Most AOA are not true AOA systems. They are "lift reserve indicators". That is they measure pressure differential in some way shape or form. The Garmin does this as does the Aspen units. Think of it and a stall warn system as the difference between an "idiot light" and a gauge. For instance on your car the idiot light tells you when the engine is hot. The gauge tells you what temp and trend and when it gets hot shows that condition.
A true AOA is a system with a vane that streamlines itself to the relative wind and is carefully calibrated. I have flown both and the lift reserve is better than nothing, but it is nothing compared to a true angle of attack unit. A true AOA indicator is so sensitive you can actually see your AOA increase flying into rain and decrease coming out of rain.
Other than the difficulty of developing mounting location and calibration IMHO it is not that much more expensive to produce a true AOA but lift reserves are the "quick and dirty" way of getting a better AOA derivative into the aircraft. Just remember it is not a true AOA system just as airspeed is a derivative and not a true system. Slips, skids, ice etc can effect in much more profound ways than a true AOA system.
A true AOA is a system with a vane that streamlines itself to the relative wind and is carefully calibrated. I have flown both and the lift reserve is better than nothing, but it is nothing compared to a true angle of attack unit. A true AOA indicator is so sensitive you can actually see your AOA increase flying into rain and decrease coming out of rain.
Other than the difficulty of developing mounting location and calibration IMHO it is not that much more expensive to produce a true AOA but lift reserves are the "quick and dirty" way of getting a better AOA derivative into the aircraft. Just remember it is not a true AOA system just as airspeed is a derivative and not a true system. Slips, skids, ice etc can effect in much more profound ways than a true AOA system.
