Quest for Performance: The Evolution of Modern Aircraft
Chapter 7: Design Trends
Power Loading
[157] The power loading data shown in figure 7.6 appear to have nearly constant values for the upper and lower bounds. Within these bounds, the transport and bomber-type aircraft have power loadings that vary from about 12 pounds per horsepower in 1928 to 8 to 10 pounds per horsepower by the 1950's. Low-performance aircraft have a higher upper bound value of the power loading of about 16 pounds per horsepower although the venerable Piper Cub J-3 had a power loading value of about 19 pounds per horsepower. The lower bound of the power loading is formed by fighter aircraft, which tend to have power loadings in the range from 5 to 6 pounds per horsepower. These low values of power loadings have, through the years, been dictated by the rate of climb and maneuvering performance characteristics required in fighter-type aircraft. Present-day general aviation aircraft have power loadings that vary from nearly 16 pounds per horsepower for the very low-performance type of pleasure or training aircraft to about 8 pounds...

chart illustrating the trends in power loading from 1920 to 1980
[158] Figure 7.6 Trends in power loading of Propeller-driven aircraft.
...per horsepower for the high-performance Beech King Air 200 (at low altitude).