Even at seven in the morning, the south Florida heat and humidity is stifling. It steams up eyeglasses and feels like an iron on your skin. An alligator lurks in a stagnant pond water between the taxiway and the runway. Perhaps a spy sent by Bell, Sikorsky’s chief competitor for the contract to build the Department of Defense’s next generation of vertical-lift aircraft.
But any resentment about Sikorsky’s decision to test its radical new S-97 Raider prototype at its facility in West Palm Beach vanishes as the helicopter’s 2,600 horsepower turbine engine spooled up on the runway and the dual, counterrotating rotors above the fuselage start spinning, out-blowing any ocean breeze.
Sikorsky touts the Raider, with its so-called compound design, including stacked (or “coaxial”) rotors and a rear-facing propeller in place of the familiar side-facing tail rotor, as fast, quiet, and highly maneuverable. The Lockheed Martin-owned defense firm is pitching it as a replacement for today’s light attack and scout helicopters, via the Army’s Future Attack Reconnaissance Aircraft program. The Raider’s big brother, the SB-1 Defiant, uses the same general setup and is in the running to replace the Army’s esteemed twin-engine Black Hawk helicopter, through the Future Vertical Lift program.
In either application, the design has a lot to prove: that it won’t introduce new challenges for pilots, that it will be affordable, reliable, and easy to maintain, and, most importantly, that it will be better in combat than any predecessor or competitor. It also needs to extend the military’s reach and capability against constantly evolving opponents.
One of the driving forces behind the Army’s hunt for fresh whirly birds is that opponents are working harder to attack military bases through the use of small drones, improvised explosives, and other strategies, so US forces are now being stationed farther away from the action. That means the aircraft need to cover more ground, faster. “To survive going in and out, you also need to be able to fly very low to hide amid the clutter—close to the ground in obstacle-rich, high-threat environments,” says Chris Van Buiten, head of Sikorsky Innovations. Today’s flight test is meant to collect data on a variety of key maneuvers, while demonstrating them to the media and some industry partners and suppliers. (Military representatives have observed previous test flights.)
During the roughly 20-minute test, the Raider shifts sideways and pirouettes around a fixed point on the ground, showing the low-speed agility enabled by the fully electric fly-by-wire controls. It holds a nose-down hover—something conventional helicopters can’t do without drifting—which improves visibility to ground objects and is made possible by the rear propeller. From the observation deck on a hangar rooftop, the aircraft seems poised and precise as the two test pilots worked through their day’s test card.
The helicopter also shows off its quiet mode, deactivating the rear propeller and flying as a conventional helo. (Sikorsky hasn’t shared a decibel count, but it’s noticeably quieter than a similarly sized conventional chopper.) Pilots Bill Fell and Christiaan Corry demonstrate fast braking, pitching the blades on that same rear prop in the opposite direction from forward flight. And then they show the aircraft’s key virtue, high-speed forward flight. They make multiple passes across the field, reaching 218 mph while sounding as much like a vintage World War II Spitfire as a 21st century future flyer. On previous flights, they’ve reached 238 mph, and the Raider can likely go even faster. By comparison, the Bell-manufactured OH-58 Kiowa, the Army’s current light scout and attack aircraft, tops out at 138 mph.
Both the dual rotor configuration and the rear pusher-prop contribute to the Raider’s fleetness of flight. The main rotors are derived from Sikorsky’s Advancing Blade Concept, a system that started development since the early 1970s but until now has been stymied by costs and technological challenges. The strategy uses two rotors spinning in opposite directions, which neutralizes a conventional helicopter’s tendency to lose lift as speeds increase. Briefly: A helicopter moving forward has blades that are always spinning in a horizontal circle and thus either advancing or retreating. As the helicopter goes faster, the retreating blades lose lift, or stall, relative to the advancing blades, since air is passing over the blade going forward at a slightly faster rate.
Having two rotor disks spinning in opposite directions balances that effect, and allows for much higher top speeds. Because the counter-rotating rotors also negate the torque typically generated on the fuselage by a single rotor, there’s no need for a conventional rear tail rotor. So in the Raider, the engineers replaced the side-facing tail rotor with a rear-facing propeller. That receives most of the power during forward flight, with the two main rotors whirling just fast enough to maintain lift, much like a wing. Conversely, the rear prop is nearly unpowered in low-speed maneuvering while the rotors are given all the power they need. (They’re able to control the helicopter completely, even if the rear propeller is damaged or destroyed.) The company honed this system in its experimental X2 helicopter from 2008 to 2011. Raider started flying in 2015, followed by the even faster and more powerful Defiant early this year.
To ensure the two rotor blades don’t hit each other, Sikorsky made them especially rigid, so they don’t bounce and flex the way conventional helicopter blades do. (There have been other coaxial helicopter designs, mostly from Russian manufacturers, but they use conventional flexible blades, as well as the more complex hubs to which the blades attach.) The rotors are also spaced a relatively wide three feet apart. Nevertheless, there was a flight test accident in August of 2017 in which the blades hit each other at low altitude, resulting in a hard landing with no injuries to the crew. The problem was traced to a software glitch that allowed the helicopter to roll much harder than it or any other helicopter should have, according to Van Buiten. “It has been fixed and with additional measures to ensure nothing like it will happen again,” he says. “No hardware changes were necessary at all.”
Speed, however, is only part of the potential appeal to military users. Another big benefit of the design is maneuverability. Test pilot Fell credits the aircraft’s flight capabilities to the rigidity of the rotor blades. “In other helicopters there’s a lag between putting the input in and getting the aerodynamic response,” he says. “But with the rigid rotor it’s pretty instantaneous.” This maneuverability helps with the low-altitude, high speed flight that Van Buiten says is crucial.
Another of the Raider’s tricks is that it can slow down quickly and without pitching the nose up, the way conventional helicopters must, since the rear propeller eliminates the need to use the main rotor as a brake. Similarly, it can accelerate without pitching forward. All of this also means the helicopter can point where it wants, when it wants, facilitating the crew’s aiming of weapons and sensors, use of rescue equipment, and whatever else they need to do.
Ultimately, the Raider will likely become one of Sikorsky’s first combat aircraft to widely incorporate autonomous or, to use the military jargon, “optionally-piloted flight.” That capability will derive from Sikorsky’s Matrix Technology system, which it’s currently being testing in Black Hawks and commercial helicopters. By the time it’s ready for wider dispersal, Raider could be in full production.
This assumes, of course, acceptance by the DoD, in spite of stiff competition from Bell, Boeing, and other manufacturers. But the aircraft also has drawn significant interest from commercial and industrial customers as well as foreign governments. So one way or another, it should be rocketing across the skies sometime in the next decade.
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