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FoilSim II Version 1.4n


This is a beta 1.4n 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. If you are an experienced user of FoilSim, or if you plan to use your browser Print command to save output, or if you are using Internet Explorer, it is recommended that you use a slightly different version of FoilSim II . This altered version does not include the following tutorial. You can also investigate the role of aerodynamics in pitching a baseball by using another interactive Java Applet called CurveBall.

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. To return to the original default conditions, click the red Reset button at the left middle of the program.


The program screen is divided into three main parts:

  1. At the top of the screen are two graphics windows. The left window is the View window in which you can display a drawing of the airfoil or wing you are designing with the flow of air around the airfoil. The right window is the Output window in which you can display a plot of the flow variables, a lift meter, or a probe to investigate the flow field. Details are given in Graphics.
  2. In the middle of the screen on the left is the control panel. You can control the type of analysis and the units used in the display by using the white menu buttons. To operate a menu button, click on the arrow to the right and make your selection from the drop menu. The interactive inputs to the program are displayed on the input panel to the right of the control panel. You can select the input panel to be displayed by using the blue and white buttons in the middle of the control panel. You can also select the type of graphical output by using the white and red buttons on the right side of the control panel. Details of the input and output variables are given below.
  3. At the bottom of the screen is a large text window in which output from the program is displayed. Output can be printed using the browser Print command. You can save the airfoil geometry and surface flow variables by pushing the Save Geom button on the control panel. You can save flow conditions and calculated lift by pushing the Save Data button on the control panel.


There are two main graphics displays. At the top left is the View window of the airfoil and flow field and at the top right is the Output plot of flow variables.

You can select the type of schematic shown in the View window by using the buttons located around the graphics window. 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 right of the window. If you get lost, pushing the red Find button will return the airfoil to the initail 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 in the Output Window. You can select the type of output by using the white and red buttons on the control panel. If you select Plots, a panel will appear 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 lower 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 conrtrol 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 input variables are located on input panels that are displayed below the Output window. You can select the input to display by using the blue input buttons on the control panel. You can choose to vary the Shape, the Size, or perform a Flight Test. To change the value of an input variable, simply move the slider. Or click on the input box, select and replace the old value, and hit Enter to send the new value to the program. By convention, input boxes have a white background and black numerals, while output boxes have a black background and green numerals.

  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 menu button 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 menu button. 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.


Interactive output from the program is displayed in output boxes. By convention, output boxes have a black background and green numerals. You can choose to display the lift or the lift coefficient by using the menu button on the control panel. The value is displayed in the output box to the right of the menu button. 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 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 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.
  2. 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.
  3. 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.
  4. 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.

Button to Download a Copy of the Program
Guided Tours


Beginner's Guide to Aerodynamics
Beginner's Guide to Propulsion
Beginner's Guide to Model Rockets
Beginner's Guide to Kites
Beginner's Guide to Aeronautics

Button to Display Aerodynamics Index Button to Display Propulsion Index Button to Display Model Rocket Index Button to Display Kite Index

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byTom Benson
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