Merged DDWFTTW in a “Real” Wind

[69dodge]

Thrust is 160 Newtons and the cart is supposed to move at 9 m/s, so it requires 1,440 Joules/sec, that is 1,440 Watts of Power.

I don't believe 9 m/s is the right speed to use here. That's the cart's speed relative to the ground. But the propeller exerts a force on the air, not on the ground. Since the propeller's thrust is moving the propeller through the air at only 2 m/s, only (160 N)(2 m/s) = 320 Watts of power is required.

That's if the propeller were 100% efficient. Llyricist supposed it was 52% efficient. So it requires (320 Watts)/(.52) = 615 Watts.

I'm having trouble understanding much of your post. I will read it again and think about it some more, but in the meantime, perhaps you should redo your analysis taking this point into account, and see how it affects the result.

 

[Subduction Zone]

professorpangloss/Harold Bricer/Christoph/schroeder and who knows what other aliases always makes the same sort of mistakes. And then he refuses to defend his errors. When he does his analysis he assumes that the cart cannot go faster than wind speed and then changes signs here or there to "prove" this.

 

[69dodge]

What can the cart do to get out of this big hole it has dug for itself? It can slow down some of that wind that is out there somewhere, but going slower than the cart. OK let it do that! 160 N of thrust and 350 watts, with F = ma, a simple back of the envelope calc says it slows down at most 72 kg of air by 2.2 m/s (72 x 2.2 ~ 160, close enough for this nonsense).

I don't understand this part. Can you show the "back of the envelope" calculation? Why must it be 72 kg and 2.2 m/s? Why not 144 kg and 1.1 m/s? Or 288 kg and 0.55 m/s? etc.

That wind will lose some energy during the slow down of 2.2 m/s, with respect to the ground it loses about 1000 joules per second, that is 1000 watts absolute Maximum.

How'd you get 1000 Joules per second? I get 1250.

(1/2)(72 kg)(9 m/s)² - (1/2)(72 kg)(6.8 m/s)² = 1251.36 Joules.

 

[humber]

Yeah, that pretty well sums it up. I mean - a sailing ship weighing hundreds of tons being pushed along by barely a ton of wind - who'd believe anything as crazy as that could work? You guys really got this science thing down.

Then you will find the air mass has more KE.

 

[humber]

JBTH - I believe humber is referring to the well known "conservation of speed" law.

JB

The gap between the submission theory and the trial results gets even bigger when other trials are examined.
I wonder if NALSA are willing to endorse that?

 

[69dodge]

Okay let's take a hypothetical cart weighing 295 kg, and give it the type of efficiency you think is appropriate and push it up to 9 m/s in a 7 m/s tail wind.

Using a crr of .02 (which is about twice that of a car tire on concrete) I'll have a rolling resistance of 57.8 Newtons. (crr * mass * 1G)

Using a Cd of .7 (the same as a Caterham open wheeler) and a frontal area of 2 m², gives a drag force of 3.43 Newtons at the 2 m/s airspeed.
(Fd = .5 * air density (1.225 standard day at sea level) * Cd * Frontal area * airspeed²)

For a total opposing force of 61.23 Newtons.

I'll let a (theoretical at this point) 4.876 meter (16 foot) diameter prop through a drive train that is 80% efficient, add an additional 83.4 Newtons of opposing force. So we have the prop, through the drive train doing negative work on the cart at the rate of 83.4 N * 9 m/s = 750 W.

I don't follow. Where does the figure of 83.4 Newtons come from?

 

[Jack by the hedge]

Then you will find the air mass has more KE.

Care to explain what you mean by that?

 

[windgrins]

Waltz me once again around the dancefloor...!

Then you will find the air mass has more KE.

JBTH

I think he means the large airmass has more KE overall than the ship. He seems to ignore the actual force on the sails. So he makes your argument for you with the BB also, but doesn't seem to realize it. If he does, he's keeping it cryptic so no one will notice.

I'm at the "Spork Point" now after watching you folks attempt in vain to take him through another round. It is clear from the last few responses that he has hit the reset point and gone back to the start. This will go on forever if you keep rising to the bait.

I say let's see something out of him before I ever respond again. Expect a round of insult and taunts from this (nowhere close to a complete list)-i.e:

'Windgrins is an annoyance,
Whoever I'm addressing that has a rational, cogent argument is an idiot of the lowest sort so the cart won't work.
The cart is trash so it won't work.
The treadmill is not equivalent.
Kinetic Energy is an absolute quantity and I can calculate it from any reference frame I like to use in my argument so the cart can't work.
Someone who didn't know what they were doing built a cart that didn't work so no cart, built by anyone, can.
Gravity doesn't work the same on moving objects (especially when moving on a treadmill.)
Your claim is Perpetual Motion (even though you say it isn't)
Your concept of reference frames is too complicated especially when you use more than one.
It's totally fair for me to flip-flop between reference frames without correct math to make my (incorrect) point.
Turning propellers can't generate thrust in still air or at all.
Turning propellers make wonderful tacking sails when used off axis, so even though this is what makes your cart work, your cart can't work.
Turning propellers generate loads of force when the relative wind is a few degrees off axis but not when directly on axis, there they generate zero thrust.
You Can't outrun your power source.
Once credible witnesses actually witness the cart working, they suddenly become non-credible since the cart can't work.
Airplanes have engines and boats move on the water so no abstracted principle applies to the cart at all, period, unless I want to use it poorly in my example.
Feedback loops are way to complicated so I just ignore them.
All the instrumentation everywhere is all wrong including tell-tales and balloons but not foam packing peanuts.
Your math is garbage. (even though I have no degrees in this and can't seem to handle the trivial.)
All the "experts" agree with me. (even though they don't and I'm misquoting them!)
None of you know anything about physics but I know everything. (even though I have no degrees in this and can't seem to handle the trivial.)
I'm an expert in microwave electronics (even though I have no degrees in this.) so your cart won't work.
I'm a rocket scientist and way smarter than those NASA folk (even though I have no degrees in this.) so your cart won't work.
Spork and JB are delusional con man and all the independent investigators with their own carts are just being hoodwinked because the cart can't work.
The cart has low aesthetic appeal to me so the cart can't work.
It's appropriate for me to use references to websites and claim them as my own thoughts, that prove the point you were making, as support of my argument.
"Proof by demonstration" by multiple independent investigators has no merit. Armchair philosophy is so much more important.
NALSA record is coming down when I contact them with my great analysis.
Sarcastic and insulting obfuscation is fine to use in an argument when I have nothing rational or relevant to say.
My alleged "Top Secret" cart is great and can go DDWFTTW but it isn't PM like your cart so your cart can't work.
The cart can't work because "I say so".
If your straightforward statement actually answers my objection, I just get to move the bar.

Now, give me more math so H and H can ignore it and insult you some more!'

Regards,

WindGrins

 

[Jack by the hedge]

Nice summary.

I resolve to try my very best not to be provoked into responding by Humber's vainglorious declarations of victory.

 

[windgrins]

A beneficial effort anyway!

Nice summary.

I resolve to try my very best not to be provoked into responding by Humber's vainglorious declarations of victory.

It's difficult (I know).

Nice effort on everyone's part and many of us benefited from the in-depth discussion.

Thanks,

WindGrins

And for Dr. Keith!

 

[3point14]

Once credible witnesses actually witness the cart working, they suddenly become non-credible since the cart can't work.

Your math is garbage. (even though I have no degrees in this and can't seem to handle the trivial.)

None of you know anything about physics but I know everything. (even though I have no degrees in this and can't seem to handle the trivial.)

I'm a rocket scientist and way smarter than those NASA folk (even though I have no degrees in this.) so your cart won't work.

"Proof by demonstration" by multiple independent investigators has no merit. Armchair philosophy is so much more important.


Regards,

WindGrins

A superb post.

I hacked it to pieces in the quote to pick out my personal favourites.

 

[windgrins]

For the sake of brevity, an incomplete list!

A superb post.

I hacked it to pieces in the quote to pick out my personal favourites.

Thanks Pi, I feel a little sad that one of my better efforts is to shred someone.

WindGrins

 

[Llyricist]

I don't follow. Where does the figure of 83.4 Newtons come from?

It's arbitrary, I arrived at it by calculating the equilibrium power level with my spreadsheet + macro, then added a couple hundred watts and rounded it to an even 750.

The spreadsheet uses the generic propeller formulas on the page I linked to a couple of times already, but here. Those formulas basically give a decent guide to what is possible to do with a propeller (except at very low speeds, where I am finding that actual propellers can do better), then you can design a propeller that will actually perform in that way, under the given circumstances, but at that point you are limited to what that propeller can do in any other circumstances.

And regarding the relative acceleration of the air you asked about above, that is also derived from those formulas.

 

[profpangloss]

Thanks Pi, I feel a little sad that one of my better efforts is to shred someone.

WindGrins

You didn't shred any of his arguments with counterpoints, all you did was launch into a long series of personal attacks and insults. Gee, I wonder if the MODS here will even notice? You out there somewhere, TRICKY?

 

[humber]

Care to explain what you mean by that?

The mass is relevant during acceleration of the boat, during which time, a great deal of air mass will pass over the sail.

 

[humber]

Thanks Pi, I feel a little sad that one of my better efforts is to shred someone.

WindGrins

You failed on all counts.

 

[profpangloss]

The mass is relevant during acceleration of the boat, during which time, a great deal of air mass will pass over the sail.

Yes, and the air mass is moving faster than the boat (DDW) containing more kinetic energy than the boat. It is thermodynamics 101. You want to increase the kinetic energy of something, hit it with something that has more kinetic energy, OR do some work on it, which requires an input of energy.
The cart is claimed to increase its kinetic energy by using its kinetic energy to do work, OR by being hit with a mass of air that has less kinetic energy than the cart has. FAIL.

 

[windgrins]

You're track record isn't so great either!

You didn't shred any of his arguments with counterpoints, all you did was launch into a long series of personal attacks and insults. Gee, I wonder if the MODS here will even notice? You out there somewhere, TRICKY?

I didn't say I shredded any arguments because frankly, I've never actually seen a relevant one from Humber. He's bought up a couple of reasonable questions before which were quickly answered by the knowledgeable.

All of my points were simply paraphrases of Humber's objections and statements so its a little hard to see how this is to be viewed as biting personal attacks.

But the two of you don't have a stellar track record for cohesive reasoned objections vs volume of words posted nor any serious consideration of any argument put forward in response to them. I find it just a bit disingenuous for this to be your objections at this point.

Regards,

WindGrins

 

[3point14]

You didn't shred any of his arguments with counterpoints, all you did was launch into a long series of personal attacks and insults. Gee, I wonder if the MODS here will even notice? You out there somewhere, TRICKY?

I disagree.

Shreded like a piece of very thin paper in a very expensive cross-cutting shredder operated by the Turtles arch nemsis.

That shredded.

 

[Michael C]

Neat trick, since the 295 kg cart @ 9 m/s has a hell of a lot more kinetic energy than that 72 kg of wind moving @ 7 m/s. Anyone here for reverse thermodynamics?

profpangloss already tried this trick at TalkRational*. He compares the fixed mass of the cart (measured in kg) with the mass of the air moving past the cart per second (measured in kg/s). Making numerical comparisons between quantities with different units is nonsensical.

He combines this meaningless comparison with a misinterpretation of the second law of thermodynamics. There is no law that states that object A cannot transfer kinetic energy to object B if B has more kinetic energy than A. If there were such a law, all sorts of simple machines wouldn't work.


*For those that don't already know, "profpangloss" is the same person as "Harold Bricer" and his sock "yevgheni" at talkrational.org. He has posted here as "Christoph" in the old "Validity of Classical Physics" threads that were split from the original ddwfttw cart thread.