A vehicle which breaks the laws of physics...

[Keith 66]

Found the catamaran vid, It didnt catch on! forgot to say other limiting factors are propellor slip & drag, windage etc, no such thing as a free lunch!
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[Gareth J]

Fascinating stuff. And a bit proud of the forum!

Watched the video and read the article and *think* I get the principal...up to the point where your propeller is accelerating air through it and creating thrust.

What I struggle to buy is the limitless acceleration idea, even in a perfect, loss free, perfect propeller, perfectly grippy tyres, indestructible design scenario. What's that limit? Is it really just down to practicalities and with a super perfect situation, you reach the next limiting factor, like supersonic wing tip issues or the fact that, while there's an ocean of wind energy available, it's not infinite and, so, at some zillionty mph, eventually, you'd slow the wind down so much, you'd run out of a useful wind/land differential to exploit....or is there some other theoretical limit, like the Betz limit for wind turbines that helps make it make sense?

 

[Dev]

2 sailboats tacking around a cylindrical plane is a really good way of visualising the principal.

 

[Munkul]

Does it work for a short time only? Do they get up to wind speed and then use drive from the wheels to spin the propeller slightly faster?

No, its at equilibrium at that speed for a sustained time.
Ie the thrust generated by the propeller equals all drag, friction and drivetrain losses when the Blackbird is going somewhere around 2.7x the wind speed at that time.

 

[spork]

Hi Rick, thanks for taking the time to join the forum and explain things properly If this is an intelligent discussion, there must be hundreds of extremely stupid discussions around the net

The stuff that's out there is absolutely tragic. Most everyone is sure it's a hoax, I'm an idiot, I don't understand the thing myself, it really is a perpetual motion machine and even I don't realize or accept it. And some of this is coming from aero and physics professors. In the old days a couple of people even offered to kick my ass for making the claims we make.

 

[spork]

Does it work for a short time only? Do they get up to wind speed and then use drive from the wheels to spin the propeller slightly faster?

Nope. It operates steady-state. It will go 2.8X wind speed for as long as the wind blows. Keep in mind that the wheels always turn the propeller, never the other way around (when configured for downwind mode). But the propeller isn't really terribly effective as a propeller until it reaches a significant fraction of wind speed.

 

[spork]

What I struggle to buy is the limitless acceleration idea, even in a perfect, loss free, perfect propeller, perfectly grippy tyres, indestructible design scenario. What's that limit? Is it really just down to practicalities and with a super perfect situation, you reach the next limiting factor, like supersonic wing tip issues or the fact that, while there's an ocean of wind energy available, it's not infinite

People often ask whether we claim limitless acceleration, and we definitely don't. It's pretty easy to say how fast it will go steady state as a function of wind speed. You could certainly do better than our 2.8X record without too much difficulty, but it would be hard to imagine anyone ever doing 10X wind speed downwind, steady-state.

On the other hand, it's interesting that it can theoretically do *any* multiple of wind speed IF the efficiency could be made arbitrarily high and the wind was slow enough (because if you start getting the blade tips up around Mach 0.3, you get compressibility effects that cause inefficiency that will limit you).

at some zillionty mph, eventually, you'd slow the wind down so much, you'd run out of a useful wind/land differential to exploit....

Yes of course. But for the purposes of discussion, I guess we assume an unlimited playa and a steady wind.

is there some other theoretical limit, like the Betz limit for wind turbines that helps make it make sense?

We're not subject to the Betz limit in its downwind configuration because the rotor is a propeller rather than a turbine. In the upwind configuration, we are subject to Betz, but in that case we "seemingly" violate that limit because we're not sitting still waiting for the wind, but going out after it. I say "seemingly" because we clearly do not violate any of the well known laws of physics and aerodynamics this thing is based on. We still don't get something for nothing.

 

[spork]

2 sailboats tacking around a cylindrical plane is a really good way of visualising the principal.

Thanks. That's actually how I got from sailboats to a vehicle that could go directly downwind faster than the wind. I wanted to post this as a brain-teaser, and doing it directly downwind felt a little more brain bendy.

 

[Munkul]

What I struggle to buy is the limitless acceleration idea, even in a perfect, loss free, perfect propeller, perfectly grippy tyres, indestructible design scenario. What's that limit?

In real life this is impossible because any physical body size has a drag coefficient above zero.
But...
Imagine the Blackbird, only with a perfectly efficient drivetrain from wheels to propeller. Imagine the propeller works perfectly without air compressibility/dynamics issues at any air speed. Imagine the whole vehicle has a zero drag coefficient.
In this case, you have a constant source of energy feeding the acceleration of the vehicle, with nothing to oppose this force. Therefore it will keep accelerating.... Faster and faster... This was Fizzy's point.

In real life even if you could zero the losses to friction/drag (which you can't) the propeller would lose effectiveness at a certain speed and the vehicle would reach equilibrium at a point dictated by the fluid dynamic properties of the propeller... If that makes sense?

 

[Gareth J]

People often ask whether we claim limitless acceleration, and we definitely don't. It's pretty easy to say how fast it will go steady state as a function of wind speed. You could certainly do better than our 2.8X record without too much difficulty, but it would be hard to imagine anyone ever doing 10X wind speed downwind, steady-state.

On the other hand, it's interesting that it can theoretically do *any* multiple of wind speed IF the efficiency could be made arbitrarily high and the wind was slow enough (because if you start getting the blade tips up around Mach 0.3, you get compressibility effects that cause inefficiency that will limit you).



Yes of course. But for the purposes of discussion, I guess we assume an unlimited playa and a steady wind.



We're not subject to the Betz limit in its downwind configuration because the rotor is a propeller rather than a turbine. In the upwind configuration, we are subject to Betz, but in that case we "seemingly" violate that limit because we're not sitting still waiting for the wind, but going out after it. I say "seemingly" because we clearly do not violate any of the well known laws of physics and aerodynamics this thing is based on. We still don't get something for nothing.

Thanks, helps a lot.

In real life this is impossible because any physical body size has a drag coefficient above zero.
But...
Imagine the Blackbird, only with a perfectly efficient drivetrain from wheels to propeller. Imagine the propeller works perfectly without air compressibility/dynamics issues at any air speed. Imagine the whole vehicle has a zero drag coefficient.
In this case, you have a constant source of energy feeding the acceleration of the vehicle, with nothing to oppose this force. Therefore it will keep accelerating.... Faster and faster... This was Fizzy's point.

In real life even if you could zero the losses to friction/drag (which you can't) the propeller would lose effectiveness at a certain speed and the vehicle would reach equilibrium at a point dictated by the fluid dynamic properties of the propeller... If that makes sense?

It does, thanks. I had considered that the prop would be optimised for some range of speed too and, short of swapping imaginary props mid acceleration (like changing gears if you like) that would also be a limiting factor

 

[Screwdriver]

Howdy folks. A friend sent me a link to this discussion. I'm Rick Cavallaro. I designed, built, and drove the Blackbird. I've seen hundreds of discussions about it, and this has to be one of the most intelligent I've seen so far. If there are any questions I can answer, I'd be more than happy to try.

Haha, that's awesome, welcome aboard! I do have a few questions for you...

It's a very common misconception that the propeller produces a sort of high-pressure air-buffer for the tailwind to push against (if that's what you mean). The reality is that the airflow through and around the propeller is identical to that of a Cessna in straight and level flight. Once past wind speed, the propeller is operating on the air entering from the front of the vehicle. But it has a huge advantage over the wheels since the propeller is advancing through the air more slowly than the wheels are advancing on the ground.

We did make the vehicle as efficient as possible within our means, but you can have a surprisingly inefficient vehicle that will still go faster than the wind - as long as you don't want to go a lot faster than the wind.

With respect to the narrow speed range, it's actually not terribly sensitive to that beyond destroying itself, and not being able to work successfully in too light of a wind (about 8mph in our case).

Aerodynamics is a dark art from my perspective and I have no idea how disturbed airflow operates. I am going to have to make a few ill informed statements to explain my current level of understanding so that I may be put right (from past experience, it may not look like that from your seat ). Also there is a bit about trees I'm not too sure about!

The airflow is "identical" to that of an aircraft? Surely then that does imply a high pressure region is generated behind the prop? The aircraft is swapping some petrol in exchange for being able to shove some wind behind it. With the Blackbird, ignoring where the energy is coming from and imagining it's "up there" flying along, disobeying the laws of physics: it must surely also be creating a large volume (or "cushion") of disturbed air in its wake?

Back to reality for a moment and we're on the ground at 1.1x wind speed. Now we take the above scenario where the prop is generating thrust "just like" your Cessna but this time within the envelope of the prevailing wind which is still pushing it along as a bluff body. My current understanding/imagination tells me the wind from behind can't even reach the huge swept area of the prop because the prop is travelling 1.1x faster. Therefore in my imagination, the prevailing wind is pushing on a cushion of air left behind in the wake of the spinning prop. The vehicle is being carried along in a protective bubble of air of its own making.

I love your description of "leverage", that really helps me to understand the dynamics. I think the tricky part which most people struggle with is the concept of relativity (no not that sort!). It is as you say, the frame of reference which is important here. I imagine for example, a tree on its side falling out of the sky. Root and branch looks for all the world like a dumbell falling perfectly level. Now change your frame of reference and imagine that same tree travelling along sideways (for some unspecified reason) at the same rate it had been "falling".

It will carry on in this manner, zooming along through still air, generating a huge wake as it is inexplicably pushed along. Now lets introduce the ground: the roots drag on the ground and the tree suddenly tips forwards (there's your "leverage" if I understand you correctly). Replacing your Blackbird with a tree on wheels so that it can be blown along, there is still appreciably more drag wanting to tip it over forwards. Instead though, we have put our tree via a clever support on a giant skateboard which diverts this drag induced turning moment into a spin (you can see where I am going with this) - gear it up and carve the tree into a propeller etc.

My frame of reference is the wind (that should be a sig. ) and I am standing still in a still wind while the planet rushes past my feet. The wind as a sentient being, sees the ground as an inconvenience as it is zooming along. In its frame of reference, the ground is "getting in the way" causing drag when suddenly it bumps into a tree on a skateboard and in that section of otherwise rough drag inducing groundplane, it can slide forwards a LOT more easily remembering that if a root touches the ground, it suddenly wants to lurch forwards. Little does it know some clever human has stuck a mechanism inside to nick some of that power otherwise lost to drag and instead turn it into a spinning prop.

 

[Screwdriver]

Imagine the whole vehicle has a zero drag coefficient.

How is the wind going to blow it along?

 

[Munkul]

How is the wind going to blow it along?

As I repeatedly said, it was a theoretical scenario that is impossible in real life for multiple reasons

 

[Screwdriver]

As I repeatedly said, it was a theoretical scenario that is impossible in real life for multiple reasons

OK but clearly this freaky and counterintuitive behaviour is not impossible. The vehicle will never achieve its theoretical ability to accelerate to unlimited velocities for much more down to earth reasons. Why it can continue to accelerate beyond wind speed at all is the fascination with this particular device.

I hope @spork comes back with some more enlightening information. What would be interesting is to discover which physical barrier is the major hurdle to overcome in optimising a practical design. It might well be aero drag for all I know but I rather suspect it will be massively affected by more mundane mechanical issues; friction and drag from the gearing (obviously), wheel scrub and of course the structural integrity of a lightweight propeller spinning at potentially huge velocities.

For example, @spork casually mentions reaching mach speed at the tips of the "propeller". That is a lot "easier" to get to than one might imagine with a large prop being blown around by wind. You only need to spin a 2m long prop at just over 2 rpm before you reach mach 1 at the tip!

So as the featherweight craft is being blown along spinning like crazy, threatening to shake itself to pieces with the prop struggling to hang onto it's physical integrity - it runs slap bang into a brick wall of sound. When I first saw the Veritasium video where our hero Derek "risked his life", I wonder if he really knew how close he was to the truth!

 

[PhillipM]

I imagine the gearbox/diff and shafts have quite some force through them given they're almost levering off each other.

 

[daleyd]

You only need to spin a 2m long prop at just over 2 rpm before you reach mach 1 at the tip!

2m impeller at 2 rpm - I reckon that’s nowhere near Mach 1?

 

[Screwdriver]

I imagine the gearbox/diff and shafts have quite some force through them given they're almost levering off each other.

Yeah I bet! There is a gigantic amount of energy in even relatively slow moving wind. Not just that there's so much of it but we forget just how heavy air is. A large room has over 100kg mass of air in it. A gentle puff of wind is shifting millions of tons of air per 100m cubic volume (two football pitches plus a bit of sky??)

 

[Screwdriver]

2m impeller at 2 rpm - I reckon that’s nowhere near Mach 1?

Work it out...

I agree though, it is surprising.

 

[daleyd]

Work it out...

I agree though, it is surprising.

0.468 mph?

 

[Seadog]

I made it 1.5kmh (but I was assuming that the prop was 4m diameter)