T-45 Aerodynamics Student Workbook
Aero Chapter 02, High-Speed Flight
BOW WAVE
While the above are occurring, the
disturbances in the area forward of the
supersonic airflow continue to send out
Subsonic
Oblique
pressure waves. As aircraft speed increases,
Shock
these waves also pile up and form a bow
Wave
Supersonic Flow
wave (Figure 23). The bow wave is so
Separation
named because it is similar to a wave in
water off the bow of a boat. As the flight
Oblique
Supersonic Flow
velocity approaches the speed of sound, the
Shock
bow wave will be at or near the leading edge.
Wave
If the leading edge is large, a large change in
Bow Wave
direction of the airflow around the object
occurs. The large direction change reduces
the velocity to subsonic and creates a
Figure 23: BOW WAVE
stagnation region off the leading edge. Until
the bow wave attaches, part of the airflow
over the airfoil is subsonic and the aircraft
Oblique
remains in the transonic flight regime. The
Shock Wave
Supersonic Flow
aircraft is not in the supersonic flight regime
until all the airflow is supersonic. A detached
bow wave possesses both normal and
oblique compression wave characteristics.
Separation
The portion of the detached bow wave
perpendicular to the leading edge is normal to
Lower
the airflow and has the characteristics of a
Supersonic
normal compression wave. A stagnation
Flow
region will be present with the accompanying
Supersonic Flow
Oblique
pressure, density, and temperature
Shock Wave
increases. Away from the leading edge, the
airflow direction change is reduced and, while
Figure 24: ATTACHED BOW WAVE
the velocity is reduced, it remains supersonic.
That portion of the bow wave is oblique to the
airflow and has characteristics of an oblique pressure wave. If the leading edge radius is small enough or
sharp, the bow wave will attach and become an oblique wave (Figure 24). An attached bow wave has
only the characteristics of an oblique wave.
COEFFICIENT OF DRAG AND TOTAL
DRAG
The formation of compression waves in
transonic and supersonic flight brings about
Total Drag
large changes in both the drag coefficient and
total drag. A rapid increase in coefficient of
drag and total drag occurs at the Force
CD
Divergent Mach Number and will increase
DRAG
until the normal waves reach the trailing edge.
COEFFICIENT
CD
As the normal wave becomes an oblique
wave, the coefficient of drag decreases but
M
remains at a level greater than the subsonic
CRIT
Force Divergent Mach Number
coefficient of drag. As the coefficient of drag
0.8
0.85
1.0
reduces, the rate of total drag increase is
1.2
VELOCITY
decreased. Figure 25 illustrates the behavior
Figure 25: COEFFICIENT OF DRAG AT FORCE
of the coefficient of drag and total drag curves.
DIVERGENT MACH NUMBER
Page 16
(7-99) Original
Previous Page
