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The S-69 (XH-59A) was a technology demonstration platform created by the pioneering Sikorsky aircraft corporation. Development of the S-69 was announced in 1972. The S-69 owes its existence to something known as the Advancing Blade Concept (ABC). This new helicopter would demonstrate and test the concept, allowing scientists and engineers to evaluate its potential. As versatile as helicopters are, the physics of rotary wing aircraft put limitations on maximum airspeed and stability. In a traditional helicopter a single main rotor provides both lift and thrust, while a tail rotor counteracts the torque generated by that main rotor. During forward flight the advancing blade produces more lift than the retreating one because of differences in relative airspeed. This introduces roll and instability. It also introduces an issue known as retreating blade stall, where at a certain speed the retreating blade’s angle of attack exceeds the critical angle, causing a dangerous flight condition. This is the main reason even the fastest single-rotor helicopters cannot exceed 230 Mph (370 Kph). There have been many compensating design features in traditional helicopters to get around this problem, such as flexible “flapping” rotor blades, but it seems that we have reached the limit of this design. The ABC design tries to get around this issue by using two rigid coaxial rotors (three-bladed in this case) that counter-rotate. The main rotor therefore generates stable lift across the rotor disc, closer to that of a fixed wing aircraft, but can still hover and perform other maneuvers that are the domain of helicopters. Since there is no need for an anti-torque tail rotor helicopters such as the S-69 use tail-mounted propulsion systems. In the S-69’s case this is a ducted fan. The first prototype was damaged in an accident, prompting further design changes. Several problems with the S-69 design were apparent. Excessive vibration was a major issue, as well as inadequate flight control technology. The first S-69 prototype was powered by a single 1825 shaft horsepower PT6T-3 Turbo Twin Pac engine. The second added two Pratt & Whitney J60-P-3A turbojet engines. In this configuration the S-69 reached speeds of 303 Mph (488 Kph), clearly outperforming what is possible with a traditional helicopter. Only two S-69s were ever built however, if the S-69/XH-59 did not lose out on the U.S. Army’s light attack helicopter contract to the ill-fated RAH-66 Commanche, things might have been very different. The ABC design did not however die with the S-69, which was followed by the X-2 and S-97 Raider, both from Sikorsky.
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