Directorly Downwind Faster than the Wind

[TinselKoala]

Mike, unfortunately your analysis is incorrect, as experiment has shown. There are plenty of links to explanations and illustrations above, so I won't add more.
But don't feel too badly; at first, due to similar errors on my part, I didn't believe it either.

[WilbyInebriated]

But don't feel too badly; at first, due to similar errors on my part, I didn't believe it either.

that's par for your course... 

[Mike Furness]

Thank you for the information, which I have studied at length.
I did indeed misunderstand the principle originally; however, having studied carefully, the answer is still NOT POSSIBLE!
To elaborate, sure, a wind generator produces power, and it's not a large step of imagination to see that if this power (velocity) is transfered to a drive motor, then SOME progress directly into wind is indeed possible. As the wind resistance increases as the square of the velocity, it's quite apparent that DIRECTLY into wind can only have a very limited velocity increase.
The walking road and sloping ramps merely confuse as the moving road furnishes the velocity which otherwise would need power to attain.
The desert scene behaved absolutely as predictable UNTIL the wind headed the machine, only then did it begin to move, and obviously could derive some very limited power from the headwind
one has to consider the very low drag and low friction wheels to see just how little energy is available to drive the machine, probably not even 100 watts.
To obtain some forward velocity from say a 30MPH DIRECT headwind is indeed possible, but accellerating to 3oMPH when there is no headwind (same as wind excess downwind) is clearly imposible, sorry (any other than directly into or directly downwind excludxed, for obvious reasons).

I return once agaiin to my original statement, no wind, no propeller turning, hence no movement; push it along a road with a stick, and it will surely stop under the same circumstances of totally self generated wind.
consider a very simple analogy, you propel a vehicle which then takes the power vested by way of it's speed; to make it even simpler for you, drive an alternator from the car's wheels, at the instant you stop external pushing, at 100% efficiency you would retrieve exactly the same energy back as you put in, NEVER more; unfortunately wind and rolling resistance would exceed the input and you would come to a standstill quite quickly!
just don't be confused by a propellor instead of an alternator, principle exactly the same!

Mike.

Mike. J. Furness

[ramset]

Mike,
"no wind no propeller turning"?

You did see the part where the telltales go slack [the no wind part],and the machine continues to accelerate ?

Chet

[helicalred]

The directly downwind, faster than the wind crowd http://www.fasterthanthewind.org/ say they have established a world record http://www.nalsa.org/DownWind.html, exceeding the wind speed by a factor of 2.8. Although they claim it is not a hoax, I think that's playing with semantics. My thought is that the whole thing is a bit of a swizz - a well stage-managed extravaganza although for what purpose other than some ego building I can't imagine as the machine doesn’t appear to have any practical value.

There are seemingly endless forums on this thing (see http://forum.mythbustersfanclub.com/index.php/topic,12948.0.html, http://forums.randi.org/showthread.php?t=128483 and http://skepticblog.org/2010/05/27/sailing-directly-downwind%E2%80%A6-faster-than-the-wind/#more-8353 if you can spare the time) the main protagonist in all the discussions being someone called "Spork" who presumably is Rick Cavallaro - the person who seems to be the "owner" of the project. Nowhere, that I can find, does he give a clear and concise explanation of how the thing works, even on the record submission http://www.nalsa.org/BlackBirdDDWSR/NALSA%20Submision%20report.pdf. Plenty of photos, video clips (some very well made and convincing), hand-waving, diversions and distractions but nothing conclusive. I'm put in mind of a stage magician. In an attempt to lend credence or authenticity or whatever to the performance there are the Major Sponsors: Joby Energy and Google, support from some obscure university and, in pride of place, a video shot by Richard Jenkins - holder of the world land speed record (Wow!). All very impressive but absolutely meaningless. And note that the speed wasn't actually measured by The North American Land Sailing Association (NALSA)  people, they were merely observers who reckoned that everything looked above board and OK.

That was four months ago. Since then nothing new on the web-site â€" no revelling in the glory of it all or cashing in on their success, they’ve just turned out the lights and gone home, it seems. Strange.

Now, for how long was this faster-than-the-wind speed sustained? Ten seconds!  I'm not convinced. Before a 10mph wind their speed was 27.7mph. In that 10 seconds they would have covered approximately 124 metres. why not go for a longer distance at a lower speed? That would look more impressive to me and would still be a record.

I don't doubt that they did reach the speed they say, but I think it is rather telling that the graphs of the velocity profile etc only show the recordings for that ten seconds; I'd be most interested to see recordings of the same measurements in the minutes before and after.

How I think it works:
As Spork says, the propeller makes no contribution in getting up to wind speed, the wheels always drive the propeller and never vice versa.
While getting up to wind speed, the propeller is 'feathered' i.e. not producing any power. However the wheels still spin it and it acts as a flywheel storing kinetic energy. I would say that the purpose of the gears seen in some of the photos is to get it spinning as fast as possible.
When wind speed is reached, this kinetic energy is tapped allowing a brief period of acceleration to an over-ground velocity greater than the wind speed by either a) disengaging the propeller from the drive chain and giving it some pitch (and consequently some thrust) or b) leaving it feathered but changing the gearing relative to the wheels so that for a short time they are driven.   

All of Spork's banter seems to me a collection of truths, half-truths and waffle with a hefty dose of obfuscation.

Cheers,
Bill