On 04/10/2010 11:18, dennis@home wrote:

but its all about energy *and* thrust, if one doesn't work the other can
be ignored.

Ah, you're back. Would you care to reply to the post I made a couple of
days ago?

Say you have a cart travelling at 20 m/s with a wind behind it blowing
at 10 m/s

The relative wind at the cart is 10 m/s to the rear.

Now to stop the wind behind you have to accelerate the air going past
the cart to 20 m/s.

Do you? Why is that?

Now put E=mv2 into the picture and work out how much energy it takes to
accelerate the air going past from 10 m/s to 20 m/s so it stops the wind
from behind.

My simple maths tells me its 4 times the energy you get from stopping
the wind .

I can also stop the wind with a wall, and I don't believe you'd claim
that takes energy. So why do you need to invent that energy input?

And while we are at it can anyone explain what the coupling is between
the energy in the air that is stopped and the cart.
Its all very well saying the energy is lost by the wind but there is no
obvious coupling to the cart which is travelling faster than the wind.

I expect that it goes into turbulence and is just lost. Lets face it an
airplane capable of doing 100 knots doesn't suddenly go 10 knots
faster flying downwind in a 10 knot wind so none of the energy released
by stopping the wind is absorbed by the plane.

Does a plane have a connection to the ground? I'm thinking generally
speaking no, and when it does it's not via an arrangement such as is
seen on those cars, merely a wheel possibly braked.

The connection to the ground is what makes this work, which is why
thinking of planes is causing you to get it all wrong.