In late July, two sleek rotorcraft raced together over the cypress wetlands of southern Florida, the pair exceeding 180 kt. as the Sikorsky S-97 Raider and Sikorsky-Boeing SB-1 Defiant high-speed helicopters flew together for the first time.
- Sikorsky S-97 Raider flights reducing risk for FARA bid
- Sikorsky Boeing SB-1 Defiant is poised for a run at high-speed goal
The flight over Sikorsky’s West Palm Beach development flight center was staged for U.S. Army acquisition chief Bruce Jette. The Raider and Defiant are competing for two of the Army’s top modernization priorities: the Future Attack Reconnaissance Aircraft (FARA) and Future Long-Range Assault Aircraft (FLRAA), respectively.
For Sikorsky, now a Lockheed Martin company, the formation flight held added significance because it came almost 10 years after its company-funded X2 technology demonstrator had set an unofficial speed record for helicopters of 250 kt. in level flight. Both the Raider and Defiant use the X2 coaxial rigid-rotor compound helicopter configuration.
Together, the X2, Raider and Defiant demonstrators represent a $1 billion investment by Sikorsky and its industry partners, now targeted squarely at winning the FARA and FLRAA.
In 2005, flush with cash from producing H-60-series helicopters for the U.S. military and export customers, Sikorsky launched the X2 program. Looking to guarantee its future with a new generation of helicopters, the company studied a wide range of designs, including tiltrotors, before deciding to revisit the coaxial rigid-rotor compound configuration it pioneered with the XH-59A Advancing Blade Concept demonstrator.
First flown in 1973, the XH-59A was fast, reaching a maximum level speed of 238 kt. But it was complex, with high fuel consumption, noise and vibration. Operating the four engines—two turboshafts for the rotors and two turbojets for propulsion—required a two-person crew.
Taking advantage of advances in technology during the intervening decades, Sikorsky simplified the concept to produce the X2: a single-seat, single-engine rotorcraft with fly-by-wire flight control, composite blades and airframe as well as active vibration control. The X2 demonstrator flew in 2008.
In 2010, after the X2 reached 250 kt., Sikorsky launched an industry-funded program to build two S-97 Raider light tactical helicopter prototypes using the configuration. Aircraft 1 flew in May 2015 and logged about 20 hr. before suffering damage during a hard landing caused by a flight-control software flaw. Aircraft 2 has now logged almost 69 hr., reaching a maximum speed of 207 kt. and angle of bank of 60 deg.
Flight testing of the S-97 is now dedicated to optimizing Sikorsky’s Raider X design for FARA, focused on tweaks to minimize drag at high speed. “Those flight hours mean someone had a question,” says Jay Macklin, Sikorsky director of Future Vertical Lift (FVL) business development. And while the S-97 is flying, Sikorsky is making progress with the Raider X prototype. “The build is on,” he says.
The Defiant, meanwhile, has logged 20 hr. of flying since taking to the air for the first time in March 2019 and has exceeded 200 kt. and a 30-deg. angle of bank. Whereas the single-engine Raider was designed for 220 kt., the twin-turboshaft Defiant is designed for 230 kt. but with a speed goal “closer to 250 kt.,” says Bill Fell, Sikorsky’s chief test pilot. A flight to achieve that speed goal is imminent, the team indicates.
Sikorsky and FLRAA teammate Boeing emphasize that both the Raider and Defiant combine high speed and maneuverability with the low-speed agility of a conventional helicopter. There are differences, however. The Raider and Defiant have no tail rotor; instead, differential torque on the coaxial rotors is used to turn at low speed. The yaw rates generated are the same and can be tailored to the mission requirement, test pilot Christian Corry says.
Both machines can fly like a helicopter at speeds of up to 150-160 kt. using conventional collective and cyclic control, Fell says. But when the tail-mounted propeller is engaged to provide propulsion, the coaxial rotors become rotating wings, Corry says, and collective pitch is automated to maximize lift and minimize drag at high speed.
Airspeed is controlled through the prop pitch, using the throttle, and flight control relies on rudders and elevators on the tail. “It’s more of an airplane than a helicopter,” Corry says. Unlike a conventional helicopter, whose nose must be pointed down or pulled up, the propulsor enables level-attitude acceleration and deceleration.
Reversing prop pitch “is like throwing a parachute out there,” Fell says. “It acts like a big brake.” This procedure is used routinely to maximize test time. Fell describes approaching the airfield in the Defiant at 180 kt., then reversing the prop and slowing rapidly while the nose stayed pointed down at the landing zone. “I was able to see everything the entire time during the approach, which you cannot do in a helicopter,” he says.
On the formation demo, the two rotorcraft stopped in about the same distance, despite the Defiant’s larger size, notes Randy Rotte, Boeing director of global sales and marketing for FVL, cargo and utility. The Defiant so far is cleared to use only half the negative prop-pitch range, Fell adds. Once the full range is cleared, “we will see much more rapid deceleration,” he says.
And whereas a tail rotor is required for control throughout the flight envelope of a conventional helicopter, on the Defiant—as on the Raider—the propulsor can be disengaged, reducing the acoustic signature and improving survivability.
High speed, not hover, drives the power requirement in both aircraft. Powered by two 4,000-shp-class Honeywell T55s, the Defiant is “loafing” at 180 kt. on less than 50% power, Fell says. “At the weights we are flying, we have hover power on one engine. That means contingency and high-hot capability,” he adds.
Even the single-engine Raider has “excess power you don’t see in a conventional helicopter,” Corry says. The competing FARA prototypes will both be powered by the 3,000-shp-class General Electric T901, a new engine that is expected to increase in power output over time. “Counter-rotating rigid versus fully articulated [rotor] provides growth potential,” Macklin says. “We can add power to the engine, and the design can take it.”
The coaxial rigid rotors also provide high control responsiveness. The 30,000-lb. Defiant “flies like a 20,000-lb. machine,” Fell says. “The crisp response from the rotors shrinks the machine.” The 11,000-lb. Raider “is more compact. It has that small, agile scout feel.” But both rotorcraft “fly the same” despite the difference in size.
“It is the entire integrated weapon system that creates survivability, but it starts with the speed, maneuverability and agility of the aircraft,” Macklin says. “It’s the packaging that provides the transformational capability,” Rotte notes. “It fundamentally changes the way you can fight—range, speed and maneuverability translate into survivability in a contested environment.”
Sikorsky on FARA and Sikorsky Boeing on FLRAA are locked in competitions with Bell that will play out over the next three years. But for Sikorsky, more than a decade after betting its future on the X2 configuration, flying the Raider and Defiant together was a milestone—and a glimpse of how the Army could use them together in multidomain operations. “Seeing them in formation seemed like a natural evolution,” Macklin says.
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