3D VORTEX LATTICE METHOD
The required strength of the bound vortex on each panel will need to be calculated by applying a surface flow boundary condition. The equation used is the usual condition of zero flow normal to the surface. For each panel the condition is applied at the 3/4 chord position along the center line of the panel. The normal velocity is made up of a freestream component and an induced flow component. This induced component is a function of strengths of all vortex panels on the wing. Thus for each panel an equation can be set up which is a linear combination of the effects of the strengths of all panels. A matrix of influence coefficients is created which is multiplied by the vortex strengths and equal to a right hand side vector of freestream effects.
The lift coefficient for the wing at a given angle of attack will be obtained by integrating the panel lift distribution. The lift on a particular panel can be found using the Kutta Law.
Therefore lift of wing is the sum of all panel lifts,
Therefore the lift induced drag of the wing is
Pitching moment about the wing root leading edge can be calculated by summing the panel lift multiplied by a moment arm which extends in the x-direction from the leading edge of the wing to the centre of the bound vortex for the panel.
Software : Vortex Lattice Method Program
The following computer program allows the user to define wing planforms and geometries. The program assumes a linear variation of planform for up to two zones and a simple planar tailplane. Zone 1 covers wing root to specified span location, then Zone 2 covers span location to wing tip. The section properties can vary linearly across each zone between wing root, mid-span and tip or a contant section can be set for each zone. The wing loading is assumed to be symmetric about the wing root. The tailplane can be set at a required distance behind the wing root with a constant chord and sweep angle. A CG position (faction of root chord behind leading) can be set so that calculated Cm coefficients will be relative to this point
The aerofoil section properties for the wing root, mid-span and tip can be specified by entering a chord length and zero lift angle. This will generate a mean line for each section which corresponds to the input section properties. An incidence angle can be set for the tailplane.
Wing geometry variables :
sc – section chord,
tc – tip chord.
The origin is at wing root leading edge and CG location is defined as a fraction of the root chord from leading edge.
The program uses the above vortex lattice method equations to obtain solutions for lift coefficient versus angle of attack, pitching moment coefficient versus angle and induced drag coefficient versus lift coefficient2. For a given angle of attack the program will display the resulting differential pressure coefficient distribution.
- Executable Script : Vortex Lattice Method (3-D)
References & Resources
"Aerodynamics for Engineering Students" Bertin and Smith, Macmillan 1966.
“Extended Vortex Lattice Model – AVL” Marc Drela, MIT,2004