The way I tend to think about this is that piston engines are more efficient on a small scale. As you increase the size of a piston engine, more and more of your power is consumed lifting and powering the piston engine itself. They become less efficient at some point. read more
There is a line where it becomes more efficient to use a turbine engine than a piston, and it seems to be right around the size of an R-44/R-66, or more broadly, the size of a helicopter when it goes from about 4 passengers to 5 or 6 with some baggage. read more
The R22 and R44 are powered by a piston engine and the turbine helicopter is powered by a sophisticated turbo shaft engine. The most significant differences are found in the power to weight ratio. For example, most piston engines will weigh twice as much as the power they can produce whereas a turbine engine only weighs about 25% of the horse power produced. read more
Selected in 1955 from 20 competing designs, it overcame early cabin configuration and insufficient power issues to become the U.S. military's first turbine-powered helicopter. Officially named the "Iroquois" by the Army, its familiar "Huey" moniker stems from its early HU-1 designation. read more