On 9/22/2010 1:26 AM, Jed Rothwell wrote:
I do not see how this can work! They are going with the wind, so if
they start to travel at the same speed as the wind, the propeller
should stop turning.
Maybe I am missing something.
Yes what an interesting puzzle! But knowing that it can be done and
armed with the clues that the device has wheels and a propeller - it
doesn't take very long to figure out how it could be done.
Consider the fact that it takes almost no energy to travel at the same
speed with the wind, and also consider that since you have access to
both the air (via the propeller) and the ground (via the wheels), and
since these things are in motion with respect to each other (the wind),
it must be possible to extract energy from that differential motion and
use it to drive the device faster than the wind. The only question is how!
Now suppose the wheels have a circumference of say 1 meter and suppose
we set the pitch of the propeller so that it also has a screw pitch of 1
meter per revolution and then suppose we connect these two things
together with a 1:1 drive shaft arrangement. Now we have the situation
that no matter what speed the device moves at *in still air*, the
propeller slices perfectly edgewise through the air and produces no
thrust or drag force via the propeller or wheels.
Now supposing we cut the shaft and interpose an electrical generator in
the shaft so that its stator is linked to the wheels and its rotor is
linked to the propeller. Now we have built a windmill generator that
works identically regardless of its motion along the ground! If there
is no wind then no matter what speed it rolls at with respect to the
air, no power is produced or consumed by the generator. But supposing
there is some wind - then again no matter what speed it rolls at with
respect to the wind - the wind produces the same differential motion
between the stator and armature and thus exactly the same power whether
it is moving or stationary!
So this is the type of device we are looking for. It can roll along at
the same speed as the wind - consuming almost negligible energy in air
friction (because it is not moving relative to the air) and only a
minute amount of energy in wheel and shaft bearing friction (thanks to
modern bearings etc) - and all the time it can be generating exactly the
same power as if it was locked to the ground with the wind driving the
generator via the propeller in the normal manner. Clearly this power is
available to be used to drive the device faster than the minimal
friction situation of rolling at the same speed as the wind.
The only remaining question is the actual detail of how this available
power can be most efficiently applied to the wheels. My guess is that
you don't bother with a generator but simply have a variable ratio gear
box between the propeller and wheels. Or simplest of all have a
variable pitch propeller!
