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Scientists/engineers - How Does The Land Yacht Blackbird Work?

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Hi all, asking this here as I know there are loads of people on this forum who are a hell of a lot cleverer than me.

My Uncle has come down to visit, and he's a very smart grumpy old man. Long story, but we were discussing where we should visit on our family trip to the USA to see the 2017 solar eclipse (instigated by my mum (his sister), who loves eclipses and has seen several already). Uncle said he'd like to do San Diego as he likes yachting and it's apparently the home of the Americas Cup. He then mentioned that Americas Cup boats don't use sails any more, they use carbon fibre 'foils'. And so to keep the chat going I thought of asking if he was aware of the land yacht Blackbird that can go directly downwind at 2.7 times the speed of the wind?

https://en.wikipedia.org/wiki/Blackbird_(land_yacht)

He was interested, but not in the way I'd anticipated: he thought it was an elaborate hoax, and was actually quite annoyed. I personally believe absolutely that the thing works as advertised, but I'm not good enough at engineering maths/theory to prove it. The Wikipedia article is completely unhelpful.

Reading further into it tonight I have had my assumptions turned on my head anyway. The indisputable fact (if you assume it is not an elaborate hoax) is that it works by having a turbine/propellor geared to a driven axle whose wheels are directly connected to the ground. I had thought that the turbine was powering the wheels when it was travelling downwind- that made sense to me as the bottom of any wheel that is in traction with the ground is stationary, whereas the wind is moving past it- but apparently I am 100% wrong. As well as getting the thing to go downwind at 2.7x the prevailing windspeed they have converted the 'propellor' to a 'turbine' and thus got it travelling upwind at 2x the prevailing wind. So of course when it's travelling downwind the wheels are driving the propellor to accelerate it forward...

If the science/mathematical proof is beyond the ken of a man who got an A in A-level Physics but only a C in maths just tell me I guess. But it would be nice to send my uncle home not thinking I'm a foolish crank...

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More info here:

http://www.bbc.com/future/story/20120727-the-wind-beneath-my-wheels

and here:

http://www.wattflyer.com/forums/showthread.php?t=58486&page=2

Apparently, boats do tack downwind faster than the wind routinely and a variation of tacking is what is happening here with the prop.

Thanks for posting it - Interesting indeed.

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hmm...certified by themselves..

As for the treadmill, well the energy used to move the thing comes from the treadmill on build up, its simply reversing the vector of the energy in the treadmill.

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More info here:

http://www.bbc.com/future/story/20120727-the-wind-beneath-my-wheels

and here:

http://www.wattflyer.com/forums/showthread.php?t=58486&page=2

Apparently, boats do tack downwind faster than the wind routinely and a variation of tacking is what is happening here with the prop.

Thanks for posting it - Interesting indeed.

gearing.

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I've never really considered that apparent wind being cancelled out by true wind before.

Sounds a bit nuts but also quite logical.

So if I am cycling at 40mph - but have a 40mph wind also behind me - I will literally feel nothing on my face ? Is that really true ?

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I've never really considered that apparent wind being cancelled out by true wind before. Sounds a bit nuts but also quite logical. So if I am cycling at 40mph - but have a 40mph wind also behind me - I will literally feel nothing on my face ? Is that really true ?

yes...any pilot will tell you, airspeed is different to groundspeed. eg, 40Kt tail wind and 100kt indicated airspeed and ground speed will be around 140Kt...turn around and your groundspeed will be 60Kt.

It is of course possible for a small aircraft to fly at 0kts compared to the ground.

On turning from tail to headwind, clearly the wind is apparently much stronger relative to the aircraft. The effect is soon cancelled out as the plane slows.

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Isn't it to do with the pressure waves caused by the propellor?

As the propellor rotates it creates energy, and hence propulsion, by creating alternating high and low pressure on opposite sides of the propellor which basically creates a cone-shaped vortex behind the craft driving it forward at speed.

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Isn't it to do with the pressure waves caused by the propellor?

As the propellor rotates it creates energy, and hence propulsion, by creating alternating high and low pressure on opposite sides of the propellor which basically creates a cone-shaped vortex behind the craft driving it forward at speed.

the pressure on each side of the blade doesnt alternate. It varies with speed and angle of attack. To turn the blade takes energy, which generates lift at the propellor blade.

It is not 100% efficient so there is a loss between the energy source and the blades output in friction, some heat and air resistance.

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the pressure on each side of the blade doesnt alternate. It varies with speed and angle of attack. To turn the blade takes energy, which generates lift at the propellor blade.

It is not 100% efficient so there is a loss between the energy source and the blades output in friction, some heat and air resistance.

Does not a propellor create alternating areas of low and high pressure on the blades as they turn/?

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Does not a propellor create alternating areas of low and high pressure on the blades as they turn/?

no. low pressure at the front, high pressure at the back...a sideways wing if you will. You may have noticed propellors are shaped with a twist too to make efficient the lift generated along its entire length...advanced props have variable pitch, sort of like adding a gearbox so engine power isnt wasted at various speeds, as the angle of attack will vary according to the forward speed of the assembly.

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I've never really considered that apparent wind being cancelled out by true wind before. Sounds a bit nuts but also quite logical. So if I am cycling at 40mph - but have a 40mph wind also behind me - I will literally feel nothing on my face ? Is that really true ?

You can still accelerate. Prepare for it the night before: I find an artichoke soup does wonders for my propulsion.

To the OP, bear in mind that as soon as you pass through the light barrier, the laws of physics reverse. Reference.

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I can't get my head round how that Blackbird thing can go downwind at more than twice the true windspeed. When it gets to windspeed surely there's no drive on the turbine? I know it does it but I can't make sense of it. It's doing my head in.

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I can't get my head round how that Blackbird thing can go downwind at more than twice the true windspeed. When it gets to windspeed surely there's no drive on the turbine? I know it does it but I can't make sense of it. It's doing my head in.

Since it's possible to move a wind-powered vehicle faster than the wind in one direction then it is, in theory, in any direction, with some suitably clever mechanism.

What I think is happening is the effects of the aerodynamics of the propellor blade (although calling it a propellor and having a different turbine for moving in to the wind is a bit I'm struggling with), see Bloo Loo's post. The key is that they aren't moving precisely downwind as they're rotating through the air. What you've got are a set of aerofoils generating lift (somewhere) in the vertical plane, which the gearing transfers to the wheels and propels the car. That's why I've a bit of an issue with it being called a propellor though, since it isn't directly providing the thrust if that's how it works.

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Since it's possible to move a wind-powered vehicle faster than the wind in one direction then it is, in theory, in any direction, with some suitably clever mechanism.

What I think is happening is the effects of the aerodynamics of the propellor blade (although calling it a propellor and having a different turbine for moving in to the wind is a bit I'm struggling with), see Bloo Loo's post. The key is that they aren't moving precisely downwind as they're rotating through the air. What you've got are a set of aerofoils generating lift (somewhere) in the vertical plane, which the gearing transfers to the wheels and propels the car. That's why I've a bit of an issue with it being called a propellor though, since it isn't directly providing the thrust if that's how it works.

odd, that when the relative wind reverses, the propellor keeps going in the same direction....wind from behind rotates a propellor one way, from the front the other way...try it with a kids windmill.

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odd, that when the relative wind reverses, the propellor keeps going in the same direction....wind from behind rotates a propellor one way, from the front the other way...try it with a kids windmill.

I've not seen any video (all seems to get blocked at work), but do blades get feathered around differently as it passes through that point?

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I've not seen any video (all seems to get blocked at work), but do blades get feathered around differently as it passes through that point?

The blades stay the same. I think there's some kind of feedback mechanism between the wheels and the blade (they are coupled). If you consider the point at which the cart reaches windspeed, there is no airflow over the prop yet it is still spinning as it is coupled to the wheels so will be creating some thrust and forward drive.

I'd like to schematic of the forces and vectors involved.

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I've not seen any video (all seems to get blocked at work), but do blades get feathered around differently as it passes through that point?

no, the man made a fixed pitch prop in his garage.

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The blades stay the same. I think there's some kind of feedback mechanism between the wheels and the blade (they are coupled). If you consider the point at which the cart reaches windspeed, there is no airflow over the prop yet it is still spinning as it is coupled to the wheels so will be creating some thrust and forward drive.

I'd like to schematic of the forces and vectors involved.

but the moment that happens the wind turns and acts as a brake on the propellor, and thus the vehicle will slow, not accelerate as they claim.

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but the moment that happens the wind turns and acts as a brake on the propellor...Its a hoax

Both before and after the wind turns the blades are still moving, and the direction of blade face relative to the air it's moving through is still to one side, whatever it is with respect to the car. Eventually it will reach a speed where the wind is blowing parallel to the blade face and stop accelerating (a bit less in practice thanks to friction). I think. Still has a bit of perpetual motion feel about it though.

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Both before and after the wind turns the blades are still moving, and the direction of blade face relative to the air it's moving through is still to one side, whatever it is with respect to the car. Eventually it will reach a speed where the wind is blowing parallel to the blade face and stop accelerating (a bit less in practice thanks to friction). I think. Still has a bit of perpetual motion feel about it though.

so, is it the road driving the blades, or the blades pushing the road?...at some stage there is a change of state. Dont forget these people claim 2.8 times the speed of the wind.

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Don't know alas.

I'm pretty confident that the concept is possible; I can think of some other means by which it could be done in theory, even though they'd almost certainly fail to work in practice. For example an electric car powered by a wind turbine and a spooled cable could easily go faster than the wind in whatever direction it liked. When the cable spools out then collapse the turbine blades, so minimal resistance, and wheel it in. Jam it into the ground and start spooling out again. Like I said very unlikely to work in practice but in theory could get you faster than the wind in any direction. Is having a suitable prop effectively doing something similar all the time?

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