"Richard Casady" wrote in message
...
On Mon, 15 Oct 2007 00:09:08 -0700, toad
wrote:

20kts of wind on the nose. Assume no friction or drag anywhere in the
system apart from the push backwards on the windmill. How fast does it
go. Show your workings.

If it is unobvious that a windmill can power a prop and proceed upwind
consider something similar on land, with a rack and the pinion on the
machine. They do make gear driven railroads, there is one at Pike's
Peak.

Why wouldn't it accellerate indefinitely with no friction anywhere in
the system. In real life, of course, props are not very efficient.

Casady

Things can go to infinity:

http://www.oddparts.com/acsi/motortut.htm

"Caution: If a DC motor suffers a loss of field (if for example, the field
power connections are broken), the DC motor will immediately begin to
accelerate to the top speed which the loading will allow. This can result in
the motor flying apart if the motor is lightly loaded. The possible loss of
field must be accounted for, particularly with shunt wound DC motors. "

Here the back EMF of the motor is redirected into the armature and the
motions speeds up infinitely until it flies apart.