Q: What are the limitations of vortex lattice prediction codes?
A: HASC utilizes the VORLAX code. VORLAX is based on linear lifting surface theory and has several prediction limitations including:
- Linear aerodynamics only.
- Induced drag prediction only.
Profile drag is not predicted by VORLAX but supporting methods for estimation of drag based on integration local section forces due to 2D skin-friction and separation have been included in the HASC Module.
- Geometry must be planar and represented by quadrilateral panels that maintain span-wise and chord-wise consistency such that trailing vortex elements do not overlap panel control points. It is possible that certain air vehicles may be difficult or impossible to fully represent.
- "Louver" control deflections.
Lattice panel deflections are by rotation about the panel center rather than an arbitrary hinge-line and this might lead to an inaccurate prediction of control power.
Q: How accurate are the results produced by SimGen?
A: The accuracy of the results produced by SimGen is dependent both on the method used and on the quality of information supplied to the module. It is very important to consider the level of accuracy that will be sufficient for your application and consider the limitations of the methods supported in SimGen.
The computational method module that is provided with SimGen (HASC Module) utilizes a vortex lattice method to predict aerodynamic forces and moments in the linear regime. For this module, accuracy depends on several factors including, the geometry model of the aircraft, the lattice spacing and density used, and the extents of the prediction. Using a well-crafted geometry model, you can expect the results for the linear angle-of-attack region for small control surface deflections and small angles of sideslip to be within 5-20% of wind tunnel measured values for longitudinal coefficients and within 10-30% for lateral-directional coefficients.
The methodology used in the HASC Module is well suited for predictions for low-speed conventional aircraft (with hinged control surfaces) that do not maneuver aggressively. Such aircraft include light sport aircraft, general aviation aircraft, and small transport aircraft. HASC is not suitable for prediction of forces and moments in the stall and post-stall or in transonic and supersonic flow regions. HASC cannot predict non-linear effects such as those due to advanced high-lift systems and large control deflections.
Shown below is a geometry model for a conventional general aviation aircraft and HASC Module results compared to wind tunnel data from NASA Technical Note D-5857 (Shivers, Fink, and Ware). These results are reasonable and sufficiently accurate for a variety of applications.
Q: Are there other Method Modules available?
A: Currently, the HASC2002 Module is the only method available and it is included with SimGen v1.0. Additional modules are under development including a module for Missile DATCOM (distribution subject to U.S. State Department International Traffic In Arms Regulation [ITAR] restrictions).
Bihrle Applied Research can work with you to develop a module for a particular computational aerodynamics code.
Q: Do I need to know a CAD program to use SimGen?
A: SimGen features a simple 2D drawing and tracing tool called AGMT (Aircraft Geometry Modeling Tool). It is very easy to use.
AGMT is specifically designed to allow users to rapidly create geometry input suitable for use by aerodynamic prediction and analysis codes.
AGMT includes the following features:
- 2D drawing primitives including polyline, Ngon, and ellipse
- Move, stretch, scale, align, mirror, and rotate modifiers
- Viewport independent background images
- Menu, toolbar, and shortcut key access to tools and modifiers
- External interface for geometry export to plug-in modules
Q: Where can I get aircraft drawings?
A: Often you can find three-view style drawings of aircraft on the World Wide Web. Another source are aircraft encyclopedias. You can use photographs if they are close to being planar (not taken at an extreme angle).
If the images you trace in AGMT are consistent in size, then you can properly scale the images using only one known size - typically wing span.
SimGen
Wright Model B Predictive Model
