Test pilot Bill Fell was zooming southward, approaching Sikorsky’s runway outside West Palm Beach, Florida, at about 190 knots when he decided that was as fast as the S-97 Raider should fly for spectators below.
The aircraft was vibrating in a way that felt different, owing to a new piece of vibration-control software loaded onto the aircraft in an ongoing search to push the compound helicopter toward 220 knots—a speed that would likely put the Raider, or a Raider derivative, within the realm of what the Army will seek for its Future Attack Reconnaissance Aircraft (FARA).
“We’re constantly testing this machine and we put a new script in our vibration control system and it didn’t pan out to where we wanted to push it to 200 knots today,” Fell later explained.
“It was a bit of a swing and miss, in that sense,” he added.
In their bright orange flight suits, Fell and co-pilot Christian Corry had already pushed the compound helicopter to perform feats that would be impossible in a traditional helicopter.
With Fell and Corry at the helm, the Raider danced pirouettes down the runway, hung still in the air with nose-down attitude and braked to a hover from level forward flight without flaring the nose. Demonstrating 190 knots in level cruise flight already significantly outpaced conventional helicopters.
The clutched pusher prop allows the aircraft to achieve such high speeds. In cruise mode, the dual main rotor discs slow down to about 85 percent of maximum rpm and act as a wing to provide lift.
“That not only aids us in high speed flight in order to reduce main road or drag, but in low speed flight, it in many respects, your acoustic signature is a lot about the tip,” Fell said. “If you’re able to decrease that tip mach [speed] … you’re able to reduce your acoustics.”
When disengaged and Raider is flying like a helicopter — it does not require a tail rotor because of the counter-rotating main rotors with differential collective — the aircraft is remarkably quiet in a hover. Company officials call that “whisper mode.”
Sikorsky is now testing the S-97 Raider almost weekly. The aircraft has amassed 55 flight hours and reached 207 knots in level cruise. In 2019, the Raider has flown about 35 hours, compared with 20 hours in the first three years of the program.
Fell has performed 60-degree banked turns and flown up to 8,000 feet. Both pilots are familiar enough with its handling characteristics that they know when to push the envelope and when to hold back, said Corry.
“We could tell that something just didn’t feel the same as it did the last time we were up around 190 knots,” he told Rotor & Wing International after the June 25 flight.
Raider is now flying at 180 knots — well past the sprint speed of a conventional helicopter — on almost every outing, Fell said. The goal was to fly at 220 knots, but the aircraft might not make that benchmark in its current configuration, Fell said.
“We’re not done by any means,” Fell said. “We’re still turning dials.”
Those “dials” are everything from the fly-by-wire controls software to continuing drag reduction of various airframe parts, Fell said. The software will be tweaked and retested as the team continues its search for speed.
Software defines the Raider as much as its pusher prop or rigid main rotors, but all its systems are digitally linked. The engine control system talks directly to the full-authority fly-by-wire controls, which operate the pitch angle of the blades and thrust from the tail rotor to achieve its groundbreaking performance capabilities. Calibrating those systems incorrectly could mean missing speed targets, as happened during the June 25 flight, or worse.
In 2017, the first prototype — the aircraft on display recently was the second one built — was destroyed because of a software error. A glitch in the code caused the flight control system to miscue during the transition from takeoff to forward flight, and the aircraft began to wobble as the computer lost control of the separation between the counter-spinning rotors. The aircraft responded to pilot inputs more powerfully than it should have, causing the aircraft to slam into the ground. The force of the impact flexed the rotors enough that they made contact and shattered, Fell explained.
“We completely understand the physics” of what happened and the code has not only been fixed but the patch has caused the rotor discs to actually flare further apart during flight, which increases drag, Fell said. The aggressive fix likely can be dialed back to gain back the lost speed, he said.
Chris Van Buiten, vice president of Sikorsky Innovations, underscored that the accident — which the company calls a hard landing – was purely a software problem that required no redesign of the rotor system. Any helicopter running that control system code would have crashed, he and Fell agreed. Fortunately, both pilots were unharmed and Sikorsky had another Raider on hand. About a year after the accident, Raider #2 resumed flight testing.
All of Raider’s critical flight control systems are triple redundant, including the airspeed indicator system, flight control computers and stability sensors.
“That’s what makes it so easy for Christian and I to fly the machine,” Fell said. “It’s got great command attitude hold. And so, when we’re sitting there in a hover, we’re not doing anything. It’s just doing it itself. It’s not a hover-hold system, it’s just we haven’t told it to do anything.”
The engineering team is constantly looking to further reduce drag and tweak the flight controls to squeeze out a few extra knots of airspeed here and there. Where Raider’s X2 predecessor proved the physics of reaching 200+ knots with a rigid-rotor compound helicopter configuration, Raider has taken another step to proving the configuration’s advantages as an attack helicopter.
A ground war threat scenario with Russia or China has the U.S. Army facing guided missiles that make easy targets of conventional helicopters. Avoiding future anti-aircraft threats will require speedy, low-level “nap-of-the-earth” flying over longer ranges. Raider is being used to design Sikorsky’s pitch for FARA that should enable that sort of performance.
In virtual anti-aircraft simulations where human subjects are trying to shoot down incoming helicopters, the S-97 has shown a 75 percent reduction in numbers of hits compare to conventional designs, owing to its speed and low noise signature, Van Buiten said.
Now, the flight test program has transitioned to the company’s FARA team, which is designing the next evolution of the X2 technology by using it as a real-world test bed for risk reduction as the formal Army program approaches.
Sikorsky formally kicked off its FARA program in January and is in the process of “optimizing the design” of the aircraft it eventually will pitch the Army. The company has not made its FARA design public, but Raider is “very representative” of what will be offered, Van Buiten said.
“We’ve essentially transitioned the asset to the FARA team to execute risk reduction for FARA,” Van Buiten said prior to the June 25 demo flight. “What are the most critical things? Risk items we can burn down. What’s the most valuable data we can give them? And um, you know, we’re not going to get into the details of our far offer, but this vehicle is very relevant in terms of its sizes, towers and attributes and 207 knots is the objective.”
Sikorsky is one of five bidders for FARA — the others are Bell, an AVX-L3 team, Boeing and Karem Aircraft — selected by the Army for competitive prototyping contracts. The service is scheduled to choose two aircraft to for a competitive prototyping effort that should result in an aircraft to fill the operational gap left by retirement of the OH-58D Kiowa Warrior. FARA will also replace a portion of the Army’s AH-64 Apache fleet.
Like Sikorsky, most of the competitors have played their FARA designs very close to the vest, going public only with general characteristics and technologies that likely will be incorporated. But unlike the rest, Sikorsky has been flying Raider for more than two years. Despite the hard landing that grounded the program for nearly a year, the company is leaps ahead of the competition in gathering data on a speedy compound helicopter in the 14,000-pound weight class.
Sikorsky’s FARA Director Tim Malia said teams based at various facilities since January have had almost direct access to the S-97 and its test team as they collaboratively develop the company’s FARA design.
If FARA engineers need data on, say, Raider’s performance characteristics during a 3-G turn, the team can have the maneuver written into a flight card and receive data in a matter of days, Malia said.
“Normally, when you start with a clean-sheet aircraft, you have to guess at all that stuff,” he said.