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Nick_wacker

Blackbird Land Yacht

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On 20/07/2018 at 09:30, Nick_wacker said:

How on earth does it sail down wind faster than the wind, surely as it matches the downwind speed the propeller stops turning.

Tricky one. Something to do with this?: The big difference in the two crafts is that in the upwind version the **turbine actually powers the wheels while in the downwind version the wheels power the **propeller.

http://www.nalsa.org/DownWind.html

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There is quite a good video showing the effect here -

I gather that before wind speed is reached the propeller acts like a sail (albeit a poor one) and after wind speed it acts like a propeller.

In terms of physics, one has to consider the forces acting on the vehicle. A constant force will cause acceleration rather than a constant velocity, so the vehicle will accelerate until the propeller or sail force equals the drag forces plus the friction forces of the mechanism, wheels and tread.

Intuitively, one would think the forces would be in equilibrium prior to the vehicle reaching wind speed, but this doesn't seem to be the case.

The interesting point is what happens at wind speed and the switch over to the propeller actually acting like a propeller. It may be that the wind speed needs to drop in order to get the propeller going. Also, at wind speed, drag forces are zero, which may help.

I'm not sure if this explains much, and as a disclaimer I could be talking complete c r a p, but thought I'd have a go!

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22 hours ago, Captain Kirk said:

The interesting point is what happens at wind speed and the switch over to the propeller actually acting like a propeller.

Maybe once at wind speed, the propeller wings twist the other way so the momentum in the wheels cause the blades to push air backwards against the wind that was originally pushing the vehicle forwards and the force of the air now in effect going the opposite direction due to being slower than the vehicle propels the wheels also.

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See if you can get your heads round this and translate from sail to propeller.

https://en.wikipedia.org/wiki/Apparent_wind

"In the foiling AC72 America's cup catamarans, the boats sail through the water at up to double the environmental wind strength. The effect of this is to radically change the apparent wind direction when sailing "downwind". In these boats the forward speed is so great that the apparent wind is always forward—at an angle that varies between 2 and 4 degrees to the wing sail. This means that AC72's are effectively tacking downwind, although at a greater angle than the normal 45-degree upwind angle, usually between 50 and 70 degrees."

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12 hours ago, Arpeggio said:

Maybe once at wind speed, the propeller wings twist the other way so the momentum in the wheels cause the blades to push air backwards against the wind that was originally pushing the vehicle forwards and the force of the air now in effect going the opposite direction due to being slower than the vehicle propels the wheels also.

I think the wheels drive the propeller prior to wind speed. I wasn't quite sure of that in my earlier post. So at wind speed it would be equivalent to a stationary vehicle with the propeller rotating and creating thrust which would propel the vehicle forward.

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On 21/07/2018 at 19:49, Arpeggio said:

Tricky one. Something to do with this?: The big difference in the two crafts is that in the upwind version the **turbine actually powers the wheels while in the downwind version the wheels power the **propeller.

http://www.nalsa.org/DownWind.html

I don't fully understand it myself, but I do know that the wind is always moving relative to the ground, and that there is always a section of the 'drive wheels' of the vehicle in contact with the ground, and stationary at the point of contact with it. The vehicle uses that connection to extract the energy of the wind by slowing it down relative to the ground. If you tried to use it on a slippery surface like an icy lake, it wouldn't work.

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The vehicle might be moving downwind but the motion of the blades relative to the wind isn't. If that's what's going on then the pitch of the blades probably needs constant changing so the ones on opposite sides don't cancel each other out.

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  • 301 Brexit, House prices and Summer 2020

    1. 1. Including the effects Brexit, where do you think average UK house prices will be relative to now in June 2020?


      • down 5% +
      • down 2.5%
      • Even
      • up 2.5%
      • up 5%



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