Multi Disciplinary Fuselage Design
At the end of the thirties, light aluminium started to take over the role of wood and steel as
primary materials in aircraft. The all-metal, stressed skin structure became the standard in the
aircraft industry. Simultaneously, the airplane configuration was adapted to the new structural
concept, as were the fabrication methods.
The results of the introduction of aluminium were rather spectacular (figure 1). Within a
relatively short period it became possible to transport more passengers over greater distances,
faster, at higher altitudes and, therefore, with more comfort. The first steps to modern air traffic
were taken. These steps marked the beginning of a long lasting evolution of the aluminium
airplane structure.
This evolution has lead to the current (over)-optimized stiffened skin structure, for which only
extensive protection measures, inspection programs and maintenance programs, can guarantee the
required comfort, reliability and durable safety level. Still, it is tried to further optimise the
aluminium stiffened skin structures at the cost of large investments by, among other ways,
applying new alloys without a clear improvement of the cost efficiency.
Figure 2 shows the technological life cycle of products in general in a simplified manner. This
cycle can be divided in four phases, i.e. development, expansion, consolidation and decay. The
aluminium stiffened skin structure is now in its consolidation phase. To give the development of
aircraft fuselages a new impulse, new concepts will have to be looked for. Only a change to new
combinations of materials and structures and the accompanying changes in manufacturing
technology can realize a jump to a new ‘S-curve’.
Starting point for the choice of new combinations of materials, structural concepts and
manufacturing technology has to be the improvement of the efficiency by integration of
functions. Figure 3 shows schematically which requirements have to be fulfilled during the
development of aircraft fuselage structures. Along the X-axis the most important material and
structural design criteria categories, which fulfilment can be related to the manufacturing of the
structure, are given. The division between X-axis and Y-axis requirements is disputable for some
items but the chosen one is useful to reveal composite design potential. Going from left to right
on the X-axis the level of integration of design features in the structure is increasing. On the Yaxis the level of integral fulfillment of design requirements during manufacturing is increasing
when going up in the diagram. The figure shows the potential for integrating functions of new
materials and structure concepts.
The current design practice is characterized by its sequential methodology. Optimisation is only
done with respect to stiffness and strength. The fulfilment of physical requirements like thermal
and acoustical insulation is done in the end with additional means, e.g. insulating and vibration
reducing means, at the cost of weight and money.
Earlier studies showed that for fuselages weight savings up to 30 % are possible when replacing
aluminium with composite materials. This weight saving can be achieved when the fuselage is
optimised for weight with regard to strength and stiffness alone. Figure 4 shows the fuselage
weight per 10 meter length for the lightest aluminium and carbon/epoxy fuselages loaded with
bending, internal pressure and shear load, optimised for strength and stiffness as function of the
frame pitch. The figure shows that the traditional frame pitch of 500 mm or 20 inches results
more or less in the lightest aluminium fuselage. The figure shows the existence of a design
window, that consists out of composite solutions with a frame pitch and skin thickness
combination that are lighter than the lightest aluminium solution. The potential weight saving can
be either fully exploited or traded off against improved damage tolerance/resistance and/or
improved acoustical and thermal insulation. This research is focussed on the integration of
acoustic and thermal insulation with stiffness and strength.
To improve the acoustic and thermal insulation of a fuselage different techniques can be applied.
A short overview of the considered insulation techniques is given below:
First the conventional insulation method. Conventional fuselages consist out of a stiffened
aluminium skin. The interior panels, which are usually mounted with shock absorbers (to avoid
stiff bridges) to the frames, form together with the skin a double wall. The advantage of a double
wall is that it performs better than a single wall of the same mass per unit area. In between the
skin and the interior panels insulation blankets are placed, which are especially efficient for high
frequency noise and thermal insulation.
A second method is the application of visco-elastic material. These materials have a high
damping factor and therefore can improve the damping performance of a wall. However the
damping properties of visco-elastic materials are highly temperature and frequency dependent.
Therefore they are best applicable on the interior panels.
A third method is the application of an active noise control system. An active noise control
system consists out of monitoring sensors and controlling actuators, which are mounted on the
structure. The sensors pick up the vibration signal that is sent to an electric control unit that
determines the required input for the controlling actuators. The active noise control system is only
applicable in the low frequency range.
A last method that is considered is the adjustment of the fuselage structure like varying the frame
pitch, stiffener pitch or the skin thickness i.e. playing within the design window of figure 4. Also
new fuselage structure concepts can be considered like the sandwich fuselages.
Each of these techniques has its advantages and disadvantages and probably an optimal solution
lies in a combination of these techniques.
To be able to find the optimal fuselage design with respect to strength, stiffness, acoustic and
thermal insulation a Design and Engineering Engine (DEE) is developed. A Design and
Engineering Engine (DEE) is an advanced tool that integrates in one flexible system all the
resources required by the product design process. It mainly consists of a set of properly
interconnected toolboxes such that automated multidisciplinary design and analysis becomes
feasible. The toolboxes can be of different nature and include different pieces of analysis
software, interface blocks, translators, pre and post processors. Figure 5 gives a typical sketch of
a DEE.
First the DEE is developed on panel level. The TU Delft was partner in the ‘SMART panel’
project coordinated by TNO TPD together with the CLC and TNO Industry Technologies from
Eindhoven. In this project the possibilities to integrate an active noise control concept into
composite panels was investigated. Within this project the TU Delft was responsible for a DEE
that can predict the sound transmission loss. An overview of this small DEE for active noise
control on panels is given in figure 6. The DEE starts with the parametric geometry model
generator by using the commercial knowledge based engineering computer package ICAD.
Knowledge based engineering is an engineering method in which knowledge about the product
(techniques used to design, to analyse, to manufacture etc.) is stored in a special product model
which represents the engineering intent behind the geometric design. The engineering intent is
programmed parametrically in design rules.
The starting point of the DEE model is a set of input parameters, which define all dimensions,
material properties and load cases of the panel, monitoring sensors and controlling actuators
(which are modelled as piezo-electric elements). Within ICAD the geometric shape of the model
is programmed parametrically. From the geometric model a meshed model is generated according
to design rules, which are also programmed in ICAD.
With the FEM package ABAQUS a dynamic analysis is performed on the meshed model
delivered by ICAD. With this analysis the dynamic response of the panel to prescribed loads is
determined. These responses are the so-called transfer functions. The evaluation tool MATLAB
is used to determine the transfer functions in the right format from the output data of the dynamic
analysis. The transfer functions are used to calibrate the TL prediction algorithms, which are
developed by TNO TPD. With the calibrated algorithms the maximal achievable sound
transmission loss by active noise control on a panel can be predicted. Experimental tests
performed at TNO TPD showed that the sound transmission loss of a panel could be improved by
+/- 6 dB in the low frequency range. See figures 7 and 8. This is a good result because in the low
frequency range it is difficult to improve the TL. Normally this can only be achieved by adding
mass. This small DEE can be used to find the optimal position of the monitoring sensors and
controlling actuators. At the moment such analysis is performed.
The next step will be to update this ‘SMART panel’ DEE with other analysis modules that
represent the other acoustic and thermal insulation techniques. All these analysis modules will be
performed with ABAQUS.
Finally the panel level DEE has to be upgraded to the fuselage DEE. To model a representative
fuselage the model should address several demands. Therefore the ICAD model of an aircraft
fuselage should be able to handle the following parts: conventional stiffened fuselages
(skin/frames/stiffeners), sandwich fuselages, double walls (skin with an interior panel), insulation
blankets in between the skin and the interior panels, application of damping improving viscoelastic layers, application of piezo electric elements for active noise control, floors, air inside the
fuselage and sound sources.
A fuselage model which should be able to contain all the before mentioned elements can be
simplified to a model that consists out of several basic building blocks. The first building block
can be a panel (Building block P, figure 9), which consist out of the skin (normal skin or
sandwich), frame surfaces and stiffener beam elements. In between the frames 3D solid elements
can be positioned which have air or insulation blanket properties. The inside of the panel can be
filled with 3D-solid air elements. Some parameters for this building block will be ‘switchparameters’ which determine if there are stiffeners, frames, visco-elastic layers, insulation
blankets, etc, yes or no. A second building block can be a floor panel (Building block F, figure 9),
which simply consist out of the floor and some reinforcement beams.
With these building blocks it is possible to model different kinds of fuselage configurations (See
figures 10 and 11). By choosing two floors also fuselages like for the Airbus A380 can be
modelled. By assigning different radiuses to the building blocks P also multi bubble fuselages
models will be possible.
When these models are finished the actual analysis and optimisation can start. By determining the
acoustical and thermal insulation performance for a number of smartly chosen fuselage
configurations, a set of response surfaces can be found. Different parameter studies can be
performed, which should make clear which configuration gives the optimal integrated
performance on strength, stiffness and acoustic & thermal insulation. Probably the optimal
fuselage configuration will be a combination of the different acoustic and thermal insulation
techniques because each technique has its own specific area of optimal performance. Of course
for a real multidisciplinary fuselage design also other design criteria as impact resistance, fatigue
etc will have to be taken into account. This will be done in this study in a more superficial way
but will certainly be a subject for future research. Anyway this tool will make it also possible to
say something about the acoustical and thermal insulation in an early stage of aircraft design.
Figure 1: Improved fuselage design
over the last 70 years. A Fokker F7
steel fuselage with plywood and fabric
covering versus the Airbus aluminium
fuselage.
(source:
www.panamair.org/Aircraft/fokkerf7.htm
www.flugzeugbilder.de
www.waoline.com/Airplane/AirbusBjet.htm)
Figure 2: Technological
life cycle of products.
(source: sandwich fuselage
design. M.J.L. van Tooren)
Figure 3: Design requirements for
fuselage structures.
(source: sandwich fuselage design. M.J.L.
van Tooren)
Figure 4: Fuselage structure
weight as function of the frame
pitch optimised for strength and
stiffness. New materials create a
design window for new solutions.
Lightest
aluminium
fuselage
Design window
Q
stability critical
p
M
Q
M
strength critical
Load case:
M = 4.4e6 Nm
p = 0.055 Mpa
Q = 600 kN
Lightest
carbon/epoxy
fuselage
Concept generator
INITIATOR
others
Cost analysis
Performance
analysis
Structural
analysis
Acoustic
Analysis
CONVERGER
&
EVALUATOR
ICAD MULTI MODEL
GENERATOR
Disciplines silos
Final
configuration
Redefinition of panel parameters
Customer
requirements
ICAD model generator
Panel & Mesh generator
ABAQUS input files
ABAQUS analysis tool
MATLAB data files
MATLAB evaluation tool
MATLAB transfer function files
TL prediction algorithms
TL results
DATA
Figure 5: Paradigm of a Design & Engineering Engine
Figure 6: schematic view of
the DEE set-up for the
SMART panel project
(source: Design and Technology in Aerospace, Flying High,
M.J.L. vanTooren).
Figure 7: Test set-up for active
noise control
Figure 8: Achieved noise
reduction with active noise control
(source: TNO TPD)
(source: TNO TPD)
F
P1
P1
P2
F1
P2
P3
P4
F1
P4
F2
P6
P3
P5
P
Figure 9: Two possible building
blocks for a fuselage model.
Figure 10: Fuselage
configuration with one floor
Figure 11: Fuselage configuration
with two floors