Sailing downwind faster than the wind

[ThinAirDesigns]

...it is almost like you think that any controlled scientific study would show that your craft doesn't work as advertised.

Here's what it looks like to me Origin:

In post #4 of this thread, math treatments by one of the foremost aerodynamicists in the world showing that DDWFTTW violates no laws of physics were posted. This MIT professor even goes so far as to say in these treatments that it is "achievable without too much difficulty".

You continue to avoid the math while accusing us of avoiding tests. Well, if the math is definitive (it is), tests to determine if physical laws are violated become unneeded and a physical device becomes an engineering exercise rather than test of physical laws.

It's as if you are afraid that if you study the theoretical proof and can't find a flaw in it, you'll have to admit that you are wrong and our craft can work as advertised

Drelas treatment is a half a dozen pages long -- get on with it and find the flaw rather than avoiding it.

JB

 

[spork]

it is almost like you think that any controlled scientific study would show that your craft doesn't work as advertised.

Right, that's why I went to the trouble to make and post a series of detailed build videos - so people could attempt to reproduce our results and prove us wrong. :roll:

 

[centsworth_II]

...We've grown a bit weary of people explaining that every test or demonstration we've done is faulty, and that if we do their one simple test, that will put the matter to rest....

One simple and really necessary control would be to run the model on the treadmill with the propeller removed. Again, excuse me if this has been done and I haven't seen it. But if it hasn't been, it really needs to be.

If my concern about friction between the wheel and its connection to the vehicle is correct, the cart will advance on the treadmill without the propeller present. If the model stays put without advancing, and advances only when the propeller is in place, that would show the advance is due to the propeller alone. Of course an intermediate case is possible.

 

[eyytee]

One simple and really necessary control would be to run the model on the treadmill with the propeller removed

Why not with the wheels removed? :roll:

If my concern about friction between the wheel and its connection to the vehicle is correct,

Of course there is friction there. But why schould it be different between treadmill and real wind?

the cart will advance on the treadmill without the propeller present.

Can we have a free body diagram of that please?

 

[nimbus]

Not gonna read this thread or anything about this contraption after all...

Thanks for the heads up. It's too bad that you don't approve of our attitude of not being responsible to jump at anyone's request to do and document any ridiculous test.

And thanks for making my point. I saw some posts where the defenders (so to speak) were politely dealing with skepticism, but later on (what I saw skimming anyway) were a bunch of angry to and fros, and frankly I don't see the sense in getting hot and bothered over a piece of wood playing with the wind. Over the internet.. When I've got a ton of work already.

You can make that out to be sanctimonious BS, or you can appreciate that the debate happening here would work better if everyone made an effort to stay on good terms and clear understanding of arguments from either side; "win/win".

 

[spork]

One simple and really necessary control would be to run the model on the treadmill with the propeller removed. Again, excuse me if this has been done and I haven't seen it. But if it hasn't been, it really needs to be.

If my concern about friction between the wheel and its connection to the vehicle is correct, the cart will advance on the treadmill without the propeller present. If the model stays put without advancing, and advances only when the propeller is in place, that would show the advance is due to the propeller alone. Of course an intermediate case is possible.

Now that's a simple enough test. I can't recall whether we've done it (I kind of think we have), but it seems clearly obvious to me that it will have to go backward with the belt in that case. If you want me to do that test and post it, I'll be glad to.

I don't see the sense in getting hot and bothered over a piece of wood playing with the wind. Over the internet.. When I've got a ton of work already.

No worries. We're all free to participate in this thread or not. I just don't need people telling me I have a bad attitude because all I've done was put 100's of hours into this, be willing to share the knowledge we've gained, but not willing to jump every time someone has an idea to do an ill-conceived test.

You can make that out to be sanctimonious BS, or you can appreciate that the debate happening here would work better if everyone made an effort to stay on good terms and clear understanding of arguments from either side; "win/win".

I can appreciate that the debate would go better if people understood that I have no responsibility to prove a case for them or do their homework. You're welcome to what I've learned on the topic - which is considerable. That doesn't make me your boy.

 

[nimbus]

And you're welcome to realize I wasn't aiming for you especially.. See what I meant?

 

[ThinAirDesigns]

So centsworth, what's the story on the 'no prop' treadmill test? Do you want the prop completely removed, or just not spinning, or ... ?

I'm also interested in what you expect to see and why.

Thanks

JB

 

[Mee_n_Mac]

Let me see if I truly understand what's going on here and do so by way of an example (inspired by a prior post) that might make more sense to origin and others.

First let me assert (hopefully truthfully) that what happens is that the wind blows across the prop and like a sail or a wing it produces some lift. This drags the craft forward. The center of the wheels are thus dragged forward but the bottom of the wheels, being in contact with the ground resist, and so the wheel turns. The wheels are "geared" to the prop and so the prop now turns. Stopping here for a moment, it seems some usefully lift is be traded for some drag and frictional losses so the prop can turn. Continuing on ... the turning prop now follows a helical path which is not directly downwind, it's on a broad reach despite the body running directly downwind.

Alternately you could envision a very wide craft (say 5000' wide) where the prop isn't a prop but a rigid sail/wing. It's tied to the wheels via a long screw gear (like some garage doors have) running the width of the craft. Everything works as above except the sail/wing now moves transversely along the long width of the body, being turned by the screw gear at its base, instead of rotating like a prop. The wing/sail moves like it would on a sailboat during a broad reach but the wide body moves DDW (unlike the body of the sailboat). Of course this would work only until the wing/sail reached the end of the wide body and then everything would screach to a halt (this is where the sailboat would tack). So the prop vehicle works better than my concept but the screw gear-wing/sail vehicle bridges the gap btw it and a sailboat.

 

[origin]

Let me see if I truly understand what's going on here and do so by way of an example (inspired by a prior post) that might make more sense to origin and others.

First let me assert (hopefully truthfully) that what happens is that the wind blows across the prop and like a sail or a wing it produces some lift. This drags the craft forward. The center of the wheels are thus dragged forward but the bottom of the wheels, being in contact with the ground resist, and so the wheel turns. The wheels are "geared" to the prop and so the prop now turns. Stopping here for a moment, it seems some usefully lift is be traded for some drag and frictional losses so the prop can turn. Continuing on ... the turning prop now follows a helical path which is not directly downwind, it's on a broad reach despite the body running directly downwind.

Alternately you could envision a very wide craft (say 5000' wide) where the prop isn't a prop but a rigid sail/wing. It's tied to the wheels via a long screw gear (like some garage doors have) running the width of the craft. Everything works as above except the sail/wing now moves transversely along the long width of the body, being turned by the screw gear at its base, instead of rotating like a prop. The wing/sail moves like it would on a sailboat during a broad reach but the wide body moves DDW (unlike the body of the sailboat). Of course this would work only until the wing/sail reached the end of the wide body and then everything would screach to a halt (this is where the sailboat would tack). So the prop vehicle works better than my concept but the screw gear-wing/sail vehicle bridges the gap btw it and a sailboat.

The problem I have with the analysis is this, if the craft is moving at 10 mph with a steady 10 mph hour tail wind then the prop is in dead air. Regardless of the helical path it is in dead air. Right?

 

[centsworth_II]

So centsworth, what's the story on the 'no prop' treadmill test? Do you want the prop completely removed, or just not spinning, or ... ? I'm also interested in what you expect to see and why.

If you want to show the prop has an effect on the treadmill results, you have to show the lack of effect without the prop.

I think removing the prop while leaving the rest alone -- letting the wheels continue to spin the prop hub -- is the best way of determining the effect of the prop alone. Another trial could be run with the wheels disconnected from the prop mechanism to determine the effect of the wheel/prop connection (without the prop).

 

[eyytee]

Alternately you could envision a very wide craft (say 5000' wide) where the prop isn't a prop but a rigid sail/wing. It's tied to the wheels via a long screw gear (like some garage doors have) running the width of the craft. Everything works as above except the sail/wing now moves transversely along the long width of the body, being turned by the screw gear at its base, instead of rotating like a prop.

Yes, this basically "unrolling" the helical path of the blade:
[youtube]http://www.youtube.com/watch?v=zPFzHoubQzg[/youtube]

Another sail related example:
[youtube]http://www.youtube.com/watch?v=IMEerIkOVZ0[/youtube]
Of course, to have the center of mass travel DDWFTTW you would add a second sail/wheel-unit, and let them tack symmetrically.

The problem I have with the analysis is this, if the craft is moving at 10 mph with a steady 10 mph hour tail wind then the prop is in dead air. Regardless of the helical path it is in dead air. Right?

Only the center of the prop is in dead air. The blades where the lift is created experience an apparent wind.

 

[origin]

Only the center of the prop is in dead air. The blades where the lift is created experience an apparent wind.

Why?

 

[eyytee]

If you want to show the prop has an effect on the treadmill results, you have to show the lack of effect without the prop.

What other external force, than prop thrust, could accelerate the cart forward against the belt? Do you understand that forces between parts of the cart cannot accelerate the entire cart?

 

[eyytee]

Only the center of the prop is in dead air. The blades where the lift is created experience an apparent wind.

Why?

Because the blades have a different velocity vector than the air mass.

 

[origin]

Only the center of the prop is in dead air. The blades where the lift is created experience an apparent wind.

Why?

Because the blades have a different velocity vector than the air mass.

You'll have to explain that a bit more.

My problem is still this.

If I am in a calm wind condition (the air molecules are not moving relative to me) with a propeller and I spin it, it will slow and stop because it is expending its energy on the calm air.

If I am moving 10 mph with a 10 mph tail wind then the air molecues are not moving relative to me or the prop (or any portion of the prop) if I spin the propeller is will expend its energy moving the molecules and will slow to a stop.

Why is this analysis not right?

 

[eyytee]

If I am in a calm wind condition (the air molecules are not moving relative to me) with a propeller and I spin it, it will slow and stop because it is expending its energy on the calm air.

In this case, with both: air & ground at rest relative to you, you have no means to power the propeller.

If I am moving 10 mph with a 10 mph tail wind then the air molecues are not moving relative to me or the prop (or any portion of the prop) if I spin the propeller is will expend its energy moving the molecules and will slow to a stop.

In this case, the ground moves relative to you, so you can extract energy from it to power the propeller.

The key is. the ground moves backwards faster than the air relative to you, so you can apply a greater force to the air, using less power than you get from the ground: power = force * velocity

 

[ThinAirDesigns]

So centsworth, what's the story on the 'no prop' treadmill test? Do you want the prop completely removed, or just not spinning, or ... ? I'm also interested in what you expect to see and why.

If you want to show the prop has an effect on the treadmill results, you have to show the lack of effect without the prop.

I think removing the prop while leaving the rest alone -- letting the wheels continue to spin the prop hub -- is the best way of determining the effect of the prop alone. Another trial could be run with the wheels disconnected from the prop mechanism to determine the effect of the wheel/prop connection (without the prop).

Fair enough centsworth -- I'll video bot hof those tests and post it once you've told me:

A: What it means if the cart still moves forwards (does this mean that ddwfttw is impossible or possible or other?)

B: What it means of the cart moves backwards (does this mean that ddwfttw is impossible or possible or other?)

C: If your answer to either of the above was "other", how is the test relevent to the issue of ddwfttw.

I love doing tests, but understanding how the results will impact a hypothesis is key to setting up a well thought out test.

I do have one more question for you --- what do you imagine would happen to a simpl Tonka Truck or other free wheeling toy vehicle if you placed it on the belt of a moving treadmill? Do you think it would go fowards or backwards?

Thanks

JB

 

[spork]

Let me see if I truly understand what's going on here and do so by way of an example (inspired by a prior post) that might make more sense to origin and others.

First let me assert (hopefully truthfully) that what happens is that the wind blows across the prop and like a sail or a wing it produces some lift. This drags the craft forward. The center of the wheels are thus dragged forward but the bottom of the wheels, being in contact with the ground resist, and so the wheel turns. The wheels are "geared" to the prop and so the prop now turns. Stopping here for a moment, it seems some usefully lift is be traded for some drag and frictional losses so the prop can turn. Continuing on ... the turning prop now follows a helical path which is not directly downwind, it's on a broad reach despite the body running directly downwind.

Alternately you could envision a very wide craft (say 5000' wide) where the prop isn't a prop but a rigid sail/wing. It's tied to the wheels via a long screw gear (like some garage doors have) running the width of the craft. Everything works as above except the sail/wing now moves transversely along the long width of the body, being turned by the screw gear at its base, instead of rotating like a prop. The wing/sail moves like it would on a sailboat during a broad reach but the wide body moves DDW (unlike the body of the sailboat). Of course this would work only until the wing/sail reached the end of the wide body and then everything would screach to a halt (this is where the sailboat would tack). So the prop vehicle works better than my concept but the screw gear-wing/sail vehicle bridges the gap btw it and a sailboat.

You got it exactly right.

 

[ThinAirDesigns]

Let me see if I truly understand what's going on here and do so by way of an example (inspired by a prior post) that might make more sense to origin and others.

First let me assert (hopefully truthfully) that what happens is that the wind blows across the prop and like a sail or a wing it produces some lift. This drags the craft forward. The center of the wheels are thus dragged forward but the bottom of the wheels, being in contact with the ground resist, and so the wheel turns. The wheels are "geared" to the prop and so the prop now turns. Stopping here for a moment, it seems some usefully lift is be traded for some drag and frictional losses so the prop can turn. Continuing on ... the turning prop now follows a helical path which is not directly downwind, it's on a broad reach despite the body running directly downwind.

Alternately you could envision a very wide craft (say 5000' wide) where the prop isn't a prop but a rigid sail/wing. It's tied to the wheels via a long screw gear (like some garage doors have) running the width of the craft. Everything works as above except the sail/wing now moves transversely along the long width of the body, being turned by the screw gear at its base, instead of rotating like a prop. The wing/sail moves like it would on a sailboat during a broad reach but the wide body moves DDW (unlike the body of the sailboat). Of course this would work only until the wing/sail reached the end of the wide body and then everything would screach to a halt (this is where the sailboat would tack). So the prop vehicle works better than my concept but the screw gear-wing/sail vehicle bridges the gap btw it and a sailboat.

Very nice -- you nailed it.

JB

 

[origin]

If I am in a calm wind condition (the air molecules are not moving relative to me) with a propeller and I spin it, it will slow and stop because it is expending its energy on the calm air.

In this case, with both: air & ground at rest relative to you, you have no means to power the propeller.

If I am moving 10 mph with a 10 mph tail wind then the air molecues are not moving relative to me or the prop (or any portion of the prop) if I spin the propeller is will expend its energy moving the molecules and will slow to a stop.

In this case, the ground moves relative to you, so you can extract energy from it to power the propeller.

The key is. the ground moves backwards faster than the air relative to you, so you can apply a greater force to the air, using less power than you get from the ground: power = force * velocity

Yes exactly. You have to get the power from somewhere. So according to what you said the momentum of the cart is the power source that spins the propeller. This power will be taken from the momentum of the cart so the cart will slow down.

 

[spork]

My problem is still this.

If I am in a calm wind condition (the air molecules are not moving relative to me) with a propeller and I spin it, it will slow and stop because it is expending its energy on the calm air.

If I am moving 10 mph with a 10 mph tail wind then the air molecues are not moving relative to me or the prop (or any portion of the prop) if I spin the propeller is will expend its energy moving the molecules and will slow to a stop.

Why is this analysis not right?

That analysis, by itself, is right. But we're addressing two different things here. First we have to address whether or not the blades of the prop are experiencing any relative wind in this situation - not whether or not they'd coast to a stop - we'll address that next.

So do you agree that the blades of the spinning prop are feeling a relative wind over their airfoil when the prop is spinning and the cart is moving directly downwind at windspeed.

 

[spork]

Yes exactly. You have to get the power from somewhere. So according to what you said the momentum of the cart is the power source that spins the propeller. This power will be taken from the momentum of the cart so the cart will slow down.

Nope, the momentum of the cart never comes into it. Let's take this in steps. Do you agree that the prop blades would be feeling a relative wind while spinning on the cart that's going directly downwind at windspeed?

 

[origin]

Yes exactly. You have to get the power from somewhere. So according to what you said the momentum of the cart is the power source that spins the propeller. This power will be taken from the momentum of the cart so the cart will slow down.

Nope, the momentum of the cart never comes into it. Let's take this in steps. Do you agree that the prop blades would be feeling a relative wind while spinning on the cart that's going directly downwind at windspeed?

They would feel exactly the same relative wind speed as a stationary cart with no wind - sure.

 

[eyytee]

So according to what you said the momentum of the cart is the power source that spins the propeller. This power will be taken from the momentum of the cart so the cart will slow down.

No, because the thrust force of the propeller is greater than the breaking force at the wheels:

The ground moves backwards faster than the air relative to you, so you can apply a greater force to the air than you apply to the ground, while using less power than you get from the ground: power = force * velocity