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In Figure 2-25B, if the wing AOA is raised to the point of
minimum controlled airspeed at which the wing begins to
stall towards the center of the wing (root area), the lift in this
area decreases dramatically. The CLW moves back a distance
“b” creating a moment to lower the nose. Therefore, the
center of lift moves behind the CG at higher angles of attack,
creating a nose-down stabilizing moment. The average lift
coeffi cient verses AOA is shown for this minimum controlled
airspeed in Figure 2-26. The root area is partially stalled and
2-14
Normal Flight at Trim Speed
High Angle of Attack at
Low Angle of Attack at High Airspeed Minimum Controlled Airspeed
Top View Side View
CLW
CLW
b
c
CLW
AT AR LT
LR
Center of lift at
same longitudinal
position as center
of gravity.
b
c
Center of lift behind center of gravity
creating nose-down moment
Center of lift ahead of center of
gravity creating nose-up moment
Stalled area not producing lift.
Center of lift of wing (CLW)
behind center of gravity.
Tip area at low angle of attack not producing lift.
Center of lift of wing (CLW) ahead of center of gravity.
CG
CG
CG
CG
CG
CLW
CLW
CLW
LW
LW
LR LT
LR
LW
A
B
C
Nose
Nose
Nose
CG
Trim
Minimum
Controlled
Airspeed
High Speed
High AOA
Low AOA
Figure 2-25. Trim, minimum controlled airspeed, and high speed pitching moments.
the tips are still fl ying. The specifi c stall characteristics of
each wing are different and this stall pattern shown here is
used for example.
Low Angles of Attack
At very low AOA, the tip chords are near zero AOA or below,
not producing any lift, as shown in Figure 2-25C. At this
point, the nose area is producing all of the lift for the wing.
The CLW moves forward a distance “c,” creating a positive
stabilizing moment to raise the nose.
Pitch Pressures
As the pilot pushes out on the control bar, this creates a pilot
input force that has a moment arm from the control bar up to
the wing hang point. [Figure 2-27]
From this pilot-induced pitch moment, the control bar is
pushed out, the nose raised, and the AOA increases an equal
amount for both the root and the tip chords. However, as
shown in Figures 2-26 and 2-28, the average CL change is
greater at the low AOA at the tip chords, while the amount
2-15
CG
CG
Arm
Pilot induced pitch moments
Pilot input force
Pilot induced pitch moments
Pilot input force
Arm
Figure 2-27. Pilot actuated pitching moment.
1.5
1.0
0.5
0
5 10 15 20 25
Angle of Attack
CL
Root
at minimum
controlled
airspeed
Tips
at minimum
controlled
airspeed
Figure 2-26. Example of AOA versus CL for wing at minimum
controlled airspeed.
1.5
1.0
0.5
0
5 10 15 20 25
Angle of Attack
Root
Tip
.12
.3
CL
Example of
Increasing AOA
3 degrees above trim
Example of trim AOA
Figure 2-28. Example AOA versus CL showing the wing increasing
AOA three degrees with the tip CL increasing more than the root.
of change of the CL is much less at higher AOA at the root
chord. Therefore, an increase in AOA for the wing results in
the tips creating a greater proportion of the lift and moving
the center of lift behind the CG, creating a negative pitching
moment to lower the nose at high AOA.
Based on the same principle, when the wing AOA is lowered
below the trim position, the tip chords’ CL decreases more
than the root chord and the center of lift for the wing moves
forward creating a positive moment to raise the nose at
lower AOA.
In situations where the pilot is fl ying in severe/extreme
turbulence, wind sheer, or the pilot is exceeding the limitations
of the aircraft, the WSC aircraft can get into a situation where
the root chord is at a negative AOA and not producing lift.
This could result in an emergency vertical dive situation, as
discussed later in the Whip Stall-Tuck-Tumble section. When
at very low angles or negative angles of attack, the WSC wing
is designed so that the wing has positive stability or a noseup
aerodynamic moment. This is accomplished by a number
of different systems (washout struts, sprogs and refl ex lines)
further explained in chapter 3 that simply keep the trailing
edge of the wing up in an emergency low/negative AOA
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Weight-Shift Control Aircraft Flying Handbook(23)
