DDWFTTW - Directly Downwind Faster Than The Wind [Archive] - Page 2

Guest625101138

06-11-2010, 12:27 AM

We finally have the downwind cart more or less completed. New aerodynamic fairings, strengthened transmission, variable pitch prop, logging of GPS and wind speed and direction, and even it's own trailer (finally).

We took it out for some testing yesterday, and had reasonably good luck with the wind conditions. We got about 2.85X wind speed in 12 to 15 mph winds.

Hopefully we'll be making a run for the record in the next few weeks.

We were also invited to give a talk on the topic at the St. Francis Yacht Club next month.

One thing we'd love to find is a small external monitor to put in front of the driver. If anyone has something old to loan or sell cheap, I'd love to hear from you.

It even looks the part now.

When will you be doing the official recorded runs?

For the next model you will need to consider the best location for the body weight.

I would not want to be in the monster at 100kph. A lot of power transfer going on.

Are there still doubters?

Rick W

sirclicksalot

06-14-2010, 02:46 AM

As with all wind-driven speed records, efficiency of the driven element (sail or propeller blade) is the limiting consideration, and that efficiency is limited by ever-increasing apparent wind lowering the angle of attack and therefore L/D. dEfficiency/dWheelspeed is negative near maximum speeds of the fastest craft; wind-driven speed records involve lowering its magnitude, period. It has nothing to do with the true wind speed. I would wager that most non-believers are so because they have not yet understood this point.

I know that has been said before in various ways (I just finished reading the entire thread), but I haven't seen anything like the following very simplified approach: basically it is Mark Drela's model (ddwe.pdf and ddw2.pdf) with all the efficiencies set to one. That may make it easier to click with some people:

http://www.boatdesign.net/forums/attachment.php?attachmentid=44294&stc=1&d=1276501323

The point of this model and that figure is where the magenta line crosses the abscissa, and it shows that if

1) the no-slip screw speed (set in cell E1) is 75% of the wheelspeed, and
2) the true windspeed is 10mph,

then the prop blades are providing thrust until the wheelspeed reaches 40mph (yellow cells, yellow line on plot), which is the wheelspeed at which the apparent wind vector wrt the blade screw (spiral? helix? I don't know the proper terminology here) is zero.

That screw-apparent wind is calculated in Column C (I think I finally got the signs right) and plotted as the magenta line. By increasing the (screw-speed/wheelspeed)=R ratio towards 100% the wheelspeed of the no-slip limit is also increased (the slope of the magenta line is 1-R); for R=90% the limit would be ten times the true windspeed. Beyond the no-slip limit the blade-apparent wind reverses (red cells) so second law considerations make such speeds impossible for any design (somone more clever than me said the three laws of thermo are 1) you can't get ahead; ) you can't break even; 3) you can't get out of the game).

This model describes a theoretical limit only. In the real world, there are losses, but dropping the wheelspeed moves us back (left from the yellow No-slip limit line) along the magenta line below the abscissa where there is apparent wind available (to the *blades*). At that point there is thrust available to balance the other losses: cart drag; rolling resistance; prop drag felt as reverse force on the wheels &c. Increasing R (e.g. blade pitch) to increase the theoretical limit also pays a penalty in efficiency so there is likely a sweet spot for R.

However, I think this diagram shows clearly, specifically in the difference in slope between the blue and magenta lines, how the wheel-driven propeller fights the loss of angle of attack i.e. it reduces the magnitude of dEfficiency/dWheelspeed.

sirclicksalot

06-14-2010, 02:55 AM

Figure 4 in the Bauer paper is wonky: there is no lift generated on a symmetrical foil at an angle of attack of zero. But I think that's a typo and doesn't otherwise affect the article (at least not the bits I understood).

spork

06-14-2010, 10:35 PM

When will you be doing the official recorded runs?

We're aiming for July 4th or thereabout. But that's coming along pretty quickly. I think it's simply a matter of coordinating with the NALSA observers at this point.

For the next model you will need to consider the best location for the body weight.

I'm not sure what you mean.

I would not want to be in the monster at 100kph. A lot of power transfer going on.

Based on our data so far it kind of looks like our best wind speed multiple for this particular cart will be somewhere in the range of 50 mph in a 15-20 mph wind. So I doubt we'll get it much faster than that. We have tow-tested it to 50.

Are there still doubters?

Yup. Some famous ones. Dan Kammen (physics PhD, Nobel prize winner, and Berkeley professor) has updated his stance to - no longer willing to take a stance. Apparently he claims that we'd somehow change the argument if he did. I don't know what that means - but I find it pretty weak.

There are several other professors in aero, M.E., and physics that still claim it's impossible. I honestly find that depressing.

sirclicksalot

06-14-2010, 11:59 PM

There are several other professors in aero, M.E., and physics that still claim it's impossible. I honestly find that depressing.

Have they actually looked at the numbers or are they just averse to believing anything they see on the web?

How much of what we see on the web do we believe?

sirclicksalot

06-15-2010, 12:13 AM

Someone's humorous link to the wiki talk:sailingfasterthan... page got me sucked in and I was up all last night. Apparently you guys are "hoaxters" and not to be trusted. I even posted a diagram. Thirty lashes with the wet noodle to whomever it was that posted that link.

Did anyone here ever watch the Planet X thing about 5 or 10 years ago? It is as amazing what people will assent to believe as what they will refuse to believe.

spork

06-15-2010, 01:00 AM

Have they actually looked at the numbers or are they just averse to believing anything they see on the web?

How much of what we see on the web do we believe?

A Berkeley student was trying very hard to get JB and I invited to speak there. We had already spoken at SJSU and Stanford. She approached Dan Kammen to invite us. He looked at the video and I think an email or two and announced that it was impossible. I don't recall the exact words, but I have a copy of his emails somewhere. As we continued to make more and more conclusive demonstrations, the student tried again to get Kammen to invite us to speak - but no-go. On Saturday a kitesurfing friend told me Kammen was her advisor, and she asked him why he refused to believe it. At this point he said he now chooses not to take a stance because we would somehow just change the argument if he did. I don't even know what that means except that he's got nothing and isn't up to admitting he got it wrong.

Another is Rhett Allain - Associate professor of physics at SELU. He writes a "physics" blog. He first discusses DDWFTTW here:
http://blog.dotphys.net/2008/12/physics-and-directly-downwind-faster-than-the-wind-dwfttw-vehicles/#more-902
And then brings it up in several more of his blogs. He arrives at some particularly bizarre conclusions in these blogs. If I remember correctly this included denying the equivalence of inertial frames. I can post the links to some of his other blogs if you'd like the full entertainment value of his thinking on the matter.

Another is Wayne Whiteman of GA Tech. Whiteman has a PhD from GA Tech (in M.E. I believe). JB and I have had several extended discussions with Whiteman by phone, email, and internet forum. He even sent me some laughable diagrams explaining his position. Never has anyone been so insulting and condescending to me and so wrong at the same time.

Here's one of my favorite quotes about Whiteman:

http://www.cdl.gatech.edu/dl/servlet/edu.gatech.dl.instructor.InstructorDetails?INSTRUCTOR_ID=2700

"He also directed and served as a senior mentor at annual workshops to improve teaching skills and enhance learning environments."

Given his astonishingly closed-minded attitude, his absolute insistance on sticking with his initial intuition no matter HOW silly his subsequent conclusions became, and his extraordinarily condescending attitude, I just shudder when contemplating the above quote. To this day he refuses to apologize for the insults he publicly levied against JB and I.

There are plenty more. Some are simply NASA engineers with a lower profile. Another is an aerospace engineering instructor who refuses to be identified (demonstrating more common sense than intellect). It goes on.

Someone's humorous link to the wiki talk:sailingfasterthan... page got me sucked in and I was up all last night. Apparently you guys are "hoaxters" and not to be trusted.

I haven't seen the link. But it should be obvious that we're hoaxters. After all, we've presented the analysis, built, tested and documented working models, posted detailed build videos so that anyone that likes can repeat our results, built several models and sent them to those that wanted to try it for themselves, and finally designed, built, demonstrated, and documented a full-scale manned DDWFTTW vehicle that does exactly as claimed - only far more successfully than anyone guessed. Meanwhile the skeptics wave their hands in the air while making broad claims of perpetual motion and violation of the 2nd law of thermodynamics. But not once has any of them actually pointed out how such a law is supposedly violated or where our analysis goes wrong. Instead, they call us hoaxters and describe OUR analyses as hand-waving.

Sadly enough, this little brain-teaser has actually changed how I look at scientists and engineers. Until this, I imagined unreasonably high standards for these groups I belong to.

That people get it wrong isn't surprising (or upsetting). That's the point of a brainteaser. How hard they cling to their initial intuition in an effort to NEVER admit they might have been wrong is what pains me somewhat. That's not science.

I even posted a diagram.

You posted a diagram? Where is the link?

FAST FRED

06-15-2010, 05:36 AM

Faster than the wind is really simple,

sail an ICEBOAT.

spork

06-15-2010, 06:31 AM

Faster than the wind is really simple,

sail an ICEBOAT.

Why not an F15?

sirclicksalot

06-15-2010, 08:19 PM

[...] You posted a diagram? Where is the link?

http://en.wikipedia.org/wiki/Talk:Sailing_faster_than_the_wind#Criticism_zero

The point of the diagram was to demonstrate how a fixed sail, on a craft moving with VMG downwind greater than the true windspeed, can extract momentum from the true wind.

spork

06-15-2010, 08:36 PM

http://en.wikipedia.org/wiki/Talk:Sailing_faster_than_the_wind#Criticism_zero

The point of the diagram was to demonstrate how a fixed sail, on a craft moving with VMG downwind greater than the true windspeed, can extract momentum from the true wind.

Ah yes. I've seen that page, but didn't recall that graph. JB has been active in that discussion, but not in editing the wiki page. Paul Beardsell is one of our latest critics - and I guess he decided to prove us wrong by changing reality - and wiki to match. Aparently he went in and did a complete hatchet job on a page that Gautier has carefully crafted and maintained for some time.

Guest625101138

06-16-2010, 01:18 AM

We're aiming for July 4th or thereabout. But that's coming along pretty quickly. I think it's simply a matter of coordinating with the NALSA observers at this point.

I'm not sure what you mean.

Based on our data so far it kind of looks like our best wind speed multiple for this particular cart will be somewhere in the range of 50 mph in a 15-20 mph wind. So I doubt we'll get it much faster than that. We have tow-tested it to 50.

Yup. Some famous ones. Dan Kammen (physics PhD, Nobel prize winner, and Berkeley professor) has updated his stance to - no longer willing to take a stance. Apparently he claims that we'd somehow change the argument if he did. I don't know what that means - but I find it pretty weak.

There are several other professors in aero, M.E., and physics that still claim it's impossible. I honestly find that depressing.

On the issue of bodyweight location it appears that there would be benefit in locating the main mass, the pilot, in a location for the best load on the rear wheels. First is even loading to counter the prop torque and second is loading in the rear to counter the forward moment from the prop pulling and the wheels dragging - avoid rear wheels lifting off and limiting power transfer.

You do not need to go beyond this forum to see quite a few who cannot actually think for themselves. Unless they have read it in a book they do not believe what their eyes tell them. It stifles creativity. In your case it pushed you to prove them wrong. There are not many willing to stand up and say they were wrong though.

Rick

spork

06-16-2010, 01:50 AM

On the issue of bodyweight location it appears that there would be benefit in locating the main mass, the pilot, in a location for the best load on the rear wheels. First is even loading to counter the prop torque and second is loading in the rear to counter the forward moment from the prop pulling and the wheels dragging - avoid rear wheels lifting off and limiting power transfer.

Ah yes. I'll admit that we didn't do a lot of in-depth analysis on weight distribution. We did extend the left axle to account for the additional prop torque at the higher speeds we were achieving (based on some observations and some basic calcs). The pilot position largely has to do with where we're willing to sit relative to the 17' spinning guillotine. I voted closer, JB voter further. I won - and thus I have to drive : )

sirclicksalot

06-18-2010, 01:42 PM

three quick queries:

1) Marchaj (Aero-hydrodynamics ...) has a formula for Vs/Vt max (Vs = boatspeed, Vt = true windspeed) of 1/sin(epsilonA) where cot(epsilonA) = L/D. If my maths are right this reduces to Vs/Vt max = SQRT(1+(L/D)^2). He does this for various foil L/D values ignoring all but the foil drag, but then goes on to talk about effective L/D for land- and ice craft (system L/D, if you will with Dfriction and Ddrag added to Dfoil for the denominator). Anyway, Vs/Vt of 2.86 gives a system L/D of 2.68 but assumes your DDWFTTW with a prop is the same as a fixed sail craft sailing at some angle to the wind. Perhaps a better number would be the speed of the blade, which with a velocity ratio of .7 to near 1.0 would add 25-40% to the 2.86 and yields system L/D as high as maybe 3.9. Since your prop blade is your only source of lift its component L/D would have to be higher to get those system L/Ds, maybe twice, say somewhere between 6 and 8. Is that consistent with what you think your prop is doing?

Or am I just mixing apples and oranges with a bogus conversion factor?

2,3) That took so much to distill into a hopefully coherent query that I forgot what the other two were.

sirclicksalot

06-18-2010, 02:40 PM

Guest625101138

06-18-2010, 10:26 PM

2) What is the design speed of the true wind after it encounters (passes from windward to leeward of) the propeller? From turbine theory you get the most energy extraction when the bucket is about half the motive fluid speed and takes half the speed off the fluid. Yes, I know it's acting as a prop, but I have been trying to imagine the path of the intersection of a true wind streamline with the blade, probably in the ground-at-rest frame, and I'm not sure it isn't like a turbine bucket (u-shaped). I'm not saying it isn't a prop driving the cart, but there is something old, something ancient, something important rattling around in the back of my mind. [there's your straight line, folks. fire at will] [#1: who's Will and why should I shoot at him?]

It is a simple propeller. It has nothing to do with a turbine. It works the same way an aeroplane can still taxi in a tail wind. The propeller pulls air in and pushes it out faster behind than the air speed in front relative to the propeller. In the case of being wind powered it slows down the airspeed over the ground.

If you go back to posts #159 and and #161 I give a table and an image of what the cart and prop are doing.

The terminal speed as a ratio of windspeed is related to the overall efficiency. The efficiency improves as it goes faster so, if you set the gearing to get to the highest terminal speed, the cart has to be push started. Alternatively the gearing can be changed on the fly using a CVT or variable pitch prop.

Rick W

sirclicksalot

06-19-2010, 10:54 AM

It is a simple propeller. It has nothing to do with a turbine. [...]

Hi Rick,

I apologize for the confusion I caused by not being clear, but I'm not a neophyte here. I've washed down the crow with the kool-aid and even before that I knew the prop is not acting a turbine. So if what I write next is not clear on first read, please take as a given that I'm not confused about prop vs. turbine and read it again. I'm influenced by Puritan writers but don't have their skill; pay particular attention to my commas which I use when I break up a thought because I am trying to qualify it - sort of like the wheels (which are the turbine buckets in the cart's reality - I told you I understood) and the prop fighting each other for a net gain. It should not take more than half a dozen reads to understand; from what I have seen of your posts you'll get it on the first or second go.

The analogy, and it is a very limited analogy, to the turbine I was trying to think about is that the true wind has a given speed before it encounters the prop, and it (the wind) has a lower speed (and a different velocity) after it encounters the prop because it has applied force to, and given energy and/or momentum to, the cart. I.e. I am looking at the cart and its sails/blades/prop as a black box and as a turbine bucket as another black box. In that limited sense and in that limited sense only, there most definitely is an analogy between the cart black box and the turbine bucket black box in that they both extract momentum and energy from available potentials that exist between the motive fluid and the ground/nozzles/shell. (Phew! I had no idea so much was implicit in my previous post; no wonder it made no sense.)

Given that very limited analogy, and possibly (probably) stretching it too far, there is a principle in impulse turbines that the theoretical upper limit on efficiency comes from reducing the incoming fluid velocity by half with the bucket moving at half the incoming fluid velocity. I was trying to figure out if there is an analogous property inherent in the DDWFTTW cart design. I just realized it's probably in the Drela papers and I missed it so I'll go read them again. It's probably the L/D limit from Marchaj applied to the prop/cart/wind geometry.

sirclicksalot

06-19-2010, 11:26 AM

The terminal speed as a ratio of windspeed is related to the overall efficiency.

... at that terminal speed.

The efficiency improves as it goes faster

I assume you mean from at the beginning i.e. startup when the prop foils are stalled. As you approach terminal speed two things happen:

1) the blade efficiency drops as the angle of attack (AOA) drops.

2) the cart overall efficiency drops both with the blade efficiency and with the increasing losses. If the overall efficiency always improved with increasing cart speed there might be no upper limit.

Maybe efficiency isn't the right term for the blade, but in the blade-apparent wind frame the prop can no longer deliver increased momentum to the cart as all the air being turned is used to overcome losses at the terminal speed, and increasing cart speed increases losses with which the prop can't keep up as it approaches feathering (luffing in cloth sailing terms i.e. low AOA).

Actually, maybe that is what I am getting at: the optimum design has the blade efficiency reaching a maximum at cart terminal speed and so blade efficiency never falls off. As you say it would seem that VP should get you most of the way there.

SamSam

06-19-2010, 12:26 PM

It is a simple propeller. It has nothing to do with a turbine. It works the same way an aeroplane can still taxi in a tail wind. The propeller pulls air in and pushes it out faster behind than the air speed in front relative to the propeller.
That's where I begin to wonder where the energy comes from if you don't have an engine supplying it. You reach wind speed, you engage friction driven wheels to a propeller that only puts out 85% of the energy put into it and
A: You keep speeding up, faster and faster.
B: you slow down
I'm thinking B.

sirclicksalot

06-19-2010, 02:48 PM

That's where I begin to wonder where the energy comes from if you don't have an engine supplying it. You reach wind speed, you engage friction driven wheels to a propeller that only puts out 85% of the energy put into it and
A: You keep speeding up, faster and faster.
B: you slow down
I'm thinking B.

Ooh! Ooh! Let me! Let me do this one!

You can go back through this thread, this has been covered at length. I'll try to summarize.

The energy comes from the velocity differential between the air and the ground. The prop blades are the cart's connection to the air, and the wheels are cart's connection to the ground.

_____High_Energy_Source_(Wind)____
|
v
+-----------+
| Cart |----->Work Extracted
+-----------+
|
|
____________v____________
Low Energy Sink (Ground)

As the cart velocity increases, its losses (prop and cart drag, rolling friction) increase as its ability to extract work (power) from the wind decreases. When the power extracted balances the losses, the cart has achieved maximum velocity.

If you really do have your thermodynamicist hat on, that should be enough to convince you why it works. The rest is in the details explaining how it works.

Wheel-Prop Energy balance (Thrust & drag):

The wheels need only provide enough energy to the prop to move the cart to leeward through (i.e. wrt) the true wind. The prop is moving at (Cartspeed-TrueWindspeed) wrt the air; the wheels are moving at Cartspeed wrt the ground. The energy to travel to leeward at true windspeed over the ground is already in the wind (ignoring losses), even if there is no prop.

Wheel-Prop Torque (Force) balance (drag):

The wheels need only provide enough force (torque) to balance the drag force of the prop in the plane of the prop rotation, in the wind that the prop sees.

The wheel-prop connection is "merely" clever gearing to make the intersection of any true wind streamline with a cart surface - specifically the prop blades - have a component that moves upwind relative to the true wind.

Basically, here is the least complicated diagram that seals it for me:

Cart
Velocity
Vector
Wrt
Ground
^<--___
Blade | ---___
| | ---___ Blade Velocity Vector Wrt Ground
v | ---___
\ | ---___
\ <-------------------------------O
\ Blade Velocity Vector wrt Cart
^ \
|
|
True
Wind
Velocity
Vector

The prop pitch and the gearing can all be adjusted by design to make the qualitative relationships above work; the point is that the prop blade, moving along the big diagonal vector wrt the ground, is able to push back against the true wind even though the cart is moving faster than the true wind.

There is an even prettier vid here:

http://i47.tinypic.com/m7xf09.gif

[created by Eytee on Wikipedia]

which is the same thing just done with with a sail and a centerboard representing the boat's connection to the two media (air on sail; water on keel) to tap the differential energy between those media. The beauty of that vid is that it shows

1) the intersection (red circle) of the sail with a true wind streamline moving to leeward slower than the true wind, and

2) the true wind (blue circles) hitting the back of the sail at that intersection, while

3) the hull and any fixed point on the entire sail itself are moving to leeward faster than the true wind.

Guest625101138

06-19-2010, 06:55 PM

.

2) the cart overall efficiency drops both with the blade efficiency and with the increasing losses. If the overall efficiency always improved with increasing cart speed there might be no upper limit.

....

The overall efficiency will not get any higher than around 75 to 80%. Hence the maximum possible vehicle speed is somewhere between 4 to 5 times windspeed.

With something that is self-starting and not CVT or variable pitch it will be less than 70%. So possible maximum in this case about 3 times windspeed.

On the matter of the propeller. It drags air in from the surrounding and accelerates it. Have a close look at the table back in #159 (reproduced here) and try to work out what is actually happening. Forget about anything to do with sails. The sail analogy is like thinking about the propellers on an aeroplane being sails. The L/D of a high aspect prop blade can be 100 but this is only one of the factors that influence the overall efficiency of a propeller.

Looking at the last column in the attached table at 10m/s. The air coming into the propeller is 5m/s. It is accelerated to 8m/s behind the prop. From an observer on the ground the air in front of the prop is moving at 5m/s. Behind the prop it is moving at 2m/s.

As an example take 12sq.m of air then the energy in the wind at 5m/s is:
E5 = 1/2 * 1.2 * 12 * 5^2 = 180W
In the slipstream of a 12sq.m propeller that slows the wind speed to 2m/s the energy is:
E2 = 1/2 *1.2 * 12 * 2^2 = 29.8W

Hence the energy extracted from the air is 150.2W.

The propeller can never reverse the airflow over the ground. This means the maximum energy that can be taken from 12sq.m with a wind speed of 5m/s is 180W. If you try to gear the propeller to make the flow negative the cart will go into the wind with propeller becoming a turbine.

Think of the propeller as a propeller not as sails. Propellers pull air in and accelerate it out. Look at the diagram in post #161. You do not think of a boat propeller as a sail or dagger board. It is a propeller. The situation with the propeller on the cart can be analysed in exactly the same way you work out what a propeller on an aeroplane or boat does.

The prop on the cart was designed using JavaProp. This is free software. You can use it to determine what efficiency it possible. Make sure you have the right Re# for the speed and chord of the blades as this has a large influence on efficiency.

Rick

sirclicksalot

06-19-2010, 08:04 PM

sirclicksalot

06-19-2010, 08:31 PM

Maybe somebody already pointed this out, but ...

I think there are some who believe a cart that could go DUWFTTW (per Bauer, 1969 and Barkla, 1971) but do not believe DDWFTTW.

I just realized that the DDWFTTW cart is also a DUGFTTG (Direct Up-Ground Faster Than The Ground) device. I.e. it goes faster through the wind against the ground than the ground is going against the wind. DUWFTTW is also DDGFTTG.

So if someone knows DUWFTTW is possible, they can't deny DUGFTTG because the mechanics are the same in the differential between two media. From DUGFTTG just swap the meaning of the media (or flip the Black Pearl and your viewing camera over) and you have DDWFTTW.

Guest625101138

06-19-2010, 10:04 PM

Thanks for providing those numbers.

Did the ThinAirDesigns team use JavaProp? Are your numbers based on their design parameters or do you think you picked similar ones?

How do you define efficiency for a prop? Is it energy-based or velocity-ratio-based or something else?

A prop and a sail do the same thing: you say toh-may-to; I say toh-mah-to; please bear with my wooden bridge syndrome (if you ask a stone mason to build you a bridge, you get a stone bridge; ask a welder and you get a steel bridge; ask a carpenter and you get ...) and I will appreciate your facility with prop equations and terminology. I do agree the propeller equations are the way to go for these numbers (thanks again), especially given the varying radii and velocities along the blade, but there is a direct analogy between sails and propellers because they both only accelerate - i.e. change the velocity vector of - the air they encounter. Remove the hull from the vid (http://i47.tinypic.com/m7xf09.gif) and you could be looking down the radius of the cart's prop.

Nothing can literally pull or drag air, a fluid with no tensile strength. By accelerating air a sail or prop can create a low pressure into which higher pressure air from the far field flows (differentially diffuses, actually, if I remember my BS&L), but I understand what you mean by pull or drag.

Thanks again,

The Thinair guys used JavaProp for their prop.

The little table is an approximation to keep it simple. It is not based on a specific propeller. I have given analysis for a DDWFTTW propeller and turbine for a boat on another thread that has windmill in the title.

Mechanical efficiency is the same for a propeller as any other mechanical device. It is the ratio of power out over power in.

A sail below stall and a propeller both make use of lifting foils. Beyond that they vary significantly. The air velocity at the tip of the blade can be 10 times or more than the farfield air velocity relative to the prop. If you are going to design a propeller you must take into account the wide variation in velocity as the radius changes. Sails only have a slight variation in velocity over their span. Sails are closer to a wing than the twisted foil of a propeller.

I am certain you would never try to explain the operation of a boat propeller by making a direct analogy to the rudder. That has about the same significance as comparing an air propeller with a sail.

Guest625101138

06-19-2010, 10:11 PM

...

So if someone knows DUWFTTW is possible, they can't deny DUGFTTG because the mechanics are the same in the differential between two media. From DUGFTTG just swap the meaning of the media (or flip the Black Pearl and your viewing camera over) and you have DDWFTTW.

You are wrong there. If you go back through this thread and the one linked here:
http://www.boatdesign.net/forums/boat-design/wind-powered-sail-less-boat-24669.html
you will see many people struggle with inertial frames of reference.

There are videos on YouTube showing DDWFTTW carts climbing up a treadmill. It seems a great number of people argue this is completely different to the cart operating in wind over ground.

Guest625101138

06-20-2010, 12:48 AM

sirclicksalot

06-20-2010, 06:46 AM

You are wrong there. If you go back through this thread and the one linked here:
http://www.boatdesign.net/forums/boat-design/wind-powered-sail-less-boat-24669.html
you will see many people struggle with inertial frames of reference.

There are videos on YouTube showing DDWFTTW carts climbing up a treadmill. It seems a great number of people argue this is completely different to the cart operating in wind over ground.

Rick W

Sadly, yes. I should have said "So if someone competent knows DUWFTTW is possible, they can't logically deny DUGFTTG ..."

sirclicksalot

06-20-2010, 07:13 AM

sirclicksalot

06-20-2010, 07:57 AM

I am certain you would never try to explain the operation of a boat propeller by making a direct analogy to the rudder. That has about the same significance as comparing an air propeller with a sail.

Actually I would explain them all exactly the same way: F = d(mv)/dt.

The rest (twist, F = INTEGRAL_r( d(m(r)v(r)/(dt.dr) dr ), etc) is an implementation detail.

A little history: my Dad worked for a division of a company that made steam turbines. He would run performance and acceptance tests of installed power plant turbines to an accuracy of a quarter percent in some cases. In a steam turbine, most of the work comes from the HP section (high pressure) where

- the rotor is large
- the blades are short
- dV/dr is zero
- there is little if any twist in the blades

In the LP section, everything is the opposite but it only adds the last few tenths of a percent to the work extracted to get the customer to pay a premium to buy your turbine instead of the next guy's.

So like I said, wooden bridge syndrome: I know it's there but if I don't have to deal with twist, as in a qualitative discussion, then I won't: d(mv)/dt does the job. Even when I get semi-quantitative I'm still only interested in an average L/D for the prop (even though I know it's probably bogus and there is no such thing in a rigorous sense) because ballpark is all I need. The 2.85 VMGDW/Vt suggests a system L/D of somewhere between 2.7 and 4 (I already have +/-25% to start with and you want me to worry about dV/dr?), which puts the prop average L/D at perhaps double that. From what you tell me about the L/D at the tip that is reasonable.

I'm not saying it's not important, in fact your analyses of the flow field are both fascinating and important. And I will get there, I just don't need it yet.

If there is one thing that all these discussions have shown it is that no one explanation works with every individual.

Thanks again.

sirclicksalot

06-20-2010, 08:29 AM

You guys are on XKCD too? When do you have time to build this thing? When do you have time for a job?

sirclicksalot

06-20-2010, 01:21 PM

http://www.boatdesign.net/forums/attachment.php?attachmentid=44488&stc=1&d=1277065046

Until you run out of string.

See also:

http://www.youtube.com/watch?v=CWmeTWPN5o4

Guest625101138

06-20-2010, 05:21 PM

........
The 2.85 VMGDW/Vt suggests a system L/D of somewhere between 2.7 and 4 (I already have +/-25% to start with and you want me to worry about dV/dr?), which puts the prop average L/D at perhaps double that. From what you tell me about the L/D at the tip that is reasonable.

....

Inferring a prop L/D from that figure is nonsense - no relevance at all. The 2.85 means the gear ratio, ground to air, is a little over 1:0.65. Also, as the acceleration in the bottom range was poor, the lowest system efficiency would not be much higher than 65%.

This value of efficiency is not easy to achieve at low rotational speed.

The efficiency improves as it goes faster but with fixed gear, the ratio to wind speed cannot be altered. It is fixed by the selected gear ratio by design. With CVT, variable pitch or push start it would be possible to take advantage of the higher efficiency at higher speed although the power handling ability of the drive needs to be up to it.

If the gear ratio was 1:0.5 it would accelerate faster but then the top speed would be a little under 2X windspeed.

So all you can infer from the windspeed ratio is the chosen gear ratio. Forget about sails and think of it as a propeller driven by the wheels. If you have trouble with that then the winch and string analogy is better than anything to do with sails.

Guest625101138

06-20-2010, 05:48 PM

....
If there is one thing that all these discussions have shown it is that no one explanation works with every individual.

Thanks again.

Yes. That is true. For some individuals NO explanation works. Some are still looking for the battery.

I think Achimedes is credited with saying something like "give me a place to stand and with a lever I will move the world". Here it is give me the right gear ratio and I will go faster than the wind down wind - simple gearing. If the ratio is way too high it will go up wind. If it is just too high it will lock up. If it is way too low it will be a lower wind speed ratio than it could be. It is mostly about the GEARING.

sirclicksalot

06-20-2010, 10:03 PM

Inferring a prop L/D from that figure is nonsense - no relevance at all.

Yes and no. There is a formula for a theoretical max velocity for a foil with no other losses:

Vs/Vt = sin(gamma) * (L/D) - cos(gamma)

for which the maximum is

Vs/Vt = SQRT(1+(L/D)^2)

at gamma = 90 + arccot(L/D) but of course with the cart prop the equivalents for "velocity of the foil" and gamma are interesting if not problematic.

spork

06-20-2010, 10:42 PM

Sorry for the delayed response. I just got back from a paragliding weekend in the mountains.

Your observation of the entire cart behaving as a turbine, despite the fact that the rotor is clearly acting as a prop, is a good one. And it's one we've reflected on a number of times when discussing the prop/turbine issue. It's yet another way this simple device brings up some very interesting questions (such as whether a sail on a sailboat acts as prop or turbine).

The overall efficiency will not get any higher than around 75 to 80%. Hence the maximum possible vehicle speed is somewhere between 4 to 5 times windspeed.

I assume you're talking about engineering realities, rather than theoretical possibilities - yes?

With something that is self-starting and not CVT or variable pitch it will be less than 70%. So possible maximum in this case about 3 times windspeed.

In that case I'm feeling pretty good. On our very first runs we had fixed pitch, no CVT, and got pretty close to 3X windspeed.

spork

06-20-2010, 10:55 PM

Mechanical efficiency is the same for a propeller as any other mechanical device. It is the ratio of power out over power in.

But it's not quite that simple. The accepted definition for prop efficiency indicates an efficiency of 0.0 for a prop on a plane sitting still on the runway. Clearly it's doing work, but not effective work on the airplane. If you wanted a house-fan, you'd find it worked just fine.

A sail below stall and a propeller both make use of lifting foils. Beyond that they vary significantly. The air velocity at the tip of the blade can be 10 times or more than the farfield air velocity relative to the prop. If you are going to design a propeller you must take into account the wide variation in velocity as the radius changes. Sails only have a slight variation in velocity over their span. Sails are closer to a wing than the twisted foil of a propeller.

This is true in practice, but in theory the two are identical. It's literally just a matter of scale. Consider two sailboats sailing around the world. Their sails form a propeller with a ludicrously large hub.

I am certain you would never try to explain the operation of a boat propeller by making a direct analogy to the rudder. That has about the same significance as comparing an air propeller with a sail.

Comparing the prop to a sail is both accurate and instructive - though it seems ridiculous in terms of scale.

...give me the right gear ratio and I will go faster than the wind down wind - simple gearing. If the ratio is way too high it will go up wind. If it is just too high it will lock up. If it is way too low it will be a lower wind speed ratio than it could be. It is mostly about the GEARING.

Well... it's about both gearing and efficiency. I can make the cart go ANY multiple of the wind speed by simply choosing the right gearing - except for one detail... the efficiency has to be ludicrously high when I start shooting for speed multiples in the 4X range.

Guest625101138

06-20-2010, 11:26 PM

...

I assume you're talking about engineering realities, rather than theoretical possibilities - yes?

In that case I'm feeling pretty good. On our very first runs we had fixed pitch, no CVT, and got pretty close to 3X windspeed.

To me engineering realities and theoretical possibilities are the same. If they are not then the theory is limited or the engineering is flawed.

You did well to get to 2.85X in the first iteration. I have not gone through the analysis of where the efficiency is lowest but it will be very difficult to get better than 70% in the low speed range with your scale.

Increase the gearing and push start it you could get maybe 4X.

Guest625101138

06-20-2010, 11:38 PM

But it's not quite that simple. The accepted definition for prop efficiency indicates an efficiency of 0.0 for a prop on a plane sitting still on the runway. Clearly it's doing work, but not effective work on the airplane. If you wanted a house-fan, you'd find it worked just fine.

...

I was referring to the whole cart system regarding the efficiency. As soon as the prop turns it is doing work and, with the wheels connected to the prop, the cart has to move when the prop is turning.

The prop may not be producing the bulk of the work initially but once you start going faster than the wind it needs to be providing all of the required thrust. This is around where the overall system efficiency is lowest and what will be limiting your gearing and potential cart to wind speed ratio.

spork

06-21-2010, 12:19 AM

To me engineering realities and theoretical possibilities are the same. If they are not then the theory is limited or the engineering is flawed.

I disagree. There are many engineering realities today that were quite impossible 30 years ago - but the theory hasn't changed.

Guest625101138

06-21-2010, 02:30 AM

I disagree. There are many engineering realities today that were quite impossible 30 years ago - but the theory hasn't changed.

The theory along with engineering is constantly evolving. Have a look at how the theory of matter has evolved over the last 30 years and how it is still evolving. The matter has not changed but our understanding of it has changed.

Theory is a model or construct of the "real" world. That construct and understanding is constantly evolving as is engineering that applies it.

Some years ago there was a common threory that the earth was flat. Now that we can get beyond it we can see that it is cose to a shpere. Some theorised that it was a shere before it was actually observed.

So theory and engineering are constantly evolving.

The situation here is that the theory of lifting foils needs to be applied to designing a propeller for a cart but the key to undertsanding is the simple relationship for power being equal to force X velocity and mechanical advantage through gearing.

As your cart picks up speed the prop slip reduces so the advance ratio approaches 1. Basically the prop threads its way through the air as if it was a screw in a tube. The tube just happens to be moving over the ground and the wheels are turning the screw so it can move faster than the tube.

Thinking about sails and lifting foils confuses the understanding. The fact that it is a propeller just introduces some small losses compared with the screw in the thread analogy.

So if you want a simple theory for DDWFTTW consider a screw geared to the ground working its way along the moving tube. Forget about sails and even propellers.

spork

06-21-2010, 02:59 AM

The theory along with engineering is constantly evolving. Have a look at how the theory of matter has evolved over the last 30 years and how it is still evolving. The matter has not changed but our understanding of it has changed.

Theory is a model or construct of the "real" world. That construct and understanding is constantly evolving as is engineering that applies it.

Some years ago there was a common threory that the earth was flat. Now that we can get beyond it we can see that it is cose to a shpere. Some theorised that it was a shere before it was actually observed.

So theory and engineering are constantly evolving.

The situation here is that the theory of lifting foils needs to be applied to designing a propeller for a cart but the key to undertsanding is the simple relationship for power being equal to force X velocity and mechanical advantage through gearing.

As your cart picks up speed the prop slip reduces so the advance ratio approaches 1. Basically the prop threads its way through the air as if it was a screw in a tube. The tube just happens to be moving over the ground and the wheels are turning the screw so it can move faster than the tube.

This seems like the extremely long way around the question. Are you suggesting that it will never be possible to make a vehicle that goes directly downwind faster than 4x to 5X, or are you saying that our current engineering methods won't allow it? If you're saying it will never be possible, you're wrong. I understand full well that theory evolves, and I think you understand that that's not what we're talking about. I think you understand that we're talking about F = M x A (at low speeds so there's no need to bring dMV/dT into it) and work = force x distance. We can use this simple theory to determine that there is no theoretical limit to the speed of the cart relative to the speed of the wind (again, keeping the cart well below mach). That's entirely different from the question of just how efficient a bearing or gear we can make. I have a hard time believing you're not understanding this distinction.

Thinking about sails and lifting foils confuses the understanding.

We don't have to talk about sails and lifting foils if that confuses you. It doesn't confuse me.

So if you want a simple theory for DDWFTTW consider a screw geared to the ground working its way along the moving tube.

Who on earth are you talking to!? I know you're not new here.

sirclicksalot

06-21-2010, 07:12 AM

Who on earth are you talking to!? I know you're not new here.

No, not exactly new; he started the thread.

I think Rick's missives are directed at me and you wandered into the middle of a pointless p*ssing contest (with predictable results - check your trouser leg;-). Being a sailor I am most comfortable understanding via foils and extrapolating to props by admittedly messy integrations across cylindrical cross sections; I assume that being an expert with props Rick is comfortable working directly with the prop equations and avoiding the mess.

I know that at the root I agree with Rick: the limit is reached when the incoming prop-relative windspeed is too high for the prop to push on it (Rick's comfort zone), which is about the same as, but a little more complicated than, the blade foils on average reaching their optimum angle of attack = cot(optimum L/D) (my comfort zone).

[Btw Rick: what is the name for that in prop jargon i.e. the other end of performance from stalling? It's like the prop is spinning but can't do anything because the far field velocity is the same as, or greater than, the - another prop term I don't know - screw speed?]

And I have to admit I am a little ADHD and am having fun getting attention by poking Rick with a stick, like this:

Me the foil boy sez:

Cart
Velocity
Vector
Wrt
Ground
^<--___
Blade | ---___
| | ---___ Blade Velocity Vector Wrt Ground
v | ---___
\ | ---___
\ <-------------------------------O
\ Blade Velocity Vector wrt Cart
^ \
|
|
True
Wind
Velocity
Vector

Rick the prop cop sez:
http://www.boatdesign.net/forums/attachments/propulsion/38631d1261654679-ddwfttw-directly-downwind-faster-than-wind-cart.png

ThinAirDesigns

06-21-2010, 10:36 AM

To me engineering realities and theoretical possibilities are the same.

Wow -- just wow.

The theoretical possiblities of the space elevator are perfectly sound. The engineering realities *currently* dictate that it's not constructable. That's about as far from "the same" as you can get.

JB

Guest625101138

06-21-2010, 04:22 PM

Wow -- just wow.

The theoretical possiblities of the space elevator are perfectly sound. The engineering realities *currently* dictate that it's not constructable. That's about as far from "the same" as you can get.

JB

Tell me what the the theory of the space elevator is.

ThinAirDesigns

06-21-2010, 07:23 PM

http://en.wikipedia.org/wiki/Space_elevator

"Current technology is not capable of manufacturing practical engineering materials that are sufficiently strong and light to build an Earth-based space elevator. Recent conceptualizations for a space elevator are notable in their plans to use carbon nanotube or boron nitride nanotube based materials as the tensile element in the tether design, since the measured strength of microscopic carbon nanotubes appears great enough to make this theoretically possible"

(But don't get stuck on this particular example as there are an infinite number of other theoretically sound solutions just waiting for engineering realities to catch up -- to keep in in the DDWFTTW realm, just go back to the two mirrored tacking ice-boats connected by a long telescoping pole with the captains chair/controls in the middle going DDW)

JB

spork

06-21-2010, 09:55 PM

...to keep in in the DDWFTTW realm, just go back to the two mirrored tacking ice-boats connected by a long telescoping pole with the captains chair/controls in the middle going DDW)

Yes - I particularly enjoyed Harold's argument that this proves nothing because "where could you ever get such a pole?" :D

Guest625101138

06-22-2010, 02:22 AM

http://en.wikipedia.org/wiki/Space_elevator

"Current technology is not capable of manufacturing practical engineering materials that are sufficiently strong and light to build an Earth-based space elevator. Recent conceptualizations for a space elevator are notable in their plans to use carbon nanotube or boron nitride nanotube based materials as the tensile element in the tether design, since the measured strength of microscopic carbon nanotubes appears great enough to make this theoretically possible"

.....
JB

This is exactly what I mean. It points out it is not theoretically possible with the current available material. The engineering and theory are perfectly aligned as good theories and engineering should be.

It is a concept that is theoretically and practically impossible with the current available materials. The concept could be theoretically and practically possible if there were materials with higher specific strength.

spork

06-22-2010, 02:32 AM

This is exactly what I mean. It points out it is not theoretically possible with the current available material.

Be honest - you're just messing with us now - right?

You do understand the difference between something like the space elevator and a perpetual motion machine I hope. One is NOT theoretically possible (perpetual motion if you're wondering), while the other IS theoretically possible. The second one is just an engineering challenge -and we may get there one day.

Please tell me you understand that some things don't necessarily defy the laws of physics, even if we don't yet have the materials and methods to build them.

Guest625101138

06-22-2010, 05:29 AM

Be honest - you're just messing with us now - right?

You do understand the difference between something like the space elevator and a perpetual motion machine I hope. One is NOT theoretically possible (perpetual motion if you're wondering), while the other IS theoretically possible. The second one is just an engineering challenge -and we may get there one day.

Please tell me you understand that some things don't necessarily defy the laws of physics, even if we don't yet have the materials and methods to build them.

Our point of difference on the elevator is that I consider a useful theory should make allowance for the limitations of the materials so it can be applied by the engineer.

A concept that relies on unobtanium is not a theory, it is a concept that may or may not have theoretical and engineering application at some future point if unobtanium is ever discovered.

A good theory will model the essential elements of what is being engineered. The DDWFTTW cart, in the first instance, is better thought of in terms of efficiency, gearing and power as force times velocity than anything to do with a lifting foil. The theory of a lifting foil has significance for the efficiency of a the propeller but that gets into detail design and is not kernell to understanding how the cart works. Bringing in lifting foils in the first instance complicates the model.

spork

06-22-2010, 07:56 AM

A concept that relies on unobtanium is not a theory...

If only we could have stopped Einstein from wasting time with his silly thought experiments. That relativity nonsense never really did pan out.

A good theory will model the essential elements of what is being engineered.

You're talking about a good development plan - not a theory. And I think you're doing that because you're choosing to dig your heels in on a completely untenable position.

The DDWFTTW cart, in the first instance, is better thought of in terms of efficiency, gearing and power as force times velocity than anything to do with a lifting foil.

Categorically incorrect. I can tell you that when I first conceived of this thing as a brainteaser, I did so coming at it from the point of view of a sailboat sailing a broad reach with downwind VMG greater than windspeed. I later learned that I wasn't the first to have conceived of such a vehicle - but I can't tell you how the first person to think of it came to this design. What I can tell you is that it's pure nonsense to tell me how it's "better thought of".

The theory of a lifting foil has significance for the efficiency of a the propeller but that gets into detail design and is not kernell to understanding how the cart works.

That is wrong.

ThinAirDesigns

06-22-2010, 10:33 AM

A concept that relies on unobtanium is not a theory, ...

LOL -- you're really just making yourself look silly now. Some of the greatest scientific theories of all time rely on "unobtanium". Been traveling at the speed of light lately?

Sorry Rick ... you're off the deep end on this one.

JB

ThinAirDesigns

06-22-2010, 10:39 AM

The DDWFTTW cart, in the first instance, is better thought of in terms of efficiency, gearing and power as force times velocity than anything to do with a lifting foil.

If it's one thing I've learn while helping to educate thousands upon thousands of people regarding DDWFTTW is that no one gets to decide what terms it "is better thought of".

Some folks laugh in your face when you mention the simple gearing of the device and then grok the aero right off the bat -for others it's the reverse. Still others come up with their own strange twisted paths of getting there.

JB

sirclicksalot

06-22-2010, 12:09 PM

It was instructive badgering back and forth with Rick about props vs foils, but he has some good points and you guys are just playing semantics, setting up straw men, and being cruel.

I like thinking about it in terms of foils, but whatever I use to bootstrap into a basic understanding, I am a fool if I don't make subsequent efforts to grok every other road I can to the same point (heck, I think anyone that has ever sat in an air-conditioned car, looks at my thermodynamic diagram, and then makes any energy conservation claim about DDWFTTW is either incompetent or stirring up trouble).

My proposition was that the velocity ratio yields a system L/D for a performance-equivalent fixed-sail device (if you know the angle of attack) which can then be extrapolated back to an equivalent sail L/D.

In this particular context, if I understand correctly, what Rick is saying is that the foil theory is already implicit in the prop theory so going back to fixed-foils is not likely to lead anywhere. He's not saying the foil approach is an invalid bootstrap (he used it in at least one of his diagrams), just that once I make the transition from bootstrap to analysis and to prop theory, which I obviously had, it is folly to try to determine an equivalent/average/bogus fixed-sail L/D because that brings back all the dVelocity/dR mess that's already been worked out in the prop theory. I suspect he is right there, and although I am still interested in comparing the two different device types (fixed sail vs prop) by analogy, perhaps L/D is not the right metric. Why not just use VmgDW/Vt itself?

My interest in L/D was that there *is* an average L/D for the prop, and messy and/or incalculable as it is, it is closest thing I have to do a sanity check (it is almost always useful to get the same numbers at least two different ways). The problem calculating L/D from V/Vt and AOA is that V and AOA are for the foil and therefore problematic for the prop case.

spork

06-22-2010, 12:40 PM

It was instructive badgering back and forth with Rick about props vs foils, but he has some good points and you guys are just playing semantics, setting up straw men, and being cruel.

You couldn't be more wrong. Rick's a bright guy, but he's made some ridiculous (and condescending) statements recently, and we're just calling him on it. There's not a straw man in sight.

You on the other hand have attacked us right out of the gate - referring to us as professional trolls.

sirclicksalot

06-22-2010, 03:19 PM

You couldn't be more wrong. Rick's a bright guy, but he's made some ridiculous (and condescending) statements recently, and we're just calling him on it. There's not a straw man in sight.

You on the other hand have attacked us right out of the gate - referring to us as professional trolls.

Doh! Hoisted by my own petard. Again.

But when I see you guys are smarter than everyone else, what else should I suspect you were doing in the Great Electronic Morass? It's not like it's really a criticism, and if it looks like a duck ...

And get it right: I think I said "career trolls;" it has a K sound in it so it's a much funnier woyd.

And it wasn't right out of the gate, it was at least a few days. For the first five minutes I was blathering bad thermo at you.

Hee hee, you guys are good.

Guest625101138

06-23-2010, 02:22 AM

....
Categorically incorrect. I can tell you that when I first conceived of this thing as a brainteaser, I did so coming at it from the point of view of a sailboat sailing a broad reach with downwind VMG greater than windspeed. I later learned that I wasn't the first to have conceived of such a vehicle - but I can't tell you how the first person to think of it came to this design. What I can tell you is that it's pure nonsense to tell me how it's "better thought of".

....

The simplest explanation I have seen on Youtube is the guy who came up with the cotton reel and ruler axamples. Nothing to do with propellers.

When I analaysed it from the AYRS cart I went straight into the power relationship assuming the propeller worked without slip. I was not thinking of sails. Bringing sails into adds unnecessary complexity.

spork

06-23-2010, 02:37 AM

The simplest explanation I have seen on Youtube is the guy who came up with the cotton reel and ruler axamples.

His videos are indeed excellent. But I have to presume you haven't seen the even simpler yo-yo examples.

Nothing to do with propellers.

Which is precisely why so many people claim that explanation has no bearing whatever on the cart.

Bringing sails into adds unnecessary complexity.

Wrong. Perhaps you don't understand that the prop blades ARE sails.

Guest625101138

06-23-2010, 02:46 AM

His videos are indeed excellent. But I have to presume you haven't seen the even simpler yo-yo examples.

Which is precisely why so many people claim that explanation has no bearing whatever on the cart.

Wrong. Perhaps you don't understand that the prop blades ARE sails.

Now I know why you have had such difficulty explaining how it works. If you had have just talked about power transfer and gearing rather than confusing it with sails you may have not got into years of debate and needed to go to the expense of building a cart to prove the point.

I have had some success convincing a dozen or so people in a few minutes. As soon as people see that something can move faster than the medium pushing it by raacting against the medium pushing it they usually get it. The simplest and most available medium on any desk is a ruler.

spork

06-23-2010, 03:04 AM

Now I know why you have had such difficulty explaining how it works. If you had have just talked about power transfer and gearing rather than confusing it with sails you may have not got into years of debate and needed to go to the expense of building a cart to prove the point.

Wow - it's really interesting just how little you do know. I have in fact described this in terms of power since long before you were on the scene. But I'm also able to understand that the prop blades are sails; so I use that as well since different people respond to different explanations.

I have had some success convincing a dozen or so people in a few minutes. As soon as people see that something can move faster than the medium pushing it by raacting against the medium pushing it they usually get it.

Wrong again. Now that people have seen a full-scale manned cart clearly going much faster than the wind, they're FAR more open to explanations rather than calling names. We've seen an EXTREME change in people's reactions in that regard. But there are still a few people that think they understand it better than the people that designed, built, and tested it. Those people continue to make innaccurate and condescending statements.

But it seems we're getting off-topic. Do tell me again about how a theory is invalid if you can't test it in the real world using only the materials found on the island.

spork

06-23-2010, 03:27 AM

Now I know why you have had such difficulty explaining how it works....I have had some success convincing a dozen or so people in a few minutes.

Here's another fun fact... On the very first page of this thread - post number 4, you pulled out your famous 4 slide proof that you worked so hard on. The proof you seemed sure no one could possibly refute. And here we are 22 pages later with people that still don't accept what you're trying to tell them. If only you'd kept it simpler.

Guest625101138

06-23-2010, 04:53 AM

Here's another fun fact... On the very first page of this thread - post number 4, you pulled out your famous 4 slide proof that you worked so hard on. The proof you seemed sure no one could possibly refute. And here we are 22 pages later with people that still don't accept what you're trying to tell them. If only you'd kept it simpler.

That is dead right. I wish I had thought of the cotton reel for each version (upwind and downwind) being pushed with a ruler. That is the simplest and most elegant explanation of the principle involved. Bringing in propellers just confuses the conceptual understanding. Thinking about sails makes it even more confusing.

If you go into this thread:
http://www.boatdesign.net/forums/projects-proposals/windmill-wind-turbine-powered-boats-how-many-out-there-they-viable-14182-7.html
and look at post #104 you will see I discussed the necessary requirements for practical upwind and downwind systems over two years ago.

That exercised prooved to me that there were not many who understood basic physics let alone the engineering of a propeller.

The cart model came a year later and again prooved to be beyond most although it has worked with a few people as a simple explanation. On fellow even said how is it possible that people cannot understand this after seeing that model.

If I was asked to demonstrate the principle now I would get a cotton reel that fitted to rails either on the large diameter or small diameter and a piece of wood to push on the large or small diameter to show how with gearing one way it will advance into the direction of pushing and the other to show it can move faster than the pushing wood. The key to understanding DDWFTTW is to understand gearing.

Rick

ThinAirDesigns

06-23-2010, 07:12 AM

If I was asked to demonstrate the principle now I would get a cotton reel that fitted to rails either on the large diameter or small diameter and a piece of wood to push on the large or small diameter to show how with gearing one way it will advance into the direction of pushing and the other to show it can move faster than the pushing wood. The key to understanding DDWFTTW is to understand gearing.

Rick

Thousands upon thousands of people (most in fact) laugh outright at your above assertion that the key is to understand gearing. People who understand gearing COLD laugh at your assertion and say "it's got nothing to do with gearing, it's got a propeller".

You're problem Rick is that you assume that the explanation that YOU find simplest and best for YOU to explain is best for everyone -- simply not true.

No one has convinced more people nor educated more people on this topic than us (not even close). No one has been actively working on it longer than us (using your example you were many years behind) Everyone learns differently. I love the gearing example, I love the sails, the yoyo, the force * distance, on and on -- if someone laughs at one and turns it off, you have to approach it another way or you have no hope of being an educator.

Remember how you GAVE UP on the original thread and 'admitted' it was all a hoax? - I suggest that you not set yourself up as a shining example of one who has the one convincing key when the record of the forum shows otherwise. You've done some excellent work, but there simply is not 'one size fits all' when it comes to this brainteaser.

JB

ThinAirDesigns

06-23-2010, 07:18 AM

For what it's worth, for experienced racing sailors, nothing (NOTHING) has been as repeatedly and instantly convincing as the following video (or related explanation). Casual sailors not so much as many of them simply won't believe that a boat (even ice-boats) can have a downwind VMG >WS.

http://www.youtube.com/watch?v=UGRFb8yNtBo

As a general rule, the 'gearing' videos and explanations are easily the most scoffed at by the average joe.

JB

A.T.

06-23-2010, 07:32 AM

That is dead right. I wish I had thought of the cotton reel for each version (upwind and downwind) being pushed with a ruler. That is the simplest and most elegant explanation of the principle involved.
I agree, but many people fail to grasp general principles and to apply them to other situations. They accept the reel but not the wind cart.

Bringing in propellers just confuses the conceptual understanding.

Some people just don't have any conceptual understanding. They need to have every single application of a principle explained.

Thinking about sails makes it even more confusing.

Actually it works for some folks, who are trapped in the "feedback loop means perpetual motion" reasoning. Instead of thinking "what drives what?" they can see the airfoils as being pushed by the air against a kinematic constraint, and squeezed out with high velocity.

spork

06-23-2010, 07:45 AM

you will see I discussed the necessary requirements for practical upwind and downwind systems over two years ago.
...
The cart model came a year later...

Wrong again. You mean to say that YOU became aware of the cart model a year later. I'm pretty sure Goodman built his physical cart to settle a running debate that stemmed from the brain teaser I had conceived of and posed much earlier still. The solution to the brain teaser was precisely the cart model you refer to. And then of course if you want to go back further, it turns out Bauer had actually built such a cart. But let's go back further still - it turns out he learned of it through a paper written by a student. Who knows how much further back it goes?

Being condescending and closed-minded doesn't fit well with getting your facts wrong.

spork

06-23-2010, 07:53 AM

Rick here are a few excerpts from PM's I've recieved warning me about engaging you.

There is a lot that Rick does not understand, but puts up plenty of smoke and mirrors by way of computer plots and cut and pasted text that support “his theories”, to give the 'impression' he does

he has been exposed many times as seriosuly lacking in understanding on many aspects, but just ignores and carries on full of his own self worth and belief.

He doesn’t accept any theory unless it is his own. He treats engineers that have been taught and know more than he, with contempt

when shown real equations of motion of real theory...
He then does the usual about face, changes tack and say his feelings and thoughts are better than any theory and arrogantly says his understanding is better than anyone’s

It is clear to many on the website he has ego problems and is so arrogant and dismissive of anything that does not align with “his theories” it is laughable. Very few bother to engage in debate with him because of this.

It is your choice whether to take him seriously, but many do not.

Watching it unfold in front of my face is so much better than simply being warned of it via PM.

A.T.

06-23-2010, 09:36 AM

I'm pretty sure Goodman built his physical cart to settle a running debate that stemmed from the brain teaser I had conceived of and posed much earlier still.
I assume, that many in the sailing community had this idea independently. John Perry described the puzzle in 2002 using this analogy:

http://www.btinternet.com/~sail/doublescrew01.jpg

Then later someone told him about Bauer:
http://www.btinternet.com/~sail/dwfttw02.htm

ThinAirDesigns

06-23-2010, 09:42 AM

I think it's really cool how the internet has connected all these disconnected dots. Very different people at different times coming up with these ideas and none of them knowing about the first guy who built one.

I sure wish I could see the student paper that got Bauer and AMO started down their path. Perhaps he'll spot something about ddwfttw someday and step up and get the deserved credit (or explain how he/she got it from some other unknown source).

JB

TeddyDiver

06-23-2010, 01:46 PM

Remember how you GAVE UP on the original thread and 'admitted' it was all a hoax?

Got to say here that the "gave up" was the real hoax and made for "peace keeping" purposes only. For that I give much credit for Rick. Some PM's were exchanged in the background..

spork

06-23-2010, 01:50 PM

Got to say here that the "gave up" was the real hoax and made for "peace keeping" purposes only. For that I give much credit for Rick. Some PM's were exchanged in the background..

Done as a joke, I'd give him credit. Otherwise I think it's shameful.

TeddyDiver

06-23-2010, 02:13 PM

That is a question I think is worth another thread.. but giving up for someone's persistence even when you know he's wrong and you right doesn't change the truth in matter like this, especially when the subject will inevitably become proven later...

The difference how people comprehend this phenomen is even more amazing than the DDWFTTW itself.. :D

spork

06-23-2010, 02:35 PM

The difference how people comprehend this phenomen is even more amazing than the DDWFTTW itself.. :D

DDWFTTW itself is moderately interesting. The number of technical questions and insights it presents is pretty impressive for such a simple device. But ultimately I agree that the sociological, psychological, and cognitive issues are far more interesting and surprising.

Guest625101138

06-23-2010, 02:44 PM

Wrong again. You mean to say that YOU became aware of the cart model a year later. I'm pretty sure Goodman built his physical cart to settle a running debate that stemmed from the brain teaser I had conceived of and posed much earlier still. The solution to the brain teaser was precisely the cart model you refer to. And then of course if you want to go back further, it turns out Bauer had actually built such a cart. But let's go back further still - it turns out he learned of it through a paper written by a student. Who knows how much further back it goes?

Being condescending and closed-minded doesn't fit well with getting your facts wrong.

On the matter of the cart I am saying that I tried various methods to explain what was going on and this one was as unsuccessful as explaining the basic physics.

The most compelling and elegant explanation is the Youtube cotton reel analogy.

spork

06-23-2010, 02:49 PM

The most compelling and elegant explanation is the Youtube cotton reel analogy.

Do you understand the problem with making unsupported statements of an absolute nature? It's simply wrong to make such an assertion.

I think the cotton reel analogy is excellent. It's a better version of the yo-yo analogy I've offered for years. But that doesn't change the fact that it's COMPLETELY non-compelling to a lot of people.

You should also consider the difference between convincing someone in person vs. convincing them on the internet. It's two completely different worlds.

Guest625101138

06-23-2010, 03:21 PM

Do you understand the problem with making unsupported statements of an absolute nature? It's simply wrong to make such an assertion.

I think the cotton reel analogy is excellent. It's a better version of the yo-yo analogy I've offered for years. But that doesn't change the fact that it's COMPLETELY non-compelling to a lot of people.

You should also consider the difference between convincing someone in person vs. convincing them on the internet. It's two completely different worlds.

I should have added it is the most compelling I have seen and would be my preferred method to explain it and wished I had seen it much earlier.

You are right about the internet as even you are not immune to some of the spurious noise and have used it an unintelligent way simply in an attempt to discredit what I am stating. You should check sources before you repeat them but that would be professional and the internet supports all sorts of poor conduct.

The internet is a good place to make contact but if you want professional exchanges it is better to do it directly. Many of the contacts I make on the net lead to long-term contact and even meeting. Some have resulted in successful collaborations over a range of ideas and successful outcomes.

Rick W

spork

06-23-2010, 03:39 PM

even you are not immune to some of the spurious noise and have used it an unintelligent way simply in an attempt to discredit what I am stating. You should check sources before you repeat them but that would be professional and the internet supports all sorts of poor conduct.

And you should check your facts before deciding whether I've checked my sources, independently come to my own conclusions, or simply taken something at face value. But that wouldn't be the simple and absolute position to take in the face of a more complex situation. I understand the world is a pretty simple place that's just black and white for some people (and facts don't count).

Many of the contacts I make on the net lead to long-term contact and even meeting. Some have resulted in successful collaborations over a range of ideas and successful outcomes.

Thanks for the keen tip. Of course that's how JB and I met over 15 years ago (on a hang gliding forum). We've since started a company together, built this goofy vehicle together, flown together, and worked together for many years. But you should definitely stick with the condescending approach. Always play to your strengths.

I should have added it is the most compelling I have seen and would be my preferred method to explain it and wished I had seen it much earlier.

That's not an "addition". That's recanting an absolute position you recently put forth.

A.T.

06-23-2010, 04:07 PM

The difference how people comprehend this phenomen is even more amazing than the DDWFTTW itself.. :D

It is interesting to see where people get stuck on this counter-intuitive stuff, and how they eventually overcome the block. This gives you new ways to understand it yourself.

Recently someone on a wiki page was absolutely convinced, that even downwind-VMG > wind-speed with a sail-craft is impossible. When he finally got it, he explained what explanation changed his mind. I made an animation based on it:

rWfEjilcJoM

Apparently for some people the above works much better than the wedge analogy I used so far:

H_OKNr120t4

I may adapt the new explanation to DDWFTTW.

spork

06-23-2010, 04:52 PM

I wonder if a video of a pair of scissors would be compelling. Maybe a pair of scissors trying to cut something hard that's sliding as you try to cut(?)

sirclicksalot

06-24-2010, 05:23 AM

It is interesting to see where people get stuck on this counter-intuitive stuff, and how they eventually overcome the block. This gives you new ways to understand it yourself.

Recently someone on a wiki page was absolutely convinced, that even downwind-VMG > wind-speed with a sail-craft is impossible. When he finally got it, he explained what explanation changed his mind. I made an animation based on it:
[...]

These videos are fantastic.

I have one more suggestion to eliminate the jumps at transitions between each reference frame segment and instead make them smooth and seamless. To do that you need three surface-boat objects:

Start with the air-at-rest frame. As the first surface moves down and the first boat moves up and to the right, a second surface-boat object drifts into place from below-left i.e. stern of new boat moves to left edge of air dotted line. When it reaches there, ...

Switch to surface-at-rest frame; first surface-boat object fades. As the air moves up and the boat moves up and right along the surface line, a third boat object drifts into place from the lower left of a third surface line above and to the left of the second surface line. When the stern of the boat reaches the air line, ....

Switch to boat/sail-at-rest frame; second surface-boat object fades. Air moves left and down; surface moves left and more down.

Draw Ground symbols attached to the fixed object for each segment e.g.

|
__|__
//////

Alternate sequences are possible and may be better; Surface, Air, Boat might work but you would need two air & boat objects (for the Surface to Ground transition the first air moves up and fades as a second air comes in from below along with the second boat) but then air is too low for the Air to Boat transition. Anyway, I leave it in your capable hands.

A.T.

06-27-2010, 09:01 AM

I have one more suggestion to eliminate the jumps at transitions between each reference frame segment and instead make them smooth and seamless.
I don't really want them seamless. It could be more confusing than helpful. I made a DDWFTTW-version now:

vVMqa7Mft0k

And here the downwind VMG version with force vectors at the end:

63hvQABLOaE

kerosene

06-27-2010, 12:10 PM

Rick here are a few excerpts from PM's I've recieved warning me about engaging you.

Isn't that a little cheap shot. I think bringing up PMs always stinks unsportsmanlike.

This has reduced to retarded name calling and while I applaud Spork's and ThinAirDesign's achievements I do wonder how is it that you guys can have web-debates with guys who agree with you.

Yes its silly for Rick to say there is no difference between theory and engineering. yet he has helped a lot of people here, and like you gone out to test his ideas of small powered boats and keeps working on them despite the nay sayers (Wouldn't be so surprised if the said PMs came from this camp).

Cheers and good luck for the NALSA tests.

you might know me as coalburner form the other forums btw.

spork

06-27-2010, 12:22 PM

Isn't that a little cheap shot. I think bringing up PMs always stinks unsportsmanlike.

I guess that's a matter of opinion. I really don't care for Rick's continued condescending and innaccurate statements about me. I didn't post a PM from Rick afterall. What's wrong with passing along something told to me?

I do wonder how is it that you guys can have web-debates with guys who agree with you.

Agreeing with me regarding DDWFTTW doesn't automatically make you good and right in everything, just as disagreeing with me on this topic doesn't make you automatically wrong about everything. Wouldn't that be a little silly and childish?

Yes its silly for Rick to say there is no difference between theory and engineering. yet he has helped a lot of people here

Oh - I guess since he's helped a lot of people here I should agree with his ridiculous statements and welcome his condescending attitude.

ThinAirDesigns

06-29-2010, 02:05 PM

... I do wonder how is it that you guys can have web-debates with guys who agree with you.

Yeah, like everyone who agrees on DDWFTTW agrees on everything. :rolleyes:

Yes its silly for Rick to say there is no difference between theory and engineering. yet he has helped a lot of people here, ...

I'm glad he has helped a lot of people here -- that's very nice of him. That doesn't give him license to make silly statements without being corrected -- that's just a silly statement on your part (now corrected).

JB

A.T.

07-01-2010, 02:54 PM

Yes its silly for Rick to say there is no difference between theory and engineering.

I think this whole argument was a semantic one. By "theory" Rick W. apparently means a "numerical investigation based on real world parameters" while others mean "physics".

spork

07-01-2010, 09:11 PM

I think this whole argument was a semantic one. By "theory" Rick W. apparently means a "numerical investigation based on real world parameters" while others mean "physics".

I disagree. It would be easy enough to go back and review the statements made - but is there any point?

ThinAirDesigns

07-05-2010, 01:36 PM

Two days of NALSA testing complete. ~16-18 speed runs in ~12-13 distinct passes -- wind was lengthwise the lake on the first day giving us room run throught 2 sequential 'traps' on one pass.

Data shows every run well over 2x. Most runs over 2.5x. Best runs above 3x. We won't know details of which runs qualify against NALSA rules until they go through the large amount of data and figure out which runs are best documented. There is a fairly comprehensive list of requirements to be met for a run to be NALSA valid and I'm certain that some runs will be disqualified if all the sensor info was not to their liking (wind switching direction too much during the run, etc).

There are reams of data for them to go through from more than 20 separate sensors (multiple gps, wind direction and wind speed sensors) on the vehicle itself, chase vehicle and lakebed, plus multiple video cameras. It will take some time for them to go through it all once they get home.

To say the least we are quite confident that a record well over 2x will be ratified by the NALSA BOD.

On the project blog (www.FasterThanTheWind.org) Pictures, video and more information to follow as we recover from the long days and long drive home.

A special thanks to the NALSA officials who put in so much time, effort and money (they wouldn't let us pay for their travel expenses) to make this happen. Thanks also to the NALSA BOD who saw something interesting in this crazy little project.

JB

Zilver

07-05-2010, 03:10 PM

Congratulations !

Could you (and Spork) now please go picking your fights somewhere else ?
Thanks in advance,

Hans

spork

07-05-2010, 03:16 PM

Congratulations !

Could you (and Spork) now please go picking your fights somewhere else ?
Thanks in advance,

Hans

We aren't picking fights here. We're here to discuss the topic and answer questions. But we're not here to take **** from people either. Thanks for your understanding - now leave.

Windmaster

07-05-2010, 03:20 PM

Congratulations !

Could you (and Spork) now please go picking your fights somewhere else ?
Thanks in advance,

Hans

It seems from this that some are against advancing human knowledge! Seemingly they would prefer to wallow in ignorance rather than learn the truth.

ThinAirDesigns

07-05-2010, 07:17 PM

Going through the data from the 18 recording sensors that were used for the test it's becoming more and more likely that NALSA has the data to ratify a record of more than 3x the speed of the wind -- perhaps as high as ~3.5x.

JB

Munter

07-05-2010, 10:22 PM

Congratulations - an impressive and well documented exercise.

spork

07-05-2010, 10:33 PM

Congratulations - an impressive and well documented exercise.

Thank you. It's become a welcome and unexpected surprise to not be accused of perpetrating a hoax, holding back information, and hand waving.

Munter

07-05-2010, 10:39 PM

btw - are you the spork who used to frequent kiteforum?
(or is there more than 1 spork?)

spork

07-06-2010, 01:56 AM

btw - are you the spork who used to frequent kiteforum?
(or is there more than 1 spork?)

I am an avid kitesurfer and used to be much more active on Kiteforum. That's one of the two sites I originally posted this brain-teaser on years ago - long before building any models (much less the manned cart). Do I know you from there?

spork

07-08-2010, 10:03 AM

WIRED posted an update on our project: http://www.wired.com/autopia/2010/07/downwind-faster-than-the-wind-possible-and-theyll-prove-it/#respond
It sounds like they plan to publish another article when last weekend's record runs become official.

ThinAirDesigns

07-09-2010, 08:31 PM

Here is a very well shot video of our very first run in El Mirage.

Renown Brit Richard Jenkins, the builder and pilot of the world speed record holding Greenbird found out about our trip to El Mirage and flew down to watch the proceedings. He shot this excellent video sitting in the back window of the chase vehicle unbeknown to us. He just posted it today. He did such a great job showing the wind instruments switching directions just as the Blackbird blew through windspeed.

As one can see from the video, the Blackbird self-starts on this run and data shows it topping out at nearly 55mph. The vehicle was still accelerating but over the radio I asked the pilot to stop as this was nearly 10mph faster than any previous run and I was worried about the integrity of the vehicle. The multiple wasn't quite one of our best as the wind was in the upper range (~20mph). We set our best multiples in a bit less wind because we could let the bird run free without fear of breakage.

http://www.youtube.com/watch?v=5CcgmpBGSCI&feature=youtube_gdata

5CcgmpBGSCI

wither

07-15-2010, 04:05 PM

Yes and no. There is a formula for a theoretical max velocity for a foil with no other losses:

Vs/Vt = sin(gamma) * (L/D) - cos(gamma)

for which the maximum is

Vs/Vt = SQRT(1+(L/D)^2)

at gamma = 90 + arccot(L/D) but of course with the cart prop the equivalents for "velocity of the foil" and gamma are interesting if not problematic.

Ummm...is there a mistake here? Would not

Vs/Vt = sin(gamma) * (L/D) + cos(gamma), where gamma is the angle off the downwind course?

sirclicksalot

07-17-2010, 03:01 AM

Ummm...is there a mistake here? Would not

Vs/Vt = sin(gamma) * (L/D) + cos(gamma), where gamma is the angle off the downwind course?

Oops, did I forget to define my terms?

Per Marchaj and others, gamma is the course angle off of upwind, so Marchaj's formula is equivalent to yours as you have gamma defined as angle off of downwind. The convention chosen is apparent in the gamma where the maximum occurs (90 + arccot(L/D)) which means max speed gamma is greater than 90, i.e. as it should be for a broad reach.

A.T.

07-18-2010, 04:59 PM

I think it's really cool how the internet has connected all these disconnected dots. Very different people at different times coming up with these ideas and none of them knowing about the first guy who built one.

Found some related old stuff:

Bernhard Schmidt (http://en.wikipedia.org/wiki/Bernhard_Schmidt) tired to patent a Windmill driven upwind boat (Gegenwindschiff) in 1925. Patent was denied due to lack of novelty. Ironically even decades later the author of Schmidt's biography Barbara Dufner thinks that "it is physically impossible to go directly upwind with an wind-powered vehicle":
http://books.google.com/books?id=Qie9-uBPPi4C&pg=PA128&lpg=PA128&dq=gegenwindschiff&source=bl&ots=xySwS3R7IR&sig=IrUJX46V2F0S_Ij0NynlR8N4uf8&hl=en&ei=Jb5BTJeCIIibOIS1zY8G&sa=X&oi=book_result&ct=result&resnum=1&ved=0CBQQ6AEwAA#v=onepage&q=gegenwindschiff&f=false (German text)

Andy Ruina wrote a Paper about DUW & DDWFTTW in 1978. He was reacting to a paper by Blackford, who claimed that even under ideal conditions you can achieve only twice the windspeed going directly upwind. The following PDF contains both papers and a very silly review(first of the two) of Ruina's paper, criticizing him for using simple mechanical analogies:
http://ruina.tam.cornell.edu/research/topics/miscellaneous/push-me_pull-you.pdf

Windmaster

07-18-2010, 08:59 PM

Found some related old stuff:

Bernhard Schmidt (http://en.wikipedia.org/wiki/Bernhard_Schmidt) tired to patent a Windmill driven upwind boat (Gegenwindschiff) in 1925. Patent was denied due to lack of novelty. Ironically even decades later the author of Schmidt's biography Barbara Dufner thinks that "it is physically impossible to go directly upwind with an wind-powered vehicle":

I'm not sure if Bernhard Schmidt was unlucky or that in those days the patent authorities were more vigilant, but in 1995 I was AWARDED a patent for a Windmill driven upwind boat.
GB2286570 (A)
See http://www.sailwings.net/rotaryhome.html

spork

07-18-2010, 11:07 PM

I'm not sure if Bernhard Schmidt was unlucky or that in those days the patent authorities were more vigilant, but in 1995 I was AWARDED a patent for a Windmill driven upwind boat.
GB2286570 (A)
See http://www.sailwings.net/rotaryhome.html

Very interesting. I'm glad you succeeded in getting the patent, but I would think that technically it would be public domain after the first applicant was denied.

Windmaster

07-19-2010, 03:09 AM

Very interesting. I'm glad you succeeded in getting the patent, but I would think that technically it would be public domain after the first applicant was denied.

Not quite so simple. Different patent offices.
Also I think my patent was on the overall layout of the design rather than the concept itself.
They cited some previous examples during examination, some, of which I believe were awarded patents.
I would recommend searching online on the patent database http://ep.espacenet.com/ you will find quite a few, most of them are vertical axis designs, although I have never heard of one of these that can successfully go against the wind.

A.T.

07-21-2010, 10:18 AM

Even more related old stuff. I mean really old (year 1335):

http://www.notechmagazine.com/2009/07/guido-vigevanos-wind-car-1335.html

http://krisdedecker.typepad.com/.a/6a00e0099229e88833011571faebab970b-pi

spork

07-21-2010, 10:28 AM

Even more related old stuff. I mean really old (year 1335):

http://www.notechmagazine.com/2009/07/guido-vigevanos-wind-car-1335.html
Luckily for Guido Vigevano there was no internet in those days.

A.T.

07-26-2010, 06:23 AM

Looks like someone will be attempting to break the Greenbird record next spring:

http://www4.lehigh.edu/news/newsarticle.aspx?Channel=%2FChannels%2FNews%3A+2010&WorkflowItemID=91ea9329-8178-49e8-b261-84b150ae3711

This could set the bar higher for the DDW-team, if they want to go after this record too.

A.T.

07-29-2010, 04:49 PM

From http://www.nalsa.org/:
Get out your slide rules and physics text books...On July 2, 2010 on El Mirage Dry Lake, Blackbird sailed directly down wind at a speed of 27.7 mph in a 10 mph wind to set a first record for the ratio of Boat Speed to true wind speed of 2.8. BlackBird was designed and built by the Thin Air Designs team (Rick Cavallaro and John Borton) and sailed by Rick. Links to follow soon.

http://www.nalsa.org/images/blackbirdbig.jpg

kerosene

07-30-2010, 02:41 PM

Congratulations!.

ThinAirDesigns

08-02-2010, 07:13 PM

NALSA DDWFTTW ratification reports are now up: www.nalsa.org

And a great article by Kimball Livingston, an editor at Sail Magazine:
http://kimballlivingston.com/?p=3971

Windmaster

08-02-2010, 11:29 PM

NALSA DDWFTTW ratification reports are now up: www.nalsa.org

And a great article by Kimball Livingston, an editor at Sail Magazine:
http://kimballlivingston.com/?p=3971

Hi
Nowhere in the above article or anywhere on the Faster than the Wind website can I see mention of Jack Goodman - a member of the Amateur Yacht Research Society who, with his downwind faster than the wind model (on youtube here: http://www.youtube.com/watch?v=aJpdWHFqHm0) showed that it was possible.

I don't think Jack was the originator of the idea (which has been talked about on AYRS channels for many years) but he was the first to build a model, film it, and demonstrate that it worked.

I am willing to be corrected on this if I have got the timescale wrong, but it seems a shame that in all the ballyhoo, the first demonstrator of this concept should be forgotten and not given any mention.

ThinAirDesigns

08-02-2010, 11:55 PM

Hi
Nowhere in the above article or anywhere on the Faster than the Wind website can I see mention of Jack Goodman - a member of the Amateur Yacht Research Society who, with his downwind faster than the wind model (on youtube here: http://www.youtube.com/watch?v=aJpdWHFqHm0) showed that it was possible.

I have no control of the article, but Jack is featured prominently on the faster than the wind website -- check out the very third post:

http://www.fasterthanthewind.org/2009/10/machinist-and-internet-hoax_03.html

I don't think Jack was the originator of the idea (which has been talked about on AYRS channels for many years) but he was the first to build a model, film it, and demonstrate that it worked.

You are correct, Jack was not the originator, nor the first to build one (see the very first post on our project blog)

By all accounts so far it was first done in the the '60s by a team of engineers from Douglas Aircraft, but even they weren't the ones who came up with the idea and as they did it to settle a bet between friends, they didn't document their exploits in a way that satisfied very many critics.

AMO Smith (google him), the Supervisor of Aerodynamics Research and Chief Aerodynamics Engineer at Douglas and one of his wind tunnel engineers, Dr. Andrew Bauer discovered the idea in a paper presented by a midwestern student who was was angling for a summer internship.

Unfortunately, no one remembers who the student was (they didn't get the intern position) so credit for the actual invention will apparently remain nebulous for all time.

Bauer believed the student was correct and AMO believed DDWFTTW to be impossible. They made a bet and Bauer assembled a small team, built and sucessfully tested the device. AMO paid off on his bet and by all acounts it was near 40 years before anyone physically tried it again (Goodman).

http://3.bp.blogspot.com/_XtHg_q76Tgs/SsUfmgmFJqI/AAAAAAAAABA/Bk6e13mXMQA/s400/Bauer.jpg

I am willing to be corrected on this if I have got the timescale wrong, but it seems a shame that in all the ballyhoo, the first demonstrator of this concept should be forgotten and not given any mention.

We give Jack HUGE credit everywhere we go and are in regular phone and email contact with him to this day. Check out the video that we did when we were trying to get this on MythBusters:

http://www.youtube.com/watch?v=_fBDcchw5nw

In history if we are remembered in any way with regard to DDWFTTW, it will be because we truly were the first group who documented it well enough to bring most rational people around. Before us most folks still called Goodman and Bauer crackpots and hoaxers -- now it's only the moonshot denier types who will never be convinced.

JB

spork

08-27-2010, 08:59 PM

New WIRED article:

http://www.wired.com/autopia/2010/08/ddwfttw/all/1

Windmaster

08-27-2010, 09:45 PM

spork

08-27-2010, 09:55 PM

You don't have to have a degree in maths or use any maths at all to see how it works.
It's really just common sense. The craft doesn't care if the ground is moving or the air is moving. (How could it know?) It's the relative movement between the two mediums that counts.
So it can make progress over the ground, driven by the wind, or it can make progress through the air driven by the ground.

I'd be interested to hear from anyone what part of the above statement they don't understand.

There is nothing amazing about it. The most amazing thing is that so many people can't understand it!

I think most people feel things should act more like a leaf blowing in the wind - but dragging on the ground. It will go a little slower than the wind over the ground, and a lot slower than the ground under the wind. Because our cart goes about 3X as fast as the wind, they claim it violates the laws of conservation of energy and momentum - but they never explain why that would be.

zawy

09-04-2010, 12:29 PM

This post explains the exact difference between powering a DDWFTTW vehicle with wind or treadmill. The "executive summary" is that there is no difference if there are no friction losses in the wind-mode. The energy from the treadmill motor is a gift to the system that overcomes some of the friction loses that would otherwise penalize the vehicle in wind-driven use.

For Vc>Vw, Dr. Bauer and Dr. Drela equations are perfectly accurate only for the treadmill. The treadmill has the following distinct acceleration advantage over the wind-car changing by the factor: (following Bauer's notation in eq 13):

[L(n-1)+L^2)]/[L(n-1)+(v1^2-v2^2)/Vw^2]

n = Vc/Vw
L= v/Vw
v = v1 + v2
v = velocity added to air mass by the prop on treadmill
v1 = same as v except for wind-machine
v2 = reduced velocity of air mass on the other side of prop for wind-machine

In the wind machine, v2 is strictly determined by friction losses in the system: air drag, rolling resistance, and transmission losses. v1 is the only part in the wind machine that adds to vehicle acceleration. Treadmill gets to use the larger v to work totally for acceleration. In the ideal wind-machine v2=0 and it is equal to the treadmill.

This makes sense because the treadmill is adding energy to the system while a wind machine has to extract it from the difference between the air and ground. Treadmill is adding energy above and beyond the difference engine. Energy and power are come entirely from the wheels on the treadmill, and entirely from the wind on the wind-machine.

To explain it intuitively: imagine wind car moving 20 mph with no wind. Naturally you use the wheels to turn the prop at a rate so that there is neither drag nor propulsion so that there is overt resistance in wheels or prop and it rolls as far as possible. Then God adds 10 mph tail wind. The prop catches the wind like a sail which adds a force to overcome friction and to accelerate. The part of the force that accelerates the car is immediately transferred to the wheels which immediately accelerates the prop rotation which causes v1 out the back of the vehicle, just like v. Whatever force was lost as friction did not get to cause this chain of events and only increases v1 (slows down the air mass in front of the prop). After working through Dr Bauer and Dr Drela's equations very carefully, I came up with the ratio above.

spork

09-04-2010, 12:54 PM

This post explains the exact difference between powering a DDWFTTW vehicle with wind or treadmill.

I can sum this up more succinctly. There is no difference.

zawy

09-04-2010, 01:11 PM

It's unfortunate you think an electrically-powered wheel is the same as a wind-powered prop. Energy flows into the prop instead of into the wheel which causes a slight difference. If you were able to do the physics and reviewed the literature, you could see the difference.

spork

09-04-2010, 01:36 PM

It's unfortunate you think an electrically-powered wheel is the same as a wind-powered prop.

It's unfortunate that you don't see what Galileo, Newton, and Einstein saw - that all inertial reference frames are equal. And that it doesn't (and can't) make any difference what caused or causes the difference in velocity between wind and surface - that's all the cart experiences after all.

Energy flows into the prop instead of into the wheel which causes a slight difference.

If you understood that energy is all about bookkeeping and that you could choose either reference frame for either case, you wouldn't make such silly statements.

If you were able to do the physics and reviewed the literature, you could see the difference.

I did the physics and WROTE the literature. There is no difference. Let me know if you want to understand it too. I'll be happy to explain it.

zawy

09-04-2010, 01:43 PM

The treadmill motor adds kinetic energy to the air mass. When operating in the wind, kinetic energy is taken out of the air mass. Einstein believed in thermodynamics, too. The direction of energy and power flow is not relative except with respect to time, and only then if it is completely reversible, but in this case it is not. As I mentioned, the 2 cases are exactly equal if there is no friction, so that then a relative point of view can be applied.

kerosene

09-04-2010, 01:57 PM

Nope. "The treadmill motor adds kinetic energy to the air mass." -this is not very clear but I think I know what you are after.

regardless - such thing as kinetic energy, speed etc. are all relative to reference point. You have no potential energy sitting on flat ground as you cannot fall anywhere. However if I dug up a 10 feet deep hole next to you - you would have potential energy that I could put in use by pushing you into the hole. All that changed was the reference point of "zero" height.

Likewise in both cases - cart on treadmill in still air or cart in still ground in moving air - the cart is slowing down the air in relation the ground.
Yes in treadmill case it accelerates it in relation to the room maybe - but "room" is absolutely irrelevant for the cart.

Only what the cart experiences is relevant.

Nature creating wind or electric motor moving the ground makes no diff. Both are external forces that obviously create the condition we can harvest the speed from. But the cart doesn't care which one is moving in relation to the spectator.

edits: typos

ThinAirDesigns

09-04-2010, 02:00 PM

zawy

09-04-2010, 02:07 PM

The direction of energy and power flow is relative with respect to time *only* if it is completely reversible. If there is friction anywhere in the system then it is not reversible. As I mentioned, the 2 cases are exactly equal if there is no friction, which is when treadmill applies exactly to the case of the wind.

kerosene

09-04-2010, 02:24 PM

Friction where? In the mechanism of the cart?

zawy

09-04-2010, 03:22 PM

Friction anywhere. They are both penalized by any friction, but to different degrees based on a difference in the pressure profile across the prop. There is not much if any difference in total pressure drop (v=v1+v2). But some is on the downwind side of the prop (v2), so it changes the net power calculation by Dr Drela's Jan 2, 2009 derivation (eq 3) where his dW/2 is Dr Bauer's v.

In Dr Bauer's eq 13, there is an s=+/-1 for the "windmill" (sail) and propeller modes (basically two different equations). With the correction, it becomes a continuous equation for a smooth transition from Vc=0 to Vc>>Vw. The treadmill derivations are not valid for Vc<Vw, but my correction combines the derivations. Dr Drela's work, as he states, is only good for Vc>Vw. To correct Dr Bauer's eq 13 for a smooth equation at all speeds:

Change
[L(n-1)+L^2)]
to
[L(|n-1|)+(v1^2-v2^2)/Vw^2]

and take out the s. v1=0 for sail mode Vc<Vw-v2. I believe it is at about Vc=Vw-v2 that v1 starts to increase and v2 starts to decrease.

There may also be an implied divide by zero in Dr Bauer's work at the transition point that this fixes.

I am not happy that the || is still in there as a carry-over from Dr Bauer's s=+/-1. I would like to make sure I am starting the physics at Vc=0 and correctly deriving Vc>Vw from there without assuming endpoint conditions. Declaring a change in pressure on only one side of the prop is the historical error.

Windmaster

09-04-2010, 03:46 PM

zawy

09-04-2010, 05:50 PM

My first post has the easy intuitive explanation at the bottom. I am typing only for those who are serious about about getting the perfect prop and prop speed.

I would like to see people stop saying the energy comes from the ground. We don't say wind farms get their energy from the ground. It's terminology that obscures how it works. It's possible to derive it from the viewpoint of the power leaving the ground, but people unknowingly use the air mass entering the prop as their stationary frame of reference rather than the air mass on the sides of the car. This is the source of the error. The thermodynamic problem I mentioned before can be resolved if the kinetic energy decrease downwind of the prop is accounted for. But it's hard to see that decrease when you think the ground is accelerating the vehicle into the air mass. This is in regard to derivations based on Earth. In regard to real treadmills, there is a tire friction that the treadmill motor overcomes, but it doesn't cause a pressure drop downwind of the prop. In a wind machine, energy is taken out of the air and a pressure drop occurs across the prop. There is also no air resistance on the treadmill, and yet this causes more pressure drop in real air.

If you don't like my assertion that it proves the treadmill is not exact, then you can still use the equation and assert that the vehicle is 100% a sail at slow speeds and 100% a prop at high speeds You can do this by letting v1 completely replace v2 at high speeds and vice versa at low speeds. Then you just have Bauer's 2 equations. But the equation allows it to be part propeller and part sail at anytime.

I did not look at Bauer and try to decide how to modify his equations to fit my beliefs. I had a belief and I used Drela's derivation to see where it would take me. I plugged the result into Bauer and it came out exactly equal to Brauer for the endpoints (all sail or all propeller). Therefore I probably did not make a math error and it adds support to my initial belief. My initial belief was that all the energy comes from the wind. After first seeing the video on Sunday and realizing it was not a hoax, i developed a model in my mind of how it should work. Luckily I did not see the treadmill example, and when I did it gave me fits as to how to explain the treadmill was the "same".

If *half* the pressure drop across the blade is due to friction losses (v2=v1), then the net force available for acceleration and overcoming the friction is about 30% less for the wind-machine verses traditional derivations (L=0.4, eff=90% for n=2 to 3.) To clarify, I am not saying this drop is due to more friction or that there is a greater pressure drop, but that when the same bulk data is plugged into the different equations, there is a noticeable difference by using the more precise equation.

The importance for anyone serious about this stuff is that if affects the choice of prop. Firstly, the intuitive view that led to this equation and that I expressed in my first post indicates that the prop should have a higher angle of attack closer to the axis in proportion to radius. This might be obvious to prop guys. Secondly, and more directly the result of the equation, is that the prop should be designed with an efficiency that matches the expected friction. In other words, selecting the prop should include the knowledge that v2 is not zero in order to get the highest efficiency out of it. Thirdly it must be realized that the prop is deriving all energy by acting like a sail at all velocities. Fourthly and most importantly the proper speed of the prop throughout its length should be such that no matter how fast the car is moving, that if there is zero wind relative to ground, then the prop should be turn that it provides no drag or thrust. In steady state this also means the wheels do not experience any force. When you push it in dead wind, it should go easily without resistance while the prop is turning.

Most of this is already pretty accurately included in the methods of Bauer. But the equation needs to be changed if you want to be precise, and the intuitive view I expressed before may be invaluable.

ThinAirDesigns

09-04-2010, 06:08 PM

Again, no.

JB

zawy

09-04-2010, 06:30 PM

You have not pointed out an error in my comments except to express your opinion. Do you have any writings on the subject I can critique for accuracy? The only thing I see on your website appears to be the discussion below. It didn't go into detail so it didn't make an error. My discussion applies when there is enough detail to derive the acceleration force at all speeds based on prop specifics.

https://docs.google.com/View?docID=0AdRsKX7aaZTPZGRnbjhkajdfMTY0aGRzNWtnaGM&revision=_latest&hgd=1

ThinAirDesigns

09-04-2010, 06:47 PM

You're arguing with Galileo, Newton and Einstein. We decided not to cover their work on our site -- it's old news.

High school physics text books are what you need.

JB

zawy

09-04-2010, 08:32 PM

How did you guys get 2.8 in 10 mph when Dr Bauer and Dr Drela's power analysis indicates only 18 pounds of thrust? I used the data below. Your estimated resistance at only 20 mph was 36 pounds.

17.5' prop
Electric generator efficiency: 85%
Electric motor efficiency: 85%
Propeller efficiency: 85%
Coefficient of rolling resistance: 0.02
Vehicle gross weight: 650 lbs
Coefficient of aerodynamic drag: 0.3
Projected frontal area: 20 sq-ft

kerosene

09-04-2010, 08:36 PM

electric generator? I thought blackbird was geared with a twisted chain - did I miss a major update?

17.5' prop
Electric generator efficiency: 85%
Electric motor efficiency: 85%
Propeller efficiency: 85%

ThinAirDesigns

09-04-2010, 08:38 PM

Electric generator efficiency: 85%
Electric motor efficiency: 85%

Replace the 72% efficiency of your above listed power transmission method with one that is well in to the 90s% and see what happens with your numbers.

There is no generator or motor on the Blackbird.

JB

zawy

09-04-2010, 09:20 PM

Now I see that I included a penalty of 20 pounds retarding force from the wheels that I shouldn't have. You should take a closer look at my equation. My predicted max force for your system came out to be exactly equal to your rolling resistance, 13 pounds. The method presented by Dr Bauer and Dr Drela predicted 18 pounds was the max.

To calculate theoretical max speed, you plug in prop eff (eff), wind speed (Vw), prop radius (R), and then keep plugging in "n" (Vc/Vw) and L (0.1->0.9) until you find the force that equals your total resistance. L is v/(2*Vw) where v is how much the prop sped up the air. This comes mostly from Bauer's paper that Wired I believed linked to, which uses the same power analysis as Dr Drela. It's also derivable from the eff calc on your web site.

pounds force = Q*[1-(n-1+L)/(n*eff)]*(L*n-L+L^2)
Q=2*rho*(pi*R^2)*Vw^2 = 0.0146*(R*Vw)^2

Change the last +L^2 to -L^2 to get my max speed correction to Dr Bauers et al work. I used rho units so that things are in ft, sec, lbs force. But the equation is good for S.I. units.

So Bauer gets 17.8 pounds and my max is 13 pounds for 2.8*Vw with 10 mph. The eq is different for slower speeds.

Bauer's paper:
http://projects.m-qp-m.us/donkeypuss/wp-content/uploads/2009/06/Bauer-Faster-Than-The-Wind-The-Ancient-Interface.pdf

zawy

09-04-2010, 09:30 PM

Can I get paid for this research? Your current set up should be able to get you up to 4x if you can get your L down to 0.25 in a 15 mph wind. Unless my equation is wrong, you should have no trouble getting 3.5 in a 13 mph wind. alternatively, you would have to increase prop diameter to 20. 13 mph and 20' is another way to get 4x. 5x is possible in 15 mph if you can get eff up to 0.88. 5.5 if you also increase prop to 20'. Efficiency is the biggest bang for your buck, but I think your equation already tells you that. What is the theoretical max for all designs? If you get a 30' prop in 20 mph with 90% efficiency then you can get 7x. Anything more than that seems difficult.

At 85% eff, a 5' prop on an airplane going downwind with a really small battery in 5 mph wind should go 15 mph with no problem. It seems like it should be a fruitful area.

zawy

09-04-2010, 10:49 PM

Dr Drela and Dr Bauer predicted blackbird's max speed was 3.4x. My correction to the equation gave the measured 2.8x. I derived it from insisting that all the energy comes from the wind. I had an initial insight that prevented me from taking prior derivations at face value.

Here is a repeat of understanding how this phenomena works. You do a wheel-to-prop gearing ratio that causes zero air resistance and zero wheel resistance when you push it in zero wind conditions. The only resistance is gear losses and acceleration of the prop which is gained back when the cart slows down. It's a wonderful little cart with little resistance. Push it at 20 mph. Everything's great, but it starts slowing down a little from normal rolling, gear, and prop efficiency friction. Now add a 10 mph tail wind. The wind thinks it is a stationary sail, so you first look at it like a SAIL no matter how fast the cart is moving. Whatever SAIL energy is lost to ANY friction is translated to a pressure drop in front of the prop (downwind side). But the *acceleration* force (non-friction) from this SAIL is transferred to the wheels which causes the prop to *immediately* accelerate which causes a pressure drop in the back. A pressure drop in the back means it has converted sail energy into prop energy. So you use sail equations for the friction losses and prop equations for the acceleration, but it initially was all SAIL energy. It's a sail boat at it's core. It's beautiful.

The primary purpose of the wheels are to tell the prop how fast to turn (or the attack angle) to achieve the above, not to provide energy. This could be done with an airplane facing downwind with a really small battery and tracking ground movement to achieve highest lift efficiency. The prop has a reverse windmill effect from the sail-action that causes a certain amount of force on the wheels. This is what prevents the speed from getting really high.

Here's another paper that treats max speed as if it were a propeller when max speed returns to being a being a fully sail case (friction losses are equal to pressure drop on sail side, not a pressure increase on the propeller side). The object is to design a sail that can act like a prop until it reaches full speed where you want it to be 100% sail again.
http://www.ewec2009proceedings.info/allfiles2/517_EWEC2009presentation.pdf

Windmaster

09-05-2010, 01:52 AM

Please set me straight on this. You seem to be saying that SAIL power is different than ROAD power. However, common sense would say that the power is the difference between the two mediums and it doesn't matter what power you call it - its the same power.

Can you therefore explain the difference between a sailboat sailing in still water in a 5mph wind, and the same boat sailing in water moving at 5mph in still air? It doesn't matter if you call it SAIL power or WATER power, isn't it the same thing?

zawy

09-05-2010, 03:25 AM

Case 1: A sail boat moving forward relative to the water at 2.5 mph with a 5 mph wind is taking energy from the air.

Case 2: A sail boat (pointing the same direction) moving backwards relative to the air at 2.5 mph is gaining energy from the water.

Notice the direction of energy flow depends on a velocity direction. If you want to say the velocity was in the same direction, then one of them has to be considered running backward in time. The direction of time determines direction of energy flow. So they are the same thing, but you have to be careful when you try to derive an equation from it. I think we should stick with the convention that the energy comes out of the softer medium so that the velocity is relative to the harder medium and *forward* motion in *forward* time.

Historically the equations for DDWFTTW have been derived for backwards time flow but they did not realize it. Said another way, they used the wrong frame of reference (downwind of the prop instead of *exactly* with the cart). Said another way, they did not realize there is still a sail component on the downwind side of the prop. This last error has its source in the belief that the energy is coming out of the ground in forward time. It's not, it's coming out of the ground only if you go backwards in time, or let the cart slow down (reverse acceleration).

At the max speed, if you place a positive pressure upwind of the prop, then you are declaring it to be a prop instead of a sail. This is the mistake researchers have been making. It's really 100% sail (v2=v, v1=0) at max speed with a negative pressure on the downwind side. So when someone declares it is a prop, his frame of reference quietly shifted from the cart to the downwind side of the prop which is moving at a slightly slower speed than the cart. So he thinks the cart can go faster than it can. Below max speed, you model the sail force as a mixture of upwind sail vacuum (v2) and downwind prop pressure (v1).

To clarify what happens at max speed: when there is no longer an acceleration, all the friction forces are canceling the sail forces. There is no longer a prop or gearing resistance force at the wheels. That force drops to zero as they get near max speed. Blackbird might be able to confirm this observation. At the same time, the pressure upwind of the prop shifts to become a negative pressure on the downwind side. This shift is what makes prior derivations less accurate.

L=0.28 seems ideal for a lot of velocities and eff=0.85 seems accurate, so I can simplify the above equation to determine max speed:
Rolling friction + air friction = Q*(0.30-0.05*Vc/Vw-0.30*Vw/Vc)

and solve for Vc. Or plug in a Vc and see how much extra sail force you have for acceleration.

Windmaster

09-05-2010, 03:32 AM

Case 1: A sail boat moving forward relative to the water at 2.5 mph with a 5 mph wind is taking energy from the air.

Case 2: A sail boat (pointing the same direction) moving backwards relative to the air at 2.5 mph is gaining energy from the water.

If you were in the sailboat, how would you know which medium you were gaining the energy from?

zawy

09-05-2010, 03:40 AM

Windmaster

09-05-2010, 03:54 AM

That's a good question because my answer seems to sum it all up best:

If you consider yourself to be traveling forward, the wind. Backward, the water. (for the particular set up I described)

In other words, the frame of reference you consider stationary determines where energy is coming from. Choosing the harder medium as the stationary one makes the most common sense.

Why do you have to choose? Why not admit there is just a difference in velocities. You seem to be making a whole lot of confusion for yourself by insisting on looking at it from some arbitrary fixed point of reference. Energy doesn't "come" from anywhere, it is just a difference in velocities between two mediums. It's really simple leverage.

zawy

09-05-2010, 04:10 AM

Windmaster

09-05-2010, 04:35 AM

I'm not confused about anything. Yes, a leverage viewpoint might be OK. I haven't thought or read about that perspective. When I do the math, I use the car as the frame of reference. But when I write down car velocity, it is forward in time and relative to the ground. That requires energy to flow out of the wind.

Your answer seems to infer that the wind is some kind of reservoir of energy of itself.
But in fact, apart from the heat energy contained within it (molecular movement) it only has energy in relation to something that is moving at a different velocity. It has no energy OF ITSELF - only in relation to something else.
The very term "wind" means moving air. If it is not moving it is not "wind" only "air".
This is why the treadmill test is EXACTLY the same as the downwind demonstration. It doesn't matter whether the ground is stationary or the air is stationary - that's only to an outside observer.

zawy

09-05-2010, 04:50 AM

"it only has energy in relation to something that is moving at a different velocity"
That's what I said. I want to view the ground as stationary because I want to derive the velocity and acceleration of the car relative to ground. Requiring this viewpoint forces the view that energy comes from the wind.

There is a subtle difference in the treadmill that I explained before. The energy input from the electrical motor to the wheel is above and beyond the actual velocity difference between the treadmill and the air. That extra energy prevents a small pressure drop in front of the prop and it is 100% acting like a prop. A real wind machine can only operate off the difference between the ground and air speed which causes a small pressure drop proportional to the friction in the wheels, air resistance, and gearing. It does not have an extra energy that is above and beyond the velocity differences. It's a small but real effect.

ThinAirDesigns

09-05-2010, 10:00 AM

If you were in the sailboat, how would you know which medium you were gaining the energy from?

zawy -- did I miss the answer to this question?

JB

TeddyDiver

09-05-2010, 10:14 AM

I want to view the ground as stationary because I want to derive the velocity and acceleration of the car relative to ground. Requiring this viewpoint forces the view that energy comes from the wind.Why you wan't to do that? You force yourself to do the observation on a spinning object. Air is 50% of the time more stationary than earth ;)

zawy

09-05-2010, 10:50 AM

zawy -- did I miss the answer to this question?
JBYes, my answer was in post 390. Direction of energy/power flow depends on chosen frame of reference combined with chosen direction for the resulting velocity/acceleration. If I switch either reference frame or redefine direction of velocity, then direction of energy/power flow reverses. An exception is heat energy (such as friction in a wind machine) which always goes towards higher entropy.

My equation above says the blackbird should have been in 13 mph winds when you reached 3.5*Vw. Was that the observed wind speed?

ThinAirDesigns

09-05-2010, 11:05 AM

Yes, my answer was in post 390.

I don't believe that answers the question he was asking, but perhaps I misunderstood his question. As clarification, I'll ask a related question:

Two sailboats:

A: One on a lake, sailing into a 10knot wind coming from the south

B: One sailing down a river (calm wind day) with the river flowing south at 10knots.

How does the boat behave differently and how can you tell?

My equation above says the blackbird should have been in 13 mph winds when you reached 3.5*Vw. Was that the observed wind speed?

The Blackbird reaches such multiples at many different wind speeds, but other than the HP limits of our drivetrain, the higher the wind speed, the higher the multiple that can be achieved.

JB

zawy

09-05-2010, 11:16 AM

A: One on a lake, sailing into a 10knot wind coming from the south

B: One sailing down a river (calm wind day) with the river flowing south at 10knots.

How does the boat behave differently and how can you tell?
I can't think of a difference.

The Blackbird reaches such multiples at many different wind speeds, but other than the HP limits of our drivetrain, the higher the wind speed, the higher the multiple that can be achieved.

JBCan the blackbird get 3.5x in less than 11.50 mph? My equation says no. I'm checking it for accuracy.

ThinAirDesigns

09-05-2010, 11:35 AM

I can't think of a difference.

Ok, we agree. I likely misunderstood his question and thus your answer. My bad.

Can the blackbird get 3.5x in less than 11.50 mph? My equation says no. I'm checking it for accuracy.

It's a question I can't answer as we haven't tested down in the single digits much. As you know it would simply depend on all the system efficiencies.

I believe our prop is also well above the 85% that you quoted. That is a typical number for a GenAv prop and ours in comparison is turning far slower and accelerating the air far less dramatically. This will give us a nice increase in efficiency.

From our performance numbers we believe we are into the 90s% with both our prop and our transmission.

JB

ThinAirDesigns

09-05-2010, 11:38 AM

zawy

09-07-2010, 10:05 AM

If I am correct in saying there is a difference between the treadmill and the wind, then the difference in acceleration is not easy to explain with words. In math it is (K-L^2)/K where K is large and L is small, so the effect is small.

The treadmill is a continuing source of energy outside of the speed difference between the treadmill and wind. This is different than taking energy out of the wind. The difference I'm proposing is based on the conservation of energy. However, to the extent the wind is also a continuing source of energy, they are identical. Obviously to a very large extent the wind *is* a continuing source of energy to the cart, so I am saying they are almost identical. The difference I'm proposing is that there is a *local* effect (slight vacuum) at the front of the prop as a result of removing energy out of the wind *locally*, which does not happen when you apply an electrical motor to the wheels in still air.

The question of if the treadmill and wind have any difference is not important to me. I just want to get the right equation for acceleration at all car and wind speeds in a real situation. If it turns out this is exactly the same as on a treadmill, then great. But I don't want to start a derivation based on an electrical motor applied to the wheels in still air because I want to make sure I'm not violating the conservation of energy even at a local level (in front of the prop).

Bauer says chain efficiency is 98% to 99%. He also says sail efficiency is about 1/0.85 = 1.17%, which I don't understand. If I'm right in saying there is a sail effect at all speeds by creating a small vacuum in front of the prop, then it could cause a prop to get higher efficiency than expected.

kerosene

09-07-2010, 10:55 AM

would you care to explain what is the fundamental difference between prop and sail (besides the spinning). If you ask me they act the same way here. before the apparent wind swings around they act like bluff bodies (parachute) but as the apparent wind starts shifting there is airflow over the blade/sail and it creates lift.

http://www.youtube.com/watch?v=UGRFb8yNtBo

Direction of energy flow is not physics terminology. Really the boat/cart only cares about speeds relative to itself. There is no difference in static boat (in terms of ground) in still air in watercurrent or boat in movement in wind. As long as apparent water flow and airflow are the same in point of view of the boat.

zawy

09-07-2010, 11:34 AM

Sail has much higher efficiency, according to Bauer. At slow speeds the blades are turning, so are they not acting like a true prop (at least a little) as well as the strong effect of being a sail at that time? I am suggesting the reverse effect is occurring at high speeds and I want the equation for it. So far I have treated the small vacuum in front of the prop as if it was affecting a prop, not as if the was a true sail, despite my emphasis on calling it all sail-derived energy. If I treat it like a true sail, then it is going to increase the "prop" efficiency. Instead I have been using the same efficiency factor for sail component (v2) and prop component (v1).

If what I'm saying wrong and the simpler equations of Bauer and Drela are correct, then if prop efficiency times chain efficiency is 0.93*0.98 = 91%, then blackbird should have been experiencing 30 pounds of air and rolling resistance (10.1 mph and 2.77x). If JB thinks 91% is accurate and says "no way it was that much" to 30 pounds of resistance, then I have support for my modification. My modification says 20 pounds resistance at max speed. JB has the closest thing available to resolve which theory might be closest.

I think the objective of the blackbird should be to make the wind speed in its wake equal to zero. That would indicate it took 100% of the wind energy (relative to ground) out of the wind.

I agree. If the frame of reference is the cart, then energy is flowing in from the wind at Vground<Vw and from the wheels when Vground>Vw. If the frame of reference is the cart, the Vc is always zero.

ThinAirDesigns

09-07-2010, 12:52 PM

Zawy, another quick question so I can make sure I totally understand your position:

When comparing the treadmill test and the tests in the natural wind, are you insisting that in one or other of the scenarios, the spinning rotor is providing the force to turn the wheels?

JB

Zawy, I couldn't find the answer to the above question in your last couple of posts.

If your still considering the question, that's cool -- just didn't want it to get lost in the shuffle.

JB

zawy

09-07-2010, 01:42 PM

JB, I didn't know how to answer the question. Let me try. As my response shows, I do not consider the treadmill and wind cases to be significantly different, if they are different. In both cases, stating with tread mill (aka wind) starting out zero and slowly increase: air resistance causes a push on car to move (sail effect), which causes tires to move, which causes prop to turn (prop effect). Notice that no prop energy was available without the air resistance. (calling it air resistance instead of "wind" makes it applicable to both wind and treadmill). In addition to this, there is a windmill effect on the prop which tries to turn it against the wheels. It is small compared to the sail effect. The prop effect is always the result of the air resistance (sail effect) in both treadmill and wind. This is how it starts at slows speeds and I do not think the physics changes for high speeds.

So I think my answer is no. The wheels are turned exclusively by the sail effect at all speeds (forces through body of cart). You can view the sail effect as caused by wind running into the prop, or as the treadmill pulling the prop back into the air resistance.

ThinAirDesigns

09-07-2010, 01:49 PM

The wheels are turned exclusively by the sail effect at all speeds (forces through body of cart).

Thanks, that is correct. We have a 'tight side' and a 'loose side' on our chain drive. The slack takeup device is of course on the loose side. At no time and in no phase of operation does the 'loose side' come under tension from the rotor attempting to drive the wheels.

What I am aiming for with these questions is to find out what of your theory can be measured and how it would be measured.

If there is ANY difference between the treadmill and the natural wind scenario, one should be able to measure it. Could you describe a way to instrument and demonstrate this difference that you claim exists?

JB

zawy

09-07-2010, 02:07 PM

Yes, air velocity meters (or pressure meters) on both sides of prop, relative to side of cart or wheels (not relative to each other). If treadmill is different like I theorize, then "downwind" side of prop (what I call front of prop) will have higher velocity than in the case of wind. You would have to set the blackbird up on "tire spinners" in relatively still air.

zawy

09-07-2010, 06:36 PM

How was the wind measured during the trial? Based on the equations below for wind above a lake, the wind at the bottom of the propeller may have been 8.7 mph, 10.1 at the axis, and 10.7 at the top. The wind speed over the area of the prop is not linear. 10.1 mph at the axis may have averaged 9.5 over the area. A treadmill and especially "tire spinners" will have an advantage because of this, unless the size of the cart is small compared to the length and width of the treadmill. The "infinite" plane of the flats slows down the wind close to ground more than the finite plane of a treadmill. It seems like the bottom of the prop would need to be 5 meters above ground for the effect to be small.

http://sts.bwk.tue.nl/drivingrain/fjrvanmook2002/node7.htm#SECTION00311000000000000000

[edit: the 10.7 to 8.7 difference prevents the ability to set set the optimum attack angle of the blades (or the rotation speed if that is adjustable). ]

ThinAirDesigns

09-07-2010, 07:49 PM

How was the wind measured during the trial?

Nalsa rules require the wind to be measured at hub height. As you note from your link, this location places the measurement at a point above the point where the speed would be the average across the propeller.

The goal of course is to race the wind that you are using for propulsion and while the average speed of said wind would occur just a bit lower than the center of the propeller, we agreed that the disadvantage to the Blackbird would be slight to measure it at hub height and that point was easy to describe and explain and so it was.

You will note however that previously and on non-record runs we have placed streamers a full 6ft above the top of the propeller, racing wind that is WAY above and faster than the wind powering us and had no problem running multiples of this wind.

http://www.youtube.com/watch?v=EEuAqq8FINw
JB