Estimated reading time 10 minutes, 36 seconds.
During a 20-minute test flight of the prototype S-97 Raider at Sikorsky’s West Palm Beach, Florida, facility — conducted in front of four journalists in the first such public demonstration — the unconventional coaxial-rotor aircraft performed routines that would make any helicopter pilot jealous.
For instance, the single-engine Raider, which Sikorsky is using as a testbed as it develops its entry for the U.S. Army’s Future Attack Reconnaissance Aircraft (FARA) competition, used reverse pitch on its rear propeller to maintain a nose-down attitude in a steady hover — as if targeting weapons or sensors to the ground, searching for injured hikers, or inspecting a landing zone. It’s a move no other helicopter can execute without drifting. Then it maneuvered briskly and perfectly above the runway, performing multiple tight patterns with an agility made possible by its rigid, stacked rotors, which rotate in opposition to each other, counteracting torque and negating the need for a tail rotor. Most other helicopters, with their hinged, flexing blades, can’t come close to that sort of precision.
Test pilot Christiaan Corry described the S-97 Raider’s performance characteristics in a way that seemed to suggest that he and his program colleague, Bill Fell, who commanded the flight, might actually have the best job in the world. “It really handles like a sportscar,” Corry said. “We don’t have an inverted oil or fuel system in this aircraft, but aerodynamically, it could fly inverted all day long. We demonstrated that in the simulator, and are proving every day that Raider can do so much in the air. The rotor system acts like a wing, and we can do these aerobatic maneuvers that just aren’t things helicopters could ever do before.”
The test seemed to validate that point. As the crew cycled through its test card on that characteristically sweltering Florida morning, we also saw Raider perform turns in half the distance of other helicopters and accelerate and brake with the fuselage completely level, thanks to the rear propeller that sits in place of the tail rotor. Whereas conventional helicopters must tilt their main rotor discs forward in order to accelerate — and backward to slow down — in the Raider, the rear propeller can be used to push the helicopter to faster speeds and also, with the blade’s pitch reversed, practically stop it on a dime. Engaging reverse pitch while simultaneously tilting the main rotors forward enables the nose-down hover. The Raider can also hover with its nose pointed skyward, by tilting the main rotors aft while generating forward thrust with the prop.
Of course, this enhanced maneuverability is actually the lesser of the payoffs from the coaxial main rotors and rear prop. The main one is significantly faster forward flight. Developed from Sikorsky’s Advancing Blade Concept, the compound design’s dual rotors neutralize the stall tendency exhibited by retreating rotor blades in conventional helicopters as speed increases. Because that instability effectively limits top speed, a helicopter with this design can fly far faster than any other. Raider’s predecessor, the X2 experimental helicopter that flew between 2008 and 2011, reached 250 knots, and Raider, which first flew in 2015, has hit 207 knots so far. (Raider’s successor is the larger and faster SB-1 Defiant being developed in partnership with Boeing through the Army’s Joint Multi-Role Technology Demonstrator program. It first flew in March.)
During the demonstration in front of the media, Fell and Corry streaked past us at 190 knots on multiple passes, with Raider behaving more like an airplane than a rotorcraft — and for that matter, sounding more like one, too. (In fact, it had a distinctly warbird-like vibe. Think Spitfire, not Black Hawk.) Although this was short of the planned 200 knots due to issues with the vibration control software, it was still staggeringly fast for a rotorcraft.
Raider’s bag of tricks goes even deeper. It can dive onto a target — also airplane-style — while using reverse pitch on the rear propeller to slow and extend the dive. The pilots can also disengage the prop and slow down the main rotors, generating a “quiet mode” when a dash of stealth is called for. Sikorsky demonstrated both of these, as well. In sum, all of Raider’s capabilities add up to a product that Chris Van Buiten, vice president of Sikorsky Innovations, feels confident meets or exceeds the Army’s requirements. “It’s well beyond the capabilities of the current fleet,” Van Buiten said. “The threat environment going in and out of combat will become more intense, so you need to be fast, maneuverable, and agile, and you need low-altitude flight so you can use the clutter to mask your signal-to-noise ratio.”