DDWFTTW - Directly Downwind Faster Than The Wind

More dead horse beating

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:





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.

 

Bauer typo

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).

 

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.

 

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?

 

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.

 

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/phy...aster-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.

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.

You posted a diagram? Where is the link?

 

Faster than the wind is really simple,

sail an ICEBOAT.

 

Why not an F15?

 

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.

 

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

 

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 : )

 

System and prop L/Ds?

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.

 

Query 2) turbine buckets

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