Down wind faster than the wind

[Thabiguy]

I initially thought that the direction of the propeller was set so that a tailwind would make it rotate in the direction that makes the wheels go forward. Only after the treadmill explanation I realised that it's the other way around.

The direction of the rotation of the propeller in response to tailwind depends on whether the device is on the ground or whether you hold it in your hand. In the former case, it rotates to make the wheels go forward, in the latter case, it rotates to make the wheels go backward.

So the initial acceleration is not caused by the rotation of the propeller, but by the drag on the entire cart.

That drag helps, but it's actually the air pushing on the propeller that accelerates the vehicle.

As the cart speeds up the propeller provides less and less drag to the wheels and at some point starts providing net thrust.

The propeller provides net forward thrust to the vehicle at all times.

 

[ThinAirDesigns]

Hethetheth:

So the initial acceleration is not caused by the rotation of the propeller, but by the drag on the entire cart.

Thabiguy:

That drag helps, but it's actually the air pushing on the propeller that accelerates the vehicle.

I believe Heth's point was that during the first phase of 'self-start' acceleration in the breeze, even with the wheels on the ground the wind still want to spin the pop (and wheels) the wrong way. It is the 'bluff body' drag of the device that overpowers the desire for the prop to spin CCW and pushes the device downwind.

On the above point, he is correct.

JB

 

[ThinAirDesigns]

Fair enough. When you say a video “where the cart rolled for two minutes untouched” I guess you mean the one were the moving tread had a piece of flat wood placed under it? Sorry but I consider this a bogus test. You are not testing on a flat surface and you are running the cart with it’s wheels on either side a hump created by the board. The cart is essentially surfing with that hump. Don’t know why you didn’t just raise or lower one end of the treadmill.

We'll now at least I understand where a bit of the confusion is coming from.

Ynot, that "piece of flat wood" in the video is actually *two* pieces of thin trim board running down each side of the treadmill. Their only purpose is to turn up the edges of the treadmill belt so that when the cart rolls to the edge it doesn't fall off. They extend under the belt only about a half inch on each side. The 'bed' of the treadmill has not been impinged and the belt is running perfectly flat -- only a "fence' has been installed.

I would prefer the treadmill to have a slight uphill incline.

In that video, it does have a slight incline -- that's why the cart isn't climbing to the top.

What is being tested by this test is whether the thrust of the propeller that is greater than the rolling resistance can be sustained.

Well, now (if you believe that the board doesn't go under the belt) you can see that the test in the video does just that for almost 2 minutes.

JB

 

[tsig]

The direction of the rotation of the propeller in response to tailwind depends on whether the device is on the ground or whether you hold it in your hand. In the former case, it rotates to make the wheels go forward, in the latter case, it rotates to make the wheels go backward.



That drag helps, but it's actually the air pushing on the propeller that accelerates the vehicle.



The propeller provides net forward thrust to the vehicle at all times.

So the propeller moves the cart, the moving cart makes the wheels turn and the turning wheels spin the propeller which moves the cart.........

 

[Thabiguy]

So the propeller moves the cart, the moving cart makes the wheels turn and the turning wheels spin the propeller which moves the cart.........

Seriously, tsig, have you read any of this thread at all?

ETA: An external force making the propeller turn faster makes the wheels turn faster, which makes the cart move forward faster. And that's it. To accelerate further, more external force making the propeller turn faster is needed. The wind provides that force.

 

[Thabiguy]

I believe Heth's point was that during the first phase of 'self-start' acceleration in the breeze, even with the wheels on the ground the wind still want to spin the pop (and wheels) the wrong way. It is the 'bluff body' drag of the device that overpowers the desire for the prop to spin CCW and pushes the device downwind.

I think technically, that's not right (the non-sliding contact with the ground is itself sufficient to transform the "desire" to spin CCW into a "desire" to spin CW, and no additional drag on the non-propelling parts is necessary), but you know what... it would probably boil down to technicalities and different ways of looking at things (as we both agree about the outcome), so let's not argue about that.

 

[GreyICE]

Darwinian elimination for you on board the battleship, then.

Stop treating the real world and high school physics as if they were the same things. I've heard of using the tools you have, but this problem is not a nail, your hammer is retarded. Velocities are all relative terms. Accelerations are all that matters. You state that you can tell the difference between a 1 kg ball striking a 10 kg ball and a 10 kg ball striking a one kg ball. You require, of course, only a reference frame. Deprived of your reference frame, you cannot tell the difference. Lets deprive you of that.

A 100,000 ton meteorite hits the earth at 105,000 km/hr. Earth is motionless here, hit by meteorite.

The same meteorite sits suspended, motionless relative to the sun (which is hardly motionless relative to the galactic core, and the core itself is not motionless relative to other galaxies). The earth, doing its usual orbit about the sun, hits it at 105,000 km/hr.

Pray tell, which collision has more energy?

 

[ThinAirDesigns]

I think technically, that's not right (the non-sliding contact with the ground is itself sufficient to transform the "desire" to spin CCW into a "desire" to spin CW, and no additional drag on the non-propelling parts is necessary), but you know what... it would probably boil down to technicalities and different ways of looking at things (as we both agree about the outcome), so let's not argue about that.

LOL -- I understand your point -- and technicalities/semantics aside, I have no doubt that in the end you and I both understand and agree on what is happening.

JB

 

[Dan O.]

Stop treating the real world and high school physics as if they were the same things.

He's not using high school physics but the world view from over 400 years ago before Newton. When humber shows that he can actually apply physics and use the math to solve problems I'll take him off ignore and try to help with any remaining misunderstandings. In the mean time, I'd appreciate it if you didn't continue to pollute this thread by quoting his gibberish.

 

[spork]

(the non-sliding contact with the ground is itself sufficient to transform the "desire" to spin CCW into a "desire" to spin CW, and no additional drag on the non-propelling parts is necessary)

Correct. But the "non-sliding" part is the whole issue. As you can see in one of our videos the abrupt gust does cause our wheels to skid (and roll backward). I think we're all in agreement on that.

 

[Modified]

Correct. But the "non-sliding" part is the whole issue. As you can see in one of our videos the abrupt gust does cause our wheels to skid (and roll backward). I think we're all in agreement on that.

Just going by intuition, I would assume that if the steady wind were strong enough the same thing would happen at takeoff.

 

[spork]

Just going by intuition, I would assume that if the steady wind were strong enough the same thing would happen at takeoff.

Yes, I would expect so. It's caused by the limited friction of the wheels against the road.

 

[Thabiguy]

Just going by intuition, I would assume that if the steady wind were strong enough the same thing would happen at takeoff.

Hmm... that's an interesting scenario. It could of course happen if the wind was strong enough (as Spork points out, the sliding friction between the wheels and the ground is limited). The question is, what would happen to the device next, in such a steady-wind situation.

Also going just by intuition, I can picture that it might settle into an equilibrium that would involve the propeller rotating in the "wrong" direction, the wheels skidding backwards, and the device crawling in some direction - forward if unwanted drag at zero speed exceeds sliding friction, backward if sliding friction exceeds unwanted drag at zero speed. Or, it's even possible that there would not be such an equilibrium, and the device could accelerate forward all the way into non-skidding mode.

Proper analysis of the situation would be rather complicated, because in the non-sliding scenario, there is only one variable describing the state of the vehicle (its speed), but in this case there are two variables (speed of the vehicle and speed of the wheels/propeller), and force affecting each depends on both. I suspect that the calculation would end up in differential equations, and I don't want to go there unless I have to. But if someone else wants to...

In any case, this is not the intended mode of operation for the OP device.

 

[ThinAirDesigns]

I will point out that on this video:

http://www.youtube.com/watch?v=kWSan2CMgos

The two opening scenes differ in couple areas key to this discussion:

In the first scene, there is a steady breeze and we have rubber bands wrapped around the wheels.

In the second, the bands have come off and it's hard plastic to pavement. Also it's a gust that hits rather than steady wind.

To me it's clear from take one what will happen in a steady breeze with a reasonably sticky interface with the pavement.

JB

 

[H'ethetheth]

I believe Heth's point was that during the first phase of 'self-start' acceleration in the breeze, even with the wheels on the ground the wind still want to spin the pop (and wheels) the wrong way. It is the 'bluff body' drag of the device that overpowers the desire for the prop to spin CCW and pushes the device downwind.

That was my point, yes.

The direction of the rotation of the propeller in response to tailwind depends on whether the device is on the ground or whether you hold it in your hand. In the former case, it rotates to make the wheels go forward, in the latter case, it rotates to make the wheels go backward.

Yes, and in a certain range of speed to windspeed, the prop will therefore exert a braking torque on the wheels, as evidenced by the cart's behaviour at very high wind speeds relative to its speed.

That drag helps, but it's actually the air pushing on the propeller that accelerates the vehicle.

I did mean the entire cart.

The propeller provides net forward thrust to the vehicle at all times.

Well, yes and no. Think of it this way: The cart is standing still; at that moment the torque exerted on the wheels by the prop wants to drive the cart backward, and the total drag on the vehicle wants to drive it forward. The net thrust of the propeller (i.e including the torque to the wheels) then depends on the propeller's aerodynamic properties, the cart's aerodynamic properties, the gearing between the prop and wheels, and the size of the wheels (asumming sufficient grip). It should be possible in principle to make this cart roll into the wind with the right parts, but I'm not sure of it's practically possible, what with real world lift to drag characteristics for propellers etc.

But you're also right that this is probably a technicality/semantics issue. Or maybe I'm wrong; that happens.

 

[spork]

Yes, and in a certain range of speed to windspeed, the prop will therefore exert a braking torque on the wheels, as evidenced by the cart's behaviour at very high wind speeds.

In fact the prop applies a braking torque on the wheels at all speeds in downwind mode.

It should be possible in principle to make this cart roll into the wind with the right parts, but I'm not sure of it's practically possible, what with real world lift to drag characteristics for propellers etc.

All we have to do is use smaller wheels and it becomes a direct upwind cart. In theory it can go directly upwind faster than the wind. In practice that would probably be difficult.

 

[H'ethetheth]

In fact the prop applies a braking torque on the wheels at all speeds in downwind mode.

Hm, good point.

All we have to do is use smaller wheels and it becomes a direct upwind cart. In theory it can go directly upwind faster than the wind. In practice that would probably be difficult.

Have you tried smaller wheels, actually?

 

[humber]

Stop treating the real world and high school physics as if they were the same things. I've heard of using the tools you have, but this problem is not a nail, your hammer is retarded. Velocities are all relative terms. Accelerations are all that matters. You state that you can tell the difference between a 1 kg ball striking a 10 kg ball and a 10 kg ball striking a one kg ball. You require, of course, only a reference frame. Deprived of your reference frame, you cannot tell the difference. Lets deprive you of that.

A 100,000 ton meteorite hits the earth at 105,000 km/hr. Earth is motionless here, hit by meteorite.

The same meteorite sits suspended, motionless relative to the sun (which is hardly motionless relative to the galactic core, and the core itself is not motionless relative to other galaxies). The earth, doing its usual orbit about the sun, hits it at 105,000 km/hr.

Pray tell, which collision has more energy?

Amongst the rhetoric, the mass of the Earth disappeared. I think its mass is greater than 100e3 tons. Case B, pray thee tell.

 

[humber]

He's not using high school physics but the world view from over 400 years ago before Newton. When humber shows that he can actually apply physics and use the math to solve problems I'll take him off ignore and try to help with any remaining misunderstandings. In the mean time, I'd appreciate it if you didn't continue to pollute this thread by quoting his gibberish.

1. Tell any non-believers they "no nothing about science"
2. If they persist, "nothing about physics"
3. Up a level, "nothing about math" ( Math is the bestest of the sciences)
4. Back to basics, "nothing about the real world"

 

[spork]

Have you tried smaller wheels, actually?

I haven't. But I should. I could go with smaller wheels, higher prop pitch, or both.