Blowhards Sailing Downwind

Udderly plecostomus. You're taking an impossible case and offering that as proof that an impossible add-on to it will boost thrust.

Think about a hot air balloon cruising at 10 MPH in a steady 10 MPH wind. This works.

Extend wheels from the balloon down to the ground and let them spin generators, to power propellers, to make the balloon go faster. This will not work. Now it's like the car.

Actually, it will.

You don't get it, you're not going to get it, so that's that.

John

engine.

Gems: "I assume that the truck can maintain 10 MPH because, well, it's a truck. And because I said it was going 10 MPH." "Use the cruise control!"

Find a physics or chemistry guy, talk to him about it. Or don't.

Your gizmo beats using a generator to supply juice to an electric motor, which then spins the generator, which feeds the motor, which spins... Your gizmo surpasses the mere 100% efficiency of that perpetual motion machine, it creates excess power out of nothingness. You'd like a Nobel Prize, no? Then publish this!

OK, continue.

And John likes that the propeller works against still air. I guess with the 10 MPH tailwind we're talking about perhaps 10-to-15 MPH groundspeeds, versus 0-to-5 MPH airspeeds, right?

A sail will push the car to the initial 10 MPH max, then one throws a switch and the powerful non-turning propeller kicks in.

However, the propeller isn't working at 0 MPH. If it is, then it isn't working, so it isn't. But if it's a good *spinning* propeller, 90% efficient, and the generator/motor stuff is all 90% efficient too, then this miracle propulsion system has wasted just 20% of the power it robbed. Is it the world's best-kept secret? ?

In the above the ground is moving by at the same speed as the ground so there's no no way to exert a larger force with the power from the ground

But with with the wind.

The ground is moving past the vehicle at a greater speed (V) than the air. so for the same power (P) taken from the wheel on the ground a larger force (F) can be exerted on the air.

so 80% of the power extracted from the ground possing at 15mph is converted into force exerted exerted against the air that passing at 5mph meaning a force

F*0.8*15/5 = F*2.4

which is over twice the force slowing the vehicle dure to the machineery driven by the wheel)

Traveling near windspeed divisor is lower which suggestes that even greater force could be exerted but I suspect at thaose speeds the propellor is much less efficient in converting energy into force.

!!!

It's a Katakana charcter and it looks like a smile, here's one with a circle. ? and the usual smile symbol ?

RELATIVE still air. In that case, V is 0, so you can get an arbitrarily large amount of force form a small amount of power.

That's how it works.

I guess with

We've been using 10 MPH wind speed (ground speed) in most of these examples. Assume that.

Yup, it turns and pushes the car to over 10 mph ground speed.

Imagine sitting on the car at 10 mph with a 10 mph tailwind. It's dead calm. From the reference frame of the car, the propeller starts off seeing a 0 mph relative wind. As noted, it takes very little power for that prop to make a good deal of thrust. That thrust pushed the car to over 0 mph wind-relative speed, which is over 10 mph ground speed.

Got the trick yet? Tere's no perpetual motion, no free energy.

No, just a silly bar-bet quality trick. But you'd lose a beer betting against it.

John

No it doesn't. The truck supplies power for both vehicles, and the truck's fuel consumption will reflect this.

Yabbut John suggests to use cruise control so you won't notice this.

OK, you explained how the Miracle Propulsion System in a tailwind places only a small drag penalty at the wheel, and produces a larger force, as thrust, at the prop.

The system will work with a tailwind OR headwind, either way. The generator and motor will automatically switch roles!

Here's how the other half works: With a headwind, airspeed is greater than groundspeed. For the same Power (P) at each end of the system, there is less Force (drag) exerted on the propeller, and more Force, as thrust, output at the wheel. Viola! The propeller now powers the wheel.

And there's almost always some wind. Every Big Rig on the highways needs to have this system installed. Ships can put propellers up in the wind and attach generators to their water propellers to reap the benefits too. Why isn't this system being used?

I almost thought I had an Aussie font installed.

Yes. The faster you push the car ahead of the truck, the more work the truck has to do to maintain 10 MPH.

Whether the 10 MPH is servoed by foot control or cruise control doesn't matter.

To see fuel consumption, look at the gas gauge. Or an MPG display, if you have one.

John

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Ok, I get the idea about the truck pushing the car faster than the truck since the jack can be geared to move 2 feet to the rear while the car moves 1 foot ahead. Therefore when the truck moves 1 foot ahead, the car moves 2 feet ahead at increased speed. So as long as the truck can keep pushing, the car will keep accelerating. Interesting idea.

But in the real world, it seems the truck cannot keep pushing against another truck that pushes in the reverse direction, when the apparent wind is zero.

Where is the force when the apparent wind is zero?

-Bill

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Not to seem too crabby, but this has been explained a zillion times already. You extract power from a wheel and feed it to the pusher prop. The wheel works at ground speed but the prop works at relative wind speed, which is less. There is a lever-like mechanical advantage, so the silly thing accelerates.

John

The truck is the wind. That was your analogy.

"Another way to look at it: instead of a tailwind, imagine a truck behind you, going 10 MPH and pushing your car."

Now you see that the truck (wind) has to do more work. So it's either more pedal for the truck, or a bigger sail for the wind. The wheel/generator source is an illusion.

That's right. What matters is the car gets the additional power from an external source, the truck or the wind.

Dodge V8 truck, I can watch the needle on the gas gauge winding down. I really need the Miracle Propulsion System.

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No, no. You just finished saying that the truck has to work harder.

"The faster you push the car ahead of the truck, the more work the truck has to do"

That was five minutes ago.

Extracting power from a wheel ultimately extracts it from the truck's engine, you already acknowledged that too.

"Naturally. It's from the truck engine."

So explain exactly WHAT a generator or pump on the wheel is good for.

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Of course. If it's pushing a car that's going 10 MPH, it has to do some work. If it's pushing a car that's pushing back, so that the car is moving 12 MPH, the truck engine has to do more work, even though the truck is still going 10 MPH.

To produce the power needed to run the hydraulic jack (in one example) or the propeller (in the other.) In other words, to provide the power that lets the car move faster than the thing (truck in one case, wind in the other) that it's pushing against. In the big picture, that wheel power comes from whatever is doing the pushing, namely the truck or the wind.

But I'm sure you're not going to get it.

John

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Sounds familiar. The jack is

(1) Speeding up car + (-1) Holding back truck

----------------------------- *ZERO*

If they were the Bentley and S-10, it would appear that the car was still going 10 MPH and the truck slowed to 8. Or they'd go 9 and 11 if they were equal weight. So the jack is pushing two masses apart at 2 MPH, no net gain.

You know that the truck has to work harder or else it slows down. What about the wind? Same deal, it has to work harder, or else slow down. Will you deploy more sail when the system turns on? That will help maintain the car's speed close to, but less than, the 10 MPH tailwind.

The Rube Goldberg setup returns most of the power that it has taken from the external source that's really pushing the car along. I get that.

You lost me. Starting over with P=F.V,

What V are we talking about?? I say V is the prop windmilling, turning BACKWARD. Some of your F has to be used up just to bring the propeller to a standstill, before it can even begin turning the right way to provide thrust. In fact, ALL of your F is used up trying to stop the windmilling prop. And you're 20% short of the F you need to stop it, it's still turning slowly the wrong way. Thrust? Not even close to it.