Aero Chapter 04, Stability
T-45 Aerodynamics Student Workbook
SPEED BRAKES
Extending the speed brakes causes the aircraft to pitch up, more so at high speed than at low speed
(Figure 70). The pitchup is caused by the extended speed brakes changing the flow pattern over the
horizontal stabilizer and increasing downward
force of the stabilator.
The speed brakes on the Goshawk were
moved to the sides of the empennage so a tail
hook could be added. The speed brake on the
predecessor to the Goshawk, the Hawk, was
on the bottom of the empennage. The Hawk
did not land with the speed brake out, but the
Goshawk needed speed brakes extended
during carrier approach to increase the engine
rpm, making the engine more responsive on
Figure 70: LONGITUDINAL STABILITY--SPEED
approach and waveoff.
BRAKE EFFECT
A speed brake-to-stabilator interconnect (SBI)
function is added to the T-45 control
STABILATOR
VANE
augmentation system to reduce pitch transients
when the speed brakes are extended or
retracted. When the speed brakes are
extended, the SBI reduces positive pitch
transients from 0.2 to 0.7 Mach and negative
pitch above 0.7 Mach. When the speed brakes
are retracted, the opposite occurs. As the
speed brakes extend, the control augmentation
system mechanically moves the stabilator to
counter the pitchup. The control stick does not
move during SBI operation. Even with SBI, the
pilot still must counter some pitch transients
during speed brake extension or retraction.
Figure 71: STABILATOR VANES (STRAKES)
STABILATOR VANES
A stabilator vane, strake, is installed on both
sides of the fuselage forward of the stabilator
and above the speed brake (Figure 71). The
purpose is to generate a vortex, which passes
below the stabilator, energizes the flow over
the lower surface of the stabilator, and keeps it
effective particularly at maximum leading edge
down stabilator deflection.
T-45 LANDING GEAR DOORS EFFECT ON
DIRECTIONAL STABILITY
As the landing gear transit, the T-45 directional
stability is reduced due to the inner main
Figure 72: LANDING GEAR DOORS EFFECT
landing gear (MLG) doors and forward nose
ON DIRECTIONAL STABILITY
landing gear (NLG) doors opening (Figure 72).
The yawing tendency is caused by the increased sail area of the opened MLG doors and the forward NLG
doors. The MLG and forward NLG doors close when the landing gear is fully extended. During
emergency gear extension, the inner MLG doors remain open and the forward nose landing gear doors
are partially closed by an electric motor. In this configuration, the aircraft is less stable directionally and
coordinated stick and rudder are required to control the slight yaw excursions that may occur.
(7-99) Original
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