multidisciplinary design and optimization of the silent...

Multidisciplinary Design and Optimization ofthe Silent Aircraft

A. Diedrich, J. Hileman, D. Tan, K. Willcox∗, Z. SpakovszkyDepartment of Aeronautics and Astronautics

Massachusetts Institute of Technology

A “silent†aircraft” is defined to be an aircraft that, in a typical urban area, is inaudi-ble outside of the airport boundary. This paper describes the creation, implementation,and use of an integrated design tool to predict and optimize the performance and costsassociated with producing a novel, commercial aircraft design with a step change in noisereduction. The silent aircraft uses a highly integrated configuration where a quiet propul-sion system is embedded in a Blended-Wing-Body type airframe. This allows the shieldingof forward radiated engine noise and the extensive use of acoustic liners. Multidisciplinaryaircraft design models, which use a combination of simple physics and empirical relations,are adapted for the silent aircraft configuration. These models are used in conjunctionwith a multidisciplinary planform optimization capability. The resulting silent aircraftdesign is assessed in terms of performance and acoustic signature. Significant componentnoise reductions can be achieved with a design that has a fuel burn competitive withnext-generation commercial aircraft. Barriers to achieving the aggressive noise goal ofthe Silent Aircraft Initiative and the associated required technology developments aredescribed.

IntroductionDesigning for noise is a highly integrated problem

that must take into account engine and airframe de-sign, aircraft operation, airline economics, and noisegeneration. This research targets the design of a novelaircraft with a radical reduction in noise – an aircraftthat, in a typical urban area, is inaudible outside ofthe airport boundary. The aircraft design and assess-ment framework described in this paper places noiseas the primary design goal, by bringing together mul-tidisciplinary design tools, noise assessment tools, andinnovative concepts such as a more closely integratedairframe and propulsion system.

Multidisciplinary design optimization (MDO) pro-vides a formal framework which simultaneously consid-ers the effects of different disciplines and their interac-tions. Exploration of the design space via optimizationalgorithms allows high-level design decisions and thequantitative assessment of trade-offs. Studies usingMDO to aid in conceptual aircraft design have beenmade in numerous areas with considerable success.1–4

Venter and Sobieszczanski-Sobieski showed that tra-ditional metrics like aircraft range can be extendedthrough variations that might otherwise seem onlyloosely coupled.5 In their study of transport wing opti-mization, optimal range was achieved not only throughoverall wing shape changes, as would be expected, butalso by a choice of construction technique wherebythe internal spar was removed while the use of skin

∗Corresponding author. 77 Massachusetts Ave. 37-447,Cambridge, MA 02139. Email: [email protected]

†“Silent” in this context does not refer to absence of acousticsources.

stiffeners was incorporated. This exploitation of theinterplay between disciplines (here, aerodynamics andstructures) is typical of MDO in conceptual design.

Another objective that is often minimized in air-craft MDO is maximum takeoff weight (MTOW). Byminimizing MTOW, designers hope to produce aninherently inexpensive aircraft by producing a smallaircraft, while incorporating the fuel weight into theobjective as part of the MTOW, so that both theacquisition costs and the operational costs are low.This technique produces particularly impressive re-sults when the coupling of the disciplines in an air-frame are strong, as in Boeing’s Blended-Wing-Body(BWB) concept.6,7 Wakayama demonstrated thatMDO codes can be used to both balance and reducethe MTOW of an aircraft simultaneously by exploitinggeometric changes to the airframe.4

More recently, environmental considerations havebeen included in MDO-based design approaches. An-toine et al. applied multidisciplinary optimization todetermine the extent to which noise can be tradedagainst other performance measures.8–10 This workshowed that, of the different figures of merit that wereoptimized (takeoff weight, operating cost, noise, ni-trous oxide emissions, and fuel burn), optimization fornoise required the greatest concessions in the otherpotential objectives. For conventional tube-and-wingaircraft with 2020 technology levels, in order to achievea cumulative 15 EPNdB decrease in total certificationnoise, operating costs rose 26%, MTOW rose 27%, fuelload rose 17%, and NOx emissions rose 33% relativeto the aircraft designed for minimum operating costs.

The extremely high cost to reduce noise is indica-

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American Institute of Aeronautics and Astronautics Paper 2006-1323

44th AIAA Aerospace Sciences Meeting and Exhibit9 - 12 January 2006, Reno, Nevada

AIAA 2006-1323

Copyright © 2006 by A. Diedrich, J. Hileman, D. Tan, K. Willcox, Z. Spakovszky. Published by the American Institute of Aeronautics and Astronautics, Inc., with permission.

tive of how poorly suited current jet aircraft are tomodification for radical reductions in noise. Thequestion arises what an aircraft would look like thathad noise as a design goal with minimal penalties inperformance and emissions. The idea of this paperis to combine a multidisciplinary planform optimiza-tion tool with first-principles and empirically-baseddesign and acoustic prediction methods to explore un-conventional, innovative airframe configurations for astep change in noise reduction and competitive perfor-mance.

At the advent of jet aviation, transports made si-multaneous steps towards lower fuel burn and lowernoise by switching to turbofans, and then to highbypass-ratio turbofans. However, the decline in noisegenerated by aircraft has always been partially coun-tered by the increased use of air travel. Now thatreductions in noise per takeoff or landing have slowed,FAA estimates predict that the two factors, increasedoperations and decreased noise per operation, willcompletely offset each other, so that no net progresswill be made in reducing societal exposure to aviationnoise (Figure 2 in Waitz et al.11). The incrementalgains made by improving the acoustic performance ofturbofans and modifying procedures to mitigate noisehave pushed current aircraft configurations close to thelimits of low noise potential.

Innovative aircraft concepts were explored throughan assessment of the feasibility of a functionally silentaircraft by Pilczer12 and Manneville et al.13 This workshowed that many challenges exist, but that a combi-nation of noise reduction technologies integrated on arevolutionary airframe could potentially achieve dras-tic noise reductions, on the order of 22.5 dB on takeoffand 30 dB on approach. These reductions would beachieved through extreme changes to airframe, en-gines, flight paths, procedures, and controls. Theywould also require the clearing of very large techno-logical and regulatory hurdles.

Alternative approaches to achieving dramatic reduc-tions in noise are proposed by Lilley,14 using a com-bination of significant increases in low speed aircraftperformance and exploitation of the inverse-square lawthat governs sound propagation. If the maximum liftcapability of aircraft can be substantially increasedthrough circulation control without increasing noise,then aircraft can decrease their takeoff and touchdownspeeds, increase departure and approach angles, andtake off and land in shorter distances. As a result,aircraft could land and take off near the mid-point ofcurrent runways, thereby maintaining a greater alti-tude outside the airport boundary. Lilley also advo-cates changing land use patterns directly adjacent toairports so that less noise-sensitive industrial-type op-erations can take over the area which is often currentlyoccupied by residences. This land-use change in com-bination with the performance changes detailed above

have the potential to reduce the noise reaching resi-dential areas by 20 dBA.14

Scope of the Paper

The work presented in this paper is unique and dif-ferent in that multidisciplinary planform optimizationis applied to an unconventional airframe configurationwith a closely integrated propulsion system to yield astep change in noise reduction at potentially minimalor no penalty in performance. This is explored throughemployment of innovative low-noise technologies andrevolutionary changes in aircraft configuration com-bined with a rigorous assessment of design trade-offs.The goals of this research are (1) to establish a multi-disciplinary design framework with noise as a primaryobjective, (2) to explore the design space and rig-orously assess the proposed aircraft configuration interms of aerodynamic performance and acoustic sig-nature, and (3) to delineate technological barriers toachieving the step change in noise reduction at perfor-mance levels competitive with next generation aircraft.

In the following section, the configuration of the pro-posed silent aircraft is detailed, followed by a descrip-tion of the multidisciplinary design and noise assess-ment tools. A performance and acoustic assessment ofthe silent aircraft design is then presented. Using thisassessment, conclusions are drawn that provide guid-ance in addressing the key barriers to achieving thegoal of a “silent aircraft”.

The Silent Aircraft ConfigurationIn the context of this work a “silent aircraft” is de-

fined to be an aircraft that, outside of the airportperimeter, is inaudible in a typical, noisy urban area.While noise is the primary design goal, the technologyand design decisions to achieve this ambitious noisedecrease are constrained to those that result in op-erating costs within the range of competition withnext-generation commercial aircraft, such as the 787.

The Silent Aircraft eXperimental (SAX) configu-ration is based upon a Blended-Wing-Body (BWB)aircraft. The BWB is a revolutionary aircraft conceptthat integrates the lifting surface, passenger cabin, en-gine inlets, and control surfaces to achieve large reduc-tions in takeoff weight, fuel burn, and installed thrust.7

The SAX configuration builds upon the aerodynamicand operational benefits of the BWB, and incorporatesdesign changes needed to achieve radical levels of noisereduction, assuming technology levels consistent witha 2030 entry into service date.

Several factors come together to position the SAXas an attractive configuration for a low-noise aircraft.One of the key characteristics is aerodynamic efficiencyof the airframe. An airframe without a separate tailand with smooth lifting surfaces and minimally ex-posed edges and cavities will be an inherently low-noisedesign. In addition, the integration of the fuselage,

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wing, and control surfaces provides an airframe thatachieves increased aerodynamic and structural effi-ciency. These characteristics will help to offset thepotentially higher operating costs associated with thesilent engine design. The SAX configuration alsopresents an excellent opportunity for shielding of en-gine noise.

The large wing area inherent in the SAX configura-tion will help to keep thrust loading (the weight of theaircraft divided by the installed thrust) reasonable, asTable 1 illustrates. Attempts at low-noise optimiza-tion of conventional configurations in the past haveyielded thrust loadings that were relatively low (mean-ing a relatively large engine thrust per unit of aircraftweight), on the order of 2.37 lb/lbf.10 This is the re-sult of using ultra-high bypass ratio (UHBR) engines,which keep takeoff noise low by using low-speed ex-haust velocities, but suffer from very large thrust lapseat altitude. Thrust lapse is the reduction in thrustfrom sea level to cruise as flight Mach number is in-creased.

Table 1 Maximum thrust loadings of various com-mercial aircraft and a BWB design.7,15

Aircraft Thrust loading (lb/lbf)737-700 3.40747-400 3.52

777-300ER 3.32A340-500 3.68

800 pax BWB7 4.45

The SAX also has an inherently high internal vol-ume due to the long, thick center section of the air-frame. This lends itself well to further integration ofthe engine and airframe. Concepts such as boundarylayer ingestion (BLI) are currently being explored andyield reductions in required thrust with benefits in fuelburn. This yields challenges in the design of low-lossinlet ducts, the control of inlet flow distortion, and theintegration of the turbomachinery. The SAX’s centerbody is thick enough to allow moderately-sized highbypass ratio turbofans to be partially embedded intothe trailing edge. The propulsion system of the SAXconfigurations discussed in this paper consists of fourUHBR engines integrated in the airframe. A distrib-uted propulsion system is currently being investigatedfor future designs.

Multidisciplinary Design OptimizationFramework

In order to explore the conceptual design space forunconventional airframe configurations and innovativelow-noise technologies, a design framework with a lowturn-around time needs to be established. As depictedin Figure 1, this multidisciplinary design optimiza-tion framework consists of a suite of noise predictionmethods, a modified version of WingMOD,4 a Boeing

proprietary MDO tool used extensively in the design ofthe BWB, and a set of first-principles and empirically-based aircraft design modules that address design is-sues for the disciplines of propulsion, aerodynamics,structures and weight, while considering mission ele-ments of takeoff, climb, and cruise.

Initial estimates of aircraft weight and performanceare provided by WingMOD to specify an initial design,complete with thrust levels, weight, and estimated fuelload. WingMOD is also used to conduct optimizationof the planform, which is then used as an input to theaircraft design modules. As described in more detailbelow, the WingMOD optimization strategy is selectedto be minimization of MTOW, since an aircraft witha given level of technology becomes quieter as theweight is reduced. The aircraft resulting from the ini-tial sizing process is evaluated over the specified designmission to determine if it has the required thrust ateach mission point and the required fuel to completethe mission. The process then iterates as shown in theloop on the right of Figure 1, considering both designand reserve missions, until the design achieves boththrust and fuel requirements. A design that meetsthe mission and performance requirements is then as-sessed for noise using empirical noise source predictionmodels and a propagation model that accounts for air-frame shielding. Iteration between the acoustic modelsand the aircraft design modules is used to explore thedesign space and complete the conceptual design. Fol-lowing this, a detailed three-dimensional aerodynamicanalysis is conducted beyond those elements shown inFigure 1. This is not within the scope of this paper; adetailed discussion of this topic can be found in Hile-man et al.16 In the following discussion, each of themain elements in the SAX design framework is out-lined.

WingMOD

As described by Wakayama,4 WingMOD is an MDOcode that incorporates performance, aerodynamics,loads, weights, balance, stability, and control consider-ations. WingMOD uses low-fidelity analyses to quicklyanalyze an aircraft over five mission configurations and26 flight conditions.

For this research, WingMOD is used in concert withthe first-principles based aircraft design tools as aplanform source, a weight model basis, and as the em-bodiment of expertise gained by others who have beenresearching blended-wing-body configurations over thepast decade. The SAX design in particular requirescontrol of certain constraints and variables withinWingMOD in order to accurately model the changesmade in moving from a BWB to the SAX configura-tion with an embedded propulsion system. Table 2lists the particular constraints and variables modifiedin order to produce the SAX designs.

WingMOD models the aircraft as a series of span-

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Fig. 1 The SAX design framework comprises three major components: (1) a set of first-principlesand empirically-based modules for aerodynamics, propulsion, weights, and mission, (2) a suite of noiseprediction methods, and (3) a version of WingMOD modified for the SAX configuration.

wise elements. A modified vortex-lattice code andmonocoque beam analysis are coupled to generate sta-tic aeroelastic loads. The aircraft is trimmed at avariety of flight conditions. Loading and induced dragare calculated from the resultant loading data, and arethen combined with profile and compressibility dragfrom empirical models. Structural weight is set byconsidering the maximum elastic loads over all of theflight conditions, and sizing the structure based uponbending strength and buckling stability. Maximum liftis determined using a critical section method, whereinit is assumed that the planform is at maximum liftwhen any individual spanwise element is at its maxi-mum coefficient of lift.4

Acoustic Models

The noise prediction is based upon a set of semi-empirical models that predict both the source strengthand the modification to the acoustic waves as theypropagate from the source. The result of the predic-tions is a footprint of ground noise for a given aircraftconfiguration and location.

The engine noise model consists of four modules: theHeidmann Fan Noise Module,17 the Stone Jet NoiseModule,18 the General Electric (GE) Turbine NoiseModule,19 and the Matta Combustion Noise Module.20

Additional information on the engine design, the pro-cedure for determining its noise emission, as well asrecent advances on its mitigation can be found in worksby Hall and Crichton.21–23

The airframe shielding of forward radiating enginenoise was approximated using a boundary elementmodel of a single monopole placed above the SAX air-frame as described in Agarwal and Dowling.24 Themitigation of engine noise by the acoustic lining of theinlet and exit ducts was estimated using informationprovided by Rolls Royce that was thought to yield aconservative estimate of the noise reduction.25 Workis ongoing to improve upon these estimates.

The airframe noise was computed as the sum of theairfoil self-noise, leading edge slat noise, elevator noise,and undercarriage noise. Airfoil self-noise, which iscreated by the boundary layer scattering at the trailingedge, was estimated using a procedure that combinesaeroacoustic aspects of Ffowcs-Williams and Hall,26

Brooks et al.,27 and Lockard and Lilley28 with bound-ary layer properties estimated from a viscous airfoildesign tool. Slat noise was estimated based upon theaverage wing chord and wing area behind the slatsusing the empirical relationships of Fink.29 Elevatornoise amplitude was estimated using the aileron noisemodel by Guo et al.30 while employing the directivity

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Table 2 Modifications to WingMOD for silent aircraft design.

Constraint/Variable Related RequirementSlat effect Model slat removal

Engine mass Model larger silent engine designEngine location Model embedded engine location, including ductingMission range Explore effect of range upon MTOW and noise

Cruise Mach number Explore effect of cruise Mach number upon MTOW and noiseCruise SFC Model performance of silent engine

Cruise altitude Explore pressure variation for cruiseWingspan Examine effects of different span loadings

Control surface actuation Examine controllability effects of control surface removal

pattern for flap noise by Chinoy.31 The undercarriagenoise was estimated using the prediction method byChinoy31 based on the strut lengths and wheel diam-eters for a pair of four wheel main bogeys and a dualwheel nose unit (the geometric parameters were basedon a comparison to other aircraft of similar size andweight). The noise models by Chinoy are laid out in asimilar manner to ANOPP32 as they are also based onthe correlations by Fink29 but include additional datato improve the empirical coefficients and directivity.

Acoustic energy was propagated from the sourceto the ground using the techniques of Evans.33 Thepropagation assumed geometric attenuation due tospherical spreading, atmospheric attenuation within astill, uniform medium, and attenuation / amplifica-tion of acoustic energy due to passage over and inci-dence onto a grassy surface. One-third octave soundpressure levels were created for a grid of ground loca-tions and a fixed aircraft location. These noise levelswere then converted to overall sound pressure levelsof A-weighted decibels (dBA) as will be shown in theacoustics results presented in the next section.

Aircraft Design Modules

The first-principles and empirically-based aircraftdesign modules build on notes by Liebeck,34 whichthemselves draw upon Shevell35 and Schaufele.36

Those methods are applicable to conventional tube-and-wing aircraft; as will be described in the following,the SAX design tool adapts each model to be appropri-ate for the SAX configuration. Empirical correlationsare used extensively to predict the size, shape, weight,and performance of an aircraft based upon the inputsspecified by the user. A list of these inputs is shownin Table 3. This list of inputs can be specified by theuser in order to produce the aircraft required or theycan be varied by an optimizer in order to drive theoutputs to a specific goal. The planform optimizationis conducted in WingMOD which is then used as aninput to the aircraft design tools. Table 3 also includesdata on the engine installation. This allows the modelto determine the effects of embedding the engine inthe airframe, which is done by changing specific fuelconsumption (SFC), engine drag, and engine weight

models to take account of varying installations.

Table 3 Inputs to the SAX design tool.

InputPayload Weight (pax + cargo)

Systems and Furnishings WeightCruise Mach Number

Cruise AltitudeDesign Range

Planform GeometryNumber of Engines

Engine Installation DragSea-level Static SFCTakeoff Field Length

Propulsion ModuleThe propulsion module is the collection of functions

that size the engine, load performance data, estimatethe engine weight, and predict the engine performanceat varying flight Mach numbers and altitudes. En-gine sizing is done based upon takeoff performanceand climb/cruise thrust requirements. The requiredthrust for takeoff is estimated based upon the require-ment that the airframe be able to accelerate to takeoffspeed, lose thrust in the most critical engine, and stillbe able to brake to a halt on the runway. This isknown as the “Balanced Field” requirement. The re-quired Takeoff Parameter (TOP) must be attained bya combination of the wing loading, takeoff lift coeffi-cient, thrust loading, and the relative air density (airdensity divided by standard sea level air density):

TOP =W/S

σCLT OT/W(1)

where TOP is the takeoff parameter, W/S is the wingloading, σ is the relative air density, CLT O is the takeofflift coefficient, and T/W is the installed thrust perunit weight of the aircraft, which is the inverse of thepreviously defined thrust loading.

Propulsion system performance prediction is donemostly offline. GasTurb 937 is used to produce tablesof available thrust and SFC at various altitudes andflight Mach numbers based upon the assumed cycle

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parameters for the silent engine design. These tablescan then be loaded into the aircraft design frameworkas matrices and used to look up the thrust availableand the SFC at any point in the mission. Figure 2shows a typical GasTurb output of thrust available andspecific fuel consumption as functions of altitude andflight Mach number. In Figure 2, the altitude curvesare the primarily vertical curves labelled with the al-titude from 0 to 18,000 meters (0 to 59,054 feet). Theflight Mach number curves are the primarily horizon-tal curves labelled with the flight Mach number from0.0 to 0.9.

Fig. 2 Gasturb-generated data showing turbofanperformance with altitude, flight Mach number.

Tables of the values in Figure 2 are created and thenloaded into the propulsion module. These tables arethen normalized to the sea-level static (SLS) values ofthrust and SFC. In this way, when the engine size ischanged in the model, the thrust available at altitudecan still be predicted as a fraction of the SLS value,and the engine performance can be predicted with thesame table. The SFC performance can similarly bescaled so that an engine with a better SLS value ofSFC can be used, assuming the variation of SFC withaltitude and flight Mach number is similar.

Aerodynamic ModuleThe aerodynamic module uses a combination of of-

fline and online calculations to estimate the CL versusα curve, CD vs. CL curve, CLmax value, and the spanefficiency, where CL and CD are the lift and drag co-efficients, respectively, and α is the angle of attack.

Lift and induced drag values are calculated offlineusing either AVL or WingMOD. AVL38 is an ex-tended vortex-lattice method that includes aerody-namic analysis, trim calculations, and stability analy-sis. As described above, WingMOD also uses a vortex-lattice aerodynamic model, which yields results con-sistent with those obtained using AVL. In either case,curves are fitted to produce lift as a function of angleof attack and induced drag as a function of lift.

The viscous drag calculation runs in line with theother functions in the framework. The planform is de-fined using a series of spanwise panels. The missiondefinition and a standard atmosphere calculator39 arethen used to determine the flight conditions at cruise.Drag for an individual section of the wing is calcu-lated35,40,41 by first determining the Reynolds numberper unit length Re(c) as

Re(c) = a · Mcruise · ρ/µ (2)

where a is the speed of sound, Mcruise is the cruiseMach number, ρ is the air density, µ is the dynamicviscosity, and c is the chord length. The Reynoldsnumber is then used to calculate the empirical skinfriction coefficient, Cf , as

Cf =0.455

log10(Re(c) · cave)(3)

where cave is the average chord length. A form factorFf , which corrects for differences from flat plate resultsby incorporating the local wing thickness, is given by

Ff = 1 + 1.8(t/c) + 50(t/c)4 (4)

where t/c is the ratio of wing thickness to chord. Fi-nally, the total skin friction drag for a section can beproduced by multiplying all of these factors:

D = q∞(Cf · Ff · Awet) (5)

where q∞ is the dynamic pressure and Awet is the wet-ted area.

Once the induced and skin friction drag have beencalculated, a compressibility correction is added to ac-count for the presence of shockwaves and local stream-tube constriction. This is typically on the order of0.0004 - 0.001 for an aerodynamically efficient aircraftoperating at high subsonic speeds. The aerodynamicmodel only adds this correction during high speedflight. The total drag coefficient is therefore given bythe sum of induced drag, viscous drag, and compress-ibility drag coefficient components.

Weight ModuleThe weight module provides estimates for struc-

tural, payload, systems, propulsion and fuel weights.The payload and systems weights are inputs, thevalues for which are based upon passenger number,configuration, engine size and number, landing gearweight, etc. They are set based upon estimates madebefore the design iteration is started. The propulsionweight is based upon the size of the engine and thetype of installation (podded nacelle, BLI inlet, etc.).Once the static sea-level thrust is set for an iteration,the weight of the engine can be computed from thatthrust value and an estimate of the thrust to weightratio for the engine type. The fuel weight is calculated

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based upon a mission simulation run during each iter-ation.

The structural weight calculation is based on em-pirical formulae, which yields a serious challenge. Agreat deal of empirical data exist for the calculationof weights of wings, tails, landing gear, and everyother component on a conventional commercial air-craft. However, the silent aircraft’s structure hasmany differences from that of conventional commer-cial aircraft. The flying wing design incorporates anon-cylindrical pressurized cabin into the center wing,highly tapered outer wings, and large winglets withcontrol surfaces at the wingtips. These structures willnot be modeled well by existing empirical fuselageand wing weight estimations. For this reason, severalWingMOD designs are used to produce a least-squaresquadratic response surface model (RSM). This surfaceis then used to predict the structural weight of thesilent aircraft. The RSM model takes the form

Wstruct = β0 · y0 + β1 · y1 + β2 · y2 . . . (6)

where Wstruct is the structural weight estimate pro-duced by the model, the βi are constant coeffi-cients, and yi are attributes of the airframe that areknown. The coefficients in (6) are determined usinga least-squares fit to WingMOD data, as described inDiedrich.42

Climb ModuleThe climb module estimates the climb rate of the

aircraft based upon the thrust available, drag pro-duced, and weight of the aircraft at an average pointin the climb which will be defined shortly. This climbrate is extrapolated to determine the fuel, time, anddistance required to reach cruising altitude.

The first step in the climb performance predictionis to set the values of altitude and speed which willbe used to model performance. In this case, a fractionof 20/35 of the cruise altitude is used as the climbaltitude. This is a fraction used by Liebeck34 as beingindicative of average climb performance. The climbvelocity is set to the limit for operations in U.S. ClassB airspace (250 knots). This limit is set because a fairamount of maneuvering is likely to be done in additionto climbing as the aircraft attempts to reach cruisealtitude. Therefore, despite the fact that the aircraftwill accelerate to speeds greater than 250 knots, it islikely to be a good indicator of the speed achievableduring a significant portion of the time the aircraftspends climbing to cruising altitude.

Once the speed and altitude are fixed, the thrustrequired for straight and level flight at these conditionsis calculated as

T rcl = qclfsum +g · mclqclπb2e

(7)

where the subscript cl represents the climb condition.T r is the thrust requirement, fsum is the flat plate

equivalent drag on the airframe, g is the gravitationalconstant, m is the mass of the aircraft, b is the span,and e is the Oswald efficiency factor.

The thrust requirement from (7) is checked againstthe propulsion module’s thrust available estimate atthis altitude and flight Mach number. Once theamount of thrust available is known, the rate of climbcan be computed. This is done by assuming that allexcess specific energy available to the aircraft is con-verted into potential energy in the form of altitude,yielding

ROC = VclT acl − T rclg · mcl

(8)

where ROC is the rate of climb, V is the velocity ofthe aircraft, and T a is the available thrust estimatefrom the propulsion module.

Once the rate of climb is calculated, the time, range,and fuel used for the climb can easily be calculatedusing the following relations:

tcl =hcr

ROC(9)

Rcl = Vcl · tcl (10)

Fcl = tcl · T acl · SFCcl (11)

where tcl is the time required to climb, hcr is the cruisealtitude, Rcl is the distance travelled during climb, Fclis the fuel burned during climb, and SFC is the thrustspecific fuel consumption calculated by the propulsionmodule.

Cruise ModuleOnce the climb module has produced values for the

fuel burned and range covered during climb, the cruisemodule is used to estimate cruise performance with theBreguet range equation

R =L

D

V

SFCln

(W0W1

)(12)

Solving for the unknown final cruise weight yields

W1 = W0e−SF C·RL/D·V (13)

In Equations (12) and (13), R is the distance re-maining to travel after the climb distance has beensubtracted from the nominal range, L/D is the cruiselift to drag ratio, V is the cruise velocity, and W0 andW1 are the cruise beginning and cruise ending weightsof the aircraft, respectively.

The cruise SFC is estimated by the propulsion mod-ule. The cruise velocity is taken from the atmosphericconditions and the cruise Mach number. The cruiseL/D value is computed by the aerodynamic moduleusing an estimated average cruise weight. Once allof these factors are computed, the final cruise weight,W1, can be computed using (13). The estimated cruise

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fuel burn is then computed as the difference betweeninitial and final cruise weight (W0 − W1).

The cruise module outputs the estimated cruise fuelburn, the thrust available during cruise, and the thrustrequired during cruise. These values are checked in themission loop to determine if the aircraft can performthe design mission.

The Silent Aircraft DesignDesign results and acoustic assessment are presented

for an aircraft that is designated SAX12. This aircraftis designed to carry 215 passengers in a multiple class,international configuration a distance of 5,000 nauticalmiles with the lightest possible design while integrat-ing constraints that represent low noise technologies.The choice here to minimize the MTOW is due to anassumption that an aircraft of a given configurationand technology level will become quieter with weightreduction. The planform of the aircraft is shown inFigure 3. The silent aircraft design utilizes the enginedesign of Hall and Crichton,21,22 which uses a BLIembedded engine installation and has a specific fuelconsumption (SFC) of 0.50 lbm/lbf-hr.

Fig. 3 The silent aircraft planform.

The choice of a BLI engine installation dictates toa certain extent the number and location of the en-gines. Figure 4 shows a possible configuration withfour engines mounted on the rear portion of the up-per wing surface using BLI inlets. The choice of foursmall engines as opposed to two large turbofans al-lows for greater embedding of the engines within theairframe. A detailed study on the choice of the propul-sion system and installation issues can be found in Halland Crichton.21 The mounting of the engines near thetrailing edge is a byproduct of the SAX design. Theengines effectively balance the airframe by offsettingthe weight of the payload and associated furnishingsand systems. The mounting location also affords ex-cellent BLI potential, as the boundary layer is fullydeveloped at the rear of the wing, so ingestion has the

greatest effect. The long exhaust ducts allow for linersto reduce rearward propagating fan and turbine noise.

Fig. 4 A 3-D rendering of the silent aircraft43

[cited Jan. 2005].

Table 4 SAX12 key performance figures.

Parameter ValuePlanform Area (ft2) 8,114

Wingspan (ft) 192MTOW (lb) 340,151

Begin Cruise L/D 21.9Cruise Mach number 0.80

Initial cruise altitude (ft) 40,000Thrust loading (lb/lbf) 2.66Wing loading (lb/ft2) 41.9

Fuel Burn (gal/nm/pax) 0.0114

Table 4 lists performance figures for SAX12 and re-veals several key results that stem from the low noiserequirements placed upon the SAX configuration. Thethrust loading for SAX12 is quite low, meaning thatthe installed thrust is high. Table 1 lists an 800-passenger BWB as having a lower installed thrust perpound of aircraft than any other aircraft in the table;however, as a result of the low noise requirements ofSAX12, the installed thrust is very high. This stemspartially from the choice of a UHBR turbofan. Sucha powerplant will achieve low jet velocity at takeoff,producing good acoustic performance, but will sufferfrom very large thrust lapse. The engines are sized bythe required thrust at top of climb, not by the requiredtakeoff thrust.

The silent aircraft design range was determinedbased on the results of a trade study performed withWingMOD. SAX aircraft designs were created forranges from 4000nm to 8000nm. In each case, the air-craft was designed to minimize MTOW. The fuel burnestimates, defined as design fuel minus reserves perdesign range and passenger capacity, of the resultingaircraft are plotted in Figure 5. Also plotted in Fig-ure 5 are the data for part of the existing fleet.44–46

The vertical lines of the data from Lee et al. show thevariation in fuel burn reported by the airlines. Thefuel burn estimate for the Boeing 787-3 aircraft wasbased upon the payload-range diagrams within Boe-ing’s latest airport planning guide, which show the

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787-3 burns 83% of the fuel of a Boeing 767-300 forthe same payload and range.47 The silent aircraft de-sign labelled SAX10 corresponds to a range of 4000nmand an earlier engine design with SFC of 0.54 lbm/lbf-hr. A detailed performance assessment of the SAX10design is given in Diedrich.42 All other SAX designsin Figure 5 use an engine with SFC of 0.50 lbm/lbf-hr.

The absolute noise level associated with the goal toreduce noise below the background noise outside theairport boundary is more easily achieved for smallersized aircraft. This suggests that the design rangespecification should be set as low as possible. However,Figure 5 shows that, for the silent aircraft configura-tion, an increase in range from 4000nm to 5000nm canbe achieved with very little increase in fuel burn. Thisis due to the fact that excess fuel volume is availablefor the 4000nm design, so that additional range can beachieved with very little increase in operating emptyweight (OEW). Figure 6 shows further that the lowfuel burn of the SAX designs is achieved by a highML/D (Mach number times lift-to-drag ratio) andthe relatively low SFC of the engine, both of whichcounteract the relatively high empty weight fraction(OEW/MTOW). For comparison, the ML/D, SFC,and empty weight fractions for a Boeing 777 are 15.5,0.55 and 0.49, respectively.46 The SAX12 fuel burn es-timate assumes no impact on thrust due to BLI; thisshould yield a conservative estimate.

Fig. 5 Comparison of the existing fleet fuelburn44–46 and projected 787-3 fuel burn47 to po-tential silent aircraft planforms of varied range andengine SFC.

The acoustic data in Figures 7, 8 and 9 show anoise assessment of SAX12. The noise contours onthe ground are plotted for the sideline position (mea-sured on takeoff), the takeoff flyover position (whenthe aircraft is about to cut back throttle position),and the approach position (when the aircraft passesover the airport boundary on approach to landing).For each position, a breakdown of the individual noisecomponents is also plotted along the trajectory. Forcomparison, the FAA reports sound levels of 76.4 and

Fig. 6 Impact of design range on empty weightfraction (OEW/MTOW) and cruise performance(ML/D).

89.3 dBA, respectively, for a Boeing 767-300 at take-off and approach certification points.48 This is a goodchoice of aircraft for comparison given its similar sizeto SAX12.

Figures 7, 8 and 9 show that, while many of thenoise components exhibit large reductions in compar-ison with current transports of the size of SAX12, theoverall noise contours on the ground do not achievethe stated goal of an aircraft inaudible outside theairport perimeter. The component noise breakdownsshow that significant aircraft noise reduction can onlybe achieved if all sources of noise are reduced dra-matically. As such, the data highlight the remainingbarriers that must be addressed to achieve a silentaircraft. These barriers will be overcome through con-figuration choices and the incorporation of low-noiseengine and airframe technologies, as discussed in moredetail below.

For the engine noise, the jet (engine exhaust) noiseis very low for both takeoff flyover and approach posi-tions. On takeoff, this low jet noise is achieved throughthe innovative silent engine design, which employs avariable exhaust nozzle.21,22 The noise from the fanexceeds the required levels, and is currently being ad-dressed through an engine redesign.23

Figures 8 and 9 show high airframe noise levels forthe SAX12 design. SAX12 utilized a conventional ap-proach trajectory of 75 m/s with a 3 degree glide slopewith conventional slats, large elevators and a tradi-tional undercarriage (four-wheel main bogeys). Sig-nificant noise reduction should be possible through theuse of a slow and steep glide slope, a faired undercar-riage, unconventional leading edge devices for high lift,and thrust vectoring for pitch trim (instead of eleva-tors). Research to incorporate these technologies tothe silent aircraft design is ongoing. Further detailson airframe noise reduction for the silent aircraft can

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Fig. 7 SAX12 sideline noise estimate (OASPL, dBA) near the airport perimeter (dotted lines) with theindividual noise components broken out along the trajectory.

Fig. 8 SAX12 takeoff flyover noise estimate (OASPL, dBA) as the aircraft passes over the airportperimeter (dotted lines) with the individual noise components broken out along the trajectory.

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Fig. 9 SAX12 approach noise estimate (OASPL, dBA) as the aircraft passes over the airport perimeter(dotted lines) with the individual noise components broken out along the trajectory.

be found in Hileman et al.16

Conclusions

A multidisciplinary design framework has been de-scribed that enables design of a novel aircraft configu-ration with a step change in noise reduction. Throughcareful selection of aircraft configuration and missionparameters, a design is achieved that has significantlyreduced noise compared with current commercial air-craft of similar size, but fuel burn competitive withnext-generation aircraft, such as the 787. While thecurrent design does not achieve the stated goal of be-ing inaudible outside the airport perimeter, acousticassessments demonstrate the further technology devel-opments that are necessary. In particular, research isongoing to reduce engine fan noise through an engineredesign, and to reduce airframe noise through a com-bination of operational procedures and quiet airframetechnologies.

Acknowledgments

The authors acknowledge the contributions of theSilent Aircraft Initiative researchers at CambridgeUniversity and MIT, and funding from the Cambridge-MIT Institute. We also gratefully acknowledge col-laborations with the Blended-Wing-Body design team,and in particular, the technical guidance of R. Liebeck,S. Wakayama, and R. Gilmore of Boeing PhantomWorks.

References1Kroo, I., “MDO applications in preliminary design: sta-

tus and directions,” AIAA Paper 97-1408, presented at 38thAIAA Structures, Structural Dynamics, and Materials Confer-ence, Kissimmee, FL, April 7-10, 1997.

2Sobieszczanski-Sobieski, J., Bartelemy, J.-F., and Giles, G.,“Aerospace engineering design by systematic decomposition andmultilevel optimization,” Proceedings of 14th Congress of theInternational Council of the Aeronautical Sciences, Toulouse,France, 1984.

3Sobieszczanski-Sobieski, J. and Haftka, R., “Multidiscipli-nary aerospace design optimization: survey of recent develop-ments,” Structural Optimization, Vol. 14, No. 1, 1997, pp. 1–23.

4Wakayama, S., “Blended-Wing-Body Optimization Prob-lem Setup,” AIAA Paper 2000-4740, 2000.

5Venter, G., and Sobieszczanski-Sobieksi, J., 2002, “Multi-disciplinary Optimization of a Transport Aircraft Wing usingParticle Swarm Optimization,” AIAA Paper 2002-5644.

6Liebeck, R. H., Page, M. A., and Rawdon, B. K., 1998,“Blended-Wing-Body Subsonic Commercial Transport,” AIAAPaper 1998-0438.

7Liebeck, R. H., “Design of the Blended Wing Body Sub-sonic Transport,” Journal of Aircraft, vol. 41, no. 1, pp. 10–25,Jan. 2004.

8Antoine, N. and Kroo, I., “Aircraft Optimization for Mini-mal Environmental Impact,” Journal of Aircraft , Vol. 41, No. 4,2004.

9Antoine, N., Aircraft design for minimal environmentalimpact , Ph.D. thesis, Stanford University, June 2004.

10Antoine, N. E., Kroo, I. M., Willcox, K., and Barter, G.,2004, “A Framework for Aircraft Conceptual Design and Envi-ronmental Performance Studies,” AIAA Paper 2004-4314.

11Waitz, I. A., Lukachko, S. P., and Lee, J. J., 2003, “MilitaryAviation and the Environment: Historical Trends and Compar-ison to Civil Aviation,” AIAA Paper 2003-2620.

12Pilczer, D., Noise Reduction Assessments and PreliminaryImplications for a Functionally Silent Aircraft , S.M. Thesis,

11 of 12


Massachusetts Institute of Technology, Cambridge, MA, June2003.

13Manneville, A., Pilczer, D., and Spakovszky, Z., “NoiseReduction Assessments and Preliminary Design Implications fora Functionally-Silent Aircraft,” AIAA Paper 2004-2925, 10thAIAA/CEAS Aeroacoustics Conference , Manchester, UnitedKingdom, May 10–12, 2004.

14Lilley, G. M., May 2004, “A Quest for Quiet Commer-cial Passenger Aircraft for Take-off and Landing,” AIAA Paper2004-2922.

15Jane’s Information Group Ltd, Jackson, P. (Editor), 2004,“Jane’s All the World’s Aircraft 2004–2005,” Thanet Press Ltd.

16Hileman, J.I., Spakovszky, Z.S., Drela, M. and Sargeant,M., “Aerodynamic and Aeroacoustic 3D Design for Silent Air-craft,” AIAA paper 2006-0241, 44th AIAA ASM Conference,Reno, Nevada, 2006.

17Heidmann, M.F. , 1975, “Interim Prediction Method forFan and Compressor Source Noise,” NASA Report TM X-71763.

18Stone, J.R. , 1974, “Interim Prediction Method for JetNoise,” NASA Report TM X-81470.

19Matta, R.K., Sandusky, G.T., Doyle, V.L. , Feb 1977,“GE Core Engine Noise Investigation – Low Emission Engines,”FAA-RD-77-4.

20Emmerling, J.J, Kozin, S.B., Matta, R.K. , Mar 1976,“Core Engine Noise Control Program. Volume III, SupplementI – Prediction Methods,” FAA-RD-74-125, III-I.

21Hall, C.A. and Crichton, D., “Engine and Installation Con-figurations for a Silent Aircraft,” ISABE-2005-1164, Proceedingsof International Symposium on Air Breathing Engines, Munich,Germany, 2005.

22Hall, C.A. and Crichton, D., “Engine Design studies fora Silent Aircraft,” GT2006-90559, to appear in Proceedings ofASME Turbo Expo, Barcelona, Spain, May 2006.

23Crichton, D., Xu, L., and Hall, C.A., “Preliminary FanDesign for a Silent Aircraft,” GT2006-90564, to appear in Pro-ceedings of ASME Turbo Expo, Barcelona, Spain, May 2006.

24Agarwal, A. and Dowling, A. P., “Low Freqency AcousticShielding of Engine Noise by the Silent Aircraft Airframe,”AIAA 2005-2996, 11th AIAA/CEAS Aeroacoustics Conference,Monterey, California, 2005.

25Law, T., “Personal communication,” January 2005.26Ffowcs Williams, J. and Hall, L., “Aerodynamic Sound

Generation by Turbulent Flow in the Vicinity of a ScatteringHalf Plane,” Journal of Fluid Mechanics, Vol. 40, 1970, pp. 657–70.

27Brooks, T.F., Pope, D.S., Marcolini, M.A., Jul 1989, “Air-foil Self-Noise and Prediction,” NASA-RP-1218.

28Lockard, D. and Lilley, G., “The Airframe Noise ReductionChallenge,” NASA TM-2004-213013, 2004.

29Fink, M.R., Mar. 1977, “Airframe Noise PredictionMethod,” FAA-RD-77-29.

30Guo, Y., Yamamoto, K., and Stoker, R., “Component-Based Empirical Model for High-Lift System Noise Prediction,”Journal of Aircraft , Vol. 40, No. 5, 2003, pp. 914–922.

31Chinoy, C., “Airframe Noise Prediction,” Engineering Sci-ences Data Unit (ESDU) Airframe Noise Prediction Manual,Item No. 90023 Amendment C, ESDU International plc, Lon-don, UK, June 2003.

32Zorumski W.E. , Feb. 1982, “Aircraft Noise Prediction Pro-gram Theoretical Manual, ” NASA Report TM-83199, Part 2.

33Evans, P., “An Introduction to Aircraft Noise Lateral At-tenuation,” Engineering Sciences Data Unit (ESDU) LateralAttenuation Manual, Item 81035, ESDU International plc, Lon-don, UK, November 1981.

34Liebeck, R. H., Sep. 2002, Lecture Notes from MIT Class,16.885J: Aircraft Systems Engineering.

35Shevell, R. S., 1989, “Fundamentals of Flight, Second Edi-tion,”, Prentice Hall, Inc.

36Schaufele, R. D., “The Elements of Aircraft PreliminaryDesign”, Aries Publications, 2000.

37Kurzke, J., 2001, “A Program to Calculate Design andOff-Design Performance of Gas Turbines,” GasTurb 9 User’sManual.

38Drela, M., http://web.mit.edu/drela/Public/web/avl/.39Kroo, I. M., Standard Atmosphere Computations, Apr.

1997, [Cited Jan 2004], http://aero.stanford.edu/stdatm.html.40Hoerner, S. F., 1981, “Fluid Dynamic Drag: Practical

Information on Aerodynamic Drag and Hydrodynamic Resis-tance,” Hoerner Fluid Dynamics.

41von Karman, T., 1934, “Turbulence and Skin Friction”,Journal of the Aeronautical Sciences, vol 1, no 1.

42Diedrich, A., The multidisciplinary design and optimiza-tion of an unconventional, extremely quiet aircraft , Master’sthesis, MIT, February 2005.

43The Silent Aircraft Initiative, [Cited Jan. 2005], “SilentAircraft Initiative ...a new approach,” http://silentaircraft.org.

44Jenkinson, L.R., Simpkin, P. and Rhodes, D., Civil AircraftDesign, AIAA Education Series, Reston VA, 1999.

45Jenkinson, L.R., Simpkin, P. and Rhodes,D., “Web Database for Civil Jet Aircraft Design,”http://www.bh.com/companions/034074152X/appendices/data-a/default.htm, Butterworth-Heinemann, UK, 1999.

46Lee, J.J., Lukachko, S.P., Waitz, I.A. and Schafer, A., “His-torical and Future Trends in Aircraft Performance, Cost andEmissions,” Annu. Rev. Energy Environ, Vol. 26, pp. 167200,2001.

47The Boeing Company, “The Boeing 787 DreamlinerAirport Compatibility,”http://boeing.com/assocproducts/aircompat/plan manuals.html,Sept. 2005.

48Federal Aviation Authority, AC36-3H, “Esti-mated Airplane Noise Levels in A-weighted Decibels,”http://www.faa.gov/about/office org/headquarters offices/aep/noise levels,Apr. 2001.

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