Going downwind faster than the wind

[spork]

Well, I just got back from D.C. and am happy to see the thread back on track.   I think it's ultra-cool that both RoofingGuy and ThinAir have such a keen grasp of the subtleties of this non-intuitive problem - particularly since they both started [firmly] on the side of common-sense.  To be honest here, I'm really hoping for a few of the doubters to join us with an open mind to hear out our lunacy.  This ain't gonna be much of a myth if it's just RoofingGuy, ThinAir and myself talking aerodynamics.

And here is my promise:  I promise to do everything I can to carefully describe how this is possible for anyone that doubts it.  I will not try to baffle anyone with B.S. or simply post inscrutable equations (except for those that want them).  I'm well aware that different people approach these sort of problems in very different ways (examples, thought experiments, math...)  There's no right way.  AND I promise to keep an open mind to any points the doubters make.

The second and third points do kind of reek of over-unity or magic. "You create your own wind and then use it to propel yourself?

I couldn't agree more.  The  2nd and 3rd points do give you that queezy feeling.  But I think three things must be said here. 

1) "making" your own wind is slightly misleading.  I understand completely, but even when you "make your own wind" you never get something for nothing.   Whether we're talking about the ice-boat on a downwind tack, or this cart going directly downwind, the relative wind that we "make ourselves" is just a part of the clever component that in the end allows us to exploit the differential speed between the ground and the true wind.

2) Along those lines, I kitesurf a LOT.  When the wind gets light we stroke our kites (on the end of 100' lines) in a big sine-wave.  There's no question this gives us lots more power.  But in this case the cost is our tacking angle.  When stroking the kite it's much harder to go upwind, or even maintain your upwind position.  Effectively, this kite stroking is nothing more than a way to let my kite experience a whole lot more of the real wind (and therefore slow down that real wind) than if I park the kite in one spot.

3) When the wind is marginal, you can definitely feel the relative wind "kick in" as you get moving.  This relative wind can get you going pretty fast.  But again, you see the cost immediately in two ways.  You can't maintain nearly as good an upwind angle; and when you try and jump you feel like a wet noodle.  In "real" wind, I can get 30+ feet in the air.  Using my relative wind to get speed works for speed, but I can't get 6' to 10' off the water.

I still maintain that the correct answer is still (usually) "no", because the question is almost always phrased such that the idea is to simply stick a big fan on a standard sailboat - no custom sails.

Just to throw a monkey wrench into it (because that's what I do), I think a properly rigged spinnaker would actually make a better thrust reverser than the "sail" in that video.

Let's imagine something silly. Let's imagine the cart is moving faster than the wind, no matter how wrong that feels instinctively. Not the same speed, but faster. What can we surmise about it?

Well if it were at the same speed as the wind, it would feel the air as being still, but if it is exceeding the wind speed then there will be a relative wind from the front.

If it were a boat, it could tack into this relative wind and maintain its overwind speed, but it would need to do so at an angle, and not directly downwind.

Interestingly enough, a boat with a downwind VMG of exactly 1.0 can maintain that speed on a tack as well.  I'm probably not telling you anything you don't know, but thought it was an interesting clarification.

DDWFTTW believers rarely argue the details of how it can be done because they can't get past the "to outrun your power source defies the laws of physics and requires more power out than in." positions. Once someone gets past their physics issues, the rest can usually be overcome.

For me one of the coolest aspects of this is that there are so many ways to approach it.  Given an inquisitive mind, and a lot of time on your hands, one way is almost sure to hit.  The one thing every approach has in common is that it has some clever way to exploit the relative motion of the real wind vs. the ground/water (unlike a typical sailboat trying to go directly downwind wing & wing).

[theSpecialist]

http://forum.mythbustersfanclub.com/index.php/topic,11448.0.html

I don't know, but I'd say you are right.  If the sail collects all of the fan-created wind by being pointed straight into the sail, then that just cancels out all of the forces involved, naturally.  However, if the sail more-or-less deflects the 'thrust' created by the fan, then you have merely created a fan-driven vehicle with a kind of deflected thrust.  So, instead of the vehicle moving forward because the thrust is pointed directly behind the craft, it instead moves forward due to the fact that the thrust has been pointed forward and is deflected off to one side by the angled sail, creating a 'tacking'  sort of condition... I think.

The discussion about a wing-effect ("lift") is interesting, yet I wonder if that would be enough alone.

To visualize this one better, I'll suggest a 'land yacht' as stated and instead of a cloth sail, let's say it is a curved kind of aluminum sheet.  Now, a turbo-prop is mounted at the rear of the vehicle and the thrust is aimed forward into the aluminum 'sail'.  This sail is angled slightly in order to swing the thrust off to one side and perhaps ever-so-slightly to the rear.  The force is translated to the mast and down to the wheels and we should get some sort of sluggish forward movement. 

Pretty much a total waste of the forces involved but I think it could look to some as a sail being 'blown' by an on-board engine.  Eh, it's just a thought.

As you can see from the above link, I supported the concept of re-directed wind (a kind of thrust actually -- as if the fan were pointed to the rear instead of being aimed at a sail and then re-directed back) ... And, I can also get onboard with the concept of out pacing the wind too.  It may turn out that the cart slowly collects the power needed to overcome the gearing involved, but once in motion (out past the opening inertia of starting up) it could attain speeds in excess of the surrounding wind.  As stated before, I am sure that kite-surfers already out pace the winds in a tacking fashion (and that has already been accepted as a given means), so this ends up being merely another form, I believe.

However, I still maintain that this has its best chance in the tacking angle.  If the prop is out ahead of the wind in a down-wind direction, I cannot understand what would turn the prop at that point.  I just may need to see it.

http://www.vista-lanzarote.com/lanzarote_fotos/kite_surfing_famara1_600.jpg
http://en.wikipedia.org/wiki/Kitesurfing#Speed_records

The current speed record over a 500 meter (1,640 ft) course, officially ratified by the World Sailing Speed Record Council, is held by Rob Douglas at 49.84 knots, which is the outright World Speed Sailing Record.

[ThinAirDesigns]

However, I still maintain that this has its best chance in the tacking angle. 

Jack's cart will work less and less efficiently the less direct down-wind you point it.  If you were to have cyclic pitch control on the prop, perhaps you could increase the angles off the wind that it would perform.

If the prop is out ahead of the wind in a down-wind direction, I cannot understand what would turn the prop at that point.  I just may need to see it.

To help you understand the above, I'll ask you a question ...

If the ice boat is "out ahead of the wind in the down-wind direction" (which is must be to beat the wind), what is it that fills the sail and propels the boat?

The answer to the above question and the answer to your concerns regarding the prop driven cart are *one and the same*.  While the chassis of the cart sees no wind at 1.0 VMG, the prop since it's tacking (spinning) sees the exact same wind as the ice boat with a VMG of 1.0. It might help to visualize the the xy path of the prop from above and notice that it's taking a zig zag (tacking) path down the course.

The issue you are having with the above is precisely why so many folks say "Impossible!".  It took some doing before I was able to understand and visualize it.

JB

[spork]

If the prop is out ahead of the wind in a down-wind direction, I cannot understand what would turn the prop at that point.  I just may need to see it.

Imagine the ice-boat on it's downwind tack.  Because it's VMG greater than 1.0 you'd assume the sail is being back-winded.  But it's not, because it's also going cross wind at 2X the wind speed (this is where the constraint of the blades on the ice come in).

Now let's imagine that the earth is not a sphere - but rather a huge cylinder.  The wind is blowing parallel to the axis of that huge cylinder - all in the same direction - all over the globe (cylinder).  Now the ice-boat never has to change tacks.  It just stays on its 45-degree downwind tack forever (or until it reaches the end of the world/cylinder).  It follows a spiraling 45-degree downwind path around the world.  No problem, right?

But let's scale the whole thing way down.  Instead of a cylindrical world, let's make a big giant threaded shaft, and stick a model ice-boat on it.  We'll constrain the ice-boat to follow the threads just as the full-size ice-boat followed the spiraling 45-degree downwind path.  In fact, let's use two model ice-boats - one on each side of the giant threaded rod.  We'll weld them together just so they balance one another out.  Now we have what looks a lot like a prop advancing down that threaded shaft, going straight downwind, faster than the wind.

All that's left to do, is to replace that shaft with an equivalent transmission so these two sails (our prop) will follow the same path as our cart advances.

[Killick]

Just to throw a monkey wrench into it (because that's what I do), I think a properly rigged spinnaker would actually make a better thrust reverser than the "sail" in that video.

Probably.  I wonder if anyone has ever done any testing with a spinnaker to see how much (if any) thrust-reverse effect you normally get from one?

[spork]

I wonder if anyone has ever done any testing with a spinnaker to see how much (if any) thrust-reverse effect you normally get from one?

Seems like a long shot.  I can't imagine any situation in which it would actually make sense to do this.  I just like it as another brain-teaser of sorts.  It's just one of those things where the "obvious" answer isn't really so obvious.

But to be honest, I'm really keen on the downwind vehicle.  I really hope some of the skeptics (or even those that are simply undecided) will give us the chance to convince them it's real.  Partly becuase it's a great discussion, and partly to keep the thread alive and get this on the show.

I really think the question that's at the heart of this is:

Do you not believe an ice-boat (or even a regular sailboat) can tack downwind faster than the wind (i.e. beat the balloon that drifts directly downwind), or is it that you don't think this relates to a vehicle that goes directly downwind?

It seems one or the other must be the sticking point for any skeptic, and I'd like to know which one to address.

[RoofingGuy]

Some couple questions to ponder:

1) How could it be faked?  (bicycle pulling it with monofilament fishing line).

2) What's the principles involved and would it necessarily require the blades to be attached to the wheels?

Just to clarify a tad...

1) we all agree (both supporters and detractors) that the actual video could very easily be faked.  The hanging point to me is more "can the cart possibly do what is claimed, or must the video have been faked?"

2) as discussed between your original questions and now, the essential principle is tacking into a relative wind, and it does necessarily require that there is an intimate connection between the wheels and the prop.




I do like spork's cylinder post.  It offers steps for people to disagree with.  Can an iceboat beat the wind?  Could an iceboat spiral around a cylinder if the wind was constantly from behind?  Is there any difference between that and a prop advancing itself along a threaded shaft if the pitch were correct?  The "big jump" then just comes from getting from the prop advancing past the wind on the threaded shaft, to the actual mechanics of the cart.

But to be honest, I'm really keen on the downwind vehicle.  I really hope some of the skeptics (or even those that are simply undecided) will give us the chance to convince them it's real.  Partly becuase it's a great discussion, and partly to keep the thread alive and get this on the show.

I really think the question that's at the heart of this is:

Do you not believe an ice-boat (or even a regular sailboat) can tack downwind faster than the wind (i.e. beat the balloon that drifts directly downwind), or is it that you don't think this relates to a vehicle that goes directly downwind?

It seems one or the other must be the sticking point for any skeptic, and I'd like to know which one to address.

To be honest, it was the latter for me.  The cart beating the wind in the configuration used was baffling to me.  It felt wrong.  I knew iceboats and sailboats could have a downwind component of motion that exceeded the wind itself, but never gave any thought to why they could do that.  And I wasn't willing to even try to understand, at first.  And it seemed to be in no way related to the cart and it's prop.

Then I set out to learn how sailboats do it (which is where I found that "Physics of Sailing" webpage).  Even then the cart looked/seemed/felt bogus to me.

The real "Aha!!" moment was recognizing that the moving cart would have a relative wind from the front if it were beating the wind, but first I had to have the other realization that the sail doesn't care what things look like externally... all it cares about is the wind it "sees", and it makes no difference to the equations what's moving and what isn't as long as there's a difference.

[spork]

I've been trying to scare up some opponents to debate this with on 3 other forums.  The good news... there are plenty of people that want to debate it.  The bad news... no one seems willing to come here to do it. 

[Mycroft]

I had to step back from this thread for a while as the whole "sailing" analogy just didn't seem "right" to me.  As I'm re-reading it, it dawned on me that this may have little to do with 'tacking' or sail analogy at all but more like an airplane in reverse!  Follow me here... if you turn the prop by hand the wheels turn in whatever ratio you have between the two sprockets.  That means that if you  can get enough torque from wind blowing from behind to the propeller so its ratio with the wheels allows them to turn faster than the wind you *might* get the effect we see in the video.  There is probably some upper limit to how fast the thing can go before the wind drag on the prop maxes out.

Just a thought and maybe you've already described this but my ADD might have kept me from seeing it.


--Mycroft

[RoofingGuy]

if you  can get enough torque from wind blowing from behind to the propeller...

I know you've been busy, but don't forget Mycroft, that if the cart were doing as claimed, the wind would be from the front.  Say the wind is 10mph and say the cart is going faster than the wind at 12mph... the cart would "see" a 2mph wind from the front, not a 10mph wind from the rear.

Just an extra wrinkle for you to puzzle over.

[spork]

Yup.  You've got the prop turning the wheels.  It's a little tricky, but the prop actually pushes the cart through its thrust bearing, and the wheels turn the prop.  Really, the wheels and belt are simply providing the same kinematic constraint that the keel does for the sailboat - but just in a twisted up way.

[spork]

Well folks, I've tried my best to drag some of the naysayers from the other forums here for a lively discussion - but to no avail.  As a result, there are some discussions taking place here:

http://www.runryder.com/helicopter/t465942p1/

and here:

http://www.hanggliding.org/viewtopic.php?t=8888&postdays=0&postorder=asc&start=0

But frankly, more and more people are seeing the light.  Not long ago this topic would result in a good 40 page rant-fest.

[ThinAirDesigns]

How very strange -- a three post exchange between Wolfyhound and myself just up and disappeared.  ??

If you wish to be involved in those tests Wolfy, just PM me.

JB

[leopard]

Explanation of the lost posts:

http://forum.mythbustersfanclub.com/index.php/topic,13049.0.html

[ThinAirDesigns]

NP Leopard.

Thanks for the note.

JB