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Topic: Direct Down Wind Faster Than The Wind (Read 24050 times) previous topic - next topic

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  • A.T.
Re: Direct Down Wind Faster Than The Wind
Reply #1575
How is a cart with a propeller traveling with a 10mph wind 3x the speed of the wind with an apparent headwind of 20mph different from a cart traveling forward into an actual 20mph headwind and utilizing that headwind to go half again faster than the wind?
The difference is obvious:

Downind faster than the wind: airspeed < groundspeed
Upwind: airspeed > groundspeed

For propulsion in steady state, energy conservation demands that you harvest energy at the faster relative speed, and generate propulsion at the slower relative speed.

This is described on page 12 of this solution sheet for a Physics Olympiad test:
http://www.aapt.org/physicsteam/2013/upload/E3-1-7-solutions.pdf

And in more detail in this paper:
http://orbit.dtu.dk/fedora/objects/orbit:55484/datastreams/file_3748519/content

Re: Direct Down Wind Faster Than The Wind
Reply #1576
lets just say the belt is infinitely long so the cart does not "know" that it wraps around some cylinders.
You don't need an infinite belt. If the cart only interacts with the straight part of the belt, it's irrelevant happens at the rollers.

I don't think the last two responses followed my point, or got the point I was trying to make regarding the "BOX" on the treadmill.

(correction, one additional unrelated post has been posted since I wrote this)

Imagine a very large and wide treadmill that we could all get on top of. Build a house on and live in. The FLOOR of the house is the treadmill. This is why I said the box does not have a bottom. The box the house the treadmill along with the cart are all being carried along at a uniform pace and don't know that they are moving.

Now set the cart down on the treadmill. We are all there and are standing around motionless, seemingly. What is going to happen?

My prediction is the cart will remain stationary and do absolutely nothing, as it would if you set the cart on the floor under normal conditions in an ordinary house on the earth.

Outside the house however, with the above scenario, the wind is raging. If some giant were to lift the house and expose us to the outside air we would find that we are traveling at a great speed relative to the earth but relative to the earth the air is motionless so we would be experiencing a great wind. Now set the cart down and it would be blown by the wind. And so would we be experiencing  wind.

Re: Direct Down Wind Faster Than The Wind
Reply #1577
lets just say the belt is infinitely long so the cart does not "know" that it wraps around some cylinders.
You don't need an infinite belt. If the cart only interacts with the straight part of the belt, it's irrelevant happens at the rollers.

I don't think the last two responses followed my point, or got the point I was trying to make regarding the "BOX" on the treadmill.

(correction, one additional unrelated post has been posted since I wrote this)

Imagine a very large and wide treadmill that we could all get on top of. Build a house on and live in. The FLOOR of the house is the treadmill. This is why I said the box does not have a bottom. The box the house the treadmill along with the cart are all being carried along at a uniform pace and don't know that they are moving.

Now set the cart down on the treadmill. We are all there and are standing around motionless, seemingly. What is going to happen?

My prediction is the cart will remain stationary and do absolutely nothing, as it would if you set the cart on the floor under normal conditions in an ordinary house on the earth.

Outside the house however, with the above scenario, the wind is raging. If some giant were to lift the house and expose us to the outside air we would find that we are traveling at a great speed relative to the earth but relative to the earth the air is motionless so we would be experiencing a great wind. Now set the cart down and it would be blown by the wind. And so would we be experiencing  wind.

OK, now I understand the situation you are describing.

Of course, in the house on the TM, the cart would do nothing because there is no wind in the house.

Outside the house, on the lawn (TM belt), There is a wind.  So the cart would work correctly as a DDWFTTW cart as long as the TM belt was moving at a speed (over the ground somewhere underneath it assuming the air was still with respect to that ground) > Vminhover.  You can imagine that you put this enormous TM in a box in an enormous truck and drive down an enormous freeway at a constant speed and it would all still work fine (even if the truck speed was such that the upper belt speed over the ground below the truck was stationary!).

That basically describes the galilean relativity of all inertial reference frames.

Windgrins :grin:
Lunatic Fringe, I know you're out there.  You've got to blame someone for your own confusion.

  • A.T.
Re: Direct Down Wind Faster Than The Wind
Reply #1578
I don't think the last two responses followed my point, or got the point I was trying to make regarding the "BOX" on the treadmill.
Yes, you basically describe Galilean Relativity, and how the Cart cannot tell the difference between airmass moving vs. surface moving - only the relative velocities matter. That's the idea behind the controlled treadmill test.

But my post 1575 is not about that, but about how downwind faster than wind and upwind, are just reversed cases of each other, and how you have to reverse power transmission between them.

Here some simple analogies for this:

https://www.youtube.com/watch?v=za_rPKSwiyc

https://www.youtube.com/watch?v=pw_B2MnMqZs
  • Last Edit: November 02, 2017, 12:24:15 AM by A.T.

Re: Direct Down Wind Faster Than The Wind
Reply #1579

Yes, you basically describe Galilean Relativity, and how the Cart cannot tell the difference between airmass moving vs. surface moving - only the relative velocities matter. That's the idea behind the controlled treadmill test.

What if you put the cart near the top of a 20 foot TM, (if there was one) and dropped on the belt from just an eight of an inch above it. Without the kinetic energy developed while holding the wheels on the moving belt, might not the cart go backwards all the way as I saw one do on a standard TM??

Re: Direct Down Wind Faster Than The Wind
Reply #1580

Yes, you basically describe Galilean Relativity, and how the Cart cannot tell the difference between airmass moving vs. surface moving - only the relative velocities matter. That's the idea behind the controlled treadmill test.

What if you put the cart near the top of a 20 foot TM, (if there was one) and dropped on the belt from just an eight of an inch above it. Without the kinetic energy developed while holding the wheels on the moving belt, might not the cart go backwards all the way as I saw one do on a standard TM??

Only for a short time until it stopped skidding and gained good traction.  As soon as the wheels/drivetrain reaches Vminhover speed and just a little bit more to overcome the linear and rotational inertia, it will start up the belt  (assuming the belt speed is > Vminhover to begin with.  We know this because a good cart will self start in adequate wind.

So as long as you have a long enough TM, it doesn't matter how hard you push it backwards, it will eventually stop and start coming up the TM.

Windgrins :grin:
  • Last Edit: November 02, 2017, 06:43:12 AM by windgrins
Lunatic Fringe, I know you're out there.  You've got to blame someone for your own confusion.

  • Brother Daniel
  • Global Moderator
  • predisposed to antagonism
Re: Direct Down Wind Faster Than The Wind
Reply #1581
Heinz poses the question:  If you have a treadmill running at v0 = 4.0 m/s, and a cart initially keeping up with the treadmill (presumably because it has been held in place to get the wheels and prop up to speed, then released), with a braking force of FB = 0.25 N, and with a transmission efficiency (from the wheels to the propeller) of τ = 0.90, what is the thrust from the propeller?

The answer depends on a bunch of other factors, of course.  But having given us a figure for FB, that helps narrow things down.

Let I be the moment of inertia of all the rotating parts of the cart, considered collectively, as measured at the wheels.  Let r be the radius of the wheels.  Let m be the mass of the cart.  We don't need these things individually, but we are interested in the ratio χ = I/(m r2).  This ratio can be just about any positive number, although I think a case with χ > 1 would be rather weird.  It will be low if the cart's mass is concentrated in the body, and somewhat higher if the mass is concentrated in the rims of the wheels and/or in the propeller.

Other things we need:  Let ρ denote the density of the air, let A denote the area of the disc swept by the propeller, and let ζ denote the propeller's "figure of merit" for the case of static thrust.

The to get the thrust force FT, we need to solve

FT3/2 / (ζ v0 (2 ρ A)1/2) + χ FT = (χ + τ) FB.

(Where does that come from?  I'll happily go through it if someone asks, although Heinz probably won't be able to follow the math.)

So if we take (say) χ = 0.50, ζ = 0.80, ρ = 1.225 kg/m3, and A = 0.126 m2 (corresponding to a propeller diameter of 0.40 m), then we get FT = 0.41 N.  Significantly greater than the braking force.

An energy accounting (in the rest frame of the room):

Rate of rotational KE gained by the cart from the treadmill:  FB v0 = 1.00 W.

Rate of rotational KE lost by the cart due to transmission inefficiency:  (1 - τ) FB v0 = 0.10 W.

Rate of rotational KE lost by the cart due to resistive torque at the prop:  FT3/2 / (ζ (2 ρ A)1/2) = 0.59 W.

Net rate of rotational KE gained by the cart:  1.00 W - 0.10 W - 0.59 W = 0.31 W.

Rate of linear KE gained by the cart:  FT v = 0, because v = 0.

Rate of KE added to air (to create a breeze behind the prop):  FT3/2 / (2 ρ A)1/2 = 0.47 W.

So altogether, the 1.00 W supplied by the treadmill breaks down into:
0.31 W gained by the cart (net gain)
0.10 W in transmission losses (from wheels to prop)
0.47 W gained by the air (as a breeze)
0.12 W in other losses (at the prop)

  • Brother Daniel
  • Global Moderator
  • predisposed to antagonism
Re: Direct Down Wind Faster Than The Wind
Reply #1582
Rate of linear KE gained by the cart:  FT v = 0, because v = 0.
I should have said:  (FT - FB) v = 0, because v = 0.

  • A.T.
Re: Direct Down Wind Faster Than The Wind
Reply #1583
But my post 1575 is not about that, but about how downwind faster than wind and upwind, are just reversed cases of each other, and how you have to reverse power transmission between them.

Here some simple analogies for this:

https://www.youtube.com/watch?v=za_rPKSwiyc

https://www.youtube.com/watch?v=pw_B2MnMqZs


And here John Denker's explanation of it:

https://www.av8n.com/physics/sailing-upwind-downwind.htm

  • A.T.
Re: Direct Down Wind Faster Than The Wind
Reply #1584
Inman Harvey, from the University of Sussex, uses DDWFTTW as one example where linear cause-effect reasoning fails.:

http://ensoseminars.com/media/recommended_reading/harvey_circles_submitted.pdf

Re: Direct Down Wind Faster Than The Wind
Reply #1585
And here John Denker's explanation of it:
https://www.av8n.com/physics/sailing-upwind-downwind.htm


I've used essentially that same example many times to convince people there's nothing stopping us from going DDWFTTW.  I typically use the case of a sailboat tacking upwind (at any speed - even a fraction of windspeed) to show that the boat is equally well described as tacking down-current faster than the current.   Because a sailboat is just two wings connected by a bobber - with one wing in each of two fluids, we know that you could equally tack DWFTTW.  From tacking DWFTTW we know it's easy to get to Directly-DWFTTW.

  • Last Edit: November 02, 2017, 05:20:51 PM by spork

Re: Direct Down Wind Faster Than The Wind
Reply #1586
Thanks for the reply, Grins. So glad to see you back. Your explanation is what I expected, and if IIRC Harold once claimed the cart would continue going forward even on an unending TM. I'm 95% sure you are right. But the absolute certainty expressed by you and most others here continues to amaze me. I mean, think of all the manor discoveries in science that were denied by the majority for quite a while, along with new claims hyped then later proven wrong.

I once offered I Retant $200 to send me a video of his cart going up a TM. His earlier videos of it going up against a bag of packing foam going ddw were only ones I've ever seen, and that lack of visual evidence will maintain my small doubts. This is especially true as I Ratant expected totally different results. He wasn't a denier. He was a very experienced model builder, specializing in planes and helicopters. That's why his videos still haunt my thinking. The foam clearly showed the cart was going DDW as the foam sailed by it.

I had hoped RR was going to erase them, but the steady wind season is over in Utah, so I've given up on that. I don;t have similar doubts about BB's exceeding ws, but still don't buy such a small prop with less weight and thus traction could do it.


  • MikeB
Re: Direct Down Wind Faster Than The Wind
Reply #1587
95 %, hmmm, some 5 % is missing.

Re: Direct Down Wind Faster Than The Wind
Reply #1588
Thanks for the reply, Grins. So glad to see you back. Your explanation is what I expected, and if IIRC Harold once claimed the cart would continue going forward even on an unending TM. I'm 95% sure you are right. But the absolute certainty expressed by you and most others here continues to amaze me.

Well, we aren't absolutely certain since we haven't seen it actually do it because we can't get our hands on a giant treadmill that is fast enough.  But I'd be willing to bet a large sum of money on it if someone produced one.  I certainly have no more doubt than if you handed me a heavy ball, and I turned it loose, it would fall to the floor.  That's reasonable certainty.  But I can always think of corner case situations like the ball is steel and someone planted a strong electromagnet in the ceiling.

I assume you meant that Harold claimed the cart would continue backwards.  It won't.  It may even go backwards for a bit if pushed hard enough.  It will then slow to a stop on the belt and start to advance forward until it is going DDWFFTW (or up the TM).  We know because we've seen these same carts self start and go DDWFTTW as well as having run a bevy of experiments that provide the full evidence for that behavior.

Interestingly, the force on the cart when pushed backwards tends to get smaller until it is moving with the belt if I'm not mistaken.  If it is pushed hard enough to go downbelt faster than the belt, the force from the wind gets larger until it is at a standstill on the belt.  But assuming the wind is above Vminhover and the cart will self start, then it starts picking up speed over the belt until it finally climbs the belt faster than the wind.  (The large BlackBird evidenced this kind of behavior).

Send me a really long TM  that can operate above Vminhover speed and I'll be glad to demonstrate it for you. :devil3:

Stay skeptical.  It's good for you!

Windgrins :grin:
  • Last Edit: November 02, 2017, 08:17:22 PM by windgrins
Lunatic Fringe, I know you're out there.  You've got to blame someone for your own confusion.

Re: Direct Down Wind Faster Than The Wind
Reply #1589
Actually, to my surprise Harold said it would keep advancing. If memory serves, his reason was somewhat similar to that of Heinz. Conversely I assumed it might well stop advancing once the kinetic energy attained holding the wheels down had run out, and then it would go backwards.

As I'm sure you know, I can't begin to grok all the physics involved in this discussion. I applaud your patience, and again ask forgiveness from those who've been irritated by my doubts.  :unsure:

Re: Direct Down Wind Faster Than The Wind
Reply #1590
Stay skeptical.  It's good for you!

It's always good to approach things with skepticism.  But at some point the evidence seems overwhelming.  Once the cart was built and demonstrated in the controlled conditions of my living room on a treadmill, demonstrated outdoors on the playa, simulated, analyzed, mathematically described, written up by Drela and others, awarded two world records by a disinterested 3rd party, used on the AAPT physics exam, and the results were repeated by others who made their own carts from videos I posted, I would think there's no room left for skepticism.

Re: Direct Down Wind Faster Than The Wind
Reply #1591
Unless you are Heinz!

  • Brother Daniel
  • Global Moderator
  • predisposed to antagonism
Re: Direct Down Wind Faster Than The Wind
Reply #1592
Heinz is credulously eager to latch onto any anti-ddwfttw idea that crosses his own mind, no matter how demonstrably stupid the idea is.  That approach doesn't deserve the label of "skepticism".  It's something quite different.

Re: Direct Down Wind Faster Than The Wind
Reply #1593
Actually, to my surprise Harold said it would keep advancing. If memory serves, his reason was somewhat similar to that of Heinz. Conversely I assumed it might well stop advancing once the kinetic energy attained holding the wheels down had run out, and then it would go backwards.

As I'm sure you know, I can't begin to grok all the physics involved in this discussion. I applaud your patience, and again ask forgiveness from those who've been irritated by my doubts.  :unsure:

Harold stated a lot of stuff, most of it nonsense.  And in general he and the H's never seemed to understand what kinetic energy means or how to successfully account for it (as it is a bookkeeping methodology based on the fundamental concepts of mass in motion with respect to something else).  Primarily anything which he thought would bolster the idea that "the cart can't work (as described)" he latched onto until proven wrong (again and again and again).  Harold, Humber, and Heinz's grasp of physics has probably been increased by 100X by participating in these threads because not one of them understood the most basic physics imaginable and even though they have been schooled, they still are below a "C" student with some fundamental lacks of understanding. 

But, I've also got to admit, that in arguing the points, my physics knowledge was increased and honed also so that's a good thing.  The difference being that when there was a bone of contention, we all argued it out and came to an agreement of the correct principles while the Hs completely ignored the outcomes.  We were willing and eager to perform real world experiments if at all possible to validate our thinking also as opposed to just making internet noise.

You may notice that several years down the road from the original topic that all reputable doubters that have admitted anything have eventually grasped the concepts that Spork espoused and now the case is being used as a tricky brainteaser for students to illustrate some fundamentals.  No academics have come forward and found any flaws in the treatments Spork has described here from the earliest days.  In fact, many of them thought it was impossible, and it violated COE, and after digging in, found out that there knowledge was also lacking.  Which bolsters the idea that one shouldn't always accept "authority" without really validating the knowledge and assumptions necessarily if a deep question arises.

You should note that the guy who overthrew the idea that "this can't work" was a guy very deeply schooled in the fundamental knowledge of the field.  (One of the morals is that doubt is good but blind doubt doesn't accomplish much.)

So again, I salute Spork and JB for such a charming and controversial as well as eye opening and contentious brainteaser!

Windgrins :grin:
  • Last Edit: November 03, 2017, 05:55:06 AM by windgrins
Lunatic Fringe, I know you're out there.  You've got to blame someone for your own confusion.

Re: Direct Down Wind Faster Than The Wind
Reply #1594
lets just say the belt is infinitely long so the cart does not "know" that it wraps around some cylinders.
You don't need an infinite belt. If the cart only interacts with the straight part of the belt, it's irrelevant happens at the rollers.

I don't think the last two responses followed my point, or got the point I was trying to make regarding the "BOX" on the treadmill.

(correction, one additional unrelated post has been posted since I wrote this)

Imagine a very large and wide treadmill that we could all get on top of. Build a house on and live in. The FLOOR of the house is the treadmill. This is why I said the box does not have a bottom. The box the house the treadmill along with the cart are all being carried along at a uniform pace and don't know that they are moving.

Now set the cart down on the treadmill. We are all there and are standing around motionless, seemingly. What is going to happen?

My prediction is the cart will remain stationary and do absolutely nothing, as it would if you set the cart on the floor under normal conditions in an ordinary house on the earth.

Outside the house however, with the above scenario, the wind is raging. If some giant were to lift the house and expose us to the outside air we would find that we are traveling at a great speed relative to the earth but relative to the earth the air is motionless so we would be experiencing a great wind. Now set the cart down and it would be blown by the wind. And so would we be experiencing  wind.

OK, now I understand the situation you are describing.

Of course, in the house on the TM, the cart would do nothing because there is no wind in the house.

Outside the house, on the lawn (TM belt), There is a wind.  So the cart would work correctly as a DDWFTTW cart as long as the TM belt was moving at a speed (over the ground somewhere underneath it assuming the air was still with respect to that ground) > Vminhover.  You can imagine that you put this enormous TM in a box in an enormous truck and drive down an enormous freeway at a constant speed and it would all still work fine (even if the truck speed was such that the upper belt speed over the ground below the truck was stationary!).

That basically describes the galilean relativity of all inertial reference frames.

Windgrins :grin:


I think that last part would be imposible. Well.. Unless the truck were driving in reverse or opposite the belt direction.

Re: Direct Down Wind Faster Than The Wind
Reply #1595
You can imagine that you put this enormous TM in a box in an enormous truck and drive down an enormous freeway at a constant speed and it would all still work fine (even if the truck speed was such that the upper belt speed over the ground below the truck was stationary!).

That basically describes the galilean relativity of all inertial reference frames.

Windgrins :grin:


I think that last part would be imposible. Well.. Unless the truck were driving in reverse or opposite the belt direction.

The treadmill is in an enclosed box moving at constant velocity with respect to the road. Whatever happens to the cart on the TM in that box is going to be the same as when it is in Spork's house. That is really the whole point. The TM can't see the ground moving and so cannot know to change physics, even if the box were made of glass.

Re: Direct Down Wind Faster Than The Wind
Reply #1596
lets just say the belt is infinitely long so the cart does not "know" that it wraps around some cylinders.
You don't need an infinite belt. If the cart only interacts with the straight part of the belt, it's irrelevant happens at the rollers.

I don't think the last two responses followed my point, or got the point I was trying to make regarding the "BOX" on the treadmill.

(correction, one additional unrelated post has been posted since I wrote this)

Imagine a very large and wide treadmill that we could all get on top of. Build a house on and live in. The FLOOR of the house is the treadmill. This is why I said the box does not have a bottom. The box the house the treadmill along with the cart are all being carried along at a uniform pace and don't know that they are moving.

Now set the cart down on the treadmill. We are all there and are standing around motionless, seemingly. What is going to happen?

My prediction is the cart will remain stationary and do absolutely nothing, as it would if you set the cart on the floor under normal conditions in an ordinary house on the earth.

Outside the house however, with the above scenario, the wind is raging. If some giant were to lift the house and expose us to the outside air we would find that we are traveling at a great speed relative to the earth but relative to the earth the air is motionless so we would be experiencing a great wind. Now set the cart down and it would be blown by the wind. And so would we be experiencing  wind.

OK, now I understand the situation you are describing.

Of course, in the house on the TM, the cart would do nothing because there is no wind in the house.

Outside the house, on the lawn (TM belt), There is a wind.  So the cart would work correctly as a DDWFTTW cart as long as the TM belt was moving at a speed (over the ground somewhere underneath it assuming the air was still with respect to that ground) > Vminhover.  You can imagine that you put this enormous TM in a box in an enormous truck and drive down an enormous freeway at a constant speed and it would all still work fine (even if the truck speed was such that the upper belt speed over the ground below the truck was stationary!).

That basically describes the galilean relativity of all inertial reference frames.

Windgrins :grin:


I think that last part would be imposible. Well.. Unless the truck were driving in reverse or opposite the belt direction.

Yep. And at the same speed as the belt in the opposite direction so the velocity over ground for the belts cancels.  That is how it would do it so you easily figured it out. 

And it is an enclosed panel truck, not a flatbed if that wasn't obvious.  Then the "wind" would be the air in the truck which is now moving over the ground at belt speed!  So it would be no motion of the upper surface of the belt with respect to the ground and the air would be moving at the truck speed over the ground (which equals the belt speed with respect to the truck (but like you say, in the opposite direction).  So the entire situation would be exactly like the cart operating on the ground with a tailwind.  This is all illustrated on the zombie forum somewhere with pictures I once drew.

The attempt to deny this concept with the associated backflips by the H's was so incredibly stupid that it is worth reading.

Windgrins :grin:
Lunatic Fringe, I know you're out there.  You've got to blame someone for your own confusion.

Re: Direct Down Wind Faster Than The Wind
Reply #1597
Curiously, if it WAS a flatbead truck, the situation being; the truck giong "backwards", belt forward, cart, belt, house, observers on belt in house in truck on road...

There would be no wind. The cart would not move. Take the house away, now there IS wind. Take the box off the truck to make it a flatbed and the wind dissapears again. The cart stops.

My original point I was trying to make was that it isn't mathematics, or it can be figured out without math, it's the actual physical exposure to the air, moving along with or not moving along with or opposite to the cart.

It is hard though to comprehend that there is an actual wind on a cart on a treadmill in a house where there is no air movement relative to tha observers.

If the cart were placed on a treadmill without holding it in place, the cart would go down the treadmill and feel wind and start up and pick up speed and eventually come back. By holding it in place that just skips that. The cart though, is traveling with an actual physical air mass or wind for all intents and purposes.

Put the cart and treadmill inside a vacuum chamber and what happens?
  • Last Edit: November 04, 2017, 07:07:43 AM by Tom Booth

  • MikeS
Re: Direct Down Wind Faster Than The Wind
Reply #1598

Put the cart and treadmill inside a vacuum chamber and what happens?
You first.

Re: Direct Down Wind Faster Than The Wind
Reply #1599

Put the cart and treadmill inside a vacuum chamber and what happens?
You first.

LOL...

Well, I would say the cart would just go cascading off the back end of the treadmill as there is not even any stationary air for the prop to work against.

Or from a sporkian point of view, no air for the prop to cut into so as to pull itself forward..
  • Last Edit: November 04, 2017, 11:30:34 AM by Tom Booth