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Quest for Performance: The Evolution
of Modern Aircraft
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- Part II: THE JET AGE
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- Chapter 10: Technology of the Jet
Airplane
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- High-Lift Systems
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- [271] Increases
in the capability of high-lift devices have always accompanied the
use of higher wing loadings. This trend has been particularly
evident in the evolution of the modern jet transport aircraft.
Data given in chapter 3 of reference 176 show that airplane maximum lift coefficients of
about 3 are being obtained in flight on modern operational jet
transport aircraft. The corresponding two-dimensional airfoil
maximum section lift coefficients for the flapped sections are
probably somewhat in excess of 4. By comparison, the data in
figure 7.5 show that airplane maximum lift coefficients slightly
in excess of 2 were being achieved by the end of World War II. The
technology for achieving two-dimensional maximum lift
coefficients, without boundary-layer control, of about 3.2 existed
at the end of World War II, as shown by the comparative data in
figure 5.3.
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- The high-lift system employed on modern
jet transport aircraft consists of an assortment of various types
of leading- and trailing-edge devices. A number of these devices
and the manner in which they are mechanically actuated are
described in reference 197. Although the detail design and relative
effectiveness of the different devices vary, the basic means by
which they increase the maximum lift coefficient remain the same.
Trailing-edge devices are designed to increase the effective angle
through which the flow is turned and thus increase the lifting
capability. Leading-edge devices are basically designed to assist
the flow in negotiating the sharp turn from the lower surface,
around the leading edge, and back for a short distance on the
upper surface, without separating. Modern high-lift devices as
used on large transport aircraft form the subject of the next few
paragraphs.
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- Two typical high-lift configurations are
shown in figure 10.25. A wing section equipped with a leading-edge
slat and a triple-slotted [272] trailing-edge flap is shown in figure
10.25(a). The trailing-edge flap deploys rearward and downward and
separates into three components. The slots in the flap allow flow
from the lower surface to the upper surface. The flow through the
slots energizes the boundary-layer flow on the top surface, which
is negotiating a positive pressure gradient, and prevents
separation and subsequent loss of lift. The detail design of the
slot contours is very critical and must be carefully worked out in
wind-tunnel studies. Both the leading- and trailing-edge devices
are completely retracted in cruising flight and are only deployed
for landing and takeoff.
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- A wing section equipped with a
leading-edge Krueger flap and a trailing-edge double-slotted flap
is shown in figure 10.25(b). The Krueger flap is somewhat less
effective than the slat but is probably simpler in mechanical
design. Some aircraft employ slats on the outboard portion of the
leading edge, where more powerful flow control is required, and
Krueger flaps on the inboard portion of the leading edge. The
double-slotted trailing-edge flap is not as powerful as the
triple-slotted flap but is mechanically simpler and easier to
implement than the triple-slotted flap. The simple single-slotted
flap is often used as a trailing-edge device. This flap consists
of a single unsegmented....
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- (a) Airfoil with triple-slotted
flap, slat, and spoiler.
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- (b) Airfoil with double-slotted
flap and Krueger flap.
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- Figure 10.25 - Typical flap
systems employed on jet-powered aircraft.
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- [273]....element that
is deployed by moving rearward and downward. Although less
effective than either of the other two types of trailing-edge
devices described, it is by far the most mechanically simple of
the three, and the aerodynamic design is the simplest. Many other
types and combinations of high-lift devices may be used on jet
transport aircraft. The types shown in figure 10.25 are only
intended to be representative of typical installations.
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- Also shown on the upper surface of the
wing in figure 10.25(a) is a spoiler in the deployed position. The
spoiler is flush with the wing surface when retracted. The action
of the spoiler in the deployed position is to "spoil" or separate
the flow downstream. The lift of the wing is therefore reduced and
the drag increased. These two aerodynamic effects are utilized in
several ways. When deployed on only one wing of an aircraft, they
cause that wing to drop and thus serve as a lateral-control
device. The wings of many jet transport aircraft employ several
spoiler elements on each wing. These elements may act
simultaneously or in reduced-number, depending on the flight
condition and the function they are intended to fulfill. Some
elements of the spoilers are frequently used in combination with
conventional ailerons to assist in lateral control. The mix
between ailerons and spoilers varies with the flight conditions
under which the aircraft is operating. For example, the dynamic
pressure corresponding to cruising flight at 35000 feet and a Mach
number of 0.8 is 223 pounds per square foot, whereas that for an
approach speed of 135 knots at sea level is 60 pounds per square
foot. The need for additional lateral-control devices for flight
at low speeds, as compared with cruising flight at high Mach
numbers, is clearly shown by the difference in the dynamic
pressure for the two flight conditions.
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- The spoilers are also used to reduce lift
and increase drag when deployed symmetrically, that is, in the
same manner on each wing. The spoilers are usually deployed in
this way immediately after touchdown on landing to assist in
stopping the aircraft. The increased aerodynamic drag serves as a
braking function for the aircraft, and the reduction in lift
increases the percentage of the aircraft weight on the runway and
thus increases the effectiveness of the wheel brakes. Many
aircraft also utilize symmetrical deployment of the spoilers in
flight to increase the rate of descent, for example, to comply
with air-traffic-control requirements during the transition from
high-altitude cruising flight to flight in the terminal
area.
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- [274] Figure 10.26 - Lower-surface view of triple-slotted
flap on Boeing 737 airplane. [NASA]
[Original photo was in color, Chris Gamble, html editor]
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- Figure 10.27 - Upper-surface
view showing triple-slotted flap and spoilers on Boeing 737
airplane. [NASA] [Original photo
was in color, Chris Gamble, html editor]
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- [275] Two views of a
triple-slotted flap installed on a Boeing 737 aircraft are shown
in figures 10.26 and 10.27. The large fairing shown on the lower
side of the wing and flap in figure 10.26 houses the mechanism for
deploying the flap. The four segments of the spoiler system
employed on each wing are shown in the deflected position in
figure 10.27. The leading-edge slat is shown in the deployed
position in figure 10.28.
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- Figure 10.28 - Lower-surface
view of leading-edge slat on Boeing 737 airplane. [NASA] [NASA] [Original photo was in color, Chris
Gamble, html editor]
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