Down wind faster than the wind

[my_wan]

This is a wind powered craft that purports to travel directly down wind faster than the wind powering it.

It has also been posted on mythbusters.
http://forum.mythbustersfanclub.com/index.php/topic,12948.0.html

In essence the prop and wheels are directly connected through a drive belt. I defended the idea on physicsforums but unfortunately spork (the OP poster) wouldn't refrain from rants about being attacked and the thread got locked twice. This guy spork made his own version.
http://www.youtube.com/watch?v=1BRvYZd81AQ&fmt=18

Why I think it works:
It wasn't presented as perpetual motion and I agree that it's not. The source of energy is well define by the difference between wind speed and ground speed. This is regardless of the motion of the craft due to that difference being maintained by a direct belt connection between the prop and the wheels. With a constant power source at a given wind speed the velocity is limited only by the friction/wind resistance on the craft itself via Newton's first law.

My debate with spork concerned how the power was distributed through craft and the effects that had on efficiency. I could well have been wrong on some points of the wind powered version. However, the specifics in that regard are likely determined by prop to wheel ratio and prop efficiency. There is a difference in the treadmill case and wind powered case irrespective of the validity of the Galilean relativity argument.

What do you think? Will it work and why?

 

[technoextreme]

The experiment doesn't prove anything because it an oddball variation of the airplane on a treadmill myth.

his is regardless of the motion of the craft due to that difference being maintained by a direct belt connection between the prop and the wheels.

Problem. Someone actually did the experiment correctly. You need to swap the direction of rotation which isn't done in this movie.

 

[humber]

The vehicle is effectively tacking the wind. The propeller's blade is presented at an angle to the prevailing wind.

 

[my_wan]

The experiment doesn't prove anything because it an oddball variation of the airplane on a treadmill myth.

True, the videos proves nothing. However, you seem to be implying that an airplane on a treadmill will not take off. Is that your claim here? If so that alone is false. Furthermore, it is not related to an airplane on a treadmill at all. An airplane has no drive connection between the prop and the wheels as in this case. An airplane is also powered by an engine. This craft is powered solely by the difference between the ground air speed and the ground, which doesn't change when the craft moves because the wheel are directly tied to the prop. Neither does this craft ever leave the ground. It can't or it loses power.

The craft being spoke of here is not designed to operate on a treadmill. The treadmill was only used by that guy as a proof of concept. The first video show what the craft is actually supposed to do.

Problem. Someone actually did the experiment correctly. You need to swap the direction of rotation which isn't done in this movie.

Who did what experiment? In this craft the wheels and the propeller are directly connected for a reason. Swap the direction of rotation of what? The connection between the wheels and the prop? Since I don't even know what you claimed and/or objected to here I'm at a loss to answer you. Presumably I need to answer it sense you called it a problem so explain.

Points of fact:
This craft has no motor and is essentially a fancy wind sail.
This craft has a direct drive connection between the prop and wheels, the wheel and prop cannot turn independently of each other (planes do not).
This craft uses force feedback to continue drawing power from the wind road differential even after the craft itself exceeds the speed of the wind.

I don't really care for the treadmill video sense it only purports to be a proof of concept rather than the actual operation. This relates to a debate I had and spoke of in the OP. The first video shows how the actual craft is supposed to perform. As a point of physical fact a windmill is perfectly capable of producing enough power to propel a vehicle faster than the wind driving the windmill. This design simply incorporates the windmill on the craft itself.

Please answer the above questions so I can even have a clue what you claimed.

 

[my_wan]

The vehicle is effectively tacking the wind. The propeller's blade is presented at an angle to the prevailing wind.

Yes there is truth to that statement. I still haven't figured out how to define the craft analytically. The force vectors get quiet complex and take on a different character at above and below wind speed. When you consider force connection between the wheel rpm and prop rpm it get even more complex. The direction of torch can go either way depending on the ratio of the connection and relative efficiencies. It appears to me in the first video that the propeller and wheel RPM is locked at a one to one ratio, or nearly so.

I can identify the source of power but until I can get a handle on the direction of the torch vectors on all the craft components I don't know how to go about maximizing efficiency.

 

[mhaze]

Yes there is truth to that statement. I still haven't figured out how to define the craft analytically. The force vectors get quiet complex and take on a different character at above and below wind speed. When you consider force connection between the wheel rpm and prop rpm it get even more complex. The direction of torch can go either way depending on the ratio of the connection and relative efficiencies. It appears to me in the first video that the propeller and wheel RPM is locked at a one to one ratio, or nearly so.

I can identify the source of power but until I can get a handle on the direction of the torch vectors on all the craft components I don't know how to go about maximizing efficiency.

Stored momentum.

 

[soylent]

However, you seem to be implying that an airplane on a treadmill will not take off.

Depending on which version of this problem you mean it may very well be true. It's certainly possible to keep an aircraft stationary on a conveyor if you purposely set out to do that and as long as you've got an idealized airplane that will not break and an idealized conveyor with unbounded traction and unbounded power output.

The wheels of the plane have some non-zero moment of inertia, which means that even if they spin completely freely and are in no way tied to the means by which the plane generates thrust the conveyor must still act on them with some non-zero force in order for them to start spinning. If the wheels are frictionless there exists some constant acceleration(not speed; acceleration) of the conveyor belt(and corresponding angular acceleration of the wheels) such that the traction force acting on the wheels is equal and opposite to the thrust of the airplane.

If there is friction in the wheels the speed of the conveyor increases until the friction torque in the wheels eventually rise up to cancel out the torque imposed on the wheel by the conveyor belt. At that point the wheel will be spinning with some constant angular velocity.

In the real world many other outcomes can be imagined(e.g. the conveyor might reach super sonic speeds and generate wind strong enough to give the air plane lift, the wheels might not have enough traction and the airplane will burn rubber and start lurching forwards, the wheels might spin so fast they fly apart etc.).

 

[humber]

@my wan
Having though it over, I would expect that efficiency alone would prevent the vehicle from reaching, let alone exceeding, the wind.
It may not be a hoax, but the measurements are so crude. Windspeed is determined by voice-over and a sock. There may well be wind bursts that exceed the average velocity of the vehicle. Is the road level? There will be some input from the wind bearing directly on the chassis, etc.

 

[Gagglegnash]

Hi

If you aren't going directly downwind, you can develop what landyachters call a, "virtual wind," which can get you going significantly faster than the wind.

I wouldn't be much surprised if the same virtual wind would work on a propeller as well as a sail.

 

[humber]

@my wan
If you look at the video of the smaller belt test, you will see that the propeller is allowed to come to speed before being released. You would expect that it would then travel along a stationary belt even if the propeller were to be a simple flywheel. It is likely that the propeller stores excess energy from wind bursts as it progresses.
Ack. Mhaze

 

[Acleron]

Just thinking of the first demo. When the vehicle's velocity reaches wind speed, it is essentially motionless with respect to the air and there should be no force on the propeller. With no force there is no acceleration. Can't see how it exceeds the windspeed.

 

[technoextreme]

True, the videos proves nothing. However, you seem to be implying that an airplane on a treadmill will not take off. Is that your claim here? If so that alone is false. Furthermore, it is not related to an airplane on a treadmill at all. An airplane has no drive connection between the prop and the wheels as in this case.

No. It isn't. It's exactly airplane on a treadmill and it's exactly what I would expect. The simplified free body diagram would prove that it is airplane on a treadmill.

This craft has a direct drive connection between the prop and wheels, the wheel and prop cannot turn independently of each other (planes do not).

And there's the problem. In order for the propeller to act like a windmill and viceversa you have to change the direction of spin of the wheels which isn't done in the video.

 

[casebro]

The concept of the propeller vanes 'tacking' is interesting.

But don't think of it as an airplane prop, think of it as a helicopter rotor. Hmmm, needs a gross amount of cyclical pitch like a helicopter rotor to get the 'tack effect' on both apparent upwind and downwind strokes of the vanes, (top and bottom of rotor). You'll want the vanes at top and at bottom in parallel planes, and in horizontal planes when horizontal. Plus the ability to twist the mast to adjust the prop for best apparent wind, and also collective pitch to get maximum power to the wheels. You'll want big blades because the wind is slower than rotors usually spin- it would look like a windmill (DUH!), or, to keep it nautical looking, a Dutch windmill with sailcloth vanes.

Generally, it would only be practical on water. The rotor would be too large for use on highways. And the cost/benefit of the mechanisms vs sails makes it impractical on the water. Marconi rigged boats go everywhere now. And unless the target is directly down wind, they tack without all the mechanism.

 

[lomiller]

It’s got no significant energy storage so power in must equal power out. Power in is a function of wind speed + craft speed and propeller efficiency. The wheel and air resistance will determine how fast the craft will go on that much power. So, basically, wherever these two curves intersect is how fast the craft will go.

The key seems to be an efficient propeller. It needs to convert a good portion of the wind resistance it creates back into power for forward propulsion. All the wind would need to do is overcome the losses in the system. Since most of the wind resistance will come from the propeller the fact that it mostly cancels should allow it to move with relatively little added power.

Optimizing the propeller is going to be a tough problem to handle analytically, but trail and error on a computer model should work just fine and it shouldn’t stray that far from what we already know from wind turbines.

Anyway, despite being an interesting little device, I’m not sure what practical use it would have. I can’t see how it could move with the wind, and it doesn’t really do anything you can’t already do with a sail driven craft, which can already exceed wind speed when they are traveling perpendicular to the wind.

 

[lomiller]

Generally, it would only be practical on water. The rotor would be too large for use on highways. And the cost/benefit of the mechanisms vs sails makes it impractical on the water. Marconi rigged boats go everywhere now. And unless the target is directly down wind, they tack without all the mechanism.

The advantage of a sail is that it’s most effective going perpendicular to the wind, which allows you to tack upwind or downwind with relative ease. This drive system would do little going with the wind, and you’d probably be better off locking it in place and treating it as a crude sail.

 

[Thabiguy]

What do you think? Will it work and why?

I think it could work - going downwind faster than the wind is directly equivalent to sailing against the wind (air -> water, ground -> air), which we know can be done. Whether this particular device can do it, I can't tell, but I don't think it matters in principle - even if this device couldn't do it, you could definitely build another one which could.

It isn't going to revolutionize transportation, but if someone can turn this mechanism into a successful toy, there is some potential money to be made here.

What I don't understand is why this thread is tagged "over unity".

 

[my_wan]

The is technically off topic but I am responding due to the claim (I think) that it is related.

Depending on which version of this problem you mean it may very well be true. It's certainly possible to keep an aircraft stationary on a conveyor if you purposely set out to do that and as long as you've got an idealized airplane that will not break and an idealized conveyor with unbounded traction and unbounded power output.

No it's not possible under any motion of the conveyor to prevent the airplane from taking off.

<snip>If the wheels are frictionless there exists some constant acceleration(not speed; acceleration) of the conveyor belt(and corresponding angular acceleration of the wheels) such that the traction force acting on the wheels is equal and opposite to the thrust of the airplane.

There may be a slight moment of inertia while accelerating the wheel but it translates into angular momentum of the wheel and places a nearly unmeasurable drag on the forward motion of the planes itself.

A plane accelerates against the air and the air alone, the same way it does after taking off. A plane does not accelerate against the wheel in any way. The small friction from the wheel is no different when the wheel is spinning or even accelerating because it just represents a tiny more acceleration. It would be like claiming that if you lowered the wheel in flight and spun it fast enough the plane would fall out of the sky.

The motor powered flight of the airplane in no way relates to the wind powered craft described in the OP. I don't even like the conveyor version of the OP craft in the second video because it doesn't even represent the actual claimed abilities. The first video does and that is the OP subject.

 

[Careyp74]

Did anyone notice that the propeller rotates the way a plane propeller rotates, and not like a generator? I mean, when the wind blows on it, the prop should go clockwise, not the counterclockwise being observed.

I also noticed after the first push when the car slowed to a stop, it started creaping forward again, but only when the wind died down. Could there be a battery pack on this thing, and the wind was too strong pushing the prop in the beginning for the motor to operate?

 

[my_wan]

@my wan
Having though it over, I would expect that efficiency alone would prevent the vehicle from reaching, let alone exceeding, the wind.

Prior to reaching wind speed the wind increases efficiency greatly. A stationary windmill can produce many times the power needed to drive the family car many times faster than the wind creating that power. The scale might be more than what's feasible on this craft design but the fundamentals are there for it to work.

It may not be a hoax, but the measurements are so crude. Windspeed is determined by voice-over and a sock. There may well be wind bursts that exceed the average velocity of the vehicle. Is the road level? There will be some input from the wind bearing directly on the chassis, etc.

Agreed. Wind variability, momentum, etc., is a major issue with reading too much into that video. However, the fundamentals of having a well defined power source to actually accomplish what is claimed is there.

Consider Newtons first law of motion. This law means that without friction you can accelerate a refrigerator for instance to near light speed with the constant power of a watch battery given enough time. The only limit to the speed of the craft then is determined by when friction and wind resistance matching the power. Up until you reach wind speed the wind resistance part actually works in your favor. Bearing and road friction can be designed extremely efficiently.

 

[my_wan]

@my wan
If you look at the video of the smaller belt test, you will see that the propeller is allowed to come to speed before being released. You would expect that it would then travel along a stationary belt even if the propeller were to be a simple flywheel. It is likely that the propeller stores excess energy from wind bursts as it progresses.
Ack. Mhaze

Yes, good point. Just one of the many reasons I don't care for the conveyor belt version. I guess I shouldn't have posted it but I felt that due to my argument with the guy that made it he deserved the recognition.