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Old 06-11-2021, 02:37 AM   #1 (permalink)
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"Faster than the wind," impossible they say but the Blackbird settles the debate

Counter-intuitive they say, that a wind-powered craft going straight downwind can go faster (no angled tacking/gybing) than the wind itself.
They built a three-wheeled land craft with a twin-bladed propeller/turbine, the Blackbird to test and prove their wind challenge. The bluff-body Blackbird first gets started by the wind from behind then the forward rolling wheels turn a geared sprocket that turns the propeller to push air rearward and provides added forward thrust! In their video you see the windsock showing the wind direction but the Blackbird's telltale streamer is waving backwards from an apparent headwind - it is moving faster the the wind.
Aerodynamicist Rick Cavallaro tweaked the Blackbird and achieved a top speed 2.86 times faster the the wind speed in 2010.

In 2012, they achieved a top speed 2.01 times faster the wind speed - headed UPWIND with a turbine rotating the opposite direction to transmit power to the wheels!

Blew my mind but it all made sense with the 'cylindrical Earth demonstration'




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Old 06-11-2021, 12:22 PM   #2 (permalink)
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over unity?

Quote:
Originally Posted by botsapper View Post
Counter-intuitive they say, that a wind-powered craft going straight downwind cannot go faster (no angled tacking/gybing) than the wind itself.
They built a three-wheeled land craft with a twin-bladed propeller/turbine, the Blackbird to test and prove their wind challenge. The bluff-body Blackbird first gets started by the wind from behind then the forward rolling wheels turn a geared sprocket that turns the propeller to push air rearward and provides added forward thrust! In their video you see the windsock showing the wind direction but the Blackbird's telltale streamer is waving backwards from an apparent headwind - it is moving faster the the wind.
Aerodynamicist Rick Cavallaro tweaked the Blackbird and achieved a top speed 2.86 times faster the the wind speed in 2010.

In 2012, they achieved a top speed 2.01 times faster the wind speed - headed UPWIND with a turbine rotating the opposite direction to transmit power to the wheels!

Blew my mind but it all made sense with the 'cylindrical Earth demonstration'


1) from the first sentence, once the craft achieves a velocity equal to the wind velocity, there can be no net force acting on the vehicle. Thrust = zero.
2) during the runup, the wheels, acting through a transmission, could be feeding power to an accelerating flywheel, storing kinetic energy, in excess of that necessary to overcome any aerodynamic or rolling-resistance road load.
3) once up to 'speed', and the blades re-configured for thrust, the flywheel could be tapped, to power the craft to some given velocity, momentarily, however, at some point, this energy would be exhausted, thrust would be lost, with deceleration.
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What is not said would be of much interest. Like, for how much time can they demonstrate this feat?
Upon opening the kimono, we may just discover some parlor trick, which has nothing to do with aerodynamics.
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Old 06-12-2021, 02:41 AM   #3 (permalink)
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aerohead -

Watch from 9:15 to 9:45, inside the craft energy always travels from the wheels to the propeller, there is no wind turbine!
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Old 06-13-2021, 01:00 AM   #4 (permalink)
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It used to blow my mind that a sailboat could sail faster than the actual wind. Then I started wrapping my head around the fact that the sail in question was acting as an airfoil.

Once I realized that this video was actually serious, I figured it probably had something to do with that effect. And it does, indirectly, as the "cylindrical world" animation shows.

-soD
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Old 06-13-2021, 11:02 AM   #5 (permalink)
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Quote:
Originally Posted by AeroMcAeroFace View Post
Freebeard is right, it is possible. https://www.wired.com/2010/06/downwi...than-the-wind/ [/IMG]
I was planning on starting a "why don't cars have sails on them?" thread later this week, but the veritasium video delayed me.

I have been thinking about the downwind faster than the wind thing for years and still don't fully understand it, but I agree that the cylindrical earth thing makes the most sense.

I don't believe it is fake at all, but very limited in application, without gearing and a variable pitch/speed propeller it seems to need a narrow band of winds to be possible.
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Old 06-16-2021, 12:41 PM   #6 (permalink)
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9:15 to

Quote:
Originally Posted by jakobnev View Post
aerohead -

Watch from 9:15 to 9:45, inside the craft energy always travels from the wheels to the propeller, there is no wind turbine!
1) I'm going to give it a 'fail'
2) the tattle-tail is on the wrong end of the car
3) there's no actual instrumentation
4) we have no idea what the local free-air velocity is
5) we have no idea what the vehicle surface velocity is
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6) the vehicle relies on aerodynamic drag to capture propulsive kinetic energy
7) there will be a mechanical loss of the propellor as a 'sail'
8) there will be a mechanical loss of the reversed vertical struts as a 'sail'
9) there will be a mechanical loss of the wheels, body, pilot, Go-Pro, and downwind excrescence as a 'sail'
10) there will be a mechanical loss of the drivetrain
11) there will be a mechanical loss of the propellor shaft
12) there will be a mechanical loss of the propellor.
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13) I suspect that turbulence downwind of the vehicle structure is inducing a low pressure which impels the tattle-tail observed orientation, fooling the observer into believing that the vehicle has exceeded the local air speed ( you can see an example of turbulent wake-induced flow reversal in the video of ' Streamlined Toyota T-100 in the Wind Tunnel ) we video-graphed this specifically for this purpose.
14) proper instrumentation will reveal the facts.
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Old 06-16-2021, 01:15 PM   #7 (permalink)
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Quote:
Originally Posted by jakobnev View Post
aerohead -

Watch from 9:15 to 9:45, inside the craft energy always travels from the wheels to the propeller, there is no wind turbine!
sorry, that isn't true, the energy doesn't always flow from the wheels to the propeller, the energy can flow from the propeller to the wheels.

in the beginning, the car isn't moving, but has a tail wind, the rotor spins, which in turn drives the wheels. the car accelerates partly due to the drag of the vehicle, and partly due to the propeller driving the wheels.

Click image for larger version

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at wind speed. so when there is no air resistance on the craft (essentially in still air in a reference frame), the wheels are driving the propeller. The propeller is driven through gearing faster and produces a small amount of thrust, This accelerates the craft slightly, at which point the craft is in a headwind, the propeller can then propel the craft upwind.

You would agree, that given the right gearing, a car could travel upwind?
you would agree, that given the right gearing, a car could travel downwind?
The hard bit to understand, is the changeover from the car going downwind to the car going upwind(or more technically downwind faster than the wind).
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Old 06-16-2021, 01:43 PM   #8 (permalink)
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energy

Quote:
Originally Posted by AeroMcAeroFace View Post
sorry, that isn't true, the energy doesn't always flow from the wheels to the propeller, the energy can flow from the propeller to the wheels.

in the beginning, the car isn't moving, but has a tail wind, the rotor spins, which in turn drives the wheels. the car accelerates partly due to the drag of the vehicle, and partly due to the propeller driving the wheels.

Attachment 30860

at wind speed. so when there is no air resistance on the craft (essentially in still air in a reference frame), the wheels are driving the propeller. The propeller is driven through gearing faster and produces a small amount of thrust, This accelerates the craft slightly, at which point the craft is in a headwind, the propeller can then propel the craft upwind.

You would agree, that given the right gearing, a car could travel upwind?
you would agree, that given the right gearing, a car could travel downwind?
The hard bit to understand, is the changeover from the car going downwind to the car going upwind(or more technically downwind faster than the wind).
1) we know that a 'propeller' is a lousy 'turbine.'
2) I didn't see a 'thrust-reverser' as found on a Lockheed C-130 Hercules, which allows the aircraft to back itself out of an airport terminal without a tug.
3) whatever energy is extracted from the 'lousy turbine' to accelerate and build kinetic, there are energy losses at every step of power transfer between blades and wheels, regardless of direction.
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4) at wind speed, there is no net force acting anywhere on the vehicle, it's at terminal velocity.
5) if the tattle is 'WRONG', and the car is actually going slower than the actual airspeed ( which is what I believe), then it's just air drag propelling the car.
6) for the blades to be producing net thrust requires over-unity, which is impossible given the already stated mechanical efficiencies between blades and wheels.
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A trip to Palo Alto, California, and NASA's largest wind tunnel would provide for a laboratory-quality test, where all parameters can be accounted for.
Until something like that, it's a 'stunt.' A parlor trick.
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Old 06-16-2021, 03:27 PM   #9 (permalink)
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Quote:
Originally Posted by aerohead View Post
4) at wind speed, there is no net force acting anywhere on the vehicle, it's at terminal velocity.

6) for the blades to be producing net thrust requires over-unity, which is impossible given the already stated mechanical efficiencies between blades and wheels.
at wind speed, the wings on the propeller are still moving, and like the sail on a tacking boat, still providing thrust.

It doesn't require over-unity, because essentially the energy is coming from the difference between air speed and ground speed.
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Old 06-16-2021, 04:05 PM   #10 (permalink)
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thrust

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Originally Posted by AeroMcAeroFace View Post
at wind speed, the wings on the propeller are still moving, and like the sail on a tacking boat, still providing thrust.

It doesn't require over-unity, because essentially the energy is coming from the difference between air speed and ground speed.
The Second Law of thermodynamics doesn't allow for the energy transferred from the blades to the air to exceed the energy coming from the wheels and inertial/ momentum energy transfer from the vehicle/passenger.
The energy at the blade-air interface must be lower than that at the road-to-tire interface due to energy loss in the tire, gears, wheel bearings, lower sprocket, chain, upper sprocket, jack-shaft bearings, then the blades themselves.
Even if the blades were 100% efficient, there wouldn't be enough energy.
And a look at Abbott and Von Doenhoff's book, Theory of Wing Sections will disabuse you of any notion about high efficiencies under high lift conditions.

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