Page 7 of 28 Sym B
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Dwg. 56201-2
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Theory of Operation
1.3
As the aircraft wing moves through the air it divides the air mass. At the center of this divided
airflow is a narrow region known as the stagnation point. The location of the stagnation point
uniquely represents the wing’s AoA.
As the AoA increases, the leading edge stagnation point moves aft chordwise on the lower
surface of the wing. The Lift Transducer senses the location of the stagnation point by means
of a spring-loaded vane. The Lift Transducer is designed to detect the location of the
stagnation point and relays this signal to the Indexer Computer.
The location of the Transducer on the wing is carefully chosen so that the sensed airflow is
optimized for all of the high lift performance regimes of the aircraft. These include near
maximum wing lift during normal and short-field takeoff and landing operations and the AoA’s
associated with optimized long range cruise and maximum endurance.
Decreasing the AoA of the wing moves the stagnation point forward (UP); increasing AoA
moves it aft (DOWN) as shown below. At a maximum aft position, wing lift rapidly decreases,
which is the AoA at which the stall occurs.
In the Aircraft Flight Manual (AFM) or Pilot Operating Handbook (POH), the speed at which
the stall occurs is given in terms of indicated airspeed. The stall speed varies with aircraft
gross weight, center of gravity, bank angle, maneuvering load and wing flap configuration. As
each of these is reflected by the movement of the stagnation point, the Lift Transducer
senses the wing’s leading edge flow field giving an accurate and repeatable indication of the
AoA.
The AoA, measured relative to stall, is displayed on the color-coded LEDs (
green/amber/red
)
on the Indexer Computer, which is mounted vertically on the instrument panel glare shield. A
pilot-selected reference mark may be set for the desired operational reference.
For Low Airspeed Awareness (LAA), high-AoA trend information supplements the aircraft’s
stall warning system by the display of two blinking
red
LEDs and a ‘Geiger counter’-like audio
output to the cockpit speaker and/or pilot headset. The audio begins when two
amber
LEDs
are illuminated and increases frequency as the AoA approaches stall.
