SERA: Making the Grumman S-2 Tracker Flyable

Single Engine Rudder Assist was an automatic system that made the short-coupled Tracker flyable in a single-engine situation.

The need to fit aboard the smaller elevators of the Essex-class carriers meant that the Grumman S2F (later redesignated S-2) Tracker had a stubby fuselage and a short moment arm which required a relatively large vertical fin to maintain control and directional stability. However, the Tracker was so short-coupled that in a single engine situation, the fin and rudder despite their size were still insufficient and an adverse yaw into a spin would result. At lower airspeeds even with both engines operating the Tracker was a handful in the carrier flight pattern.

To compensate, Grumman designed what was called the SERA- Single Engine Rudder Assist. If you look closely at the fin and rudder of the Tracker, the rudder appears to be two vertical sections and a prominent trim tab.

During normal flight, only the rearmost section of the rudder and its trim tab acted as a conventional rudder and the section of rudder closest to the fin acted as a rudder trimmer driven by an electric screwjack.

During an engine out situation as well as during routine takeoff and landing, the SERA system was activated and the section that in cruise flight acted as a rudder trimmer switched over to hydraulic activation and acted with the rearmost section of the rudder to act together, effectively doubling the area of the rudder during those critical areas of flight. It could be activated by the pilots at slower speeds but a differential slipstream sensor could activate it automatically in an engine out situation.

With the SERA activated, the full two-section rudder traveled an impressive 40 degrees on each side, allowing the Tracker to have a minimum control speed of 85 knots with one engine out.

Images: US Navy