A major research program launched three years ago by the European Union has identified open rotors and natural laminar flow as key technologies to be taken forward in the Clean Sky joint technology initiative and potentially into the mooted replacement for the Airbus A320.
The e30 million ($47 million) new aircraft concepts research (NACRE) program began in April 2005 to pursue the Vision 2020 efficiency, affordability and environmental objectives framed by the advisory council for aeronautical research in Europe (ACARE). Last week in London participants gathered to share some of the key findings so far.
João Frotta of Airbus future projects serves as NACRE project coordinator. He said the environment accounts for fully 50 percent of the program’s budget. Targets include a reduction of 10 dB per operation in noise and a 25 percent cut in fuel burn.
To structure their research, the participants identified three generic concept aircraft. These, Frotta said, are like the concept cars that automobile manufacturers use to explore ideas that ultimately find their way into production vehicles.
The Proactive Green concept for an aircraft capable of carrying 180 passengers 3,000 nautical miles is configured to minimize the effect of aviation operations and manufacturing on the environment. It has been examined in terms of two baseline designs.
Pro-Green 1 would have a comparatively modest speed of Mach 0.76, an initial cruise altitude of 35,000 feet and an approach speed of 135 knots. It features a forward-swept wing for natural laminar flow, with no slats to preserve a clean leading edge, and Snecma-designed contrafan engines mounted above the tail, where the Avro Lancaster-style horizontal and vertical tail surfaces shield the noise from the ground.
For Pro-Green 2 the performance is relaxed further, with a cruise speed of Mach 0.74 and an initial cruise altitude of 31,000 feet. The wing is a turbulent design with high aspect ratio and low sweep, providing a 20 percent improvement in lift-to-drag ratio at both high and low speeds. The tail configuration is similar, but propulsion in this case comes from Rolls-Royce engines driving contra-rotating open rotors designed by Dowty Propellers.
The Passenger Friendly concept uses a blended wing body (BWB) to provide maximum internal space and comfort. It embraces a family of three flying wings developed by Russia’s Central Hydrodynamic Research Institute, carrying 581, 748 or 991 passengers, and envisions H-cylinder, V-cylinder or lens-shape pressurized cabins with the aerodynamic shape wrapped around them. The BWB work is coming to an end, however, and will not be carried forward into Clean Sky.
The V-tailed Simple Flying Bus, on the other hand, accentuates return on investment over the lifetime of the aircraft and serves primarily as an innovative evaluation platform.
Within those overall concepts the 36 participating organizations from 13 European countries are looking at novel ideas for lifting surfaces, powerplant installations and fuselage structures. The various technologies are integrated at the major aircraft component level–wing, empennage and powerplant system–and then the components are integrated into the aircraft concepts. They are then assessed in terms of three ACARE long-term challenges, namely quality and affordability, environmental impact and air transport system efficiency.
The main NACRE building blocks to be carried forward into Clean Sky are natural laminar flow and open rotors, Frotta said. Natural laminar flow dispenses with the perforations that have been tried in the past to reduce drag by keeping the airflow over the wings smooth rather than turbulent.
Open rotors were added to the NACRE work package when it became clear that the ACARE targets would be unattainable with turbofans, said John Whurr, who represented Rolls-Royce future programs engineering. The work has been extremely valuable to the company, he added, helping it understand how novel engine configurations mitigate the potential detrimental effects on performance of the U-tail.