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The largest single engine propeller driven aircraft is the Grumman AF-2S Guardian, a sub chaser of the Korean War era. It grossed at 25,500 lbs and was powered by a Pratt & Whitney R2800 engine of 2400 horsepower. It also had the largest Wing span for a single engine reciprocating aircraft at 60' 8".
The Douglas Skyraider and Vietnam version called the Spad are the most powerful piston driven propeller aircraft with highest load carrying capacity but not the largest. The Grumman AF-2S Guardian Torpedo Bomber of the Korean War era had slightly less horsepower with the Pratt Whitney R2800 engine, but it was a larger airplane and for purely piston powered reciprocating aircraft is the largest known single engine aircraft