On Sun, 22 Jan 2006 at 20:18:32 in message
, Brian Whatcott
wrote:

But think about planes with (some) jet engines,
these can be constant THRUST.

That means, the faster they go, the more HP they put out!
(A reason why jets on slow planes is not a great idea)

No, is not quite that easy. Another way of looking at force is that it
is rate of change of momentum.

In a simplified way the thrust of a jet engine comes from the change of
momentum from the air captured by the engine to the momentum of the air
that leaves the back of the engine. If you think of a Turbofan engine
then the fan is not so different from a propeller. The internal
efficiency of the two types of engine is somewhat different.

In any case the greatest propulsive efficiency comes from a momentum
change of a large mass of air with a very small velocity change.
--
David CL Francis