DDWFTTW - Tests.

[Dan O.]

This depends on the advance ratio and efficiencies of the drive-train etc. I'm pretty sure it is possible to build a DDWFTTW cart whose prop efficiency is less than 50%.

You're right there. The 50% comes from the solution for the case where the air is expelled from the prop at the speed of the ground. Of course, if more air is accelerated less, you get the same momentum change with less energy and as it turns out requires less efficiency from the prop.

 

[sol invictus]

Of course the ball can never travel faster than the overall speed of the moving surface because it doesn't have the facility to do this. A cart does in the form of a propeller.

Precisely.

Perhaps "achieved or developed energy" would be a better term than "stored energy". If the prop was sufficiently highly geared to the wheels and the speed of the treadmill was fast enough I imagine the cart could be made to fly vertically with the thrust of the prop. This doesn't mean it could do this continuously however. I have seen my cart both hover and progress against the turntable. My main question is can it do this continuously? If it can I don't currently see any reason why this doesn't mean it can travel DDFTTW. I am more interested in physically testing than discussing this.

I'm all in favor in doing experiments, but I don't think you've thought through the alternative very carefully. Once you see the cart accelerate from wind speed to faster than the wind, it's over. That means the forces when it's at wind speed aren't balanced, and instead make it go DWFTTW. Done.

 

[ynot]

Precisely.



I'm all in favor in doing experiments, but I don't think you've thought through the alternative very carefully. Once you see the cart accelerate from wind speed to faster than the wind, it's over. That means the forces when it's at wind speed aren't balanced, and instead make it go DWFTTW. Done.

I don't agree. As I said earlier - " If the prop was sufficiently highly geared to the wheels and the speed of the treadmill was fast enough I imagine the cart could be made to fly vertically with the thrust of the prop. This doesn't mean it could do this continuously however."

There is no argument (from me at least) that the thrust of a prop can be made to exceed the rolling resistance by "artificially" holding the wheel(s) that power it against a moving surface until the prop develops enough thrust to do so. There is a doubt in mind however that this can be sustained in a closed system. Holding the prop against the moving surface isn't a closed system and enough time must be given to let any "advantages" gained by "artificially" holding the cart against the moving surface to dissipate. If I conduct the tests correctly the cart should be able to hover and travel against the turntable indefinitely. Jumping of the ground doesn't prove that you can sustain levitation.

 

[sol invictus]

I don't agree. As I said earlier - " If the prop was sufficiently highly geared to the wheels and the speed of the treadmill was fast enough I imagine the cart could be made to fly vertically with the thrust of the prop. This doesn't mean it could do this continuously however."

Sure - but so what? First, the situation isn't comparable, because if the cart lifts off it loses its power source, which doesn't happen in the normal configuration. It's obvious instead what will happen - it will accelerate until it reaches a speed where the forces balance, where it will come to a steady state at that speed. Second, the claim was merely that the cart will go DDWFTTW. Once you see that it accelerates from windspeed, the claim has been demonstrated.

And even if something somehow went wrong at a higher speed (like the wheels started to slip, and it fell back to slower than windspeed or something) that would merely demonstrate an engineering flaw in the design.

 

[ynot]

Sure - but so what? First, the situation isn't comparable, because if the cart lifts off it loses its power source, which doesn't happen in the normal configuration. It's obvious instead what will happen - it will accelerate until it reaches a speed where the forces balance, where it will come to a steady state at that speed. Second, the claim was merely that the cart will go DDWFTTW. Once you see that it accelerates from windspeed, the claim has been demonstrated.

And even if something somehow went wrong at a higher speed (like the wheels started to slip, and it fell back to slower than windspeed or something) that would merely demonstrate an engineering flaw in the design.

The claim is NOT merely that a thing can travel DDWFTTW. Anyone can make a brick travel DDWFTTW if they throw it hard enough. The claim is that that it can travel DDWFTTW using the immediate force of the wind only (no storage) and that it can do it continuously/sustainably. If this is not your claim please say so.

If the cart starts to travel against the turntable/treadmill it should continue to do so as the conditions remain the same. Any wheel slip should happen then if it's going to happen later.

 

[mender]

On the treadmill you're starting the cart at wind speed in a steady stats. If the cart moves forward at all for even a short time, it is moving faster than the wind. This is exactly the same as a cyclist holding on to the side of a vehicle while the vehicle gets up to a certain speed. When the cyclist is stationary relative to the vehicle, it doesn't matter how long he stays holding onto the vehicle before he releases and starts pedaling. If he can go faster than the vehicle by pedaling, he does it whether he has been holding on for five seconds or five minutes. There is no way that energy can be be stored to increase the speed of the cart simply by holding the cart in position.

Doing it for two seconds is sufficient to prove that the cyclist can exceed the speed of the vehicle after releasing. The cart isn't subject to "tiring out" (maybe seizing up but that's a mechanical issue), so one can say that five seconds is as good as five hours or five years as long as the cart stays mechanically intact. Continuing to test past the actual event is just showing how long it will go DDWFTTW, not whether it can or not.

My street car has reached a top speed of just over 150 mph. It went that fast for about five seconds before I backed off. Holding that speed for five minutes is a test of the cooling and lubrication systems, not the horsepower of the engine.

 

[ynot]

On the treadmill you're starting the cart at wind speed in a steady stats. If the cart moves forward at all for even a short time, it is moving faster than the wind. This is exactly the same as a cyclist holding on to the side of a vehicle while the vehicle gets up to a certain speed. When the cyclist is stationary relative to the vehicle, it doesn't matter how long he stays holding onto the vehicle before he releases and starts pedaling. If he can go faster than the vehicle by pedaling, he does it whether he has been holding on for five seconds or five minutes. There is no way that energy can be be stored to increase the speed of the cart simply by holding the cart in position.

Doing it for two seconds is sufficient to prove that the cyclist can exceed the speed of the vehicle after releasing. The cart isn't subject to "tiring out" (maybe seizing up but that's a mechanical issue), so one can say that five seconds is as good as five hours or five years as long as the cart stays mechanically intact. Continuing to test past the actual event is just showing how long it will go DDWFTTW, not whether it can or not.

My street car has reached a top speed of just over 150 mph. It went that fast for about five seconds before I backed off. Holding that speed for five minutes is a test of the cooling and lubrication systems, not the horsepower of the engine.

Stopping the cart from moving backwards with the moving surface is the essentially same as pushing the cart up to wind speed. Stopping the cart from also moving forward with the moving surface is essentially the same as pushing the cart to faster than wind speed. In other words you are saying that if you push the cart to a speed faster than wind speed it is going faster then the wind. This is self-obvious and has nothing to do with the claim being made.

The cyclist analogy is more relevant to the marble demonstration than a prop driven cart. Like the marble the cyclist can't travel any faster than the thing that is powering it. A cart with a prop can. Pedaling is not getting energy from the vehicle and is like fitting a motor to the cart.

ETA - Have you built your hi-tech turntable yet?

 

[Dan O.]

Ynot, I think I see what your concern is. You think that the energy stored in the rotating prop is enough to give the cart a slight boost after it is released and if the wheels also loose traction after the release there won't be any drag to slow it down again.

If this is what you think, you are right.

So how do you plan to determine if the cart is steady state or still running off excess stored energy?

 

[ynot]

Ynot, I think I see what your concern is. You think that the energy stored in the rotating prop is enough to give the cart a slight boost after it is released and if the wheels also loose traction after the release there won't be any drag to slow it down again.

If this is what you think, you are right.

So how do you plan to determine if the cart is steady state or still running off excess stored energy?

Not quite what I think. The "battle" is between the thrust of the prop + cart KE and rolling resistance. If the cart is held "stationary" (backwards AND forwards) on the moving surface that is supplying it with energy then (as long as the prop efficiency, gearing and surface speed etc are sufficient) it will develop more than enough prop thrust to merely hover and it will progress. This only represents sustainable DDWFTTW if it is self-sustainable.

If the cart is set traveling at a constant speed counter to the movement of the surface then whatever wheel slip, gearing friction, headwind or whatever other inefficiencies there are should be pretty much constant. If it starts moving it should be able to continue to do so if the claim made is correct. If not the claim is incorrect.

As I said earlier, I plan to have the turntable spinning at a constant speed that allows the cart to slowly progress against it and run it for a long period to see if it losses energy to rolling resistance. As the guys at NASA say - "It's not rocket scie . . . oh . . . yes it is!

 

[spork]

Just before releasing the cart you're holding it at exactly wind speed steady-state. At that point it had better already be pushing itself forward. If it is, it will go forward faster than the wind steady-state guaranteed. Any issue of wheel slip is a mechanical limitation, not a theoretical limitation. We can replace the wheels and surface with rack and pinion - problem solved.

Also, there is no way the cart can be pushing forward while held in place steady-state by your hand because of stored energy. Incidentally, both experiment and analysis shows that stored energy is not a necessary piece of the puzzle at all.

 

[ynot]

Just before releasing the cart you're holding it at exactly wind speed steady-state. At that point it had better already be pushing itself forward. If it is, it will go forward faster than the wind steady-state guaranteed. Any issue of wheel slip is a mechanical limitation, not a theoretical limitation. We can replace the wheels and surface with rack and pinion - problem solved.

Also, there is no way the cart can be pushing forward while held in place steady-state by your hand because of stored energy. Incidentally, both experiment and analysis shows that stored energy is not a necessary piece of the puzzle at all.

Do you think in the test I've outline in my last post that the cart will constantly progress against the turntable?

 

[mender]

Stopping the cart from moving backwards with the moving surface is the essentially same as pushing the cart up to wind speed. Stopping the cart from also moving forward with the moving surface is essentially the same as pushing the cart to faster than wind speed. In other words you are saying that if you push the cart to a speed faster than wind speed it is going faster then the wind. This is self-obvious and has nothing to do with the claim being made.

In a word, no. Stopping the cart from moving forward is keeping it from accelerating past wind speed. By keeping it at a steady state speed before releasing, you ensure that the cart can only go faster if there is an unbalanced forward force acting on the cart.

This is absolutely wrong:
"Stopping the cart from also moving forward with the moving surface is essentially the same as pushing the cart to faster than wind speed."

First of all, you are keeping the cart from moving forward against the direction of the moving surface. You are keeping the cart from moving in order to have a baseline speed. When you release the cart, the motion from that point indicates what the cart is capable of achieving steady state. If it moves forward, it can go faster than the wind. If it stays in position, it can only match the wind speed. If it falls back, it is no better than a big marble.

There is no way to store energy that could accelerate the cart despite what Dan O. said. If the wheels start slipping as soon as you release the cart, it will go backward, not forward.

No table yet, but we better establish what the results of the testing signify if we expect the test to be of any value.

 

[ynot]

In a word, no. Stopping the cart from moving forward is keeping it from accelerating past wind speed. By keeping it at a steady state speed before releasing, you ensure that the cart can only go faster if there is an unbalanced forward force acting on the cart.

This is absolutely wrong:
"Stopping the cart from also moving forward with the moving surface is essentially the same as pushing the cart to faster than wind speed."

First of all, you are keeping the cart from moving forward against the direction of the moving surface. You are keeping the cart from moving in order to have a baseline speed. When you release the cart, the motion from that point indicates what the cart is capable of achieving steady state. If it moves forward, it can go faster than the wind. If it stays in position, it can only match the wind speed. If it falls back, it is no better than a big marble.

There is no way to store energy that could accelerate the cart despite what Dan O. said. If the wheels start slipping as soon as you release the cart, it will go backward, not forward.

No table yet, but we better establish what the results of the testing signify if we expect the test to be of any value.

If the cart is being prevented from moving forward against the moving surface when it "wants" to move forward then it is "artificially" being held at wind speed as the prop develops even more thrust. For the period that the cart is being held stationary on the moving surface all rolling resistance is effectively being removed and the prop can develop more thrust than it could with that rolling resistance. If the moving surface had enough power and speed and the prop was sufficiently highly geared to the wheel(s) and the prop was large enough, then a brick without wheels could be made to progress against the moving surface for a short period. I may develop a test to prove this if anyone disagrees.

Perhaps before we establish what the results of the testing signify, exactly what the claim is should be specified.

 

[mender]

For the period that the cart is being held stationary on the moving surface all rolling resistance is effectively being removed and the prop can develop more thrust than it could with that rolling resistance.

No, the rolling resistance is the same for that speed. The prop doesn't develop any more thrust than it would when at that speed as it accelerates through that speed. The instant of release is essentially a snapshot of the cart when it is at wind speed.

When you spin up one of those little flywheel powered toy cars, the highest speed that it can get to is the same as the speed you pushed it to - no higher!

Same thing applies to the cart. There is no way to store any extra energy or force that can accelerate the cart past wind speed if the cart isn't already capable of achieving DDWFTTW travel. If the cart moves forward when released, the case is proven. It's as easy as that. Longer testing won't show a different result unless there is a mechanical problem that crops up during testing.

Is there any disagreement with that?

 

[ynot]

No, the rolling resistance is the same for that speed. The prop doesn't develop any more thrust than it would when at that speed as it accelerates through that speed. The instant of release is essentially a snapshot of the cart when it is at wind speed.

When you spin up one of those little flywheel powered toy cars, the highest speed that it can get to is the same as the speed you pushed it to - no higher!

Same thing applies to the cart. There is no way to store any extra energy or force that can accelerate the cart past wind speed if the cart isn't already capable of achieving DDWFTTW travel. If the cart moves forward when released, the case is proven. It's as easy as that. Longer testing won't show a different result unless there is a mechanical problem that crops up during testing.

Is there any disagreement with that?

I agree that when the cart is at a certain speed a particular amount of rolling resitance is established and is relatively constant. I'm talking about the cart being being able overcome rolling resistance to reach that speed however, not when it has reached that speed. When the cart "wants" to move forward it has reached it's ability to do so. If it is restricted from doing so it will develop more thrust than it needs to move forward and when released will move forward faster than it would have.

 

[mender]

If the moving surface had enough power and speed and the prop was sufficiently highly geared to the wheel(s) and the prop was large enough, then a brick without wheels could be made to progress against the moving surface for a short period. I may develop a test to prove this if anyone disagrees.

I disagree 100%. Think about what you wrote. If your brick has no wheels, the prop has no way of making use of the relative motion of the ground and the air. That's like saying that you can make a sailboat that has no sail and by towing it behind another boat then releasing it, can somehow have the sailboat move faster than the boat that was towing it.

In order for the cart (or a brick) to work, the prop has to be connected to wheels.

 

[mender]

If it is restricted from doing so it will develop more thrust than it needs to move forward and when released will move forward faster than it would have.

No. If it is restricted from moving forward, it doesn't develop more thrust as a result. It has exactly the same thrust at 10 mph when held as it does as it accelerates through 10 mph. The cart has only been interrupted in that process by holding it at 10 mph or more correctly placed into the exact same condition. When released it moves to the speed that it is capable of. No faster than it would have.

How could it?

 

[ynot]

I disagree 100%. Think about what you wrote. If your brick has no wheels, the prop has no way of making use of the relative motion of the ground and the air. That's like saying that you can make a sailboat that has no sail and by towing it behind another boat then releasing it, can somehow have the sailboat move faster than the boat that was towing it.

In order for the cart (or a brick) to work, the prop has to be connected to wheels.

It is possible to attach a wheel to a brick that powers a propeller but the brick doesn't ride on it. As I said I might build a working model to demonstrate what I mean.

 

[ynot]

No. If it is restricted from moving forward, it doesn't develop more thrust as a result. It has exactly the same thrust at 10 mph when held as it does as it accelerates through 10 mph. The cart has only been interrupted in that process by holding it at 10 mph or more correctly placed into the exact same condition. When released it moves to the speed that it is capable of. No faster than it would have.

How could it?

One of the factors that determines whether a cart can reach or exceed the speed of a moving surface is rolling resistance. A cart that could never reach the speed of the moving surface if it wasn't artificially held stationary to it can if it is. In other words, a wheel that is held against a moving surface has to reach the speed of the surface and the propeller has to reach it's full thrust potential at that speed. This not the case if the wheel has to accelerate up to the speed of the moving surface without being forced to do so by being held. Once again I think there are experiments I can conduct to demonstrate this.

 

[sol invictus]

When the cart "wants" to move forward it has reached it's ability to do so. If it is restricted from doing so it will develop more thrust than it needs to move forward and when released will move forward faster than it would have.

That's wrong, but it's good - now we know where your confusion is coming from.

When the cart's wheels are spinning at a certain speed, the prop will supply a certain amount of thrust. At the same time friction in the wheels will provide a braking force. At wind speed there's no air resistance, so the only other force acting is the one supplied by your hand holding it in place (if it's there).

So consider two situations - your hand was holding the cart in place but has just released it this instant, versus the cart started from a slower speed and is accelerating, and at this instant it is at windspeed. In both situations the wheels are spinning at the same rate, therefore so is the prop, and therefore the force from the prop and the friction in the wheels are identical. If the cart accelerates in the first case, it will accelerate in the second. There is no "stored energy" present in the first that isn't there in the second.