Ducati Monster Forum

Details in the link. Short version is that someone has built a wind powered cart that can travel faster than, um, the wind

http://blog.makezine.com/archive/2010/11/downwind_faster_than_the_wind_black.html (http://blog.makezine.com/archive/2010/11/downwind_faster_than_the_wind_black.html)

I am surprised so many people thought it was a hoax.  It is not hard to understand how it could be done.  They are just amplifying the wind speed to rotary speed through mechanical ratios.

mitt

Effin brilliant.

i bet those same people think that 7800rpm on ur tach means ur wheel is spinning 7800 times a minute.

Put me in the skeptic camp.  [coffee]

But does the plane take off?







 [evil]

Is Randall driving it?

wait it doesn't?  hrm..
 





;)

Once you are traveling faster than the wind isn't the wind force then coming from the opposite direction?

Which is to stay is if you are traveling a constant speed that is the same as the speed of the wind you wouldn't feel the wind.

So unless that thing is storing power and must spend time at sub-wind speed speeds, I call bullshit.
Sac

change pitch of the blades to have the oncoming wind of it's forward motion power it once it exceeds tailwind?
friction losses would negate something I'd thing though otherwise it's a perpetual motion machine  :P

the more I think about it now I am skeptical too - maybe there is something else going on there like maybe they vehicle is at a slight angle to the wind or something.

mitt

It could tack going downwind, and net velocity (velocity made good to sailboat folks) could be greater than windspeed.
Maybe 50% more than windspeed....

Directly downwind at 2.8X windspeed, no way.

i'm still waiting on one of you engineering types to build a cart to test your theories like these guys:

Downwind Faster than the Wind (DWFTTW) Myth Challenge (http://www.youtube.com/watch?v=xHsXcHoJu-A#normal)

It's like a piston.  How does a force acting on a piece of metal moving up and down translate into forward motion?  It's the |magnitude| of the speed differential between the fan and the wind that is what drives the cart.

Really it's the magnitude of the speed differential between the ground and the wind.  The cart acts as a rectifying bridge, in electrical terms, between ground and wind, and the potential energy difference is fed back into a force that is acting in the same direction (or opposite depending on your point of view) that the phenomenon is occurring.  It is a feed back loop that stabilizes once the usable power resulting from channeling the potential energy difference between ground and wind equals to the resisting friction forces, which act against the feedback power.  The feedback loop is initially unstable since the resulting forces cause the cart to accelerate, which motion in turn changes the equations for the feedback power being generated, causing the cart to accelerate differently, causing the feedback power to shift, etc.

Now build shit tons of these and let's put all wind on Earth to a stop!

I don't know if anybody else picked up on this, but when it first starts to move, the propeller is spinning backwards.

The wind isn't spinning the propeller in the beginning.  The propeller is spinning because it's being driven by the wheels turning.  The propeller at this point is acting like a sail and the wind it simply pushing the cart along.  The propeller turning is merely a by-product of being geared to the wheels.

Then as soon as there's a headwind, the wind is now driving the propeller and the cart is now wheel driven.  At this point the cart starts accelerating much much faster.

It's not perpetual motion because eventually air resistance and rolling resistance equal the gear magnified force that wind is transferring to the rear wheels and it reaches a constant speed.


What I haven't worked out yet is how the transition is made from tailwind to headwind.  It seems like there needs to be some sort of stored energy to push the cart through this zero relative airspeed period.

Unless by this point the propeller is spinning fast enough that it's producing thrust and has enough rotational momentum (which I guess would be the stored energy I mentioned) to continue turning fast enough and long enough to drive it into a headwind state.

The turbulence of the wind causes it to pop.

Just like an electric motor.

Wouldn't the cart still have to be going at least 101% of the wind speed before there is this turbulence you speak of. How does that happen by a mere push from a rear wind?

sac

maybe the driver ate Mexican for lunch and is providing some added wind thrust

[retracted]

Bah.  Put me down for skepticism.

----

I can't put my finger on why, but I have trouble reading any of this without feeling like I'm being snowed.  It's like someone's asking you a riddle and the careful phrasing of the question misleads you into going down the wrong mental path.  "Why is the horizontal reflection of a mirror reversed, but the vertical reflection is not?"  Someone's being tricky and too many not-quite-smart-enough internet wizards are claiming to know the answers.

If the Hydroptere can reach speeds in excess of the wind, why can't it be done on land? (I know jack nor shit about this stuff)


Hydrofoil Speed record (http://www.youtube.com/watch?v=jCuP-XHefTo#normal)

The Hydroptere is not going directly downwind. Nevertheless, I'm a believer, I have my theories, but I'll wait to discuss them with my aero professor tomorrow before making any bold claims. It does seem physically plausible though.

that's at an angle to the wind. this "directly downwind" challenge is a specifically different set of circumstances.

Derby  ;D

I too remain skeptical. Time to break out the pad and pen and think about it in more detail.

Real Wind force is not a smooth theoretically consistent vector  force that pushes the cart along according to some equation.  The air molecules are smashing around everywhere, and they build up in groups.  It is very chaotic.  When you talk about velocity and acceleration of the cart, fortunately you can simplify it with the average force that the wind makes; however, in this case I think the chaos plays an important role in helping the cart transition stages.

Let's say the cart is going the same speed as the average wind speed.  The cart moves smoother and more consistently than the wind because 1) it is more dense mass-wise, and 2) the rotational momentum built up in the mechanics.  So the cart follows the average wind speed more closely than the local wind surrounding the cart does.  A strong cluster of wind molecules builds up and acts on the "sail" to drive the cart forward.  The cluster disappears, and there is a vacuum behind it.  Or better yet there is another strong cluster of wind moving the opposite direction.  This causes a negative pressure on the air stream.  The cart has too much momentum built up in its gearing/mechanical system to be very effected by the negative pressure, plus it was just pushed ahead by that forward moving force cluster that has left a "force hole" behind.  The air stream, localized around the cart, slows down significantly.  The cart was going the same speed as the average wind and hasn't slowed down, but the local wind slowed down for the moment, so the cart is now going faster than the local wind.  Now the cart is moving downwind faster than the local wind.  We know the local wind speed will center around the average flow over time, so the local wind is going to pick up again.  If the local wind picks up, it will bring the cart with it.  As the wind picks up again, the cart remains in the downwind-faster-than-wind stage, and it will continue to accelerate into that stage until the power it harvests from the wind is equal to the power required to  keep it moving.

This is how you make money in a failing stock market.

It is a propeller that spins like propellers do: opposite to the torque from the air. If it would spin in accordance with the torque from the air it would be a turbine.

Correct so far

No, that wouldn't work. The rotor would push the air forwards and accelerate the true wind. To extract wind energy you have to slow down the true wind. When going downwind that means always push the air backwards with the propeller.

The idea with a turbine driven by the apparent headwind above wind-speed would in fact create more true wind and thus energy out of nothing. It cannot work continuously.

There is no transition. Nothing changes in the power transmission at wind-speed. The wheels always turn the propeller.

What did the aero professor say?

No, there is clearly a transition.

The cart goes from traveling slower than the windspeed to faster than the windspeed.

You can't argue that no change from a tailwind state to a headwind state occurs.

The direction of the power transmission doesn't change at windspeed. The wheels always turn the propeller.

The switch of apparent tailwind though zero to apparent headwind at the chassis is not that important (just for drag). What really matters is the apparent wind at the rotating airfoils, which never drops to zero. At the airfoils the apparent wind changes it's angle to the travel direction right from the start continuously while the vehicle accelerates. At wind-speed it comes at 90deg to travel direction, which easily allows acceleration with the right orientation of the airfoil.

It's all relative... seriously. There is only ONE transition, from not moving to moving.  Everything else is based on wind deflection.  I'll also add at this time that I'm a believer in Newton-based lift as opposed to Bernoulli-based lift, talk about a controversy! So, before I explain it, lets set some definitions

Beta = Blade angle = for this example, 10 degrees relative to tangential
N = Normal Wind: Relative wind vector in direction of travel (downwind)
T = Tangential Wind: Relative wind vector opposite of blade rotation (sideways)
R = Relative wind: Wind vector relative to the blade.  Combination of T and R
M = Magnitude of R
Theta = Angle of relative, with respect to tangential
Delta = Wind deflection in degrees

(http://img96.imageshack.us/img96/9738/20110227121253387.th.jpg) (http://img96.imageshack.us/i/20110227121253387.jpg/)

Uploaded with ImageShack.us (http://imageshack.us)

Stage 1: Cart standing still.
N = (1,0)
T = (0,0)
R = (1,0)
M = 1
Theta = -90

The cart is literally PUSHED by the true winds, causing it to roll forward and advance to the next stage

Stage 2: Cart moving forward, normal wind still positive (air still faster than cart).  For simplicity, normal magnitude is equal to tangential magnitude
N = (.5,0)
T = (0,.5)
R = (.5,.5)
M = 0.707
Theta = -45
Delta = -55

The wind is deflected (Beta - Theta) 55 degrees.  Regardless of whether the wind changes speed, it's direction has changed, and thus has accelerated.  This is where the force comes from (important!)

Stage 3: Normal wind is zero (cart moving same speed as air)
N = 0,0
T = 0,1
R = 0,1
M = 1
Theta = 0
Delta = 10

The wind is still being deflected, so a force is still being applied, but a substantially smaller force.

Stage 4: Cart is no longer accelerating
N = -0.2 , 0
T = 0 , 1.2
R = -0.2 , 1.2
M = 1.21
Theta = 10
Delta = 0

The wind is no longer deflected, so no force is being applied.  Cart stops accelerating.

QED

Now, granted I didn't account for drag on the vehicle, a more efficient curved blade (this analysis is basicaly assuming the blade is a flat sheet), or the fact that different points on a real blade would have different angles and tangential velocities.  All in all, experience in fluids tells me that this is a workable model, and if all other factors were introduced, similar results would be possible.

Like I said before, true wind speed doesn't matter... As Einstein would say, it's all relative.

Anybody else sometimes get an oily film on the water when they take a crap?

Yup. It's usually a bad day when that happens. I like hot wings. They don't necessarily like me.

I much prefer the solid "volcanic island" craps, where you deposit enough to break the surface of the water. At that point, I feel as tho I've really accomplished something.

Also, I think it works. You're "storing" energy in the form of momentum in the fan blades until you get past the wind speed, at which point you start using your own speed to drive the fan blades/wheels.

If that were the case... then this (@ about 4:00 in) wouldn't work: Downwind Faster than the Wind (DWFTTW) Myth Challenge (http://www.youtube.com/watch?v=xHsXcHoJu-A#normal)

Just imagine..

years ago..

no internet..

people had to actually get off their fat asses and go BUY a magazine or pick up a flyer to start believing stuff like this.

Watch for about 30 seconds longer. Their model uses a "pusher" fan blade, which the original vehicle does not. Their "model" works, as I would expect it to. It pushes itself up the treadmill via air pressure from the fan blades, where as the original vehicle uses a wind "driven" propeller to drive an axle, clawing its way across the ground.

My assertion of storing energy in the form of momentum is directed at Sac Duc's "how does it go from slower than the wind to faster than the wind" argument. At a standstill, the "driven" propeller acts as a sail to a tailwind. While the vehicle is going slower than the tail wind, the propeller is actually fighting the wheels and robbing power.  When the wheels are driving it at the same speed as the wind, because of the gear ratio and the pitch of the blades, the propeller is already being driven by a headwind, imparting more driving force to the wheels and increasing acceleration, powering it through that "dead spot."

The original carts blades are set up to pull the vehicle.  The leading edge is clearly forward, as seen in the picture below, and the blades are curved to be driven by the wheels, not by the wind as easily seen at the beginning of the video below the picture
(http://www.cars-show.org/wp-content/uploads/2010/11/blackbird-wind-cart-powered-by-only-by-the-wind-01.jpg)
DDW.mov (http://www.youtube.com/watch?v=5CcgmpBGSCI&feature=player_embedded#normal)

The difference between the model and the cart is the same as the difference between these two airplanes:
(http://www.youngeagles.org/photos/gallery/Monoplanes/RutanLongEZ-B_1J.jpg)
(http://www.lancair.com/Main/235_gallery_images/Miked.jpg)


Yes, from the cart's perspective, the cart goes from slower than the wind to faster than the wind.  But, from the fan blade's perspective the wind is always moving from the leading edge to the trailing edge except from stand-still, so there never is a transition from slower than the wind to faster than the wind with respect to the fan.

I suppose the best way to convince you that the wheels are driving the prop as opposed to a turbine driving the wheels is their website itself... we engineers are pretty picky about what turbomachine devices are called.  A fan or propeller pushes air, whereas a turbine is pushed by the air.  They (the engineers behind this) call it a prop:
http://www.fasterthanthewind.org/2009/12/for-next-phase-assembly-and-layup-of.html (http://www.fasterthanthewind.org/2009/12/for-next-phase-assembly-and-layup-of.html)

No, you cannot use the energy stored in prop rotation to accelerate past wind speed. To extract that stored energy you would have to slow down the prop rotation, but that would slow down the wheels and thus the entire cart.

No, using your own speed to accelerate yourself doesn't work. But slowing down the true wind to accelerate yourself does work.

No, the rotor is never driven by the apparent headwind above windspeed. I explained why on the last page: It would create more wind, instead of extracting wind energy. They also have ratchets that prevent the prop from driving the wheels. Only the wheels can drive the propeller.

There is no dead spot. The apparent wind at the rotating propeller blades never drops to zero. It always creates lift that pushes the cart forward.

Punx Clever explained all very well but I have a minor nitpick:

According to Newtons 3rd Law both rotors push the air with the same but opposite force as the air pushes them, so "who is pushing whom" is not a good criteria, as it is physically irrelevant. The most general difference between turbine and propeller is rather:

- Turbine rotates in accordance with the torque exerted by the air.
- Propeller rotates opposite to the torque exerted by the air.

There are other less general definitions, based on the direction of energy transfer, but they are frame dependent and fail in special cases, like a propeller working in reversed flow (as is the case here below windspeed).

That is correct. The rotor always turns opposite to the torque exerted by the air, and in accordance with the torque transmitted from the wheels.

Minor details. Potato - potato

Wind turns fan.

fan turns wheel

wheel twists big rubber band

big rubber band eventually twists enough

goes faster than wind.

easy

my kid has pull back toys, same principle

Look at the videos. It turns opposite to the torque from the air.

The vehicle was inspected by officials from the North American Land Sailing Association (NALSA) before the record runs:
http://www.nalsa.org/DownWind.html (http://www.nalsa.org/DownWind.html)

Quote from their report:
http://www.nalsa.org/BlackBirdDDWSR/Observers%20ReportNALSA%20C4BlackbirdJuly2and%2032010.pdf (http://www.nalsa.org/BlackBirdDDWSR/Observers%20ReportNALSA%20C4BlackbirdJuly2and%2032010.pdf)

it was an invisible rubber band

and how do you know it was turning opposite to the torque from the air?  which way was the air torquing?

NALSA?  really?  a bunch of old farts who invent a sport in 1972 can't be bought? 

people worry about the FDA and the FBI, but nooooo.. when NALSA says something it much be holy writ!

Everybody missed it.
Soapbox derby. [evil]
Apply this to a soapbox derby car, you'd win everytime until either it was outlawed or a competetor built a better one.  ;D

Or invisible dwarfs pushing it, or a giant invisible magnet pulling it... or simply a wind powered vehicle.

you forgot to reply to the substantive part of my post:

I assume that using wind power is already outlawed in soapbox races.

If you want to believe in some conspiracy theories, I won't stop you. From the physics standpoint there is no necessity to fake it.

Found another pic of a wind powered vehicle, not sure how fast they were going.

(http://i19.photobucket.com/albums/b158/emerald_skies/Funny%20Macros/dq0n48.gif)

You know, I threw my hand in this debate...  then I realized that the guy who ressurected it from the dead has only posted in this thread... then I stopped caring.

 [laugh] [laugh] [laugh]

ding ding ding ding!
(http://i19.photobucket.com/albums/b158/emerald_skies/Funny%20Macros/dq0n48.gif)

Conventional sail craft on water can achieve a downwind-velocity-made-good of 2 x true windspeed. On land or ice it goes to above 3 x true windspeed.

If you understand how tacking with downwind VMG greater than windspeed works, then it is very easy to understand this vehicle. The rotating propeller blades do the tacking, while the hull stays on a constant direct downwind course:

Sail_to_Prop.mov (http://www.youtube.com/watch?v=UGRFb8yNtBo#normal)

I sail above windspeed occasionally.    I wouldn't call anything that does 2x windspeed "conventional".    And vmg?   Show me the boat that does 2x windspeed vmg to a downwind mark.   

I completely understand sailing and iceboating.  I even get that gybing downwind there will be (brief) periods of time when the boat is going Ddw faster than true windspeed before heading up to keep up speed. 

What's so difficult is that at windspeed Ddw there is no air moving across any part of your vehicle.  So that's a hump to get past in real life or in my head. 

Btw, what kind of bike you ride?    :)

On water it is not the normal case. On land and ice it is very common.

(http://upload.wikimedia.org/wikipedia/commons/thumb/0/0f/USA-17-flying-cropped.jpg/170px-USA-17-flying-cropped.jpg)
Quote from http://en.wikipedia.org/wiki/Sailing_faster_than_the_wind#Speed_made_good (http://en.wikipedia.org/wiki/Sailing_faster_than_the_wind#Speed_made_good)
Sources are linked in the article.

For ice boats see page 4 of:
http://www.nalsa.org/Articles/Cetus/Iceboat%20Sailing%20Performance-Cetus.pdf (http://www.nalsa.org/Articles/Cetus/Iceboat%20Sailing%20Performance-Cetus.pdf)

The propeller blades do not need to gybe. They are on a continuous helical tack and never go DDW, just the hull goes DDW.

That is not true for the propeller blades which move relative to the hull.

You are used to sails which don't move continuously relative to the hull, and therefore experience the same apparent wind as the hull. Here the apparent wind at the airfoils is different and never zero.

wired magazine this month has a great article on this... along controversy with all the people saying that it cant work even after the full scale model worked... haha

anyone get the idea this guy doesn't even own a bike at all?

Yup

He is the guy with the funny hat in the begining of the desert video.  

Often...

if you stop feeding them they go away.

I am buying the explanation myself.   It makes sense that using a prop is like sailing across the wind, just instead of a x y z coordinate solution it is a x, theta, R polar coordinate model.


mitt

Here it is:
http://www.wired.com/magazine/2011/02/ff_fasterthanwind/all/1 (http://www.wired.com/magazine/2011/02/ff_fasterthanwind/all/1)