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Interactive Kite Modeler


This is a beta 1.3a version of the KiteModeler 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 design and flight of several kites. 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 kite design.


With this software you can study the physics and math which describe the flight of a kite. You can choose from several types of kites and change the shape, size, and materials to produce your own design. You can change the values of different variables which affect the design and immediately see the new flight characteristics. The program tells you if your design is stable or not and also computes a prediction of how high your kite will fly.

If you are an experienced user, you can use another version of KiteModeler which does not include this tutorial. 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 Reset button in the view window.


The program screen is divided into four main parts:

  1. On the left of the screen is a graphics window in which you can display a drawing of the kite you are designing, trimming, or flying. Details are given in GRAPHICS.
  2. On the right of the screen at the top is the control and output panel. You can select the "Design" or the "Trim" mode by clicking on the buttons at the top. The "Trim" mode will also allow you to "Fly" your kite. The selected mode is indicated by a dark colored button. Depending on your choice, different input panels appear at the bottom and different output variables are shown at the top. Output boxes are shown with a black background, input boxes with a white background. Details of the INPUT and OUTPUT variables are given below.
  3. On the right of the screen at the bottom are the input panels. 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. Some input panels contain menu buttons. To operate a menu button, click on the arrow to the right and make your selection from the drop menu.
  4. 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 kite geometry and flight conditions by pushing the Save button at the bottom of the view window.


The View window for the kite is shown on the left of the program screen. You can change the size of the image by using the slider at the far left. Just click and drag on the yellow bar to make the image larger or smaller. If your kite design or trim is, unstable a red and yellow banner appears over the graphics. You should then change either a Shape input or a Trim input until the banner disappears.

You can perform the calculations in either English units (default) or metric units by clicking on the button in the upper left of the view window.

There are three views available to the user; Front, Side, and Field, as indicated by the buttons at the top of the window. The selected button is indicated by a yellow background. By default, the Front view is shown in Design Mode, the Side view in Trim Mode, and the Field view in Fly Mode. However, you can override the default by using the "Select View->" button. Click on this button, then click on the desired view. You can use any view at any time in the design, trim, and flight sequence as long as the "Select View->" button is yellow. To return to the default views, just click on the "Select View->" button again.

  1. Front shows the kite as viewed looking perpendicular to the surface. This view is the default during the "Design" of your kite. The geometric input parameters are shown to the right and below the kite drawing. Surfaces are shown as white areas, while the frame is shown as black lines. The center of gravity is shown as a red dot and the center of pressure as a green dot. The tail of the kite is given as a green line at the bottom.
  2. Side shows the kite as viewed looking along the surface. This view is the default during the "Trim" phase of your kite. The wind is blowing over the kite from left to right and the magnitude of the wind is shown by a small green arrow at the bottom. The kite surface is again shown as a white area, and the tail as a cyan line at the right. The length of the tail (T) is set on the trim input panel. The kite surface is inclined to the horizontal at the angle of attack of the kite. The bridle of the kite is given by the white line below the kite surface. The bridle length (B) is set on the trim input panel. The control line is shown by a magenta line from the lower left. The control line is attached to the bridle at the bridle point (knot) shown as a white dot. The distance of the knot from the base of the kite is denoted by (K) and is shown as a yellow line below the bridle. This distance is also set on the trim input panel. This drawing changes as you trim the kite by changing the length of the bridle or the location of the knot. The lift, drag, and weight forces that are acting on the kite during flight are displayed as vectors (arrows) with the lengths proportional to one another. You can change the length of all the force vectors by using the "Scale" slider at the bottom of the window.
  3. Field shows the kite flying with the control line running from you to the kite. This view is recommended during the "Flight" phase of your kite. As before, the wind blows from left to right over the kite. A white line shows the length and shape of the control line. (Notice that the line sags under its own weight and may even touch the ground.) The kite flies at some distance "X" from your feet and at some height "Y" above the ground. The values of these variables are shown on the output panel. If you let out line, the kite may leave the view window. You can change the size of the display by using the "Scale" slide at the bottom.


Output variables are displayed at the top right of the program screen. In most cases, these output variables are calculated by the computer program using mathematical equations. We have created hyperlinks to other web pages which describe the exact equations used in this program. Just click on the variable listed below to view these equations.

If the "Design" mode is chosen, the following variables are displayed. The Weight of the kite is calculated based on the input geometry and the materials selected. The weight is displayed in ounces or gram-weights and does not include the weight of the control line. The aerodynamic Lift and Drag are computed by the program and displayed in ounces or gram-weights. The trimmed Angle of attack is displayed in degrees and may be either input by the user or calculated by the program. If the value is calculated by the program, the output numerals are green and the value is calculated to eliminate any torque on the kite. If the value is input, the output numerals are yellow to warn the user that the resulting flight conditions may be unstable. The program calculates the net Torque on the kite about the bridle point. In stable flight the value is zero (in this program it is computed to a very small but non-zero number). The units for torque are ounces-inch. Using the "Side" view of the kite, a positive torque is clockwise and a negative torque is counter-clockwise about the bridle point. The Center of Gravity (Cg) and Center of Pressure (Cp) are the average location of the weight and aerodynamics forces, respectively. They are computed by the program in inches or centimeters from the bottom of the kite as shown in the "Front" view. The Surface Area of all the material and the length of all the components of the Frame needed to make your kite are computed based on your geometric input and the type of kite selected. These values are used in the computation of the weight and Cg. The Aspect Ratio is a geometric parameter which is used in the calculation of the lift and drag forces. The Height-Y is the predicted altitude at which your kite flies. This value depends on the flight characteristics of the kite, the wind velocity, and the amount of control line that you use. The value is computed in feet or meters and is shown on the "Field" view.

If the "Trim" mode is selected, three additional variables are displayed. The Wind velocity in feet or meters per second is the value which you set in "Fly" mode. The X distance is the horizontal distance of the kite from your location during flight. This value is related to the Height-Y and is also displayed on the "Field" view. The Tension in the line is displayed in ounces or grams. This is computed based on the forces on the kite and the weight of the line.


Input variables are displayed on input panels at the lower right of the screen. There are four sets of input variables which depend on the program mode.

  1. In the "Design" mode, the default input panel is the Shape panel. On the Shape panel you first select the type of kite to be designed by using the drop menu at the upper right. You can choose from five basic designs. Depending on the design, you then vary the height, H1 and H2, and width parameters, W1 and W2, to alter the shape of your kite. The "Front" view shows the meaning of these variables and also indicates if your design is unstable. There are default geometries for each kite based on the width W1. If you change the type of kite, you will get the new kite default with the current value of W1 held constant. Pushing the Reset button on the view panel will return the default geometry with W1 = 10 inches.
  2. In the "Design" mode, you can also change the material properties of all of the parts of your kite. Push the white "Material" button to display the Material input panel. On the Material panel you change the material used for the Surfaces, Frame, Tail, and control Line by using the drop menu at the far right of each part. Each material has a characteristic "density" which is displayed in the black output box. This density is used to compute the weight and Cg of your kite and the sag of the control line. For each part you can Specify your own material density by making this selection on the drop menu. The black output box then becomes a white input box and you can enter your own data as described above.
  3. In the "Trim" mode, the default input panel is the Trim panel. On the Trim panel you can select to have the program compute the trimmed angle of attack or your can input your own flight angle of attack by using the drop menu at the upper right. If you decide to input your angle of attack, you then input the Angle in degrees. The "Side" view shows the inclination of your kite relative to the wind. The most important variables used to trim your kite are the Bridle and Knot variables. Bridle gives the total length of the bridle string in inches or centimeters. Knot gives the length along the bridle string from the base of the kite to the attachment of the control line. These two variables affect the trim angle of the kite which affects the magnitude of the aerodynamic forces. Because the kite pivots about the knot in flight, the location of the knot has a large effect on the stability of the kite. The length of the Tail also affects stability and trim by shifting the Cg.
  4. In the "Fly" mode, you can set the Wind speed in feet or meters per second and the length of the control Line in feet or meters. The weight of the control line is calculated and causes the line to sag as shown in the "Field" view. Eventually a point is reached where the line touches the ground at your feet. The program then ignores any additional line input that you make.


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

  1. On 1 Oct 02, version 1.3 was released. This version includes the output text field at the bottom of the program, the default geometries for all the kites, the Reset feature, the line tension output, and a fixed bug in the line weight.
  2. On 29 July 02, version 1.2 was released. This version includes English and Metric units and the Twin-Trap kite geometry.
  3. On 28 Mar 02, version 1.1 was released.


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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