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substantial loss of altitude.
• Thrust can be increased to accelerate the airplane
to a speed above VMD to reestablish steady flight
conditions. It should be remembered that the
amount of thrust required will be quite large. 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, VMD in the clean
configuration is normally at a speed of about 1.3 VS.
[Figure 15-12] Flight below VMD on a piston engine
airplane is well identified and predictable. In contrast,
in a jet airplane flight in the area of VMD (typically 1.5
– 1.6 VS) 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
Minimum
Power
Required
L/DMAX
Figure 15-12. Thrust and power required curves.
Ch 15.qxd 5/7/04 10:22 AM Page 15-10
15-11
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 no well defined maximum
coefficient and has the ability to fly well beyond this
maximum buildup even though lift is lost. The drag
curves (which are not depicted in figure 15-13) are
approximately the reverse of the lift curves shown, in
that a rapid increase in drag component may be
expected with an increase in the angle of attack of a
sweptwing airplane.
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.
On a conventional straight wing/low tailplane airplane,
the weight of the airplane acts downwards forward of
the lift acting upwards, producing a need for a
balancing force acting downwards from the tailplane.
As speed is reduced by gentle up elevator deflection,
the static stability of the airplane causes a nosedown
tendency. This is countered by further up elevator to
keep the nose coming up and the speed decreasing. As
the pitch attitude increases, the low set tail is immersed
in the wing wake, which is slightly turbulent, low
energy air. The accompanying aerodynamic buffeting
serves as a warning of impending stall. The reduced
effectiveness of the tail prevents the pilot from forcing
the airplane into a deeper stall. [Figure 15-14] The
conventional straight wing airplane conforms to the
familiar nosedown pitching tendency at the stall and
gives the entire airplane a fairly pronounced nosedown
pitch. At the moment of stall, the wing wake passes
more or less straight rearward and passes above the
tail. The tail is now immersed in high energy air where
it experiences a sharp increase in positive angle of
attack causing upward lift. This lift then assists the
nosedown pitch and decrease in wing angle of attack
essential to stall recovery.
In a sweptwing jet with a T-tail and rear fuselage
mounted engines, the two qualities that are different
from its straight wing low tailplane counterpart are the
pitching tendency of the airplane as the stall develops
and the loss of tail effectiveness at the stall. The
handling qualities down to the stall are much the same
as the straight wing airplane except that the high, T-tail
remains clear of the wing wake and provides little or
no warning in the form of a pre-stall buffet. Also, the
tail is fully effective during the speed reduction
　
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