DDWFTTW

[zahc]

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

My brain is angry.

Using a wind-powered vehicle, it appears that traveling directly downwind faster than wind-speed, in steady state, is quite possible.

[MechAg94]

I am curious exactly how this is supposed to work.  I am skeptical until I know that.  It seems very questionable at first glance assuming you are talking about a steady wind.  Given a steady wind, I can't think of any way you could keep accelerating once you matched wind speed unless you had some way of storing energy to use to continue accelerating.  Does the propeller provide that?  Was it a steady wind?

I guess it seems to say you are getting more then 100% efficiency in energy transfer so I want to know how they are doing it.

[geronimotwo]

can...not...fathom.....must...be...hoax...

[drewtam]

Not a hoax. There is video of that full size working model on the intertubes, and mathematical explanations.
The key part to this thing is that the wheels are geared to the prop. So the vehicle is working off the difference in speed between the air and ground.

[MechAg94]

The claim is they can do it consistently into the wind, but the video is too short to see if it can be sustained.  It is so short it looks like he could just be using the prop as a flywheel and gearing in the acceleration.  He didn't maintain speed very long. 

The only way I can see sustained speed is if the prop uses the then head wind to maintain motion as well.  The one video calls it a feedback loop between the prop and the wheels fed by the wind.  However, once you lose tail wind, you lose the energy input and the feedback loop is not 100% efficient and should lose speed.  If you can use the prop momentum to get up well past wind speed, you then get some head wind to use as an energy source.  I can't see it any other way.

[Chuck Dye]

TANSTAAFL

[ThinAirDesigns]

Hi folks.  I'm JB and I'm one of two primary designer/builders of the OP device.  Hope you don't mind that I dropped in.  If I've crashed a party, let me know and I'll be gone.

If it's OK that I hang around, I'll respond to individual posts in a bit, but before I do I
thought I'd post a few answers to FAQs we often get asked.

Oh, and there is no free lunch needed -- we are polite and natural law abiding. :-)

Basics:
-- the vehicle does not use stored energy to accelerate
-- at all speeds the device is slowing down the wind relative to the ground
-- 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 is equipped with a fixed ratio transmission
-- it will take off from a standing start on it's own.
-- we sometimes give it a push start to save time during testing
-- there's no "null point' of any sort at windspeed.
-- It will maintain a speed well 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 directly upwind rather than down

[Doggy Daddy]

-- 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).

Okay, now for the second time in a week or so, someone on this board is making me think when I didn't really feel like doing so.  Intuitively, this is magic.  I await the science.   

DD

[MechAg94]

I am missing something also.  If the prop is turned by the wheels only and not the other way around, how does it start moving in the first place?  Something catches the wind and turns it into motion initially.  The prop is the biggest thing on the cart.  Lacking more information, I cannot believe the prop does not turn the wheels at all.  How would it go upwind in that case? 

Regarding the mention of no let off in acceleration and such:  At the point of matching wind speed, there is a net zero wind speed.  How do you maintain acceleration without using stored energy to push past that point somehow?

[ThinAirDesigns]

I am missing something also.  If the prop is turned by the wheels only and not the other way around, how does it start moving in the first place?  Something catches the wind and turns it into motion initially.  The prop is the biggest thing on the cart.

Initially it is merely the bluff drag of the entire device that starts the vehicle rolling dead downwind.  The flat plate area of both the chassis and the propeller is sufficient to begin a slowly accelerating roll.  (Just as a matter of reference, the prop may be the dominating visual feature of the vehicle, but it's flat plate area is small compared to the rest of the vehicle).

In what may seem even more odd, when the vehicle has a relative tailwind (startup phase) the angled propeller blade are actually trying to rotate the rotor in the opposite direction than the wheels are forcing it to turn.  Check out the following video: Pay particular attention to the direction of propeller rotation and blade pitch at ~0:15 seconds into the video.  Considering that the vehicle is in a pronounced tailwind at this point (see 0:20 for wind direction) it's not hard to figure out that this wind is trying to turn the angled blades CCW (from the rear). The bluff drag of the entire vehicle acting through the wheels is forcing the rotor to turn CW.

http://www.fasterthanthewind.org/2010/07/video-from-richard-jenkins-world-land.html
     

Lacking more information, I cannot believe the prop does not turn the wheels at all.

What more information would you like to have before you can believe that the prop never turns the wheels.  Would it help for you to see the picture of the large ratchet on the prop shaft that simply doesn't allow the propeller to drive the wheels?

http://www.fasterthanthewind.org/2010/04/not-this-time.html
http://www.fasterthanthewind.org/2010/05/higher-torque-upgrades.html

How would it go upwind in that case?

This vehicle in its current configuration doesn’t go upwind (into true wind)

Regarding the mention of no let off in acceleration and such:  At the point of matching wind speed, there is a net zero wind speed.

Yes, it’s true that at exactly wind speed there is no relative wind over the vehicle – that doesn’t mean the true wind has stopped blowing however.  If you are extracting energy from the true wind (as we are) rather than the relative wind over the vehicle, it matters not that there is no relative wind.

That last point is KEY to understanding the vehicle.  People are very used to seeing devices that extract energy from the relative motion between the air and the device.  This is how a turbine works.  They are not used to a vehicle that can extract energy from the motion between the air and the ground independent of the speed of the device itself.  The Blackbird does just that through the use of its propeller.
 

How do you maintain acceleration without using stored energy to push past that point somehow?

Because we are not extracting energy from the relative wind, but rather from the true wind, there is no “point” to which you refer.  The true wind blows no matter what speed the vehicle runs and by slowing that wind down, energy can be extracted at any speed.  That is what I meant in my FAQs by the statement “there's no "null point' of any sort at wind speed.”

JB

[AmbulanceDriver]

Ok....

That's really cool...   I'm suitably impressed.   

Mind you, I'm completely mind-boggled, but I'm impressed.  :)

[Doggy Daddy]

How much tailwind does it require to work?  (this has probably been answered elsewhere, but I don't see it)

DD

[ThinAirDesigns]

How much tailwind does it require to work?  (this has probably been answered elsewhere, but I don't see it)

DD

It will operate (meaning go faster than the wind) in winds as light as 6-7mph.  It will not self start however until the winds get up to very near 10mph.  The vehicle is too slippery to self start at lower wind speeds.  If we needed to run in lighter winds regularly, we could install a retractable drag flap on the chassis that would allow it to start rolling in the lighter winds and still not impact top speed.

JB

[zahc]

Now that I believe the device works, I have a related question.

Land sailers can, of course, go much faster than wind speed by traveling at an angle to the wind and using the sail as an airfoil.

So, in a 10mph wind, a land sailer could travel probably 40mph at some angle downwind. In a steady state, without turning, is the component of said land sailer's velocity which is resolved in the directly downwind direction greater than the speed of the wind?

In other words, if you had a uniform 10mph wind, and you had to travel 1000 miles directly downwind, would this DDWFTTW device be faster than a land sailer?

[mtnbkr]

Hi folks.  I'm JB and I'm one of two primary designer/builders of the OP device.  Hope you don't mind that I dropped in.  If I've crashed a party, let me know and I'll be gone.

If it's OK that I hang around, I'll respond to individual posts in a bit, but before I do I
thought I'd post a few answers to FAQs we often get asked.

By all means, stay and make yourself at home.  We're a general discussion forum.  New blood and new ideas are always welcome.

Chris

[Perd Hapley]

Basics:
-- the vehicle does not use stored energy to accelerate
-- at all speeds the device is slowing down the wind relative to the ground
-- 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 is equipped with a fixed ratio transmission
-- it will take off from a standing start on it's own.
-- we sometimes give it a push start to save time during testing
-- there's no "null point' of any sort at windspeed.
-- It will maintain a speed well 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 directly upwind rather than down

Enough with the trivia; is XM an option? How about leather seats? Most importantly, if I'm driving one of these, must the Prius-folk follow me, kissing my oh-so-faint carbon footprints?

[kgbsquirrel]

Really neat stuff.

ThinAirDesigns, do you have plans to replace the chain and sprocket with something more akin to a geared drive bar for sturdiness? Also have you considered a manually adjusted variable pitch propeller? It seems like that could be used for three things. First, helping in start forward motion (reverse pitch of the blades) using the propeller to actually drive the wheels forward while you still have relative wind flowing forward over the vehicle, then shifting to the normal driving pitch. Second, helping slow the vehicle (zero angle to the wind, so it is just beating air and in a manner acting as a drive train brake for the wheels) so the wheel brakes are not fighting the propulsion to bring it to a halt. And third, use to help control speed by reducing the angle of attack in order to limit the wind energy being fed back into the system. Any particular thoughts?

[Hawkmoon]

Oh, and there is no free lunch needed -- we are polite and natural law abiding. :-)

Basics:
-- the vehicle does not use stored energy to accelerate
-- at all speeds the device is slowing down the wind relative to the ground
-- 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 is equipped with a fixed ratio transmission
-- it will take off from a standing start on it's own.
-- we sometimes give it a push start to save time during testing
-- there's no "null point' of any sort at windspeed.
-- It will maintain a speed well 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 directly upwind rather than down

Unless the upwind version operates on a totally different principle than the downwind version, your premise is impossible. If the propeller (the "rotor") does not power the wheels, then the wind is pushing the chassis directly. In an upwind scenario, the wind is going to push the chassis backward, yet you want us to believe that the wheels will generate sufficient energy to power the propeller to reverse this, and propel the chassis upwind.

You are proposing the equivalent of a perpetual motion machine. It isn't possible.

[ThinAirDesigns]

Unless the upwind version operates on a totally different principle than the downwind version, your premise is impossible.

You have an excellent grasp of the physics Hawkmoon to pick up on that.  Kudos

The upwind vehicle does *indeed* have to operate on a completely different principle than the downwind vehicle.   When going upwind, the rotor *must* drive the wheels (just as you present).

There's no conflict in my list of basics however -- my list was describing the Blackbird, the vehicle featured in the OP.  The Blackbird is configured as a downwind vehicle and as such the wheels always drive the rotor and it has no capabilities to move upwind.  It's also true that with a simple gear ratio change the Blackbird would move directly upwind -- but then the rotor would be driving the wheels as you point out.
Now, a simple gear change will do just as I say, but it won't create a "practical" (if that term has any meaning in this case LOL) upwind vehicle. In the case of the Blackbird, changing the gearing (or simply placing smaller drive wheels on it) will cause it to *back up* directly into the wind.

Here's what going on in this downwind/upwind gearing issue: when the vehicle has a relative tailwind (startup phase) the angled propeller blade are actually trying to rotate the rotor in the opposite direction than the wheels are forcing it to turn. Check out the following video: Pay particular attention to the direction of propeller rotation and blade pitch at ~0:15 seconds into the video. Considering that the vehicle is in a pronounced tailwind at this point (see 0:20 for wind direction) it's not hard to figure out that this wind is trying to turn the angled blades CCW (from the rear). The bluff drag of the entire vehicle acting through the wheels is forcing the rotor to turn CW.

http://www.fasterthanthewind.org/2010/07/video-from-richard-jenkins-world-land.html

The winner in this 'force on the blades vs force on the wheels' battle is determined by the gearing between the prop and the wheels. Adjust the ratio in one direction and the blades will win, the rotor will turn CCW and the vehicle will go (back up) directly upwind. Adjust it the other way and the wheels win the battle and force the rotor to turn CW and downwind we go.

To make a truly efficient upwind vehicle, the propeller blades need to be replaced with turbine blades (different camber location on the blades). We will be building such a rotor over this winter and intend to set a new directly upwind record in the spring. The current record is about 60% of wind speed and we believe we can rather easily bump this up to greater than 100%

You are proposing the equivalent of a perpetual motion machine. It isn't possible.

Post my explanation above, I'd be interested to know if you still feel the same way.

Thanks

JB

[Doggy Daddy]

Do you think it would be acceptably accurate to describe the mechanics as being a force multiplier?  I've been stumbling over the "perpetual motion" idea myself.  It seems as if you're saying that even with no wind, if the cart were "given a push", even by hand, at enough speed the prop would start to spin as a result of the wheels turning, and continue to propel it even after the manual shove ended.  OTOH, perhaps we should think of the mechanics serving more as a lever or maybe a block and tackle.  Force must be applied, the mechanism amplifies that force.

The ground speed does not equal wind speed + prop speed. Ground speed equals a multiple of wind speed.  Close?

DD

[drewtam]

Now that I believe the device works, I have a related question.

Land sailers can, of course, go much faster than wind speed by traveling at an angle to the wind and using the sail as an airfoil.

So, in a 10mph wind, a land sailer could travel probably 40mph at some angle downwind. In a steady state, without turning, is the component of said land sailer's velocity which is resolved in the directly downwind direction greater than the speed of the wind?

In other words, if you had a uniform 10mph wind, and you had to travel 1000 miles directly downwind, would this DDWFTTW device be faster than a land sailer?

Zahc, when I first read about the design, I found out that the seed ideas/principles came from tacking across the wind. The way I understand it, this principle of design will be roughly equivalent to tacking in a very efficient manner. In other words, yes, its faster than a land sailor going downwind.

[ThinAirDesigns]

In other words, if you had a uniform 10mph wind, and you had to travel 1000 miles directly downwind, would this DDWFTTW device be faster than a land sailer?

zahc -- what I'm going to say next is the product of a lot of speculation, but also a lot of performance measurements taken from our vehicle and also land yachts.

The Blackbird is very crude and built on a VERY small budget (under $15k).  It was built by two guys who had never built a propeller in their lives -- in fact there were a LOT of things we did on this project we had never done before.  Point being ... it's rough and we believe we could do significantly better next round (which there may well not be).

We believe that only the very best few land yachts out there could maybe beat the Blackbird in your hypothetical race.  That race however would be between a high evolved craft (the top performing land yacht) and a barely developed concept vehicle (the Blackbird).  We're convinced that with the proper budget and our increased knowledge, the DDWFTTW vehicle would win handily.

Again, speculation.

JB

[zahc]

http://www.youtube.com/watch?v=k-trDF8Yldc

This video helped me to accept DDWFTTW travel a little bit.

[ThinAirDesigns]

ThinAirDesigns, do you have plans to replace the chain and sprocket with something more akin to a geared drive bar for sturdiness?

Currently we have no plans to spend that sort of effort on the Blackbird.  While we are converting it to set the upwind record next spring, Blackbird will likely never again see a wind behind it - we've taken it as far as we wish to from a hobby standpoint.

Also have you considered a manually adjusted variable pitch propeller?

While originally it was not, Blackbird has been equipped with such a mechanism for a while now.  The construction blog at www.fasterthanthewind.org will show how we did this.

We did this for many of the reasons you describe, however we did not allow the pitch to adjust so far that the rotor would drive the wheels.  While this would have indeed helped get the vehicle up to speed faster, it would have precluded the use of the prop shaft ratchet which was key to meeting the NALSA rule of not being able to use stored energy for acceleration.

JB

[CNYCacher]

Blackbird will likely never again see a wind behind it

It never did