Sailing downwind faster than the wind

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spork

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ThinAirDesigns":3p7xxuvs said:
Yes eyytee, I alway love the "it's got nothing to do with sailboats" display of ignorance. As that animation (and all the math and vector analysis) so easily shows, the only difference between the airfoil on the reaching boat and the airfoil on the cart is the diameter of the circle.

JB

Better still is the fact that sailboats is precisely how I came to the idea of a DDWFTTW vehicle. I wondered how I could take a sailboat tacking with downwind VMG greater than wind speed, and use that idea to go DIRECTLY downwind faster than windspeed. It fairly quickly occured to me to let the sail spiral in a continuous downwind circle. All that was left was to provide the kinematic constraint to make it follow that path. Gearing it to the wheels is a pretty obvious approach.

But I suppose it might look different to someone who knows all about physics. :roll:
 
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spork

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I wonder if origin figured out that maybe we're not the morons he claimed we are - and if he followed that thought to its logical conclusion?
 
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ThinAirDesigns

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I'd love for him to tell Drela that the airfoils on the cart have nothing whatsoever to do with the sails on a sailboat and see the response. LOL

JB
 
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wick07

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spork":2m1ktvzp said:
One of the main problems I'm having with the derivation is that you're mixing units. The thrust from the propeller will be a simple force. The "force" required to turn the prop will be a force at a specified distance (e.g. 10 ft-lbs).

True, I did do a little hand-waving here to keep it simple. Look at it from this perspective:

Wheels have "wheel-power" equal to 100% wheel-power units (what this unit is in the real world doesn't matter)
The generator can convert 100% "wheel-power" into 100% "gen-power" units (again, what this unit is doesn't matter) The prop can convert 100% "gen-power" to 100% thrust (once again, the unit is immaterial)

I was able to ignore the units because in the ideal case it is the relationship between these things that is important. In other words, what "wheel-power" power is doesn't matter, since the relationship between that and "gen-power" and thrust will be linear. If 100% "wheel-power" created X thrust, then .5X thrust will take 50% "wheel-power". All my "math" is in thrust units (who cares what the units are since they cancel out in the comparison anyway).

I admit it is a sloppy way to do science, but this is a back-of-the-envelope calculation used to demonstrate a point, not give a numerical answer. But thanks for calling me on it, I should have listed my assumptions out up-front.

spork":2m1ktvzp said:
In this case "m" has to be "m-dot" or mass flow rate (i.e. mass/time).

Once again, my bad for a little hand-waiving. In both the cases I assumed "m" would not change. I was specifically trying to make everything a constant except the velocities. Is this a bad assumption? (I suspect it may be)

spork":2m1ktvzp said:
Again, F1 is force and power is force x velocity.

Once again it doesn't matter what F's units are, since I am comparing F to F. As long as I do not try to directly compare F to any other values other than F, I should be okay dropping units.

Assuming an ideal prop, the thrust created by the prop will be equivalent to the power used to turn the prop. No loss.

Sorry, I keep saying "equal to" and "equivalent" when I actually mean "proportional to". The bottom line is, if the prop is 100% efficient there should be no loss, 100%=100%.

We can't look at this as the prop "blocking" the tailwind. The prop is immersed in the fluid and operating in the normal way a prop does in air.

I was thinking about this last night, because I wasn't satisfied with my own reasoning. The tailwind must have a effect on the props ability to create thrust. There are only 2 forces (simplified) acting on the cart. A retarding force, and an accelerating force. The retarding force consists drag, internal friction in the system, and the power being taken by the generator to turn the prop (BTW, I am using "generator" as shorthand for "whatever is transferring power from the wheels to the prop be it mechanical or other"). The accelerating force can come from wind blowing directly on the cart, and thrust from the prop.

I am going to drop direct wind, since this is only going to be a player a <wind speed. This model specifically has to work above winds speed, so direct wind shouldn't play a part. Also I am going to drop internal friction & drag, because our system is ideal.

So, now our only forces acting on the system are power taken by the generator, and thrust created by the prop. Power taken by the generator must be constant at a given speed, so I can't change that. Therefore, somehow the tailwind is increasing thrust created by the prop. Which brings me back to the thrust equation.

spork":2m1ktvzp said:
With a tailwind, the prop is operating closer to the static case (think of a plane sitting still on the runway and revving its engine). This gives a greater AOA (angle of attack) of the blades on the air vs. blades trying to get a bite into air that's already flowing rapidly through the prop disk. Thus greater thrust.

The second way to look at it is this... power out = thrust x free_stream_velocity. The power out can never be more than the power in. With a tail-wind, the free_stream_velocity is the vehicle_velocity - the speed of the tail wind. So a given power can give me more thrust in this case.

That is what I was getting at. But I was breaking the first rule of physics: "Never derive in public."

There should be a way to get that result from the thrust equation (which is where I think it ultimately comes from), and it will probably involve breaking each velocity into it's component winds. There will be true wind, and wind induced by the motion of the card. Together these will combine to make a relative wind, but that relative wind will obviously change depending on the tailwind. This is where I predict we will find our answer.

spork":2m1ktvzp said:
The power available however, has nothing to due with true or relative winds. The power available is related to the speed of the cart (which hasn't changed), not the wind speed.

Yes, the power available is equal to the speed of the cart times the retarding force on the wheels from "turning the generator".

If I have increased thrust because of a tailwind, that means I need less input power to the prop to achieve the same thrust as was achieved in the NO TAILWIND case. But I still have the same power available.

Exactly!

This means I have excess power in the TAILWIND case! I will end up accelerating!

Right again.

My top speed will now be limited by my ability to utilize this excess power. The more efficient I make my machine, the higher the possible speed. This has become an engineering problem. Note that this is not perpetual motion.

Right on all counts.

We both agree that this is what the real world case is (or at least I am taking your word on it). I am just trying to figure out why it is the case. I think that was also origin's issue. Without a good "why" it is hard to accept.

spork":2m1ktvzp said:
As we get faster and faster, the tailwind effect will decrease.

I don't think this is true - but perhaps in a relative sense.

At some point the internal resistance of the system will overwhelm it and we will have reached an effective no-tailwind condition. At that point the system will be in equilibrium again.

This is right except for the "effective no-tailwind".
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I stated that poorly I guess. The tailwind is only one component of the relative winds experienced by the system. At some point, even in the theoretical "ideal" model, those other components will eventually overwhelm the effect of the tailwind. It will still be there and won't have decreased. But as you get going faster you will have diminishing returns until it become negligible. This is entirely wild conjecture on my part (just to be clear).
 
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ThinAirDesigns

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origin":1oryygn4 said:
ThinAirDesigns":1oryygn4 said:
origin":1oryygn4 said:

Some folk are slower at it than others.

JB

My thoughts exactly. :D

Well, it's good you at least recognize your slowness. Now if you can just apply that to a 1HP problem, you might get somewhere slowly.

JB
 
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origin

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Well, it's good you at least recognize your slowness. Now if you can just apply that to a 1HP problem, you might get somewhere slowly.

JB

Ha, that coming a guy that still can't intelligently explain how his little cart works after 2 years of trying... talk about slow. ;)
 
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ThinAirDesigns

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origin":102lriy0 said:
Ha, that coming a guy that still can't intelligently explain how his little cart works after 2 years of trying... talk about slow. ;)

It can't work -- it violates the most basic laws of physics ... remember? :lol: :lol: :lol:

By definition it's impossible to explain something in a convincing manner to someone who ignorantly insists it's not possible.

JB
 
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ThinAirDesigns

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BTW origin ... how is that education coming? -- have you been brought up to speed on basic High School physics yet? Are you now able to determine what it *actually* possible and not possible?

Let's keep our eye on the prize here ... you so far have claimed that something being done every day is impossible and defies the the most basic laws of physics. At some point (if your honest) you're going to have to admit that you were taking a wrong position and that you were arguing from ignorance. Once you make that step forwards, I suggest you take another look at the 1HP problem you are so carefully avoiding -- you'll find the error there is yours as well.

JB
 
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origin

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ThinAirDesigns":24yyy2p0 said:
origin":24yyy2p0 said:
Ha, that coming a guy that still can't intelligently explain how his little cart works after 2 years of trying... talk about slow. ;)

It can't work -- it violates the most basic laws of physics ... remember? :lol: :lol: :lol:

By definition it's impossible to explain something in a convincing manner to someone who flatly insists it's not posssible.

JB

The problem is you are clearly ignorant of physics so are unable to explain much of anything in a logical manner. Dr. Drela seems to explain things rather clearly.
 
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origin

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ThinAirDesigns":208k7ll1 said:
BTW origin ... how is that education coming? -- have you been brought up to speed on basic High School physics yet? Are you now able to determine what it *actually* possible and not possible?

Let's keep our eye on the prize here ... you so far have claimed that something being done every day is impossible and defies the the most basic laws of physics. At some point (if your honest) you're going to have to admit that you were taking a wrong position and that you were arguing from ignorance.

If anyone wants to explore the claim of the down wind contraptions, my advice is to look at some real physics in the Dr. Drelas papers and to avoid the inane ramplings of the 3 stooges.

Once you make that step forwards, I suggest you take another look at the 1HP problem you are so carefully avoiding -- you'll find the error there is yours as well.

JB

I can't decide if it is ignorance or dishonesty that has resulted in you ignoring what was clearly laid out in the original 2 hp scenario. Maybe it's both. ;)
 
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ThinAirDesigns

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origin":1ko3aq1a said:
he problem is you are clearly ignorant of physics so are unable to explain much of anything in a logical manner. Dr. Drela seems to explain things rather clearly.

So in other words, you were completely wrong, didn't understand the basics of high school physics and insisted on declaring DDWFTTW to be a violation of these simple laws -- and at the same time want to claim that those of us who insisted that no such violation existed are clearly ignorant of physics. :lol: :lol:

Good luck selling that hogwash.

JB
 
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ThinAirDesigns

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origin":5zucykvv said:
If anyone wants to explore the claim of the down wind contraptions, my advice is to look at some real physics in the Dr. Drelas papers and to avoid the inane ramplings of the 3 stooges.

If anyone wants to explore High School physics and algebra, my advice is to look for folks who can accurately execute them and avoid the inane ramblings of someone who claims for pages that the possible is flatly impossible

JB
 
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ThinAirDesigns

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origin":2vvl0p0r said:
I can't decide if it is ignorance or dishonesty that has resulted in you ignoring what was clearly laid out in the original 2 hp scenario. Maybe it's both. ;)

Unfortunately for you, there is no "2HP" scenario ... only a guy/gal (you) who *on their own* insists on doubling the 1HP actually required and continues to run away from walking through the problem step by step so he/she can see his/her mistake.

Just as a fun example of how wrong you are to double that HP ... if doubling it (and leaving the rest the same) were the correct application, ddwfttw would violate the laws of physics -- and as you now know ... it doesn't. No better evidence needed of your wrongness (again).

JB
 
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eyytee

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origin":2btit399 said:
If anyone wants to explore the claim of the down wind contraptions, my advice is to look at some real physics in the Dr. Drelas papers
This is quite a leap from:

origin":2btit399 said:
I looked at Drela's analysis and it is very strange to say the least.
I'm happy to hear you finally make some progress understanding the material.... after only 10 pages of arguing against Drela's conclusion.
 
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origin

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ThinAirDesigns":2kwj4m15 said:
origin":2kwj4m15 said:
I can't decide if it is ignorance or dishonesty that has resulted in you ignoring what was clearly laid out in the original 2 hp scenario. Maybe it's both. ;)

Unfortunately for you, there is no "2HP" scenario ... only a guy/gal (you) who *on their own* insists on doubling the 1HP actually required and continues to run away from walking through the problem step by step so he/she can see his/her mistake.

Just as a fun example of how wrong you are to double that HP ... if doubling it (and leaving the rest the same) were the correct application, ddwfttw would violate the laws of physics -- and as you now know ... it doesn't. No better evidence needed of your wrongness (again).

JB

Alright Moe, I will give you the benefit of the doubt - you're ignorant not dishonest.
 
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origin

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eyytee":3sciqeli said:
origin":3sciqeli said:
If anyone wants to explore the claim of the down wind contraptions, my advice is to look at some real physics in the Dr. Drelas papers
This is quite a leap from:

origin":3sciqeli said:
I looked at Drela's analysis and it is very strange to say the least.
I'm happy to hear you finally make some progress understanding the material.... after only 10 pages of arguing against Drela's conclusion.

I didn't spend 10 pages arguing over Dr. Drela's conclusion, Larry, I was arguing against you and the other 2 stooges convoluted, mubbling, stumbling attempts at explaining physics.

Where the heck is Curly? I haven't been called a troll all day! ;)
 
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ThinAirDesigns

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Here, we'll prove that you don't need any help at all to be a total idiot -- your first post before we explained *anything* to you.

origin":313rgriv said:
The problem with this particular vehicle is that the propeller turns when the following wind is faster than the vehicle.

Stupidly wrong (as you now know) -- the propeller turns *all* the time the vehicle is moving.

If there is no relative wind then the propeller will not rotate.

Stupidly wrong (as you now know) -- the propeller rotates just fine the entire time the vehicle is moving.

If the vehicle is faster than the wind then the propeller will try to spin in the opposite direction - it will be a drag on the vehicle.

Stupidly wrong (as you now know) -- the propeller is attempting to spin the same direction the entire time, above and below windspeed.

A properly positioned sail on a vehicle will beat the propellered contraption every time.

Stupidly wrong (as you now know) -- no matter how you position a traditional sail, it can't propell the vehicle up to wind speed, let alone above.

There you go -- you brought your total ignorance with you and still wear it proud.

JB
 
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spork

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origin started out wrong and insulting. It's hard to come back from that. Impossible if you're the sort that just can't admit you were wrong.
 
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RCP

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I'm curious, origin. What do you think is happening in the videos showing the Blackbird going downwind with the streamers going straight back into the wind? Also, the tests they ran at Ivanpah were observed by Bob Dill, a NALSA official who was involved in certifying the Greenbird's world speed record. Does it seem likely to you he could have fooled someone with his experience? And what do you have to say about the Michael C ruler video?
 
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ThinAirDesigns

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It funny as hell ... Origin is caught between a rock and a hard place:

A: If he openly admits that a vehicle like this actually goes or can go DDWFTTW, it's plain to see that he totally screwed up on the simple and basic physics principles involved and posted his own steaming pile of total wrongness from the beginning.

B: If he doesn't admit that a vehicle like this actually goes or can go DDWFTTW, he knows he's wrong and he's written enough to know that we know he knows he's wrong, but he doesn't get to blame his steaming pile of total wrongness on someone else.

Pick your poison dude -- right now you're just making yourself like an ignorant fool either way.

JB
 
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spork

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ThinAirDesigns":1f3u3wa7 said:
It funny as hell ... Origin is caught between a rock and a hard place:

A: If he openly admits that a vehicle like this actually goes or can go DDWFTTW, it's plain to see that he totally screwed up on the simple and basic physics principles involved and posted his own steaming pile of total wrongness from the beginning.

B: If he doesn't admit that a vehicle like this actually goes or can go DDWFTTW, he knows he's wrong and he's written enough to know that we know he knows he's wrong, but he doesn't get to blame his steaming pile of total wrongness on someone else.

Pick your poison dude -- right now you're just making yourself like an ignorant fool either way.

JB

You're not very familiar with trollage. All he has to do is say: "hey - it's a brainteaser - lots of people get it wrong at first. And to make it worse, you idiots were confusing the issue further with your completely messed up explanations".

I think we can count on that - despite the fact that our explanations are identical to what Drela later presented.
 
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