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flight envelope is known as “coffin corner.”
Mach buffet occurs as a result of supersonic airflow on
the wing. Stall buffet occurs at angles of attack that
produce airflow disturbances (burbling) over the upper
surface of the wing which decreases lift. As density
altitude increases, the angle of attack that is required to
produce an airflow disturbance over the top of the wing
is reduced until the density altitude is reached where
Mach buffet and stall buffet converge (coffin corner).
When this phenomenon is encountered, serious consequences
may result causing loss of airplane control.
Increasing either gross weight or load factor (G factor)
will increase the low speed buffet and decrease Mach
buffet speeds. A typical jet airplane flying at 51,000
feet altitude at 1.0 G may encounter Mach buffet
slightly above the airplane’s MMO (.82 Mach) and low
speed buffet at .60 Mach. However, only 1.4 G (an
increase of only 0.4 G) may bring on buffet at the optimum
speed of .73 Mach and any change in airspeed,
bank angle, or gust loading may reduce this straightand-
level flight 1.4 G protection to no protection at all.
Consequently, a maximum cruising flight altitude must
be selected which will allow sufficient buffet margin
for necessary maneuvering and for gust conditions
likely to be encountered. Therefore, it is important for
pilots to be familiar with the use of charts showing
cruise maneuver and buffet limits. [Figure 15-11]
The transitioning pilot must bear in mind that the
maneuverability of the jet airplane is particularly critical,
especially at the high altitudes. Some jet airplanes
have a very narrow span between the high and low
speed buffets. One airspeed that the pilot should have
firmly fixed in memory is the manufacturer’s recommended
gust penetration speed for the particular make
and model airplane. This speed is normally the speed
that would give the greatest margin between the high
and low speed buffets, and may be considerably higher
1 2 3 .1 .2 .3 .4 .5 .6 .7 .8
Load Factor – G Indicated Mach Number
Sea Level
5,000
10,000
15,000
20,000
Pressure Altitude – 25,000 Ft.
30,000
35,000
40,00040,000
45,00045,000
MMO
19,000 Lbs.
11,000
18,000
17,000
16,000
15,000
14,000
13,000
12,000
A
C B
D
Figure 15-11. Mach buffet boundary chart.
Ch 15.qxd 5/7/04 10:22 AM Page 15-9
15-10
than design maneuvering speed (VA). This means that,
unlike piston airplanes, there are times when a jet
airplane should be flown in excess of VA during
encounters with turbulence. Pilots operating airplanes
at high speeds must be adequately trained to operate
them safely. This training cannot be complete until
pilots are thoroughly educated in the critical aspects of
the aerodynamic factors pertinent to Mach flight at
high altitudes.
LOW SPEED FLIGHT
The jet airplane wing, designed primarily for high
speed flight, has relatively poor low speed
characteristics. As opposed to the normal piston
powered airplane, the jet wing has less area, a lower
aspect ratio (long chord/short span), and thin airfoil
shape—all of which amount to less lift. The sweptwing
is additionally penalized at low speeds because the
effective lift, which is perpendicular to the leading
edge, is always less than the airspeed of the airplane
itself. In other words, the airflow on the sweptwing has
the effect of persuading the wing into believing that it
is flying slower than it actually is, but the wing consequently
suffers a loss of lift for a given airspeed at a
given angle of attack.
The first real consequence of poor lift at low speeds is
a high stall speed. The second consequence of poor lift
at low speeds is the manner in which lift and drag vary
with speed in the lower ranges. As a jet airplane is
slowed toward its minimum drag speed (VMD or
L/DMAX), total drag increases at a much greater rate
than lift, resulting in a sinking flightpath. If the pilot
attempts to increase lift by increasing pitch attitude,
airspeed will be further reduced resulting in a further
increase in drag and sink rate as the airplane slides up
the back side of the power curve. The sink rate can be
arrested in one of two ways:
• Pitch attitude can be substantially reduced to
reduce the angle of attack and allow the airplane
to accelerate to a speed above VMD, where steady
flight conditions can be reestablished. This
procedure, however, will invariably result in a
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