DDWFTTW - Tests.

[John Freestone]

Ynot, how you getting on with that new design? I'm itching to know more about it.

 

[recursive prophet]

Me too, Y'not. I'd also like to know if perchance you've ever attempted to weigh the pressure of the guidance arm at point of contact on the disk, and if you've ever tried running it without the prop? Just curios.

 

[ynot]

Have been very busy with business and travelling lately so haven’t had any spare time to spend on the new circular wind tunnel. Hope to be able to get some time to work on it next week.

Can’t see any purpose to weighing the pressure of the guidance arm at point of contact on the disk or running the cart without a prop. Was going to run it with a flywheel in place of the prop at one stage but what I was wanting to find out has been answered by other tests.

DDWFTTW seems to have lost interest on this forum.

 

[recursive prophet]

Thanks for taking the time to answer, Y'not. Surprised your eyes haven't been tingling. People talkin bout you, dude. I forgot to invite you to the party cause you never really posted on the Validity of Classical Physics Thread. The debate with humber and Christoph continues, with a couple new non-believers joining the fray.

Anyway, if you get a chance I would like to know the load on the restraining arm. Your video is linked often, and the Fr has become one of issues. Also, did you ever mention minimum disk speed required, and how do you measure that?

If you get a chance you really should drop in on the new DDWFTTW thread. It started on March 4, and is already up to 2,704 posts shattering all previous records for the forum, maybe the entire site. Pt. 3 began on March 10. Do stop by. John F, spork, Tad, mender, the whole gang are all goin srtong. And of course, we're all here..because we're not all there. --rp

 

[ynot]

Thanks for taking the time to answer, Y'not. Surprised your eyes haven't been tingling. People talkin bout you, dude. I forgot to invite you to the party cause you never really posted on the Validity of Classical Physics Thread. The debate with humber and Christoph continues, with a couple new non-believers joining the fray.

Anyway, if you get a chance I would like to know the load on the restraining arm. Your video is linked often, and the Fr has become one of issues. Also, did you ever mention minimum disk speed required, and how do you measure that?

If you get a chance you really should drop in on the new DDWFTTW thread. It started on March 4, and is already up to 2,704 posts shattering all previous records for the forum, maybe the entire site. Pt. 3 began on March 10. Do stop by. John F, spork, Tad, mender, the whole gang are all goin srtong. And of course, we're all here..because we're not all there. --rp

So people haven’t lost interest in DDWFTTW they have just moved to another forum. Had a quick look at the “Dawkins” thread and it’s not a “party” that interests me much. I would rather see quality of posts rather than quantity. Couldn’t see where people were “talkin bout” me. Did you mean that forum/thread or some other?

Have found some time to do some work on the circular wind tunnel and things are progessing well. Thanks to John Freestone and yourself for reminding me to keep at it.

 

[recursive prophet]

You know, I should have linked you back a few pages on that thread Y. I just sent where we were at that moment, which was a low point. I think there's also been some interesting discussion, and other perspectives added by several RDFies new to the subject. As testing has become the major issue of late, perhaps some of it will drift back here. The question of various types of wind tunnels has also come up, so I'll pass on that you're working on one.

Have to confess that after following this topic from my_wan's OP, I am still not completely convinced the cart can perform as claimed in actual wind on the ground, though given all the converts do lean in that direction. Have you tried gearing the propeller to function as a turbine just to see what happens? And again when time permits I would like to know the arm load on the wheel.--rp

PS. Yeah, you were mentioned by name several times and many links to your video-at least a dozen-as the steady state question kept coming up. Oh, and again what speed is the wheel/disk going, and what is the minimum speed required for the prop to advance??

 

[Subduction Zone]

rp and others, yes the action on the Dawkin's website can get a bit vitriolic to say the least. And humber and Christoph are as full of nonsense as ever. But it does allow us to sometimes polish the arguments for DDWFTTW. For example did you see spork's analogy with the cart on the treadmill on the truck. I reprised it later and Michael C added a very nice illustration. If you have any questions about the validity of the treadmill tests this should answer them for you. No math or complex engineering knowledge needed. Even a total tyro to physics should be able to understand it. Take another look there rp, I will keep a civil "toungue" on my keyboard when I answer you.

 

[Michael C]

Have you tried gearing the propeller to function as a turbine just to see what happens?

Here's a prediction based on both theory and praxis: in this configuration the cart will reach a constant speed less than that of the wind. From the observer's point of view, this will mean that the cart will be seen to be going backwards on the turntable at a constant rate, in the same direction as the turntable but not as fast as the turntable.

And again when time permits I would like to know the arm load on the wheel.--rp

What do you mean? The weight that the arm adds to the wheel? Or the tension in the arm? Or something else?

I guess you are remembering humber's comments about the tether arm: something about it providing a reaction force. I have already explained why the tether arm cannot contribute to acceleration in the direction of travel. It's somewhere in the first thread on Dawkins, but I'll happily go through some details again if you have specific questions.

Oh, and again what speed is the wheel/disk going, and what is the minimum speed required for the prop to advance??

There will be a minimum wind speed for any particular DDWFTTW cart: for winds under this minimum speed the cart will not run faster than the wind. The actual minimum speed will depend on details of the cart: size of propeller, efficiency of propeller, frictional resistance in the axles... The minimum speed will not be the same for all DDWFTTW carts.

This is nothing surprising: many machines have a minimum speed necessary in order to perform a particular function. Any particular plane has its own minimum speed necessary in order to take off: move it forward at less than this speed and it'll happily roll along the ground, but it won't fly.

What I'm wondering here is: why do you want to know the minimum necessary speed for ynot's cart? Are you trying to work out what minimum speed is necessary for "real wind" trials? Since ynot's cart is quite different from spork's, or any other cart that does not need to be tethered, there's no guarantee that the minimum speeds will be the same, or even similar.

 

[recursive prophet]

@SZ: Thanks for the help, and you are definitely not one of the egregious offenders on RDF. You have to get really down and dirty to stand out in that crew. To me spork’s treadmill on a truck makes as little sense as humber sees in it. The ‘electric powered ground’ is still driving the prop instead of the reverse as it is in the wind. You still wouldn’t have the resistance you would as the prop attempted to exceed wind speed while turning the cart’s wheels on the ‘real, stationary ground.’ I just can’t grok the equivalence despite all I’ve read trying to explain it.

What I'm wondering here is: why do you want to know the minimum necessary speed for ynot's cart? Are you trying to work out what minimum speed is necessary for "real wind" trials? Since ynot's cart is quite different from spork's, or any other cart that does not need to be tethered, there's no guarantee that the minimum speeds will be the same, or even similar.

Michael: I remember your explanation of y’not’s wheel, but what I still fail to grok is how you can be so certain the wheels turning the prop will be the same as the prop driving the wheels. I asked about this last night on RDF but my questions got lost in the ‘pleasantry exchanges.’

I believe the addition of the tether arm solves one problem while creating another. I would still like to know how much tension it adds and what the weight is of the prop and wheel on the disk. You have argued about the difference between a tethered balloon and an un-tethered one in the wind, yet here claim the tether makes no difference. Personally I still think we should forget balloons, boats, and treadmills and focus on carts on the ground in the wind. I realize such tests come with their own set of problems, but hopefully you will agree little has been tried to date beyond Goodman’s very simple and IMV unscientific experiment.

 

[Michael C]

Michael: I remember your explanation of y’not’s wheel, but what I still fail to grok is how you can be so certain the wheels turning the prop will be the same as the prop driving the wheels. I asked about this last night on RDF but my questions got lost in the ‘pleasantry exchanges.’

I'm not sure what you mean here. The prop is always being turned by the wheels.

I believe the addition of the tether arm solves one problem while creating another. I would still like to know how much tension it adds and what the weight is of the prop and wheel on the disk. You have argued about the difference between a tethered balloon and an un-tethered one in the wind, yet here claim the tether makes no difference.

The difference is the direction is which the tether is pulling. In the case of the tethered balloon, the tether can exert a force against the direction of movement of the balloon. In the case of ynot's cart, the tether is always acting at right angles to the direction of movement.

A force at right angles to the direction of movement does not produce an acceleration in the direction of movement.

All the tether does here is to constrain the movement to be circular. It doesn't matter how much tension is present in the tether arm, this tension cannot accelerate the circular movement.

 

[Subduction Zone]

rp, the point about putting the treadmill on the truck outside is to show you that the surface of the treadmill inside is exactly like the ground outside. The top of the treadmill on the truck is stationary with respect to the ground. So the wind blowing over it is not due to the treadmills motion, as it is with the inside treadmill, the wind blowing over it is the "real wind". I hope you can see that.

 

[recursive prophet]

I'm not sure what you mean here. The prop is always being turned by the wheels.

Lets stop right here. I've seen this stated before and still don't understand the claim. I can see how the wheels turn the cart on the belt, but unlike the belt the ground isn't moving in reference to the wheels of the cart. It is stationary, so how can it be the wheels turning the prop? I thought the wind was doing that, and the prop then turned the wheels when on the ground. Glad this came up as it is a major factor in my lack of understanding. Please explain.

 

[ThinAirDesigns]

Lets stop right here. I've seen this stated before and still don't understand the claim. I can see how the wheels turn the cart on the belt, but unlike the belt the ground isn't moving in reference to the wheels of the cart. It is stationary, so how can it be the wheels turning the prop?

A: When the cart is on the treadmill set at 10mph and the cart is sitting still relative to the room, the wheels are moving relative to the rolling surface at 10mph. You would probably agree that if you put a speedometer driven off of those wheels you would see 10mph on the speedometer.

B: When the cart is on the ground and is moving downwind at 10mph, the wheels are moving relative to the ground at 10mph. You would probably agree that if you put a speedometer driven off of those wheels you would see 10mph on the speedometer.

I simply can't comprehend what you can possibly mean by "...unlike the belt the ground isn't moving in reference to the wheels of the cart. It is stationary, ..."

In both cases, the wheels are moving the same speed across the rolling surface.

I thought the wind was doing that, and the prop then turned the wheels when on the ground.

You need to stop thinking that -- even if you don't understand *how* it can be, I promise you it does you no good whatsoever to continue to attempt to grok it the other way.

JB

 

[John Freestone]

Lets stop right here. I've seen this stated before and still don't understand the claim. I can see how the wheels turn the cart on the belt, but unlike the belt the ground isn't moving in reference to the wheels of the cart. It is stationary, so how can it be the wheels turning the prop? I thought the wind was doing that, and the prop then turned the wheels when on the ground. Glad this came up as it is a major factor in my lack of understanding. Please explain.

Hello RP. Are you sitting comfortably?

The prop blades are set at such an angle that they aren't allowed to act as turbine blades. As you can see in the last (cart) video of spork's, when lifted off the ground, the tailwind blows the prop round as a turbine, but when put on the ground (in this case he uses the treadmill turned off) it switches direction. Now, why is that? What does it mean? Well, in the first instance there is virtually no load on the prop-turbine (i.e. there is nothing stopping the rest of the machine and the wheels turning in any direction the prop-turbine is tending to turn them. It's worth pointing out, of course, that the prop-turbine (or "spinny thing" (ST) as I've been calling it there) isn't actually one or the other. The same gizmo can be used as either. Ok, so we've got our first bit of information: nothing stopping the wheels go in either direction, and the wind does what wind would do to a "windmill" - a turbine generating electricity, for instance.

Next thing to note is that, if you were to look at the wheels during that turbining motion, while they're off the ground, they would be turning backwards. If you got hold of the wheels and turned them backwards, the spinny thing would go in the turbine direction. If you turned the wheels forward - by which I obviously mean the direction they would turn in if the cart was going forwards on them! - the ST turns as a prop.

So why does it turn as a prop when you put it down and blow wind at it from the rear? Well, because the whole vehicle is getting blown along. Do you see? If there was just a flat disk there instead, or the blades were turned to point in the "across the road" direction (part of a disk), this being called having a pitch of zero, then it's spinning wouldn't make any difference at all. If it were removed - just a cart on some wheels - it would still be blown along the ground, like a shopping cart can be seen to do in a carpark, even though it would take a little more wind power to shift it.

In the normal configuration, there is a force just like that (despite the fact that most of the wind resistance on a well-designed cart comes from the prop itself, and ALSO DESPITE the fact that the ST would, given free rein, like to turn the turbine way). So, what actually happens is that the whole cart is pushed along the road, and that overcomes the tendency of the blades to be turned as a turbine, making it act like a prop.

Think of just a wind turbine, like one of those massive ones generating electricity. Imagine it in your mind's eye, how the wind passes the blades, and because of the angle of the blades - see that angle - the air pushes each blade round in a particular direction - just one way because of that sloping angle of the blade. Now, imagine if someone got hold of the gears inside and pushed it round the "wrong" way. It's a hell of a battle - the wind's trying to turn a turbine, and we're forcing the poor thing to do the opposite. It's like that, only that "wrong-way" force is coming from the fact that the whole cart is being pushed along the ground, and that is - THE GROUND IS - turning the wheels.

Now, check that capitals bit out. If instead of having wind blowing the cart along, you keep the air still, you can get exactly the same effect by moving the ground backwards instead - the cart doesn't feel any different forces. Now to our biased human minds, these things look very different indeed, but really they are no different except in the details outside of that motion. The cart doesn't touch anything except the surface it's on and the air its in. Does that help? That's the stupid thing about humber's and C's endless rant about the belt pushing the wheels, driving the wheels. If they got their heads round the above, they'd see that the cart is pushed along the road and the road drives the wheels, just the same (in all relevant details).

Going back to where I started, if you've understood that, it is the pitch chosen for the prop blades that makes that possible. There's a lot of detail about that elsewhere, but basically it's kept fairly shallow (across-the-road, a short pitch) - if the slope was made gradually steeper, the blades turning more towards the down-the-road direction, it will get to a point where the blunt pushing force - the trolley-dash force - of the wind got too weak to overcome the turbine tendency. The turbining would win the battle, and, because there's no alternative, the wheels would turn backwards, and the cart would actually back up into the wind (actually, for me, that was much easier to see than DDWFTTW).

I know you've also puzzled about how, at windspeed, there can be any puff left in the wind. I don't know if you've got that bit yet, but let me know if you'd like me to have a go at that part as well.

ynot - no pressure, I was just curious. I guess I can relax and know that if you got round to it you'd post, so I needn't keep asking.

 

[John Freestone]

I simply can't comprehend what you can possibly mean by "...unlike the belt the ground isn't moving in reference to the wheels of the cart. It is stationary, ..."

In both cases, the wheels are moving the same speed across the rolling surface.

JB, I think it's just that simple human bias we mortals have - some, like humber, can never get over it, or if they do, they pretend never to have done so. RP is less susceptible, I hope, but makes that mistake here.

RP, I think when you go back to the road situation in your mind's eye, you remember the motionless (balanced) cart on the treadmill, "put it" on the road, still motionless, and that gives no relative velocity between road and wheels. But that situation of "still" for the road-cart is when it's stopped - the wind is whistling past it from behind, and the brake's on. We're waiting to get going when the wheels aren't moving!

You have to remember that the treadmill only demonstrates windspeed and near windspeed, and transfer that same situation back if you're going to compare it with the road. So the treadmill models the road cart whizzing along at windspeed.

 

[ThinAirDesigns]

JB, I think it's just that simple human bias we mortals have - some, like humber, can never get over it, or if they do, they pretend never to have done so. RP is less susceptible, I hope, but makes that mistake here.

I guess so, but it's really freaky -- if RCP were driving his car next to the cart as it went down the street he could look at his own car's speedometer and clearly see that is says "10mph" (or kph). I just don't get here his statement comes from that on the street the cart's wheels are stationary.

JB

 

[John Freestone]

I guess so, but it's really freaky -- if RCP were driving his car next to the cart as it went down the street he could look at his own car's speedometer and clearly see that is says "10mph" (or kph). I just don't get here his statement comes from that on the street the cart's wheels are stationary.

JB

Hey, we're starting a psych-RP thread! Sorry, RP. Maybe you'll take it as a compliment. Reading the whole piece of text and what came before, it looks like this is part of his confusion about how on earth the cart gets going at all on the road, because the ground isn't powering it, and he's been persuaded by those pesky persuaders that if it's driven by the wind instead, that can only be done if it's turbining. I think that my long post earlier might bridge that gap if we don't distract him from it too much.

What amazes me is how many times the content of that must have been written on these threads, and that I wrote two long posts explaining that to C only last week or thereabouts - one with a diagram of a mechanical screw instead of the prop, IIRC. So I'll not hold my breath. However, the noise to signal ratio around here could mean people like RP aren't entirely to blame if they miss something central.

You know, what you, spork et al really need is a central place, er website they call 'em, where all of the correct physics and explanations and the most relevant analogues can be collected - instead of this mess of forums - or is there such a place and I've missed it? Then newbies can just be given one link and it could save an awful lot of bother. I did complain myself in the earlier thread that I was having to ask questions and get answers and and raise my internal mental objections to the bits of explanation I was getting (which was sometimes seen as more critical than it was meant), in amongst a raging argument, but nobody was explaining what the answer was clearly, top down, this is how it works.... That, as I observered, was ok, because it was meant to be a brainteaser. But this has been going on a long time now. We've had the reveal moment. People need a clearer explanation that's always readily available. This repetition and drip feeding is a problem now, IMO.

 

[ThinAirDesigns]

You know, what you, spork et al really need is a central place, er website they call 'em, where all of the correct physics and explanations and the most relevant analogues can be collected - instead of this mess of forums - or is there such a place and I've missed it?

http://dwfttw.blogspot.com/

JB

 

[mender]

RP, if you don't mind I'll try out another way of explaining how the cart works to you. I feel that this thread might be a better place to do that. I'd like to go through this without humber or Christoph butting it if possible. Once we get through this, I may try it on them if they'll cooperate. I feel that you may have some of the same blind spots as them but you definitely are much more relaxed about that! From the questions you're asking now, I'd say you're ready for the chewy parts!

Most of this will be idealized, i.e., no losses or slippage unless specified. Also, we'll ignore any influence by the wind on the body of the cart and say that only the prop feels the wind. We'll adjust that later.

Let's have humber and his bike participate in this example since he wants a test on this. Let's build a cart with a variable speed transmission and a clutch between the wheels and the prop for the test. The wind will be a nice steady 12 ft/second tailwind.

First, set the cart in the wind with the transmission in neutral, with the cart not moving and allowing the prop to spin freely. What speed does the prop turn at? That depends on two things: the speed of the air going by, and the pitch of the prop. The air is going by at 12 ft per second as verified by the on-board anemometer, and the prop has a pitch of 8 inches per revolution. That means that the prop will spin freely at 18 times per second or 1080 rpm.

An important thing to remember is that if the propeller is turning at this freewheeling speed, it is neither taking energy out of nor putting energy into the air as it moves by. Slowing down the prop from that speed will take energy from the air (cart speeds up). Speeding up the prop above the freewheeling speed take away energy from the cart (cart slows down). More on that later.

Back to our cart, sitting on the ground with the 12 ft/sec wind blowing on it. The prop is spinning at 1080 rpm. Humber hooks up a tether to his bike and starts pedaling downwind.

As humber gets to 3 ft/second, let's freeze the picture for a moment. The cart is moving at 3 ft/sec. Let's say that the wheels turn once per foot,which gives us a wheel rpm of 3 x 60 or 180. The on-board anemometer says that the air is going at 9 ft/sec., which means that the freewheeling speed of the prop is now only 810 rpm. You decide to see what the gear ratio would need to be if you engaged the transmission. Easy enough: 180/810 is .22:1.

Now we unfreeze and let humber get up to 6 ft/sec and repeat. Now the relative air speed is only 6 ft/sec, the prop's freewheeling speed is 540 and the wheels are turning at 360 rpm. The gear ratio needed to keep everything turning at the same rpm is 360/540 or .66:1.

Next speed is 9 ft/sec. The relative air speed is verified by the on-board anemometer as being 3 ft/sec, the prop only turns 270 rpm and the wheels are turning at 540 rpm. Now we see a big jump in the gear ratio needed to engage the transmission without issues, or 2:1. The next step is even bigger.

Okay, now humber is riding at 12 ft/second or wind speed, the relative airspeed is 0 ft/sec so the prop stops turning, and the wheels are turning at 720 rpm. Now what is the gear ratio needed? Well, the ratio is infinite but don't dwell too much on that at the moment, the important part is that the prop is stopped.

Just to be thorough, humber decides to pedal faster and gets to 15 ft/sec. Now the prop is turning again but the other way! Not a big surprise but an important point. The rpm of the prop is again 270 rpm but the other way, and the wheels are turning at 900 rpm. The gear ratio is 3.33, on its way back down from infinity but on the other side - we'll call this the positive side since we have to go faster than the wind to need this ratio. That of course would make the other side negative, since that side only applies to below wind speed.

Humber is a wild man, now he gets to 18 ft/second! The prop is turning faster because it now sees a relative air speed of 6 ft/sec with an rpm 540 rpm in the plus direction. The wheels are turning at 1080 rpm, so the gear ratio needs to be 2:1. This trend of gear ratios continues downward as the cart goes faster and faster. The exact ratio is not that important now but we'll need that if we get into advance ratios.

RP, I hope the concept came through, that the prop has a speed and direction that it wants to turn depending on how fast the cart is moving along the ground in a steady tailwind. Below wind speed, the prop wants to turn one way and the gear ratio has to be negative; above wind speed it wants to turn the other so the gear ratio has to be positive. That's what spork's second last video showed, the difference between a slower than wind cart and a faster than wind cart.

At this point, we aren't taking any energy out of the wind, just going along with the wind speed, Humber is doing all the work for us! All we're after is the way the prop has to turn when the cart is below, at, and above wind speed. Christoph acknowledges this by saying that the cart's gearing can't suddenly change the direction of prop rotation when the cart goes faster than wind speed. We'll get into the power part after you have this done pat.

I hope that was clear. Please ask about any stage that isn't and we'll start there. Also, please let me know if this helped at all or was just confusing. No pressure!

P.S. Sorry for the long post!

 

[recursive prophet]

Mender: Thanks so much for the thoughtful reply. It does remind me a bit of Maxwell’s Demon with your infinitely geared transmission. For as humber and Chris point out, why wouldn’t the turbine act as a brake at 15fps? It would be pushing against the direction the cart is moving. So please do jump ahead, and to cover the point Chris makes about the sudden change in rotation, and how it couldn't overcome the resistance of the wind from the opposite direction once wind speed is exceeded?

Sorry if I’m still missing the point. I just can’t help but wonder how relevant the example is to an actual cart, which only has one gear. As an aside, did you catch the article in the March Popular Mechanics on the prop car races in Stuttgart? They do have a lot of gears, and a prop that swivels, yet there was no mention of going downwind faster than the wind. I recall the winning car averaged about 15mph in the race. A lot of tacking going on I’m sure.

TAD and John: Sorry I missed your replies when I responded to mender earlier. Think you screw example was the best yet, JB, in helping me grok what's happening. Anyway, thanks for the online 'intervention,' and good job on the blogsite JB. For just a modest monthly payment of $100 I won't tell humber about it. :lol: