Explain this? Vehicle travels faster than the wind. My head hurts trying to figure this out.

[WobblyElvis]

I like trying to understand things like this but my brain has met it's match. Can anyone explain this?
There are many videos on this "blackbird" vehicle traveling fast than the wind. Here is one video but it's long.

 

[oil pan 4]

If dude put in the work to build that rig and it runs faster than the wind I'm just going to go with it.

 

[goingplacesanddoingstuff]

The more the wind pushes into the vehicle, the more it pushes against the wind. Positive feedback loop, acceleration.

 

[dumkid]

It goes faster than the wind, but with less torque than an ordinary sail. No free energy but a cool concept that could be useful for something... Maybe

 

[MRC01]

That is a freaky and wonderful example of how even simple things can be counterintuitive. The universe is stranger than we think.

 

[PandaBear]

I think it is only possible if you shoot something out of the car faster than wind but not the whole car going faster. If this work it would have been build already but unfortunately not.

 

[honeeagle]

what ,never seen a iceboat doin 80 mph in a 29 knot breeze ?

 

[Allaneddie]

Sailboats do this all the time.

 

[MRC01]

Sailboats do this all the time.

Sure, but a sailboat's fastest point of sail is not generally directly downwind. Not even close to that, but roughly perpendicular to the wind.
https://sailawayblog.com/fastest-point-of-sail/
And that means your fastest point of sail is not necessarily in the direction you want to go. So reaching your destination as quickly as possible is a tradeoff between speed and direction, or point of sail.

 

[KGMtech]

Sure, but a sailboat's fastest point of sail is not generally directly downwind. Not even close to that, but roughly perpendicular to the wind.
https://sailawayblog.com/fastest-point-of-sail/
And that means your fastest point of sail is not necessarily in the direction you want to go. So reaching your destination as quickly as possible is a tradeoff between speed and direction, or point of sail.

I like the cut of your Jib....I always wanted to say that! Spent my youth tacking sailboats in English Bay Vancouver dodging freighters just because. Beam reach was an easy sail but slow, so it was reserved for when wind direction permitted and crew was tired.

 

[97prizm]

I like trying to understand things like this but my brain has met it's match. Can anyone explain this?
There are many videos on this "blackbird" vehicle traveling fast than the wind. Here is one video but it's long.

Ahh veritasium. He's got a great channel

 

[dumkid]

Sure, but a sailboat's fastest point of sail is not generally directly downwind. Not even close to that, but roughly perpendicular to the wind.
https://sailawayblog.com/fastest-point-of-sail/
And that means your fastest point of sail is not necessarily in the direction you want to go. So reaching your destination as quickly as possible is a tradeoff between speed and direction, or point of sail.

The propeller is like the sail. It moves across the wind, providing extra speed

 

[WobblyElvis]

If this works, if you were to give it a push in no wind conditions, would it go on forever? why-why not?

 

[dumkid]

If this works, if you were to give it a push in no wind conditions, would it go on forever? why-why not?

No because there's no wind to push it forward

 

[mk378]

A famous cover from 1970s Popular Mechanics proposed a cargo ship powered by wind turbines, and claimed it would have better performance than a sailboat. I don't think any such ships were ever built.

 

[OVERKILL]

A famous cover from 1970s Popular Mechanics proposed a cargo ship powered by wind turbines, and claimed it would have better performance than a sailboat. I don't think any such ships were ever built.

Not sure about the sailboat comparison but a large cargo ship power plant is like 30-40MW IIRC, and there's absolutely no way you are fitting 10x 4MW wind turbines on one.

 

[OVERKILL]

The more the wind pushes into the vehicle, the more it pushes against the wind. Positive feedback loop, acceleration.

Yup, it's the thrust from the propeller pushing against the wind that's pushing from behind the vehicle so you end up with an overall speed that's faster than just the wind's speed before the forces are compounded, at least that's how it computes for me. Aerodynamics would of course be critical for this to work as the vehicle has to be "slippery" enough that its friction doesn't cancel out or overpower the rest of it. The treadmill "demos" don't really illustrate this, because there's no wind, they are supplying energy to the system via the motion of the treadmill, which is like his wheels and 2x4 example, where he's supply the energy by pushing the 2x4. There is neither of these direct couplings of energy to the actual vehicle.

 

[MRC01]

Different people get this in different ways. I can't see it being positive feedback because that opens the question why isn't it a perpetual motion machine? Or as @WobblyElvis says, if you push it to get it started in no-wind conditions, it won't run. It does require wind to push it.

At first I didn't "get" the treadmill example but I think I do now. Remember that it is an inertial reference frame if it's moving at a constant speed (and it is). Now imagine the treadmill the size of the entire desert (or Earth), the prop cart moving forward on the treadmill is going "downwind" faster than the wind.

Another way to think about this is that because of the tailwind, the wheels are moving relative to the ground, much faster than the prop is moving through the air. Suppose the cart is moving at 20 miles per hour with a tailwind of 15 miles per hour. So its wheels are moving at 20 mph relative to the ground, while the prop is moving at 5 mph relative to the surrounding air. Power is thrust * speed and speed is relative. If the power at each is the same, the wheels have low thrust at high speed while the prop has high thrust at low speed. This difference relies on the relative motion of the wind with the ground, thus cannot work unless there is wind. It is not a perpetual motion machine.

 

[goingplacesanddoingstuff]

Yup, it's the thrust from the propeller pushing against the wind that's pushing from behind the vehicle so you end up with an overall speed that's faster than just the wind's speed before the forces are compounded, at least that's how it computes for me. Aerodynamics would of course be critical for this to work as the vehicle has to be "slippery" enough that its friction doesn't cancel out or overpower the rest of it. The treadmill "demos" don't really illustrate this, because there's no wind, they are supplying energy to the system via the motion of the treadmill, which is like his wheels and 2x4 example, where he's supply the energy by pushing the 2x4. There is neither of these direct couplings of energy to the actual vehicle.

It’s so interesting because, when traveling with the wind, the vehicle relies on the “traditional” push from the wind to get started. During the very initial phase, more aerodynamic drag could actually be beneficial. Then, as the propeller spins faster, the vehicle would benefit more and more from being more aerodynamic.

I wonder where that inflection point is. I think that’s a big source of the mind-bending nature of this.

 

[OVERKILL]

It’s so interesting because, when traveling with the wind, the vehicle relies on the “traditional” push from the wind to get started.

Yes, which is why the treadmill example doesn't really illustrate the effect.

During the very initial phase, more aerodynamic drag could actually be beneficial.

Yes, make the vehicle more like a sail, though I suspect with such a light and aerodynamic unit, the blades themselves are enough of a "sail" to get it moving.

Then, as the propeller spins faster, the vehicle would benefit more and more from being more aerodynamic.

I wonder where that inflection point is. I think that’s a big source of the mind-bending nature of this.

Would be an interesting calculation. My immediate reaction is to think it would be very soon, but then I quickly realized it's not that simple, and would depend on how windy it is, as to how much of a benefit the "sail" aspect would be in compromising on aerodynamics for more surface area for the wind to contact, vs the opposite of making the vehicle more slippery so that less thrust is required by the prop for a given speed.