On Thu, 13 Dec 2007 11:13:59 -0800 (PST), peter <[email hidden]>
Quoted message said:A Muzi said:
Every kid who had a Molor Products "Cosmic Whirl" just _knew_ it made
the bike go faster:http://www.deltiology.com/00000images/cosmic.gif
(discontinued product AFAIK)
Of course no one here, incl. gaby, ever claimed that the Cosmic Whirl
(CW) would make a bicycle faster. But if it's mounted in front of the
rider in such a way that it doesn't add any frontal area then it won't
make the bike have any more wind resistance either. I.e. there's added
drag on the CW but correspondingly less drag on the rider right behind
it so the sum remains the same. So making the CW spin takes nothing
away from the speed of the bike.
Dear Peter,
You're falling into the same perpetual motion mistake.
Where does the power come from to spin the Cosmic Whirl?
The rider's legs, of course.
Remember the three laws of thermodynamics, easy enough even for laymen
when described in terms of gambling against the devil in hell.
1) You can't win. (You never get more energy than you start with. You
can't get 101 watts out of a 100 watt system.)
2) You can't break even. (You always lose some energy. You can't even
get 100 watts out of that 100 watt system--frictional losses are
losses.)
3) You can't even quit. (Just sitting there, a closed system tends
toward entropy.)
You _can_ improve the aerodynamics to reduce drag, often in a
non-intuitive fashion, but the propeller isn't likely to function that
way--it will likely just create more chaotic turbulence and increase
drag.
The classic example is the trip wire on the face of a sphere, which
seems as if it ought to increase drag, but actually reduces drag by
clothing the sphere with a thin layer of turbulent air that in crude
terms greases the trailing part of the sphere.
The effect is shown in the two famous photos at the bottom of this
page:
http://www.princeton.edu/~asmits/Bicycle_web/blunt.html
The flow is from left to right. The left-hand sphere has no trip wire
and shows a large flare of turbulence, corresponding to drag. The
right-hand sphere has a trip wire (just a wire circle stuck on the
left side of the sphere) that creates a thin boundary layer of
turbulence that "greases" the surface and greatly reduces drag.
The same principle is used on golf balls. No matter which way the ball
flies and spins, the dimples provide the tiny layer of turbulence to
reduce the drag.
A propeller, on the other hand, doesn't provide a thin layer of useful
turbulence. It just kicks up a chaotic storm--which requires more
energy than smooth flow.
The ideal aerodynamic effect is a tapered double-ended knitting needle
that pushes as little air as possible gently and slowly outward and
then lets the same air close again just as gently and slowly, leaving
no eddies or turbulence behind--any turbulence means that power has
been wasted accelerating the mass of the air, not the vehicle.
That's why canoes and kayaks are double-ended--the pointy front end
parts the water gently, and the pointy back end lets the water close
gently, leaving scarcely any wake. Put a propeller on the front or
back, and it would just add to the drag, spinning the water into
useless swirls.
That said, I believe that a Cosmic Whirl would indeed make a bicycle
faster, but Einstein died before he could complete the proof. :-)
Cheers,
Carl Fogel
