Sailing downwind faster than the wind

[spork]

I don't understand why it is so important that we all believe it is possible.

And I don't understand why it's so important for you to believe it's not possible. I presume it's simply because that was your initial intuitive reaction, and now no amount of logic or proof will be accepted.

 

[origin]

Lets look at the scenario again:

Sure - but let's use the actual scenario I presented, not the new one you made up.

Spork agreed that this is what he was trying to say.

No - spork said "That's not precisely my scenario, but it embodies the principle of my scenario, and yes - it's correct."

That in no way suggests that your scenario is "what I was trying to say". Listen origin, you're free to choose not to accept math, physics, or reality - but I prefer that you not put words in my mouth.

Have we shown that you can travel faster than the wind going directly down wind - clearly not, this scenario soesn't even directly address it.

Indeed THAT scenario does not directly address DDWFTTW. I presume this is precisely why you chose it over the scenario that I laid out quite clearly.

I certainly was not trying to put words in your mouth or change the scenario in a way that would invalidate your point. I do not see how your craft could work - but I do not want to try to introduce any 'tricks' to invalidate your original scenario. That is why I asked if what I was saying was correct - the only reason I did that was to try to take some of the confounding factors out of the equation. Apparently that changed the problem which I was specifically trying not to do.

Like I said your craft doesn't make sense to me - but I could be wrong there certainly may be some effect occuring that I am missing. I have no problem changing my mind if the facts warrant it. So let me go back and look at your precise scenario.

 

[origin]

Here is your original scenario:

Let's say our cart is moving along at 55 ft/sec (in the ballpark of 40 mph). And let's say it's taking 10 lbs of force at the road surface to turn the wheels. Without worrying just yet about the fact that this would slow our cart down, wouldn't you agree that I'm putting exactly 1 horsepower into the cart (550 ft-lbs/sec)? In other words, let's assume for now that I'm towing the thing behind my car. I could hook up a generator to the cart's wheels and that generator could put out 1 h.p. of electricity (if it were 100% efficient). And yes - that energy would have to come from my car towing the cart.

So we are towing the cart at 40 mph and it takes 1Hp to overcome the air drag and the friction from the wheels and bearings.
So now we hook up a 1 Hp generator to the wheels. This means that it will take an input 2 Hp to maintain a constant speed, because now we have air drag, friction from the wheels and CEMF from the generator.


Then you added

Can be pulled by my cart - or by an engine on the cart itself.
- When I pull that cart at 55 ft/sec, with a 10 lb retarding force on the tires, I can generate 1 H.P. by connecting a generator to those tires. Wind never comes into this in any way yet. I have no prop or windage. We can worry later about the coefficient of drag of the chassis.
- I can take the 0.6 H.P from the 1 H.P. generated by my generator to turn my prop to create 10 lbs of thrust in a 22.5 ft/sec wind. That thrust is just as good at the 10 lbs my car is pulling the cart with, so I can cut the tow line and I'm good to go.
- This leaves me with 0.4 excess H.P. to handle the real-world drag, losses, and innefficiencies.

So the above analysis has some problems:

If there is a tail wind of 20 mph then the effective head wind will be about 20 mph. The wheel friction won't change but my guess is most of the retarding force would be from the air drag so lets just cut in half the forces and say that the power needed to overcome that drag is .5 Hp.

So to maintain your speed you need to put 1.5 Hp into the cart. So if you take 1Hp from the generator with a prop effeciency of 85% the power developed from the propeller will be .85 Hp.

This will not be enough to overcome the 1.5 Hp of retarding force so the cart will slow.

 

[ThinAirDesigns]

Origin, would you mind telling me which of the following you disagree with:

A: A force of 10lbs, pulling forwards on our theoretical vehicle at 55ft sec, can be used to generate 1 H.P. at the drive axle (ignoring generation losses, etc. for the moment).

B: A vehicle traveling at 55ft sec, in a 22.5ft sec tailing will be experiencing a 22.5ft sec. headwind.

C: An 85% efficient propeller can turn 0.6 H.P. into 10lbs of thrust in a 22.5ft sec. headwind.

D: 1 > 0.6

Thanks

JB

 

[origin]

Origin, would you mind telling me which of the following you disagree with:

A: A force of 10lbs, pulling forwards on our theoretical vehicle at 55ft sec, can be used to generate 1 H.P. at the drive axle (ignoring generation losses, etc. for the moment).

B: A vehicle traveling at 55ft sec, in a 22.5ft sec tailing will be experiencing a 22.5ft sec. headwind.

C: An 85% efficient propeller can turn 0.6 H.P. into 10lbs of thrust in a 22.5ft sec. headwind.

D: 1 > 0.6

Thanks

JB

See the above discussion.

Edited to add: If you ignore real world losses you can do almost anything on paper!

 

[ThinAirDesigns]

See the above discussion.

You don't answer that question directly in the above discussion. Would you mind just telling me which of those (A, B, C, or D) you disagree with? It would take only a few seconds of your time.

Thanks

JB

 

[origin]

See the above discussion.

You don't answer those questions in the above discussion. Would you mind just telling me which of those (A, B, C, or D) you disagree with? It would take only a few seconds of your time.

Thanks

JB

If you ignore the generator losses which would account for 50% of the retarding force I guess that your 4 points are valid.

 

[ThinAirDesigns]

If you ignore the generator losses which would account for 50% of the retarding force I guess that your 4 points are valid.

Thank you.

Yes, since our 85% efficient prop produces 10lbs of thrust using 0.6 H.P and our wheels can produce 1.0 HP from the same 10lbs of thrust, we now have clearly demonstrated that in a tailwind, we can get more power from the wheels than the prop uses to produce the needed thrust.

Before moving on to the real world losses (transmission efficiency, rolling resistance and aero drag) I would stop right here and note that after the agreement we have on the above, the problem of DDWFTTW is no longer a "physics problem" -- that is we have just demonstrated through equation that no laws are violated. The problem is now simply an "engineering problem" or a "materials problem" -- if we can build a vehicle efficient enough, light enough and slippery enough to turn only 0.4 H.P to heat, we have a valid *2x* ddwfttw vehicle.

Agreed?

Thanks.

JB

 

[origin]

If you ignore the generator losses which would account for 50% of the retarding force I guess that your 4 points are valid.

Thank you.

Yes, since our 85% efficient prop produces 10lbs of thrust using 0.6 H.P and our wheels can produce 1.0 HP from the same 10lbs of thrust, we now have clearly demonstrated that in a tailwind, we can get more power from the wheels than the prop uses to produce the needed thrust.

Before moving on to the real world losses (transmission efficiency, rolling resistance and aero drag) I would stop right here and note that after the agreement we have on the above, the problem of DDWFTTW is no longer a "physics problem" -- that is we have just demonstrated through equation that no laws are violated. The problem is now simply an "engineering problem" or a "materials problem" -- if we can build a vehicle efficient enough, light enough and slippery enough to turn only 0.4 H.P to heat, we have a valid *2x* ddwfttw vehicle.

Agreed?

Thanks.

JB

Are joking? Of course I don't agree. You are not allowed to ignore parts of an equations to prove a physics problem.

Try this one. I have an electric car, so if I hook up a generator to the car I can produce electricity to run the car. So ignoring the generator losses the car can run forever without refueling. Tada - it is not a physics problem it is just an engineering problem.

Unbelievable. :roll:

 

[eyytee]

If you ignore the generator losses which would account for 50% of the retarding force I guess that your 4 points are valid.

If you have to assume a certain amount of loses to show that it cannot work, then you basically admit that it is not entirely impossible but rather a question of efficiency.

You are not allowed to ignore parts of an equations to prove a physics problem.

If you want to prove that it is entirely impossible then you have to show it assuming arbitrarily small but non-zero loses.

A direct mechanical gearing is far more efficient than an a generator + motor used here just as an example, But even with rather high losses of more than 50% you still could go faster than the wind with the right gearing, just not much faster.

I have an electric car, so if I hook up a generator to the car I can produce electricity to run the car. So ignoring the generator losses the car can run forever without refueling

Yes but even with the smallest loses it will not work. You don't need to assume 50% loss to show that.

 

[origin]

I was asked to look specifically at the scenario laid out by spork. I beleive I have done that. I would appreciate a comment on that.

 

[eyytee]

I was asked to look specifically at the scenario laid out by spork. I beleive I have done that.

The comment is: You can show any machine to fail by assuming high enough losses.

Drela uses realistic losses to show that DDWFTTW is easy to achieve with a wheeled vehicle, and experiments confirm this.

 

[spork]

I was asked to look specifically at the scenario laid out by spork. I beleive I have done that. I would appreciate a comment on that.

You did not do that. Once again you changed the scenario.

ETA: I'll tell you what... you give me the losses you think are reasonable. Then I'll give you the scenario in which I still go DDWFTTW with your parameters. For this we'll skip the generator and motor, and instead use a stricttly mechanical transmission between wheels and prop. So give me these:

Rolling resistance (our cart is about 0.01)
Transmission efficiency (I think we're about 95%)
Prop efficiency (I don't recall, but l'll assume ours is 85%)
Coefficient of drag of the overall vehicle not including the prop (a typical car is on the order of .35)
Projected frontal area without the prop (10 sq-ft?)

I included our rough numbers as a guide, but you should give me the parameters you think are reasonable. Then I'll lay out a scenario in which they're ALL accounted for, we'll be going directly downwind faster than the wind, and we'll be accelerating.

Fair enough?

 

[ThinAirDesigns]

Try this one. I have an electric car, so if I hook up a generator to the car I can produce electricity to run the car. So ignoring the generator losses the car can run forever without refueling. Tada - it is not a physics problem it is just an engineering problem.

Like we have done for you, please show us the numbers from your above vehicle which demonstrate that you can get more power from the generator than you need at the motor. Then and ONLY then will you have an engineering problem. Above all you have is a physics quandry.

You have agreed *explicitly* with the numbers which show that in a tailwind, the generator produces more power than the motor needs to move the vehicle. That excess in energy is the difference between an engineering problem and a physics problem.

Again, in a physics problem if you have *any* losses you are screwed. In an engineering problem you must only minimize them and can still afford to have some. In the presented scenario, the vehicle can afford 0.4HP in losses and STILL GO TWICE THE SPEED OF THE WIND.

JB

 

[ThinAirDesigns]

Rolling resistance (our cart is about 0.01)
Transmission efficiency (I think we're about 95%)
Prop efficiency (I don't recall, but l'll assume ours is 85%)
Coefficient of drag of the overall vehicle not including the prop (a typical car is on the order of .35)
Projected frontal area without the prop (10 sq-ft?)

Origin, I'm going to give you a little 'heads up' on this scenario from spork:

You can make the transmission efficiency 50% if you want (or any other numbers)
You can make the prop efficiency 50% if you want (or any other number)
You can make the Cd 1.0 if you want (or any other number)
You can make the frontal area 100sq-ft (or any other numbers)

Do all that and by simply calculating the drag on the wheels and the HP requirements of the prop spork can give you the wind speed and vehicle speed showing DDWFTTW. The above scenario might be going 100.01mph in a 100mph tailwind, but it will always be there.

JB

 

[origin]

I was asked to look specifically at the scenario laid out by spork. I beleive I have done that. I would appreciate a comment on that.

You did not do that. Once again you changed the scenario.

ETA: I'll tell you what... you give me the losses you think are reasonable. Then I'll give you the scenario in which I still go DDWFTTW with your parameters. For this we'll skip the generator and motor, and instead use a stricttly mechanical transmission between wheels and prop. So give me these:

Rolling resistance (our cart is about 0.01)
Transmission efficiency (I think we're about 95%)
Prop efficiency (I don't recall, but l'll assume ours is 85%)
Coefficient of drag of the overall vehicle not including the prop (a typical car is on the order of .35)
Projected frontal area without the prop (10 sq-ft?)

I included our rough numbers as a guide, but you should give me the parameters you think are reasonable. Then I'll lay out a scenario in which they're ALL accounted for, we'll be going directly downwind faster than the wind, and we'll be accelerating.

Fair enough?

Nah, we are just going round and round, 11 pages is plenty for me. My curiosity is peaked a bit, so I will take the time to look at the analysis of MIT guy.

 

[ThinAirDesigns]

Origin is the absolute classic case of "conclude first, and then ignore all numbers (and physical evidence) that threaten conclusion".

The math shows that in *every* tailwind scenario, more power can be generated at the wheels then is needed by the propeller. The more inefficient the vehicle, the higher the wind must be and the smaller the multiple above windspeed it will travel, but the numbers can show a positive delta every time.

Wind power works.

JB

 

[trumptor]

Now, if the vehicle is traveling faster than the wind, then, relative to the vehicle, the wind is blowing against it.

So if this is the case, the vehicle should be able to continue indefinitely, because even if you were to slow the wind to a stop, the vehicle would feel a wind against it because it is already in motion.

I could understand a burst of acceleration propelling the vehicle to a velocity greater than the windspeed temporarily, but not continuously.

 

[ThinAirDesigns]

Now, if the vehicle is traveling faster than the wind, then, relative to the vehicle, the wind is blowing against it.

Yes, it feels this relative headwind but it's not powered by this relative headwind. It's still powered by the relative motion between the air and the ground (wind). If the wind stops, the vehicle stops.

JB

 

[spork]

Nah, we are just going round and round, 11 pages is plenty for me. My curiosity is peaked a bit, so I will take the time to look at the analysis of MIT guy.

Yeah, that's what I suspected. We showed you that it's possible with a very simple scenario. So you decided to change the scenario. I then offered to run a scenario for whatever real-world numbers you care to use.

I don't think we're going in circles at all. I think you've finally noticed that you're in a corner - so you don't want to play anymore. No problem.

You can look at the analysis from the "MIT guy" if you like; but if you can't follow mine, you don't have a chance with his. You do however have the upside that he's not here to ask you how it's going.

 

[SubductionZone]

origin, you threatened to look at Drela's paper a week ago. Once again, if you do not understand the explanations that have been given to you in several ways here you will never understand Drela's paper. And one more time, you keep asking "Where does the cart get its energy from?" All you have to do is to analyze what happens to the energy of the air it interacts with relative to the ground. The cart leaves behind a trail of air that is moving slower relative to the ground than it was before the cart ran through it, where do you think all of that energy went?

 

[spork]

origin, you threatened to look at Drela's paper a week ago. Once again, if you do not understand the explanations that have been given to you in several ways here you will never understand Drela's paper. And one more time, you keep asking "Where does the cart get its energy from?" All you have to do is to analyze what happens to the energy of the air it interacts with relative to the ground. The cart leaves behind a trail of air that is moving slower relative to the ground than it was before the cart ran through it, where do you think all of that energy went?

You're beating a horse that's left the building. The threat to study "that MIT guy's" paper was simply the closest thing he had to a graceful exit. He's admitted he can't follow Drela's treatment, and demonstrated that he can't or won't follow the simple analysis I've presented (basically equivalent to Drela's minus the fancy terms and subscripts). If you look at his activity, you'll see that he's bouncing around on other threads at the moment - threads that are clearly less stressful.

 

[origin]

I looked at Drela's analysis and it is very strange to say the least. The very first part of the analysis he says that the boat is moving with velocity V, with the implicit assumption that the boat is at steady state. He then uses the equation for a powered propeller which is P=V x F x (eff). The V is the boat velocity - the wind speed. What is powering the propeller, well the power is from the turbine blades in the water. The turbine in the water is being powered by relative motion of the water or the speed of the boat. What is powering the boat, it is the propeller.

To me this presents a problem. The turbine powers the propeller, which moves the boat, which powers the turbine, which powers the propeller, which powers the turbine, which....

So where is the power coming from to maintain the speed of the boat? There must be external power coming into the system to maintain the velocity where is this adressed? I mean this is kind of key.

Hey Spork you said that I changed the last scenario - I see no way that I changed your scenario except that I didn't ignore real life losses - please tell me where I changed your scenario.

 

[spork]

I looked at Drela's analysis and it is very strange to say the least.

Yeah, he probably made some obvious mistakes that you caught. Drela ("that MIT guy" to you) is quite possibly the most reknowned aerodynamicist on the planet - but he can't seem to get a simple one-page analysis right.

To me this presents a problem. The turbine powers the propeller, which moves the boat, which powers the turbine, which powers the propeller, which powers the turbine, which....

Interesting... if only someone could explain this in simple terms without all those fancy symbols (as I did just a few posts back for you).

So where is the power coming from to maintain the speed of the boat?

See above.

Hey Spork you said that I changed the last scenario - I see no way that I changed your scenario except that I didn't ignore real life losses - please tell me where I changed your scenario.

So there are two inherent problems with this...

1) You either did change it or you didn't. You claim you only changed it to avoid ignoring real-world losses. Did you change my scenario or not? But before you answer, keep in mind, that giant cart that I sit in on the lakebed in the desert seems to go about 3X windspeed, directly downwind, in the real world - with real-world losses. My analysis predicts that behavior. Yours predicts that's impossible.

2) You claimed there's no point to discussing this with me as we're "going around in circles".

If you want to have an honest discussion about this, and are genuinely interested in coming to a common conclusion (i.e. willing to change your mind given the appropriate evidence), I'm happy to discuss it. Let me know.

 

[origin]

Fine, how did I change the scenario you presented?