concept design of an ultra low noise

8/14/2019 Concept Design of an Ultra Low Noise

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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 ofan 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

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 offand during climb, and then be optimised for minimum fuel burn in cruise

throughout the climb, the thrust, nozzle settings and climb rate are optimised

for low noise, subject to meeting legal requirements

long engine exhaust ducts provide space for extensive acoustic liners to

absorb the sound


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On approach, the airframe on a conventional aircraft is as noisy as the engines. Onthe 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. Thereare 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 manydisciplines 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


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Design for low fuel burn

The airframe is highly efficient. It is an all-lifting design. Although based initially onthe 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 atail, 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 otherall-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 theflow 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-bodyas well as the wings. The airframe makes use of a novel centre-body

shape with leading edge carving. This balances the aerodynamic forceswithout 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: 20.1Boeing PW BWB ML/D : 17-18

Boeing 777 ML/D : 17.0

ML/D = (Mach number) X (Lift) (Drag)


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1. Centrebody with leading edgecarving

2. Winglet rudder

3. Elevons

4. Centrebody boundary layeringested

5. Thrust vectoring, variable areanozzle

6. Deployable drooped leading edge

7. Faired undercarriage

Cruise:

Mach number: 0.8Altitude: 40,000 45,000 ft

Lift/Drag: 25.1 23.5Static margin: 5.9% 9.5%

Span: 221.6 ft (incl. winglet)

Gross Area: 8,998 ft2

MTOW (Maximum Take-Off Weight):

332,560 lbs


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Centre of Gravity travel: 0.4 m

OEW (Operational empty weight): 207,660

lbsStructure: 104,870 lbs

Payload: 51,600 lbsFuel: 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 distributedpropulsion system is designed to ingest the boundary layer on the aircraft

centrebody which reduces the fuel burn. The multiple small fan design is easier toembed in the airframe, and leads to reduced weight and nacelle drag. It also

enhances boundary layer ingestion, thereby improving fuel efficiency, and the low fantip speeds lead to low noise. The engine has an ultra-high bypass ratio 18.3 at take-

off 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 stageLow Pressure Turbine

5. transmission system to transitpower from Low Pressure Turbine


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to Fans

6. 3 high capacity,

low speed Fans

Fan Diameter: 1.20 mEngine 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

ratio48.8 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, 40degreesbehind (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)
http://cdn.streamcdn.com/realplayer.rpm?mt=cmi&fn=FLYOVER_behind.mp3http://cdn.streamcdn.com/realplayer.rpm?mt=cmi&fn=FLYOVER_behind.mp3http://cdn.streamcdn.com/realplayer.rpm?mt=cmi&fn=FLYOVER_behind.mp3http://cdn.streamcdn.com/realplayer.rpm?mt=cmi&fn=FLYOVER_behind.mp3


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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 engine3. 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: aperimeter 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 LondonHeathrow 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

Extensive liners

Airframe shielding

Airframe design for enhanced low speed performance
http://cdn.streamcdn.com/realplayer.rpm?mt=cmi&fn=FLYOVER_ahead.mp3http://cdn.streamcdn.com/realplayer.rpm?mt=cmi&fn=FLYOVER_ahead.mp3http://cdn.streamcdn.com/realplayer.rpm?mt=cmi&fn=FLYOVER_ahead.mp3http://cdn.streamcdn.com/realplayer.rpm?mt=cmi&fn=FLYOVER_ahead.mp3


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Cut-back noise estimate

Challenge at Outset:

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

Solution:

Takeoff power management and variable area nozzle

Extensive liners

Airframe shielding

Airframe design for enhanced low speed performance


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SAX-40 noise overview

Temperature: ISA+12?C

Approach noise estimate

Challenge at outset:

Airframe, fan, and turbine noise.


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Solution:

No flaps or slats.

Displaced threshold.

Undercarriage fairing.

Airframe design for enhanced low speed performance.

Deployable drooped leading edge.

Low noise LPT design.

Trailing edge brushes.

Low engine idle thrust.


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What is the predicted fuel burn of the 'Silent' Aircraft?

In addition to quiet, analysis suggests high fuel efficiency.

passenger miles per

UK gallonMach * Lift/Drag

SAX-40 ~149 20.1

Toyota Prius Hybrid

Car~144 w/ 2 people -

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.


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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 theflaps (labelled B below), slats and undercarriage (labelled A below) is unsteady and

generates sound (Image showing the strength of the sound sources, courtesy ofNLR). The jet noise contribution is labelled C.


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What can we do about these noise sources?

We will not achieve our noise target with engines hanging underneath wings - weneed 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 thewings, the sound tends to be

reflected downwards.

(video to follow on 7 November)

The noise made by engines abovethe wings is shielded from

listeners on the ground.

(video to follow on 7 November)

We can also use extensive acoustic liners in the inlet and exit engine ducts to absorbengine noise.

All airframe noise sources are cut by reducing approach speed, and so there arebenefits from flying the final approach more slowly.

There are also benefits from reducing the engine fan speed and the jet velocity, sincetheir noise increases significantly with speed.


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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 aspossible (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 segmentsin conventional step down approaches.


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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 trialsprogramme 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 AreaNavigation (allowing the procedure to be programmed into the aircraft Flight

Management System to optimise the approach path) and Low Power/Low Drag (tokeep the aircraft in a clean aerodynamic configuration)

The procedures comprise a set of waypoints with:

a lateral profile to allow low population exposure to noise

vertical constraints which assist the achievement of a CDA vertical profile

speed constraints designed to achieve low power/low drag


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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 alsoindications of areas where further improvements could be sought.

Further details will be released shortly.


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Future challenges

The Silent aircraft is currently a conceptual design. There are manychallenges 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, themechanical design of thrust vectoring and variable area nozzle, and detailedassessment of the low speed aerodynamic performance.

The project has also clearly identified these challenges and identified a path toaddress them.

concept design of an ultra low noise

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