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WaxRubiks
2010-Jul-16, 08:52 PM
Can a vehicle be built which can go directly downwind, faster than the wind (DDWFTTW), powered only by the wind, steady state? Thin Air Designs, in collaboration with the San Jose State University Aero department, along with generous corporate sponsors intend to definitively answer this question. Follow our quest.
http://www.fasterthanthewind.org/

Don't quite know how that works.

although I have a vague feeling about how it works.

IsaacKuo
2010-Jul-16, 09:17 PM
There's nothing inherently mysterious about how it works--it's just highly counterintuitive that it does indeed work in the obvious way.

It's well known that you can sail downwind faster than the wind by sailing at an angle to the wind. So, in principle, it should be possible to "wrap" that diagonal path into a helix so you can sail straight downwind faster than the wind. Either way, you're simply taking advantage of the velocity difference between the wind and the ground.

JohnD
2010-Jul-16, 11:24 PM
Any dinghy sailor will tell you that it is possible to sail faster than the wind.
You need a dinghy that will plane over the water, as otherwise speed is limited by the boat's overall length.
Then a 'broad reach', at right angles to the wind, will get you sailing very fast indeed.
The sail accelerates the wind as it passes across the surface, more on the leeward side, so Bernouille extracts energy from the wind.
That thrust is at an angle to both wind aand boat direction, so it needs skill to direct boat and trim sail to get the best result.
Downwind, a boat cannot sail faster than the wind, as the sail is completely stalled.

But the dinghy sailor will also tell you that as you increase your speed over the water, the wind direction changes.
A planing dinghy that can use a spinnaker, can set its mainsail to the opposite side, for a dead-downwind sail, or much, much better take a broad reach, to one side of downwind and set the main on the same side as the spinner. Then, not only speed effect , but slot effect raise the wind speed over the sail even more.

This device is a clever combination of that effect, the autogyro effect to get it though the zero wind point and, I think, reverse propellor feathering. Certainly the blades are at a different angle when it's 'sailing' downwind

Ice boats, with much less drag than even a planing dinghy, can go even faster. See: http://www.windjet.co.uk/index.php?option=content&task=view&id=15&Itemid=50 and this, "The most spectacular claim is that by John D. Buckstaff, who in 1938, apparently clocked 143 mph (230 km/h), in a 72 mph wind on Lake Winnebago in Wisconsin (USA)" though the fastest modern claim is 126mph (http://blog.greenbird.co.uk/ the Greenbird blog of the current WSR holder Richard Jenkins)

John

cjl
2010-Jul-17, 03:00 AM
Yep - the ongoing argument over at the Physics Forums (which the full scale car finally settles) was whether it was possible to go directly downwind faster than the wind. That's what this was meant to do, and it does so quite impressively.

clop
2010-Jul-17, 12:21 PM
I'm going to assume that the wind is blowing from behind the vehicle. Maybe it's just me but the blades seem to be angled the wrong way for the rotation of the propeller.

clop

IsaacKuo
2010-Jul-17, 12:29 PM
Think of it this way--the wind pushes the vehicle forward. This forces the wheels to rotate, which in turn rotate the propeller via the drive chain. The drive chain has mechanical advantage, so the force of this torque exceeds the "wrong way" force of the wind wanting to rotate the propeller the other direction.

The reason this vehicle works is very counterintuitive, but one way to try and comprehend it is to reverse the roles of wind and ground. If you look at it in the frame of reference of the wind, the goal is to sail "upstream" against the ground. In this frame of reference, it's the wind which isn't moving, and it's the ground which the vehicle "sails" against.

mike alexander
2010-Jul-17, 05:39 PM
Once you realize you can sail into the wind, sailing faster than the wind doesn't seem any stranger.

clop
2010-Jul-17, 10:07 PM
We are supposed to believe that, "Once the vehicle is moving directly downwind at the same speed as the wind it uses lift from the propeller (being driven by the wheels) to continue accelerating and outrun the wind."

Can anyone explain to me how this is any different to the vehicle rolling along an enormous moving treadmill (with zero relative speed to the floor the treadmill is standing on) in a room with no wind, and accelerating forward along the treadmill without ever slowing down and moving backwards relative to the floor.

clop

cjl
2010-Jul-17, 10:23 PM
We are supposed to believe that, "Once the vehicle is moving directly downwind at the same speed as the wind it uses lift from the propeller (being driven by the wheels) to continue accelerating and outrun the wind."

Can anyone explain to me how this is any different to the vehicle rolling along an enormous moving treadmill (with zero relative speed to the floor the treadmill is standing on) in a room with no wind, and accelerating forward along the treadmill without ever slowing down and moving backwards relative to the floor.

clop

It's exactly the same, actually. This vehicle would move forwards along a giant treadmill if it were initially stationary relative to the surrounding air.

clop
2010-Jul-17, 10:24 PM
It's exactly the same, actually. This vehicle would move forwards along a giant treadmill if it were initially stationary relative to the surrounding air.

For how long?

clop

loktky
2010-Jul-17, 10:31 PM
This technology was invented in the Great War on the Spit Fire British fighter plane.

cjl
2010-Jul-17, 10:39 PM
For how long?

clop

Indefinitely. It can hold it as a steady state condition.

clop
2010-Jul-17, 10:48 PM
Indefinitely. It can hold it as a steady state condition.

You're telling me that it can move forward indefinitely, relative to the floor, along a moving treadmill, directly into an effective headwind.

That's pretty funny.

clop

cjl
2010-Jul-17, 11:04 PM
You're telling me that it can move forward indefinitely, relative to the floor, along a moving treadmill, directly into an effective headwind.

That's pretty funny.

clop
And yet it's true.

Amazing how that works, isn't it?

(There are some videos of small scale models on youtube doing exactly that on a treadmill, though they tend to be limited by both the length of the treadmill and their ability to not go off of either side).

clop
2010-Jul-17, 11:17 PM
And yet it's true.

Amazing how that works, isn't it?

(There are some videos of small scale models on youtube doing exactly that on a treadmill, though they tend to be limited by both the length of the treadmill and their ability to not go off of either side).

This video seems to illustrate it very well.

But I still don't get it. Maybe they've got the treadmill ramped downhill slightly. There are wooden blocks under the back of it.

clop

cjl
2010-Jul-17, 11:23 PM
This video seems to illustrate it very well.

But I still don't get it. Maybe they've got the treadmill ramped downhill slightly. There are wooden blocks under the back of it.

clop

It shouldn't be ramped downhill - the wooden blocks are probably to level it (most treadmills have a slight upslope to them). Here's one which shows the treadmill is level:

IsaacKuo
2010-Jul-17, 11:26 PM
You're telling me that it can move forward indefinitely, relative to the floor, along a moving treadmill, directly into an effective headwind.
Yep. A much older device which does something similar is used to pull a boat upriver. In this case, the "ground" is a rope attached to a bridge or a pole or something. The other end of the rope wraps around an axle on the boat between two water wheels. As the water wheels rotate, the rope wraps around the axle and the boat moves upriver.

There's another way to view this device, if you're familiar with traditional sailing. Each blade of the prop can be considered a small sail that is forced to move along a particular helix angle due to how the prop is geared to the road wheels. It's like a traditional land sail which is steered at a particular angle to the wind. A traditional land sail would be moving along a straight line that's not directly downwind. This unusual sail instead moves along a helix on a virtual cylinder suspended above the ground.

This way of looking at things helps if you're familiar with how traditional land sails can greatly exceed wind speed by moving at an angle that's not directly downwind.

chrlzs
2010-Jul-18, 12:10 AM
How long before it turns up in CT forums as an over-unity energy device? :(

mugaliens
2010-Jul-19, 06:48 AM
I seems counterintuitive, but only because we're used to sailboats, the vast majority of which have significant hull drag and insufficient sail area to go faster than the wind driving it.

The land sailing speed record (http://en.wikipedia.org/wiki/Land_sailing#Speed_record), however, is 126.1 mph, achieved in winds fluctuating between 30 and 50 mph. That record was set by Richard Jenkins, the photographer of the video in the OP.

This works because the sail is not going directly downwind, but rather, is angling off the wind. While the craft shown in the video is going directly downwind, it's propeller (it's "sail") is not. It, too, is angling off the wind, the same as ice yachts and land yachts.

As Isaac mentions, the sail of a land or ice yacht angles off the wind in a straight line, while the sail of this vessel angles off in a helical pattern. This technique isn't new, as it's been employed in Darrieus (eggbeater) wind turbines for eighty years. Graphic (http://en.wikipedia.org/wiki/File:Darrieus.jpg).

Sailplanes employ it as well, achieving 70 kts forward velocity with sink rates of between 1 kt and 2 kts.

clop
2010-Jul-19, 07:31 AM
I seems counterintuitive, but only because we're used to sailboats, the vast majority of which have significant hull drag and insufficient sail area to go faster than the wind driving it.

The land sailing speed record (http://en.wikipedia.org/wiki/Land_sailing#Speed_record), however, is 126.1 mph, achieved in winds fluctuating between 30 and 50 mph. That record was set by Richard Jenkins, the photographer of the video in the OP.

This works because the sail is not going directly downwind, but rather, is angling off the wind. While the craft shown in the video is going directly downwind, it's propeller (it's "sail") is not. It, too, is angling off the wind, the same as ice yachts and land yachts.

As Isaac mentions, the sail of a land or ice yacht angles off the wind in a straight line, while the sail of this vessel angles off in a helical pattern. This technique isn't new, as it's been employed in Darrieus (eggbeater) wind turbines for eighty years. Graphic (http://en.wikipedia.org/wiki/File:Darrieus.jpg).

Sailplanes employ it as well, achieving 70 kts forward velocity with sink rates of between 1 kt and 2 kts.

I don't think the land yacht was doing 126.1mph directly downwind. It would be interesting to know the downwind and sidewind vector components of its velocity at this speed.

You say, "This works because the sail is not going directly downwind, but rather, is angling off the wind. While the craft shown in the video is going directly downwind, it's propeller (it's "sail") is not."

It can't be a propeller and a sail at the same time can it? I thought I read that the propeller is supposed to be driven by the wheels not the wind.

clop

WaxRubiks
2010-Jul-19, 10:06 AM
yeah, I don't think it can be just the helix idea of the blades as sails traveling side ways as even if they have a high perpendicular-to-the-wind vector they are still moving faster than the wind along the wind direction.

G O R T
2010-Jul-19, 12:11 PM
The land yacht is understandable: Force from a large collector is vectored to overcome ground friction + directional wind drag.

This idea requires the energy collected fom the wheels to blow air backward with more force than the wheel drag losses.

Can it? Explain.

WaxRubiks
2010-Jul-19, 03:14 PM
I see, so it uses the energy from the wheels to drive the drive the fan and push it faster(once it has got up to speed and is traveling around the same speed as the wind), and somehow this makes use of the fact that the wind is traveling over the ground to obtain its energy from?

ETA: does any body know if the blades change much, or reverse direction, or something, in the process....?....

ETA2: maybe it makes use of the fact that it is pushing against air which is already moving...somehow...force times distance..something or other.

IsaacKuo
2010-Jul-19, 03:40 PM
yeah, I don't think it can be just the helix idea of the blades as sails traveling side ways as even if they have a high perpendicular-to-the-wind vector they are still moving faster than the wind along the wind direction.
The downwind component of a land sail in broad reach can indeed exceed the wind's own speed. Thus, a land sail which zig-zags downwind in broad reaches can go faster than a land sail which goes straight downwind.

I know this is counterintuitive, but try to think of it in the reference frame of the wind. In the reference frame of the wind, it's the ground which is moving and the goal is for the vehicle to "sail" against the direction of the ground. In this reference frame, the sail acts like the keel, and the wheels act like the sail.

So, let's suppose the wind is moving to the North. In the new reference frame, the wind isn't moving but the ground is moving to the South. The goal is to sail Northward despite the Southward movement of the ground. The "keel" can be set at, say, NW, while the wheels are set at NNW. The Southward movement of the ground against the wheels imparts a force perpendicular to the wheels (in the WSW direction). The WSW force is closer to NW than SE, so it propels the vehicle forward in the NW direction.

IsaacKuo
2010-Jul-19, 03:42 PM
ETA: does any body know if the blades change much, or reverse direction, or something, in the process....?....
The blades seem to be set at a fixed angle during the run, with the gear ratio to the wheels also fixed. As counterintuitive as this is, it works just fine in theory (as well as in practice, evidently).

WaxRubiks
2010-Jul-19, 04:29 PM
I sort of see.....this reminds me of the trick where you push in the backwards direction of a bicycle pedal, with your hand as you kneel on the groud, yet the bicycle goes forwards...

ETA: ok got that wrong; the bike moves backward, but the pedal feels like it goes in the opposite direction to your push.....

ThinAirDesigns
2010-Jul-19, 06:44 PM
Hi gang. I'm JB -- one of the two designer/builders of the vehicle in question.

Unfortunately I'll be traveling most of today and tomorrow, but in the end if there are still questions I'll be happy to answer them.

spork
2010-Jul-19, 08:47 PM
I see, so it uses the energy from the wheels to drive the drive the fan and push it faster(once it has got up to speed and is traveling around the same speed as the wind), and somehow this makes use of the fact that the wind is traveling over the ground to obtain its energy from?

I'm the "other guy". Designer, builder, and driver of the Blackbird. Yes, it uses the fact that the vehicle is moving faster over the surface than it is through the air. Thus the energy that can be "harvested" by the wheels is greater than the energy needed to drive the prop to create the same thrust as the drag created on the wheels to harvest that energy. It's basically just a lever - trading a small force over a large distance for a larger force over a smaller distance - no magic of course.

ETA: does any body know if the blades change much, or reverse direction, or something, in the process....?....

The direction of rotation of the blades never changes during normal operation. The blades are always turned by the wheels, never the other way around. We have added on-the-fly variable pitch for more rapid initial acceleration (which is still dog slow), but we've run it many times with fixed pitch. It self starts just fine, and gets up to better than 2.5X windspeed in that configuration.

ETA2: maybe it makes use of the fact that it is pushing against air which is already moving...somehow...force times distance..something or other.

This is a very common (and understandable) misconception. Just like a Cessna in flight, the flow around the vehicle and through the prop disk is no different because of the tailwind. It's not building a "cushion of air" against which the wind pushes. It's immersed in a moving mass of air, and it's screwing itself forward through that airmass.

Incidentally, this group has by far the best understanding of any group we've encountered so far.

mugaliens
2010-Jul-20, 08:25 AM
Welcome to the forum, gents! Nice design!

spork
2010-Jul-20, 02:25 PM
Thanks! It's a pleasure to see a group of folks that have this thing reasonably well figured out - and discussing it civily.

Sam5
2010-Jul-21, 12:04 AM
Thanks! It's a pleasure to see a group of folks that have this thing reasonably well figured out - and discussing it civily.

Does the vehicle have to be ultra light, or could you put a seat on it for a passenger?

spork
2010-Jul-21, 12:10 AM
Does the vehicle have to be ultra light, or could you put a seat on it for a passenger?

Many of the articles reported that the vehicle was ultra-light and that it was built primarily out of foam. Both are not accurate. But for what it's worth, most articles reported that the prop turns the wheels - which is just flat wrong.

The cart weighs about 450 lbs empty, and carries me up front (an additional 200 lbs). The weight has little to do with the top speed, but does affect the acceleration for obvious reasons.

I'm sure we could put a passenger behind me without changing the top speed performance all that much.

mugaliens
2010-Jul-21, 08:21 AM
What's next? A design that'll reach directly upwind? I'd think that one would require the prop to turn the wheels.

IsaacKuo
2010-Jul-21, 10:19 AM
What's next? A design that'll reach directly upwind? I'd think that one would require the prop to turn the wheels.
Even though this is an essentially similar challenge, it's not at all controversial because it's intuitively obvious how it works. You simply have a windmill that's geared to the wheels with sufficient mechanical advantage to overcome the drag.

spork
2010-Jul-21, 02:55 PM
Even though this is an essentially similar challenge, it's not at all controversial because it's intuitively obvious how it works. You simply have a windmill that's geared to the wheels with sufficient mechanical advantage to overcome the drag.

There have been aero and physics PhD's that assure us that's also impossible - despite the annual upwind race in the Netherlands.

swampyankee
2010-Jul-21, 11:26 PM
I'm an aero guy, but not a PhD. I do, however, know one of the sharpest aero guys in the business. Maybe I'll email him the link.

I remain very skeptical of the claim.

spork
2010-Jul-22, 01:50 AM
I'm an aero guy, but not a PhD. I do, however, know one of the sharpest aero guys in the business. Maybe I'll email him the link.

I remain very skeptical of the claim.

We welcome skeptics. We kind of don't care for the "deniers" that simply shout us down and insult us. I'm reasonably confident I can explain this to your satisfaction and convince you that it works and why it works - if you care to go through it. Have you seen the various videos we've posted of both the models and the manned vehicle? If so, do you think we're pulling a hoax, or just fooling ourselves?

spork
2010-Jul-23, 08:05 AM
I'm an aero guy, but not a PhD. I do, however, know one of the sharpest aero guys in the business. Maybe I'll email him the link.

I remain very skeptical of the claim.

Swampman, did you talk to your aero buddy about this question? Are you not interested in having the discussion that I think will convince you?

swampyankee
2010-Jul-23, 11:42 AM
Swampman, did you talke to your aero buddy about this question? Are you not interested in having the discussion that I think will convince you?

I've not had a chance yet.

Subduction Zone
2010-Jul-26, 03:48 PM
I see by your location that you are not too far away from MIT swampyankee. If, by any chance, your aero buddy is Mark Drela your discussion could be very short. He is a professor of aerodynamics there and has his own theoretical design for a boat that sails downwind faster than the wind.

mugaliens
2010-Jul-27, 03:44 AM
Even though this is an essentially similar challenge, it's not at all controversial because it's intuitively obvious how it works. You simply have a windmill that's geared to the wheels with sufficient mechanical advantage to overcome the drag.

My question was rhetorical, Isaac. I'm well aware of how both designs work, and the addition of variable pitch would render the craft capable of upwind work, as well as increase its downwind acceleration quite a bit.

spork
2010-Jul-29, 05:11 AM
I've not had a chance yet.

Well, NALSA has made the official announcement. We established the first world record by going directly downwind at 2.8X wind speed. Have you still not had a chance to ask your buddy if it's possible? If so, what would it take to convince you?

HenrikOlsen
2010-Jul-29, 10:46 PM
The trick to understanding what's going on is that that thing on top of the car isn't a windmill, it's a pusher propeller driven by the wheels.

There's nothing against the mainstream about how it gets its energy, as it leaves behind a wake of air moving slower that it did before the car passed.

With variable pitch it would likely start faster, as it could then use the prop as a windmill for the slow part, but it's clear it isn't needed once it's gotten up in speed.

WaxRubiks
2010-Jul-30, 02:35 AM
There's nothing against the mainstream about how it gets its energy, as it leaves behind a wake of air moving slower that it did before the car passed.

Yes, thanks....that makes it easier to understand....I wasn't quite sure where the energy came from before..well, it must be the wind, but couldn't quite see how.

The other thing is that the whole thing acts as a sail, to start with, if I understand it right...and that the propeller doesn't drive the wheels(at the start, or at any time), as the propeller turns against the wind...I assume...for the blades to be angled correctly, in order to be able to be driven by the wheels, later on.

spork
2010-Aug-07, 02:29 AM
the whole thing acts as a sail, to start with, if I understand it right...and that the propeller doesn't drive the wheels(at the start, or at any time), as the propeller turns against the wind...I assume...for the blades to be angled correctly, in order to be able to be driven by the wheels, later on.

That's all correct. We now have a variable pitch prop on it so we can have the prop fight being turned less during the initial bluff-body phase, but we can't drive the wheels with the prop because we have a ratchet on the axle.