Purists become realists in quest to understand helicopter dynamics.
Dr. Inderjit Chopra, Ph.D., has spent 30 years of his professional life trying to understand helicopter dynamics and how they affect vibration and noise. He’s nowhere near finished. "We can predict helicopter vibration with 50 percent accuracy," Chopra said, "and we cannot predict noise at all."
While today’s design practices obviously produce safe aircraft, Chopra said the lack of basic research shows up in the performance area. Because the prediction models can be off by a factor of 10, engineers must over-design the vehicle for safety reasons, making conservative assumptions on the loads the blade will experience and the noise generated. Chopra said that it is not uncommon to waste 10 percent of the useful load in vibration suppression devices for the cabin alone. "We extrapolate to understand the behavior," he said, "and hope there will not be any surprises." Though new technologies are making their way to the market in incremental doses, the effect of a lack of basic research makes for longer development times and increased expenses due to trial and error design and suboptimal performance.
Chopra’s comments reflect the unsettling state of affairs in the U.S. helicopter industry, especially coming from one of the preeminent researchers in the sector. Chopra, 60, is the director of the Alfred Gessow Rotorcraft Center at the University of Maryland at College Park, one of three universities designated as NASA/U.S. Army rotary wing Centers of Excellence (COE). Along with other government and industry contracts, Chopra’s team of 72 graduate students, 10 faculty and 10 research scientists and visiting professors are investigating a plethora of futuristic and contemporary topics from micro-hovering vehicles to vibration and noise-reducing smart structures and self-adapting composite rotors. Their research budget this year is bumping up against the $5 million mark, about 20 percent of which is from major helicopter manufacturers and the other 80 percent from government sources.
Like that of his staff, Chopra’s resume is voluminous. His includes a doctorate from the Massachusetts Institute of Technology plus four years spent as a researcher at the NASA Ames/Stanford University Joint Institute of Aeronautics and Acoustics. After coming to the University of Maryland, he has led three Army-funded "smart structures" programs while helping, as a professor and advisor, more than 88 students earn their aerospace engineering masters and doctorate degrees.
With weakening rotorcraft research budgets at NASA and production priorities being the utmost urgency for engineering staff at manufacturers, university shops like Chopra’s are increasingly having to transition from no-strings-attached generators of ideas to the last line of defense for U.S. helicopter manufacturers striving to keep pace with new technologies emerging in other countries. For campuses, that has led to a mental transition from idea-oriented to product-oriented and sometimes has meant holding off on publishing certain competition sensitive findings in manufacturer-sponsored research in order to preserve the funding stream. "In the older days, universities were purists," said Chopra. "That’s changing now, people are becoming realists. We work with industry and try to safeguard their interests."
The fact that even the best minds–not only at Maryland, but at the other Centers of Excellence at Penn State and Georgia Tech as well–aren’t better able to predict noise and vibration characteristics in the design phase in this day and age is testament to the complexity of the mechanics of a spinning rotor in a moving air stream, a situation that can involve transonic flow on the advancing blade tips, dynamic stall and reversed flow on the retreating side of the disk, highly-yawed flows on both sides of the disk, blade-vortex interactions and mechanical coupling between the rotor, transmission and airframe. Chopra said that it is also a symptom of a faltering national priority for basic helicopter research in light of diminishing budgets at NASA, previously the home of such research. The state of affairs is becoming painfully clear to the Department of Defense and the Federal Aviation Administration in several new initiatives for advanced rotorcraft, programs that will largely be undoable until more basic research is completed.
Rhett Flater, executive director and legal counsel for the American Helicopter Society (AHS) International, said the DOD’s anticipated need in 2015 for a very heavy lift vertical takeoff and landing (VTOL) vehicle with a 20- to 24-ton payload, 1,000 km. range, 30 percent reduction in empty weight and reduced vibration environment from today’s helicopters should be a wakeup call for technology investment. Asked by the DOD what was needed to achieve those goals, AHS answered: A $2 billion investment in technology–$500 million a year for four years. "They weren’t happy with our response," said Flater. "They want to have a capability by 2015. We said we probably can’t develop it until 2020 because of a lack of basic research."
A similar wakeup call recently occurred on the civil side. Flater said the FAA just completed a congressionally mandated report investigating funding needed to realize an advanced technology helicopter with 80 percent reduction in takeoff and landing noise, a factor of 10 reduction in vibration, 30 percent reduction in empty weight, reduction in accident rate to that of commercial airlines and operations in zero ceiling/visibility–all in a 10-year timeframe. Again, the answer was not cheap. According to Flater, the FAA found that a $3.4 billion investment in technology would be needed, with the first three years allotted for "foundational research" and basic prototyping and testing; the remaining four to seven years for complex testing and validation of two prototype aircraft.
In either case, the numbers are far higher than today’s technology investment, which largely comes from a four decades-old 50/50 cost sharing agreement between the Army and NASA to fund efforts like Chopra’s. Until 2002, NASA and the Army had each contributed what amounted to $60 million in funding per year, with major helicopter manufacturers as a whole contributing a similar amount. However, in its 2002 budget request, NASA had zeroed the contribution. Flater said AHS and others were able to get the funding restored, though at reduced levels. Currently NASA and the Army are committed to investing a minimum of $15 million a year from both organizations through 2008.
"NASA effectively eliminated its rotorcraft R&D efforts beginning in 2002," said Michael C. Romanowski, assistant vice president for civil aviation at the Aerospace Industries Association. "The Europeans are actively pursuing the goal they put forward in Vision 2020 to become the world’s leader in all aspects of civil aviation–and they’ve backed this up with significant support for aeronautics R&D and other assistance. The effects of their efforts are shown by the increasing number of international rotorcraft competitions won by European rotorcraft manufacturers."
As to how to best spend the money that is available to NASA, Flater said the goals are to reduce the empty weight, create lighter, stronger structures, fly-by-wire and fly-by-light control systems and advanced rotor technologies like bearingless rotors and individual blade control with smart flaps, areas where the Europeans are leading the charge. Chopra and others say the NH90, made by the consortium of Agusta, Eurocopter and Fokker, and featuring a titanium hub with elastomeric bearings, four composite blades, an automatic monitoring and diagnostic system, and fly-by-wire controls, is the most advanced helicopter to date. The most advanced for the United States was to be the Boeing/Sikorsky Comanche Reconnaissance/Attack helicopter, though the program was cancelled in February. The Comanche featured a five-bladed composite bearingless rotor, triple-redundant fly-by-wire control system, and was designed in part with analytical tools developed by Chopra’s team at Maryland. Even with its advances though, Chorpa said the Comanche represented 1980s technology, compared with 1990s technology for the NH-90.
Priorities at Maryland mirror Flater’s wish list and include technologies to help today’s fleet, including far-flung but analytically sound ideas for the longer term. Chopra has high aspirations for the overall impact to future helicopters, including a 60 percent reduction in vibration, 85 percent aeromechanics prediction effectiveness (predicting vibration from models), 24 percent increase in maximum rotor blade loading, 45 percent reduction in maintenance costs, 50 percent reduction in perceived noise and a 50 percent reduction in development cycle time through "next generation" design tools.
Chopra and the Maryland COE are perhaps best known for their "aeromechanics" expertise and work on the so-called "smart rotor" technologies developed over the past 10 years. They hit the headlines in May when Boeing whirl-tested an MD 900 Explorer five-bladed rotor with an active vibration-reduction system. Researchers have targeted the main rotor for improvement because in addition to being the root of all good in a helicopter (lift), it is also the main contributor of vibration and unwanted forces to the fuselage, making for more frequent inspections, higher maintenance costs for short-lived components and an uncomfortable ride.
To reduce vibration, smart rotor technologies make use of trailing edge flaps with "smart actuators" or active blade-twisting with embedded piezoceramic actuators, or actuators designed to produce opposing waves to nullify vibration waves. The basic idea is to sample numerous lightweight surface-mounted or embedded sensors at various locations along the blade and in the cabin, run the data through a control system and modify the local shape of the airfoil with surface-mounted or embedded actuators on the blade, canceling out the vibration. For rotor-tracking tab adjustments, researchers are experimenting with active shaped-memory alloys, slow-reacting devices can trim the rotor with the application of voltage.
The MD 900 Explorer smart rotor system design was conceived at MIT (Maryland had a competing design) but all of the actuator development was done at Maryland, said Chopra. During the whirl testing of the five-bladed rotor at Boeing’s Mesa test facility in Arizona, Boeing reported an 80 percent reduction in vibration levels, though Chopra said DARPA "didn’t show much excitement" about the results.
The pushes to some extent mimic the priorities for the European Union states, but the United States may be in the catch-up mode. Eurocopter last year announced a four-pronged approach to new technology for its helicopters, including abbreviated development programs via enhanced modeling tools and computational fluid dynamics in lieu of wind tunnel testing; reduced maintenance costs and lighter materials though composites, bearingless main rotors, and reduced external and internal noise and vibration using active rotor control, in addition to all-weather operations.
While U.S. manufactures are incrementally upgrading today’s designs, Chopra said the rate of technology transfer is slower than it could be. "If they could add 40 percent (of the new technologies)," he said, "today they add 5 percent." The result of the technology lag is that "you don’t have design efficiency in the new product," said Chopra. "You can’t compete." To stay on the leading edge of technology, Maryland is investigating a wide variety of product and analysis enhancements, including swashplateless rotor designs, micro hovering vehicles and ways to use computational fluid dynamics to model helicopter noise.
Given that funding is unlikely to leap in today’s fiscal environment, the question becomes how campuses like Maryland can best help in the fight for technology dominance. Along with working directly with industry and writing technical papers, Chopra said the most significant contribution possibly is a freshly trained engineer. "The best way to get technology to them is our graduate students," Chopra said. "They take their ideas with them."