User Guide

AERODYNAMICS AND G FORCES
CHAPTER25
25-7
GUN CROSS
FLIGHT PATH MARKER
The fuselage reference line is a line that extends straight out of the body of the aircraft. This line is
also the path that the bullets take out of the gun. The gun cross in the HUD shows the path of the
bullets along with the fuselage reference line.
The difference between that gun cross and the flight path marker is the AOA. (These concepts are
discussed in Chapter 1: Learning How to Fly.) Just remember that AOA is related to lift. As AOA
goes up, so does lift. As the jet slows down, the pilot must raise the AOA to stay in level flight. The
reason for this is that total lift produced must be equal to weight in order to maintain level flight.
Since lift is directly proportional to AOA and airspeed, as you slow down you will lose lift if you do
not raise the AOA of the aircraft. With the increase in AOA, lift will also increase until the critical
AOA of the aircraft is reached. At this point, lift will stop increasing and will actually level off or
decrease. This point in the AOA curve is called the stall AOA and is depicted in the figure below.
Lift
AOA
Stall AOA Stall AOA
of F-16
Swept wing F-16
has small loss of lift
past stall AOA
Straight wing
aircraft
Straight wing aircraft
Dramatic loss of lift
past stall AOA
F-16