A manned glider, the D.1, with provision for fitting engines and propellers, was constructed under great secrecy and, in July 1907, was taken to Blair Atholl in the Scottish Highlands for flight testing. On its one successful flight, Capper flew it for just long enough to demonstrate its stability before crashing into a wall. It was repaired and fitted with its powered chassis, but was damaged on its first and only attempted flight when the takeoff trolley veered off course.