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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 13: Jet Transports
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- New Transports of the
1980's
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- [448] The year 1982
saw the introduction into airline service of an entirely new
American-built jet transport, and a second new aircraft by the
same manufacturer began service in 1983. Neither aircraft is of
the "jumbo-jet" size such as discussed above. Both are designed to
supplement and ultimately replace current transports in use on
medium- and short-range stage lengths. In the design of both
aircraft, increased fuel costs led to great emphasis on improved
flight efficiency and careful matching of range-payload
capabilities to specific airline needs. A few details of the two
new aircraft are given below.
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- Boeing 767-200
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- With a first flight date in September
1981, the Boeing 767-200 entered airline service in the late
summer of 1982. The aircraft is shown in figures 13.27 and 13.28,
and physical and performance data for one version of the aircraft
are given in table
VII.
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- The Boeing 767-200 is a 290-passenger,
double-aisle, wide-body airliner designed to replace the aging
Boeing 707 and McDonnell Douglas DC-8 transports now used on
domestic and foreign medium range route segments. Average stage
lengths over which the aircraft will be operated are estimated by
Boeing to lie between 850 and 1150...
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- [449] Figure 13.27 - Rollout of Boeing 767-200 wide-body,
twin-engine airliner. [mfr]
[Original photo was in color, Chris Gamble, html editor]
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- ....miles. Maximum range is, of course,
much greater and includes a nonstop United States coast-to-coast
capability; the twin-engine 767-200 is not intended for long
over-ocean flights.
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- As can be seen in figures 13.27 and 13.28,
configuration of the Boeing 767-200 is conventional with the wing
located in the low position at the bottom of the fuselage and with
one of the two engines pylon mounted beneath each wing. Location
of the engines under the wing, rather than to the rear of the
fuselage, allows the horizontal tall to be mounted in the low
position. As discussed in chapter 10, a low tall position is helpful in minimizing
pitching-moment nonlinearities that are often characteristic of
sweptback wings at angles of attack in the vicinity of the stall.
The main landing gear consists of two struts, each with a
four-wheel bogie, that retract inward into the wing root.
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- Although of conventional configuration,
the detailed aerodynamic design of the 767-200 is highly refined,
as might be expected by the nearly 25 000 hours of wind-tunnel
time required in the development of the aircraft. To place this
wind-tunnel effort in perspective, 14 000 and 4000 wind-tunnel
hours were expended in developing the Boeing 747 and 727,
respectively.
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- The Boeing 767-200 has been widely
advertised as being much...
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- [450] Figure 13.28 - In-flight view of Boeing 767-200
airliner. [mfr] [Original photo was
in color, Chris Gamble, html editor]
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- ....more fuel efficient than earlier
generations of jet transports. Although the careful aerodynamic
design just mentioned contributes to the efficiency of the
aircraft, the high-bypass-ratio turbofan engines employed on the
767-200 are primarily responsible for its high fuel efficiency. At
present, the 767-200 is offered with two versions of both the
Pratt & Whitney JT9D and the General Electric CF6 turbofan
engines. Both of these engines are in the 48 000- to 50 000-pound
thrust range and have bypass ratios between 4.5 and 5.0 and
compressor pressure ratios between 25 and 30. Specific fuel
consumption of these engines, expressed in pounds of fuel per
pound of thrust per hour, is between 20 and 25 percent lower than
that of the Pratt & Whitney JT3D engine that powers both the
McDonnell Douglas DC-8 and the Boeing 707.
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- Comparison of certain characteristics of
the 767-200 with those of the older Boeing 707-320B is of
interest. Examination of the data in table VII shows that the wings of the two aircraft are nearly
the same size, with only small differences in area and span.
Sweepback angle and aspect ratio of the new 767-200 are 31.5°
and 8.0, respectively, as compared with 35° and 7.1 for the
707-320B. These differences in wing geometry would be expected to
increase aerodynamic efficiency by a small amount. Incorporated in
the wing of the 767-200 is a new Boeing-developed
supercritical-type airfoil section. The basic technology of the
supercritical airfoil section was pioneered by Richard T. Whitcomb
of the NASA Langley Research Center. Use of such sections allows
increased wing thickness ratio without corresponding reductions
[451]
in the Mach number at which large adverse
compressibility effects begin to occur. Reduced wing structural
weight, increased aspect ratio and reduced wing sweepback angle-or
some combination of the three-are accordingly possible.
Incorporation of this new type of airfoil section on the Boeing
767-200 contributes to high overall
efficiency.
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- High-lift devices on the wing consist of
full-span leading-edge slats
and a combination of both single-
and double-slotted flaps on the trailing edge, with the
double-slotted flaps placed on the inboard part of the wing.
Inboard and outboard ailerons in combination with spoilers are
used for lateral control. When deployed symmetrically, the
spoilers help decelerate the aircraft on the landing rollout and
aid in rapid in-flight descents. An elevator and adjustable
stabilizer are used for longitudinal control, and a conventional
rudder is provided for control about the yaw axis. All controls
are of the fully powered, irreversible type.
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- New techniques for navigation and flight
control are used on the 767-200. These techniques herald an
entirely new relationship between the aircraft and the flight
crew. An automatic flight control system coupled with a computer
allows storage of an entire flight plan and gives automatic
guidance and control of the aircraft from takeoff to landing.
Included in the system are not only navigation functions but
vertical flight-path control to minimize fuel consumption. To a
large extent, the traditional electromechanical instrumentation
has also been replaced by more simple cathode-ray-tube displays
that provide different types of information at the command of the
crew. A detailed description of this new equipment and its use are
beyond the scope of the present discussion. Let it be noted,
however, that aircraft such as the 767-200 may herald the end to
most hands-on flying of transport aircraft and introduce an age in
which the pilot is increasingly a button-pushing systems manager.
Automatic flight management techniques such as those employed in
the new Boeing transports will certainly result in more efficient
fuel utilization in future airline operations.
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- All versions of the 767-200 can
accommodate a maximum of 290 passengers seated in a 7-abreast
double-aisle configuration. The aircraft is now offered in six
variants, with gross weights falling in the range from 302 000 to
337 000 pounds. Listed in table VII are the characteristics of the 337 000-pound
version of the aircraft, which is powered by two Pratt &
Whitney JT9D-7R4E turbofans of 50 000 pounds thrust each. This
particular variant of the aircraft, available in 1984, has nearly
the same gross weight as the Boeing 707-320B but [452] carries about
100 more passengers over a much shorter range. Maximum cruising
speed of the 767-200 is about 40 miles per hour slower than that
of the 707-320B, and takeoff and landing field lengths of the new
aircraft are significantly shorter than those of the 707. These
differences in speed and field length reflect the differing
requirements of a long-range aircraft designed for international
operations and one designed for medium-range domestic use.
Aerodynamic efficiency of the 767-200 can be judged by the maximum
lift-drag ratio, estimated to be about 18. The larger ratio of
wetted area to wing area of the 767-200, as compared with that of
the 707-320B, results in a value of (L/D) max somewhat lower
than that of the older aircraft. The much larger passenger
capacity and more efficient engines, however, make the new
aircraft more efficient in terms of cost-per-seat-mile.
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- The Boeing 767-200 has just entered
airline service, and although its characteristics seem highly
promising, its ultimate place in the spectrum of successful
transport aircraft has yet to be determined.
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- Boeing 757-200
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- The second new Boeing jetliner of the
1980's, designated the 757-200, made its initial flight in
February 1982 and is scheduled to enter airline service in the
spring of 1983. Intended as a fuel-efficient replacement for the
long-lived Boeing 727 on short-range route segments, the 757-200
can accommodate as many as 239 passengers in a single-aisle
six-abreast cabin arrangement. Average route segments are expected
to be about 575 miles or less and to require less than 2 hours'
flight time. A narrow six-abreast single-aisle configuration
usually has slightly less wetted area, and thus less drag, than a
six-abreast twin-aisle arrangement designed for the same number of
passengers. Apparently, passengers are willing to accept the
single-aisle layout for short flights but prefer the more spacious
wide-body design for flight times greater than several
hours.
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- The first 757-200, photographed on the
occasion of its rollout in January 1982, is shown in figure 13.29,
and data for one version of the aircraft are given in table VII. Configuration of this twin-engine aircraft is seen
to be very similar to that of the 757-200 shown in figures 13.27
and 13.28; however, the data in table VII show that the 757-200 has a smaller wing of less
sweepback angle and is much the lighter of the two
aircraft.
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- Like the 767-200, the fuel efficiency of
the 757-200 derives largely from the high-bypass-ratio turbofan
engines employed on the aircraft.
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- [453] Figure 13.29 - Rollout of Boeing 757-200
narrow-body, twin-engine airliner. [mfr] [Original photo was in color, Chris Gamble,
html editor]
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- Listed in reference 150 are the characteristics of three versions of the
757-200. The aircraft currently in production are powered by the
Rolls-Royce RB211-535C engines of bypass ratio 4.36 and thrust of
37 400 pounds. By the end of 1984, the aircraft will also be
available with the Rolls-Royce RB211-535E4 engine of 40 000 pounds
thrust, or with the all-new Pratt & Whitney 2037 turbofan of
40 000 pounds thrust. All three of these engine types offer a 20-
to 25-percent reduction in cruise-specific fuel consumption as
compared with the various versions of the Pratt & Whitney JT8D
engine that powers the 727.
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- Many of the design features of the 767-200
described above are incorporated in the 757-200. The same
supercritical airfoil section is employed in the wings of both
aircraft. The high-lift and lateral control systems of the two
aircraft are nearly the same, although some differences are
evident in the trailing-edge flaps and ailerons. In contrast with
the 767-200, the trailing-edge high-lift system of the 757-200
consists almost entirely of double-slotted flaps, and no inboard
ailerons are used. Cockpit layout and automatic flight control and
navigation systems are essentially identical on the two
aircraft.
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- Gross weight of the version of the 757-200
now in production is 221 000 pounds. With the more advanced
Rolls-Royce and Pratt & Whitney engines, gross weight will be
241 000 pounds. The data in table VII are for the heavier version of the aircraft powered
with Pratt & [454] Whitney 2037
engines. As compared with the 727-200 it has been designed to
replace, the 757-200 is shown by the data in table VII to be larger and heavier and to have a larger
passenger capacity. Wing sweepback angle of the new aircraft is
7° less than that of the 727-200, and the maximum cruising
speed is 49 miles per hour lower than that of the older aircraft.
This speed differential is relatively unimportant on the
short-range segments for which the aircraft is intended; also, it
reduces fuel consumption.
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- As with the larger 767-200, only time and
experience will measure the success of the 757-200 in airline
operation and in the domestic and international
marketplace.
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