## Q-talk 85 - Angle of Incidence Measurement Tool

Paul Buckley, England

Provided that you have accurate level boards still attached to the upper surfaces of your wing, I found the following method of setting angles of incidence to be very easy and accurate. If a circle has a circumference of 360 inches, then one inch of its arc would be one degree, and its radius would be 57.28 inches. Divide the radius by four, that gives 14.32 inches. Therefore, on a circle with a radius of 14.32”, one inch of circumference equals 4 degrees, or 1/4” = 1 degree.

A builders ‘roofing’ square [obtainable very cheaply at any hardware store] is ‘L’ shaped, the long side being 24” and the short side 16”. If the long side is placed on the level board, with the short side hanging down in front of the wing, then all that is needed is a plumb bob pivoted at the top of the short side, with a length of 14.32”, running against a short scale at the bottom, where 1/4” 1 degree. For the plumb line I used piano wire [from a model airplane shop], bent at right angles at the top and pushed through a small drilled hole to act as the pivot, and ground to a point at the bottom, care being taken to ensure that the length was 14.32”. The ‘bob’ is a heavy [relatively] piece of brass tube glued to the piano wire 1/2” from the bottom. The scale is a small piece of aluminum, scribed in 1/4 degree increments and stuck in place with double sided tape whilst the long side was resting on a known horizontal surface.

Using this easily manufactured gadget, it is a cinch to set your flying surfaces to within 1/8 of a degree, but it must be calibrated using a known, truly horizontal, surface before using!

Hope this is of help, and BTW check my math first!

ED Note: Good use of simple tools and basic math to deal with a complex issue.

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