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some airplanes in a shallow climb. The handling
qualities in a jet can change drastically when the
maximum operating speeds are exceeded.
High speed airplanes designed for subsonic flight are
limited to some Mach number below the speed of
sound to avoid the formation of shock waves that begin
to develop as the airplane nears Mach 1.0. These shock
waves (and the adverse effects associated with them)
can occur when the airplane speed is substantially
below Mach 1.0. The Mach speed at which some
portion of the airflow over the wing first equals Mach
1.0 is termed the critical Mach number (MACHCRIT).
This is also the speed at which a shock wave first
appears on the airplane.
There is no particular problem associated with the
acceleration of the airflow up to the point where Mach
1.0 is encountered; however, a shock wave is formed at
the point where the airflow suddenly returns to
subsonic flow. This shock wave becomes more severe
and moves aft on the wing as speed of the wing is
increased, and eventually flow separation occurs
behind the well-developed shock wave. [Figure 15-9]
If allowed to progress well beyond the MMO for the
airplane, this separation of air behind the shock wave
can result in severe buffeting and possible loss of
control or “upset.”
Because of the changing center of lift of the wing
resulting from the movement of the shock wave, the
pilot will experience pitch change tendencies as the
airplane moves through the transonic speeds up to and
exceeding MMO. [Figure 15-10]
For example, as the graph in figure 15-10 illustrates,
initially as speed is increased up to Mach .72 the wing
develops an increasing amount of lift requiring a nosedown
force or trim to maintain level flight. With
increased speed and the aft movement of the shock
wave, the wing’s center of pressure also moves aft
causing the start of a nosedown tendency or “tuck.” By
Mach .83 the nosedown forces are well developed to a
point where a total of 70 pounds of back pressure are
required to hold the nose up. If allowed to progress
unchecked, Mach tuck may eventually occur.
Although Mach tuck develops gradually, if it is
Figure 15-8. Jet airspeed indicator.
M=.72 (Critical Mach Number)
M=.77
Supersonic
Flow
M=.82
Normal Shock Wave
Subsonic
Possible
Separation
Supersonic
Flow
Normal Shock
Normal Shock
Separation
Maximum Local Velocity
Is Less Than Sonic
Figure 15-9.Transonic flow patterns.
70
60
50
40
30
20
10
0
10
20
0
0.3 0.4 0.5 0.6 0.7 0.8 0.9
Stick Force in Pounds
Mach Number
Pull
Push
Figure 15-10. Example of Stick Forces vs. Mach Number in a
typical jet airplane.
Ch 15.qxd 5/7/04 10:22 AM Page 15-7
15-8
allowed to progress significantly, the center of
pressure can move so far rearward that there is no
longer enough elevator authority available to
counteract it, and the airplane could enter a steep,
sometimes unrecoverable dive.
An alert pilot would have observed the high airspeed
indications, experienced the onset of buffeting, and
responded to aural warning devices long before
encountering the extreme stick forces shown.
However, in the event that corrective action is not
taken and the nose allowed to drop, increasing airspeed
even further, the situation could rapidly become
dangerous. As the Mach speed increases beyond the
airplane’s MMO, the effects of flow separation and
turbulence behind the shock wave become more
severe. Eventually, the most powerful forces causing
Mach tuck are a result of the buffeting and lack of
effective downwash on the horizontal stabilizer
because of the disturbed airflow over the wing. This is
the primary reason for the development of the T-tail
configuration on some jet airplanes, which places the
horizontal stabilizer as far as practical from the
turbulence of the wings. Also, because of the critical
aspects of high-altitude/high-Mach flight, most jet
airplanes capable of operating in the Mach speed
ranges are designed with some form of trim and
autopilot Mach compensating device (stick puller) to
alert the pilot to inadvertent excursions beyond its
certificated MMO.
RECOVERY FROM OVERSPEED
CONDITIONS
The simplest remedy for an overspeed condition is to
ensure that the situation never occurs in the first place.
For this reason, the pilot must be aware of all the
conditions that could lead to exceeding the airplane’s
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AIRPLANE FLYING HANDBOOK 飞机飞行手册下(73)
