Aero Chapter 02, High-Speed Flight
T-45 Aerodynamics Student Workbook
standard day temperature of -48F (-44.4C). Compressibility effects will be experienced at lower
airspeeds as altitude is increased. Mach Number is used to describe the relationship of airspeed to the
speed of sound…M = TAS /SOS where:
M = Mach Number
TAS = True Airspeed
SOS = Speed of Sound
COMPRESSION WAVES
In our discussion we will consider the effect of airflow only on an airfoil section. Keep in mind that airflow
about an aircraft is considerably more complicated.
NORMAL COMPRESSION WAVES
MAXIMUM LOCAL VELOCITY IS LESS THAN SONIC
Initially all airflow about the airfoil is less than
the speed of sound (Figure 17). The airfoil is
in the subsonic flight regime. However,
velocity of the airflow about the airfoil is
greater than the velocity of the free airstream.
At some flight velocity, less than Mach 1,
Figure 17: COMPRESSION WAVE AT MACH 0.50
there will be an area of local sonic flow on the
airfoil (Figure 18). This is the “Critical Mach
Number” (Mcrit) of the airfoil. Critical Mach
FLIGHT MACH IS LESS THAN ONE
Number is defined as the flight Mach number
Sonic Flow (m = 1)
where there is first evidence of sonic flow on
Flow Accelerates to above Flight Mach
the aircraft. It is an important reference point
because it is the beginning of the transonic
flight regime. All compression waves and
phenomenon occur at a Mach number
greater than Mcrit. As the airflow accelerates,
MCRIT = .80 Mach for T-45
the area of supersonic airflow increases.
Pressure waves created in the area of
Figure 18: COMPRESSION WAVE AT MACH 0.80
supersonic airflow begin to pile up against the
aft moving airflow. This “piling up” forms a
weak normal compression wave. “NORMAL”
Normal Shock Wave
is a mathematical term meaning
Supersonic
Flow
perpendicular. Thus, a normal compression
wave is perpendicular to the airflow. A
Possible Separation
Subsonic
supersonic airstream traversing a normal
shock wave experiences a rapid reduction in
velocity to subsonic speed. If the Mach
number ahead of the compression wave is
Force Divergent Mach Number = 0.85 for T-45
Mach 1.2 or less, the velocity of the airflow
Figure 19: COMPRESSION WAVE AT MACH 0.85
after the normal wave is approximately the
reciprocal of Mach 0.85 (Figure 19). With the
great reduction in velocity comes an increase in static pressure, density, and temperature. A great deal of
kinetic energy is converted into unusable heat.
The possibility of boundary layer separation from the trailing edge of the airfoil and the degree of that
separation relates directly to the aerodynamic design of the airfoil and the existent angle of attack.
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