1848

I watched that earlier today, and I’m still pretty fuzzy on how it works. I guess I acknowledge that it DOES, but the whole wheels-turn-the-prop thing is escaping me.

1849

it is definitely counter intuitive.

So one of the core principle of Newtonian motion, objects in motion will stay in motion and objects at rest will stay at rest, unless acted on by another force.

So once the vehicle is moving, the friction between the ground and the wheels will create a force acting on the car, which can be used to do work. In this case turn a fan.

Now here’s the thing. Normally air moving across a surface will also apply a force. By positioning the wind behind the vehicle it is applying a force behind the vehicle. This, in turn, means the force applied by the wind, and force against the wheels, are not of equivalent magnitude.

Clear so far? It’s basically the same principle as sails so far. Wind provides force forward, drag provides force backwards, when those forces equalize acceleration stops.

But here’s the tricky part. The thing that is not normally applied. That drag force applied to the wheels is used to provide work to turn a fan. That fan, in turn, applies a force against the air which accelerates the vehicle. Essentially pushing against the air that is pushing the vehicle.

Now where I get hung up is the math. There is simply an intuitive leap that I am trying to make sense of the numbers on, namely that of reference frames. From the reference frame of the vehicle, here is what I see when the speed is equal to the wind speed.

At that point the forward movement of the car is applying force X against the wheels. If the vehicle is moving at 10 MPH, and however many RPMs on the wheel, the drag force is going to be some value Y. Now, nominally, that value is not some fixed amount. I mean just think about it, if the speed is constant, then changing the drag on the wheels doesn’t directly impact the speed. If the vehicle is being pushed, then doubling the drag simply doubles the work required to maintain the speed.

So if you increase the size of the fan, you increase the amount of work that goes into spinning it. But you also increase the amount of work the fan is producing.

It seems, in theory, that the amount of work applied at the wheels should be equal to the work output at the blades. I mean that roughly should make sense, right? So why does it still accelerate?

Here’s what I see happening. And as best as I can articulate it.

Normally a vehicle drag force would go into work slowing the vehicle. However in this case it is not. That drag force is being converted and translated into the blades. Most of the rotational energy of the wheel is being converted, and only a very small amount turned into actual drag friction. What the blades are doing is, from the reference point of the car, increasing the apparent wind speed behind them. So a 10MPH wind, from the reference of the vehicle, is actually increased across the largest surface of the vehicle. The apparent wind speed is higher, because the fan is spinning. So at that point of reference the wind isn’t a 10MPH back wind, it is 20 or 30, because the fans are moving air. So the wheels are applying a force at 10MPH, but the apparent wind speed at the fan is 20MPH. And this is how they are able to go faster than the wind.

It’s really goofy, and seems like it would only work in very special circumstances (i.e. a vehicle with low surface drag, low friction wheels, and light body), but if you can design it well enough apparently you can eliminate enough energy loss to make it possible.

1850

So here’s my one-sentence stab at it, see if I’m on base:

Because the wheels are not free-spinning, but instead are both providing resistance and transferring that resistance into an energy-capturing system that also aids acceleration, the input force of the wind is able to be captured for longer because the relative power of the system doesn’t decrease 1:1 with the wind as their speeds merge.

?

Or even shorter, because the wheels keep the cart going slower than the wind even while they capture more energy because of the higher differential.

1851

I haven’t watched the video yet, but sailboats travel faster close-hauled (angling into the wind) than running downwind and you can definitely feel the wind on your face while while using the wind and only the wind as a motive force.

1852

This is a different beast, and if I’m not mistaken, while sailboats can travel faster than windspeed, they can’t do it in the same direction as the wind. That’s the super-non-intuitive thing happening with Blackbird.

1853

Well that was one of the references originally. To consider the two propellers as two sail boats circling on a cylindrical surface. Basically each blade can be envisioned as a single close hauled sail ship in a way to understand how they can run faster than the wind.

1854

But the small model shows that it’s the wheels, not the prop, that’s doing the work. There’s no wind in that treadmill experiment.

1855

Here’s another video with some math

It’s not the wind speed, per se, but the differential between wind speed and ground speed. From the reference frame of the car still air and moving ground is no different than moving air and still ground. To the vehicle both are moving at different speeds

This is why relativistic effects are so hard to grasp. We are used to one absolute frame, when the apparent effects are more about the relative reference frame of the object acted upon. This is why time dilation as you approach light speed is a thing, just at a far less extreme level.

1856

Also here’s one from the person who designed the scale model used in the latest video, with some more technical details on what is needed to work

1857

That’s the one to which I was referring, there’s no wind in that system externally, only what the prop generates. I’m content to stay mystified. I get that the energy going into the system is coming from her holding the wheels down until the prop spins up, I just don’t get the acceleration from there.

1858

It’s all about reference frames. From the perspective of the vehicle, these two scenarios are exactly identical.

Ground stationary
Wind 10mph
Vehicle moving 10mph into the wind

And

Ground moving at 10mph
Wind stationary
Vehicle stationary

They both, from the reference of the vehicle, look like this:

image
image1237×828 62.4 KB

As best as I can draw it on my phone

1859

I learned cool bat science! On tic tok no less! You can too!

1860

I get that, 10mph coming in from either source, but where is the acceleration? If you’re going directly downwind in a perfect frictionless sailboat then you will go 10mph and no faster. If you are perfectly frictionless then a 10mph treadmill under you will scoot by at 10mph and not affect your position in the world. Yes?

How did it go faster?

1861

And this is the part that is incorrect.

It is well documented, and known, that a sailboat can tack faster downwind than the wind is blowing.

And that’s the weird and tricky part. It all has to do with moving at angles (they do this by going not straight downwind) and the fan blades are able to replicate this.

1862

Did some deeper digging and now I get it. Using lift forces the new negative wind forces, or front vs. rear wind forces are still enough to overcome the drag of the system. Still pretty fuzzy but I see where it is happening now. More specifically I’ve done a bit of sailing and while it’s great fun to lean the boat over and tack into the wind, it isn’t at all obvious that the overall travel forward is greater than the windspeed backward, but looking into the current superships it apparently is. Thus the wheels provide on land what the keel provides on the water, a resistance to negative forces that translates back into forward forces.

1863

Swarms of synchronous fireflies are rather like melting ice, or at least that’s how Raphael Sarfati, a physicist, sees it. Ice remains solid until it warms to a certain temperature and becomes a liquid. Likewise, a loose swarm fireflies will flash the lanterns in their abdomens randomly. But when the swarm reaches a certain density, the fireflies begin to blink in unison.

“Above that threshold, it is almost perfect synchronization,” with rhythmic, coordinated waves of light, said Sarfati, a postdoctoral associate at the University of Colorado, Boulder.

1864

Damn do I miss fireflies. We had them in our backyard in Brooklyn. Every summer. Never saw any here.

1865

Growing up in the south I saw them all the time as a boy, but when I lived in the FL panhandle or down in the Tampa area I never saw them. Up here in north Mississippi I see them pretty much every night in early summer. I’ve never seen a synchronous swarm before, but I’ve heard it’s a magical sight.

1866

Nor I. Not enough density I guess.

1867

So, I have an advanced engineering degree and minored in physics as an undergrad. I’ve taken a full 3.5 years of university physics classes. I’ve always been fascinated by modern physics, have read a bunch of books on relativity and QM, watched many many PBS Space Time videos. In an attempt to get a more rigorous understanding the principles and motivations behind modern theories, I’ve started dipping my toes into the math… and what I discovered is that even though I’ve taken a whole bunch of university physics courses, none of them covered classical mechanics with the rigor necessary to tackle even fundamental ideas in QM. Lagrangian mechanics, Hamiltonian mechanics, Noether’s theorem, the principle of least action, and the mathematical foundations necessary to gain an intuitive grasp of what they’re saying. None were part of my education. I kind of feel let down. These have been the foundation of physics for 150 years.

Leonard Suskind’s lectures (and book version) are a great summary of classical mechanics for anyone interested: