Silent Passenger Aircraft No Longer a Flight of Fancy

LONDON, UK, November 9, 2006 (ENS) - The design for a silent, environmentally friendly passenger aircraft unveiled in London this week bridges the gap between increasing demand for air transport and public concern over noise and pollution, say researchers.

Sponsored by the United Kingdom government, 40 researchers from Cambridge University and the Massachusetts Institute of Technology, MIT, began working on the Silent Aircraft Initiative three years ago. On Monday at the Royal Aeronautical Society they presented a conceptual design for a plane designed to carry 215 passengers.

The overall shape incorporates the wings and engines into the body of the plane with air intake for the engines mounted above the aircraft in long ducts, muffled with acoustic liners to reduce the noise.

The plane is designed to be so quiet that it would be barely audible beyond the perimeter of an urban airport. It also has the potential to be more fuel efficient.


An all-lifting design, as pictured here, has many benefits enabling a closer integration of airframe and engine than the traditional tube and wing design. This is one design under consideration by the Silent Aircraft Initiative. (Photo courtesy MIT-Cambridge Institute)
In a typical flight, the proposed plane is predicted to achieve 124 passenger miles (200 kilometers) per gallon, almost 25 percent more than current aircraft, according to Edward Greitzer, professor of Aeronautics and Astronautics at MIT. By comparison, the Toyota Prius hybrid car carrying two passengers achieves 120 passenger-miles per gallon.

"Public concern about noise is a major constraint on expansion of aircraft operations," said Greitzer.

"The silent aircraft can help address this concern and thus aid in meeting the increasing passenger demand for air transport."

The International Air Transport Authority, IATA, predicts air travel will almost double in the next 20 years. The UK's Department of Transport forecasts that between 2005 and 2020 the number of terminal passengers at UK airports will grow from 229 million to 401 million.


Aircraft on the tarmac at London's Heathrow Airport. Air travel is forecast to double by 2026. (Photo by Ian Britton courtesy FreeFoto)
While IATA says airline fuel efficiency improved 20 percent in the last decade, greater fuel efficiency can only help tackle emissions of carbon dioxide and other greenhouse gases blamed for global warming.

The Silent Aircraft Initiative addresses three major noise sources: the engines, the undercarriage, and the airframe the physical structure of the plane.

The engines are an obvious source of noise, with their huge fans at the front, fast moving compressors and turbines at their core and hot, fast-moving exhaust air flowing out of the rear, creating turbulence and noise as it mixes with the still air around it.

But, once the engines are throttled back for landing, about half the noise comes from the flow of air over the airframe. By embedding the engines in the aircraft, instead of under the wings, the body of the plane itself shields the ground from noise.

Other key features of the design include an overall shape that integrates body and wings into a single flying wing. Both the body and wings provide lift, allowing a slower approach and takeoff, which would reduce noise. The shape also improves fuel efficiency.

The elimination of the flaps, or hinged rear sections, on each wing is another key feature. These flaps are a major source of airframe noise when a plane takes off and lands.

A variable-size jet nozzle that allows slower jet propulsion during takeoff and landing but is efficient cruising at higher speeds is also part of the new design.


Edward Greitzer is associate professor and deputy director of the Massachusetts Institute of Technology Department of Aeronautics and Astronautics' Vehicle Technologies division. (Photo courtesy MIT)
"One major technical challenge is the integration of the propulsion system with the aircraft," Greitzer said.

"The propulsion system, with engines embedded in the fuselage, is different than for traditional civil aircraft, in which the engines are located in nacelles below the wing," he said. "This presents a different set of issues to the designer."

Numerous companies are also involved in the project, which aims to develop aircraft that will fly by 2030. Engineers at Rolls-Royce in Derby, England, for instance, are helping to produce a completely new engine design, which balances the reduced jet speed required for noise reduction against the level of thrust required for take-off.

Greitzer praised the collaboration between MIT, Cambridge University and their industrial partners.

"Collaboration and teaming occurred in essentially all aspects of the project," he said. "The Silent Aircraft Initiative has been very much an enterprise in which the whole is greater than the sum of the separate parts."