Downwind Faster than the Wind - Successful Run by manned cart

[trapezeartist]

I am trying to keep an open mind on this, even though this has all the hallmarks of a perpetual motion machine.

Snow Leopard keeps telling us that it is not driven by apparent wind but by the difference in speed between wind and ground. Therefore it would seem that the trolley needs to be driven by a windmill that is fixed to the ground.

Nevertheless, let's try to stick with it, and perhaps SL will be kind enough to tell me if each of my following assumptions is true.
The trolley starts off moving dead downwind at less than wind speed. It does this because the wind is blowing from behind and rotating the airscrew, which in turn drives the wheels to propel the trolley forwards. Am I right so far, SL?

Later, the trolley is moving faster than the wind. What is actually pushing the trolley along? As I understand SL's explanations, the forward motion of the trolley and resulting rotation of the wheels is driving the the airscrew, which functions as a propellor to push the trolley along. Am I still right?

In the two conditions above, is the pitch of the airscrew and the gear ratio the same? Or is one or both adjusted as the trolley accelerates?

 

[mikemonty]

On what basis are you suggesting that the system is subcritically damped?

Im suggesting that at rest the force on the airscrew is minimal - just enough to start the vehicle.
As the airscrew starts to work it starts to build a pressure "cushion" upwind that the wind can act against. A positive feedback loop increasing accelleration and taking speed up to windspeed and beyond.

As that cushion increases towards its maximum, determined by pitch,speed, available power and friction, the cart moves forwards at greater than windspeed (it appears in the video) against it, decellerating until it finds equilibrium.

If DWFTTW worls that equilibrium will be above windspeed, if not, then below windspeed.

The damping is, as ever, down to friction. Even with a variable pitch prop there is a limit to its efficiency.

 

[Ubergeekian]

I am trying to keep an open mind on this, even though this has all the hallmarks of a perpetual motion machine.

What hallmarks are those?

The trolley starts off moving dead downwind at less than wind speed. It does this because the wind is blowing from behind and rotating the airscrew, which in turn drives the wheels to propel the trolley forwards. Am I right so far, SL?

I'm afraid that you're dead wrong - but don't worry, as you have made a common mistake. The cart starts moving downwind for exactly the sae reason that a loose Avon inflatable moves downwind - simple drag as the wind passes over it.

As it moves, though, the wheels are turned and in turn (ha!) drive the propeller which is set up to blow the cart downwind faster. That's crucial bit (a) - the propeller pushes the cart along.

If you gave the cart a shove to start it in still air you would still have a retarding force from the wheels and an accelerating force from the prop, but the former would always be bigger and the system would slow to a halt. It's different when a wind is blowing, though, because then the thrust at the prop increases. If it helps you can think of the prop blowing back against the wind blowing forwards, with a net force resulting from both of these. This is crucial bit (b).

It's very counterintuitive at first!

In the two conditions above, is the pitch of the airscrew and the gear ratio the same? Or is one or both adjusted as the trolley accelerates?

I've seen references to variable pitch props but don't know if one is used here. The system would accelerate fastest with variable overall drive ratios, and varying the prop pitch is probably the best way to do this - gearboxes in the drive would be significantly lossy.

 

[RAI]

Congratulations to the team.

Nice to see they have increased their estimate of achievable speed from 2 x wind speed to 3 x wind speed dead down wind. That puts their cart in the same class as BMW Oracle or Alinghi but not yet up with the ice yachts.

Maybe SnowLeopard could re-publish his little vector diagrams again. The sceptics might look at them a little more carefully this time.

Or are they going to say the results are invalid because it's not the 1st of April yet?

 

[Blueboatman]

'' of course it will take forever to test a perpetual motion machine, caruthers''

 

[snowleopard]

Snow Leopard keeps telling us that it is not driven by apparent wind but by the difference in speed between wind and ground. Therefore it would seem that the trolley needs to be driven by a windmill that is fixed to the ground.

Not fixed to the ground or it wouldn't move! Connected to the ground by the wheels geared to the prop so if can use the relative speeds of air and ground

Nevertheless, let's try to stick with it, and perhaps SL will be kind enough to tell me if each of my following assumptions is true.
The trolley starts off moving dead downwind at less than wind speed. It does this because the wind is blowing from behind and rotating the airscrew, which in turn drives the wheels to propel the trolley forwards. Am I right so far, SL?

No, quite wrong. The very first impetus is given by the windage of the whole machine (or sometimes a gentle nudge). Once it starts to move the prop is turning in the opposite direction to the way the wind would drive it if it was free-turning (as several sceptics have already pointed out).

Later, the trolley is moving faster than the wind. What is actually pushing the trolley along? As I understand SL's explanations, the forward motion of the trolley and resulting rotation of the wheels is driving the the airscrew, which functions as a propellor to push the trolley along. Am I still right?

On that point, yes

In the two conditions above, is the pitch of the airscrew and the gear ratio the same? Or is one or both adjusted as the trolley accelerates?

This vehicle has adjustable-pitch blades and the driver is increasing the angle of attack as the speed increases. Models with fixed-pitch props do work but are less efficient as the prop only works optimally at one speed.

And for the benefit of the 'perpetual motion' contingent, it wouldn't move at all in a calm as there is no air movement relative to the surface which is its source of energy.

 

[Tommyrot]

Congratulations to the team, must have been difficult to convince anyone to help fund it.

 

[snowleopard]

Maybe SnowLeopard could re-publish his little vector diagrams again. The sceptics might look at them a little more carefully this time.

OK, but you can lead a horse to water....

V Velocity of vehicle
W Velocity of headwind
V-W headwind experienced by vehicle (V>W)
R rotational movement of prop blade
A apparent wind experienced by prop blade (not moving directly down wind!)
F force generated by air flow over prop blade

 

[shaunksb]

No it doesn't

I'm afraid that you're dead wrong - but don't worry, as you have made a common mistake. The cart starts moving downwind for exactly the sae reason that a loose Avon inflatable moves downwind - simple drag as the wind passes over it.

Is this a Troll?

So the drag off the little carty thing is more than the drag of the great big propeller? So much more its actually has enough power to turn it the wrong way when moving off. That's a lot of drag!

The cart is "aerodynamic" because in the back he has got his dads starter moter hooked up to all hi mates ipod batteries with his mothers washing machine belt driving the prop forwards.

THE EMPEROR HAS NO CLOTHES.

 

[Ubergeekian]

Is this a Troll?

Nope.

So the drag off the little carty thing is more than the drag of the great big propeller? So much more its actually has enough power to turn it the wrong way when moving off. That's a lot of drag!

The propeller produces thrust, not drag, remember.

THE EMPEROR HAS NO CLOTHES.

Whatever.

 

[snowleopard]

So the drag off the little carty thing is more than the drag of the great big propeller? So much more its actually has enough power to turn it the wrong way when moving off. That's a lot of drag!

At the start the 'great big propeller' is sitting there stationary with a wind of maybe 10 knots blowing over it. That's a fair bit of drag in my book, certainly enough to push the whole rig forwards and start the prop turning slowly. As the cart starts to move the blade is turning very slowly, the aerodynamic drag slowing the turning of the blades is very little so all the windage force has to overcome is: rolling resistance of the wheels, friction in the drive train and the inertia of the cart and blades.

 

[Keen_Ed]

THE EMPEROR HAS NO CLOTHES.

So please tell us what you're seeing in the video.

 

[shaunksb]

This is fascinating but now I'm late for work!

Forget all that "faster than the wind" malarkey, just explain to me how it goes in the first place at below wind speeds.

 

[snowleopard]

just explain to me how it goes in the first place at below wind speeds.

Post #31

 

[shaunksb]

Nope.



The propeller produces thrust, not drag, remember.

yes from the washing machine belt.

 

[shaunksb]

At the start the 'great big propeller' is sitting there stationary with a wind of maybe 10 knots blowing over it. That's a fair bit of drag in my book, certainly enough to push the whole rig forwards and start the prop turning slowly. As the cart starts to move the blade is turning very slowly, the aerodynamic drag slowing the turning of the blades is very little so all the windage force has to overcome is: rolling resistance of the wheels, friction in the drive train and the inertia of the cart and blades.

Are you lot watching the same video as me?

 

[bbg]

OK, but you can lead a horse to water....

V Velocity of vehicle
W Velocity of headwind
V-W headwind experienced by vehicle (V>W)
R rotational movement of prop blade
A apparent wind experienced by prop blade (not moving directly down wind!)
F force generated by air flow over prop blade

I won't get into the physics of the vectors themselves, but two observations:
Your example shows the velocity of the cart being greater than the velocity of the wind - i.e. below wind speed.
You have failed to include one important force vector: the backwards vector (i.e. 180 degrees from the cart direction). This is the force required in order to turn the propeller.

The question is whether the reduction in cart velocity that is required in order to turn the prop can be more than offset by the thrust produced by the prop. I have to say I didn't believe it was possible. I remain sceptical despite having seen the video. As we all know, it is easy to fool they eye. I can think of several ways the video could have been faked, or could appear to show DDWFTTW without actually achieving it (without intentional faking).

I would want to see data such from synchronised and calibrated instruments mounted on the cart (esp speed, in addition to the wind data), mounted at various heights and at several stations along the track, and on the chase vehicle. Two chase vehicles with ribbons on them would also give additional information as would, as someone has said, balloons blowing downwind.

 

[snowleopard]

You have failed to include one important force vector: the backwards vector (i.e. 180 degrees from the cart direction). This is the force required in order to turn the propeller.

My diagram doesn't show lots of vectors that are important to the whole machine. As you say the force turning the prop, provided from the wheels, the rolling resistance of the cart, the frictional losses in the drive train, the wind resistance of the cart and so on.

The diagram is merely showing how the prop blade is producing a forward thrust in a situation where the apparent wind on the cart is coming from ahead.

 

[RAI]

Are you lot watching the same video as me?

Yes.

At the start, the pressure, or drag, of the wind on the entire structure including propeller is trying to push the cart forwards. To move it forwards, either the wheels must turn or slip. As the road to wheel friction is great, the wheels must turn. To turn they must turn the propeller through the sprocket-chain-sprocket connection. The propeller would prefer to turn in the opposite direction because of the lift on its blades. (Only the blades are fairly stalled at this angle of incidence, so lots of drag and not much lift.) As the friction on the wheels has a mechanical advantage over the lift induced torque on the propeller, the cart can move forward with the propeller turning to oppose the wind.

 

[dk]

I don't see friction in this schematic! Surely the most important force on the vehicle.

Like many others brought up with a traditional scientific education you will have to put me in the sceptics box. I would be more curious if we saw the vehicle continue for a considerable period, when I believe it would settle down to a velocity just below the true wind speed - just my opinion, but I'll stick with it until I see different!