Airplane Flying Handbook

C
19,000 lbs
18,000 lbs
17,000 lbs
16,000 lbs
15,000 lbs
14,000 lbs
13,000 lbs
12,000 lbs
11,000 lbs
5,000
Sea Level
10,000
15,000
20,000
30,000
Pressure Altitude –25,000 ft
35,000
B
M MO
40,000
45,000
D
A
1 2 3 .1 .2 .3 .4 .5 .6 .7 .8
Load Factor –G
Indicated Mach Number
Figure 15-11. Mach buffet boundary chart.
in drag and sink rate as the airplane slides up the back side
of the power-required curve. The sink rate can be arrested
in one of two ways:
• Pitch attitude can be substantially reduced to reduce
the AOA and allow the airplane to accelerate to a
speed above V MD , where steady flight conditions
can be reestablished. This procedure, however, will
invariably result in a substantial loss of altitude.
• Thrust can be increased to accelerate the airplane
to a speed above V MD to reestablish steady flight
conditions. The amount of thrust must be sufficient to
accelerate the airplane and regain altitude lost. Also,
if the airplane has slid a long way up the back side of
the power required (drag) curve, drag will be very high
and a very large amount of thrust will be required.
In a typical piston engine airplane, V MD in the clean
configuration is normally at a speed of about 1.3 VS.
[Figure 15-12] Flight below V MD on a piston engine airplane
is well identified and predictable. In contrast, in a jet airplane
flight in the area of V MD (typically 1.5 – 1.6 V S ) does not
normally produce any noticeable changes in flying qualities
other than a lack of speed stability—a condition where a
decrease in speed leads to an increase in drag which leads to
a further decrease in speed and hence a speed divergence. A
pilot who is not cognizant of a developing speed divergence
may find a serious sink rate developing at a constant power
setting, and a pitch attitude that appears to be normal. The
fact that drag increases more rapidly than lift, causing a
sinking flightpath, is one of the most important aspects of
jet airplane flying qualities.
Stalls
The stalling characteristics of the sweptwing jet airplane can
vary considerably from those of the normal straight wing
airplane. The greatest difference that will be noticeable to
the pilot is the lift developed vs. angle of attack. An increase
in angle of attack of the straight wing produces a substantial
and constantly increasing lift vector up to its maximum
coefficient of lift, and soon thereafter flow separation (stall)
occurs with a rapid deterioration of lift.
By contrast, the sweptwing produces a much more gradual
buildup of lift with a less well-defined maximum coefficient.
This less-defined peak also means that a swept wing may not
have as dramatic loss of lift at angles of attack beyond its
maximum lift coefficient. However, these high-lift conditions
are accompanied by high drag, which results in a high rate
of descent. [Figure 15-13]
The differences in the stall characteristics between a
conventional straight wing/low tailplane (non T-tail) airplane
and a sweptwing T-tail airplane center around two main areas.
• The basic pitching tendency of the airplane at the stall.
• Tail effectiveness in stall recovery.
15-11