Sailing downwind faster than the wind

[spork]

WIRED posted an update on our project: http://www.wired.com/autopia/2010/07/do ... t/#respond
It sounds like they plan to publish another article when last weekend's record runs become official.

 

[eyytee]

NALSA confirms 2.8 wind speed DDW

From http://www.nalsa.org

Get out your slide rules and physics text books...On July 2, 2010 on El Mirage Dry Lake, Blackbird sailed directly down wind at a speed of 27.7 mph in a 10 mph wind to set a first record for the ratio of Boat Speed to true wind speed of 2.8. BlackBird was designed and built by the Thin Air Designs team (Rick Cavallaro and John Borton) and sailed by Rick. Links to follow soon.

More info:
http://www.fasterthanthewind.org

 

[ThinAirDesigns]

Nalsa has now ratified a new World Record for the Blackbird of 2.8x the speed of the wind while traveling dead downwind using only wind power and no stored energy.

Record ratification reports at: http://www.nalsa.org.

And a great article by Kimball Livingston, an editor at Sail Magazine:

http://kimballlivingston.com/?p=3971

JB

 

[eyytee]

It will however go 100 times the wind speed in no wind.

I looked at the NALSA rules for true-wind-multiple-DDW, and I don't think they closed that loophole. :mrgreen:

EDIT: They demand acceleration during the measurement period, so my nice brick here will not beat you cart

 

[spork]

Another WIRED article. This one they invited JB and I to submit:

http://www.wired.com/autopia/2010/08/ddwfttw/all/1

 

[unclefred]

Another WIRED article. This one they invited JB and I to submit:

http://www.wired.com/autopia/2010/08/ddwfttw/all/1

From the WIRED article:

“Forget “plane on a treadmill,” what about DDWFTTW on a treadmill? It turns out that if you want to test this device in a wind tunnel you turn the wind off. No, really.”

Not true. Running a model on a treadmill with the wind off simulates running at the same speed as the wind, not running faster than the wind. For example, let’s assume a 10 mph wind and the cart running at 2.8 times the wind speed. The cart would then be moving 28 mph. Thus a person on the cart would be experiencing an 18 mph head wind. To simulate this, one needs to run the tread mill at 28 mph and the wind tunnel at 18 mph. The model must be running at 28 mph on the treadmill (and thus standing still) while the wind is pushing it from the front.

 

[spork]

Not true.

On a standard treadmill we can't (and don't claim to) determine the cart's top speed, only the claim that it can go faster than the wind. If it advances on the treadmill at all, it's doing this. Our little cart bounds forward on the treadmill - even up a positive incline.

 

[unclefred]

Not true.

On a standard treadmill we can't (and don't claim to) determine the cart's top speed, only the claim that it can go faster than the wind. If it advances on the treadmill at all, it's doing this. Our little cart bounds forward on the treadmill - even up a positive incline.

I never said anything about top speed.

A treadmill (with no air blowing across it) does not simulate going faster than the wind. It simulates going the same speed as the wind. If you are going faster than the wind, then there is a headwind to the cart. If you are stationary on a treadmill there is no headwind. However, I will agree that when you go slightly faster than the treadmill then you are going slightly faster than the wind. If this is significant or not, depends on your definition of "slightly".

Is it acceptable to go only 1% faster or do you need to go 2.8x faster than the wind to claim the treadmill demonstrates the effect? It probably depends on who you are talking to. To go 10% faster than the wind you must advance 10% faster than the treadmill is running. To go 2x faster than the wind you must advance just as fast as the treadmill is running the opposite direction. I also agree that if you incline the treadmill then you are adding a backwards force to the car and this might simulate a headwind. I suppose that one could calculate the wind force and figure what angle would match that.

Even if it is not "faster than the wind", I still see the treadmill demonstration itself as absolutely amazing. The treadmill proves that when the wind is doing absolutely nothing (the air is stationary with respect to the cart and thus not pushing in any way forward or backward), yet work is being done by moving the cart on the treadmill and overcoming friction. First impression is that it violates physics and is not possible, yet here it is. Just explaining the zero wind treadmill demonstration is a challenge. Really cool. Nice job.

 

[spork]

I'd guess that we maybe had the cart doing 50% faster than belt speed at times (5 mph forward on a belt moving 10 mph). We always realized that we couldn't determine top speed on a short treadmill, but we felt that showed plenty of margin to demonstrate DDWFTTW.

Unfortunately, even our outdoor runs with the manned vehicle (and corresponding world record) won't convince the true power-deniers. But I'm OK with that.

 

[eyytee]

Is it acceptable to go only 1% faster or do you need to go 2.8x faster than the wind to claim the treadmill demonstrates the effect?

Any acceleration forward against the treadmill direction (from initially stationary relative to air) demonstrates DDWFTTW under controlled conditions. It shows that at wind speed you can still produce a net-thrust that accelerates you forward. That means that your top speed will be somewhere above wind speed.

I also agree that if you incline the treadmill then you are adding a backwards force to the car and this might simulate a headwind. I suppose that one could calculate the wind force and figure what angle would match that.

You can place a fan in front of the treadmill, that creates an apparent headwind. This way you can simulate any wind multiple, without the cart falling down.

(the air is stationary with respect to the cart and thus not pushing in any way forward or backward),

The air is pushing the cart forward. Simply because the cart is pushing the air backwards. (Newton's 3rd law)

 

[csmyth3025]

I read the links and I can understand how the prop can drive the vehicle with a tail wind. When the vehicle approaches wind speed, I can see how the momentum of the vehicle can, in turn, keep the prop rotating for a short while. Beyond this point I'm at a loss to understand what's going on. Is this for real, or is it some sort of elaborate prank?

Chris

 

[eyytee]

I read the links and I can understand how the prop can drive the vehicle with a tail wind. When the vehicle approaches wind speed, I can see how the momentum of the vehicle can, in turn, keep the prop rotating for a short while. Beyond this point I'm at a loss to understand what's going on.

Nothing changes at windspeed, about the way the cart works. Here a visualization of the steady state above windspeed, seen from 3 different reference frames, with force diagrams and energy balance:

[youtube]http://www.youtube.com/watch?v=FqJOVHHf6mQ[/youtube]

 

[spork]

I read the links and I can understand how the prop can drive the vehicle with a tail wind. When the vehicle approaches wind speed, I can see how the momentum of the vehicle can, in turn, keep the prop rotating for a short while. Beyond this point I'm at a loss to understand what's going on. Is this for real, or is it some sort of elaborate prank?

Chris

We never rely on the momentum of the cart. This thing can pull a trailer below, at, or above wind speed. And it's not a prank (elaborate or otherwise). The secret is simply that the wheels are going over the road faster than the cart is moving through the air (due to the tailwind). This allows us to produce more thrust with the prop than we create drag on the wheels to produce that thrust. And no - it won't work in no wind.

 

[unclefred]

Is it acceptable to go only 1% faster or do you need to go 2.8x faster than the wind to claim the treadmill demonstrates the effect?

Any acceleration forward against the treadmill direction (from initially stationary relative to air) demonstrates DDWFTTW under controlled conditions. It shows that at wind speed you can still produce a net-thrust that accelerates you forward. That means that your top speed will be somewhere above wind speed.

I also agree that if you incline the treadmill then you are adding a backwards force to the car and this might simulate a headwind. I suppose that one could calculate the wind force and figure what angle would match that.

You can place a fan in front of the treadmill, that creates an apparent headwind. This way you can simulate any wind multiple, without the cart falling down.

(the air is stationary with respect to the cart and thus not pushing in any way forward or backward),

The air is pushing the cart forward. Simply because the cart is pushing the air backwards. (Newton's 3rd law)

I agree that any forward motion is faster than the wind, even 1% faster is still faster.

"You can place a fan in front of the treadmill, that creates an apparent headwind". Yes, you could do that, but they DID NOT do that. Thus the best their demonstration achieved is slightly faster than the wind. It does not demonstrate significantly faster than the wind (or 2x or 3x or anything else). This is not to demean the treadmill. It is still a mind boggling demonstration and a challenge to explain.

You said that with the cart moving at the speed of the wind, "the air is pushing the cart forward. Simply because the cart is pushing the air backwards. (Newton's 3rd law)" I assume from your statement that you are considering the blades to be blowing air backward just like a fan. Thus blowing air backward propels the cart forward. Is that your assumption?
- - - Uncle fred

 

[spork]

"You can place a fan in front of the treadmill, that creates an apparent headwind". Yes, you could do that, but they DID NOT do that.

We certainly did. You just didn't watch the right video.

Thus the best their demonstration achieved is slightly faster than the wind. It does not demonstrate significantly faster than the wind

We have videos that probably show the cart advancing at 5 mph on a 10 mph treadmill. I'd call that significantly faster.

You said that with the cart moving at the speed of the wind, "the air is pushing the cart forward. Simply because the cart is pushing the air backwards. (Newton's 3rd law)" I assume from your statement that you are considering the blades to be blowing air backward just like a fan. Thus blowing air backward propels the cart forward. Is that your assumption?

It's not an assumption. That's exactly what's happening. It's easily demonstrated (as we have shown in at least one of our videos), and it's easy to explain analytically (which we've also done).

 

[csmyth3025]

Thanks for the explanation and video spork and eyytee. I don't have the background to really understand how this counterintuitive contraption works, but seeing is believing. My congratulations to you for thinking this up and then making it work despite those who said it's impossible.

Chris

 

[eyytee]

I don't have the background to really understand how this counterintuitive contraption works, but seeing is believing.

You don't need any background to understand how it works in general. It is not much different from a simple lever:

[youtube]http://www.youtube.com/watch?v=g8bxXRQtcMY[/youtube]

Which can also work continuously:


[youtube]http://www.youtube.com/watch?v=Ufk6HVWdSzE[/youtube]

 

[ThinAirDesigns]

From the WIRED article:

“Forget “plane on a treadmill,” what about DDWFTTW on a treadmill? It turns out that if you want to test this device in a wind tunnel you turn the wind off. No, really.”

Not true.

Yes, true.

Running a model on a treadmill with the wind off simulates running at the same speed as the wind, not running faster than the wind.

Not if the vehicle is moving forwards on the treadmill (which ours did).

JB

 

[ThinAirDesigns]

"You can place a fan in front of the treadmill, that creates an apparent headwind". Yes, you could do that, but they DID NOT do that.

You really should stop saying what we did and did not do until you learn what we actually did and did not do.

There has been video up for a year or two where we place a fan in front of the treadmill creating an apparent headwind.

JB

 

[vattas]

It's funny how my windsurfing buddies, when I showed them BUFC video, accepted it as completely natural thing. Reaction - "Cool!" Maybe they're used to the thought, that in sailing there are other contributing factors than pure wind.
I wonder if Formula board can go downwind faster than the wind (VMG). It would be interesting to analyze GPS tracks of some competitions, unfortunately it's hard to find relevant wind data for specific moment...

 

[spork]

It's funny how my windsurfing buddies, when I showed them BUFC video, accepted it as completely natural thing. Reaction - "Cool!" Maybe they're used to the thought, that in sailing there are other contributing factors than pure wind.
I wonder if Formula board can go downwind faster than the wind (VMG). It would be interesting to analyze GPS tracks of some competitions, unfortunately it's hard to find relevant wind data for specific moment...

I think that's because most windsurfers (like most other people) have a hard time arguing with experimental real-world evidence. When we said this could be done, but hadn't yet built working models, there were plenty of windsurfers around to call us kooks. Keep in mind, I'm an old windsurfer and kitesurfer. For us, this whole thing started when I posted it on a kitesurfing forum. As we later learned Andrew Bauer beat us to it by about 40 years however.

 

[dgwalker]

I looked at the Propellor wind driven vehicle. Not new. It has been done before. What is forgotten here is that the wind passing the propellor speeds up as it speeds up generating more speed till the air resistance equals the forward propulsion. The faster the propellor goes the more unslowed air it cuts into. The secret of modern wind driven turbines is to cut across unslowed air hence the low number of blades ussually two or three. The slow start is most certainly due to wrong gearing to the wheels. A gearbox would fix this.

 

[spork]

I looked at the Propellor wind driven vehicle. Not new. It has been done before.

Thanks for letting us in on the secret. You'll also find this bit of information in the first few seconds of our video and on the first page of our blog. After proposing this as a brain-teaser we learned that at least one other person had already demonstrated one (Andrew Bauer). We also learned that he learned about it from a student's paper. But we haven't learned the name of that student yet.

What is forgotten here is that the wind passing the propellor speeds up as it speeds up generating more speed...

Read that back to me again. And then tell me who it was forgotten by. We understand quite well how this works. And it does NOT work by the wind turning the prop. The wheels turn the prop.

The slow start is most certainly due to wrong gearing to the wheels. A gearbox would fix this.

You should consider not speaking of that which you don't understand. Feel free to ask us about the gearing if you like.

 

[vattas]

BTW, have you tried to run not directly downwind, but on a broad reach, setting propeller accordingly? I wonder if it's possible to reach greater speeds this way...

 

[spork]

BTW, have you tried to run not directly downwind, but on a broad reach, setting propeller accordingly? I wonder if it's possible to reach greater speeds this way...

We've never "tried" to do that, but have been forced to run on a broad reach during runway testing occasionally. Both theory and experimental results show that we do best directly downwind. The only way to set prop pitch accordingly for this course would be to have a cyclic pitch that allowed each blade to change pitch over each rotation.