The Grumman SF was a two man scout aircraft based on the similar Grumman FF-1 two seat fighter, and that was ordered in slightly larger numbers.
Grumman’s first orders from the US Navy were for floats to convert landplanes into amphibians. These included a novel retractable undercarriage, which pulled up into the side of the floats, and the Navy expressed an interest in installing this form of undercarriage on their existing fighters, including the Boeing F4B. Grumman preferred to design their own fighter, and submitted a design on 10 March 1930. A prototype was ordered on 28 March 1931, and this made its maiden flight on 29 December 1931. The Grumman FF-1 was a two seat biplane, with an enclosed cockpit and fully retractable undercarriage, and was faster than the Navy’s single seat fighters of 1931.
On 9 June 1931 the Navy ordered a single prototype of a two seat scout based on the FF-1, as the XSF-1 (several months before the prototype FF-1 made its maiden flight). In order to increase its range and endurance one of the fixed forward firing guns was removed and another 45 gallons of fuel added. It was powered by a 750hp Wright R-1820-78 Cyclone air cooled radial engine with a Townend ring cowling and individual exhaust stubs for each cylinder. Like the FF it had an all-metal semi-monocoque fuselage, while the wings had an aluminium structure with fabric covering. On the tail the horizontal stabilisers could be moved from -1 to +5 degrees. The prototype had external aileron mass balances.
The prototype of the XSF-1 didn’t make its maiden flight until 20 August 1932, eight months after the XFF-1 had taken to the air. Ten days later it was flown to Anacostia for trials, where it remained until it was struck off in 1938. The XSF-1 lived up to expectations, and the Navy ordered thirty-four production aircraft, as the SF-1.
Production aircraft removed the external mass balances (to reduce drag), and replaced the Townend ring with a longer chord NACA cowling and an exhaust collector ring. Early aircraft used the same engine as the prototype, but later production used a 750hp Wright R-1820-84. Production aircraft were five mph faster than the FF-1 fighter variant, with a top speed of 206mph.
The last aircraft in the SF-1 production run was used to test the 650hp fourteen cylinder Pratt & Whitney R-1820-78 radial engine, becoming the XSF-2. A new streamlined engine cowling was used, similar to that of the F2F-1. The aircraft carried two .30in machine guns, one flexibly mounted gun in the rear cockpit and one fixed forward firing gun on the right of the engine cowling.
The prototype made its maiden flight on 26 November 1934, and reached a top speed of 202mph, impressively close to that of the SF-1 with an extra 100hp. The aircraft went to Anacostia on 3 December, where it was modified to have a streamlined fairing behind the rear cockpit and a rear canopy that slided back over the fairing. The XSF-2 was involved in accidents in April 1939 and December 1940 and then withdrawn from service.
Deliveries of the SF-1 began on 15 February 1934 and ended on 12 July 1934 after 33 aircraft had been built. The last aircraft was built as the XSF-2 prototype.
One squadron, Scouting Three (VS-3B) on the Lexington (CV-2) converted to use the SF-1. Another five fighter squadrons (VF-1B, VF-2B, VF-3B, VF-5B and VF-6B) and Bombing Two and Bombing Five each received a single aircraft, which was used as a squadron liaison aircraft and as a navigation and communications aircraft to lead squadron formations on longer flights.
The SF-1 only remained in front line service for just over a year. They were then withdraw and sent to the Navy and Marine Air Reserve, with aircraft based at Brooklyn (4), Anacostia (4), Long Beach (6), Oakland (7) and Seattle (5). By December 1941 only eleven aircraft remained. After they were withdrawn from flying duties they were used as ground instructional airframes.
Engine: Wright R-1820-78 or -84
Crew: 2 (pilot and observer/ gunner)
Span: 34ft 6in
Length: 24ft 6in
Height: 11ft 1in
Max speed: 206mph
Armament: One fixed forward firing 0.3in guns and one flexibly mounted 0.3in gun