Sikorsky’s S-97 Raider light-attack prototype has exceeded 200 kt in level forward flight and is expected to continue ramping up to at least 220 kt, a speed unachievable in a traditional helicopter.
Experimental Test Pilot Bill Fell said the Raider, which features dual coaxial rigid main rotors for lift and an aft propulsor, is flying regularly at the Sikorsky Development Flight Center in Palm Beach, Florida.
“We’re getting into our stride and increasing the speed envelope with nearly every flight,” he told a few aviation reporters during an Oct. 4 telephone interview. “We’re continuing to expand the envelope, so we’re going to go faster than we’ve already gone.”
Raider is based on Sikorsky’s X2 technology that set speed records for a rotorcraft, but the S-97’s top speed is still technically unknown. It was originally designed to fly 230 kt in level forward flight “clean,” meaning without any external weapons or payload. With weapons, it was supposed to fly 220 kt, Fell said.
“We’re on a path that we intend to go over 220 knots in level flight, but where exactly we get to will depend a lot on what we learn over the next few months,” he said. “I think it’s something above 220 knots, but where above that it’s hard to predict at this point.”
Raider’s maneuver envelope is also steadily widening as pilots put more load factor on the aircraft, increase to steeper bank angles of bank and test other performance characteristics, he said.
As flight testing progresses, engineers are making modifications to the physical structure of the aircraft to reduce drag and vibration as well as tweaking the complex flight control software that enables its fly-by-wire controls.
“We’re still actively testing and modifying the aircraft,” Fell said. “As we have discovery and learn new information, we’re making tweaks to the machine. … We’re making some drag improvements to the machine to make it a little bit slicker. We’re also actively making changes to the … vibration-control system and we always evaluate the flight controls on every flight.”
Candidates for drag reduction include the fairing or “sail” between the rotors, the fairings on the rotors and “regular aircraft stuff” like gaps between the doors, antennas and other sources of drag. The active vibration control software that dampens vibrations from the airframe and engines is also being constantly tweaked to reduce wear on the aircraft and better stabilize the cockpit, Fell said.
“The Sikorsky S-97 Raider flight test program is exceeding expectations, demonstrating Raider’s revolutionary speed, maneuverability and agility,” said Tim Malia, Sikorsky's director of Future Vertical Lift Light. “X2 technology represents a suite of technologies needed for the future fight, enabling the warfighter to engage in high-intensity conflict anytime, anywhere as a member of a complex, multi-domain team.”
Raider incorporates Sikorsky’s advanced fly-by-wire flight controls, vehicle management systems and systems integration that were developed during the X2 program, the previous Comanche armed-scout helicopter development program and the U.S. Marine Corps’ CH-53K King Stallion heavy-lift helicopter.
“That has performed as well as we can expect,” he said.
As impressive as the Raider’s speed is, its ability to decelerate rapidly without stalling or pitching nose-up is equally important to U.S. Army pilots who might one day fly the aircraft in combat. The aft prop can be used to decelerate the aircraft in level flight, much as a commercial airliner does with reverse thrust when landing on a runway.
“We do that in flight,” Fell said. “You’re not worried about stalling the aircraft. You’ve got the two counter-rotating rotors. You also have this incredible sense of deceleration where you’re not having to bring the nose 30-degrees nose-up where you can’t see anything out front.”
Sikorsky, now owned by Lockheed Martin, has formally announced Raider as a contender for the U.S. Army’s Future Attack Reconnaissance Aircraft, or FARA.
A former OH-58D Kiowa Warrior pilot, Fell said the Raider’s maneuverability will greatly increase the effectiveness, lethality and survivability of armed scout missions. Using the propulsor as an air break, the aircraft can loiter nose-low and train weapons on a target without rapidly accelerating toward the target, as in a traditional helicopter.
“As you come in on a gun run, you can do some pretty impressive pitch-up and pitch-up type stuff. … You get that real nose-low attitude that gives you a nice, tight rounds impact area … the more nose-low you are,” he said. “We put negative impact on the target while we do that and it’s essentially a parachute. We’re not hovering at 30 to 40 degrees nose low, but we’re also not accelerating rapidly at the target. At that point, when you’re done, you’re able to reverse that pitch, move it from negative to positive and quickly egress the target area and get set up to do it again.”