American Institute of Aeronautics and Astronautics
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45th AIAA Aerospace Sciences Meeting and Exhibit Special Session – Towards A Silent Aircraft
Jan 8-11, 2007, Reno, Nevada
Airframe Design for “Silent Aircraft”
J. I. Hileman
*
, Z. S. Spakovszky
†
, M. Drela
‡
Gas Turbine Laboratory, Massachusetts Institute of Technology, Cambridge, MA, 02139
M. A. Sargeant
§
Cambridge University, Cambridge CB2 1PZ, UK
The noise goal of the Silent Aircraft Initiative, a collaborative effort between industry,
academia and government agencies led by Cambridge University and MIT, demands an
airframe design with noise as a prime design variable. This poses a number of design
challenges and the necessary design philosophy inherently cuts across multiple disciplines
involving aerodynamics, structures, acoustics, mission analysis and operations, and
dynamics and control. This paper discusses a novel design methodology synthesizing first
principles analysis and high-fidelity simulations, and presents the conceptual design of an
aircraft with a calculated noise level of 62 dBA at the airport perimeter. This is near the
background noise in a well populated area, making the aircraft imperceptible to the human
ear on takeoff and landing. The all-lifting airframe of the conceptual aircraft design also has
the potential for a reduced fuel burn of 124 passenger-miles per gallon, a 25% improvement
compared to existing commercial aircraft. A key enabling technology in this conceptual
design is the aerodynamic shaping of the airframe centerbody which is the main focus of this
paper. Design requirements and challenges are identified and the resulting aerodynamic
design is discussed in depth. The paper concludes with suggestions for continued research on
enabling technologies for quiet commercial aircraft.
I. Introduction
HE heretofore unasked technical question what an aircraft would look like that had noise as one of the primary
design variables calls for a “clean-sheet” approach and a design philosophy aimed at a step change in noise
reduction. While the aircraft noise during take-off is dominated by the turbulent mixing noise of the high-speed jet,
it is the airframe that creates most of the noise during approach and landing. To reduce the aircraft noise below the
background noise level of a well populated area, it is clear that the airframe and the propulsion system must be
highly integrated
1
and that the airframe design must consider aircraft operations for slow and steep climb-outs and
approaches to the airfield.
2,3
Furthermore, the undercarriage must be simple and faired, and high-lift and drag must
be generated quietly. A candidate configuration with the above characteristics is the Silent Aircraft eXperimental
design SAX-40, as shown in Figure 1. The conceptual aircraft design uses a blended-wing-body type airframe
4,5
with an embedded, boundary layer ingesting, distributed propulsion system, discussed in depth in a companion
paper.
6
The details of the engine design can be found in Hall and Crichton
7,8
and de la Rosa Blanca et al.
9
The engine
inlets are mounted above the airframe to provide shielding of forward radiating engine noise
10
while the embedding
of the propulsion system in the centerbody enables the use of extensive acoustic liners.
11
As depicted in Figure 1, the airframe design incorporates a number of technologies necessary to achieve the step
change in noise reduction. The all-lifting, smooth airframe was designed for advanced low speed capability to
reduce noise and efficient cruise performance to improve fuel burn. The details of the aerodynamic design are the
focus of this paper and are discussed at length. A simple and faired undercarriage in combination with reduced
approach velocities mitigates the noise generated by unsteady flow structures around the landing gear and struts as
discussed in Quayle et al.
12,13
To achieve the low approach velocities, deployable drooped leading edges are used in
combination with the advanced airframe design. The necessary drag for a quiet approach profile is generated via
*
Research Engineer, Department of Aeronautics and Astronautics, 77 Massachusetts Ave, Member AIAA.
†
Associate Professor, Department of Aeronautics and Astronautics, 77 Massachusetts Ave, Member AIAA.
‡
Professor, Department of Aeronautics and Astronautics, 77 Massachusetts Ave, Fellow AIAA.
§
Ph.D. Student, Engineering Department, Trumpington Street, Member AIAA.
T
45th AIAA Aerospace Sciences Meeting and Exhibit
8 - 11 January 2007, Reno, Nevada
AIAA 2007-453
Copyright © 2007 by The Cambridge-MIT Institute. Published by the American Institute of Aeronautics and Astronautics, Inc., with permission.
American Institute of Aeronautics and Astronautics
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increased levels of induced drag through an inefficient lift distribution over the all-lifting airframe during approach.
This is achieved via a combination of upward deflected elevons and vectored thrust. Although not used on the
conceptual aircraft design presented here, other quiet drag concepts were investigated which are potentially
applicable for conventional aircraft configurations. For example the acoustic signature of perforated drag plates is
reported in Sakaliyski et al.
14
and a novel, quiet engine airbrake concept based on steady swirling flow to generate
pressure drag is discussed in Shah et al.
15
The airframe trailing edges are acoustically treated by deploying brushes
to reduce the airfoil self-noise. This concept is similar to the quiet flight of the owl where the feathers are used to
reduce the flow noise of the wings as reported by Lilley.
6
A noise reduction of about 4 dB was experimentally
demonstrated by Herr and Dobrzynski
17
using trailing edge brushes on a scale model aircraft wing.
The present paper focuses on the detailed airframe design for a step change in noise reduction and improved fuel
burn. More specifically, the objectives are to (1) introduce a newly developed quasi-three dimensional aerodynamic
airframe design methodology based on the above ideas and concepts, (2) validate the methodology using three-
dimensional Navier-Stokes simulations of a candidate airframe design, and (3) define and optimize a conceptual
aircraft design for low noise and improved fuel efficiency by combining the methodology with noise assessment
tools. The resulting conceptual aircraft design, SAX-40, yields a calculated noise level at the airport perimeter of 63
dBA and has the potential for a fuel burn of 124 passenger-miles per gallon, a 25% improvement compared to
existing commercial aircraft. Given the high risk of the technologies used, SAX-40 meets the objectives of a “silent”
and fuel efficient conceptual aircraft design.
The paper is organized as follows. The design requirements and challenges for a “silent” and fuel efficient
aircraft are discussed first. Next, the key features of the aerodynamic airframe design are outlined, elucidating how a
step change in noise reduction and enhanced aerodynamic performance are achieved. The evolution of the airframe
design along with the characteristics of three generations of designs is briefly summarized. The airframe design
methodology and framework used in the last generation of designs is then described in detail. Next, the established
aerodynamic design framework is validated using a three-dimensional Navier-Stokes calculation of a candidate
airframe design. The framework is then used to optimize for low noise and improved fuel efficiency, and the
resulting design, SAX-40, is discussed in detail. Last, the findings and conclusions are summarized and an outlook
on future work is given.
II. Key Challenges and Enabling Concepts
A key airframe design requirement necessary to achieve the approach noise goal is the capability of the aircraft
to fly a slow approach profile. The sound pressure levels of the airframe noise sources scale with 1/r
2
and u
n
where r
Figure 1. Silent Aircraft eXperimental design SAX-40.
221.6 ft / 67.54 m
144.3 ft / 43.98 m
35.4 ft / 10.79 m
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is the distance between source and observer, and u is the approach velocity. The exponent n is 5 or 6 depending on
whether the noise stems from scattering of turbulent structures near edges or acoustic dipoles. The scaling law thus
suggests that the noise at the observer location can be reduced by using a slow approach profile and by landing
further into the runway
3,18
to keep the aircraft at higher altitude when crossing the airport perimeter. This requires a
low stall speed of the airframe and correspondingly increased amounts of drag. The low approach speed determines
the landing field length, which combined with the runway length, sets the threshold displacement. Although the
conceptual design is strongly governed by noise considerations, fuel economy and emission levels must be
competitive with next generation aircraft. This requirement raises the question whether trade-offs between noise and
fuel burn need to be made and, if so, what the potential penalty for noise reduction is. The paper demonstrates that,
by taking advantage of the all-lifting configuration and by aerodynamically shaping the airframe centerbody, both a
reduction in noise and an improvement in fuel burn can be achieved.
A. Major Challenges
The above requirements introduce major design challenges. The first challenge is to achieve competitive cruise
performance while maintaining effective low speed aerodynamic characteristics. For a given aircraft weight, either
the area or the lift coefficient need to be increased during landing to reduce the approach velocity. This demands
variable wing geometry such as for example conventional flaps and slats which are inherently noisy and must thus
be avoided. Circulation control
16,18
is one possible option to achieve enhanced high lift capability without a variable
wing geometry but the impacts of weight and complexity of the flow control system on overall performance and
cruise efficiency need yet to be assessed in detail. The idea adopted here is to avoid this complexity and to
incorporate passive circulation control in the aerodynamic design of the all-lifting airframe by optimally shaping its
centerbody.
In order to achieve the noise goal, the lifting surfaces must be smooth and the undercarriage needs to be simple
and faired. This inherently reduces the drag on approach which poses another challenge in the design of a low noise
aircraft. The drag required for a slow approach profile must be generated in quiet ways. The concept used here is to
increase the induced drag by setting up an inefficient but relatively quiet lift distribution over the airframe during
approach.
Another major challenge lies in trimming and rotating a tailless airframe such as the all-lifting configuration
considered here. Pitch trim and static stability can be achieved without a tail but require reflexed airfoils on the
centerbody.
4
The major drawbacks thereof are a penalty in cruise performance and relatively large control surfaces
and actuation power to facilitate rotation. As discussed next, aerodynamically shaping the leading edge region of the
centerbody enables pitch trim and static stability without the use of reflexed airfoils or canards.
B. Key Airframe Design Feature
It is important to note that the holistic approach and the integrated system design of SAX-40 are crucial to
achieve the noise goal and to improve fuel burn. In this, the all-lifting airframe incorporates a key design feature that
distinguishes the conceptual aircraft design presented here from other blended-wing body type concepts. As depicted
in Figure 1, the leading edge region of the centerbody is aerodynamically shaped and the all-lifting airframe is
optimized to generate a lift distribution that (i) balances aerodynamic moments for pitch trim and provides a 5 to
10% static stability margin while avoiding a horizontal tail lifting surface and reflexed airfoils, (ii) achieves an
elliptical span load on cruise yielding a 15% improvement in ML/D compared to current blended-wing body aircraft
designs, and (iii) increases the induced drag on approach via elevon deflection and vectored thrust, reducing the stall
speed by 28% compared to currently operating airframes.
The in-depth analysis of this advanced airframe design and the underlying aerodynamic characteristics are the
subject of this paper and are discussed next.
III. Airframe Design Evolution
The SAX-40 aircraft design is the culmination of an iterative design process which, in retrospect, evolved from
three major aircraft design generations. In each generation the assessment tools were further developed to improve
fidelity and the redesigns were aimed at closing the gap between the estimated aircraft performance and the design
goals. In conclusion of each of these major design steps, technical reviews were held with the Boeing Company and
Rolls Royce plc. This section highlights the major characteristics and outcomes of the design evolution.
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A. First Generation SAX Design
The first generation of SAX designs utilized a modified version of Boeing’s Multi-disciplinary Design
Optimization code WingMOD
4,19
where the objective function for the optimizer was focused on minimizing takeoff
weight. This design process culminated in the SAX-12 planform,
5
As shown in Figure 2 on the left, the
configuration incorporates four boundary layer diverting Granta-252 engines.
7,8
The cruise altitude, Mach number,
range, and passenger capacity were held constant for SAX-12 and subsequent designs. The aircraft design was
calculated to have an MTOW of 340,150 lb, a fuel burn of 88 passenger-miles per gallon (based on a passenger
weight of 220 lbs), and maximum noise levels at the airport perimeter of 80 and 83 dBA during takeoff and
approach, respectively.
5
Considerable challenges remained before the noise goal could be achieved; chief among
them was the lack of a methodology to optimize the airframe shape for low noise. Thus a clear need was the
capability to define the three-dimensional geometry of the airframe and a novel airfoil stack. The SAX-12 planform
shape, airfoil thickness distribution, minimum cabin size, rear spar location, and mission were carried over as
starting points in the next generation of aircraft design. In addition, WingMOD was used to create the structure
weight response-surface-model that was used throughout the design process.
B. Second Generation SAX Design
The focus of the second generation of SAX designs was the development and validation of a quasi-3D airframe
design methodology with inverse design capabilities. A first version of this methodology was previously reported by
the authors
20
and improvements will be discussed in Section IV. For the second generation of designs, this
methodology was used to achieve a significant reduction in noise by reducing the stall speed. This resulted in
aerodynamic shaping of the centerbody leading edge with supercritical profiles designed for the outer-wing sections.
The design process started with SAX-15 and culminated in the SAX-29 planform, shown in Figure 2 in the center.
This design incorporated a boundary layer ingesting, distributed propulsion system based on three engine clusters.
Each engine cluster consisted of a single gas generator driving three fans. To assess the methodology and the
effectiveness of the centerbody aerodynamics, a three-dimensional Navier Stokes calculation was carried out for the
SAX-29 airframe at Boeing Phantom Works. The details of the analysis are presented in Section V. The quasi-3D
design methodology was successfully validated such that the airfoil profiles and detailed centerbody shape of the
SAX-29 design were used in subsequent airframe designs.
C. Third Generation SAX Design
The third and last generation of designs focused on further refinement of the aerodynamics and the weight
models by taking full advantage of the optimization capability of the design methodology. A gradient based
optimization of the outer wing shape was used to minimize a cost function combining approach noise and fuel burn
as metrics. The outcome of the optimization culminated in the SAX-40 planform, shown in Figure 2 on the right and
discussed at length in Section VI. Similar to the second generation SAX-29 design, SAX-40 incorporates three
Granta-3401 boundary layer ingesting engine clusters. The distributed propulsion system consists of three gas
generators and nine fans. Engine and transmission system design details can be found in de la Rosa Blanco et al.
8
and the integration of the propulsion system into the airframe is discussed in Plas et al.
6
The SAX-40 aircraft design
was calculated to have an MTOW of 332,560 lb, a fuel burn of 124 passenger-miles per gallon (based on a
passenger weight of 240 lbs), and maximum noise at the airport perimeter of 63 dBA.
2,3
Figure 2: Three major generations of conceptual aircraft designs: SAX-12, SAX-20, and SAX-40.
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D. Design Comparison
As the SAX design evolved, significant
gains in ML/D were achieved and the
approach velocity was reduced while
increasing the planform area as tabulated in
Figure 3. Most of the improvement in ML/D
can be attributed to the aerodynamic shaping
and cambering of the centerbody leading
edge which enabled a nearly elliptical lift
distribution. In addition, as shown in Figure
3, the optimization process increased the
planform area, slightly unswept the wings
and grew the span, yielding a reduction in
stall speed. The full optimization of the
three-dimensional airframe geometry
demonstrates that a configuration with both
lowered noise emission and improved fuel
burn can be achieved. This was not clear
prior to the optimization as it was
hypothesized that cruise performance
penalties would have to be incurred for
reduced approach noise.
20
IV. Technical Approach – Quasi-3D Design Methodology
The unconventional airframe configuration yields a highly three-dimensional aerodynamic design problem
which requires a three-dimensional analysis to capture the centerbody aerodynamics. The involved computations are
too costly to fully explore the design space with viscous three-dimensional calculations so a framework with a faster
turnaround time but yet adequate fidelity was developed. Building on previous work by the authors, a quasi-3D
design methodology was refined combining a two-dimensional vortex lattice method with sectional viscous airfoil
analyses and empirical drag estimates of the three-dimensional centerbody, enabling rapid design iterations and
optimization. At every major design change during this iterative process a fully three-dimensional flow assessment
was conducted. A three-dimensional vortex panel method and Euler calculation of the entire airframe were carried
out to assess the loading of the airfoils and shock strength obtained from the quasi-3D design methodology. To
validate the overall framework and procedures, a three-dimensional Navier Stokes calculation was conducted and
the results demonstrated good agreement with the established quasi-3D design methodology. An outline of the
design methodology is given in this section and the details of the validation are discussed in Section V.
The quasi-3D design methodology, schematically shown in Figure 4, can be broken into three main parts, (i)
airframe creation, (ii) cruise performance analysis, and (iii) low-speed performance analysis. The three-dimensional
airframe shape is created from an airfoil profile stack and planform shape. This planform must enclose the spar box
and is assessed over five mission points: takeoff rotation, takeoff climb-out, begin cruise, end cruise, and approach.
The methodology iteratively estimates the aerodynamic performance using the procedure outlined previously by the
authors. The design framework estimates stall and landing speed, landing field length, and elevon deflection / thrust
vectoring requirements for pitch trim during approach and landing. During take-off, the elevon deflection / thrust
vectoring requirements are assessed for rotation, and the aerodynamic performance is estimated during climb-out.
This analysis guided the propulsion system design as described in more detail in Crichton et al.
2
and also provided
an estimate for the airframe noise during take-off and approach.
The aerodynamic design framework discussed in the present paper differs from the previous version in a number
of ways. The wing twist was defined over three segments with non-zero twist at the aircraft centerline, and the wave
drag of the outer wings was estimated using MSES, a compressible, two-dimensional airfoil analysis tool. The
trimmed stall speed of the aircraft was estimated by combining a two-dimensional vortex lattice approach (AVL)
and a viscous airfoil analysis (XFoil). In this approach the aircraft angle of attack and elevator deflection for trim
were iterated until the maximum airfoil sectional lift coefficient was reached.
To improve the assessment of aircraft weight, the following modifications to the weight models were
implemented. The operating empty weight of the aircraft was estimated using an empirical model for the fixed
Figure 3: Evolution of SAX planform and aircraft
performance.