Since the dawn of the jet age 60 years ago, aerospace engineers have been on a quest for quieter, more environmentally friendly flying machines. And, with air travel expected to increase dramatically over the next 20 years, noise pollution will become a hot issue, especially in heavily populated urban areas.

A team of researchers at Cambridge University (Cambridge, England) and the Massachusetts Institute of Technology (Cambridge, MA) claim they have developed a solution: The silent aircraft. In addition to dramatically reducing the noise experienced by people in the vicinity of airports, the highly efficient design promises to improve in-flight fuel economy by 25 percent compared to current aircraft.

“The design is intended for the next-generation of aircraft, for entry into service in 2030,” says Ann Dowling, a professor of mechanical engineering at Cambridge. The aircraft would generate a noise of 63 decibels outside the perimeter of airports, which is 25 decibels quieter than current aircraft. In addition, it would generate 149 passenger-miles per gallon of fuel vs. 120 for the best current aircraft in this range and size. “This is equivalent to a Toyota Prius hybrid car carrying two passengers,” claims Dowling.

The team of aerospace engineers focused on creating new designs for airframes and engines, because half of the noise from a landing plane comes from its airframe. Their SAX-40 aircraft features a radical design that pushes traditional boundaries. For instance:

• The overall shape of the aircraft is a single flying wing. This allows the body to provide lift as well as the wings, allowing a slower approach, which reduces noise. The unique tailless, blended-wing shape also improves fuel efficiency when the plane is at cruising speed and altitude.
• Wing flaps and slats have been eliminated, because they are a major source of airframe noise when a plane is landing.
• The undercarriage has been simplified and its aerodynamics improved. Aerodynamic fairings around the landing gear divert airflow, which reduces drag and dampens noise.
• The engines are mounted on the top of the aircraft, which screens much of the noise from the ground.
• Ultra-high bypass engines are used. The engines have variable size jet nozzles to allow slower jet propulsion during takeoff and climb for low noise. They can be optimized for maximum efficiency during cruise, which requires higher jet speeds.

“The airframe is highly efficient,” explains Dowling. “It is an ‘all-lifting’ design. Although based initially on the blended-wing-body concept, it makes use of a novel center-body shape, with leading-edge carving. This balances the aerodynamic forces without the need for a tail, and enables an optimal wing design with an elliptical lift distribution and low cruise drag.”

The resulting lift-to-drag-ratio of 25-to-1 is 33 percent better than current aircraft. Weight and drag are reduced by embedding the engines in the airframe. The aircraft wake is further reduced by ingesting the air near the aircraft into the engines.

The aircraft would be propelled by three jet engines. Each engine has a single core, driving three high-capacity low-speed fans. Long exhaust ducts use acoustic liners to dampen noise.

“This distributed propulsion system is designed to ingest the boundary layer on the aircraft center body, which reduces the fuel burn,” says Dowling. “The multiple small fan design is easier to embed in the airframe, and leads to reduced weight and nacelle drag. It also enhances boundary layer ingestion, thereby improving fuel efficiency, and the low fan tip speeds lead to low noise.”

The silent aircraft is currently a conceptual design. “There are many challenges that would have to be overcome before it could become a reality in the 2030 time frame,” warns Dowling. “These include the technical challenges of the propulsion system and airframe integration; structural analysis and manufacturability of the noncircular pressure vessel; the mechanical design of the thrust-vectoring and variable-area nozzle; and detailed assessment of low-speed aerodynamic performance.”

But, the design has grabbed the attention of aircraft manufacturers. “This collaboration has stretched our imagination and generated some noise-mitigation ideas that we will be able to study for potential future use,” says Jim Morris, vice president of engineering and manufacturing at Boeing Commercial Airplanes (Seattle), which helped fund the project.