(Interested SLF, not a pro pilot)
Originally Posted by underfire
Perhaps someone can explain why this is a vane outside the aircraft, rather than measured with the gyro, or some other level instrument inside?
That's because AoA doesn't stay the same. It depends on static and dynamic loads, airspeed (indicated!) and air density. It doesn't depend on the position or direction the aircraft is pointing in relation to anything fixed (e.g. ground).
Static load is the mass of the aircraft (doesn't stay the same, either - fuel gets burned off during the flight)
Dynamic load is the acceleration in any direction (usually vertical relative to the cabin floor e.g. what a passenger feels when he gets pressed into his seat during takeoff, turns, etc.).
All of these (and possibly some others I forgot) combine to give you an angle of attack.
Usually, in a small aircraft, you fly your maneuvres with an eye on the airspeed indicator to keep it within a speed range that was determined to be safe. However, that is only necessary because the small plane lacks an AoA indicator. (Actually, the stall warning horn is a very rudimentary AoA indicator)
For a given wing profile, the stall always occurs at the same AoA (
AFAIK, for the wing on a Cessna 172 that is about 17 degrees). The stall speed is just the speed at which the critical angle usually is reached. (AFAIR, the 172 has a rated stall speed of 55kn, however, there have been accidents where pilots did steep turns at (let's say) 80kn, stalled and lost control because of the higher dynamic load in the turn.
So, having a vane outside that gets turned by the airflow is a very simple but very effective way to find out the exact angle of the air flow relative to the aircraft.
(I hope I explained this reasonable accurately.. :-)