# Background

The Tornado code is a vortex lattice method programmed to be used in conceptual aircraft design and in aerodynamics education

The work on the code begun in 1999 at the Department of Aeronautics at the Royal Institute of Technology, in Stockholm, Sweden. The first version was released in 2001 together with the users manual and code description. The work done was a part of the masters Thesis of Tomas Melin, the code developer.

The aircraft geometry in Tornado is fully three dimensional with a flexible, free-stream following wake. Tornado allows a user to define most types of contemporary aircraft designs with multiple wings both cranked and twisted with multiple control surfaces. Each wing may have taper of both camber and chord. The Tornado solver solves for forces and moments, from which the aerodynamic coefficients are computed. Aerodynamic derivatives can be calculated with respect to: angle of attack, angle of sideslip, roll-, pitch-, and yaw- rotations, and control surface deflection. If necessary all of these conditions may be applied simultaneously. Any user may edit the program and design add-on tools as the program is coded in MATLAB (tm), and the source code is provided under the GNU-General Public License.

The core method stems from [Moran], but is modified according to [Melin] in order to accommodate a three dimensional solution and trailing edge control surfaces. The most notable change is the extension of the theory of the horseshoe vortex into the vortex-sling concept. The vortex sling is essentially a seven segment vortex line, which, for each panel, starts in the infinity behind the aircraft, reaches the trailing edge, moves upstream to the hinge line of the trailing edge control surface, then forward to the quarter chord line of the panel in question, going across the panel and then back downstream in an analogous way. The issue of the wake passing trough the geometry at certain flight conditions is resolved by a collocation point proximity detection routine which automatically removes the influence from a vortex thread passing too close to a collocation point.

The code is generally distributed in the hope that it will be useful, but without any warranty; without even the implied warranty of merchantability or fitness for a particular purpose. However, validation comparisons have been made by [Melin] in which the code output is compared with experimental data. The test case is a Cessna 172, for which the Cessna Aircraft Company have released flight test data [Cessna] and for which computation results from AVL and PMarc, a vortex lattice method and a panel method respectively, was available.

# Software Features

Tornado allows for a wide variety of aircraft morphologies, the list below shows some of the available features. Please also have a look in the screenshots section.

Wing layout:
Each wing is built up of quadrilateral partitions with individual: Sweep, dihedral, twist, taper, trailing edge control surface, camber, NACA 4-digits and coordinate library wing profiles.

An aircraft is built up by multiple wings which can have a full 3D orientation with multiple control surfaces on ech wing . The wing may be  cranked and have variable mesh layout. Almost any concievable aircraft geometry can be described with the Tornado program.

Solver features

• Explicit forces in Newton's
• Treffz's plane analysis
• International standard atmosphere
• Velocity in TAS, CAS or Mach
• Compressibility corrections for high subsonic Mach numbers
• Trimmed polars
• Stability derivatives with respect to:
• Pitch and yaw
• Angular rates in pitch, roll and yaw.
• Control surface power derivatives
• Parameter sweep
• Dynamic pitch and yaw derivatives.

# References

[Cessna] L.L Leisher et al, Stability derivatives of Cessna aircraft, Cessna Aircraft company. 1957.
[Melin] Melin.T, Tornado, a vortex lattice MATLAB implementation for Linear Aerodynamic Wing applications", Masters thesis, Royal Institute of Technology (KTH),Sweden, December 2000.
[Moran] Moran, J "Computational Fluid Dynamics", Weily&Sons, 1984
[SP-405] Anon, "Vortex-Lattice Utilization" NASA SP-405. Publication: Vortex-Lattice Utilization. NASA SP-405, 424 pages, published by NASA, Washington, D.C., 197