Good point about the autorotating helicopter. My other point about the size of sail needed to move a regular is now moot. My point about the land yacht was to see how large of a sail was needed to move a regular yacht. So my next question is ... How much more effective than a regular sail would your spinning rotor be?

Okay now as I think about it more the answer to my last question is "It depends". I thought back to your helicopter reference and realized that if you locked the rotating prop (a senseless thing to do in normal operation) it would not do much better than freefall. The faster the blade spins the more "efficient" it is sail wise.

quote:

Originally posted by roofingguy:
If it were "clearly" directly abeam, would it not be pointing either towards or away from the camera?

Only if the camera is directly abeam, which it is not.

At the point about 17 seconds in, where the cart stops and they have to give it another push, the tell-tale shows the wind from the side. Maybe it doesn't look that way to you. After more than 25 years of watching tell-tales (started sailing in 1980) I can tell you that it is EXTREMELY clear to me that the wind is coming from well off to the side of the cart.

Here's where I'm with Denverd0n:

The video proves nothing. There are a hundred ways to fake that video.

Here's where I part with Denverd0n:

He says the vehicle is not traveling directly downwind, but rather it achieving it's speed because it's going at an angle

Ok, that's his opinion and for the sake of the following perspective, I'll go with it:

Here's the problem with Denverd0n's position ... If he understood the physics of the cart he would realize that the less directly downwind this cart goes, the less effective it is. Wind at an angle *hurts* this device and makes it run slower.

Perhaps the video is faked (there's no reason for it to be, but still ... perhaps), but the reason it's going faster than the wind is NOT because it may be running at an angle to the wind.

JB

quote:

Originally posted by BubbleBoy:
He says the vehicle is not traveling directly downwind, but rather it achieving it's speed because it's going at an angle

Nope. Sorry. You're completely wrong. That is NOT what I said.

All I have said is that the vehicle is not traveling directly downwind, or even mostly downwind. Period. I never said anything about how it achieves its speed. In fact, I specifically said that I do not dispute that the cart CAN go downwind.

I just find it a bit ridiculous that so many people post this same video and claim that it is proof that a cart can go directly downwind faster than the wind. Since the cart is NOT going directly downwind, this video proves absolutely nothing of the sort! That is all I've said.

All the cart video shows is that the propellor is an energy conserving device, similar to a flywheel, since the wind wasn't constant. Note he didn't publish any videos of the the treadmill test.

quote:

Originally posted by jeffareid:
All the cart video shows is that the propellor is an energy conserving device, similar to a flywheel, since the wind wasn't constant. Note he didn't publish any videos of the the treadmill test.

So you're saying that it collects energy in the flywheel(prop) during gusts and then uses that energy during the lulls?

You sound certain in your quote. Are you really certain?

JB

quote:

All the cart video shows is that the propellor is an energy conserving device, similar to a flywheel, since the wind wasn't constant. Note he didn't publish any videos of the the treadmill test.

To be fair he didn't publish any videos of anything. He did that video for himself as he and his friends were debating this same point. One of his friends posted the video - originally on another server. It overloaded that server and was taken down. Subsequently someone else posted it on YouTube. Jack never has.

I spoke to Jack yesterday about the treadmill test. You're right, he doesn't have it on video. The prop is hanging on his wall (it's sort of a work of art, made of Butternut wood if I recall correctly) and the cart is in his attic. He said he might get his friend with the video camera to help him do a video of it on the treadmill. But he also warned me that the treadmill test is a bit ugly because that cart (thinner wheelbase) has no steering - so they have to sort of bump it to keep it from rolling off.

But the only reason Jack's videos don't seem like proof positive to everyone is because he's not trying to prove anything to everyone. If he were, he'd be the one on here making all the noise.

Me:

quote:

Here's where I part with Denverd0n:

He says the vehicle is not traveling directly downwind, but rather it achieving it's speed because it's going at an angle

Denverd0n:

quote:

Nope. Sorry. You're completely wrong. That is NOT what I said.

All I have said is that the vehicle is not traveling directly downwind, or even mostly downwind. Period. I never said anything about how it achieves its speed.

My goodness, how is it that people think they can post on the record all over the Disco site and then claim they didn't say things they did.

Let's remember in particular this little quote of Denverd0n's:

quote:

I never said anything about how it achieves its speed

Ok, heres a quote from Denverd0n on the other and bigger Disco DDWFTTW thread (referenced earlier)

Denverd0n:

quote:

So, he gives it a push and it goes off at an angle to the wind, fast enough for the telltale to eventually stream straight backwards. But remember that the wind is still coming from off to the side! Big deal! Sailors have been doing this for 500 years now!

Well, looky there -- it works because it's at an angle ... just like the sailors have been doing for 500 years now!

Denver, there're plenty more quotes from your other posts where like the above, you *insist* that this is nothing new, just another example of angling across the wind to beat the wind.

Frankly I don't understand why you are now being so defensive about the "it's just going at an angle" position. Though I disagree with you, it's a perfectly reasonable position to take.

JB

quote:

I do not dispute that it is possible to go downwind faster than the wind, but that video proves absolutely nothing of the sort!

Don, I'm not sure if I understand your position. Are you saying you believe it might be possible for such a cart (or any wind powered vehicle) to go *directly* downwind faster than the wind, or are you saying that it may be able to go faster than the wind, but not with a downwind component faster than the wind?

For the record, I'm not talking about this specific cart and/or video.

Time to get a life, BubbleBoy. That quote is from a different thread and... I'm not even sure HOW long ago! Months, at least. In any case, spork convinced me. I believe it is possible for the cart to go downwind. I've said that now three times (or is it four?) in this thread.

In THIS thread I never said that it gets its motive force from the fact that it is going crosswind. Okay? Is that good enough for you? Or are you next going to dig up something I wrote 40 years ago and try to use that to "prove" I don't have a complete grasp of physics? SHEESH!

Regardless of all of that, the cart in the video is NOT going downwind, and so to post that video and claim that it proves that the cart can go downwind faster than the wind is just DUMB! It is the EXPLANATION of how the cart works that is compelling. The video is just a confusing and pointless distraction.

quote:

At the point about 17 seconds in, where the cart stops and they have to give it another push, the tell-tale shows the wind from the side. Maybe it doesn't look that way to you.

You're right, to me that particular moment in the video shows a gust of wind from the side, but isn't as indicative of the wind direction as just before that point, when the camera is farther behind the cart and clearly shows that the wind is much less than 20 degrees to the road.

Keep in mind that the post that the streamer is on is off to the side of the cart, not directly behind it.

As Bubbleboy said, how does the cart manage to travel fast enough to have the streamer pointing what looks to me to be straight back once it is up to speed? Any cross wind would cause the streamer to point off to the side, which it certainly doesn't to me.

But the main thing is that you agree that the cart can go directly downwind faster than the wind, which wasn't the case almost three months ago at the end of the other thread. Which of the explanations helped convince you?

The video shows what a DWFTTW cart would look like when it was traveling faster than the wind. That to me is reason enough to include it in a discussion of such a vehicle.

Any others left to convince?

quote:

Any others left to convince?

We've got a guy on the R/C heli forum that rivals the dude that threatened to fight me over this.

The O.P. jeffareid knows who I'm talking about. Maybe he could invite him over. I'd do it myself, but he despises me.

spork, I always agreed that the cart can sail directly downwind faster than the wind, but my reason why was different from yours.

Reluctantly, I finally agreed with you. But now, after thinking this through, I'm going to challenge your idea of why it works. I'll post my reasoning later tonight.

One issue with the cart and it's propeller is induced wash. Upstream of a propeller, the air is accelerated (both linear and angular), so by the time the air reaches the blades of a propeller, the apparent wind on the blades is greatly reduced. To compensate for this a higher angle of attack is required along with a higher amount of drag.

Another issue is the tips of a propeller also operate in and induced vortice wash.

The linear aspect is mentioned in this example of a helicopter in hover: helicopter hovering

In the case of the cart, the goal was to get the propeller to operate in an apparent wind sitation similar to landsails or icesails, but the induced wash reduces the apparent wind greatly.

quote:

I just find it a bit ridiculous that so many people post this same video and claim that it is proof that a cart can go directly downwind faster than the wind.

Oh really?

I could have sworn the OP states:

quote:

More controversial is this propeller based device that claims to get a similar "apparent wind" effect (using the difference between wind and ground speed as a power source) that can go downwind faster than the wind.

I see no offer of "proof". I see a question of the claims and the physics involved.

quote:

Since the cart is NOT going directly downwind, this video proves absolutely nothing of the sort! That is all I've said.

We also disagree on what the video shows. I see mostly downwind with occasional gusts and/or lulls. But if the cart *could* do what is claimed of it, whether the video shows it distinctly or not, then where's the "controversy"?

A cross wind should help, and probably did in that video, because it "washes" away the induced wash going into a propeller. It's the reason that helicopters in forwards flight require less power than while hovering. However with a cross wind, then the cart is no longer going directly downwind, which defeats the point of the cart.

quote:

Originally posted by denverd0n:
You're completely wrong. That is NOT what I said.

I never said anything about how it achieves its speed.

quote:

Time to get a life, BubbleBoy. That quote is from a different thread and... I'm not even sure HOW long ago! Months, at least.

LOL Denver, I haven't responded to the above quickly -- I spent a bit of time doing the research as I wanted to be very fair to you.

As it turns out, it's very well established that the period of time called "Months, at least" falls well within the purview of "never". No, really - it isn't even close to the edge of "never". I mean, it could be *years* and "never" still isn't going to work.

LOL. And as to your "different thread" excuse ... yeah, a thread regarding the *exact same video* -- what a difference *that* is.

It's really funny how you can tell me that I am "completely wrong" ... that's fine. But when I point out and document that in reality it is you who is completely wrong, then I'm the one who needs to "get a life".

Mirror?

JB

A cross wind will hurt performance. It will mean that without the cyclic pitch of a heli, it is impossible to optimize the angle of the sail/prop as it tacks back and forth.

As it is shown, the device is mechanically constrained to the same sail and ground track angle for both tacks.

JB

quote:

Originally posted by jeffareid:
A cross wind should help, and probably did in that video, because it "washes" away the induced wash going into a propeller. It's the reason that helicopters in forwards flight require less power than while hovering. However with a cross wind, then the cart is no longer going directly downwind, which defeats the point of the cart.

For the record...

- My original purpose in starting the original thread was to show that it IS POSSIBLE to make a wind-powered vehicle that can go downwind faster than the wind. That has now been shown in a number of ways including very simple analogies, recorded GPS data, testimony of ice-sailors, to mention a few. Ultimately, the goal is to have the Mythbusters take this on as a segment.

- I like the cart. I know HOW it works, I know THAT it works. I DON'T claim the video is PROOF that it works.

- Despite all the speculation about steady winds, cross-winds, and inertial frames of reference, the cart works just fine on a treadmill.

- A boat CAN tack into a relative wind (i.e. the boat can tack on a day when the wind is dead calm relative to the shore, but the boat is in a current). The boat can outpace the current that propels it in this case.