Concept Design Of An Ultra Low Noise

Silent Aircraft http://silentaircraft.org/ Concept design of an ultra low noise, fuel efficient aircraft The concept aircraft SAX-40 (Silent Aircraft eXperimental) is a result of an iterative design process (SAX-01 to SAX-40) to achieve low noise and improved fuel burn.

We predict: •

149 passenger-miles per UK gallon of fuel (compared with about 120 for the best current aircraft in this range and size). This is equivalent to the Toyota Prius Hybrid car carrying two passengers.

•

A noise of 63 dBA outside airport perimeter. This is some 25dB quieter than current aircraft.

Why does the 'Silent' Aircraft concept design look like this?

Design for low noise At take-off the engines are the largest sources of noise from an aircraft. The Silent Aircraft Initiative low noise target is achieved by: •

installing the engines embedded within the fuselage with intakes above the wings to shield much of the engine noise from listeners on the ground

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novel ultra-high bypass engines with a variable-area exit nozzle. This means the engines can operate for low noise with low speed exhaust jets at take off and during climb, and then be optimised for minimum fuel burn in cruise

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throughout the climb, the thrust, nozzle settings and climb rate are optimised for low noise, subject to meeting legal requirements

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long engine exhaust ducts provide space for extensive acoustic liners to absorb the sound

On approach, the airframe on a conventional aircraft is as noisy as the engines. On the ‘Silent’ Aircraft the approach noise is reduced by: •

•

an airframe design that enables the aircraft to approach at lower speed and so land further down the runway. Further reduction in airframe noise on approach noise is due to o

low-noise fairings on the undercarriage

o

advanced airfoil trailing edge treatment

o

a deployable drooped leading edge on the wings and vectored thrust, which are used to enable low-speed flight without more noise. There are no flaps or slats

the engines are designed for low noise, have a low idle thrust and they are able to spool up quickly if a go-around is necessary

Low noise is therefore not achieved by a single design feature but results from many disciplines integrated into the design and operation of a noise-minimising aircraft system.

1. Advanced airfoil trailing edge treatment 2. Airframe shielding of forward propagating engine noise 3. Exit nozzles rotate to provide thrust vectoring in combination with the elevons this gives quiet drag via increased induced drag 4. Optimised extensive liners for low engine noise 5. Variable area exhaust nozzle and ultra-high bypass ratio engines at take-off for low jet noise 6. Deployable drooped leading edge for quiet approach 7. Faired, low noise undercarriage for quiet approach 8. Advanced centrebody design enables a low approach speed, thereby reducing the airframe noise sources on approach 9. Engines have a low idle thrust enabling low approach speed

Design for low fuel burn The airframe is highly efficient. It is an ‘all-lifting’ design. Although based initially on the Blended-Wing-Body concept, it makes use of a novel centre-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 some 10% better than other all-lifting designs such as the Blended-Wing-Body and about 33% better than current aircraft. The 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. Careful attention has been given to the inlet duct design to minimise the flow distortion at the fan face. Finally, the area of the exit nozzle is set to operate the engines at optimum efficiency throughout cruise.

1. Variable area exhaust nozzles tuning engine for optimum cruise efficiency 2. Embedded, boundary layer ingesting, distributed propulsion system for reduced fuel burn 3. Advanced centrebody design for excellent lift to drag ratio 4. Elliptical lift distribution at cruise for excellent lift to drag ratio

Major design features of the conceptual aircraft

The aircraft (SAX-40) The aircraft is an ‘all-lifting’ design, producing lift on the centre-body as well as the wings. The airframe makes use of a novel centre-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. Range: 5,000 nm Number of passengers: 215 (3 class)

Cruise ML/D: SAX-40: 20.1 Boeing PW BWB ML/D : 17-18 Boeing 777 ML/D : 17.0 ML/D = (Mach number) X (Lift) ÷ (Drag)

1. Centrebody with leading edge carving

2. Winglet rudder

3. Elevons

4. Centrebody boundary layer ingested

5. Thrust vectoring, variable area nozzle

6. Deployable drooped leading edge

7. Faired undercarriage

Cruise: Mach number: 0.8 Altitude: 40,000 – 45,000 ft Lift/Drag: 25.1 – 23.5 Static margin: 5.9% – 9.5%

Span: 221.6 ft (incl. winglet) Gross Area: 8,998 ft2 MTOW (Maximum Take-Off Weight): 332,560 lbs

Centre of Gravity travel: 0.4 m OEW (Operational empty weight): 207,660 lbs Structure: 104,870 lbs Payload: 51,600 lbs Fuel: 73,310 lbs

The engines (GRANTA – 3401) The aircraft has three novel engines - the engine type is called GRANTA – 3401. Each engine has a single core, driving three high capacity low speed fans. This distributed propulsion system is designed to ingest the boundary layer on the aircraft centrebody which reduces the fuel burn. 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 engine has an ultra-high bypass ratio 18.3 at takeoff for low jet noise, 12.3 at top of climb for good efficiency.

1. axial-radial compressor

2. extensive acoustics liners

3. variable area nozzle

4. low noise 5 stage Low Pressure Turbine

5. transmission system to transit power from Low Pressure Turbine

to Fans

6. 3 high capacity, low speed Fans

Fan Diameter: 1.20 m Engine Length: 2.46 m Cruise Fuel Flow: 0.86 kg/s Bare weight: 6,566 lbs / engine Installed weight: 12,058 lbs / engine

Top of climb Take-off Fan pressure ratio

1.50

1.19

Bypass ratio

12.3

18.3

Overall pressure ratio

48.8

24.2

What is the 'Silent' Aircraft predicted to sound like?

Conventional Engine compared with the Concept Engine (Granta-3401) Simulated sound files constructed from the predicted sound for FLY-OVER condition, 40degrees behind (3-sec each) (you will need RealPlayer to listen to this sound file) 1. Modern conventional engine 2. GRANTA 3401 bare engine 3. GRANTA 3401 (with shielding) 4. GRANTA 3401 (with liners) 5. GRANTA 3401 (with shielding and liners)

for FLY-OVER condition, 40degrees ahead (3-sec each) (you will need RealPlayer to listen to this sound file) 1. Modern conventional engine 2. GRANTA 3401 bare engine 3. GRANTA 3401 (with shielding ) 4. GRANTA 3401 (with liners) 5. GRANTA 3401 (with shielding and liners)

The noise from SAX taking off / landing at a hypothetical runway, typical of a large international commercial airport, has been predicted. The airport we consider has: a perimeter 1km from the start of the runway, a 3.0 km long runway, with the airport perimeter a further 1.0 km from the end of the runway. The airport width is 0.45 km either side of the runway. For comparison the corresponding figures for London Heathrow are 0.7 km, 3.9 km long runway, 1.0 km, and 0.45 km to either side.

Distances for takeoff noise analysis

Temperature: ISA+12?C

Sideline noise estimate Challenge at Outset: •

Sideline noise dominated by jet and fan buzz-saw

Solution: •

High thrust and low jet velocity using variable area nozzle

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Extensive liners

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Airframe shielding

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Airframe design for enhanced low speed performance

Cut-back noise estimate Challenge at Outset: •

Jet noise reduction with steep, low speed climb-out.

Solution: •

Takeoff power management and variable area nozzle

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Extensive liners

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Airframe shielding

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Airframe design for enhanced low speed performance

SAX-40 noise overview

Temperature: ISA+12?C

Approach noise estimate Challenge at outset: •

Airframe, fan, and turbine noise.

Solution: •

No flaps or slats.

•

Displaced threshold.

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Undercarriage fairing.

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Airframe design for enhanced low speed performance.

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Deployable drooped leading edge.

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Low noise LPT design.

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Trailing edge brushes.

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Low engine idle thrust.

What is the predicted fuel burn of the 'Silent' Aircraft? In addition to quiet, analysis suggests high fuel efficiency. passenger miles per Mach * Lift/Drag UK gallon ~149 20.1

SAX-40 Toyota Prius Hybrid ~144 w/ 2 people Car Boeing 777 103 - 121 17.0 Boeing 707 55 - 70 13.5

Non-SAX data cited from Lee, Lukachko, Waitz, and Schafer (2001)

Emission predictions: total carbon and NOx Low noise solution expected to have low pollutant emission Low pollutant emission primarily a result of low aircraft fuel burn.

Total CO2 emission is 89.5 g per passenger-nm Total NOX emission is 0.22 g per passenger-nm

Why are aircraft noisy and what can be done about it?

What generates noise on conventional aircraft? From the engine, the main sources of noise are the fan (labelled A in the diagram to the right), and the high speed propulsive jet (labelled B):

On approach, the airframe makes as much noise as the engine. The flow over the flaps (labelled B below), slats and undercarriage (labelled A below) is unsteady and generates sound (Image showing the strength of the sound sources, courtesy of NLR). The jet noise contribution is labelled C.

What can we do about these noise sources? We will not achieve our noise target with engines hanging underneath wings - we need a greater integration of airframe and engine. For example, using the airframe to shield the engine noise from listeners on the ground. The videos below illustrate the effects of shielding: With engines underneath the wings, the sound tends to be reflected downwards.

The noise made by engines above the wings is shielded from listeners on the ground.

(video to follow on 7 November) (video to follow on 7 November) We can also use extensive acoustic liners in the inlet and exit engine ducts to absorb engine noise. All airframe noise sources are cut by reducing approach speed, and so there are benefits from flying the final approach more slowly. There are also benefits from reducing the engine fan speed and the jet velocity, since their noise increases significantly with speed.

Low noise approaches A variety of techniques can be employed to reduce the noise impacts of aircraft as they approach an airport, including:

 keeping the aircraft high for as long as possible (increasing the distance from the aircraft noise sources to the ground)

 keeping the aircraft at low engine power for as long as possible (reducing engine noise)

 keeping the aircraft in a clean aerodynamic configuration for as long as possible (reducing airframe noise)

 minimising overflight of highly populated or sensitive areas

Continuous Descent Approaches (CDAs) One effective technique is called Continuous Descent Approach (CDA). This technique keeps aircraft higher and at lower thrust for longer by eliminating the level segments in conventional “step down” approaches.

Significant noise, fuel burn and emissions benefits can result, but there can be potential impacts on air traffic control & flight crew procedures.

'Silent' Aircraft advanced CDA flight trials In order to investigate the potential benefits and challenges with advanced CDAs in the operational system, the SAI Operations team has been coordinating a flight trials programme involving a large number of KIC partners from airports, air traffic control, regulators, operators, and suppliers. A set of advanced CDA procedures were developed for a regional UK airport which also incorporated other low noise “best practice” techniques of Precision Area Navigation (allowing the procedure to be programmed into the aircraft Flight Management System to optimise the approach path) and Low Power/Low Drag (to keep the aircraft in a clean aerodynamic configuration) The procedures comprise a set of waypoints with:

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a lateral profile to allow low population exposure to noise

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vertical constraints which assist the achievement of a CDA vertical profile

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speed constraints designed to achieve low power/low drag

Flight trials of the procedures have been ongoing with a variety of aircraft types. Initial results show promising reductions of noise, fuel burn and emissions, and also indications of areas where further improvements could be sought. Further details will be released shortly.

Future challenges 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. These include market viability, financing, societal acceptance, aircraft certification, as well as the technical challenges of the propulsion system / airframe integration, structural analysis and manufacturability of non-circular pressure vessel, the mechanical design of thrust vectoring and variable area nozzle, and detailed assessment of the low speed aerodynamic performance. The project has also clearly identified these challenges and identified a path to address them.