If these applets don’t display properly, try reloading the page. If that doesn’t work, you need to install a Java plug-in for your browser, available at Sun’s download site. If you are using Chrome, some users have reported that the Java Applet Host steals focus from the browser, not allowing you to actually use the applets. This isn’t an issue in Firefox or Safari.

Introduction

This online tool is an implementation based on Phillip Dalton’s Patent for a Plotting and Computing Device (US Patent No. 2,097,116 – June 1936). One previous implementation that is commonly known is in the form of the E-6B Flight Computer. Pilots use this tool to compute the heading correction required when a wind is blowing across their intended track. It is also used to compute the resulting ground speed after taking wind into account.

How to Use the Applet

1. Rotate the transparent window (by clicking and dragging on the transparent window) until the wind direction is at the top of the disc.
2. Place a wind mark (by double-clicking) at a position directly upward from the grommet (the centre hole on the transparent disc), at a distance upward from the grommet equal to the wind speed. For example, if the wind speed was 24 knots and you have the grommet centred over the 100 knot arc, the dot should be placed on the 124 knot arc. (This step is made simpler if you position the grommet over the 100 knot arc.)
3. Rotate the transparent window until your intended track direction is at the top of the disc.
4. Slide the entire frame (by clicking and dragging the left or right side of the circular frame) so that the wind mark from Step 2 lies along the speed arc that matches your chosen airspeed. You must not rotate the transparent window during this step, only slide the frame up or down.
5. Now, the grommet lies on top of the arc associated with your resultant ground speed.
6. Also, the angle left or right of centre at which your wind mark is positioned is the wind correction angle.

Pages 29-36 of [1] gives another presentation of instructions and a walkthrough of one example problem.

Circular Slide Rule

This is a circular slide rule, like is found on the reverse side of many flight computers and on pilot watches.