Introduced in 1938, the Lycoming flat four provided solid power -- 50 horsepower at 2,300 rpm. It was unique among the fours that appeared in the late 1930s thanks to its one-piece cast construction -- all four cylinders were cast with the crankcase. From its modest power beginnings, two later models were developed that produced 60 then 75 horsepower. It did not gain the wide usage of its competitor models from Continental or Franklin.
Among the famous Second World War aircraft using the Franklin flat four were the L-5 and L-13 observer/utility planes.