Downwind Faster than the Wind - Successful Run by manned cart

From the SJSU Blog:

On Monday, March 22, 2010 the San Jose State University project overwhelmingly succeeded in demonstrating 'direct downwind faster than the wind' on the Ivanpah dry lakebed south of Las Vegas, Neveda.

There were numerous NALSA (North American Land Sailing Association) officials in attendance and one NALSA BOD member (Bob Dill) was there each of these runs and collected some of his own rough wind and speed data. All collected video and data corroborate the personal witnesses.

To be clear, this was NOT a NALSA sanctioned event but was presented as a demonstration to the NALSA BOD that the vehicle was capable of ddwfttw. Post demonstration, the team is working out the details with NALSA for rules and instrumentation related to any upcoming official NALSA ratified tests and records.

Pending NALSA BOD approval, the team expects to be able to certify a record according to these upcoming rules in the range of 3 times the speed of the wind powering the craft.





From Nikos J. Mourtos, Ph.D.
Professor
Mechanical & Aerospace Engineering
San Jose State University:

We will definitely document results in a number of ways. My experience with people is that they will believe what they want to believe regardless of evidence. Not much we can do there.

I tend to agree with Dr. Mourtos.
 
Pieromme

I don't have time to catch up on all the threads until tonight, but one question to you as you seem to know all the answers: if another force were to start the kart off - say a long healthy shove, would the kart then continue to move forward under its own power. If so, then you have invented perpetual motion! There's no other word for it - if it runs faster than the wind for any appreciable amount of time it is creating its own energy from somewhere!
 
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Well done to the team.
This whole debate has been almost as interesting in terms of psychology as in engineering. The power of the human tbeing o not change his or her mind despite arguments and eventually evidence to the contrary, is immense. Now if we can only harness that power and build a machine that can go faster than it then we could leave all these people behind!
 
dk said:
I don't have time to catch up on all the threads until tonight, but one question to you as you seem to know all the answers: if another force were to start the kart off - say a long healthy shove, would the kart then continue to move forward under its own power.
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No.

If so, then you have invented perpetual motion! There's no other word for it - if it runs faster than the wind for any appreciable amount of time it is creating its own energy from somewhere!
Click to expand...

Why do you think that?
 
dk said:
Pieromme

I don't have time to catch up on all the threads until tonight, but one question to you as you seem to know all the answers: if another force were to start the kart off - say a long healthy shove, would the kart then continue to move forward under its own power. If so, then you have invented perpetual motion! There's no other word for it - if it runs faster than the wind for any appreciable amount of time it is creating its own energy from somewhere!
Click to expand...

No, it would slow down and stop, in the same way that if you stop peddling on a bicycle you'll slow down and stop. No wind = no motive power = no movement.
 
dk said:
Pieromme

I don't have time to catch up on all the threads until tonight, but one question to you as you seem to know all the answers: if another force were to start the kart off - say a long healthy shove, would the kart then continue to move forward under its own power. If so, then you have invented perpetual motion! There's no other word for it - if it runs faster than the wind for any appreciable amount of time it is creating its own energy from somewhere!
Click to expand...

Hi - I don't have "all" the answers but this is how it seems to me.

No, your "other force" would not cause the cart to keep going. If there was no wind blowing the cart would grind to a halt.
The energy comes from the differential between the wind and the ground. Just as sailing is possible because of the differential between the velocity of the wind and the water.
As far as the cart is concerned it is not able to "see" which medium is moving - the ground or the air. If you are sitting in the cart and it reaches windspeed there is no apparent wind. But the difference in the two mediums still exists, because the ground is moving beneath. The cart simply takes the energy from the moving ground, sends it to the prop, which pushes the air backwards and you start to move through the air medium, which is (from an outside perspective) faster than the wind downwind.

What people have difficulty with is when you say "moving ground" - they say "but the ground is not moving"!
Answer: The ground is not moving if you are standing on the ground. But if you are sitting in the cart the ground is moving. It's just a different frame of reference.
It's not perpetual motion.
Latest evidence seems to show it does work.
 
Robwhelton said:
Well done to the team.
This whole debate has been almost as interesting in terms of psychology as in engineering. The power of the human tbeing o not change his or her mind despite arguments and eventually evidence to the contrary, is immense. Now if we can only harness that power and build a machine that can go faster than it then we could leave all these people behind!
Click to expand...

I thought of going over to see this machine in action and see how they had hidden the motor, Unfortunately the only transport I can find is one of these heavier-than-air machines and I know they don't work so I'm just going to have to sit in a cornfield and hitch a ride from some aliens when they land to make a crop circle.
 
Been doing my head in overnight, but I now think the above by Pierrome is correct. As long as the wind blows, there will always be a difference between the ground speed, and hence the speed of rotation of wheels and prop, and the apparent speed of the air over the rotating prop blades. The wheels are always driving the prop. Forget the apparent airspeed over the vehicle; its the moving prop blades doing the work, and the sailing broad reach faster than the wind analogy is right. I think! .
 
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What happens if....

I once was a sceptic but now I see, having read all the threads (including the ones on the physics forum) but I have yet to see the video. Once you get the head round the fact that at no point does the prop drive the wheels it becomes very logical. What happens if the wind drops to zero once the cart is up to speed? Logic says it should stop (if it didn't you would have a pmm) but I can't figure out why, or is it because the air coming off the blades has nothing to "push" against.

Hmmmm Bob
 
chiefeng said:
What happens if the wind drops to zero once the cart is up to speed? Logic says it should stop (if it didn't you would have a pmm) but I can't figure out why, or is it because the air coming off the blades has nothing to "push" against.
Click to expand...

Here are a couple of ways of looking at it that may help:

- The source of the energy to drive the cart comes from the difference in velocity between the wind and the surface. Take that away and there is no source of energy so there is no more drive and friction will cause it to slow down and stop.

That's an explanation that causes a lot of people difficulty so here's another:
foil1.jpg


In this diagram of the air flow over the prop blade, the blade has a positive angle of attack relative to the apparent wind which allows it to produce drive. Take away the true wind and the apparent wind it the reverse of the prop's motion so the angle of attack is now negative and the prop can no longer produce drive.

No drive forward, friction & drag backwards = retardation. Vehicle slows to a stop, no perpetual motion, no mystery.
 
Actually, yes it does. But it's adjustable "on the ground" rather than "in flight". So it's a case of setting the pitch for each run. They plan to make it "in flight" adjustable.
 
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I was more than happy with the mechanics of the cart and how the propeller works, but it took me a while to work out the conservation of energy. The way I see it is this.

Place a post at the start line. This is the datum.

Assume the cart is travelling at exactly the same speed as the wind away from the post.

Place an imaginary box around the cart, including the air around the cart.

The kinetic energy in that box is the 1/2 mv^2 of the air plus the 1/2mv^2 of the air where v is the velocity at which the air and the cart travel away from the post. Since the cart is traveling at the windspeed, v is the same in both cases.

But we know that friction in bearings, wheels etc mean that heat is released into the box. So the energy within the box increases. But that means energy is not conserved, it is created and against the laws of physics. To conserve energy, some of the kinetic energy of the cart and or air must be reduced and the cart will slow down.

But the propeller acts on some of the air in the box blowing it backwards (relative to the cart). So a portion of air in the box is not traveling away from the starting post at the same velocity as the cart, it is traveling away from the starting post at a lower velocity. Therefore that air has less kinetic energy relative to the starting post. It is that kinetic energy which goes to overcome friction - the energy balance is heat energy gained because of friction = loss of kinetic energy in the air that is slowed by the propeller.
 
snowleopard said:
I thought of going over to see this machine in action and see how they had hidden the motor, Unfortunately the only transport I can find is one of these heavier-than-air machines and I know they don't work so I'm just going to have to sit in a cornfield and hitch a ride from some aliens when they land to make a crop circle.
Click to expand...

No - aeroplanes work - really. Its all to do with the opposing forces of thrust, drag, lift..........................AAAhhhh! Your giving me back some banter aren't you?

I like that - jump on in...........................



just make sure you can swim!
 
Please can we stop now?

Ok time to clear this up.

Point 1 - The treadmill demonstration.

This works by using the kinetic energy stored in the wheels/prop blades to move the cart forward once it is up to speed. If there is a half decent thrust from the prop this will initially push it forwards as the thrust is greater than the friction in the cart but it will very slowly start to slow down. Once it reaches equilibrium and then starts to move backwards it will stop quite quickly as it will be moving backwards in the air and this will add significant drag to the prop.


To prove this drop the little kiddies an email and ask them to put the cart on the treadmill when the treadmill is stationary. Then switch the treadmill on.

I GUARANTEE the cart will fall off the back of the treadmill EVERY time.


I'll clear up the other bits later.
 
As ever, these discussions are getting bogged down in detail.
Forget about propellors , gearing variable pitch etc and enclose whatever clever mechanism you can think of in an open ended wheeled box which allows free flow of the wind from front to back.
No matter what mechanism you put in the box,it will accelerate downwind until, at best, it achieves the same velocity as the wind..... end of story...unless you can find some extra force from some additional energy source
You are stuck with Newtons Law that a mass will continue at uniform velocity unless some external force is applied.
You have used up all your external forces, the wind..... and your argument!
 
Another way to look at SL's vector diagram.

The force on the propeller blade can be resolved into lift and drag. However it can also be resolved into along track and cross track components. The along track component is the thrust driving the cart forward. The cross track component is opposing the propeller's rotation. This later component is countered by the torque provide from the wheels.

As long as the torque from the wheels is greater than the cross track component (summed over both blades x radius) the propeller can spin faster.

As long as the down track component is greater than the sum of all forces opposing forward motion, the cart can accelerate. The biggest of these forces is the resistance of the drive wheels, which stems from the cross track force on the blades.

All rather circular, but it means if the blade's aerofoil produces a lot more lift than drag, this approximates to a lot more along track force than cross track force. So we have a lot more force pushing forwards than force opposing forward motion. The cart can accelerate.

However, higher forward speed changes the angles in SL's vector diagram, which means the propeller reaches a point where it is producing less lift for greater drag. (Were the propeller adjustable in pitch, this need not happen.)

BUT - the vectors in the along track force and cross track force force change with the apparent wind vector too, lift is contributing more to cross track and drag is contributing more to slowing the vehicle.

Consequently, a limit is reached where the along track force is just balancing the cross track force expressed in terms of torque through the wheels and gearbox.
 
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