Car Goes Downwind 2.5 Times Faster Than the Wind

[ThinAirDesigns]

Hi folks. I'm one of the two primary designers/builders of the vehicle(s) in question.

I'll try to go back through the posts on the thread and come up with some relevent comments over the next bit, but in the mean time feel free to fire away with any questions you might have.

Basics:
-- wind power only ... no stored energy is used for acceleration ever
-- the spinning rotor is a propeller, not a turbine
-- the wheels provide the torque to turn the rotor (always).
-- the rotor does not provide the torque to turn the wheels (ever).
-- it will take off from a standing start on it's own.
-- initially it is just the bluff drag of the entire device that starts it rolling
-- we sometimes push it up to speed to save time during testing
-- there's no "null point' at windspeed.
-- given a rolling surface, it will maintain it's speed 2-3x above wind speed indefinitely
-- By design, this particular one works best when aimed directly downwind.
-- One can be built to go faster than the wind in any direction
-- A simple gearing change will cause it to go upwind rather than down

We have lots of videos if this site allows me (a new member) to post links - perhaps someone can tell me if this is OK ... I don't want to 'un-welcome' myself by doing something uncool.

JB

 

[Papa]

Welcome, JB,

-- given a rolling surface, it will maintain it's speed 2-3x above wind speed indefinitely

This almost sounds like perpetual motion.

1. Assuming foot assisted launch in absent of tailwind, will it increase in speed on its own?

2. In its present configuration and aerodynamic shape, what is it theoretical top speed?

 

[swbluto]

Well, I might suspect perpetual motion designers would appropriately have "thinAir" in their name.

(Just joking.)

I think it's already been mentioned but nothing will happen in still air. I suspect the "2-3" times the airspeed might be the "average" airspeed. I suspect what might be happening is that it's taking advantage of airspeed peaks, and that minimal drag and abundance of kinetic energy causes it to minimize energy loss until the next peak.

 

[ThinAirDesigns]

This almost sounds like perpetual motion.

Agreed. But when the wind stops it stops. We've checked.

1. Assuming foot assisted launch in absent of tailwind, will it increase in speed on its own?

You can push it to any speed you wish any way you wish and absent tailwind it will promptly coast to a dead stop.

2. In its present configuration and aerodynamic shape, what is it theoretical top speed?

That's a bit of an odd question -- in it's present configuration it breaks the chain at about 55mph ... I suppose you could call that a "theoretical" as well as practical limitation on top speed. I figure you are probably asking a different question though -- "what is the theoretical top speed of a vehicle of this design?" Is that what you are asking? The answer to that question is "there isn't one" (subsonic at least). There are *many* practical limitations on top speed however.

We've exceeded 3x many times now with the Blackbird -- most recently under the watchful eye and instrumentation of the folks from NALSA.org. NALSA is the organization that ratifies wind powered land speed records in North America -- most recently the new 126mph record by Brit Richard Jenkins. We figure with an effort better funded than our simple garage project (we had small sponsorship dollars from Joby Energy and Google) 4x would be pretty achievable but consider 5x to require something of a moonshot effort.

JB

 

[ThinAirDesigns]

I suspect the "2-3" times the airspeed might be the "average" airspeed. I suspect what might be happening is that it's taking advantage of airspeed peaks, and that minimal drag and abundance of kinetic energy causes it to minimize energy loss until the next peak.

The NALSA rules that we ran to last weekend preclude using peak gusts to power the vehicle to an average speed less than those peaks. We had to not only beat the wind, be also had to be faster than the highest peak gust for a run to be valid. With 18 recording sensors on the vehicle and the dry lakebed, not one of 18 runs over two days failed to qualify against that standard. We were typically above 2x the highest gust recorded during the run.

NALSA will be issuing their report in the next few weeks once the examiner gets it written and it's ratified by their full BOD. Here is a report from the Greenbird record showing the sort of detail they include: http://www.nalsa.org/MeasuremantReport/MeasuremantReport.html

In short, it's not powered by gusts and coasting between them -- it will achieve and maintain 2-3+ multiples of the wind speed, steady state in smooth winds.

JB

 

[Papa]

1. Assuming foot assisted launch in absent of tailwind, will it increase in speed on its own?

You can push it to any speed you wish any way you wish and absent tailwind it will promptly coast to a dead stop.

Ok.. so please correct me if I'm in error;

The fundamental requirement here, is relative motion between the immediate and natural air currents and the surface the vehicle resting on - tailwind or headwind being optimal? yes?

 

[ThinAirDesigns]

The fundamental requirement here, is relative motion between the immediate and natural air currents and the surface the vehicle resting on - tailwind or headwind being optimal? yes?

Well, very close: There is no requirement that the air currents be "natural", only that they be relative to the rolling surface. One can test this device on a treadmill rather than in the natural wind -- other the obvious limitation of belt space, the results from going up a 10mph treadmill in a still air room is the same as the results from a 10mph tailwind.

As to headwind/tailwind, our device is currently configured to perform best in a direct tailwind, though a simple gearing change (or smaller tires) will cause it to back directly upwind. Though none of ours is configured in such a way, one can be built that will exceed wind speed it any direction.

JB

 

[Papa]

The fundamental requirement here, is relative motion between the immediate and natural air currents and the surface the vehicle resting on - tailwind or headwind being optimal? yes?

Well, very close: There is no requirement that the air currents be "natural", only that they be relative to the rolling surface. One can test this device on a treadmill rather than in the natural wind -- other the obvious limitation of belt space, the results from going up a 10mph treadmill in a still air room is the same as the results from a 10mph tailwind.

Well, 'natural' in the contexts of the vehicle's intended purpose. I haven't read the rules, but suspect that NALSA would frown on 'record attempts' at Ames Research Center (home of the largest wind tunnel).

 

[etard]

Great to have you on here JB, and congrats it seems you silenced alot of doubters last weekend. I will be following site to see if I can't catch you out in Barstow next time, if you ever make it out there. Awesome writeup by Richard Jenkins, freakin hilarious!

It appears you are using bicycle or moped components, how much does the Blackbird wiegh? Also, is there any possibility that you could surpass Richard Jenkins 126mph run? Say the wind is going 50 mph, with a stouter setup, could you reach the record, or is there some greater losses in efficiency at greater speeds?

 

[John in CR]

Welcome, JB,

-- given a rolling surface, it will maintain it's speed 2-3x above wind speed indefinitely

This almost sounds like perpetual motion.

1. Assuming foot assisted launch in absent of tailwind, will it increase in speed on its own?

2. In its present configuration and aerodynamic shape, what is it theoretical top speed?

Papa,

It won't work without wind. He said it would work into the wind with a gearing change, but also then the prop would be acting like a turbine and the prop would drive the wheels for into the wind use. Since a boat can work directly into the wind with a similar arrangement despite the losses of a propeller in the water, I think this one would work surprisingly well into the wind, though not nearly as well as with the wind. It's wind driven all right, just not through a traditional mechanism.

Are there wind tunnels that big, because there's no way to calculate the aero drag? This thing's limit is friction, mostly in the prop I would guess, and of course the mechanical limits of the prop. The power available is obviously based on wind speed and size of the prop.

 

[John in CR]

JB,

Congrats on what you guys have accomplished. Is one of those involved the person who had the unmanned small wooden version running on the street a few years ago? It took a while for me to understand the concept back then, and that clip wasn't proof for me that it actually worked. It's great to see that I didn't waste my time wrapping my noggin around the concept.

Any chance you can tell us the wheel and prop diameters, and the gearing ration?

John

 

[johnrobholmes]

-- the spinning rotor is a propeller, not a turbine
-- the wheels provide the torque to turn the rotor (always).
-- the rotor does not provide the torque to turn the wheels (ever).

We have lots of videos if this site allows me (a new member) to post links - perhaps someone can tell me if this is OK ... I don't want to 'un-welcome' myself by doing something uncool.

JB

Go ahead and link away, there are no rules against spreading knowledge here!

So I had things a bit backwards. The wheels turn the prop that pulls the vehicle? Now I am actually more confused about how it keeps running above windspeed, common sense would tell me that the prop can't make more power than the wheels are using to turn it. I gotta me missing something.

 

[John in CR]

...common sense would tell me that the prop can't make more power than the wheels are using to turn it. I gotta me missing something.

In a closed system it can't. The difference is the wind. It's like a plane flying faster with the wind with no extra fuel consumption because, the extra power is coming from the wind, except in this case the wind's power goes into the wheel, which in turn turns the prop which pushes against the wind.

http://www.fasterthanthewind.org/

 

[swbluto]

Let's say you're right at the speed of the wind.

For there to be a further increase in speed, the thrust of propeller has to be more than the energy taken from its kinetic energy. However, that propeller is deriving its thrust from the ground. So you're converting kinetic energy into "thrust energy"; you just need to create more 'thrust energy' than the kinetic energy invested. If the kinetic energy was the only thing you derived energy to turn the prop, then the prop's output would be less than the amount of kinetic energy taken and you'd slow down. This is because a propeller and its gearing isn't perfectly efficient. As you'd slow down, the drag would increase from the wind and you'd stabilize at some speed less than the wind speed.




That's what sounds like the impossible claim. But, the thrust energy also is increased by the wind itself. With a given propeller speed, a tailwind is going to help you produce more "thrust" then less of a tailwind (As evidenced by going faster). So, thus, the thrust is composed from sapping the existing kinetic energy plus the wind's energy, so it seems possible for the thrust's output to exceed kinetic energy's input, thus causing a net increase in kinetic energy and thus speed past the wind's speed.

(But, I have to admit I'm waiting for the "We fooled you all along!" line. )

ANOTHER way of looking it is that if you were standing still and there was no wind, you wouldn't go anywhere. Now, if for some reason, the propeller was spinning, the prop would produce thrust and thus you'd go somewhere. That's exactly analogous to being at the speed of the wind. From the reference of the person to the wind, the wind is 0 mph and they are standing still (relative to the wind), but yet their prop is spinning so they start going faster than 0 mph or the wind's speed.

So.... cool. Could this be applied to airplanes? It seems wind speeds are a bit higher up there in the atmosphere, but airplanes have to deal with lift which ground-based vehicles get to conveniently ignore since the ground provides it.

 

[ThinAirDesigns]

Great to have you on here JB, and congrats it seems you silenced alot of doubters last weekend. I will be following site to see if I can't catch you out in Barstow next time, if you ever make it out there. Awesome writeup by Richard Jenkins, freakin hilarious!

Thanks and yes, when our project started, we were considered such crackpots that Jenkins wouldn't even answer our emails -- literally.

It appears you are using bicycle or moped components, how much does the Blackbird wiegh?

Many standard and recumbent components. Just over 400lbs.

Also, is there any possibility that you could surpass Richard Jenkins 126mph run? Say the wind is going 50 mph, with a stouter setup, could you reach the record, or is there some greater losses in efficiency at greater speeds?

This vehicle was designed for 2x in an 18-20mph wind and we pushed it's top speed to over 150% of design speed but really don't want to press our luck beyond that. Are the design principles sound and could a vehicle suitable for higher speeds be constructed on those principles? -- we believe so. There are no losses inherent to this design that put a cap on it competing for the outright record. Someone would have to come along with the money and motivation -- we built this on a very small budget and put close to 2,000hr of unpaid time into it and we can't afford to continue down an even more expensive path using that 'business model'.

JB

 

[ThinAirDesigns]

I haven't read the rules, but suspect that NALSA would frown on 'record attempts' at Ames Research Center (home of the largest wind tunnel).

Hi Papa -- I apologize, when you were referring to "natural currents", I thought you were commenting on the vehicles ability to function. In the context you meant it you were absolutely correct, NALSA isn't going to ratify a wind tunnel record.

JB

 

[Papa]

He said it would work into the wind with a gearing change, but also then the prop would be acting like a turbine and the prop would drive the wheels for into the wind use.

Which was my original interpretation. But then JB also stated,

" -- the wheels provide the torque to turn the rotor (always).
"-- the rotor does not provide the torque to turn the wheels (ever).",

which further clouds the analysis unless the other parameters are clarified.

The power available is obviously based on wind speed and size of the prop.

Relative wind speed, yes, but IMO, it's the conquering the efficiency losses that's the challenge here, which I would assume embraces a host of possible design modifications and endless testing... not just prop diameter.

 

[ThinAirDesigns]

Any chance you can tell us the wheel and prop diameters, and the gearing ration?

Wheel diameter = 27"
Drive axle sprocket = 23t
Prop axle sprocket = 65t
Prop pitch: 216" at chord line, 280" at zero lift line.
Prop diameter = 17'

JB

 

[Papa]

I haven't read the rules, but suspect that NALSA would frown on 'record attempts' at Ames Research Center (home of the largest wind tunnel).

Hi Papa -- I apologize, when you were referring to "natural currents", I thought you were commenting on the vehicles ability to function. In the context you meant it you were absolutely correct, NALSA isn't going to ratify a wind tunnel record.

JB

Noted and appreciated, but no apology is necessary JB. I guess I need to use more of these

Some of my questions were surreptitiously driven by, "where can I apply this" on my daily driver.

 

[ThinAirDesigns]

He said it would work into the wind with a gearing change, but also then the prop would be acting like a turbine and the prop would drive the wheels for into the wind use.

This statement is correct. Merely reducing the size of the rear tires will cause the vehicle to literally back up into the wind when the brakes are released. The blades in this configuration are indeed working as simple (though inefficient) turbine blades.

Which was my original interpretation. But then JB also stated,

" -- the wheels provide the torque to turn the rotor (always).
"-- the rotor does not provide the torque to turn the wheels (ever).",

which further clouds the analysis unless the other parameters are clarified.

Again, my above statements are correct (in context). We configure this vehicle ONLY for DDW operation, thus the rotor *never* provides the torque to turn the wheels. *IF* we were to configure it for upwind operation, those statements would reverse.

JB

 

[ThinAirDesigns]

So.... cool. Could this be applied to airplanes?

Depends if you're talking theoretical or practical. Energy is there for taking anytime there is differential velocities, but the device must have some sort of 'hook' into both media.

For a downwind airplane to take advantage of the relative velocity between air and ground it would have to drag a cart along the ground, collecting energy at this point and transmit this energy up the tether to the airplane propeller. Theoretical? - yes. Practical? - no.

JB

 

[TylerDurden]

Believe it or not...

 

[spork]

Shoot! Now that we've made Ripley's people are sure to think it's a hoax.

 

[shorttyd]

This was in Wired Mag. http://www.wired.com/autopia/2010/06/downwind-faster-than-the-wind/ There is video. Seems like it would be possible to do on a bike, maybe carbon fiber?

 

[Thud]

Hmmnn......
I see the Joby name on the mast, & I know them boyz were on the cutting edge of airfoil design for capturing windy energy.
Is their a connection i missed somewhere in this thread?
Cool project.