As fundamental as it sounds, measuring AOA is surprisingly difficult. Local flow in an about an airplane must be carefully studied as it relates to actual free stream AOA, and once defined, is accurate for only that aerodynamic shape. Even something as mundane as lowering flaps on an airplane can alter local airflow such that prior measurements become inaccurate. This means that a vendor cannot simply produce a "one size fits all" device for measuring AOA on several different airplanes.

Most of the available AOA indicators sold for full scale airplanes don't actually measure AOA in degrees. They are simply on-off devices that turn on when the airplane approaches the stall angle (described in prior post) or they are analog indicators that provide the pilot with some relative idea of where he is in the AOA range of the airplane's envelope. Such indicators typically have ranges marked in green, yellow and red, but don't actually read out in degrees. In all cases, they have to be calibrated to that particular airplane due to the specifics of local flow peculiar to that aerodynamic shape.

The only somewhat accurate means of measuring AOA without detailed knowledge of the local flow is with a probe that extends well forward into undisturbed air. Rule of thumb placement of such probes is that they should be at least 1 1/2 local chord lengths ahead of the wing leading edge (if wing mounted) or at least one body diameter ahead if mounted on the nose of the fuselage. The probe can consist of a vane/potentiometer affair or a blunt cylinder with an array of static ports, each monitored for local air pressure. Needless to say, any such probe extending out from the nose or wing leading edge is highly subject to handling damage.

Some simple yet accurate means of measuring AOA is still off in the future somewhere, but it certainly would be welcome in R/C flying as well as in full scale. If such a device had been feasible years ago we might not be relying on airspeed indicators so much today.

Dick