Considering that Eurocopter aims to enter production with a high-speed compound helicopter based on the X3 by the end of the decade, the clock is ticking on figuring it all out.
X2: The Need for Speed
Sikorsky’s approach to the rotorcraft max speed conundrum is, by almost any measure, complicated; yet it’s also arguably a more elegant solution and, the company hopes, a more effective one too. As we’ve already discussed, VTOL rotorcraft flight requires a compromise between hover performance and forward speed. The efficiency versus speed equation especially penalizes low-disk-loading aircraft, such as conventional helicopters, which hover efficiently and have good low-speed controllability and relatively low downwash. With the X2, Sikorsky says it is focused on increasing cruise speed without compromising the attributes that make helicopters so valuable.
At the heart of the X2 design is an integrated fly-by-wire system that allows the engine, rotor and “propulsor” (the rear-facing propeller mounted at the tail) to operate efficiently together, with full control of rotor rpm throughout the flight envelope. The key to making the concept work is the X2’s twin-coaxial, high-lift-to-drag rigid main rotor blades, which spin opposite of each other and thereby ensure that an advancing or retreating blade never has the upper hand in the lift equation and that the whole system is about as efficient as it can be.
Sikorsky announced the development of the X2 technology demonstrator in 2005 and flew the prototype for the first time in August 2008. The 23rd and final flight was conducted in July 2011 after Sikorsky explored the X2’s full envelope from hover to a maximum cruise speed of 253 knots in level flight — an unofficial record for a conventional helicopter. Having achieved its goal of flying the X2 demonstrator helicopter at more than twice the average top speed of a conventional helicopter, Sikorsky will next design, build and fly two more X2 technology demonstrators, one of which it will hand over to the U.S. military for flight test and evaluation. Both will become the baseline prototypes of an all-new light tactical helicopter called the Sikorsky S-97 Raider.
While military customers are clearly Sikorsky’s aim with the X2 and S-97 programs, the company says the experience gained in their development will lay the groundwork for a commercial variant of the design, which could hit the market in five to seven years after the military version enters production. The big unanswered question is how much more a helicopter based on X2 technologies would cost than, say, a stock S-76. Sikorsky isn’t saying, probably because it doesn’t yet know. What’s clear is the leap won’t come cheap. But such a product will make up for the difference in price, it is hoped, by providing better high and hot hover capability, improved maneuverability and agility, less noise and, of course, much faster cruise speeds.
A final point about Sikorsky’s entry in the high-speed-helo sweepstakes: The X2 demonstrator is a small, two-person machine, and that has led some observers to question whether helicopters based on the design will be limited in size. Sikorsky is adamant that the technology will be scalable through a “full range” of weight- and passenger-carrying capabilities. Insiders say Sikorsky has drawn up plans for several X2 technology aircraft with passenger capabilities ranging from one to (if you can believe it) 100 passengers.