T-45 Aerodynamics Student Workbook
Aero Chapter 02, High-Speed Flight
TRANSONIC AND SUPERSONIC EFFECTS ON STABILITY
Any of several phenomena may be encountered as an aircraft accelerates into the transonic flight regime.
They include buffeting, tuck under or Mach tuck, and wing drop or shock stall.
Buffeting
Buffet of the fuselage is caused by the turbulence associated with separation of the
boundary layers as normal compression waves form. Buffeting will occur if the separated
boundary layer is close to the fuselage or interferes with the horizontal tail.
Tuck under
As supersonic airflow is achieved over the wing, the aerodynamic center will start to move
aft. The effect of the aft movement is to lengthen the lift-to-CG moment arm. As a result,
nose down pitch or “tuck under” occurs. Tuck under is also known as Mach tuck. Normally,
tuck under is not a sudden or violent action but rather a gradual nose down tendency as the
aerodynamic center moves aft.
Wing drop
When compression waves form on the wing unevenly due to either physical differences in
the wings or the attitude of the aircraft, separation of the airflow over one wing may occur
before the other. That wing will produce less lift and a wing drop will occur. If separation of
the airflow occurs over both wings simultaneously, the aircraft will pitch nosedown as in a
typical low airspeed stall.
Depending on the aircraft design,
any of these phenomenon may or
Compression Wave
may not occur. However, if any
Vortex
erratic movements should occur,
they will be more severe when the
Critical Mach Number is exceeded
at low altitudes. At lower altitudes,
the dynamic pressure for a given
Mach Number is greater and
results in greater changes in
aerodynamic forces.
Other erratic movements
associated with transonic flight may
include random yaw caused by the
greater wave drag on one wing and
Smooth
degradation of flight control
Lift
effectiveness due to compression
Transonic
Distribution
wave-induced separation. Lateral
Lift
Lift
stability will degrade if lift
Coefficient
Distribution
distribution over the wing is
affected by shock wave formation
(Figure 26). As an aircraft is
accelerated into the supersonic
flight regime, longitudinal stability
will increase as the aerodynamic
Root
Tip
Span Location
center moves aft.
Figure 26: TRANSONIC STABILITY
(7-99) Original
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