AIRCRAFT SYSTEM SIMULATION FOR PRELIMINARY DESIGN

AIRCRAFT SYSTEM SIMULATION FOR PRELIMINARY DESIGN
28TH INTERNATIONAL CONGRESS OF THE AERONAUTICAL SCIENCES
AIRCRAFT SYSTEM SIMULATION FOR PRELIMINARY
DESIGN
Petter Krus, Robert Braun, Peter Nordin, and Björn Eriksson
Linköping University, SE-58183 Linköping, Sweden
[email protected]
Keywords: aircraft conceptual design, system modeling, mission simulation
Abstract
Developments in computational hardware and
simulation software have come to a point where
it is possible to use whole mission simulation in
a framework for conceptual/preliminary design.
This paper is about the implementation of full
system
simulation
software
for
conceptual/preliminary aircraft design. It is
based on the new Hopsan NG simulation
package, developed at the Linköping University.
The Hopsan NG software is implemented in
C++. Hopsan NG is the first simulation
software that has support for multi-core
simulation for high speed simulation of multi
domain systems.
In this paper this is demonstrated on a
flight simulation model with subsystems, such as
control surface actuators.
1 Introduction
Traditionally aircraft conceptual design involves
very few aspects of system design. There is,
however, a great advantage if system subsystem
design also can be involved early on. There are
several reasons for that. Modern aircraft are
very compact so system installation is important
to take into consideration, and this cannot
properly be done unless there is a preliminary
design of the systems. Furthermore, energy
efficiency and the impact of subsystem on the
energy efficiency of the whole aircraft are
becoming important in order to select the proper
subsystem concept. The aim here is to allow the
whole aircraft to be simulated with its
subsystem as early in preliminary design as
possible, and then use this model to develop the
design further. In this way the simulation model
becomes a point of convergence for the aircraft
conceptual design as well as the conceptual
design of the subsystem as well as a tool for
further development in preliminary design.
Using
simulation
performance
characteristics at the whole aircraft level can be
evaluated very straightforwardly, and things like
trim drag are accounted for as a byproduct.
Furthermore, as the design progress into sub
system design, systems such as actuation
systems, fuel systems etc, can also be included
and verified together in the actual flight profile.
Using simulation models of whole aircraft with
subsystems, it is then possible to do design
analysis, e.g. sensitivity analysis and trade of
analysis, as well as design optimization. In this
paper this is demonstrated on a flight simulation
model with subsystems, such as control surface
actuators.
In recent years there has been a lot of
development in methods and tools suitable for
simulation of systems. One example is the
Hopsan-NG simulation package developed at
Linköping University. This means for example
that it is possible to model basic aircraft
systems, such as hydraulic system, air system
and fuel system, much more efficiently than
before and that a lot of systems can even be
simulated in real time or faster than real time. In
this paper it is shown how subsystem models
also can be coupled to models of flight
dynamics, propulsion, and flight control, to
produce a more complete aircraft system model.
Such a model can be used already in
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KRUS P., et al.
preliminary design, thus allowing the
preliminary subsystem designs to be designed
concurrently with the aircraft layout.
One problem when dealing with large
complex systems, however, is that most
simulation packages rely on centralized
integration algorithms that scale rather poorly
with respect to system size. For large-scale
systems it is an advantage if the system can be
partitioned in such a way that the parts can be
evaluated with only a minimum of interaction.
The reason that centralized solver dominates is
most likely, that until recently, the typical
system simulation has been of a moderate size.
In this paper, distributed solvers with linear
scaling properties is used, which means that
simulation speeds, orders of magnitude higher
than real time, can be achieved for system
simulation Furthermore, development in single
processor performance is leveling out. On the
other hand, multi-core architectures have
become the norm. However, using conventional
simulation software, the simulation can only use
one core for the simulation, thus not exploiting
that potential. The distributed modeling
approach used in here, however, makes it
intrinsically suitable for multi-core simulation,
and Hopsan already has that capability
implemented.
2 Distributed modelling
A very suitable method for modelling and
simulation of large complex dynamic systems is
represented by distributed modelling using
transmission line elements. The origin of this
concept goes back at least to Auslander 1968 [1]
who first introduced transmission lines (or bilateral delay lines). This method evolves
naturally for calculation of pressures when
pipelines are modelled with distributed
parameters. This approach was adopted for
simulation of fluid power systems with long
lines in the HYTRAN program [2]. already in
the seventies. The method can be generalised to
both mechanical and electrical systems.
A related method is the transmission line
modelling method (TLM) presented by Johns
and O'Brien (Ref. [2]) for simulation of
electrical networks.
Johns and O'Brien pointed out that an
important
aspect
of
modelling
using
transmission line elements is that most of the
numerical errors introduced by an ordinary
solver are avoided. The errors made due to the
introduction of transmission line elements, are
better described as modelling errors.
An attractive feature with this is that laws of
conservation of mass and energy still hold for
the solution, since there always exist a plausible
physical system for the model although the line
lengths may vary compared to the original
system. This also implies that the user may
tolerate a larger numerical error since, generally,
quite large modelling errors are present anyway
(errors of the order of 10% are generally
considered acceptable from an engineering point
of view).
A key feature of the transmission line is the
finite signal propagation speed (speed of sound)
of the signal travelling through the line. This
means that events at one component do not
affect another immediately. Using this approach
the subsystems can be solved independently in
each time step in contrast to using conventional
solver. The implementation of the numerical
solver of differential algebraic equations can
therefore be implemented in the subsystem
rather than at a central level. This approach is
sometimes referred to as distributed modelling
for several reasons. The simulation of waves in
transmission lines means that distributed
parameters are used in the lines, secondly it
allows for distributed solver of differential
algebraic equations in component and
subsystems. There is also the possibility of
using distributed processing by allocating
subsystems to different processors.
In this way the system is divided into
subsystems that generally are of limited size and
the very robust method for DAE:s described
earlier, can be used.
2.1 The unit transmission line
In transmission line modelling the basic
dynamic element is the unit transmission line. In
the Hopsan package this is used to connect
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AIRCRAFT SYSTEM SIMULATION FOR PRELIMINARY DESIGN
different components to each other. In the
general case it can be used to model both
capacitances and inductances. In the Hopsanpackage, however, it is used only to represent
capacitances (oil volumes and mechanical
springs).
q1
q2
p1
p2
Figure 1. Transmission line
The complete set of equations that describes
a lossless transmission line are:
(1)
p1 (t )  p2 (t  T )  Z c q1 (t )  Z c q2 (t  T )
p2 (t )  p1 (t  T )  Z c q2 (t )  Z c q1 (t  T )
(2)
(3)
c2 (t )  p1 (t  T )  Z c q1 (t  T )
(4)
Here c is the wave variables that represent
information that has been transmitted from the
other side of the transmission line. With these,
the following set of equations is obtained.
p1 (t )  c2 (t )  Z c q1 (t )
(5)
p2 (t )  c1 (t )  Z c q2 (t )
p2 (t )  p2 (t  2T )  Z c [q1 (t  2T )  q1 (T )] (7)
Compared to the trapezoidal method for
integration (h is the time step).
1
(8)
yh +t = yt + h  f ut , t + f uh +t , h + t
2
where
y° = f u, t
(9)
These equations are the same if T  h / 2
Here Zc is the characteristic impedance of the
line, p and q are pressures and flows
respectively. T is the time delay in the line. Note
that the main property of these equations is the
time delay they introduce in the communication
between the ends.
Introducing
c1 (t )  p2 (t  T )  Z c q2 (t  T )
An interesting observation is found if c2 in
Eq. (18) is substituted with Eq. (19) and the
outlet at 2 is blocked.
(6)
The relationship between flow entering a
volume and the pressure can be written as:
q
(10)
p° =
C
where C is the capacitance. Identification yields
h
(11)
Z =
c
C
The implication of this is that if we use the
trapezoidal method to integrate pressure in a
volume (capacitance) between two components,
this corresponds to introducing a short pipe
instead of a pure capacitance. This can also be
extended to other domains such as mechanical
and electric.
Laminar restrictor
q1
c1

2Z
2Zcc
c2
Delay h
cr1
ci2
Zc

Delay h
ci1
Zc
cr2
2Z
2Zcc
q2
Figure 2. Block diagram of transmission line.
With this method the system can be
partitioned since there is no direct
communication between the two sides of the
transmission line, there is always a time delay
that can be used to partition the model. This also
means that it is possible to use it for parallel
simulation as in [15], [5], and [4].
3 Differential algebraic systems
In order to solve the dynamics of the individual
components and subsystems any type of solver
can be used. A general approach to represent a
system is to represent it as a differential
algebraic system. This also allows for algebraic
loops.
F x, x° , t = 0
(12)
where x is the variable vector.
However, Eq. (1) implies that the system
essentially has to be written in state space form,
something that may be considered as too
limited. Many relationships are usually given in
3
KRUS P., et al.
transfer function form, which makes it more
natural to allow for higher derivatives. The
system can then instead be expressed as
d n y 
 d y d2 y
,
F y,
, .... ,
, t = 0
d t d t2
d tn 

(13)
2
h  xt + xh + t
A more effective way of using the
trapezoidal rule is to reformulate it in the form
known as the bilinear transform.
d
dt
=
2 1 - q - 1 
h 1 + q - 1 
(15)
where q in this context represents the time
displacement operator such that:
qy  y (h  t )
(16)
Using the bilinear transform in Eq. (4) means
that it can be rewritten as a function G of y and
old states.
G ( y (t ), y (t  h),..., y (t  nh))  0
(17)
When solving the system all the old values y(th)... y(t-n h) can be regarded as constants since
they have already been established in previous
time steps. It is therefore rewritten as:
G ( y (t ), t )  0
∑Gi  yk t
(18)
In order to solve this system of equations in a
numerically stable way, the Jacobian matrix is
needed, which is defined as:
(19)
∑ yj
The equation can then be solved numerically
using Newton-Raphson iteration.
yk +1t = yk t - Jk t- 1 G yk t
This also has the advantage that the variable
vector is reduced, since y is shorter than x, y
contains a subset of the states in x. It should,
however, be pointed out that it is only possible
to impose strong non-linearities (such as
limitations on the state variables) represented in
the y vector. Also all variables that are of any
interest must be included in the y vector,
otherwise they will not be computed explicitly.
In order to solve the dynamic part of the
system in a numerically stable way, the
trapezoidal rule can be used. Using the
trapezoidal rule (or bilinear transform) the time
differential is solved as:
1
(14)
xh + t = xt +
Jijk =
(20)
Since an iterative procedure is used, there is a
potential for performance loss due to the
number of iteration needed to solve the system.
However, since the values from the previous
time step can be used as start values.
y0t = yt - h
J0t = J t - h
(21)
If the system is linear, the system can be
solved in only one iteration, and it is usually
sufficient with only one iteration even for nonlinear systems, especially if a small time step is
used. There are, however, situations when input
signals changes suddenly, e.g. a valve is
changed step wise during one time step that
requires more than one iteration. In practice,
however, it has been found that two iterations
increase the tolerance against non-linearities
dramatically, while a further increase to three
iterations gives only minor improvement. Two
iterations have therefore been found to be
something near to an optimum for almost all
situations. For implementation it is better to use
LU-decomposition rather than using the matrix
inverse of the Jacobian.
Provided the system is reasonably linear
(slow variation of J, Eq. (9) is an A-stable
method. However, in reality, rather large
variations of J can be tolerated. Even pure
discontinuities can also be handled satisfactory
using the above approach, when fixed-time step
is used (as in real-time simulation).
Eq. (9) also illustrates a dilemma associated
with all numerically stable methods. They need
knowledge of the Jacobian, and if the system is
stiff and highly non-linear this must be updated
(and inverted) very often. We also realize that
the computational burden is much more than
linearly dependent on the size of the system.
This makes these methods unsuitable for large
4
AIRCRAFT SYSTEM SIMULATION FOR PRELIMINARY DESIGN
problems. Eq. (9) is, however, very effective for
solving small systems which makes it very
suitable for solving subsystems.
Instead of using equation (9) directly it is
wise to replace the inverse of the Jacobian by
using LU-decomposition instead, there are also
a few other actions that can be done in order to
further enhance the efficiency of the solver.
4 Modelling and simulation of an aircraft
As an example a small unmanned jet power
aircraft is simulated. The control surfaces are
powered by conventional hydraulic servo
actuators. There is an attitude control unit that is
fed by an external altitude controller. There is a
fully non-linear six degree of freedom flight
dynamics model with a non-linear aerodynamic
model.
4 Flight dynamics and aerodynamic model
Figure 3. Scheme of transformations
Figure 3. shows a scheme of the
transformations involved. The differential
algebraic system DAE is transformed into time
discrete form using bilinear transform. This
method needs the Jacobian of the system. This
can, however, be obtained analytically using
symbolic math packages such as Mathematica,
and a model generator has been written in the
Mathematica symbolic manipulating language
called Compgen, which takes the acausal
component description similar to Modelica,
[8][12], and transforms it into a component
model implementation in C++ that can be used
for simulation. This approach is described in
more detail in Ref. [5] (although for generating
Fortran models for the old Hopsan package).
Work is also progressing to integrate this
modeling capability based on Modelica and free
software directly into the Hopsan package.
Packages for symbolic math can also be used
to facilitate the modelling work in other ways,
so that the user can concentrate on formulating
the equations rather than solving them. The
6DOF-flight dynamics model is an ideal
example since it involves a lot of co-ordinate
transforms that can be set up entirely in the
Mathematica environment. Using the Compgen
tool a model for the Hopsan-NG simulation
package can then be generated.
The flight dynamics model is here based on a 6
degree of freedom rigid body model that is
connecteted to a aerodynamic model. The
aerodynamic model can have different number
of wings, with an arbitrary number of control
surfaces, and a body with its characteristics.
Alternatively all wings can be lumped into one
wing if data for a whole aircraft is available.
However, for this case the stall characteristics
will not be correct, since there is only one lift
curve for the whole configuration. The control
surfaces are modeled both with a linear increase
of lift force with deflection and the
corresponding increase in induced drag. There is
also a cross coupling effect of drag for control
surfaces on the same wing e.g. ailerons and
flaps.
Data for the aerodynamic model can be
obtained in the early design phases from panel
codes and/or handbook formulas, or some panel
code. It is here based on a static version of the
model presented in [9], although the unsteady
effects can of course also be included.
Linear region,
(panel code)
Fully separated
flow
Figure 4. Non-linear aerodynamic model.
5
KRUS P., et al.
In this way effect of trim drag on range is
automatically included, and the effect of
reduced weight as fuel is consumed.
.
.
.
where the linearized aerodynamic stiffness and
is calculated as
(25)
M 2
k 

Note that this linearization is only used for
the numerical solver, and does not affect the
steady state value of the moment.
4.1 Flight control
Figure 5. Screen shot of Aircraft system
simulation model.
The connection between the actuators and the
aircraft
control
surfaces
are
through
transmission line elements that models the
aerodynamic stiffness of the control surfaces.
The moment at the actuator link for the elevator,
can be calculated as:
M 1 (t )  c (t  h)  Z c  (t )
(22)
c (t )  M 2 ( , ,  , q)
(23)
Figure 6. Model of aero load on rudder as a
(non-linear) spring.
Here M 2 is the moment from the
aerodynamic forces, which is a function of
surface angle  , aircraft pitch angle  , and
angle of attack  , and dynamic pressure q. M 1
is the corresponding moment at the actuator
hinge, Ideally they are the same, but through the
separation with the transmission line element
they becomes slightly different. M 2 is a nonlinear function and hence not a purely linear
spring. The characteristic impedance is
calculated as
(24)
Z c  hk
For simulation in preliminary design of an
unmanned aircraft the flight control system is
not defined yet. However, in order to perform a
flight simulation it is necessary to have some
control system in and/or flight control system,
or to represent the pilot in case of a manned
aircraft. The requirement on this is that it should
be easy to set up while still give reasonable
performance to evaluate the aircraft at the level
of detail present in preliminary design.
The flight control system is divided into an
attitude control system and an attitude control
system. The attitude control system is cascaded
with the attitude control system. From the point
of view of the attitude control system the plant
will be an integral action system as the altitude
becomes the integral of the attitude time speed
(for small angles), in the low frequence region.
For this kind of system there exist a very simple
controller that is based on a PI controller but
with a lead filter in the feedback path. This is
here referred to as a PI-lead controller. This
produce a pole placement controller with only
real poles it is then only necessary to have the
speed as a design parameter, which is known.
Figure 7. Altitude controller
The parameters are set as:
6
AIRCRAFT SYSTEM SIMULATION FOR PRELIMINARY DESIGN
K1  a / k x
(26)
11  a
21  a / 2
22  a
This yield the closed loop response as:
1
y
s
a
1
yref
(27)
4.2 Subsystem modelling
In order to deal with complexity it is
necessary to be able to model in a hierarchical
way. This is done by introducing subsystems. In
this example the hydraulic actuation system is
one subsystem representing the control surface
actuators and the hydraulic supply unit. The
hydraulic supply system is represented by the
pressure controlled pump, a pressurized tank
(represented by an asymmetric piston in the
model), and an accumulator.
Furthermore, for small attitude angles:
(28)
v
h 
s
Which means that
kx  v
However, in order to avoid very high gains at
low speeds a minimum value of kx is set.
Furthermore, to have realistic altitude
changes, it is useful to put limitations on the
requested pitch angle, both in positive and
negative direction.
The attitude controller is defined as:
uailerons  Kv ( K   limit[ K  , max , max ]) (29)
uelevator  Kv Kelev
urud  Kv ( Krud ( )  Kd  Rb )
where  represents the error in the respective
attitude angles. Rb is the rotational speed around
the vehicle vertical axis that is used for yaw
damping. Of course damping terms on the other
axis can also be introduced if needed, such as
for the pitch angle if the aircraft is an unstable
configuration. Since the gain of the plant (the
aircraft) varies with the dynamic pressure. Kv is
a speed dependent term that is defined as:
Kv 
U 02
U 02  v 2
Figure 8. Model of the hydraulic system with a
constant pressure controlled pump with pressurized
tank (piston) and accumulator.
The servo actuators are also modeled as
subsystems with servo valve, piston and
linkage.
(30)
where U 0 is a low speed that prevents the gain
from becoming very large.
Figure 9. Model of the servo actuator subsystem.
7
KRUS P., et al.
The redundancy has not been modeled here,
but it is very straightforward to include as well.
layer of control have to be implemented in the
form of a mission controller. This is essentially
a event based model based on Functional Flow
Block Diagram (FFB). In this way it is possible
to include a hierarchical description of event
based systems. A transition to the next state is
triggered if the aircraft gets close enough to a
way point. This will generate reference heading,
reference altitude and velocity for the next stage
of the mission.
Figure 10. The aircraft attitudes during an Smaneuver.
Figure 11. Angular position and reference position
of the rudder actuator.
This simulation model is part of an integrated
framework for aircraft design developed at
Linköping University. In order to be part of a
work flow, the system simulation needs to be
able to integrate with other software. HopsanNG models can be exported for simulation
within Matlab Simulink. Furthermore, the flight
dynamics model has a parameter set that is very
general, which means that coefficients can be
obtained from other sources. Data is stored in
XML for simple conversion and import of data.
Figure 12. System model for Whole system
simulation.
4.2 Whole mission simulation
This model can also be simulated throughout a
mission. In addition mission simulation also
need a mission control unit where the flight is
controlled using waypoints. In this way also
performance measures such as fuel consumption
can be assessed. Furthermore, with this kind of
simulation full mission system analysis can be
performed, including sensitivity analysis and
studying the influence of uncertainty in
parameters.
To simulate a whole mission another
Figure 13. Mission controller based on state
machines.
There is also a subsystem to introduce time
acceleration. I sections where attitudes are close
to constant, time is accelerated. This has a
similar effect to variable time step, but the
difference is that the internal time step is
constant but that the rate of states related to
distance traveled and consumed flow are
multiplied by a a large factor (here it is 100)
8
AIRCRAFT SYSTEM SIMULATION FOR PRELIMINARY DESIGN
which means that what determines the
simulation time is essentially the transient part
of the mission.
4.3 Simulation results
Using the simulation model a simulation of a
mission can be performed several times faster
than real time PC. This means that is possible to
use the model for design analysis such as
sensitivity analysis. Furthermore, it is even
possible to use it for optimization of some
design parameters.
Figure 14. Simulated flight path. Altitude and
lattitude as a function of longitude.
relative
influence
on
the
functional
characteristics, such as fuel consumption and
mission time, is indicated.
5 Conclusions
In this paper the use of flight simulation,
including subsystems, for design evaluation of
the whole aircraft in the preliminary design
phase, is demonstrated using the Hopsan NG
simulation package. In this way system
performance and subsystem interaction can be
studied very early. Since efficient, hi-speed
simulation of a complex system is desirable,
robust simulation is a requirement. A key
element is the method of using bi-lateral delay
lines for partitioning of complex system models.
In this way it is possible to use highly robust
distributed solvers on subsystems that are then
connected to each other using the bi-lateral
delay lines, for system simulation. This means
that highly robust models are achieved where
very large time steps can be used. As a
consequence fairly detailed simulation models
can be used for whole mission simulation
already at the preliminary design stage, for
system optimization and design analysis.
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Figure 15. Consumed fuel as a funciton of time.
4.4 Influence of uncertainty in coefficient
values
One way to manage the required accuracy of the
coefficients involved in the simulation is to
study the influence of removing the uncertainty
of one coefficient (uncertainty variable), as in
[16] and [17]. This is done through sensitivity
analysis and is presented in a matrix where the
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