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geometrical properties (wing span, wing area, mean aerodynamic chord, etc.),
the mass of the airplane, and the moments and products of inertia1,
4. and the matrix GM2, which contains the inertia coefficients.
The exact definitions of these variables can be found in appendix C.
The purpose of MODBUILD is to build these variables and save them to a datafile.
By default, the model parameters will be stored in AIRCRAFT.DAT in the subdirectory
DATA. The saved data can easily be retrieved by means of the utility DATLOAD,
which has been described in section 12.4.2. Since the default FDC distribution already
includes AIRCRAFT.DAT, it will normally not be necessary to run MODBUILD,
unless the datafile is inadvertently deleted. Please do not be mislead by the name
MODBUILD, the utility does not actually ‘build’ complete models, it merely defines
their parameters!
If it is desired to change the values of one or more model parameters (including
mass and/or mass-distribution), the source file MODBUILD.M must be edited,
as MODBUILD does not feature a user-interface to change parameter values interactively.
However, this is less of an inconvenience than it may seem, because the user
is assisted in identifying the individual parameters by the comment lines in MODBUILD.
M, and changing parameters usually won’t be required very often.2 The file
MODBUILD.M has been included in the AIRCRAFT directory, to reflect its close relation
to the aircraft model itself.
It should be realized that the parameter structure used for the SIMULINK model of
the Beaver is only one possible implementation of many. Obviously, there is a direct
relation between the structure of the aircraft-dependent parts in this simulation model
(see chapter 8) and the shape of the parameter matrices. Models of other aircraft
may require different data structures. For instance, many models use a much more
elaborate set of aerodynamic tables than the Beaver model.
Theoretically, it might be possible to create stringent guidelines for the shapes
of data structures, to streamline future model enhancements. However, this has not
(yet) been considered for the FDC toolbox, as flexibility is deemed more important.
The current SIMULINK implementation of the aircraft model uses a generic overall
framework with a series of aircraft-dependent and aircraft-independent blocks that
function as ‘black boxes’.
The exceptions to this rule (‘no formal definition of data structures’) are the geometry
and mass-distribution variables GM1 and GM2, because they are used for the
generic aircraft-independent parts of the aircraft model. The shape of these variables
is in fact stipulated by the structure of the subsystem Aircraft Equations of Motion,
which basically forms the core of the aircraft simulation model.
1The airplane’s mass and the moments and products of inertia are treated as model parameters, as
these properties are assumed to remain constant during the motions of interest.
2If MODBUILD.M is edited to implement new parameter values, it is recommended to save this macro
to a new file, e.g. MODBUILD1.M. If some parameters are changed more frequently, it may be more
convenient to include some degree of user-interaction in the MODBUILD program.
12.6. Model-specific helper functions 219
Since the mass and mass-distribution are assumed to be constant during simulations,
MODBUILD also computes the inertia coefficients from equation (2.29) by substituting
the pre-specified values of the moments and products of inertia into the aircraftindependent
equations from table 2.2. Future versions of the toolbox may feature
continuous computation of the geometrical and/or mass properties during simulations,
which will make it possible to simulate vehicles with non-constant geometry
or significant sudden changes in mass and/or mass-distribution, e.g. airplanes with
a variable wing-sweep or airplanes that drop significant loads in-flight.
In order to implement a different aircraft model within this framework, we must
adapt or replace the aircraft-dependent parts, (e.g. the aerodynamic and propulsive
models), perhaps changing some signal lines too.
There are no formal requirements for the data structure of the parameters for
these aircraft-dependent blocks: any data format that is supported by MATLAB and
SIMULINK may be used, be it scalar constants, matrices, vectors, arrays, structures,
or any combination of these. While it may be useful to use the current implementations
of Aeromod and Engmod (and the corresponding parameter definitions) as a
guideline or starting point, this is not required.
However, it is recommended to re-use at least the MODBUILD.M code that defines
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FDC 1.4 – A SIMULINK Toolbox for Flight Dynamics and Contro(99)
