By the end of 1925, all nine C-1s ordered were built and delivered to the U.S. Army Air Service. Eight aircraft entered operational service and one was retained for use in various test programs. The second C-1 built, S/N 25-426, was selected and modified several times during a relatively short duration test program (less than six months). Because of several significant changes to the aircraft, it was redesignated C-1A.
One of the major modifications involved replacing the standard Liberty engine with a geared version which allowed the engine to operate at lower rpm while maintaining propeller rpm. Additionally, several cowling and engine exhaust systems were tested to determine configurations which yielded the lowest drag and most efficient cooling.
The rudder and vertical stabilizer of the C-1A were also modified. To reduce the pilot force required to yaw the aircraft, a balanced rudder was added. The balanced rudder had a small section extending forward of the hinge line which reduced the force required to hold the rudder to the left or right.
The C-1A was also temporarily fitted with skis for performance testing in winter conditions.
After the test programs were completed, the C-1A was refitted with its original C-1 engine and cowling and entered operational service. The balanced rudder was kept however and the basic design was incorporated into the C-1C -- a slightly larger and improved version of the C-1 built in 1926-27.
First plane using C designation
C-1 with engine change
TECHNICAL NOTES: Engine: Geared Liberty V-1650-5 of 420 hp Maximum speed: 116 mph Cruising speed: 85 mph Range: 385 miles Service ceiling: 14,850 ft. Span: 56 ft. 7 in. Length: 35 ft. 4 in. Height: 14 ft. 0 in. Weight: 6,445 lbs. loaded Crew: Two (pilot and flight mechanic) Cargo/passenger capacity: Six passengers or approx. 2,500 lbs. of cargo Serial Number: 25-426