Sikorsky’s S-97 Raider advancing blade helicopter prototype has
taken flight at their test center in Florida:

Sikorsky’s “big bet” on the future of rotorcraft took a sizable step forward on May 22, when the S-97 Raider high-speed helicopter made its first flight, almost eight decades after founder Igor Sikorsky set the mold for the helicopter by flying the VS-300.

With its single main rotor and anti-torque tail rotor, the VS-300 solidified a configuration that has come to dominate the rotorcraft market, but also set a speed limit of around 150 kt. that has not changed significantly for 30 years.

With its rigid coaxial rotors and pusher propulsor, Sikorsky believes the Raider and subsequent designs can change the vertical-lift market by offering twice the cruise speed while retaining the low-speed attributes of conventional helicopters, but with higher hot-and-high performance, efficiency and maneuverability, and lower noise, vibration and pilot workload.

The aircraft flown on May 22 is the first of two prototypes of the Raider light tactical helicopter being built under a $200 million industry effort funded by Sikorsky and its supplier partners. This follows on from the $50 million company-funded X2 Technology Demonstrator, which flew 23 times from 2008-11 and exceeded its speed goal of 250 kt.

On its first flight at Sikorsky’s development flight center in West Palm Beach, Florida, the Raider flew for an hour, versus the 30 min. planned, says Mark Miller, vice president of research and engineering, completing three takeoffs and landings, and forward, rearward and sideward flight. The aircraft was flown by Raider chief pilot Bill Fell, with X2 test pilot Kevin Bredenbeck as co-pilot.

Where the 6,000-lb.-gross-weight X2 proved the physics of the rigid coaxial-rotor compound helicopter, Miller says, the production-representative 11,400-lb. Raiders are intended to show their operational effectiveness through customer demonstrations. They will also reduce risk for the 30,000-lb. SB-1 Defiant Sikorsky is building with Boeing for the U.S. Army’s Joint Multi Role technology demonstration.

Rolled out in October, the first Raider had completed 36 hr. of shakedown ground runs since February. This culminated in an untethered ground run on May 20, clearing the aircraft for flight. On the hour-long first sortie, which was limited only by fuel, all 97 points on an “aggressive” test card were completed, including piloted frequency sweeps in all axes, something normally too risky for a first flight, says Fell.

Initially, the Raider is flying with its triplex fly-by-wire flight control system in back-up degraded mode to reduce complexity and focus on basic airworthiness in the low-speed regime. “There is phenomenal control power with the rigid rotors,” he says. “You can make an input in roll or pitch and the aircraft responds immediately. And with no tail rotor, you do not have to manage the pedals.”

………

The Raider will be flown to 140-150 kt. in pure helicopter mode, says Miller. Toward the end of Phase 1, software will be upgraded to Block 2, bringing in the variable-pitch propulsor and articulating tail to increase speed and enable the full flight envelope. Hub and inter-rotor fairings will be fitted to reduce drag.

Phases 1 and 2 will demo the hover KPP carrying the equivalent of six troops and two crew, as well as an endurance objective. Phase 2 will focus on demonstrating—and likely exceeding—the speed objective when fitted with stub wings carrying weapons. “Raider is a balanced design optimized for more than 220 kt. fully weaponized, but the inherent speed of the configuration is more than 250 kt.,” says Miller. “That’s 100 kt. faster than anything else.”

Phase 3 will demo the maneuverability potential of the rigid coaxial rotor and propulsor. In addition to enabling level-attitude acceleration and deceleration and pushing the helicopter to higher forward speeds, the variable-pitch propeller can be used to produce reverse thrust, enabling the Raider to “hang on the prop” to point sensors and weapons toward the ground.

………

The Raider will be flown to 140-150 kt. in pure helicopter mode, says Miller. Toward the end of Phase 1, software will be upgraded to Block 2, bringing in the variable-pitch propulsor and articulating tail to increase speed and enable the full flight envelope. Hub and inter-rotor fairings will be fitted to reduce drag.

[Aircraft 1 is instrumented for envelope expansion, Hub and inter-rotor “sail” fairings for high speed are not yet fitted. Credit: Sikorsky]
Aircraft 1 is instrumented for envelope expansion, Hub and inter-rotor “sail” fairings for high speed are not yet fitted. Credit: Sikorsky

Phases 1 and 2 will demo the hover KPP carrying the equivalent of six troops and two crew, as well as an endurance objective. Phase 2 will focus on demonstrating—and likely exceeding—the speed objective when fitted with stub wings carrying weapons. “Raider is a balanced design optimized for more than 220 kt. fully weaponized, but the inherent speed of the configuration is more than 250 kt.,” says Miller. “That’s 100 kt. faster than anything else.”

Phase 3 will demo the maneuverability potential of the rigid coaxial rotor and propulsor. In addition to enabling level-attitude acceleration and deceleration and pushing the helicopter to higher forward speeds, the variable-pitch propeller can be used to produce reverse thrust, enabling the Raider to “hang on the prop” to point sensors and weapons toward the ground.

The early tests will involve low, conventional helicopter, speed.  As the test moves to higher speeds, fairings will apply to the mast and rotor hubs.

Right now, there are two competing technologies for high speed vertical lift for the US military,  advancing blade, and tilt rotor.

Tilt rotor appears to offer superior performance in horizontal flight, at the cost of increased down wash issues, less capable low speed handling, and a larger ground envelope.

I’m inclined to go with the advancing blade.  I think that it is an inherently safer technology (autorotation), and the implementation seems to involve a lot less “bleeding edge” technology.  (The V-22’s high pressure hydraulic system, necessary to keep weight down, has been a maintenance nightmare).