OT. New design "Wind Turbine"

[Chris Hogg]

Ah. That sort of synchronisation...

[The Natural Philosopher]

Mmm. it's actually the swept area that counts, because of the interference between the bits of air.

There is an apocryphal tail of someone who pioneered the water screw against the paddles of the day and his one complete turn screw actually smashed itself on a log, and the boat doubled its speed.

Its easily possible to make fans or propellers or screws whose blades cover the whole swept area. The fact is they dont work better.

IIRC its not the same case if you can duct the fan - like a turbofan jet.

So it must be all about removing the 'interference' from the air going past outside the propellor as against the air inside.

Betz's law covers it for sure, but why it is so and deriving it from first principles of physics is something I take on trust.

[harry]

There is a "smoke ring" vortex round normal propellors and fans. The duct eliminates this "short circuiting" to a large extent.