Download: flag.rar
Movie: flag.mov In this first example I have set up a flag blowing in the wind. To make this work you need to have several elements setup.

  1. The flag mesh with enough vertices that it will deform the way that you would like.
  2. A wind force.
  3. A gravity force.

Setup:

  1. Add flex to the flag and add both the wind and gravity to the Forces rollout in flex.
  2. Turn off Use Chase Springs and Use Weights.
  3. Set the solver to Runge-Kutta4 with Samples set to 5. Runge-Kutta4 is a more accurate solver then Euler but will be slower.
  4. Change to SO mode Edge Vertices and select all the vertices that are on the leading edge of the flag. These are the ones that touch the flag poll and are not to move. Or you could select just the top and bottom ones to make it look as though the flag is only tied at the two corners.
  5. Change to SO mode Weights & Springs and select all the verts in the flag.
  6. In the Advanced Springs rollout Enable Advanced Springs checkbox.
    1. In the Advanced Springs rollout open the Option dialog.
      1. Set the Spring type to Hold Shape Springs.
      2. Change the Hold Shape Radius spinner to a value around 18 for the file that I have provided. You need to play with this setting to make sure that you get the right amount of springs. To few and the flag will not keep it shape, to many and it will either blow up or be far to stiff. Note: The second time that you open this dialog it will tall you what the average edge length of the flag is, set the Hold Shape Radius to about three times the average.
      3. Press OK.

    2. Press the Add Springs Button. You can turn on the Show Springs check box and you will see all the springs in the viewport.
    3. Note the Suggested Strength, in this case it will be around 0.038.
    4. Set the Shape Str and the Shape Sway to a value the same as the Suggested Strength to start with.
    5. Turn on Hold Length and set the percent value to around 28%.

  7. With the Shape Str at the suggested strength the flag might not hold together correctly. If this is the case start to increase the Shape Str a bit at a time until the flag starts to hold it shape better.
  8. Adjust the Turbulence in the Wind force to help blow the flag around.

Notes:
Hold Length will try to stop the edges from stretching. At a value of 0% this can cause the flag to blow up as it will start to fold the flag in on itself. Let the flag stretch a little and it will start to act more naturally.
Shape Sway should generally be less then Shape Str and you should start both around the Suggested Strength value and adjust it in very small amounts from there.
Flex Amount needs to stay at a value of 1.0. If changed you will get some interesting results.
Use Chase Springs can be turned on and used to slow the effect. Set Strength to 0.0 and then adjust Sway to the desired value. Remember that Sway is really dampening, the higher the value the more the effect is damped.
There is no way to calculate self intersections so you will see the flag pass through it self. If you want to get far more accurate simulations use the cloth modifier instead.