Sikorsky's S-97 X2 Raider is changing shape, slightly, as it moves from full-scale mockup to flying prototypes. Here are the latest images - to put some scale on the 3-view, rotor diameter is now 34ft and length nose-to-tail 35.9ft.Graphics: Sikorsky
The most obvious change from the mockup Sikorsky has toured round the shows is the switch from tricycle to taildragger landing gear. That makes the revised tail, with the tailwheel on the ventral fin, look more like that of Sikorsky's X2 demonstrator.
The glasshouse has also changed, as expected, with smaller windows. The 3,000shp-class engine now breathes through small intakes either side of the rotor pylon and exhausts through a stealthy infrared suppressor on top of the aft end of the tailboom.
Briefing the X2 technology demonstration program on the show floor at Quad-A in Nashville this week, project test pilot Kevin Bredenbeck provided some interesting insights. One is that the unofficial helicopter speed record of 250kt set on 15 September was faster than expected.
It was predicted the X2 would reach 235kt at 70% torque, when in fact it accelerated all the way to 253kt in level flight - and that was without the drag-reducing inter-hub fairing that was planned to be fitted (and is seen on the X2 Raider above). When aircraft attitude was reduced from 3deg to 1deg nose-up, the X2 accelerated to 263kt "and was en route to 270kt" when Sikorsky called it a day.
Increased vibration apparent at 263kt was caused by the advancing-blade tips exceeding their critical Mach number, and Bredenbeck believes the X2 could have flown another 15-20kt faster had they reduced the rotor speed. "At 70% torque we had 200-300shp left," he says. The X2 may yet get a chance to show its maximum speed as one last demonstration flight is on the cards.
As for the X2 Raider, Sikorsky says it's not all about being twice as fast as a conventional helicopter. Hot/high hover performance is increased 150%, endurance doubled, payload increased by 40% and acoustic detection range halved (with prop disengaged) in an aircraft 15% smaller than an equivalent conventional armed scout.
Sikorsky hopes to prove its claims when the prototypes fly in 2014, and to get a chance to compete for the US Army's Armed Aerial Scout requirement to replace the OH-58D Kiowa Warrior. If a development contract was awarded in 2014, the Raider could be operational by 2025, the company says.