Merged DDWFTTW in a “Real” Wind

[profpangloss]

Are you serious?

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.

Since the drive train is only 80% efficient the prop only gains 600 W of power from its negative work. At his level of power and this airspeed the theoretical maximum efficiency for a prop of this size is 64.5%, so I will give it 12% less than that, which is more than I should (see this page for formulas and explanations). So the prop efficiency is 52%. 600 W * .52 = 312 W of power to the air.

312 W / 2 m/s = 156 Newtons of thrust.

with a total opposing force of 83.4 Newtons from the prop's rotational drag and drive train,
plus 57 Newtons of rolling resistance,
plus 3.43 Newtons of aerodynamic drag
equals 143.83 Newtons
acting against a thrust of 156 Newtons, giving a net acceleration force of 12.17 Newtons.

There you have it. A total piece of crap that can accelerate beyond wind speed directly down wind.

Seriously, are you serious? This is what your faith in DDWFTTW rests on, and there are people out there who actually believe in this? There are people out there who look at this so-called “math” and don’t fall over themselves laughing?
OK, I’m going to make this short as possible and just use nice round numbers to bring a bit of enlightenment to the masses.
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.
You started out with 750 Watts at the wheels, only the Good Lord knows where it came, from (ground power) but I know the only place it could have come from is the cart’s very own kinetic energy, assuming it is somehow already moving at 9 m/s without explanation of how it got there. So, at 295 kg it would have 12 kilo joules of ke, right? Now you steal maybe 850 of that to get your 750 watts at the wheels ( nothing is 100% efficient, you know) Scorecard reads, minus 100, gone to heat and unrecoverable. (bye bye). Next, you send that up the long twisted chain held in place with pieces of skateboard, and 600 Watts arrives at the propeller shaft, so you lost another 150 Joules. (not looking good is it?) Now the propeller gets into the act, and with 52% efficiency (your number, not mine) it puts 312 watts of power into working on the air. Ah, lets make that 350 watts just to give this thing a fighting chance. Scorecard reads minus 500 watts. That is minus 500 Joules per second of precious kinetic energy and you know what that means don’t you? The cart is slowing down. What do propose as a solution to this quagmire? In order to push with that thrust of 160 N and move at 9 m/s, the cart needs 1,440 watts of power. After burning off 500 watts, it has 350 watts at the propeller to do some magic on the air. Now think about where we are here: the cart wants 1,440 watts to do what you claim, but it has already lost 500 watts trying. Now it needs 1, 940 watts to do what you claim and it has a grand total of 350 watts to do it with. BLEAK, VERY BLEAK.
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). 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. IF that ALL went to the cart, the cart would have 1,350 watts when it needs 1,940 watts. But as we all know, no process is 100% efficient and there is no way all of that wind energy goes into the cart but it doesn’t matter anyway as the cart is dead meat, slowing down and it will not stop slowing down until it is well below the wind speed.
Still some believers out there?

I agree with one thing you said A total piece of crap

 

[jduffy77]

Seriously, are you serious? This is what your faith in DDWFTTW rests on, and there are people out there who actually believe in this? There are people out there who look at this so-called “math” and don’t fall over themselves laughing?
OK, I’m going to make this short as possible and just use nice round numbers to bring a bit of enlightenment to the masses.
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.
You started out with 750 Watts at the wheels, only the Good Lord knows where it came, from (ground power) but I know the only place it could have come from is the cart’s very own kinetic energy, assuming it is somehow already moving at 9 m/s without explanation of how it got there. So, at 295 kg it would have 12 kilo joules of ke, right? Now you steal maybe 850 of that to get your 750 watts at the wheels ( nothing is 100% efficient, you know) Scorecard reads, minus 100, gone to heat and unrecoverable. (bye bye). Next, you send that up the long twisted chain held in place with pieces of skateboard, and 600 Watts arrives at the propeller shaft, so you lost another 150 Joules. (not looking good is it?) Now the propeller gets into the act, and with 52% efficiency (your number, not mine) it puts 312 watts of power into working on the air. Ah, lets make that 350 watts just to give this thing a fighting chance. Scorecard reads minus 500 watts. That is minus 500 Joules per second of precious kinetic energy and you know what that means don’t you? The cart is slowing down. What do propose as a solution to this quagmire? In order to push with that thrust of 160 N and move at 9 m/s, the cart needs 1,440 watts of power. After burning off 500 watts, it has 350 watts at the propeller to do some magic on the air. Now think about where we are here: the cart wants 1,440 watts to do what you claim, but it has already lost 500 watts trying. Now it needs 1, 940 watts to do what you claim and it has a grand total of 350 watts to do it with. BLEAK, VERY BLEAK.
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). 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. IF that ALL went to the cart, the cart would have 1,350 watts when it needs 1,940 watts. But as we all know, no process is 100% efficient and there is no way all of that wind energy goes into the cart but it doesn’t matter anyway as the cart is dead meat, slowing down and it will not stop slowing down until it is well below the wind speed.
Still some believers out there?

You have been shown over and over again where you are making your mistake. I think the patience shown by posters whose knowledge of the subject dwarfs your own has been amazing.

While most of us have long since moved on to talking about why you don't want to git rid of your ignorance, some continue to try and educate you. I guess they should be commended, but for my part I believe you know you are wrong but still post as a way of toying with the people who were right all along. Your error is so glaring obvious that this is the only explanation.

ETA: For any newcomers BTW, the reason he always posts his math in the long drawn out single paragraph prose format instead of line by line formulas is to make it easier for him to hide reversed signs and double counting. If you care (and have the patience) all you need to do is re-write it in a more normal way and they will become obvious. This famous riddle does exactly the same thing:

3 men go into a hotel to check in for a conference and get a room to freshen up in. The man behind the desk said the room is $30, so each man paid $10 and went to the room.

A while later the man behind the desk realized the room was only $25, so he sent the bellboy to the 3 guys' room with $5 leftover. On the way the bellboy couldn't figure out how to split $5 evenly between 3 men, so he gave each man a $1 and kept the other $2 for himself. This meant that the 3 men each paid $9 for the room, which is a total of $27; add the $2 that the bellboy kept = $29.

Where is the other dollar?

 

[humber]

You have been shown over and over again where you are making your mistake. I think the patience shown by posters whose knowledge of the subject dwarfs your own has been amazing.

While most of us have long since moved on to talking about why you don't want to git rid of your ignorance, some continue to try and educate you. I guess they should be commended, for my part I believe you know you are wrong but still post as a way of toying with the people who were right all along. Your error is so glaring obvious that this is the only explanation.

You won't find Spork coming to the rescue, no to explain the difference between the theory and the trial results, if only because he has taken you for granted.

 

[profpangloss]

You have been shown over and over again where you are making your mistake. I think the patience shown by posters whose knowledge of the subject dwarfs your own has been amazing.

While most of us have long since moved on to talking about why you don't want to git rid of your ignorance, some continue to try and educate you. I guess they should be commended, but for my part I believe you know you are wrong but still post as a way of toying with the people who were right all along. Your error is so glaring obvious that this is the only explanation.

ETA: For any newcomers BTW, the reason he always posts his math in the long drawn out single paragraph prose format instead of line by line formulas is to make it easier for him to hide reversed signs and double counting. If you care (and have the patience) all you need to do is re-write it in a more normal way and they will become obvious. This famous riddle does exactly the same thing:

3 men go into a hotel to check in for a conference and get a room to freshen up in. The man behind the desk said the room is $30, so each man paid $10 and went to the room.

A while later the man behind the desk realized the room was only $25, so he sent the bellboy to the 3 guys' room with $5 leftover. On the way the bellboy couldn't figure out how to split $5 evenly between 3 men, so he gave each man a $1 and kept the other $2 for himself. This meant that the 3 men each paid $9 for the room, which is a total of $27; add the $2 that the bellboy kept = $29.

Where is the other dollar?

Hey, while you are here, maybe instead of saying I made an obvious mistake, and adding a lot of insults as well, just maybe you can actually point to the mistake? Just an idea for you to try out.

 

[tobermoryphil]

Prop efficiency is nearer 90% than 52%- unless you are using one of Humber's little props! Have another go with Javaprop, or try Prop.Designer.

 

[jadebox]

... it would have 12 kilo joules of ke, right? Now you steal maybe 850 of that to get your 750 watts at the wheels ....

Okay ... you've already lost me. You subtracted 850 "somethings" from 12,000 joules and got 750 watts. What is the unit of measure for the 850 somethings? And how does 12,000 - 850 equal 750? You've obviously left something out. Llyricist was very careful to document each step in his calculations. Your argument would be easier to follow if you did the same.

-- Roger

 

[69dodge]

Hey, while you are here, maybe instead of saying I made an obvious mistake, and adding a lot of insults as well, just maybe you can actually point to the mistake?

That is a reasonable request. I don't have time right now, but I will try to do it later. However, may I note that you didn't point out a mistake in Llyricist's analysis either. You just posted an entirely different analysis of your own. Perhaps you could point out his mistake.

 

[ThinAirDesigns]

However, may I note that you didn't point out a mistake in Llyricist's analysis either. You just posted an entirely different analysis of your own. Perhaps you could point out his mistake.

Exactly what I was going to post.

+1

JB

PS: quite a number of us have already posted the mistakes that Harold makes in his 'treatement' as he has previously posted it. Jduffy mentioned the easiest to spot.

 

[humber]

Humber doesn't do terms like "net". He hears large torque numbers at the prop shaft an imagines a Corvette spinning it's big fat tires on pavement.

JB

It is 32N, if you like it or not, where as your laughable "How fast does it go, Mister?" appeals to a Corvette, means well over 1000N.
Excellent harness video by the way. It is you, because one comment refers to "bubble-boy"

 

[Llyricist]

Seriously, are you serious? This is what your faith in DDWFTTW rests on, and there are people out there who actually believe in this? There are people out there who look at this so-called “math” and don’t fall over themselves laughing?

Way to out yourself.... again.

OK, I’m going to make this short as possible and just use nice round numbers to bring a bit of enlightenment to the masses.
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.

okay so far.

You started out with 750 Watts at the wheels,

Wrong, I started out with 750W of negative work on the cart.

only the Good Lord knows where it came, from (ground power) but I know the only place it could have come from is the cart’s very own kinetic energy,

Wrong again, the propeller, through the drive train, is doing negative work on the cart at that rate.

assuming it is somehow already moving at 9 m/s without explanation of how it got there.

Somehow I knew you weren't going to print and study what I wrote, this is the second thing that indicates you didn't even read it.

I said it was pushed up to that speed in one of the first sentences.

So, at 295 kg it would have 12 kilo joules of ke, right? Now you steal maybe 850 of that to get your 750 watts at the wheels ( nothing is 100% efficient, you know)

The cart's KE has nothing to do with where the energy comes from. And you are just making up a loss here.

Scorecard reads, minus 100, gone to heat and unrecoverable. (bye bye). Next, you send that up the long twisted chain held in place with pieces of skateboard, and 600 Watts arrives at the propeller shaft, so you lost another 150 Joules. (not looking good is it?)

No, your usual Inverse Enron style bookkeeping looks just as crappy as ever.

I'm not going to bother with the rest, I'm just going to do the energy balance correctly.

In the frame of reference where the cart is momentarily at rest ("the cart's frame"), it is indeed a trivial matter to extract power from the ground, since the mass of the ground moving at 9 m/s has, well lots and lots of KE. but that's not relevant, since I already couched it in terms of the "ground's frame" with the negative work statement.

The cart's mechanism works like any positive feedback loop, and like any positive feedback loop, it would quickly run down to nothing due to losses, unless there is a source of energy replenishing those losses.

In this case the mechanical losses of the feedback loop amount to 150 J / s. With an additional 117 J / s of aerodynamic losses at the prop, which is the difference between the maximum theoretical efficiency of the prop and the efficiency I chose. (The effective "Engine Power" the thrust I used works out to is 483 W).

So that is 267 J / s that needs to replenished to the feedback loop, but there are more losses to account for, namely the aerodynamic drag and Rolling resistance (which you double counted above, duh).

So we have the aero drag causing a loss of 31 J / s, bringing it to 298 J/s. And rolling resistance causing a loss of 521 J /s, bringing it up to 819 J/s.

Also, there is the matter of the cart accelerating, which requires it gain that KE from somewhere. The force accelerating the cart is actually 11.377 Newtons, using un-rounded numbers in the calcs. Which gives an acceleration of 0.038 M/s². So the cart has a KE of 11,947.5 J before, and 12,050.1 J after, gaining ~102 J, bringing our energy requirement up to 921 J.

Using my numbers for thrust and Engine power in the prop formulas, I get a Delta v of 2.2 m/s, a mass of air of about 71 kg. And a KE loss of 920 J.

Close enough, especially considering I didn't account for the acceleration of the cart in the air mass calculation.

As for total work, 156 * 9 = 1404 J. 920 J + 483 J = 1403 J, close enough.

 

[EHocking]

So, now Hummer is saying the cart works but spork didn't explain it properly?

There were some goalposts around here somewhere . . .

Feel free to use this one I created for a particularly persistent UFO believer on this forum

 

[jduffy77]

Hey, while you are here, maybe instead of saying I made an obvious mistake, and adding a lot of insults as well, just maybe you can actually point to the mistake? Just an idea for you to try out.

Your request would be completely reasonable if this were the first time you have posted the same thing. If it were the 10th time I might still consider it to be at least somewhat reasonable. But since your simple errors have been pointed out now hundreds of times to do so again would be acting like a trained monkey. I believe that is what you want so all I did was make it easier for anyone who wants to find your errors for themselves. After all it is pretty trivial to find them.

 

[humber]

Where is the other dollar?

In the wishing well in the lobby, put there to extract money from gullible believers.

 

[Myriad]

Hey, while you are here, maybe instead of saying I made an obvious mistake, and adding a lot of insults as well, just maybe you can actually point to the mistake? Just an idea for you to try out.

Okay.

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.

Not true. Thrust of 160 Newtons at a speed of 9 m/s provides 1,440 Watts of power. That implies nothing about what the cart needs to run at 9 m/s or to generate that thrust or anything else.

That's a rather fundamental distinction for what you're trying to demonstrate, so it's probably best if you were to start over from there.

Respectfully,
Myriad

 

[humber]

Exactly what I was going to post.

+1

JB

PS: quite a number of us have already posted the mistakes that Harold makes in his 'treatement' as he has previously posted it. Jduffy mentioned the easiest to spot.

At 9m/s the cart's KE is 1/2*195kg*9^2 = 7897J
From the trial, a = 0.11ms-2, so one second later it is 1/2*195kg*9.11^2 = 8091J
An increase of 194J. The input form the wheels is 1440J/sec, so the cart's overall efficiency would be 194/1440 = 13.5%, if that were at all possible.

 

[humber]

Feel free to use this one I created for a particularly persistent UFO believer on this forum

[qimg]http://www.internationalskeptics.com/forums/imagehosting/26614a76ec3f84da4.gif[/qimg]

Another cultist trait, and one that betrays true faith. The Master is right, even when he's wrong. The trials contradict the cart's ddw theory, so it can't work as Spork claims; but can as a sail cart.
UFO's are not impossible, just unlikely. That makes the UFO believers probably wrong, but at least rational to some degree, whereas the ddw cart is impossible. UFO believers can't be shown that UFO's don't exist, but the trials show that at least this ddw cart doesn't.
Jeer at them all you like, but the joke is well and truly on you.

 

[Llyricist]

Prop efficiency is nearer 90% than 52%-

Actually, at that particular speed and power, it is not. but it would be closer to the maximum theoretical (0 loss) of 64.5% than the 12.5% I used.

unless you are using one of Humber's little props! Have another go with Javaprop, or try Prop.Designer.

The formulas I have been using (from here) are actually proving very close to the Java Prop results, as long as Push up way closer to the max theoretical efficiency from them.

 

[jduffy77]

Another cultist trait, and one that betrays true faith. The Master is right, even when he's wrong. The trials contradict the cart's ddw theory, so it can't work as Spork claims; but can as a sail cart.
UFO's are not impossible, just unlikely. That makes the UFO believers probably wrong, but at least rational to some degree, whereas the ddw cart is impossible. UFO believers can't be shown that UFO's don't exist, but the trials show that at least this ddw cart doesn't.
Jeer at them all you like, but the joke is well and truly on you.

This is one of Humber's reset posts. He makes a statement. It is logically dismantled. He then makes it again as if the intervening posts did not exist.
The cycle continues.

 

[Subduction Zone]

How long ago was it that I predicted profpangloss' next step was to make a ridiculous "proof" where he double counts, changes signs and other errors to prove that the cart does not work?

Thank you Harold for showing your true colors, or is that Christoph here?

 

[humber]

This is one of Humber's reset posts. He makes a statement. It is logically dismantled. He then makes it again as if the intervening posts did not exist.
The cycle continues.

Why don't you take it up? They can test your cart, as published in Make, while they are at it. A free lesson.
Your "math" has shot itself down. Your method predicts enormous net forces, that change with cart speed, but the constant acceleration of the cart seen in the trial says small and constant net force.
And your explanation is?

It takes a Biblical level of interpretation to extract Dr Kieth's conclusion, and your support for it, from the above post.