Aero Chapter 03, High AOA/Stall/Spin
T-45 Aerodynamics Student Workbook
GEOMETRIC TWIST
Geometric twist is the twist of an
airfoil having different geometric
Constant
angles of attack at different
Incidence
Wing
spanwise stations. In the early
1990s, the T-45 stalled dramatically
at the tips and rolled off violently.
CL
Modifications were made to make
the root stall and tip stall occur at
approximately the same time. The
incidence of the outboard leading
edge was modified with 3.5 degrees
of geometric twist to make the tip
stall at a higher AOA (Figure 49).
Combined effect of the geometric
AOA in Degrees
3 1/2
twist and the stall strips is that the
root and tip stall at approximately
Figure 49: EFFECT OF GEOMETRIC TWIST
the same time (Figure 50). The pilot
is now able to control the aircraft
into the stall, and only a 25-30 degree roll-off occurs at the stall.
Root Without
Stall Strips
Tip Without
Geometric Twist
CL
AOA
Root With
Tip With
Stall Strips
Geometric Twist
Adding Stall Strips to
C
the Root Causes the
L
Root to Stall Earlier (At
a Lower AOA)
Adding Geometric
Twist Causes the Tip to
Stall Later (At a Higher
AOA)
AOA
3 1/2
Figure 50: EFFECT OF STALL STRIPS AND GEOMETRIC TWIST
Page 30
(7-99) Original
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