind is ethereal or even omnipresent.

I wish in this day and age we could easily sketch diagrams at each other. Or annotate photos. But we live in this backward text based world of computers.

 

[Pierrome]

Now that explanation I like...I'd probably love it even more if I understood how it might explain how a cart that accelerates downwind (at exactly 90 degrees to,say, a 12 knot wind) until it reaches, say, 15 knots isn't travelling into a headwind of 3 knots. In other words, a cart that is moving at a speed greater than the wind wouldn't be going downwind.

Nobody has yet answered that question.

Nobody answered because the answer is obvious.

The cart in the above example is going downwind faster than the wind WITH RESPECT TO THE ROAD.

But going upwind WITH RESPECT TO ITSELF.

That's why people don't believe this. They are unable to see things from two different points of view.

I said this twice before and I say it again. The cart doesn't "know" whether the air is stationary and the ground is moving or, whether the ground is stationary and the air moving.

If anyone can explain how the cart could "know" any different I would like to hear it.
We, as outside observers know which is static and which is moving because we are standing on the earth, which we normally consider to be static.

However, when we are in our cars, the earth moves past us and we are standing still.
Think of the moment when you are on a big ship when it starts away from the dock. For a moment, you believe the dock is moving away and the ship is standing still.
Or when you're in a traffic queue and the vehicle next to you moves backwards a little and then you check your foot is on the brake because you think you are moving forward.

Remember what Einstein said: "What time does Grand Central Station reach the next train": IT'S ALL RELATIVE!

 

[RAI]

RAI I'm on your side but I would like to point out your slight error. I have pointed this out before.
The propeller on the downwind cart is not analagous with the wing on BMW Oracle because on BMW Oracle the wing is the COLLECTOR of energy from the wind and the boat is "driven" by it's keel, and on the cart the propeller is the driving force through the air and the wheels are the COLLECTORS or energy from the road.

I'm with you on the "its possible" view. But to me, an airfoil in an airflow obeys the same physics independent of whether it happens to be part of a wind turbine, a propeller, a sailing vessels or on a cart. It results in a force with orthogonal components that we call lift and drag.

So, even when the cart is rolling down wind at wind speed, its rotating propeller blades are still producing lift and drag.

 

[Ubergeekian]

Yes, you could, but it would have to have a large vertical dimension to get sufficient differential.

Dynamic soaring is a way of exploiting vertical variations of horizontal wind strength. It requires very very accurate flying very very close to the ground, and last time I checked no human glider pilot had done it, or not for long. Sea birds, however, use it all the time.

 

[Ubergeekian]

On the contrary, if the wind is blowing at 10k and the boat is travelling at 10k there is NO wind.

I know you're new here, and probably haven't had time to read all the posts but ... so what? A propeller works perfectly well in still air, and the important thing to realise is that the propeller is blowing back against the wind.

Let me give an example of this mechanism. Imagine a helicopter climbing at 1kt vertically above a field on a hot summer's day. A thermal kicks off in the field, giving a vertical air velocity of 10kt. The helicopter is now climbing at 11kt ... faster than the wind.

That is the crucial mechanism here: a propeller working against a bulk air movement produces more thrust than one operating in still air.

 

[snowleopard]

No wonder there's confusion here!

.... a cart that accelerates downwind (at exactly 90 degrees to,say, a 12 knot wind) until it reaches, say, 15 knots isn't travelling into a headwind of 3 knots....

under those circumstances the cart experiences an apparent wind of 19 knots striking it at an angle of 39° to its forward direction.

1. OK, 12kt wind coming from the North and going south and a vehicle travelling North at 15kts. So the apparent wind is 3kts head wind.

And this one has a headwind of 27 knots.

 

[Ubergeekian]

I believe you when you say it's possible (to go directly to windward) but I can't agree that it's the same thing but the other way round. The faster you go to windward, the greater the power of the wind - quite the opposite of the experiment referred to here.

And the faster you go downwind the greater the power of the water. It really is precisely the same problem with an inversion and a coordinate shift.

 

[halfway]

snowleopard: It was 1.30 in the morning give me a chance.

Correction:

1. OK, 12kts wind coming from the South and going North and a vehicle travelling North at 15kts. So the apparent wind is 3kts head wind.

I will correct last nights effort.

 

[snowleopard]

snowleopard: It was 1.30 in the morning give me a chance.

Posting without due care and attention. That's 3 points on your licence.

 

[PaulJS]

I know you're new here, and probably haven't had time to read all the posts but ... so what? A propeller works perfectly well in still air, and the important thing to realise is that the propeller is blowing back against the wind.

Let me give an example of this mechanism. Imagine a helicopter climbing at 1kt vertically above a field on a hot summer's day. A thermal kicks off in the field, giving a vertical air velocity of 10kt. The helicopter is now climbing at 11kt ... faster than the wind.

That is the crucial mechanism here: a propeller working against a bulk air movement produces more thrust than one operating in still air.

Hi Ian,
This is fine until the helicopter runs out of fuel and then it will drop! Even if we assume that the helicopter is 100% efficient the fact that the rotor is moving through the air will exert friction on it (which it must in order to create any force) thus either slowing it until it stops and the helicopter plummets earthward or until the helicopter is falling down through the updraft at a rapid enough rate to maintain a steady speed of autorotation.
I think...

 

[mikemonty]

No, it's very relevant.

The treadmill isn't going anywhere either. But the belt is moving.



But what happens if you ride off your rollers and the first thing to touch the floor are your bike wheels?

The rollers are moving too - difference is they are powered by the bike - not an external source.

What happens is that you stop - dead. The wheels are the only things that have any energy/motion in this scenario and they, by virtue of their tiny mass, add little to forward motion.

I have done this many times - it takes a lot of concentration to stay on a rolling road type set of rollers.

If the wheels had been made of solid cast iron the effect would be different.

 

[mono]

No wonder there's confusion here!

under those circumstances the cart experiences an apparent wind of 19 knots striking it at an angle of 39° to its forward direction.

No confusion here - though I can see how my right angle reference was confusing. Strike that from the records will you Miss Smythe. I am referring of course, as I have made plain until my aberration, to a cart/boat/pram/etc moving directly downwind.

 

[Ubergeekian]

Hi Ian,
This is fine until the helicopter runs out of fuel and then it will drop! Even if we assume that the helicopter is 100% efficient the fact that the rotor is moving through the air will exert friction on it (which it must in order to create any force)

Sure. But it illustrates the two important principles here:

1. The wind can do work on a propeller with a relative velocity downwind and
2. The amount of work the wind can do is completely independent of the shaft work needed to turn the propeller

Once the helicopter is climbing (or hovering) relative to the air, vertical air motion can do work on the helicopter without any additional fuel being used. And that's the key with the cart or boat: getting reaction work out of the prop for a lot less shaft work.

In still air, you see, it's easy. If the propeller exerts a thrust T and is moving at v through the air, the reaction work T x v can't be more than the shaft work needed to turn the propeller - and in the case of the hovering helicopter, the reaction work is zero.

However, when the propeller is working against wind, the game changes, and it's possible to get more work out than you put in. Or, before the conservation of energy zealots wet their knickers, it's possible to have the wind do some work on the propeller too.

I'll leave it as an exercise for the reader to calculate the 'efficiency' (power out over power in) of a gyroglider climbing in a thermal ...

As several of us have posted, there is absolutely no problem with fundamental laws regarding travel downwind faster than the wind. Those who mutter about conservation of energy or the second law of thermodynamics just haven't thought about it clearly.

Practical implementation is a different matter, of course, and is clearly not going to be easy.