United Technologies subsidiary Sikorsky Aircraft announced at the American Helicopter Society annual forum earlier this month that it plans to build an experimental helicopter using a coaxial main rotor system that it says will achieve cruise speeds well above that of conventional helicopters. Coaxial helicopters have two counterrotating rotors on the same vertical axis.
Sikorsky plans to build its X2 technology demonstrator helicopter at its Schweizer Aircraft subsidiary in Elmira, New York. Here in Paris on Monday, Jeff Pino, Sikorsky senior vice president of marketing, promised the X2 demonstrator would fly by the end of next year. He also said preliminary design work for the tandem-seat demonstrator has been completed and parts fabrication has already begun. Before the end of this year, Pino said, Schweizer will fly a 333 fitted with the same Honeywell fly-by-wire system that will be used in the demonstrator.
The fastest helicopters today cruise comfortably at about 160 knots. Pino said the X2, powered by a single LHTEC T-800 turboshaft engine (the cancelled RAH-66 Comanche’s powerplant), will cruise at 250 knots. By comparison, the Bell/Agusta BA609 civil tiltrotor has an expected maximum cruise speed of 275 knots, although it has not yet flown in airplane mode. Bell Boeing’s V-22 military tiltrotor has demonstrated its maximum cruise speed of 275 knots, but as tiltrotors, both the V-22 and 609 are not conventional helicopters.
In developing its proposed line of X2 technology helicopters, Sikorsky is building on several decades of company research, specifically mentioning the XH-59A Advancing Blade Concept demonstrator, which showed high speed was possible with a coaxial helicopter and auxiliary propulsion; the Cypher UAV, which expanded company knowledge of the flight control laws in fly-by-wire aircraft with coaxial rotors; and the Comanche, which developed company expertise in composite rotors and advanced transmission design.
A computer rendering of the X2 demonstrator shows a pusher prop on the tail, which Sikorsky describes as part of an integrated auxiliary propulsion system. Interestingly, a proposed follow-on to the XH-59A was to have a pusher prop as well, but the Advancing Blade Concept program lost funding before it was built. The XH-59A, boosted by two 3,300-pound-thrust turbojet engines, attained 240 knots in cruise and 260 knots in a dive.
Sikorsky, which is funding the project, is using the X2 moniker to describe a class of coaxial helicopters and to imply the multiplying effects (X2 meaning “times two”) of applying several advanced technologies to the design. These technologies include improvements to the rotor blades, flight control laws, transmission and active vibration control. Pino said the design is scalable and can be applied to a wide range of requirements for UAVs to heavy-lift helicopters. He added that the demonstrator itself is not planned to become a product.