What is a vortex lattice code?
2-D case – approximating an airfoil as a flat plate at some angle of attack with a vortex filament, with strength , at the ¼ chord position and a control point at the ¾ chord position
pos t o a d a co t o po t at t e ¾ c o d pos t o (Weissinger’s approximation).
The lift (L’) and down wash (wi) and corresponding down wash velocity can be computed using the “Biot-Savart Law” (See McCormick(5) or Dreier(6) for more detail)
To produce a chord wise lift distribution simple add more vortex filaments and control points
filaments and control points
[5] McCormick, B.W., “Aerodynamics, Aeronautics and Flight Mechanics,” Wiley, New
3-D Aerodynamics: AVL Vortex Lattice Code
What is a vortex lattice code?
3-D case – Expanding on the infinite flat plat assumption to a finite thin wing can be derived assuming two wing tip vortices extending from the wing quarter chord of each aft connected e te d g o t e g qua te c o d o eac a t co ected with a bounding vortex along the wing ¼ chord. Resulting in
“Horse Shoe” vortex with strength .
Through summing the wash effects of each vortex at a central control point at mid span and ¾ chord the lift and induced drag control point at mid span and ¾ chord the lift and induced drag for this wing can be approximated for small angles of attack.
Moving this lift vector to the ¼ chord produces the wing pitching moment.
3-D Aerodynamics: AVL Vortex Lattice Code
What is a vortex lattice code?
To produce a span wise lift distribution add more horse shoe vortices at the wing ¼ chord
To produce a chord wise and span wise distribution add horse shoe vortices at various chord locations
3-D Aerodynamics: AVL Vortex Lattice Code
How to operate AVL: AVL is a command driven (similar to x-foil) which reads geometry and weight data from data files.
Getting Started: place “avl.exe” in the “runs” folder and follow the. Double click avl.exe
t e oub e c c a e e
The remainder of this introduction is a visualization of the
“session1.txt” file which summarizes the basic commands to operate AVL
operate AVL.
3-D Aerodynamics: AVL Vortex Lattice Code
To Load and visualize the aircraft geometry:
1) Load the geometry file “vanilla.avl”
2) Change to the “.OPER” directory
3-D Aerodynamics: AVL Vortex Lattice Code
To Load and visualize the aircraft geometry:
3) Type “G” to bring up a wire frame plot of the geometry 4) Type “K” to bring up keyboard commands for
manipulating the plot
3-D Aerodynamics: AVL Vortex Lattice Code
To Run AVL for a specified flight condition:
1) From the “OPER” directory type “M” to modify the flight conditions
2) Enter the first letter of the variable you wish to specify 1)) Example: “MN” for Mach number MNa p e o ac u be
3-D Aerodynamics: AVL Vortex Lattice Code
To Run AVL for a specified flight condition:
4) AVL does not appear to run angle of attack or mach number sweeps as done with DATCOM or Linair (If you find a way let me know!). There for you can specifically define the angle of attach, side-slip angle, etc., through de e t e a g e o attac , s de s p a g e, etc , t oug the constraint table which appears in when you are in the “OPER” directory
5) Set the angle of attack to 4.0 degrees by typing “A” to select angle of attack. Then Select the variable which AVL will use to set the angle of attack. Select angle of attach by entering “A” and finally enter the angle of y g y g attack 4.0 deg
Note: AOA can be constrained by any variable listed (Like CL)
3-D Aerodynamics: AVL Vortex Lattice Code
To Run AVL for a specified flight condition:
6) Type “x” to execute the vortex lattice code. The following screen will appear
6) Repeat the process for every flight condition or constraint
3-D Aerodynamics: AVL Vortex Lattice Code
To Visualize Output with AVL:
1) To Visualize the Lift distribution for all lifting surfaces type “T” for the Trefftx plane plot.
Wing Lift Distribution
Horizontal Tail Lift Distribution
Downwash angle distribution
3-D Aerodynamics: AVL Vortex Lattice Code
To Visualize Output with AVL:
1) To Visualize the pressure distribution for all lifting surfaces type “G” to return to the geometry plot 1)) Type “LO” to visualize the wing loading (pressure ype O to sua e t e g oad g (p essu e
distribution)
Wing Lift Distribution
Horizontal Tail Lift Distribution
Downwash angle distribution
3-D Aerodynamics: AVL Vortex Lattice Code
To Dump Output AVL to data files:
1) To output
1) Stability derivatives type “ST” or “SB”
2)) Total forcesota o ces “FT”
3) Surface forces “FN”
(wing, horizontal tail, etc.)
4) Strip forces (lift distribution) “FS”
5) Element forces (control points) “FE”
6) Strip shear moments “VM”
6) Strip shear, moments VM (drag and pitching moments)
7) Hinge moments “HM”
2) For each command it will prompt you to provide a file name to write the output.
Horizontal Tail Lift Distribution
Downwash angle distribution
3-D Aerodynamics: AVL Vortex Lattice Code
To Build an AVL model of the Citation X:
1) Start with the “bd2.avl” as a template for a wing, body horizontal and vertical tail configuration.
2)) Modify the reference areas mach number center of od y t e e e e ce a eas ac u be ce te o gravity references and CD0 (Vortex lattice methods only predict induced drag!!!!!!)
3) Modify the fuselage according the description in AVL’s Users guide (avl doc txt)
Users guide (avl_doc.txt) Notes:
1) Fuselages and nacelles are modeled as
uncambered bodies of revolution with circular cross-sections. Therefore, for the Citation X use the Top view to determine the cross-sectional radius at each x-station.
2) Fuselage bodies are input in the same manner airfoils (i.e. specify radii from tail to nose across the top of the fuselage followed by the radii from p g y nose to tail along the bottom surface
4) For each lifting surface (wing, horizontal tail, vertical tail, etc. specify
5) For each command it will prompt you to provide a file name to write the output.
3-D Aerodynamics: AVL Vortex Lattice Code
To Build an AVL model of the Citation X:
4) For each lifting surface (wing, horizontal tail, vertical tail, etc.) any number of chordwise locations (root, tip, mid-span, etc.) specify Leading edge x, y, z location, chord length, incidence angle and airfoil ordinate file c o d e gt , c de ce a g e a d a o o d ate e according to the AVL users guide.
Notes:
1) The AVL can read the same airfoil data files as x-foil
2) You may use one or more airfoils for this model 2) You may use one or more airfoils for this model.
-0.1 Mean camber line
GIII BL 145 XLE3 Mean camber line
GIII BL 45
0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90 1.00
x/c y/c
Upper Lower Mean camber line
Cessna 7500
4.3 Aerodynamics for performance methodology Performance Team Aerodynamic Requirements:
The performance team requires the aerodynamic drag polar and lift curve to predict range, take-off and landing field length, climb gradients, time to climb, service ceiling, etc.
etc.