I don't understand how this car works.

@DrewDad,

Quote:

Thrust from the propeller has to be less than drag from the wheels.

Your mistake, DrewDad, is that you keep telling yourself that the tailwind doesn't effect the thrust from the propeller. I think if you could accept that the tailwind does interact with the propeller (even when the car is either going at an equal speed with the tail wind, or faster then the tailwind), you would get over the hurdle to your understanding.

In fact, the propeller pushes air backwards into the tailwind. (Think of this as a conservation of momentum problem if it helps... the propeller is changing the momentum of the air in the tailwind). The faster the tailwind, the more thrust you get from pushing against it. (How else can I word this).

1 Reply

@DrewDad,

DrewDad wrote:

If the car cannot accelerate from rest when there is no relative wind, then I don't see how it can accelerate from a point when there is slack wind, even if the device is moving..

That of course is a key question and you appear to be stuck on a common misconception -- that we are extracting energy from the relative motion between the air and the vehicle -- we are not. We are extracting energy from the relative motion between the air and the *ground*.

When there is no motion between the air and the rolling surface, there is no energy to extract and thus the vehicle sits still. Once there is said relative motion, even when the vehicle is at exactly wind speed and has no wind moving over its chassis, the motion between air and ground remains and thus remains as a potential power source. It is this source we tap.

JB

2 Replies

@ThinAirDesigns,

I've read that explanation before (and actually replicated that explanation on this thread), but I'm afraid I don't understand how the device accomplishes this.

0 Replies

@ebrown p,

ebrown p wrote:

Your mistake, DrewDad, is that you keep telling yourself that the tailwind doesn't effect the thrust from the propeller. I think if you could accept that the tailwind does interact with the propeller (even when the car is either going at an equal speed with the tail wind, or faster then the tailwind), you would get over the hurdle to your understanding.

At any point when the device is moving at the same speed as the wind, there is no tailwind. Or perhaps I should say, no force impacting the car because of the wind speed relative to the car.

ebrown p wrote:

In fact, the propeller pushes air backwards into the tailwind. (Think of this as a conservation of momentum problem if it helps... the propeller is changing the momentum of the air in the tailwind). The faster the tailwind, the more thrust you get from pushing against it. (How else can I word this).

Perhaps a diagram, with forces delineated.

2 Replies

@ThinAirDesigns,

I have a question unrelated to proving that this effect happens... (I already understand why it works, so I don't need any more convincing).

What do you see as real-world marketable applications for this design/effect?

0 Replies

@DrewDad,

Quote:

At any point when the device is moving at the same speed as the wind, there is no tailwind.

At which point the spinning of the propeller provides a significant thrust which means there is an acceleration! That is why there is no null-point.

2 Replies

@ebrown p,

ebrown p wrote:

Quote:

At any point when the device is moving at the same speed as the wind, there is no tailwind.

At which point the spinning of the propeller provides a significant thrust which means there is an acceleration! That is why there is no null-point.

At which point the drag of the wheels against the ground provides significant counter-thrust, which means you're pulling energy out of your butt.

1 Reply

@DrewDad,

Quote:

At which point the drag of the wheels against the ground provides significant counter-thrust, which means you're pulling energy out of your butt.

You are counting the "drag" of the wheels twice. The tail wind can bring the car up to the 13mph speed without the propeller.

1 Reply

@ebrown p,

According to the person who claims to have designed this device, the wheels always provide motive force to the propeller. The drag from the wheels has to exceed the motive force provided by the propeller.

1 Reply

@DrewDad,

Quote:

The drag from the wheels has to exceed the motive force provided by the propeller.

YES! But this motive force is pushing against the tailwind. It is added to the tailwind.

2 Replies

@ebrown p,

ebrown p wrote:

Quote:
At which point the drag of the wheels against the ground provides significant counter-thrust, which means you're pulling energy out of your butt.

You are counting the "drag" of the wheels twice. The tail wind can bring the car up to the 13mph speed without the propeller.

Er... How am I counting the drag of the wheels twice?

0 Replies

@ebrown p,

But at the point where you reach a speed equal to the wind, there is no more tailwind. (Relative to the car.)

1 Reply

@DrewDad,

Quote:

But at the point where you reach a speed equal to the wind, there is no more tailwind.

Sure (relative to the car there is no more tailwind)... But now there is a spinning propeller accelerating the car.

1 Reply

@ebrown p,

ebrown p wrote:

Quote:
The drag from the wheels has to exceed the motive force provided by the propeller.

YES! But this motive force is pushing against the tailwind. It is added to the tailwind.

And the force from the wheels to keep the propeller moving is subtracted from the equation.

1 Reply

@ebrown p,

ebrown p wrote:

Sure (relative to the car there is no more tailwind)... But now there is a spinning propeller accelerating the car.

And drag from the wheels decelerating the car. And we've already covered that the drag from the wheels is always greater than the force imparted by the propeller.

You're still pulling energy out of your butt.

1 Reply

@DrewDad,

DrewDad wrote:

I've read that explanation before (and actually replicated that explanation on this thread), but I'm afraid I don't understand how the device accomplishes this.

Ok, I'll give that a go.

DrewDad wrote:

At any point when the device is moving at the same speed as the wind, there is no tailwind. Or perhaps I should say, no force impacting the car because of the wind speed relative to the car.

Correct DD -- when we are going the same speed as the wind (or faster) we simply can't wait for the wind to hit us -- as you point out it *can't* impact us. What do we do then ... WE HIT BACK!!

Literally, that's what we do -- a turbine is a passive device in that it relies on the action of others (the force of the wind) to hit it, offers resistance and slows down the air through this resistance. A propeller is an *active* device which says in essence "hell with you hitting me ... I'm going to reach around in front of you and smack YOU backwards, forcing you to slow down proactively".

Now, we can talk about where the energy comes from to power that propeller to smack the air, but if you are wondering how we take air that is already moving slower than we are and slow it down even more --- that's how.

JB

2 Replies

@DrewDad,

Quote:

And we've already covered that the drag from the wheels is always greater than the force imparted by the propeller.

I don't think we have in fact covered this in depth; just asserted it.

I think the propeller works in two ways: 1, it provides motive force to push the car forward. Second, it increases the surface area for the wind to push against, basically increasing the size of the sail without adding any additional weight. Not only that but the tips of the propellers ought to be constantly tacking despite the direct tailwind - so the new surface area is more efficient then the original one.

Am I right on this, inventor guy?

Cycloptichorn

0 Replies

@DrewDad,

Of course. The work from the wheels to keep to propeller moving balances out the work done by the propeller pushing air backwards. (edited to make the physics terms a bit more accurate).

You ignoring the fact that the propeller is pushing backwards against a tailwind. This tailwind adds to the thrust of the propeller. This makes all the difference.

0 Replies

@ThinAirDesigns,

ThinAirDesigns wrote:

Now, we can talk about where the energy comes from to power that propeller to smack the air,

Yes, that's exactly what I'm talking about.

Work equals force times distance. The motive force provided by the propeller is traveling over the same distance as the force acting against the wheels.

So motive force forward equals motive force backward. Both forces are acting on the same mass.

So acceleration forward equals acceleration backward.

1 Reply

@DrewDad,

Quote:

acceleration forward equals acceleration backwards

You realize that you aren't making any sense, right?

1 Reply

I don't understand how this car works.

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