Small design changes focus of seminar

 - December 28, 2007, 7:20 AM

Rotorcraft design has reached a plateau and advancements are taking place in incremental steps rather than as major breakthroughs. That was the prevailing message of a day-long workshop about the past, present and future of rotorcraft held at the University of Maryland’s Alfred Gessow Rotorcraft Center.

“You don’t see a lot of brand-new vehicles here,” said Barry Lakinsmith, acting director of the U.S. Army’s Aeroflightdynamics Directorate. The Directorate’s work is focusing on improving military rotorcraft mission effectiveness by increasing range, payload and speed.

Some of the technologies that the Directorate is exploring in tandem with NASA, DARPA and the Gessow Center are active rotors combined with blowing rotors. Active rotors employ internal actuators to reposition the rotor blades in flight, while blowing rotors generate additional lift and lower drag using air blown from inside the blades onto the blades’ lifting surfaces.

Eventually, Lakinsmith said, fly-by-wire and fly-by-light technology will operate active rotors. By 2010, rotorcraft designers will incorporate adaptive vehicle management systems, which combine sophisticated rotorcraft health monitoring with active controls to optimize the vehicle’s configuration in all phases of flight.

Another promising area of research is variable-diameter or morphable rotor systems, a technology under study at Penn State’s Vertical Lift Research Center. Ed Smith, director of the center, said that multiple actuators inside rotor blades could be used to vary blade length, cutting engine power requirements by 12 to 15 percent compared with conventional fixed- length rotor systems.

The Army’s Research Laboratory Vehicle Technology Directorate and many of the University of Maryland graduate students and engineers are exploring the other end of the rotorcraft spectrum– tiny machines that can operate with little or no operator assistance in hazardous environments.

Current Operations
On a more practical level, researchers discussed developments that will help today’s helicopter pilots fly more safely and efficiently. Germany’s DLR (aerospace center) is working on a flight director that will help pilots maintain better control while flying with sling loads, according to Berend van der Wall, head of the DLR’s simulation in wind-tunnel program. Van der Wall said he has been discussing the system with helicopter manufacturers. Omri Rand, dean and professor at Technion-Israel Institute of Technology, is also working on a sling-load stability and control system that would match load movement to helicopter movement.

Another problem pilots face is “brownout,” or loss of visual reference when the rotors blow up sand and dust. Gordon Leishman, Minta Martin professor of engineering at the University of Maryland, is working on modeling the brownout problem and showed animations of simulated brownout conditions that his team developed. Losses due to brown-outs have cost operators more than half a billion dollars in the past few years, he said.

“Sediment uplift is probably the most interesting problem from a scientific standpoint,” Leishman said. “Most models are quite primitive.” In researching the problem and simulating brownouts, Leishman found that dust and sand particles move primarily by bouncing around in unpredictable ways. When looking at these particles as a fluids study, it becomes clear that the particles both couple and collide with each other. “It’s a complicated aerodynamic sediment problem. Much more foundational research is needed,” he said.

Helicopter manufacturer Sikor- sky is pushing ahead with research into how modern health-management systems can benefit operators, especially those flying in rough environments. Andy Bernhard, branch chief of Fleet Management Engineering, explained how the company is using health usage management systems (HUMS) to catch problems well before they cause breakdowns in the field.

Bernhard said that in one case his team was able to detect an overheated hanger bearing during production flight testing of a helicopter before delivery. The problem was caused by a misaligned tailboom and drive shaft, a unique glitch that occurred only on that particular helicopter. Finding this problem–which could not have been detected that early without HUMS–prevented the defect from causing trouble in the field and helped engineers understand that it was isolated to a single helicopter and didn’t require grounding the entire fleet.

HUMS is helping Sikorsky add metrics when new problems crop up. In another case, a tailrotor gearbox bearing seized, but the HUMS didn’t note the problem before it happened. Engineers added a new metric to capture that problem, then found another helicopter with the same trouble–well before the bearing seized.

When technicians found a cracked gearbox carrier plate in a Sikorsky Black Hawk, Bernhard’s team ran ground tests on the gearbox to see if HUMS made by other suppliers could detect the crack, and none could, he said. The team then created a test crack to modify the Sikorsky HUMS so that it could detect the flaw before it caused a gearbox failure.

During initial development of Sikorsky’s S-92, the first commercial helicopter with built-in HUMS, gearbox engineers initially resisted the idea of using HUMS, according to Bernhard. “The Sikorsky gearbox people said, ‘Our gearboxes don’t fail,’” he recalled. But now they realize the benefits that HUMS offers. “HUMS is useful only when it is used,” he said.  

Tiltrotor Technology

For those inclined to see tiltrotor technology as a dead-end, Mark Nixon, director of the Army’s Research Laboratory Vehicle Technology Directorate, summarized research on the joint Bell/Army/NASA quad tiltrotor. The quad is a double-wing, four-engine C-130-size aircraft with two tiltrotors on each side. Wind tunnel testing on a one-fifth-scale model was completed in September 2006 and research continues.

Ken Rosen, founding partner of Aero-Science Technology Associates, challenged the rotorcraft industry to take tiltrotor technology to the next level. “Today’s tiltrotors don’t lift very well,” he said. “There’s an opportunity here for this entire industry to take on the tiltrotor and do it justice.”

He suggested that tiltrotors employ morphing technology so that rotor blades could extend or retract to optimize lift in varying configurations. This would help eliminate one of the tiltrotor’s main drawbacks–its inability to take off like an airplane. By shortening the blades during airplane mode, a morphable tiltrotor could carry a much greater load and take off horizontally, then extend the blades after transitioning into helicopter mode for vertical flight.