I've always thought of those turbo compound radials as the 'missing link' between piston engines & turbine.
Sort of a jet engine with a high frequency, intermittent ignition & low efficiency turbine.
If they could've left the valves open, kept the flame lit & improved the efficiency of the power recovery turbine it would pretty much be a turbo-prop.
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The Super Constellation: The best 3 engine aircraft Qantas ever operated.