On 01/10/2010 18:16, Ronald Raygun wrote:

The exactly-at-windspeed scenario is a bit of a crazy special case in
that there is no air resistance; if the wheels are also frictionless,
then the car doesn't actually need any power to remain at its speed.
Therefore the presumption that the wheels must generate 1hp and the
prop must therefore provide 10lb of thrust is a bit silly, since
clearly the prop really doesn't need to provide any thrust, and so
it's perfectly all right to run it at 0hp, and our generator will
happily supply 0hp using 0lb of wheel drag.

Yes, but the freewheeling case isn't interesting. Introducing the wheel
driven prop is what makes it interesting.

You've introduced the assumption that the car must be at steady state -
there's no reason for that to be the case. Drop that assumption, work
with the forces and see where you get.