Thanks alot for good anwers!
Would it be true, then, that if a turbo jet engine and a turbo prop engine, which produces the same amount of thrust, and were both designed be most efficient at a relatively low altitude (let's say FL150) the FF would be practically the same? Or would a turbo prop still be more fuel efficient?
..I guess it probably still would vary with TAS, as turbo jets would be more efficient at high TAS than a turbo prop?
. .Nick.