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A wind-powered vehicle that can travel twice as fast as the wind itself

businessinsider.com

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Re: A wind-powered vehicle that can travel twice as fast as the wind itself

#111

Post by karmakaze »

sopooneo wrote: post #16
michaelt wrote: post #13

> The secret to Blackbird, Cavallaro explained, is that once the wind gets the vehicle going, its wheels start to turn the propeller blades — they're connected to the blades by a chain. As the vehicle speeds up, its wheels turn the propeller faster and faster. The propeller blades, in turn, act as a fan, pushing more air behind the land yacht and thrusting it forward. That's a terrible explanation - no wonder people…

You're right that it falls far short, but it corrects one of the fundamental misunderstandings many people have about the vehicle: the wind does not spin the propeller, the wheels do.

A better way of looking at this is to not think of 'the propeller' as being in one plane for the wind to 'push'. Since the prop is rotating, consider the 'airspeed' of the surface of a blade. You can have zero airspeed when the vehicle is moving forward faster than the wind. Similarly the airspeed of the blade is slower (actually negative) than the vehicle's airspeed which allows the wind to push the blade surface when the vehicle is moving faster than the wind.

If you think of a longitudinal vertical plane you can calculate the speed of a point on the cross-section of the blade moving backward relative to the vehicle as a function of the rotation speed. As the vehicle goes faster, so too does the prop surface 'move' backward.

Re: A wind-powered vehicle that can travel twice as fast as the wind itself

#112

Post by Akronymus »

goldenkey wrote: post #100
Akronymus wrote: post #96

Earlier quoted context omitted.

It seems I misunderstood the thread. I somehow thought we were talking about sails in space rather than in atmo.

So what were you originally referring to? Solar wind in space? I'm honestly confused now.

I was actually. I rhought the parent I originally replied to was saying that solar sails aren't useful for large payloads in space.

Re: A wind-powered vehicle that can travel twice as fast as the wind itself

#113

Post by pkulak »

munchler wrote: post #93
marcodiego wrote: post #61

Earlier quoted context omitted.

I really would like a simple explanation of how that is possible. AIUI, when speed is stable, the wheels turning provide power to the propeller. But if the propeller pushes the car, which in turn makes the wheels turn faster making the car faster than the wind itself, what makes it different from perpetual motion machines? I would love a diagram showing how the power flows in this system.

Here's a simple explanation: Let's assume that the windspeed is 10 km/hr. We all agree that the wind can push this vehicle so that it is now going 10 km/hr downwind, right? From the vehicle's point of view, the wind is now 0 km/hr. But its wheels are turning, which turns the propeller, which behaves like a fan, pushing the vehicle forward in the still air, so that it is now traveling faster than the wind. This isn't…

The problem with this explanation, at least to me, is that it doesn't require any wind at all. Why not just give it a push in still air and have to take off into the distance forever?

EDIT: I think it's clicking. It's because the air and the ground are moving relative to the craft at different speeds. So if the ground and air were still, there would be no speed difference to draw energy from.

Re: A wind-powered vehicle that can travel twice as fast as the wind itself

#114

Post by [deleted] »

glitchc wrote: post #98
dmurray wrote: post #34

Earlier quoted context omitted.

Having 9.5 million subscribers on a science-based YouTube channel is definitely more notable than having gone to university, or indeed having a PhD. I think the writer picked the correct description here.

It's a sad indictment of the times when people feel that having a million subscribers (read: being popular) is more notable than having a PhD (read: expert training) in a relevant field. It's no wonder anti-vaccine material is so widespread. Someone famous on Youtube said it was bad, it must be true!

[deleted]

Re: A wind-powered vehicle that can travel twice as fast as the wind itself

#115

Post by lizardmancan »

marcodiego wrote: post #61
michaelt wrote: post #13

> The secret to Blackbird, Cavallaro explained, is that once the wind gets the vehicle going, its wheels start to turn the propeller blades — they're connected to the blades by a chain. As the vehicle speeds up, its wheels turn the propeller faster and faster. The propeller blades, in turn, act as a fan, pushing more air behind the land yacht and thrusting it forward. That's a terrible explanation - no wonder people…

I really would like a simple explanation of how that is possible. AIUI, when speed is stable, the wheels turning provide power to the propeller. But if the propeller pushes the car, which in turn makes the wheels turn faster making the car faster than the wind itself, what makes it different from perpetual motion machines? I would love a diagram showing how the power flows in this system.

imagine the thing has no wheels. the wind provides an energy source.

now imagine a sail cart. you can put a dynamo on the wheel and extract power

now imagine a sail boat sailing close to the wind. it can go much much faster than the wind speed

now imagine a batter powerd propeller pushing a cart forwards.

now imagine the battery powered prop is pushing against the wind. a+b

now remove the battery and extract the power (a) from the wheels.

Re: A wind-powered vehicle that can travel twice as fast as the wind itself

#116

Post by LeifCarrotson »

IgorPartola wrote: post #89
LeifCarrotson wrote: post #62

Earlier quoted context omitted.

The wheel friction and gearing allows the vehicle to tie the effective velocity of the sail (actually a propeller) to the velocity of the ground. Energy can be extracted from the interface between the ground at one velocity and the air moving at another velocity. A sail tied to the ground and extending into this wind will exert a force according to the drag equation: Force = 1/2 x drag coefficient x density x area x…

In your static example how does the turbine get back to the front of the vehicle? Does the force of the wind push it there? If so, if the wind is blowing at 10 knots but the base of the vehicle is moving at 20 then as far as the turbine is concerned it is actually being pushed backwards, no? Or is the idea that when you lift the anchor the vehicle slows to below wind speed while the turbine gets pushed?

For that 'spherical cow' example, assume that the vehicle has batteries to average out the duty cycle while the turbine is going back to the front. Fold the mast down flat and stow the blades so it has minimal air resistance, assume it has minimal weight, and let an electric motor zip it back to the front of the vehicle at 100 knots ground-speed or 90 knots into the wind, if you're moving forward at 20 knots this gives you an 80% duty cycle.

The critical thing to glean from the example is that when it's anchored, the 10 knot ground speed wind is generating power at the turbine, and this has absolutely nothing to do with the speed of the vehicle.

Yes, in the real world, there are all kinds of problems with actually building such a vehicle, and the folding and weight and efficiency and timing and air resistance and rigidity and friction and so on would make it hard to actually get it to work. But these are all purely engineering problems, the physics obviously work!

Re: A wind-powered vehicle that can travel twice as fast as the wind itself

#117

Post by mumblemumble »

raisedbyninjas wrote: post #74
mumblemumble wrote: post #44

Earlier quoted context omitted.

Any wind that's strong enough to overcome static friction would do the trick, and I can't see why 10mph wouldn't be enough. The small models that you can build at home (sorry, can't find the plans anymore; this was 10 years ago now) do not require a push to get started.

The fan blades are an airfoil, which have a minimum speed to create the lifting effect.

Lift isn't really a part of how this gets started. Regular old pushing works on airfoils, too.

Re: A wind-powered vehicle that can travel twice as fast as the wind itself

#118

Post by kube-system »

glitchc wrote: post #98
dmurray wrote: post #34

Earlier quoted context omitted.

Having 9.5 million subscribers on a science-based YouTube channel is definitely more notable than having gone to university, or indeed having a PhD. I think the writer picked the correct description here.

It's a sad indictment of the times when people feel that having a million subscribers (read: being popular) is more notable than having a PhD (read: expert training) in a relevant field. It's no wonder anti-vaccine material is so widespread. Someone famous on Youtube said it was bad, it must be true!

I think it's just a simple matter of the relative exclusivity of those accomplishments.

If I asked you who Woodrow Wilson was, what would you respond with?

Re: A wind-powered vehicle that can travel twice as fast as the wind itself

#119

Post by jfoutz »

mumblemumble wrote: post #50
Valgrim wrote: post #28

Earlier quoted context omitted.

Then what is spinning the wheels?

Asking what is spinning what is maybe framing it in a way that impedes understanding. A quick summary of the basic forces involved are: The wind pushes the propeller, the propeller pushes the wheels, the wheels push the ground, the ground pushes the wheels, the wheels push the propeller, and the propeller pushes the wind. Or perhaps it's better to say that everything is dynamically interacting with everything else. I…

Sailboats are one I struggled with, especially sailing into the wind.

I think my intuition there came from a boat going the same direction as the wind, say, north at 1 meter/sec. now the boat can't go faster than 1 m/sec, but if you turn 45 degrees to the east you can travel sqrt(2) m/sec, because you're still going north at 1/ms and the boat gets pushed along to the east as well.

the closer you get to perpendicular to wind, the faster you can go.

I still don't quite get sailing into the wind, but I'd imagine this wheel and propeller system works the same way. The vehicle motion restricts moving to just a line, the wind pushes along the x axis, so the vehicle can sort of zoom along in the y direction, much faster than moving along x.

__edit__

oh, I see, it's the difference between ground and wind. a sailboat couldn't do this but something clever with propellers and linkages probably could

Re: A wind-powered vehicle that can travel twice as fast as the wind itself

#120

Post by tshaddox »

marcodiego wrote: post #61
michaelt wrote: post #13

> The secret to Blackbird, Cavallaro explained, is that once the wind gets the vehicle going, its wheels start to turn the propeller blades — they're connected to the blades by a chain. As the vehicle speeds up, its wheels turn the propeller faster and faster. The propeller blades, in turn, act as a fan, pushing more air behind the land yacht and thrusting it forward. That's a terrible explanation - no wonder people…

I really would like a simple explanation of how that is possible. AIUI, when speed is stable, the wheels turning provide power to the propeller. But if the propeller pushes the car, which in turn makes the wheels turn faster making the car faster than the wind itself, what makes it different from perpetual motion machines? I would love a diagram showing how the power flows in this system.

It’s certainly not perpetual motion, because the vehicle would quickly come to a stop if the wind speed (relative to the ground) went to zero.
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