In 1910, when Igor Sikorsky turned his attention from experimental helicopters to fixed-wing aircraft, the airplanes in use were single-engined. They were unsafe for sustained use. Sikorsky's solution was to build multi-engined airplanes which would continue to fly in the event of an engine failure and would also be big and powerful enough to meet the goal of carrying passengers and freight over long distances.
There were two crucial challenges: to create enough engine power to lift and carry the aircraft's weight plus its payload, and to maintain the pilot's control when an engine failed. The failure of an engine immediately disrupts the aircraft's balance and steering. Sikorsky invented an automatic control device that used tail wing rudders to restore directional control if an engine failed.
The rudders, operable by the pilot's foot pedal, are mounted vertically on the tail of the aircraft. In normal operation, the airstream effect across the rudders is zero and the pilot may use them for steering. On engine failure, the plane veers and the disturbed airstream across the rudders immediately and automatically stabilizes the aircraft.