This is a beta 1.5a version of the FoilSim II program, and you are invited to participate in the beta testing. If you find errors in the program or would like to suggest improvements, please send an e-mail to

This page contains an interactive Java applet to explore the various factors which affect the lift of an airfoil. All of the information presented by the applet are available within the Beginner's Guide to Aerodynamics. You should start with the slide describing the factors that affect lift.

FoilSim II

With this software you can investigate how an aircraft wing produces lift by changing the values of different factors that affect lift.

There are several different versions of FoilSim II which require different levels of experience with the package, knowledge of aerodynamics, and computer technology. This web page contains the off-line student version of the program. It includes an off-line user's manual which describes the various options available in the program. More experienced users can select a version of the program which does not include these instructions and loads faster on your computer.


This program is designed to be interactive, so you have to work with the program. There are a variety of choices which you must make regarding the analysis and the display of results by using a choice box. A choice box has a descriptive word displayed and an arrow at the right of the box. To make a choice, click on the arrow, hold down and drag to make your selection. The current values of the design variables are presented to you in boxes. By convention, a white box with black numbers is an input box and you can change the value of the number. A black box with yellow numbers is an output box and the value is computed by the program. To change the value in an input box, select the box by moving the cursor into the box and clicking the mouse, then backspace over the old number, enter a new number, then hit the Enter key on your keyboard. You must hit Enter to send the new value to the program. For most input variables you can also use a slider located next to the input box. Click on the slider bar, hold down and drag the slider bar to change values, or you can click on the arrows at either end of the slider. At any time, to return to the original default conditions, click the red Reset button at the upper right of the program.

If you see only a grey box at the top of this page, be sure that Java is enabled in your browser. If Java is enabled, and you are using the Windows XP operating system, you need to get a newer version of Java. Go to this link:, try the "Download It Now" button, and then select "Yes" when the download box from Sun pops up.

If you experience difficulties when using the sliders to change variables, simply click away from the slider and then back to it. If the arrows on the end of the sliders disappear, click in the areas where the left and right arrow images should appear, and they should reappear.


The program screen is divided into four main parts:

  1. On the top left side of the screen is the View Window. The view window includes a graphic of the airfoil that you are designing and several buttons which control the graphic. Details of the window are given in the Graphics section of this page.
  2. On the upper right side of the screen is the Control Panel. The control panel holds several choice buttons which control the analysis. You can select the type of analysis, the type of input and output to be displayed, and the units to be used in the calculations. You will always see the computed lift or lift coefficient displayed. Details of the input and output variables are given below.
  3. On the lower right side of the screen is the Output Window. The output can be presented as graphs of airfoil performance, a probe which you can move through the flowfield, a lift meter, or printed numerical values of certain parameters. You select the type of output displayed by using the choice button labeled "Output:" on the upper right panel.
  4. On the lower left side of the screen is the Input Window. Various input panels are displayed in this window. You select the input panel by using the choice button labeled "Input:" on the upper right panel.


The View Window contains a schematic drawing of the wing that you are designing and some buttons to control the schematic drawing. Possible choices are colored blue while the selected option is colored yellow.

  1. Edge shows the wing as viewed looking along the leading edge. The cross section appears as an airfoil or circle with the flow going from left to right. You can move the picture within the window by moving the cursor into the window, holding down the left mouse button, and dragging the airfoil to a new location. You can also "Zoom" in close to the airfoil by using the green slider at the left of the window. If you get lost, pushing the red Find button will return the airfoil to the initial position.
  2. Top shows the wing as viewed from above looking down onto the planform. The flow is from bottom to top. Since FoilSim only solves for the two-dimensional flow past objects, you won't see any flow in this view. It is provided only to display the geometry of the wing area.
  3. Side - 3D shows an orthographic projection of the three dimensional wing.
  4. Moving shows particles flowing past the airfoil. The particles are drawn as line "traces". The inclination of the trace is at the local flow angle, and the left most part of the trace is the particle location. Particles are being periodically released from a constant streamwise location upstream of the airfoil.The streamwise distance between any two particles is proportional to the local velocity.
  5. Frozen is a snapshot of the moving particles. In this view, you can change the streamwise release point of the particles by moving the cursor into the view window, holding down the left mouse, and dragging the particles to the left or right. In this view, the particles are color coded by the time at which they are released. (All of the yellow particles were released at the same streamwise location at the same time.)
  6. Streamlines are collections of the particle traces to form a solid flow line.
  7. Geometry shows only the geometry of the wing or airfoil with some descriptive labels but with no flow field.


There are several different output options available for the Output Window at the lower right. You select the type of output by using the red choice box on the control panel. If you select Plot, a graph will appear in the window. The type of graph is described below. Plot Selection provides a panel in the output window which allows you to select the type of plot. If you display a plot and begin to change the input variables, it may become necessary to rescale the plot axes by pushing the white Rescale button at the upper left of the window. The types of available plots are listed below.

  1. The default plot is the Surface Pressure. The yellow line will be a plot of the lower surface pressure, and the white line a plot of the upper surface pressure. For reference, the green line shows the value of free stream pressure.
  2. You may also display the Surface Velocity. As with the pressure, the yellow line will be a plot of the lower surface, and the white line a plot of the upper surface. If the Stall Model is used and the wing stalls, these plots are not available.
  3. The remaining plot choices show Lift or Lift Coefficient -Cl versus each of the input variables. For these plots, the current value of the flow conditions is shown as a red dot on the plot.

The Lift Meter shows the current value of lift (or lift coefficient) displayed in scientific notation. The meter is displayed in the output window and moves as the lift is varied.

The Probe lets you explore the flow around the wing. A probe control panel appears in the output window when you select "Probe" from the control panel. By default, the probe is turned off. You turn the probe on by pushing one of the white buttons on the probe panel. The probe itself will then appear in the view window. You change the location of the probe using the sliders to the left and below the gauge on the probe panel. The value of the pressure or the velocity at the location of the probe tip (magenta ball on the view window) is displayed on the gauge. Or a green trail of "smoke" is swept downstream from the probe location. You turn the probe off by using the red button located above the gauge.

The Performance option provides a written list of important input and computed variables in the Output Window. You have two options for performance output; the Data option gives the computed lift and flow conditions, the Geom option shows the coordinates of the airfoil geometry and the local value of velocity and pressure. These numbers correspond to the plots of velocity and pressure described above.

Some additional output from the program is displayed on the control panel and some input panels. You can choose to display the lift or the lift coefficient by using the choice box on the control panel. The value is displayed in the output box to the right of the choice box. Lift may be expressed in either English or metric units (pounds or Newtons). On the Flight Test input panel is a group of output boxes that give the atmospheric conditions of the air. The pressure, temperature, and density will change depending on the altitude and planetary inputs. On the Size input panel the geometric aspect ratio of the wing is displayed. The aspect ratio is defined to be the square of the span divided by the wing area and is included here for later calculations of wing drag.


The input variables are located on input panels that are displayed at the lower left. You can select the input to display by using the blue choice box on the control panel. You can choose to vary the Shape, the Size, or perform a Flight Test.

  1. If you choose to Flight Test (the default choice), you can change the value of lift by varying the speed, or the altitude. You can flight test your wing on the earth (default), on Mars, or in the water by using the choice box at the upper right. You can also choose to specify your own values of temperature and pressure for air, or to specify your own fluid by providing a value of the fluid density.
  2. If you choose Shape/Angle, you can select a classic airfoil shape, an ellipse, or a thin plate cross section by using the choice box. You can change the camber, thickness, or the angle of attack of the cross section. The definitions of these geometric variables are given on the wing geometry web page. You can also choose to investigate the lift created by a rotating cylinder, or a spinning ball. For these problems you must specify the spin rate and radius.
  3. If you choose Size, you can vary the layout of the wing. You can change the chord, span, or the wing area. The ratio of the span to the chord is called the Aspect Ratio and this parameter also has an effect on the lift of the wing. If you have selected a cylinder or ball shape, this input panel is not used.


The Learning Technologies Project will continue to improve and update FoilSim II based on user input. Changes from previous versions of the program include:

  1. On 6 Feb 04, version 1.5a was released. This version includes a slightly different layout of the program. The large text box is now included as an output option. This is the first version which is available as an undergraduate version and as a Java application. The undergraduate version version includes some additional options to better understand the analysis. The application allows printed output from the program.
  2. On 7 Oct 02, version 1.4n was released. This version includes the effects of aspect ratio on lift and some small modifications suggested by users, including the "Rescale" option on the plots.
  3. On 1 Dec 00, version 1.4 was released. This version includes some additional shapes (cylinder and ball), larger graphics output selections (density and lift coefficient plots), different environments (water, non-standard earth atmosphere, and specified fluid density) and a different layout of the view panel. Corrections have also been made to the particle traces.
  4. On 28 July 00, version 1.3 was released. This version includes some additional airfoil shapes (ellipse and plate), larger graphics output, and a (hopefully) cleaner layout of input panels.
  5. On 28 Feb 00, a stall model was added. This model is invoked by using the blue on white Input Choices button. The default analysis mode is Ideal Flow, in which the stall model is turned off. If you choose Stall Model and the angle of attack is greater than 10 degrees, you will see the flow begin to separate from the surface of the airfoil, as it does on real airfoils. The lift will continue to increase up to about 15 degrees; then it will decrease. This behavior is observed on real airfoils as seen on the slide describing the effects of angle of attack on lift.