June 18, 2012
When Boeing executives talk about sticking to the basics in their 737 MAX reengining program, they are not kidding.
Besides
the improvements they expect from the new aircraft's CFM Leap-1B
engines, Boeing is keeping a tight rein on the technology risks of the
biggest upgrade to the 737 since the Next Generation series was launched
nearly 20 years ago. To make the MAX work, Boeing needs to distinguish
it from rival Airbus's A320NEO while raising the benchmark of what its
customers can expect in performance and reliability.
The MAX
must be sufficiently advanced to achieve double-digit percentage point
improvements in fuel burn and operating efficiency over the NG. Engines
and aerodynamic improvement carry most of that burden, but there are
numerous technology swap-outs that the company might include in MAX to
make the airplane more attractive.
However, all of them come at a price,
not the least of which is their potential disruption of a finely honed
manufacturing process at Boeing's Renton single-aisle jet factory south
of Seattle, which is midway through the biggest increase in 737
production rates in history.
It is essential that Boeing get
right this next phase in the four-decade 737 story. The family is a
priceless asset and must remain so. Boeing expects 70% of all aircraft
sales in the next two decades to be single-aisle transports.
Since
deciding last August against its New Small Airplane (NSA) project in
favor of the MAX, Boeing has been cautiously working out just how far it
needs to go with technology advances in order to compete with the NEO.
The company's designers felt they had a game-changing prospect in the
NSA that would trump a basic engine upgrade to the A320. At first, so
did Boeing's customers.
But as fuel costs rose, they began asking for
relief sooner than the NSA would be available. The overwhelmingly
positive response Airbus gained from the NEO—it quickly shot past the
1,000-order mark—pushed Boeing to shelve the NSA.
With the MAX,
Boeing wants to avoid the temptation of adding cost and complexity to
the 737 program; it needs to stay focused on what airlines value most,
says the MAX's chief project engineer, Michael Teal. “Customers are
looking for improved economics,” he says.
Teal comes to the MAX
from the Boeing 747-8 where, as chief engineer, he witnessed firsthand
what can happen when unexpected issues turn a fast-tracked derivative
into an over-budget development marathon. Those harsh lessons are
keeping the MAX team focused on its development schedule.
Boeing Commercial Airplanes President and CEO Jim Albaugh has hinted
that a service entry for the first MAX might be brought forward from the
official goal of the fourth quarter of 2017.
But Teal says that is only
“if we can.” There will be no overconfident promises like those made
early in the 787 and 747-8 programs that erupted into embarrassing
schedule lapses. “I was on those phone calls in 2008, and I didn't like”
them, he says.
Boeing will spend the rest of this year
“getting the final concept done” before marching on to a firm
configuration in mid-2013, Teal says. Design will take place in 2014,
assembly in 2015 and first flight in 2016. The 737-800-sized MAX 8 is to
be produced first.
This plan reflects the evolution of airline
orders for single-aisle jets. In the early days of the 737NG program,
the 126-149-seat 737-700 was easily the best-seller, in no small part
because of its popularity with launch customer Southwest Airlines. Like
many others, Southwest—also the MAX launch customer—is now ordering
bigger single aisles, having moved up to the 162-189-seat 737-800.
This
up-gauging trend is widespread. As of May, there were 1,415 orders for
the 737-700, which entered service in December 1997; for the -800, which
came on the scene in April 1998, there were 4,053—more than all
737-100s, -200s, -300s, -400s, -500s and 600s combined.
There is
a bigger backlog for the 737-900ER, the closest thing Boeing has to a
757 replacement, than for the -700, even though it entered service a
half-decade later. Consequently, the MAX 9 will be the next to enter
service, in 2018, and the MAX 7 will follow in 2019.
After
considering a huge range of design options—including split trailing
edges and hybrid laminar flow—Boeing's choices for the MAX underscore
how it is restricting itself to a strict diet to assure as smooth a
production transition as possible to the MAX from the NG.
The
CFM Leap-1B engine is the principal reason Boeing anticipates a 13%
reduction in fuel burn compared to the 2012-standard Next Generation
737.
The engine's fan is expected to slightly exceed 69 in. in diameter,
so the Leap is larger and heavier than the CFM56-7B. But that weight is
more than offset by the Leap's larger 8.5:1 bypass ratio, which will
contribute an anticipated 11% fuel burn benefit (see p. 61). Lower drag
in the aft fuselage and introduction of novel “dual-feather” winglets
account for the rest.
The all-important engine installation is
an evolution for the 737 and builds on the mounting design used for the
787. The installation moves the engine “a little forward and up,” says
Teal.
By cantilevering the engine out ahead of the wing, Boeing is
avoiding the need for a dry bay above the engine, thereby preserving
fuel volume. “It's not new technology, but we wanted a little more room
under the nacelle,” says Teal. The bottom of the MAX nacelle will be 17
in. off the tarmac, 1 in. less than an NG's.
The nose-wheel
landing gear is 8 in. longer than the 737's and prompted Boeing to move
the front bulkhead of the nose undercarriage bay—the “doghouse”—and an
associated inspection hatch forward about 8 in.
The leg extension also
means that an aerodynamic fairing is required to accommodate the bulge
of the nose wheel. “We're trying to minimize this, and we're still
trying to make it smaller,” says Teal. Nonetheless, he says the
aerodynamic impact is negligible.
Building on the 787 program's
application of a natural laminar flow (NLF) nacelle, Boeing is “looking
at opportunities as to how we can keep the NLF attached” in the MAX,
Teal says. But the option of a hybrid laminar flow control system (HLFC)
for the vertical fin will not be undertaken.
It was originally studied
as part of the interim “737NG Plus” upgrade that was to be a gap-filler
between the 737NG and NSA. “There is the complexity [in the design] and
the build of it, as well as [its] questionable value on short flights,”
Teal explains. The HLFC system has been developed as a drag-saving
device for the stretched 787-9 and, pending full development, is
expected to be offered as a 787-8 performance upgrade.
The
recently announced dual-feather winglet is the most distinguished
external feature of the MAX. The baseline blended winglet is credited
with providing a 3-4% fuel-burn improvement over a 737 without winglets.
Boeing expects the MAX's feathered design to save up to 5.5% in fuel
burn, or the equivalent of an additional 1-1.5% above the 737NG
standard, says Teal.
The feathered winglet integrates a downward-tilted version of
Boeing's raked-tip configuration with a more conventional winglet. Its
ground clearance of 10 ft. 2 in. will be 2 in. greater than the NG's
winglet.
The new design has undergone low-speed wind tunnel
tests at Qinetiq's U.K. facility and Boeing's transonic tunnel. “The
data showed it performed as expected,” says Teal.
The winglet design
team will perform further work on the concept as the fuselage's design
details and build plans firm up in 2013. The dihedral of the winglet,
combined with anhedral of the modified raked tip, assures that the MAX's
wing span is within the “Code C” gate size of the 737NG family.
The
fuselage's aerodynamic cleanup is focused on the very aft section just
behind the auxiliary power unit (APU). This marks the first tail cone
taper revision since the 737's original short, stubby fuselage design
emerged in the 1960s. The redesign eliminates the need for the vortex
generators that current models use to “help calm down the airflow,” says
Teal. Extending the cone in a 787-style allows the flow to “clean up
nicely,” according to computational fluid dynamics analysis.
A new
low-drag APU inlet also will be integrated into the tail, while a
horizontal root fillet fairing, or “strakelet,” will be added to reduce
drag around the empennage.
Although APU upgrades were considered
as part of the original 737NG Plus package, Teal says none are planned
for the MAX. Instead, system changes will focus on the adoption of
fly-by-wire actuated wing spoilers and a digitally controlled engine
bleed system for the environmental control system.
“The air conditioning
packs are not changing, but the control for getting bleed air is going
digital,” he says. The new spoiler system will save weight and
installation costs.
Since the spoilers also will be connected
directly to the flight control system, they can be used for maneuver
load alleviation (MLA). By symmetrically deflecting the spoilers under
certain conditions, wing-bending loads are reduced. This allows use of a
slightly lighter wingbox.
Other wing changes were considered, including
an improved trailing edge for better low-speed handling. While aspects
of these studies, such as a mini-split flap, are expected to be tested
as part of an upcoming EcoDemonstrator program, Teal says the MAX will
not use them. “I don't think we need them,” he says.
To handle
higher loads associated with the MAX's heavier operating weight, the
airframe will be locally strengthened with regauging of skins, spars and
structures in the fuselage, empennage, wing and landing gear. “If you
have heavier engines, this increases the torsion loads into the body and
these are reacted through pickle forks,” Teal says, referring to
structures in the wing-fuselage join area. The existing design will be
retained but “just gauged up” for the MAX, he adds.
Also under
consideration is the replacement of the longitudinal beam—called a
crease beam—which, in the dual-lobe configuration of the 737 fuselage,
works with the floor beams to smooth out-of-plane loads at the
intersection of the two lobes. “As we work through the certification
basis, if the decompression analysis works out, there might be an
opportunity to go to a one-piece truss,” Teal says.
By the time
the MAX enters production in Renton, Boeing expects the factory to be
producing 42 airplanes per month from the plant's two final assembly
lines. Changes needed to accommodate the new airplane are still being
considered, but the general goal is for MAX fuselages to flow seamlessly
down the line with the NG's. Early planning includes the possibility of
shifting an engine buildup area off Line 1 in Renton's Building 4-82
elsewhere to make room for a proving line for early MAX production.
Spirit
AeroSystems provides the 737's fuselages from Wichita and is still in
the early planning stages for what accommodations will be necessary for
the MAX. But Vice President Forrest Urban, who leads MAX integration as
head of advanced projects, says only minor tooling changes are
anticipated. The company wants to avoid significant changes to the
assembly process in its big Plant 2.
Fuselage alterations, such
as in Section 48 or at the doghouse, will be accommodated offline and
brought to the plant's final assembly, Urban says. This same approach is
used for Boeing's P-8 Poseidon maritime patrol aircraft, which is based
on the 737NG fuselage. Urban expects changes for the MAX to be less
extensive than those for the P-8.
“We think the NG is the most efficient, highest-quality production
process anywhere,” he says. To keep it that way, the company will turn
to its Spirit Exact design-build software process to smooth the MAX's
transition into the 737 line.
As of May, Boeing had recorded 451
MAX orders. The new program is leading the 737's charge past the
10,000-total-order mark. As of last week, Boeing was within 221 orders
of that milestone, which no other commercial jet has reached. The
company does not expect the head start Airbus achieved with the NEO to
affect the sales balance between the A320 and 737 over the long run.
To
European reporters, Vice President Randy Tinseth, Boeing's head of
marketing, said the MAX will build on the 737's “higher lease rates,
higher 'fair market' values and higher residual values” to attract
orders.
With the MAX order count growing, the marketing heat is
on for both manufacturers as they head toward next month's Farnborough
air show.
aviationweek.com
