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Aviation week and space technology
Wraps Come Off U.S. Navy's First Tailless, Stealthy Unmanned Aircraft
Northrop Grumman's X-47B unmanned combat air system (UCAS)
demonstrator reveals an unprecedented emphasis on all-aspect stealth
for the maritime environment.
Unveiled at the company's Palmdale, Calif., site on Dec. 16, the X-47B
is the U.S. Navy's first dedicated stealth aircraft since the ill-
fated General Dynamics/McDonnell Douglas A-12, canceled in 1991. The
X-47B is designed to demonstrate technology for a naval UCAS that
would perform the stealthy strike mission originally intended for the
A-12, but with the much-increased range and endurance of an unmanned
The program's focus on low observability (LO) at sea potentially puts
the Navy on a fast track to catch up with the U.S. Air Force in
stealth design. But to be a viable option for the Navy's emerging F/A-
XX requirement for a 2025-timeframe strike aircraft, the UCAS must
show that it can replace a manned aircraft on carrier flight decks.
"She is as stealthy as she looks," says Scott Winship, Northrop
Grumman's Naval UCAS program manager. The tailless flying wing
combines "all-spectrum, all-aspect" low observability with the
ruggedness required for day-to-day shipboard operations, he adds.
Incorporating the B-2A bomber's low radar cross-section (RCS) design
aspects with the systems and control features of the much smaller
X-47A Pegasus demonstrator, the X-47B makes use of a wing-fold system
derived from the A-12.
The first air vehicle, AV-1, is scheduled to fly on Nov. 11, 2009,
while AV-2 will be completed around December 2009. Both will be used
to evaluate the viability of an unmanned combat aircraft in carrier
operations; the first X-47B carrier landing is expected in November
The outer 16 ft. of each wing folds up 135 deg. to reduce the overall
62.1-ft. wingspan to 30.9 ft. Achieving this with a wing-fold
mechanism that did not compromise the RCS with a "bulge" in the outer
mold line of the wing "was one of the more difficult design problems,"
says Winship. The same requirement drove the A-12 designers to develop
a "double roll" hinging mechanism that was modified by Northrop
Grumman to keep the X-47B wing skin smooth on both sides. The A-12
never advanced beyond the mockup stage.
The wing hinge line and narrowly slotted fold zone, although visible
on AV-1, are expected to be given LO treatment with blade seals
similar to those developed to cover the gaps between the edges of the
elevons and ailerons as well as the wing trailing-edge "island"
supporting structure. These were not shown at the rollout because of
their proprietary design. The edges of the island will also be inlaid
with conventional radar-defeating diamond, or cat's-eyes, shapes, says
Winship. GKN Aerospace developed the X-47B's composite skins, covers
and doors, and was responsible for the design, tooling and manufacture
of the outboard wing and the forward center fuselage section.
GKN also developed large blade seals to minimize potential radar
returns from the trailing-edge recess cavity exposed during aileron
movement. Described simply by Winship as a "new material," the radar-
absorbing material used in the broad seal is flexible and spring-
mounted to maintain tension over the forward section of the aileron.
The seal therefore moves with the aileron, but snaps shut to become
flush with the wing surface with the ailerons in a neutral position.
Roll and yaw control is also executed with very large spoiler panels
measuring 3 X 5 ft. on the upper wing surface forward of the ailerons.
Elevons can be drooped by 20 deg. for landing, but no deflection is
required for the catapult launch, says Winship. "We generate lift in a
hurry, so much so that we can launch off in any direction," he adds.
The company believes that being able to launch from the deck,
regardless of wind direction, significantly increases operational
flexibility as the carrier does not need to alter course into the
wind. "I don't know of any other aircraft that can do that," he says.
Lockheed Martin was responsible for refining the detailed low-
observable features of the leading and trailing edges, control
surfaces and engine inlet. The work was largely perfected on a full-
scale RCS pole model at Lockheed Martin's Helendale measurement
facility in the Mojave desert, about 25 mi. from Palmdale.
The shift to the larger, winged configuration of the X-47B away from
the sharp, diamond-kite-shaped X-47A - with a 55-deg. backward sweep
on the leading edge and a 35-deg. forward sweep on the trailing edge -
did not force Northrop Grumman "to give up on stealth," says Winship,
who describes the larger UCAS as a "six-pointer instead of a four-
pointer." The X-47B's wing extensions provide greater range and
superior flying qualities compared with the unstable X-47A, which
Winship called a "Flying Dorito" after the shape of a popular brand of
tortilla chip. The A-12 was also given the same nickname.
Structural proof tests will be conducted in a loads test rig scheduled
to run from March to May 2009. The tests will simulate carrier landing
and critical flight loads, as well as check the structure for catapult
and arrestment loads, fuel system integrity and control-surface
freedom under load. From June 2009 onward, engine run-ups and taxi
tests should take place prior to the vehicle's transfer to nearby
Edwards AFB for flight tests. The X-47B may fly the short distance or
"be trucked," he adds.
Following first flight, AV-1 will undergo a year-long envelope-
expansion test period at Edwards before being ferried to the naval
test center at NAS Patuxent River, Md. This phase will focus on
working up to carrier demonstrations, and will include catapult
certification tests at Lakehurst Naval Air Engineering Station, N.J.
Further tests will include a period at Norfolk, Va., where it will be
craned onto the deck of a Nimitz-class carrier for dock-side taxi
A final phase at Patuxent will be followed by a November 2011 landing
on a carrier at sea. This is likely to be the USS Truman, which "right
now looks as if it's going to be in the right place at the right
time," says Navy N-UCAS program manager Capt. Martin Deppe.