How Does DDWFTTW Work and What Are Its Key Principles?

[rorix_bw]

It's very simple -- the rear axle is connected to the prop shaft through a long twisted bicycle chain.JB

Ya I saw that but you can't see if there are clutches or diffs. Are the rotation rates locked together?

 

[rcgldr]

Are you guys really saying that the cart is tugging the ground forward somewhat as it moves forward it as opposed to using it as a reaction mass?

Yes, the wheels are driving the prop which accelerates the relative headwind. The reaction force from the air results in an opposing torque on the prop which is transferred to the tires, which apply a forwards force onto the ground. The ground applies a backwards force onto the tires, and this force resists the forward motion of the cart. However the air applies a greater forwards force onto the propeller (due to the effective gearing that we've been calling advance ratio), so the cart is propelled forward (as long as there is a tailwind wrt ground).

 

[mender]

It actually looks to me from the photos that the wheels aid in rotating the prop.

That is correct; the wheels drive the prop.

 

[rorix_bw]

In one of the older threads, I think someone created a parts list for the small treadmill carts, that included the model prop used and differential from some radio control car.

Oh cool, will have a tinker. No large flat areas of land near me, but there is water. :-)

EDIT: Looks a lot harder to make it work on water though. Maybe needs a lake. I don't have a lake :-( This is probably doomed to failure!

 

[kmarinas86]

[Well it turns out that there is potential energy right there.] (Although, technically, at a closer inspection, we are dealing with kinetic energy of tiny gaseous particles that is at least to some degree contained by external forces.) [Some will like to call it "pressure energy" - fine. In this thread I've already called it "P-V" energy.]

If by this you mean that the cart uses the kinetic energy of the wind, you are correct. The cart takes KE out of the moving air mass (wind) by slowing it down relative to the ground with the propeller. After the cart passes through the air mass, the air that has been affected by the propeller will be moving slower showing that KE was transferred to the cart.

The KE of the moving air mass is the energy source for the cart.

So it seems that much of the disagreement about the "potential energy" I was invoking to explain the phenomenon was merely a semantic issue.

 

[kmarinas86]

I've been talking about the (+)30 mph (with respect to the ground) frame, in which the cart initially is moving 30 mph (with respect to the ground), and thus initially stationary at this point, and where the wind and the ground were moving in the (-) direction. This would imply that the center of momentum frame is not this 30 mph frame which I speak of. I cannot see how the wind (20 mph in my example) would be moving in the opposite direction as the ground in this moving frame.

If the 30 mph cart is used as the reference for the frame, the ground would be seen moving back at 30 mph while the air mass would be moving back at 10 mph.

Is that how you see it?

Yes.

 

[mender]

So it seems that much of the disagreement about the "potential energy" I was invoking to explain the phenomenon was merely a semantic issue.

Could be; definitions are pretty important!

So we're now on the same page re: the KE of the air mass being used as the energy source? And by your last reply, the frame of reference scenario is clear also.

Moving on; how do you see the cart working at this point? Just want to make sure that is clear as well.

 

[jduffy77]

So it seems that much of the disagreement about the "potential energy" I was invoking to explain the phenomenon was merely a semantic issue.

I am still afraid it was a bit more than semantics:

https://www.physicsforums.com/showpost.php?p=3691985&postcount=33

https://www.physicsforums.com/showpost.php?p=3692118&postcount=45

Could you re-visit the description in post 33? Unless you change that a bit it still seems to me there is some confusion.

 

[ThinAirDesign]

Ya I saw that but you can't see if there are clutches or diffs. Are the rotation rates locked together?

On the small models, direct drive. On the larger Blackbird, there are ratchets which only come into play of you brake quickly. The ratchets keep the mass of the rotating propeller from shearing off the drive if one has to stop quickly in an emergency. Under normal operation is is effectively a direct drive system as the ratchets never click.

JB

 

[ThinAirDesign]

So it seems that much of the disagreement about the "potential energy" I was invoking to explain the phenomenon was merely a semantic issue.

-No.

 

[kmarinas86]

Could be; definitions are pretty important!

So we're now on the same page re: the KE of the air mass being used as the energy source? And by your last reply, the frame of reference scenario is clear as well.

Moving on; how do you see the cart working at this point? Just want to make sure that is clear as well.

Yes.

Outside the context of this problem, I have even considered that the very idea potential energy, in general is an illusion manifested by kinetic energy contained below a certain bound of observation. For this problem, I have seem to adopted the notion of potential energy.

I can now imagine the paths of, say two particles of air, moving in two hyperbolic paths, approaching at a minimum and diverging away (like the "hourglass" figure). The particles change direction, but not necessarily their speed. The result of the latter divergence is to expand the volume occupied by two particles, allowing force to be applied to a surface, which is itself moving in the forward direction (from the point of view of the center of momentum of "two particles+surface") while the center of mass of the other two particles (taken separately from the surface) drifts in this backward direction. Of course, an "hourglass" shape is not required, I'm just using a common visual reference that people can relate to so I don't have draw a picture of random interactions.

Then all I have to do is increase the number of such interactions to extrapolate how the kinetic energy can be absorbed by the sail even though overall mass of air is moving in the opposite direction in the center of momentum frame.

 

[kmarinas86]

-No.

Wow, thanks a lot!

 

[ThinAirDesign]

Wow, thanks a lot!

{blushing}
It's the least I can do ... really, it is.

JB

 

[mender]

Yes.

Outside the context of this problem, I have even considered the very idea potential energy, in general is an illusion manifested by kinetic energy contained below a certain bound of observation. For this problem, I have seem to adopted the notion of potential energy.

I can now imagine the paths of, say two particles of air, moving in two hyperbolic paths, approaching at a minimum and diverging away (like the "hourglass" figure). The particles change direction, but not necessarily their speed. The result of the latter divergence is to expand the volume occupied by two particles, allowing force to be applied to a surface, which is itself moving in the forward direction (from the point of view of the center of momentum of "two particles+surface") while the other two particles whose sum drifts in this backward direction.

Then all I have to do is increase the number of such interactions to extrapolate how the kinetic energy can work on the sail in the opposite direction of the overall drift even though it is moving in the opposite direction.

Interesting theory if I'm understanding what you're saying but I see some issues with it.

It sounds like you're saying that a flat surface that is moving faster than the air mass can somehow generate pressure behind it. Yes?

As I said, if I'm understanding what you're saying.

 

[mender]

{blushing}
It's the least I can do ... really, it is.

JB

 

[kmarinas86]

Interesting theory if I'm understanding what you're saying but I see some issues with it.

It sounds like you're saying that a flat surface that is moving faster than the air mass can somehow generate pressure behind it. Yes?

As I said, if I'm understanding what you're saying.

A non-flat surface to be sure. A glider experiencing a slight downdraft during flight can still have lift fighting against it. But as for a flat surface? No, I'm not trying to say that. Blowing over a piece of paper does reduce the pressure on one side though. I can't imagine how it would increase the pressure on the other though unless some kind of funnel were present. (Okay, now that I think about it, I do think it is possible. The relatively unaffected air mass that is somehow behind the vehicle prop combined with the angle of attack of the blades can create a "funnel cup" to increase the pressure (if the air is compressing that is), or perhaps instead it merely transfers existing force momentum towards more focused "areas" that are projected onto the sail and the reacting mass, such that the average pressure (integrated over a closed surface) doesn't really change. Sorry for my ignorance of the actual technical terms!)

 

[A.T.]

What is the force "on" the ground?

The horizontal force exerted by the wheels on the ground.

Are you guys really saying that the cart is tugging the ground forward

The cart is exerting a forward force on the ground.

The only way I can see that happening is if the wheels slip, or if something non-rotating like skis are used.

Nope. The wheels can be rolling without slippage and still exert horizontal forces on the ground.

 

[kmarinas86]

The horizontal force exerted by the wheels on the ground.

The cart is exerting a forward force on the ground.

Nope. The wheels can be rolling without slippage and still exert horizontal forces on the ground.

So do you think the wind pushes both the cart and the ground even when using wheels?

I can imagine that it would if the wheels were "locked" so that they could not rotate.

I guess I should just go out and say that whether or not the ground experiences a force will depend on what is connected to the wheels and what is driving them.

If the wind can go in the opposite direction (backwards) that it applies a force to something else (forwards), maybe it is also true for a car moving forwards applying a force to something else (backwards).

For a car, I definitely do expect that the ground is a reaction mass to the car's acceleration. But maybe it is different for wind or even propeller thrust in general? What do you think? What would the reason for that be? (I can almost explain it, but I don't know what to say without (perhaps) going into more semantics issues.)

 

[mender]

(Okay, now that I think about it, I do think it is possible. The relatively unaffected air mass that is somehow behind the vehicle prop combined with the angle of attack of the blades can create a "funnel cup" to increase the pressure (if the air is compressing that is), or perhaps instead it merely transfers existing force momentum towards more focused "areas" that are projected onto the sail and the reacting mass, such that the average pressure (integrated over a closed surface) doesn't really change.

Or more simply, the angle of the attack of the prop plus its rotational speed allows the prop to push the air back as the prop moves through the air mass (by definition moving faster than the air), pushing the cart forward.

I think you're making things a little too complicated!

 

[mender]

So do you think the wind pushes both the cart and the ground even when using wheels?

I can imagine that it would if the wheels were "locked" so that they could not rotate.

I guess I should just go out and say that whether or not the ground experiences a force will depend on what the wheels do.

If the wind can go in the opposite direction (backwards) that it applies a force to something else (forwards), maybe it is also true for a car moving forwards applying a force to something else (backwards).

For a car, I definitely do expect that the ground is a reaction mass to the car's acceleration. But maybe it is different for wind or even propeller thrust in general? What do you think? What would the reason for that be?

If by reaction mass you mean that the wheels are pushing the cart forward by exerting a force on the ground, you are wrong!

The prop is pushing the cart forward by exerting a force on the air; for the prop to be able to generate that force, the wheels have to exert a force on the ground in the opposite direction (think about levers). The prop generates more force against the air than the wheels generate against the ground, so the net force is pushing the cart forward is greater and the cart accelerates.

 

[A.T.]

So do you think the wind pushes both the cart and the ground even when using wheels?

Yes. The air pushes the ground downwind via the cart.

I can imagine that it would if the wheels were "locked" so that they could not rotate.

No, rolling wheels can also exert horizontal forces.

For a car, I definitely do expect that the ground is a reaction mass to the car's acceleration.

When you gently use the brakes in a car the wheels are not sliding. And yet the car slows down. So there must be a horizontal force backward on the wheels, and an equal but opposite force forward on the ground.

 

[mender]

So do you think the wind pushes both the cart and the ground even when using wheels?

Yes.

This is very clear when the cart is stopped (wheels locked with brakes for example). If the brakes are released slightly so that the cart rolls, the wind is exerting a force on the cart which still exerts a force on the ground, meaning that the wind indirectly is pushing the ground. Good so far?

Duplication of posts, and I have to get some work done! I'll check in later but remember you're in good hands here!

 

[kmarinas86]

Or more simply, the angle of the attack of the prop plus its rotational speed allows the prop to push the air back as the prop moves through the air mass (by definition moving faster than the air), pushing the cart forward.

I think you're making things a little too complicated!

It's just hard for me to think of this wind as being a perfectly homogeneous entity while trying to explain why this is possible.

The problem was of course whether or not the prop had to do additional work on the particles of air to make the air push against the prop, combined with the fact that we are trying to explain the acceleration of the craft purely using wind. Well I know that average speed of air particles is always faster than the wind they make up (for it to equal it would be like having a condition of absolute zero for the wind, which is impossible). A simple deflection of a momentum doesn't require kinetic work (because its changing the direction only). So a few particles here and there can actually move faster than the craft temporarily, pushing the cart, while particles kicked back are moving slower.

 

[mender]

It's just hard for me to think of this wind as being a perfectly homogenous entity while trying to explain why this is possible.

The problem was of course whether or not the prop had to do additional work on the particles of air to make the air push against the prop, combined with the fact that we are trying to explain the acceleration of the craft purely using wind. Well I know that average speed of air particles is always faster than the wind they make up (for it to equal it would be like having a condition of absolute zero for the wind, which is impossible). A simple deflection of a momentum doesn't require kinetic work (because its changing the direction only). So a few particles here and there can actually move faster than the craft temporarily, pushing the car, while particles kicked back are moving slower.

Yup, too complicated!

Go back to thinking about a homogeneous air mass moving at a uniform speed rather than the individual particles; macro rather than micro! The way the cart works is explainable without invoking Brownian movement!

It's just hard for me to think of this wind as being a perfectly homogenous entity while trying to explain why this is possible.

That's 'cause you don't have the right explanation yet! As I said, it's much simpler than this, so pull out another sheet of paper and have another go!

 

[kmarinas86]

Yup, too complicated!

Go back to thinking about a homogeneous air mass moving at a uniform speed rather than the individual particles; macro rather than micro! The way the cart works is explainable without invoking Brownian movement!

If a train moves at 120 mph and I throw a curve ball at (initially) -60 mph, if the ball does not reach close enough to the train at the rear of one the train cars to hit the train, then it does not help the train to accelerate. Instead it will just pass by. (The exception is if it was a really good curve ball and could return to the back!)

Let's pretend for a moment that this ball is the "homogeneous air mass". How can the ball hit the back of a train car after that kind of throw? I can imagine it hitting the edge of the face of the back of one the train cars. But onto the face of it? No, I cannot imagine that. The ball is supposed to be the donor of energy, not the train. What could the train do to the ball to allow the ball to contribute net kinetic energy to the train? Bounce between train cars? There's no net gain for the train to be found there. A net loss of kinetic energy for the train is to be had if the ball settles down on the train itself, or if it bounces forward faster than the train as it leaves.

That's why wind to me has got to be inhomogeneous in order to explain how this DDTWFTTW stuff works.

 

[A.T.]

The way the cart works is explainable without invoking Brownian movement!

And sailboats can be explained without General Relativity and bending of space time. Believe us kmarinas.

 

[mender]

That's why wind to me has got to be inhomogeneous in order to explain how this DDTWFTTW stuff works.

That's 'cause you don't have the right explanation yet! As I said, it's much simpler than this, so let's pull out another sheet of paper and have another go!

Okay. When an airplane flies through the air (homogeneous air mass), does the prop on the plane move faster than the air around it? Of course it does. Is the prop able to exert a force on the air while it is moving faster than the air? Of course it is; that's what a prop is designed to do and it does it quite well.

Do you agree that a prop can exert a force on an air mass as it is moving through (and therefore faster than) the air mass?

 

[A.T.]

Let's pretend for a moment that this ball is the "homogeneous air mass". How does the ball hit the back of the train"? I can imagine it hitting the edge of the face of the back of one the train cars. But onto the face of it? No, I cannot imagine that.

This animation shows how the "ball" (air particle) hits the back of the propeller blade:

https://www.youtube.com/watch?v=FqJOVHHf6mQ

Here the version without lateral movement:

https://www.youtube.com/watch?v=Ufk6HVWdSzE

 

[kmarinas86]

This animation shows how the "ball" (air particle) hits the back of the propeller blade:

https://www.youtube.com/watch?v=FqJOVHHf6mQ

Here the version without lateral movement:

https://www.youtube.com/watch?v=Ufk6HVWdSzE

Okay. So the slant of the propeller blades combined with its rotation makes it look like the propeller is moving slower than it actually is.

Well, okay then, that does make sense.

 

[A.T.]

Okay. So the slant of the propeller combined with its rotation makes it look like the propeller is moving slower than it actually is.

Yes. The intersection point of the blade surface with the path of the particle is moving slower than the particle. Even if the cart is moving faster than the particle.

In the paddle wheel example it is more obvious that the lower paddles move slower than the air, even if the center of the wheel is moving faster.