Merged DDWFTTW in a “Real” Wind

[casebro]

This thread isn’t for a debate on whether DDWFTTW works or not. It’s for those that know it works and how it works to provide a simple as possible description of how it works ...

If you have any questions regarding my description please feel free to ask. If you consider anything is incorrect in my description please feel free to point it out

I'm not debating whether it works. I accept the demos.

I do proposed a different theory as to HOW it works, since your simplistic theory is perpetual motion.

 

[ynot]

Thanks for posting in an appropriate way.

Thanks for starting this thread ynot. It was lonely being the only dummy on the other ones, despite spork's affable and unlimited patience with me in explaining it all.

You do Humber a disservice

So if the wind isn't actually turning the prop, where is it's energy being transferred to? The frame of Blackbird doesn't present a very big sail, and yours on the turntable even less so. Given that all the energy comes from the wind, where is the transfer point? Explain just how the difference between the air and ground speeds make this possible.

The wind IS turning the propeller, it’s just not doing so directly as a turbine (windmill). It’s doing so via the wheels as a propeller (fan).
The frame of all the equipment shown in successful demonstrations is obviously sufficient as a “sail” to move the carts downwind (clearly shown by practical demonstrations). The only thing that acts to stop the cart moving downwind is that the wind acting directly on the propeller “wants” to use it a turbine and “pull” the cart upwind via the wheels. That the cart as a sail “wins” over the propeller as a turbine is because of the gearing between the wheels and propeller. If the propeller was sufficiently low-geared to the wheels the cart would move directly upwind. I have seen a Youtube clip of a small cart doing exactly this but can’t find it again.
Sorry but I don’t have time to continue at present.

 

[quarky]

A sphere has the highest volume/surface are of any form, so is unlikely to be beaten as a basic shape.
A serious obstacle would be keeping the load line within the skin. Without internal pressure, the balloon is like an arch, so the load line must remain within the material or it will collapse.

Yes, the sphere is the container of choice. Yet, a sphere of 1" diameter has a very high % of surface area/vol. The deal gets better as the radius grows...up to a point.

Not sure what you meant about the load line and such.
I think I grasp structures, in general.
Are we in danger of de-railing the thread?
I hate to get yelled at by mods.

Yet, I don't have enough to start a new thread...and all the smart mechanical engineers are already here.

 

[ynot]

I'm not debating whether it works. I accept the demos.

Just to be very clear - Are you saying that you accept that DDWFTTW works?

I do proposed a different theory as to HOW it works, since your simplistic theory is perpetual motion.

I think my description makes it very clear that a wind provides all the driving force to the cart - “A “driving wind” blows the cart directly downwind”. It could only be perpetual motion therefore if there was a perpetual wind.

 

[ThinAirDesigns]

Are we in danger of de-railing the thread?
I hate to get yelled at by mods.

I'm sure no one but the mods would care about a physics related derail since the whole point of the thread is to see what enertaining physics nonsense can be created by the hidiots.

JB

 

[quarky]

I'm sure no one but the mods would care about a physics related derail since the whole point of the thread is to see what enertaining physics nonsense can be created by the hidiots.

JB

Thanks. That was what I was thinking.

 

[mender]

How about you pick up that baton and have a go, and see how you get on against my reply?

http://demonstrations.wolfram.com/BallRollingWithoutSlippingInsideAVerticalCylinder/

 

[vwgub]

I do believe that theory IS perpetual motion, in that the cart, moving forward, spins the prop, which pushes the cart forward.

My own theory is that the wheel acts as a governor, keeping the prop spinning at just the right range of relative speed. In that range, the moving air creates lower air pressure on the leading edge of the prop, the higher pressure on the side of the prop pushes both the prop around, and the cart forward.

A spinning prop has more drag than a locked one. That is well know in sail boats with auxiliary engines, where in the captain will leave the engine in gear when not using it to prevent the prop from spinning.

This all may be a distinction without a difference though.

But food for thought: Why can't it also go cross wind? If all that matters is having the wheel turn the prop? In my theory, the cross wind would disturb the delicate balance of prop rpms to air flow.

AND IF IT IS SIMPLY THE WHEEL DRIVING THE PROP. WHY ISN'T INFINITE SPEED ATTAINABLE?

Wrong. L.

 

[ynot]

Very sorry I misread your post.

I do believe that theory IS perpetual motion, in that the cart, moving forward, spins the prop, which pushes the cart forward.

But the cart moves forward ONLY because a wind pushes it. If the wind stops the carts stops.

My own theory is that the wheel acts as a governor, keeping the prop spinning at just the right range of relative speed. In that range, the moving air creates lower air pressure on the leading edge of the prop, the higher pressure on the side of the prop pushes both the prop around, and the cart forward.

In any range of speed a propeller creates thrust that is a combination of “pulling” and “pushing”. The propeller thrust pushes the cart forward but it doesn’t push the propeller around. If a propeller could push itself around it really would be perpetual motion.

A spinning prop has more drag than a locked one. That is well know in sail boats with auxiliary engines, where in the captain will leave the engine in gear when not using it to prevent the prop from spinning.

So a spinning propeller acts better as a sail for the wind to push the overall cart downwind than a non-spinning one.

AND IF IT IS SIMPLY THE WHEEL DRIVING THE PROP. WHY ISN'T INFINITE SPEED ATTAINABLE?

It ISN’T simply the wheel spinning the propeller. It’s a WIND that pushes a cart, that turns a wheel, that spins a propeller. Everything is dependent on there being a wind and the speed of the wind and the efficiencies of the cart determine the attainable speed.

 

[windgrins]

Vacuum balloons

Yes. As Lyricist points out, a partial vacuum.
As humber points out below, its a surface area/volume ratio dilemma, as so many are.

As radius increases, the vol/surface area ratio gets much friendlier...but does it make analogous demands on the structural requirements?

In other words, is bigger better? Me thinks so. Yet, at a certain size, the weight of the atmosphere becomes an issue...more at the bottom of the balloon than at the top. This is assuming a crude geodesic sphere. Unlike a membrane without an underlying structure, the vacuum balloon would presumably remain roughly spherical.

Good architecture would allow for this anomaly; struts would be sized for the zone of air pressure they were exposed to.

At radius of 1/2 mile, for instance, if all the air could be removed, the weight removed would allow the floating of 1' or more of reinforced concrete.
This is clearly not do-able, as per limits of materials; expansion/contraction coefficients, etc, etc.

I've gone off topic, but hopefully its a welcome respite from the pointless bickering.
Risking more off-topic scolding, I wonder if spork and Tad have ever pondered the vacuum balloon?
I've been thinking about it , off and on, for 40 years.

(Not proud of that either...my wife wants me to mow the lawn and so forth.)

Hi Quarky,

Good questions. Basically, air exerts a pressure of 14.7 pounds/square inch at sea level over the area no matter how big the area. A perfect sphere distributes this force consistently. The material has to be able to withstand this amount of pressure per square inch and as it becomes larger, the total pressure over the sphere becomes larger. The buoyancy of the vacuum balloon is defined by the amount of weight of material that would otherwise fill the sphere if it was open.

So you can see, at pure vacuum, the weight displaced for every cubic meter of space is a little over a kilogram (about 2.2 pounds) but this cubic meter sphere must weigh less than this to have buoyancy. The larger it is the, worse the problem becomes as you have a total net force much greater the bigger the sphere. The ratio of material to vacuum is about 2 lbs of shell for every cubic meter for a positive buoyancy balloon. Lowering the pressure by only 5 PSI means you have much less air displaced and so one has to lower the shell weight commensurately. Since the volume of a sphere increases faster than the surface area, Buckminster Fuller proposed that a city sized sphere could have a small differential pressure and still provide lift. These weren't vacuum balloons but interesting reading nonetheless.

It is a tough problem but apparently achievable at the edge of material science. One needs a good reason to go to this much trouble and engineering to achieve this. I'm sure there probably are suitable applications but I can't think of any currently. It's pretty cool though.

I hope this helps.

WindGrins

 

[69dodge]

It ISN’T simply the wheel spinning the propeller. It’s a WIND that pushes a cart, that turns a wheel, that spins a propeller.

But when the cart is going faster than the wind, the wind isn't pushing the cart forward; it's pushing it backward. So why does the cart keep going forward, causing the wheels to turn and drive the propeller?

 

[windgrins]

Not off the Rails

Yes, the sphere is the container of choice. Yet, a sphere of 1" diameter has a very high % of surface area/vol. The deal gets better as the radius grows...up to a point.

Not sure what you meant about the load line and such.
I think I grasp structures, in general.
Are we in danger of de-railing the thread?
I hate to get yelled at by mods.

Yet, I don't have enough to start a new thread...and all the smart mechanical engineers are already here.

I don't think you have to worry too much about derailing this thread with pure physics questions. Everyone seems happy to entertain them and they certainly have some applicability to the fundamental topic, even if tangential.

The guy who started this whole topic-Spork, posed it as a physics/aerodynamics/sailing brainteaser and things that are along these lines seem just fine. Ask away!

WindGrins

 

[vwgub]

But when the cart is going faster than the wind, the wind isn't pushing the cart forward; it's pushing it backward. So why does the cart keep going forward, causing the wheels to turn and drive the propeller?

Wrong. L. Read the original thread.

 

[windgrins]

Humber has it! Quarky explanation!

A sphere has the highest volume/surface are of any form, so is unlikely to be beaten as a basic shape.
A serious obstacle would be keeping the load line within the skin. Without internal pressure, the balloon is like an arch, so the load line must remain within the material or it will collapse.

Humber is basically correct here. What he means about the load line is that all this force I've described earlier has to be transmitted through the skin of the sphere. If the actual vessel gets just a little bit away from the perfectly spherical shape, the force will cause the vessel to be relying upon material strength other than pure pressure (shear, etc.) Most materials that would be considered for resisting pressure would perform better under pure compression than they do in shear conditions. So the material in an imperfect sphere configuration is much closer to massive failure if the forces change from perfect compression to a shearing condition caused by this fundamental asymmetry of the vessel.

Or to put it all in simple language, a slightly out of round sphere is much easier to collapse than a perfect one.

I hope this helps.

WindGrins

 

[69dodge]

Wrong.

Can you be a bit more specific?

What did I say that was wrong?

Read the original thread.

Perhaps I will.

 

[vwgub]

Can you be a bit more specific?

What did I say that was wrong?



Perhaps I will.

The mechanism is counterintuitive. Read the original thread to get a handle on it. I still have a problem understanding it.

 

[ynot]

But when the cart is going faster than the wind, the wind isn't pushing the cart forward; it's pushing it backward. So why does the cart keep going forward, causing the wheels to turn and drive the propeller?

Good question and probably the hardest to understand and explain. In simple terms there are two winds in play. There is a driving wind and a propeller wind that pushes backwards against the driving wind. The speed of both winds combined is faster than the speed of the driving wind alone. The driving wind isn’t just pushing against the body of the cart, it’s also pushing against the propeller wind which is also pushing back against it. When the cart reaches and exceeds the speed of the driving wind the propeller wind is effectively still going slower than the cart and driving wind because it’s moving backwards relative to the cart and driving wind. When the cart reaches and exceeds the speed of the driving wind the propeller wind and the driving wind are still pushing against each other. Does that help?

 

[macdoc]

how about you just say

The forward motion is the sum of the vectors provided by the constant tacking of the prop.
Summed into the driving wheels by way of the chain.

For anyone that understands how a sailboat or an iceboat can sail into the wind the - the rapid tacking providing forward motion is easily understood. ( even if it is downwind, the prop is tacking providing crosswind vectors )

An iceboat can get downwind extremely fast and this just rapid tacking of that principle tho a rather inefficient one.

There is nothing mysterious except the explanations that go around.

TANSTAAFL

 

[ynot]

It is also claimed BB goes fastest moving directly downwind. Unlike sail craft, wind at angles slows it down. Can you explain how and why? And remember: idiots guide to props level. I've read many explanations for these questions. They seem logical, but based on assumptions that with my limited physics background are difficult to assess.

With reference to my last post - Two winds pushing against each other do so more effectively when they directly oppose each other.

 

[ynot]

how about you just say

The forward motion is the sum of the vectors provided by the constant tacking of the prop.
Summed into the driving wheels by way of the chain.

For anyone that understands how a sailboat or an iceboat can sail into the wind the - the rapid tacking providing forward motion is easily understood. ( even if it is downwind, the prop is tacking providing crosswind vectors )

An iceboat can get downwind extremely fast and this just rapid tacking of that principle tho a rather inefficient one.

There is nothing mysterious except the explanations that go around.

TANSTAAFL

You think that represents “a non-technical, simple, plain English description” for “Dummies”?