Scale is everything in rotorcraft: What works at one size might not at another, because of complex interactions between aerodynamics, propulsion, structures and control. That is why the U.S. Army plans to fly competing high-speed rotorcraft demonstrators before deciding which direction to take in replacing its utility and attack helicopters beginning in the mid-2030s.
There are four options for the Army's planned Joint Multi-Role (JMR) technology demonstrators. Bell Helicopter and Karem Aircraft each offer tiltrotors. AVX Aircraft offers a coaxial-rotor compound helicopter, as does a Sikorsky/Boeing team. Following completion of preliminary design, a downselect to two aircraft is planned for July 2014, leading to first flights at the end of fiscal 2017.
For Sikorsky, the challenge is to prove its coaxial rigid-rotor technology can be scaled up from the 8,000-lb. X2 Technology demonstrator that exceeded 250 kt. in 2010, through the 11,400-lb. S-97 Raider light tactical helicopter prototypes now being built to fly in 2014, to the 230-kt. Sikorsky/Boeing SB-1 Defiant JMR demonstrator—precursor to the Army's planned 30,000-lb.-class Future Vertical Lift (FVL) medium utility rotorcraft to replace Sikorsky's UH-60 Black Hawk.
“Our key goal is to show the technology is scalable,” says Samir Mehta, president of Sikorsky Military Systems. “X2 proved the basic physics and design, Raider is the next weight class up and JMR the next after that. With each increase in size, questions come with the technology. We want to leave no doubt, so will build our JMR demonstrator to full size.”
AVX plans to fly its coaxial-rotor/ducted-fan design at 75% scale, while Bell's V-280 tiltrotor demonstrator would be “92%-plus” scale, says CEO John Garrison. Abe Karem, designer of the Predator unmanned aircraft, is tight-lipped about his TR36TD variable-speed tiltrotor, but says he is “not known for building things subscale.”
Winning one of the JMR air-vehicle demonstrator contracts would vindicate Sikorsky's decision to make speed one of three pillars of its research and development strategy—the others being autonomy and intelligence—and focus on the coaxial rigid-rotor configuration first tested in the 1970s with the XH-59 Advancing Blade Concept (ABC) experimental helicopter and re-invented as the X2 by integrating the latest available technology.
Conventional helicopters are not fast because, with the rotor flying edgewise, as forward speed rises, the airflow velocity increases over the advancing blade until the tip goes supersonic and decreases over the retreating blade until it stalls. ABC overcame this by generating lift on the advancing sides of contra-rotating coaxial rotors, which were slowed to delay shockwave formation at the tips, and off-loading the retreating sides to delay blade stall. The rotors were stiff so they could be close together to reduce drag.