Put the Tape On Before the Cracks Show Up
The AV-8B Harrier engine inlet ring issues that have been
readiness degraders for the jump jet since 1983 have been solved
with a simple fix that provides a cost benefit and maintenance
avoidance for the fleet.
Dr. Dave Barrett
thought he had a solution based on existing technology and
experience gained from research completed by NAWCAD engineers more
than a decade ago. But Barrett, airframe technology team lead, knew
he had a hard sell on his hands when he suggested that the only
thing needed to stop the out-of-control vibrations and eventual
sonic-induced fatigue in the Harrier’s engine inlet ring was
a little damping.
The Harrier’s inlet ring, an aluminum sheet-metal
duct-like structure running from the frame of the aircraft to the
face of the engine, was experiencing excessive vibration, causing
the inlet ring skin, fasteners and stiffeners to crack.
Looks simple, in retrospect...
"The engine was causing the structure to resonate," said Troy
Hullander, aircraft structural integrity manager for the AV-8B
Weapons System (PMA-257). "Basically at a critical point in the
engine operation, it would make the structure 'sing.' It would
spike up and increase the vibrations in the structure to the point
where we would induce cycles at an extremely rapid rate [and]
causing widespread cracking. Originally [the cracks] started at
just the stiffeners on the structure."
An attempt at a fix by
the original Harrier manufacturer, McDonnell-Douglas Aircraft
Corporation, in 1983 during the AV-8B’s full-scale production
program, was handled by adding more stiffeners and increasing the
ring’s wall thickness.
As the fleet Harriers’ flight hours increased, the engine
inlet cracking returned. The structure would crack at the engine
inlet stiffeners, fasteners and skins. As the cracking of these
components became a foreign object damage risk, the policy was to
repair the cracks as they were found. This issue quickly became a
high maintenance cost and fleet-readiness degrader.
A "tiger team" was assembled to attack the issues with the
Harrier engine inlet ring. The tiger team included participants
from the AV-8B Fleet Support Team and the Structures, Dynamics,
Structural Technology, and Materials and Processing groups within
NAVAIR and Boeing, which acquired the McDonnell-Douglas Aircraft
Corporation and the Harrier program in 1997.
"We came up with 15 different possible fixes," Hullander said,
"from the easy to the absolutely ridiculous. Then we started
down-selecting. That’s when I started getting educated from
Dave about damping systems."
The proposed solution that came out of the tiger team
wasn’t damping, though – at least not at first.
The team took the classical approach to solve the cracking issue
in the engine inlet by stiffening the structure. A modification
concept was developed that increased the stiffness of the structure
and showed promise. However, to install the modification would have
required the removal of over 250 fasteners and a majority of the
stiffeners, and replaced them with stronger parts. A concern arose
within the team that the amount of work that had to be performed to
the engine inlet during modification might cause more damage to the
structure than the vibratory loads of the environment.
'Moving' the problem
isn't the same as 'fixing' it.
But Barrett and Hullander both said that when complex systems
are subjected to vibratory loads, the addition of more material is
not necessarily beneficial and can "chase" the cracking problem to
a new location.
"With the results of Dave’s [in-lab independent research]
studies [on damping] we convinced the program office and they
invested a small amount of money for us to do some design and
verification work," Hullander said.
John Hill, an engineer from NAVAIR Depot North Island on
rotation here at Pax River, set up a team with Jennifer Elmore of
the Airframe Technology Team, and Rimi Rivera and Andy Guy from the
Materials Division. They set up and conducted durability studies to
make sure the configuration the team chose would, once stuck, stay
stuck.
Wriley Gay of NAVAIR Depot Cherry Point provided an inlet ring
and fuselage hulk for test. With the help of Doug Walling, also of
Cherry Point, and through an existing Air Force contract, they sent
the test articles to the University of Dayton Research Institute to
come up with the design and perform the initial testing for
validation and verification.
The Dayton Institute came up with a design that involved
adhering the damping tile to the inlet ring with a visco-elastic
tape. The damping material converts the kinetic energy of the
structure to heat, and thereby reduces the vibration level.
"We went out and modified two aircraft," Hullander said. "One
aircraft now has 120 hours on it. It went over to Iraq. One hundred
twenty hours on it and [the damping tile] stuck."
Hearing that news for
the first time, Barrett said, "That’s a pretty good demo."
And because of the success of the demo, Hullander said, they have
been given the go-ahead to incorporate the damping tiles on the
rest of the fleet, which involves Navy Harriers and Harriers from
four other countries as well.
"Based on the success of this program our division is pursuing
another engineering effort," Barrett said. He has been helping the
S-3 Viking program folks with an inlet problem the Viking is
experiencing.
The Harrier’s engine inlet ring success story is an
example of what the national NAVAIR team can do, Barrett said. One
small investment made in the late 1980s at the Warminster
Independent Research program, headed by Dr. Asha Varma, is paying
great dividends as it already saved one project and four countries
millions of dollars. And, more programs are planning to use damping
as their cost-saving fix.
"The small investment made will give us immense returns on the
investment, and also reduce the readiness problems we have with the
fleet," Hullander.
[Thanks to Jim Jenkins, NAS Patuxent River Public Affairs
Department --ed.]