In layman's terms:
Low to medium speed efficiency is greatly improved by enlarging the fan/prop. For instance, a very good example is a helicopter - the bigger the rotor diameter, the less HP it needs to fly. Same is true for aircraft. This is exactly the reason why all the new "fuel efficient" turbofans get bigger and bigger in diameter (look at the GENx and the Trent 1000's). The core is just as inefficient as it's always been (it's still a simple turbojet in the middle), but the bigger the fan is at the front, the more of that can be turned into thrust for the same amount of fuel. Now, obviously, at too large a prop/fan diameter the penalties of weight become a problem. But as mentioned - there is no real difference in performance between a turboprop and a fanjet of the same size. And the two will eventually grow even closer than they are today. Just witness the fanjet blade count steadily going down for every new generation...
The notion that props are inherently slower than jets is also false. The Piaggio Avanti flies a lot faster than a Citation burning 40% less fuel. Why? Because it moves more air.
Last edited by AdamFrisch; 4th November 2012 at 21:18.