Just a quick diagram to illustrate the relationship between the chord length of a control surface deflection, the radius of the control surface to the hinge, and the angle. You can use this to calculate the angle of deflection using only a ruler, on many 3D planes at least.




Of course, you could always laser print or buy a protractor, too.

But if you have a calculator, and ruler, and know how to take an arcsine, it is fairly simple. (Warning: Make sure your calculator is in degrees mode and not radians.)