Three prototypes proved the S-4B model and this carried the Gnome rotary engine of 110 horsepower. Some 100 of this mark were ordered during the summer of 1917 with the U.S. Army taking a stock of 97 of the aircraft while the U.S. Navy added another six in floatplane configuration as the "S-5". The S-4B became the definitive advanced flight trainer for the U.S. air service for a good part of the war until succeeded by the S-4C mark. These were given Gnome B-9 series engines of 80 horsepower and were represented through six prototypes followed by some 461 serial aircraft, four also added (again, in floatplane arrangement) to the USN stable. After the 52nd example came off the line, the C-mark was updated with the Le Rhone 9C engine of 80 horsepower.
With the 9C engine unit, the aircraft could make top speeds nearing 100 miles per hour and fly up to a ceiling of 15,000 feet. Endurance was a 2.5 hour window giving good range for a trainer type. A single air-cooled 0.30 caliber Marlin machine gun could be installed for gunnery training.
In service, the aircraft became known as "Tommy".
A post-war attempt to sell the military on a new model, the S-4E, with revised tail unit and Le Rhone 9J 110 horsepower powerplant failed, leading to this aircraft being re-engined with an Aeromarine unit of 135 horsepower and flown by air racer Basil Rowe as the "Space-Eater"
The S-4 proved a quite the success for both the American Army and Navy during the wartime period as its numbers and popularity would eventually show. Their availability in number naturally led to many being acquired by way of surplus at the civilian market level in the post-war years where they continued on as trainers, racers, aerobatic performers, and flyable Hollywood props - such was their versatility. Many of these post-war models adopted the Curtiss OX-5 engine for drive power instead of the original military fits.
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