"Faster than the wind," impossible they say but the Blackbird settles the debate

[some_other_dave]

If I may take another tack here-- (see what I did there?)

Do you believe that a boat can sail faster than the wind speed? Your arguments would seem to say that no, they cannot.

If not, then you need to research sails, because there are boats that can do so.

If you believe that they can, then do you think it stands to reason that the same sort of phenomenon might be used to allow a vehicle to move downwind faster than the wind? Including a lot of losses along the way?

-soD

[oil pan 4]

The guy that built the machine says it can be done.
Aerohead says it can't done.
Who should we believe?

[ennored]

Originally Posted by AeroMcAeroFace

It doesn't require over-unity, because essentially the energy is coming from the difference between air speed and ground speed.

That's how I understand it. Traveling at say, 10 MPH, with a 7 MPH tailwind, you have 10 MPH of groundspeed to overcome a 3 MPH headwind.

[jakobnev]

The energy at the blade-air interface must be lower than that at the road-to-tire interface due to energy loss in the tire, gears, wheel bearings, lower sprocket, chain, upper sprocket, jack-shaft bearings, then the blades themselves.

Sit down and enjoy this moment of enlightenment.

Power is force times velocity.

For the wheels it's simple: The velocity is just the difference in velocity of the wheels and the ground.

For the propeller it is just as simple: The velocity is just the difference in speed between the propeller and... (this is the facepalm moment!) the air!

The propeller doesn't give a hoot about ground speed!


Let's say we wanted to go 10m/s down a 5m/s wind and were transferring 100N of force.

We harvest 100N*10m/s = 1kW at the wheels.

The useful power of our propeller needs to be: 100N*5m/s = 500W.

Ergo (I love that word) we need a total system efficiency of 50%, including all losses, that is very very much not over unity, no magic needed!

Propeller efficiencies can be 82-92% according to the internet.

[RedDevil]

Oh crikey. We already had a thread on this.

Ah well. I'll repeat my explanation in concise form:
The propeller on the craft is slowly turning backwards.
Its blades form a surface that still moves forward with the craft, but less fast than the car itself.
The wind still exerts pressure on that surface, even if the rest of the car is moving faster than the wind.

The propeller is driven by the wheels, but as the gearing reduces the speed the forward pressure on the blades is larger than the backward force on the wheels through those gears needed to spin the propeller.
As all other sources of friction are pretty minimal, that's enough to make the craft accelerate.

The important point is that the propeller spins slower than the wind. If it spins faster than the wind then the car would move backwards, straight into the wind.

Originally Posted by AeroMcAeroFace

...
in the beginning, the car isn't moving, but has a tail wind, the rotor spins, which in turn drives the wheels. the car accelerates partly due to the drag of the vehicle, and partly due to the propeller driving the wheels.

That is exactly what is NOT happening. The rotor spins against the wind!
It does not drive the wheels at any moment. instead it exerts a force on the wheels fighting its motion (but losing), or rather the ground forces the wheels to turn which through the gearing drives the propeller to turn slowly against the wind.
It is just the wind blowing against the blades of the propeller which moves it forward, like a sail would.

If the blades were fixed it would act just like a sail.

Now we add a mechanism that very slowly turns the blades backward.
It would still be mainly a sail.

It will remain working like a sail until you set the ratio such that the propeller moves the air backwards as fast as the craft moves forward, in which case the forces cancel each other out and the craft wouldn't move at all.

[AeroMcAeroFace]

"Faster than the wind," impossible they say but the Blackbird settles the debate

I think aerohead begs to differ on this.

That is exactly what is NOT happening. The rotor spins against the wind!
It does not drive the wheels at any moment. instead it exerts a force on the wheels fighting its motion (but losing), or rather the ground forces the wheels to turn which through the gearing drives the propeller to turn slowly against the wind.
It is just the wind blowing against the blades of the propeller which moves it forward, like a sail would.

So you are saying that the car would accelerate faster if the blades were stationary and not connected to the wheels? At least in the beginning?

[RedDevil]

Originally Posted by AeroMcAeroFace

So you are saying that the car would accelerate faster if the blades were stationary and not connected to the wheels? At least in the beginning?

Yes! That's it!
Suppose the gearing is set up in such a way that the propeller blades push the air back at half the speed of the craft.
Then of all the force on the propeller trying to push it forward half gets lost by the gearing turning the blades against it. When the rolling resistance and other friction sources are less than that, there is still something left to push it forward.

Once you see it it really blows the mind.

When in motion the propeller blades wing effect kicks in and they become more effective, like as if the surface of the sail increases. So a stationary propeller is good to get rolling but you want it to turn immediately after.

The craft in the video has adjustable gearing. I guess in low wind you have to reduce the gearing as the other sources of friction are relatively large. And maybe you want oversized gearing to be able to brake or even back up against the wind.

[AeroMcAeroFace]

I am not saying you are wrong, but they have a variable pitch propeller and according to this paper it seems that it is preferable for thrust to be generated from the propeller to the wheels rather than parachuting up to speed.http://www.ijsrp.org/research-paper-...srp-p16135.pdf and the paper makes no mention of drag from the propeller pushing it up to speed.

In 2006, Jack Goodman [8] explained that the correct gearing
of the propeller to the wheels of the car will speed up the car if
energy is available from a tailwind.

[aerohead]

Originally Posted by some_other_dave

If I may take another tack here-- (see what I did there?)

Do you believe that a boat can sail faster than the wind speed? Your arguments would seem to say that no, they cannot.

If not, then you need to research sails, because there are boats that can do so.

If you believe that they can, then do you think it stands to reason that the same sort of phenomenon might be used to allow a vehicle to move downwind faster than the wind? Including a lot of losses along the way?

-soD

The only corollary which applies for 'with-the-wind' would be flying a spinnaker, and the sailboat fails to reach parity with the wind velocity.

[aerohead]

Originally Posted by oil pan 4

The guy that built the machine says it can be done.
Aerohead says it can't done.
Who should we believe?

'Says' isn't equivalent to 'proves.'
The video provided no 'proof.'
It does provide 'illusion.'
It should never be about 'belief,' it should be about demonstrating scientific rigor.
Which wasn't.
Laboratory testing would tell the tale.