ORIGINAL: HarryC
ORIGINAL: Gaspar
this is the reason why a turbofan is more efficient than a pure jet, so, looking from this perspective, the engine with lower exhaust speed win...
The efficiency is due to a crucial difference between the equations for momentum and energy.
Thrust is the reaction to momentum imparted to the air and is simply mass * velocity. So a large mass at slow speed and a small mass at high speed can have the same velocity. If you double the mass you can halve the velocity and still get the same thrust.
The kinetic energy of the exhaust can only come from energy supplied by the engine, i.e burning fuel. But the energy is related to the square of the velocity, whereas the thrust is related to the straight velocity. If you move twice as much air at half the speed then the mass term doubles but the velocity term is cut to a mere quarter, so the energy of the exhaust is cut in half and the energy required to be put in is thus cut in half, for the same thrust. That is why turboprops, and fan engines which are simply props in a duct driven by a jet engine, are so popular at subsonic speeds - it takes less fuel to get the same thrust.
I dont want to a smart ass but I think you meant to say same momentum not velocity in your third sentance.
Very interesting thread thanks HarryC for the informative posts....so in conclusion are we saying that gaspar is right and that at the sort of speeds we fly 200mph ish the residual thrust of an engine with higher EGV is not sufficient to accelerate the airframe any more than the engine with the same thrust but lower EGV and we would have to be flying at speeds approaching the exhaust gas velocity to see the differance?