Speed comes at a cost and, as Bell and Sikorsky/Boeing design fast rotorcraft demonstrators for the U.S. Army, to justify the price they must prove their designs will do more than cross the battlefield more swiftly than today’s helicopters.
Their competing Joint Multi Role (JMR) demonstrators are set to fly in 2017, but Sikorsky will have an early opportunity to showcase all its rigid coaxial-rotor compound helicopter configuration can do when its 220-kt. S-97 Raider armed scout prototype flies.
The first industry funded S-97 rolled out Oct. 2 (see photo) and is planned to fly this year. A second will be used for customer demonstrations soon after the first Raider has opened up the flight envelope. Sikorsky is already designing demonstrations with prospective customers, to show the military utility of higher speed, but also the design’s “unique flight characteristics.”
Sikorsky’s X2 configuration comprises contra-rotating rotors with hingeless hubs and stiff blades for hover efficiency and low-speed agility. The rigid rotors are closely spaced, hubs and shaft faired, to minimize drag at high speed. Coaxial rotors eliminate the tailrotor. Instead the engine also drives a pusher propulsor via a clutch so the propeller can be disengaged at low speed to increase safety and reduce noise.
This integration of coaxial rotors and pusher propulsor gives the Raider its unique characteristics. The 11,000-lb. helicopter will hover out-of-ground effect at 10,000 ft. on a 95F day, compared with 4,000 ft. for the Army’s 5,500-lb. Bell OH-58D Kiowa Warrior armed scout. The Raider will cruise at 220 kt. carrying external stores—faster when clean and light—while the OH-58D can be limited to just 90 kt. in hot/high conditions, Sikorsky says.
Credit: Sikorsky Aircraft
But the variable-pitch propulsor, active elevons and rudders on the tail, and rotor disks that can tilt together or differentially give the S-97—and therefore also the Sikorsky/Boeing SB-1 Defiant JMR demonstrator—“more control degrees of freedom” than a conventional helicopter, says Andreas Bernhard, Raider chief engineer.
To move into forward flight from the hover, a traditional helicopter tilts its rotor disk down and the fuselage follows, leaving the pilots looking at the ground as it accelerates. Using the pusher propeller, the Raider can lift into a hover and accelerate “in a level attitude to 200 kt. at the end of the runway, giving the pilots complete situational awareness of the environment around them,” Bernhard says.
At low speed, the propulsor’s forward- and reverse-thrust capability “allows us to decouple the aircraft’s attitude from its trajectory, to point it in directions different to where it is flying,” Bernhard says. “We can hang on the prop and drop the nose, or sit on the prop and pull the nose up, which allows us to sweep a cone with the sensors and weapons.”
In level flight, rudders and elevons allow different trim states that enable aircraft attitude to be adjusted to reduce drag or optimize the sensor or weapon field of regard, he says. The Raider will also be able to pull -higher-g maneuvers at higher speeds than conventional helicopters, although Sikorsky is not revealing its maneuverability targets for the aircraft.
But it is not just Sikorsky (and Boeing) that must prove the value of a new configuration. Bell has to show that a high-speed tiltrotor can match a helicopter in hover capability and low-speed agility. The V-22 Osprey has proved a tiltotor is fast, and can decelerate and accelerate quickly to increase survivability into and out of the landing zone. But the V-22 has higher disk loading and lower hover efficiency than a helicopter.
Compared to the V-22, the 280-kt. V-280 Valor JMR demonstrator will be simpler and lighter, with lower disk loading and longer wing for greater hover and cruise efficiency. But Bell does not have a prototype to display before the V-280 flies. So it has teamed withTextron sister company TRU Simulation & Training to build a high-fidelity marketing simulator. “We want to get more pilots in, so they can understand how to fly a tiltrotor, its acceleration and deceleration and low-speed agility,” says Keith Flail, Bell’s Future Vertical Lift program director.