Page 44 - EAA78.Newsletter.Archives.(February.2017-July.2021)

Experimental Aircraft Association
Delaware Valley, Pennsylvania
Doylestown Airport (KDYL)
3879 Old Easton Rd.
Doylestown, PA 18902
Meets: Last Wed each of month (7:30 PM)
CHAPTER CHATTER
Chapter Number 78
What's the Big Deal About Angle of Attack?
The AOA provides the perfect approach angle, making the
airspeed indicator almost irrelevant.
An angle of attack indicator offers a visual indication of
the amount of lift the wing is generating at a given
airspeed or angle of bank. The AOA delivers critical
information visually or through an aural tone to indicate
the actual safety margin above an aerodynamic stall.
AOAs are created around one of two systems, either a
lift reserve indicator or a normalized AOA. An LRI is
normally accurate in a single configuration, usually
near the approach angle of attack. The advantage of a
normalized AOA is that the angle of attack
measurement is accurate in all aircraft configurations.
Both AOA configurations consist of a heated wing
probe that looks similar to a standard pitot tube. Both
AoA versions also require an air-data computer and a
visual cockpit indicator.
July, 2017 (4th Edition)
Volume 41, Number 07
Flying Through History
The AOA tube is constructed with two tiny machined
holes to create differential pressure sources. One hole
is bore-sighted at the front of the AOA tube along the
longitudinal axis, while the second, located at the
bottom of the AOA probe, is pointed downward at a 45-
degree angle to act as a reference source.
Garmin’s normalized AOA includes an additional static-
air-pressure source to allow the air-data computer to
calculate airspeed and air density, important in
calculating precise AOA. The addition of the static-
pressure reference input makes the AOA indication
accurate regardless of the airplane’s weight, speed or
configuration. This comes in handy particularly in steep
turns when a pilot normally adds back pressure to
maintain altitude, something that also increases the
angle of attack and the airplane’s stall speed. In order
to ensure accurate data, each AOA must be calibrated
for the specific aircraft in which it is installed.
On a normalized AOA, air pressure passes from the
probe through flexible tubes in the wing until it reaches
three pressure sensors co-located with the air-data
computer’s microprocessor. It is here the translation
occurs to create the electronic signals sent to the
cockpit indicator.
The AOA indicator, in our example measuring
approximately 3.2 inches high by 1.4 inches wide, is
often mounted on the top of the glareshield, where it
can be easily seen through the pilot’s peripheral vision
during most flight conditions.
In cruise, the AOA is dark. As the angle of attack
increases, the AOA indicator (shown on the opposite
page)first illuminates the green bar near the display’s
bottom. As the angle of attack increases, additional
green bars light up until the green dot — called the
donut — illuminates, indicating the proper final
approach path angle. If the angle of attack continues to
increase, the green bars will turn yellow as a caution
and, finally, red as the wing approaches a stall.