|December 1, 2004
Today I finished running the tubing for the upper and lower ports of the Angle of Attack instrument. Pictured below are the lower (left) and upper (you guess which side) units. There is really nothing sophisticated about these. They are simply designed to provide a sealed chamber on the inside of the wing at the point of pressure measurement. Small (#60) holes are drilled through the wing skin for pressure measurement and these units are attached behind the holes. The pressure lines attach to the units and run to the small computer.
Here is the bottom unit attached through the skin. The #60 hole is already drilled in place. I used some RTV to seal around the unit per instructions.
The tubing is very light weight and needs to be tied off to something so it doesn't flop around, yet it can't be tied down too tightly or bent sharply. I just looped it around and used pull-ties barely snug. A little RTV will keep them from sliding up or down on the tubing.
The two units are aligned similarily (equal distances from the leading edge) and set apart a few inches spanwise. Note that the upper unit has a fuel drain on the bottom. Of course, it won't be draining fuel. A small hole is drilled in the underside of the wing so that water can be drained from it using a short piece of piano wire. The fundemental principle of this setup is to measure the comparative pressures of the bottom and top wing at equal points along the airfoil. That is converted into relative angle of attack as an indicator of stall point, best climb, glide etc. It's not all that simple and other factors are computed to keep the reading consistant, but that's roughly how it works.