After three years of design work, a team of researchers has unveiled what it believes is a “credible concept” for an airliner that is inaudible outside a typical airport. The Silent Aircraft Initiative (SAI), led by the Cambridge-MIT Institute, detailed the design, operating concept and business case at the Royal Aeronautical Society in London today.
A video clip from the presentation showing the aircraft in flight can be found at the bottom of this article.
The SAX-40 is a 2025-timeframe, 215-passenger blended wing-body with a sound level outside the airport perimeter of 63dBA – less than road traffic – and a cumulative noise level more than 75EPNdB below Chapter 4, compared with 12EPNdB below for the Airbus A380. The goal was also to design an efficient aircraft, with the SAX-40’s fuel burn per passenger projected at about 80% that of a Boeing 787.
Technologies include a cambered lifting centrebody for low approach speed and efficient cruise; an embedded, distributed propulsion system with ultra-high bypass engines and variable-area, thrust-vectoring nozzles; a flapless wing with continuous-mouldline deployable drooped leading-edges and elevons with trailing-edge brushes to reduce approach noise; and faired landing gear to eliminate noise sources.
Funded by the CMI and NASA Langley, the clean-sheet design effort involved around 40 researchers, and covered airframe optimisation, noise mitigation, landing gear design, engine cycle studies, propulsion integration, quiet operations, regulatory considerations and an economic analysis. The design underwent “non-advocate” reviews by Boeing, Rolls-Royce and others.
Acknowledging some of the technologies are high-risk, principally distributed propulsion, the team developed a lower-risk design, the SAX-L/R1, with three podded ultra-high bypass engines. This is moderately noisier on take-off because of fan rearward noise – a cumulative 12EPNdB above the SAX-40, but still 60EPNdB below Chapter 4.
With three-class seating for 215 passengers, the SAX-40 is 44m (144.3ft) long, with a 67.5m span and 150,800kg (332,600lb) maximum take-off weight. Cruise speed is Mach 0.8 and range 9,250km (5,000nm). Power is provided by three “Granta-3401 clusters” – each comprising a core engine driving three 1.2m-diameter fans, resulting in a bypass ratio at take-off of 18.3:1, compared with 9.5 for General Electric’s latest GEnx.
The centrebody design transforms the fuselage into a lifting surface for improved low-speed performance. Its cambered leading edge balances the tailless aircraft to provide a low approach speed with minimum performance penalty in the cruise, where the blended wing-body configuration provides an elliptical lift distribution for low drag.
Drooping the wing leading-edges provides high lift without slat noise, while eliminating flaps removes an intense source of airframe noise, but requires a large wing area and high angle-of-attack to achieve a low approach speed. Split elevons open during the approach to deploy trailing-edge brushes that scatter wake turbulence and reduce noise. Eliminating surface details from the landing gear reduces high-frequency noise, while mid-frequency noise is reduced by partially enclosing the axles and wheels.
Because fairing the gear and eliminating the flaps removes sources of drag normally used to slow the aircraft, the SAX-40 needs “significant quiet drag generation” on the approach. This is achieved by using a combination of elevon deflection and thrust-vectoring to increase induced drag, the resulting high approach angle-of-attack requiring a cockpit display to provide runway visibility.
Embedding the propulsion system enhances airframe shielding and virtually eliminates engine forward noise on the ground. The upper-surface inlets are positioned to ingest the centrebody boundary layer for improved propulsive efficiency and reduced fuel burn. Distributed propulsion allows use of small-diameter fans and extended acoustic liners ahead of and behind the engine.
The Granta-3401 cluster designed for the SAX-40 has a core engine with fan, axial-radial compressor and five-stage low-pressure turbine (LPT), which drives the outer pair of fans via a transmission. Detailed design of the high-capacity, low-speed forward swept fan, low-noise LPT and transmission has been completed, and the installed weight of each cluster is estimated at 5,470kg and cruise fuel flow at 0.86kg/s.
Nozzle area varies, increasing during take-off to provide high mass flow and low jet velocity, and reducing at the top of climb and in the cruise to provide the required thrust. Opening the nozzle fully on the approach enables an ultra-low engine speed and idle thrust, reducing both rearward fan noise and the airframe drag required to slow the aircraft. The tailless SAX-40 needs thrust vectoring for take-off rotation and pitch trim.
The SAI team also investigated quiet operations, including a slow, continuous-descent approach to a threshold displaced 1km down the runway, increasing the aircraft’s altitude at the airport perimeter. A thrust-managed take-off – a steep, low-speed climb-out with continuously varying thrust, nozzle area and climb angle – would maintain a set noise level outside the airport boundary, but requires three or more engines.
Economic studies compared the SAX-40 with the 767, 787, a 2020-timeframe airliner and the lower-risk SAX-L/R1 assuming moderate and major changes to noise regulations and landing fees. At $160 million, the aircraft would be more profitable under all regulatory scenarios, the only one profitable in the worst case, and more profitable than the 787 even with 30% higher maintenance costs. But increase the price to $200 million, and the SAX-40 outperforms the 787 only in the worst scenario.
Further work is needed to address challenges including cabin design, propulsion/airframe integration, the non-circular pressure vessel, variable-area/thrust-vectoring nozzle and low-speed aerodynamics. Inlet design, engine access, transmission system and a fan able to cope with distortion caused by boundary-layer flow are all concerns, as are the weight, cooling and inspection issues posed by faired gear. But, says the SAI: “We think we have a credible conceptual design given the high-risk technologies used."