there's some pretty good comments on slashdot

http://tech.slashdot.org/story/10/11/06/1959249/Going-Faster-Than-the-Wind-=
In-a-Wind-Powered-Cart

including some actual maths if you're so inclined.

DougM

On Sat, Nov 6, 2010 at 6:22 PM, RussellMc <apptechnz@gmail.com> wrote:

> If I start watching this sort of stuff I'll die on Monday :-).
> So, without having seen the videos or read the comments apart from a quic=
k
> skim, here's my 1st take - see if it makes sense.
>
> Short:  He's essentially cheating :-) . The car is going directly down wi=
nd
> - the blade essentially isn't. Vector sum of velocities generates lift
> whose
> primary component is downwind.
>
> Long:  An airfoil can travel at speeds greater than the air stream which
> derives it by creating an apparent wind velocity vector which is the vect=
or
> sum of its own motion and the incident wind. Modern WTs (Wind Turbines)
> typically run a5 say 5:1 - 7:1 TSR (tip speed to wind speed ratio). You c=
an
> run at 10:1 or even higher but noise becomes severe and blade erosion
> becomes severe  due to the high blade velocities.
>
> Ina typical HAWT (horizontal axis WT) the wind passes through the blade,
> the
> blade travels at approaching 90 degrees to the wind (the difference being
> the AOT (angle of attack). The instantaneous change of speed of the blade
> is
> not directly related to wind speed but to the lift forces on the blade
> which
> in turn are related to the vector sum of its own current velocity and air
> velocity.
>
> In the current situation the vector sum as far as the car is concerned =
=3D
> Vforward - Vwind.
> If the wing is angles (aerodynamically if not visually physically) to the
> stream it generates lift.
> If Vforward is at least double Vwind then net forward velocity over the
> airfoil is >0.
>
> Getting to this happy state seems hard, at first glance. I have not seen
> the
> videos, but I'd expect it easier if they started at an angle and pulled
> onto
> downwind as speed exceeded twice windspeed. Maybe not.
>
> If their system had a potential TSR of say 10:1 then they may be able to
> get
> say 8+:1 ws.
>
> The above seems to be able to argue that you can run downwind at high spe=
ed
> with NO wind at all  and that the less wind there is the better :-). I'd
> have to draw myself pictures to see if the above makes sense and I'm not
> going to start !!! There has to be energy input from somewhere and airfoi=
l
> needs to derive a velocity vector which is different to the available spe=
ed
> somewhere else within its system (no flow, no lift). Explain it to me :-)
>
>
>        Russell
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