Advances in blade design
Rotorcraft OEMs tackle noise and speed issues through improvements to main disc and antitorque.
Relieving airport congestion
Sikorsky X2 demonstrator undergoing flight trials. The coaxial main rotor is augmented by a pusher engine designed to validate 250-kt cruise speeds.
In a Nov 2000 presentation to the Intl Powered Lift Conference in Arlington VA, NASA estimated that a civil tiltrotor could reduce noise impact over traditional helicopter designs by an average of 12.5 dB.
This reduction would be achieved through low-noise approach profiles and a reduced overall noise footprint. The agency estimated that tiltrotor technology could achieve forward speeds of 350–400 kts by around 2022.
Reducing congestion at airports will be aided in the future by the advances of NextGen in managing traffic from these reliever sites by large commercial tiltrotors as well as cruise efficient short takeoff and landing (CE-STOL) aircraft operating from shorter runways at outlying airports.
NextGen’s capabilities to reduce separation criteria with ADS-B equipped aircraft, especially during IMC, will help negate the need for designing and constructing new airports, which will in any case be vigorously opposed by both environmental groups and city and state governments.
FBW and future designs
BA 609 tiltrotor in flight. The first new category rating since 1946 will be issued when the tiltrotor enters service labeled “powered lift.”
As seen in recent trade show offerings, cockpit and flight control technologies are moving toward FBW and, eventually, fiberoptic or fly-by-light (FBL) applications.
Sikorsky successfully flew the H92 (an S92 variant) outfitted with FBW flight controls in 2007, and the National Research Council of Canada flew a modified Bell 412 in 2001, all intended for civilian applications.
As blade technologies advance further, the benefits of FBW/FBL will become increasingly useful for both controllability and noise mitigation efforts. Future design for all rotor disc aerodynamics will incorporate features adapted from current use, such as active blowing and boundary layer modification (taken from high-bypass fan engines such as GEnx and the Rolls-Royce Trent series, scheduled for the Boeing 787 and in use on the 777, respectively).
Likewise, dynamic vibration and noise control will employ active and passive tuning modules to reduce fatigue on both passenger and airframe. Swashplates would be eliminated from rotor head design and would incorporate active “material morphing” blade geometries to reduce BVI and improve efficiency almost an order of magnitude.
Fenestron history and technology
The fenestron (or fan-in-fin shrouded rotor) has been evolving over the past 35 years and remains an integral part of noise mitigation efforts. Using an even number of blades spaced unevenly to reduce noise, the shrouded anti-torque mechanism was designed by Sud Aviation Aérodynamique in Marignane, France under Paul Fabre and René Mouilée. Eurocopter’s history of the fenestron has evolved to where the unit is standard on the EC120/130, EC135 and EC155. The distinctive design does not allow for significantly higher forward speeds but provides true aerodynamic induced-drag reduction. Certain wind vectors affect fenestron effectivity, but these can be avoided through proper training and avoidance, and the design has proved to be trouble-free from both a pilot and maintenance support standpoint.
Another unusual source for noise mitigation issues is coming from the UAS fleet. For example, the remotely piloted Boeing A160 Hummingbird uses variable main rotor speed to reduce noise impact, as well as an asymmetrical airfoil and lightweight composites.
The A160 has a range of 2500 nm with endurance in excess of 24 hrs and can carry a 300-lb payload. It uses operator input to adjust main rotor speeds during different phases of flight for optimized efficiencies, eg, lower speeds in cruise and higher for takeoff and landing.
Like other military applications, this technology will flow to the civilian sector, as seen recently with Sikorsky introducing a 2-speed main rotor for the S76D. As with the S92, health usage monitoring systems (HUMS) are becoming more common and can provide real-time datalink capability to maintenance personnel about impending failures and excessive wear.
Future designs will have advanced sensors located throughout the airframe and report status and predictive time-remaining for critical and rotable parts to crew and maintenance base. This “intelligent operator interface” (IOI) will add a layer of safety and allow a flightcrew to decide to divert or land short of destination depending on the nature of the impending failure.
Likewise, noncritical components could be ordered by maintenance staff and replaced during scheduled downtime. Self-monitoring, adaptive and reconfigurable self-healing systems are known as bioanalogous distributed systems. Engineers have been designing them for a decade.
Recent advances in nanotechnologies (combined with dynamic piezoelectrics) will enable aerospace engineers to incorporate them into future design. Honeywell’s forecast shows a sharp increase in sales about 10 years hence which may coincide with a mature nanotechnology application.
Rotorcraft are designed for work that cannot be accomplished by any other means. Faster speeds mean lower operating costs per cycle, but for law enforcement and HEMS they also mean more lives saved.
Population growth will continue to affect how the rotorcraft community adapts to pressures for reducing noise and environmental impact—the solutions are complex and expensive. Thus far, the use of composites, piezoelectrics and variable-speed main rotor designs have helped considerably, but the future needs commitments from municipalities to allow more heliports and vertiports in urban areas to reduce congestion at traditional airports.
The city of São Paulo SP, Brazil is an excellent example of this, and the Brazilian government has installed numerous sites in this sprawling city of nearly 20 million. In the US, heliports are closing, bowing to public pressure from noise and fear of catastrophe if an aircraft should crash into a densely populated area.
This type of thinking is politically motivated and shortsighted—the number of lives that could be saved in disaster relief would more than offset the small increase in traffic. The general public once viewed HEMS as an annoyance, but they are in high regard today, thanks to the millions of lived saved.
Likewise for law enforcement and military search and rescue during civilian disasters. Education and awareness are key issues—when the public realize the benefits, tiltrotors and coaxial designs will flourish and the next generation of rotorcraft will continue to thrive.
David Bjellos is the aviation manager for a private corporation whose flight department was the first in south Florida to achieve IS-BAO certification. The company operates a Gulfstream IVSP, a Dassault Falcon 2000, 2 Bell 407s and a Eurocopter EC120.
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