DDWFTTW - Directly Downwind Faster Than The Wind

[spork]

Originally Posted by SamSam

In post #140, reference is made to another forum http://talkrational.org/showthread.p...758#post610758 where Spork offers 10 to 1, in favor of doubters, whether this is possible or not. I'll put $20 on that. I still say it won't work.

I definitely offered 10:1 many times, but I believe it was my $100K against your $10K. I couldn't find the reference in post #140, so I don't recall if I said I'd take any bet no matter the size. I kind of feel like I'm owed $20 just for going to look for the post.

Can you at least make it a little bit more interesting? A few hundred?

[ThinAirDesigns]

Samsam, thanks for your post and interest -- please do keep an eye on the project blog (and this thread) for updates.

Whoever brought up the Brennen torpedo as an example of the principle is correct ... remove the steam engine from the cable and merely anchor same cable to the bottom of a river and the torpedo will go downcurrent faster than the current. The torpedo has no idea what is forcing the cable to unreel -- the current or the steam engine.

Same with the cart on the threadmill -- while the treadmill has the electric motor, the wind has the sun -- and the cart can't tell the difference.

Once the airfoils on the cart are spinning above their stall speed, they act only as simple propeller blades, no funny/fancy 'both' behavior.

Anyway, I'm quite certain that Rick covered all of the above in the other thread and I'm also quite certain that my mere assertions here do not add anything to that body of discussion so I'll not bore you with more unless you ask for such.

Yes, you are in some pretty extensive company when it comes to doubters. We recently hosted the head of wind tunnel testing for NASA Ames (home of the worlds largest wind tunnel) at our company (for reasons other than ddwfttw) and at the end of our conference we posed the little brainteaser to him and he assured us that it wasn't possible.

As a brainteaser, it's supposed to be counterintuitive -- and this one is in spades. Stay tuned -- you'll enjoy the show (and lose some money if you bet spork).

JB

[kerosene]

Originally Posted by SamSam

Much has been made of the small cart on the treadmill proving the concept, but again, it only proves that a small cart powered by a comparatively massive electric treadmill motor can go faster than the treadmill.

Please bear with me and try to read with open mind. And feel free to respond. The treadmill creates following situation - ground moves in relation to the air. Objectively looking this is exactly the same as air moving in relation to the ground. The forces acting on a cart traveling straight are exactly the same if the treadmill moves the mat 5mph in still air as if the air was moving 5 mph over static ground. Do you agree with that - if not Please explain. (1)

Yes the treadmill has huge electric motor but it is not used directly to move the cart - the same way a cart outside is not creating the wind - the weather is. The treadmill motor is used to create the same condition - movement between two media - ground and air.

Originally Posted by SamSam

and seems to me it will have to be a turbine and propeller at the same time to accomplish your goals. The propeller will have to capture the power from the wind and in turn has to power the same propeller to bite into the wind to pull it forward.

Now that certainly wouldn't work - what you are suggesting is two props geared together - one turbine and one thrusting prop. Goal being that the thrust is bigger than the drag from turbine. All will agree that it will not work. The problem - it is working in one media - you can only harvest work from difference between two speeds. A boat traveling down a river with its current cannot drop in a turbine and expect to harvest energy (now should they drop an anchor they could). Same way you cannot build a wind powered aircraft (lets leave the dynamic soaring stuff out for one sec).

You cannot leverage the force from one media - you need a difference. You need the ground and the air - and this is not snake oil hoccus poccus sales talk. You can harvest more speed than the speed of wind going directly downwind - this statement does not violate laws of physics. Do you agree that the statement does not violate physics? (2) The energy is harvested from the wind - no mystical energy creation here.


Lets try another angle here.

1st - and please read this carefully (as 90% of teh skeptics failed to understand this simple fact about the cart before they decided its impossible) The cart's wheels are forced to rotate as the cart is pushed forward, the wheels are geared to the propeller that creates thrust (backwards - to accelerate the cart forward). That is the principle of the ddwfttw cart. Do we agree? wheels harvest work that is put into thrusting propeller - ok? (3)

Now for sake of simplicity lets forget all losses and imagine following situation. 5mph wind cart traveling 5mph (exact speed of wind) what do you suggest will happen? (4)


even a dummy cart with nothing but wheels would travel the speed of wind (we pretend no friction losses from wheels). Traveling slower than the wind would mean work being done against it.

On our cart the wheels have the propeller spinning - thus creating a forward pushing force - will the car accelerate or slow down. If it slows down what makes it happen - after all slowing down would mean that something is working against the wind?

Will it keep traveling at exactly the speed of wind (as a dummy cart do just because of the air drag). Would it really do this? even though it has a propeller causing blowing air against the wind?


Also - do you agree that ice boats can reach down wind marker (by tacking) faster than a balloon floating with the wind can? (5)

This has been documented over and again as a fact - an ice boat can cover distance downwind faster than the wind it just cannot do it in straight line as there would be no apparent wind to power the sail. If we acknowledge that ice boat can do this then we acknowledge that there is no perpetual motion going on. You can indeed travel faster than the wind - just that regular sail only works at off angles as it needs to keep the apparent wind.
Do you agree with this? (6)

It would absolutely great if you could answer the numbered points. This would help understand where the thinking departs from what we are trying to explain.

[A.T.]

Originally Posted by SamSam

I'll put $20 on that. I still say it won't work.

Before you bet any money against spork you should have a look at some animations that explain DDWFTTW, and are intuitively easy to understand:

1)

The claim that "you cannot extract energy from the wind, if you are at rest to the air" is easily disproved by this mechanical analogy to a simple lever:
http://www.youtube.com/v/g8bxXRQtcMY&autoplay=1&loop=1

That can also be made continuously:
http://www.youtube.com/v/Ufk6HVWdSzE&autoplay=1&loop=1

2)

It is a fact that iceboats can tack indirectly downwind faster then the wind. To explain this an analogy to a "squeezed wedge" is sometimes used:
http://www.youtube.com/v/H_OKNr120t4&autoplay=1&loop=1

The same analogy can be used to explain the propeller carts:
http://www.youtube.com/v/zPFzHoubQzg&autoplay=1&loop=1

The only difference here: only the sail (= propeller blade) is constrained to move indirectly downwind, while the rest of the vehicle moves directly downwind.

Just trying to save you 20 bucks.

[ThinAirDesigns]

We completed an excellent weekend of dyno testing a few days ago. Overall system efficiency was well above our (admittedly conservative) expectations. We even managed to create a little (nearly) harmless chaos which goes with any good test session.

Two weeks from now we'll be at Ivanpah at the NALSA land sailing event held there.

www.fasterthanthewind.org

JB

[Guest625101138]

I noticed that you broke a few bits. It is better to do this where it can be fixed easily than in some place where equipment is scarce.

I thought the test set up was clever. Of course as far as the skeptics go it proves nothing. You were using a 6.5HP Honda to provide the power - of course it would work doing this. The fact that you had to hold it back does not come into their calculation!

I look forward to some live test data. Hope it all holds together.

Rick W

[Windmaster]

If it pulls ok when the wheels are driven by the motor.

Then I think it is bound to work when the wheels are driven by the road in the environment of the "still air" provided by the wind. (neat way of thinking of it - don't you agree!)

Just hope it holds up and the wind is not too blustery.

[spork]

Originally Posted by Windmaster

If it pulls ok when the wheels are driven by the motor.

Then I think it is bound to work when the wheels are driven by the road.

That's our plan. The dyno or treadmill can tell us whether it will go downwind faster than the wind, but it can't tell us whether it can get itself to windspeed, or how much faster than the wind it will go. If we're able to get all the measurements we want (prop torque, thrust, etc.) we should be able to answer those questions. Either way, I expect we'll learn a lot more on the dry lake bed in a couple weeks. Fingers crossed.

[360weatherbound]

Its all witchcraft................

[ThinAirDesigns]

Originally Posted by Rick

I noticed that you broke a few bits. It is better to do this where it can be fixed easily than in some place where equipment is scarce.

Exactly Rick -- many critics (and even supporters) thought that the break that ended the day was some sort of disaster. Nothing could be further from the truth.

We had continued through the day to up the rpms, ratios and then the pitch settings, pushing more and more HP through the prop to watch the vehicles behaviors. Towards the end we knew we were near double design loads but that's how you figure out your weak points.

Hopefully that session (and another one this weekend) will make our desert trip far more producive and trouble free. Breaking something when you are 50ft from the milling machine that made the original part rather than the far end of a thousand mile journey actually makes one pretty freakin' happy.

JB

[SamSam]

Originally Posted by spork

That's our plan. The dyno or treadmill can tell us whether it will go downwind faster than the wind, but it can't tell us whether it can get itself to windspeed, or how much faster than the wind it will go. If we're able to get all the measurements we want (prop torque, thrust, etc.) we should be able to answer those questions. Either way, I expect we'll learn a lot more on the dry lake bed in a couple weeks. Fingers crossed.

I'm not concerned about self starting ability or even if you tow it to faster than windspeed and then let it go. My concern is the necessary fragility of the machine and the seemingly unstable design. It looks like that if you get very much off of directly downwind, all the force applied so very high up will topple the narrow tricycle design. But I guess the drag on the rear wheels will have a drogue parachute effect, which will help.

Now that I think about it, once the threshold of windspeed is reached and bested, what is the driving force of the vehicle? Is the propeller pulling or pushing the vehicle forward or is it the wheels driving it forward? Since on the treadmill, the wheels were driving the propeller, I'm guessing on the salt flats with wind, the propeller will be driving the wheels.

I'm sure I will be informed of the error of my ways. Good luck.

[A.T.]

Originally Posted by SamSam

Is the propeller pulling or pushing the vehicle forward or is it the wheels driving it forward?

The wheels are breaking (backwards force), but the forward force on the prop is higher due to two reasons combined:
a) prop is rotating driven by the wheels (feedback loop)
b) air is moving relative to the ground (external power input)

Many people see just a) and think this must be a closed-loop-perpetual motion. But it isn't. The rotation of the prop is just "supporting" the power harvesting from the air-ground relative motion. The prop could never push the cart alone, without the air moving relative to the ground (and therefore moving so slow relative to the cart).

Originally Posted by SamSam

I'm guessing on the salt flats with wind, the propeller will be driving the wheels.

No, the ddwfttw-cart works just like on the treadmill: wheels drive the prop.

'Propeller driving the wheels' is a different case: a turbine cart, which can go directly upwind. But the two cases are symmetrical:
http://www.youtube.com/watch?v=za_rPKSwiyc

[ThinAirDesigns]

Originally Posted by SamSam

It looks like that if you get very much off of directly downwind, all the force applied so very high up will topple the narrow tricycle design.

You're underestimating it's stability (and perhaps its weight) -- it's designed to withstand a 30mph gust from the side and still be well within margin.

Now that I think about it, once the threshold of windspeed is reached and bested, what is the driving force of the vehicle?

The driving force for the vehicle remains the same from start to finish ... the relative motion of the air over the ground. Air that was once going 15mph over the ground is only going say 10mph over the ground once the vehicle passes. This is no different than a sailboat on a broad reach.

Is the propeller pulling or pushing the vehicle forward or is it the wheels driving it forward? Since on the treadmill, the wheels were driving the propeller, I'm guessing on the salt flats with wind, the propeller will be driving the wheels.

The wheels are *always* driving the propeller.

I'm sure I will be informed of the error of my ways. Good luck.

The vehicle will do just that. And thanks for the best wishes.

JB

[ThinAirDesigns]

It's getting harder to be a doubter.

http://www.youtube.com/watch?v=7SYvg40NHtc

[spork]

Originally Posted by ThinAirDesigns

It's getting harder to be a doubter.

http://www.youtube.com/watch?v=7SYvg40NHtc

You underestimate the doubters.