Faster than the wind going downwind?

[svetz]

Watched this, it's a little mind-bending and while I trust the you-tuber, I think they got spoofed.

The claim is the prop blows backward against the wind and the prop is driven by the vehicle's forward motion. That just seems to break the laws of conservation of energy to me.

I get how boats on a broad reach can be moving faster than the wind, that makes great sense.

Is seeing believing? I don't think so. I suspect what he experienced was the craft surging faster than the wind due to the previous acceleration (stored energy in the mass of the craft and the spinning prop); had it kept going it would have slowed back to a steady-state. That doesn't explain the model on the treadmill that was talked about, but I haven't seen that video.

Update:
So, I changed my vote to "real". There's a peer-reviewed paper saying it's possible (see post #6), and finally, in post #20 an illustration and explanation I can finally wrap my noggin around. Great claims require great proof, it just took me a while to get there.

 

[snoobler]

I am dubious of the claim. You might be right, and it could simply be a change of wind speed. The transition from fore to aft was very abrupt.

The air flowing over the top of the vehicle is moving faster and creates a low pressure region just like the top of an airplane wing. That could easily suck the streamer backwards. Having the streamer higher up or out to the sides in the free stream would be more convincing.

He made a later claim that they attained 2.8X wind speed, and I'm just not buying it. That was 11 years ago per wiki. One would think that if this were legit, there would be some more work on it.

Probably a battery and a motor on that prop.

 

[Q-Dog]

So, the prop works like the rotating wing on a helicopter. But it is powered by the wheels as the vehicle moves forward. And it can only go in whatever direction the wind is blowing?

Question. Since the prop is pushing against the wind, would it be possible to reverse the direction and have the wind spin the prop to drive the wheels and thus the vehicle into the wind?

 

[JimJr]

Air planes and jets take off in to the wind on a aircraft carrier

 

[Q-Dog]

Air planes and jets take off in to the wind on a aircraft carrier

Yes, but airplanes and jets launching off of a carrier also have engines and a catapult to throw them into the air. This thing has no engine.

 

[svetz]

Still can't wrap my noggin around it, but there was a paper published in the International Journal of Scientific Research in 2013 that concluded:

It is theoretically possible to build a wind driven vehicle that can go in the downwind direction faster than the free stream wind speed (using a propeller in the air). There does not exist a definite upper limit for vehicles of this kind. As long as efficiencies are improved, the velocities would also increase unasymptotically. The calculations above show that it was possible to go downwind even 2.5 times the speed of wind in a wind propelled vehicle.

 

[svetz]

Watched the treadmill video:

The model is restrained, because it's restrained the treadmill turns the wheels which brings the propellor up to speed. The prop provides additional forward force, so of course, the model moves forward when released.

But the "trick" is they keep pushing the model back. That puts energy into the system to spin the prop back up to speed so it can push forward again. I suspect if they didn't do that and the treadmill was long enough, it would eventually reach steady-state and fall off the back of the treadmill. So... still not convinced.

 

[Supervstech]

His explanation in the video made sense to me.

 

[svetz]

Question. Since the prop is pushing against the wind, would it be possible to reverse the direction and have the wind spin the prop to drive the wheels and thus the vehicle into the wind?

It should be able to, that's the energy captured by the turbine converted to power the vehicle. E.g., turbine makes watts, the watts run the vehicle's electric motor.

 

[svetz]

His explanation in the video made sense to me. I want to believe

 

[Q-Dog]

It should be able to, that's the energy captured by the turbine converted to power the vehicle. E.g., turbine makes watts, the watts run the vehicle's electric motor.

There is no electric motor in this contraption, right? Just mechanical linkages, chains, gears ,etc?

 

[Supervstech]

Description said gears and chains only connected to the prop.

 

[svetz]

There is no electric motor in this contraption, right? Just mechanical linkages, chains, gears ,etc?

Sorry, I tossed in the electric motor as a visualization for your question in #3 as to if a prop could power a vehicle into the wind, was trying to explain it as an energy source (wind) and energy consumer (electric motor).

 

[Substrate]

Reminds me of the time when a guy tried to get me to invest in his line of portable squirrel-cage fans you strap to the top of your vehicle's roof to charge your external battery.

Version 2 was indeed the turbo-fan concept. Just strap one to the roof and enjoy all the free energy just by driving! He was never allowed to look at my Winstons ever again...

So yes indeed - if taken to court he could prove that it would charge. Practical? Not so much.

 

[Sillyputty]

Watched this, it's a little mind-bending and while I trust the you-tuber, I think they got spoofed.

The claim is the prop blows backward against the wind and the prop is driven by the vehicle's forward motion. That just seems to break the laws of conservation of energy to me.

I get how boats on a broad reach can be moving faster than the wind, that makes great sense.

Is seeing believing? I don't think so. I suspect what he experienced was the craft surging faster than the wind due to the previous acceleration (stored energy in the mass of the craft and the spinning prop); had it kept going it would have slowed back to a steady-state. That doesn't explain the model on the treadmill that was talked about, but I haven't seen that video.

Fascinating. It seems legit, in that while you're moving as fast as the ambient volume of wind is, you're still always "in" the wind, which has mass and can be affected by the propeller in order to add propulsion. So long as the countervailing forces of drag don't exceed that amount of force gained, you can go faster.

 

[sunshine]

True...its on ytube!
Maybe the prop is acting like a funnel, the same wind gust effect you get in cities between tall buildings. The driver now knows what its like to go through a tx valve!!
Also the frames holding the prop up are shaped very similar to those high speed sail boats and the angle would add that side wind effect while travelling straight downwind and their narrowing towards the top, the props centre, maybe significant.
All of above and a gearing to the wheels above the static!

 

[Delmar]

The basic premise can be proved by a sailboat tacking upwind. The component of the wind must be at an angle to the direction of the boat.

The prop provides the required tack angle. The gearing from the prop drives the wheels, thus providing forward thrust. Rewatch this illustration that explains it best, especially making the earth into a cylinder:

 

[wattmatters]

The component of the wind must be at an angle to the direction of the boat fan's blades.

Which is achieved by the angle the blades are pitched at.

 

[svetz]

The prop provides the required tack angle. The gearing from the prop drives the wheels, thus providing forward thrust. Rewatch this illustration that explains it best, especially making the earth into a cylinder

I rejected their cute illustration of two sailboats on a cylindrical world as a propellor because sailboats on a broad reach still have an apparent windspeed. However, a sailboat on a run moving at windspeed has a zero apparent windspeed. A turbine at zero apparent windspeed does not turn, regardless of pitch.

The other factor is that their propellor isn't affected by the force of the wind as a sail is. The propellor is turned by the wheels and pushes back against the wind. So, that whole analogy made no sense to me.

While it's unlikely a peer reviewed paper is wrong (I cited one above, see post #6), I just can't wrap my noggin around how it can work in a sustained fashion and believe I have a satisfactory explanation for why the treadmill experiment is flawed (see#7).

But, let's walk through a thought experiment... possibly I can prove myself wrong and come to understand it.

An airboat with a tail-wind is faster than an airboat with a headwind. This is similar to their model in that they are using a prop to push air backwards as force to push them forward. The tailwind/headwind is a variable resistance depending on speed.

Where the difference occurs is that the force for turning the propellor comes from the wheels which ultimately comes from the wind. Once the wind is high enough, the vehicle starts to move forward and the propellor turns.

So the vehicle picks up to windspeed and the pitch is such that it provides no force, that is it is spinning freely. Now the vehicle has significant energy stored in both forward momentum and the flywheel that the propellor is. You alter the pitch and like an ice-skater entering a spin, the craft surges forward as it converts the existing flywheel energy into force.

The vehicle picks up speed, which provides more energy to the propellor, so in theory it goes even faster.

That's where things break down for me. While the vehicle moves slower than the wind, the system gains energy from the wind. Once the vehicle moves faster than the wind, the system must similarly lose energy. As the vehicle moves with the wind the apparent wind speed drops and the system can gain less energy from it.

I suppose the real question, and where I'm possibly missing it, is what is the apparent wind speed? The wind is "blowing" against a backwards stream of air from the propellor. So, while the flag sees an apparent wind reverse, does the system? If the "apparent" windspeed at the point the force is applied is greater than 0, then the craft should continue to move forward. So, no real resolution for me.

But hey, I believe the earth is round and that we landed on the moon, so two out of three ain't bad, right?

 

[svetz]

Thought Experiment Two: How I came to love the DDWFTTW

(Update: The force vector is backwards for FPropEnergy, it's a drag).
​

Let's assume 0 energy losses across the board for this. I think the above illustration accounts for all the forces.

When the propellor is applying no force, Then Fwind = 0 when the Car is moving at wind speed.
(This is where I went wrong, FApparentWind = 0, FWind is still the same, that is > 0)

Let's engage the propellor pitch such that it adds 1 mph to the vehicle speed.
Fdrag & FProp are both now > 0, since the wheels provide energy FProp Energy must be > 0.

The surge of acceleration from the video or the treadmill is easy to account for as the spinning prop at 0 pitch acts like a flywheel with stored energy. But, can it sustain faster-than-wind speed once that energy is depleted? To do so, assuming no losses, then:

FProp Energy must be >= than FDrag + Fprop.​

One saving grace is that FDrag isn't linear to apparent wind speed, at low speeds it is minuscule. So, let's zero out FDrag, but recognize that as the vehicle goes faster it becomes a killer. So now FProp Energy must be >= Fprop.

The theory is that the wheels are spinning so fast that there is excess energy to FProp., which appears to be true as with no losses:

FProp Energy = (Fprop + Fwind) - Fprop, or FProp Energy = FWind.​

That is, despite the apparent windspeed, the force turning the wheels is still the actual windspeed + vehicle speed.
So energy is still available to the system even when the apparent wind speed is <= 0.

I think where I was getting hung up before was that when the car was at the apparent wind speed, therefore Fwind = 0, so nothing to push the car with. But, since the wheels are in contact with the ground, they do see a force (Fprop + Fwind).

UPDATE: So close... see a better explanation in #39