@Dennis@home:
Repeating a suggestion from my previous post:
"Rather than some version of "Oh, no you can't", would one of the critics actually step up and document an actual flaw in "A", "B" or "C" "
You know dennis - you look a bit silly simply ranting that we're liars, fools, and hoaxters without even attempting to tell us specifically how you believe we violate any law of physics. If there is a problem with the analysis JB posted point it out. It's about the simplest derivation you could hope for.
Perhaps that's because we think its true. Do you feel we need to argue against the truth?
This is also true.
Because you don't do all three. You are travelling at 2.9 times the speed of the wind. You claim to slow down the wind. The only way to slow down the real wind would be for you to push wind back at >2.9 times the speed you are travelling at. So you are now going to tell me that the energy needed to get that pushing back wind up to >2.9 times the real wind is less than the energy you get by slowing the real wind down. You do not make any sense with your argument.
Citing three effects isn't the same as actually using them.
I understand that it can extract energy from the drive belt to power the prop so it moves in the opposite direction to the belt. maybe its you that doesn't understand your cart?
You are making claims that aren't true and expecting people to agree.
You are the one that doesn't know what you are talking about. the very fact that you think a rolling road is the same as moving air shows that you don't have a clue.
Well I think you are either an idiot or think everyone else is.
The fact that you are wrong is not my problem either.
>
Ignoring Dennis, who delights in proving how wrong he can be on a regular basis, I have to say I found the explanation of how it works a bit opaque. I'm pretty sure I've got it, but the text and diagrams didn't really help that.
And metric just makes things easier :-)
It does require a bit of a twist in the way of thinking about it - that the wheels drive the windmill, not the other way around as you'd expect!
Hey, they're Americans. They like to make things hard for themselves by mixing units. Just ask NASA...
Rick, JB, if you want another challenge... there's a bunch of guys over on uk.rec.sailing who don't believe you either. Which bearing in mind that most of them have seen fast boats tacking downwind at more than windspeed is impressive. It's come up before, but never with first hand experience - and that's why I didn't need persuading. The streamers on the video I saw persuaded me it was no fake - the effects were too good for Hollywood!
Andy
You think you got it? and you think I am wrong? Why don't you explain it then?
Trouble is it needs the connection to the ground that a boat doesn't have, so having a sailing background will naturally confuse things.
Just wondering how a multi-blade prop would change the balance of power required to drive it vs thrust that the slowed air imparts to the craft?
The best explanation for me is that there's three things involved, and hence two relative velocities, which allows us to extract energy.
Let's try a different drive method - not a prop. Instead, we'll have the road and a very long conveyor belt.
I've got a little car, with 6 wheels - 4 straddling the belt, 2 on the belt.
Can I arrange a drive mechanism such that my little car can go faster than the conveyor belt? (no engines, stored energy, etc, just simple mechanical connections)
(actually, this is all quite like the various cotton reel problems covered in 1st year physics)
I'm imagining something involving planetary gears, we're getting dangerously close to Pious territory now ...
Clive, there a a BUNCH of ways to describe what's happening here. We've found that some people respond to one description and think all others are nonsense, and others find one of the others useful. We'll be happy to answer your questions and tailor the discussion to help bring it across. Just let us know what's making sense and what's not making sense to you. The energy analysis is very compelling to me at one level, but it's also useful for me to think of each blade of the propeller as being on a continuous broad reach - spiraling downwind faster than the wind.
I disagree. Whether the vehicle is a boat or land vehicle it exploits the energy available at the wind/surface interface. In the case of a boat the keel just acts as a wing underwater while the sail acts as a wing sticking up in the air. We know there are sailboats that can tack their way downwind much faster than the wind, so all we need to do is connect two such boats side-by-side with a long telescoping pole. Now they can go downwind staying on alternate tacks and the whole contraption constitutes a "boat" that goes directly downwind faster than the wind.
Spool video:
I prefer working with the forces rather energy - I think introducing time into it unnecessarily complicates things.
Playing with the dynamics of the propellor also doesn't help me - see how I've started my own explanation. The propellor comes later.
Another counterintuitive aspect to this thing is that the propeller acts just like any other propeller. It's tempting to think of the wind pushing on a cushion of air being produced by the prop, but this isn't the case. The propeller simply pulls its way through a (moving) mass of air in exactly the same way an aircraft prop does. There are trade-offs with number of blades, pitch, diameter, etc. Each affects prop efficiency and practical engineering aspects (like having a thick enough camber to contain the spar).
If you prefer the forces approach I can post a vector diagram that shows how an ice boat can achieve a downwind VMG (velocity made good) faster than wind speed on a broad reach. This works strictly on lift and drag of the sail and keel/blades. From there you can imagine the two ice-boats side-by-side connected by a telescoping pole, or you can picture two ice boats circling their way down a cylindrical earth on a continuous broad reach. In this case their sails are doing exactly the same job as our prop - and in the same manner.
Go for it.
Here's a simple vector analysis:
Here is the vector diagram for an ice-boat that=92s maintaining a course such that its downwind velocity component is faster than the wind. I've assumed a boat going 45 degrees downwind at a speed of twice the wind speed. The ticket is to then compute the necessary L/D of the sail and keel to make this possible (this would have the boat making a downwind velocity component of 1.414 times the wind-speed).
Given the wind velocity and the boat velocity, we can easily compute the apparent wind over the sail. From this we see the direction of lift and drag on the sail. What we need to do to make sure this sailing configuration is possible is to insure that "alpha" is small enough so that the resultant force has a forward pointing component (relative to the boat). In this case alpha would have to be 45 - 16.3 degrees or 28.7 degrees (or less). That relates to a sail whose L/D is
1.83. Obviously this is easily achievable. However, I've assumed a keel with infinite L/D in this case. The drawing gets a little more cluttered if we include the keel's L/D. So I'll try to describe it in words. I'm going to assume an L/D of 10:1 for the keel (easily done with an ice-boat). This will trim 5.7 degrees off of my 28.7 degree budget. That leaves me with an allowable 23 degrees(L/D =3D 2.35:1) to achieve this configuration. So with a keel that has a 10:1 L/D and a sail/boat that has an overall 2.35:1 or better L/D this configuration can be achieved (we can achieve a downwind component about 40 percent faster than the wind speed).HomeOwnersHub website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.