Re: sailing upwind

[brian whatcott <inet@INTELLISYS.NET>]

At 13:24 12/31/99 -0500, John D wrote:

> Actually it was on wheels.  The air-turbine used the propeller from a model
> airplane.  The linkage was implemented using his kid's construction-toy
> kit, which included a couple of what I'm guessing to be 4:1 gearbox pods,
> for an overall mechanical advantage of 16:1 if vague memory serves....
> The air came from a blow dryer.
>
> The result was quite impressive.  It took a pretty stiff breeze to overcome
> static friction and turbine blade stall, but once the thing started moving
> it moved quite smartly.  Everyone who saw it couldn't help but smile to see
> the thing attracted to the blower.
>
> It left no doubt that the aquatic model would work just fine, especially if
> it had some angle-of-attack control on the turbine blades to help with
> starting.


This was a truly splendid puzzle.
 Nobody I assume, disagrees that momentum is conserved in this model?

And so we appear to have a paradox: backward momentum is reduced through
 the air prop's disk, and forward momentum is gained by the car's forward
movement. A net gain in momentum, one could suppose?

This is fully the equal of New Scientist's perpetually moving bicycle
wheel at a science oriented show some years ago!
 It perhaps illustrates that physicists are not the best folks to
detect sleight of hand from magicians.

This is what I think you should consider.
 1) It was an aircraft model propeller (This is optimised to increase
the momentum of the airstream, not to extract momentum)
 2) It needed a stiff breeze from a fan. (This is a far from uniform
source, naturally enough)

Method: Play a stiff breeze into a *part* of an airplane prop.

 The driven surface is inefficiently driven.
 The remainder of the disk drives (relatively still) air rearwards.

Net result: an *increase* in momentum through the integrated prop
disk surface.

This allows a surplus thrust force for propulsion.
Wonderful!



brian whatcott <inet@intellisys.net>
Altus OK