A prerequisite for realistic prediction of the S&C behavior and sizing of the FCS (Flight Control System) is the availability of complete and accurate aerodata (i.e. the S&C database).
The task is to build a tabular model for the aerodynamic forces and moments on the airframe by simulation. The geometry should be represented in a way to be parameterized by a small number, say O(100), parameters with intuitive interpretation.
Traditionally, wind-tunnel measurements are used to ﬁll look-up tables of forces and moments over the ﬂight envelope but wind-tunnel models become available only late in the design cycle. To date, most engineering tools for aircraft design rely on handbook methods or linear ﬂuid mechanics assumptions. The latter methods provide low cost reliable aerodata as long as the aircraft remains well within the limits of the ﬂight envelope. However, current trends in aircraft design towards augmented-stability and expanded ﬂight envelopes require an accurate description of the non-linear ﬂight-dynamic behavior of the aircraft. The obvious option is to use Computational Fluid Dynamics (CFD) early in the design cycle. It has the predictive capability to generate data but the computational cost is problematic, particularly if done by brute force: a calculation for every entry in the table. Fortunately methods are available that can reduce the computational cost.
There are essentially three issues. The computational models considered here range from handbook methods (USAF Digital DATCOM), through linear singularity methods (Vortex Lattice Method, Panel Methods such as dwfSolve ) to full non-linear Euler and RANS (Reynolds-averaged Navier–Stokes method) compressible ﬂow CFD packages.
Each of the tools has a range of validity which can be exploited to keep the computational cost down:
- The lower ﬁdelity Tier I models are acceptable for low angles of attack and low speed
- Tier I+ Euler model extends the predictable region of the envelope by capturing compressibility eﬀects
- tier II RANS models include also viscous eﬀects. A challenge is to approach automatic volume mesh generation for tier I+, with geometries including control surface deﬂections.
Therefore CEASIOM’s Aerodynamic module develops along with these three elements. A range of computational tools are available into CEASIOM:
USAF digital DATCOM handbook methods is linked to CEASIOM as the basic tier I method.
TORNADO, a vortex-lattice method (VLM) for conceptual aircraft design and education has been integrated into CEASIOM as the main tier I tool. TORNADO allows a user to define most types of contemporary aircraft designs with multiple wings, both cranked and twisted with multiple control surfaces located at the trailing edge. Each wing is permitted to have unique definitions of both camber and chord. The TORNADO solver computes forces, moments, and the associated aerodynamic coefficients. The aerodynamic derivatives can be calculated with respect to: angle of attack, angle of sideslip, rollpitch-yaw rotations, and control surface deflections.
To account for viscous effects, CEASIOM provides a correction to the steady vortex lattice method by the strip theory that combines the linear potential results with the 2D viscous airfoil code XFOIL. A basic unsteady version of TORNADO is currently under development in CEASIOM.
The Tier II geometry models require high-quality surfaces with all relevant details. Such high-quality geometry models can be created by SUMO and sent as IGES ﬁles to full-ﬂedged mesh generator systems such as Ansys ICEM CFD. For existing aircraft, data, including a CAD model, may be available for validation experiments and modification exercises. The approximation of a given CAD geometry by the geo.xml format is not a well defined task. It is currently done ”manually” by the engineer, by extracting cross sections etc. as native SUMO input, or, with even more radical shape approximation, by adapting the O(100) parameters of geo.xml to ”best ﬁt” the CAD surface data.
No tier II CFD tools are currently embedded in CEASIOM because users are mainly interested in coupling their own RANS CFD tools. Therefore, only standard interfaces and ﬁle formats are defined in CEASIOM to which different RANS solvers can be coupled.