After training, he was assigned to 66 Squadron to fly a Sopwith Pup.
In 1915, Sopwith produced a personal aircraft for the company's test pilot Harry Hawker, a single-seat, tractor biplane powered by a 50 hp Gnome rotary engine.
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It was flown by the noted aces Edward Grange and Robert A. Little, both of whom scored victories with the aircraft.
Sopwith Camel | Sopwith Pup | Sopwith Aviation Company | Sopwith 1½ Strutter | Pete the Pup | Thomas Sopwith | The Under-Pup | Sopwith Dolphin | A Pup Named Scooby-Doo | Alpha Pup Records | Sopwith Triplane | Sopwith Snipe | Sopwith Snapper | Sopwith 1913 Circuit of Britain floatplane | PUP | Beagle Pup |
He did not open his victory roll until 23 January 1917, after his transfer to 8 Naval, when he drove an Albatros D.III down out of control while piloting a Sopwith Pup.
On 2 February 1917, Kosmahl shot down a Sopwith Pup piloted by acting Flight Lieutenant W.E.Traynor of 8 squadron RNAS over Hermies.
He scored his first aerial victory on 21 July; at 1530 hours, he shot down a Sopwith Pup from No. 66 Squadron Royal Flying Corps over Noordschote.
This also included No. 55 Training Squadron utilising the Avro 504, Camel, Sopwith Pup and the Royal Aircraft Factory S.E.5A.