AEROSPACETESTINGINTERNATIONAL.COMMARCH 2015 |^89
Harrier engine upgrade z
which were often exacerbated by
damage from material drawn into
the engine during operation. From
completed analysis, it was determined
that the dynamic mode most likely to
cause issues was a part speed stall
flutter mode, which gives rise to high
stress levels at several locations on
the blade.
The importance of this mode was
further underlined by an analysis of
the operating regime of the Harrier
aircraft, which indicated that
considerable time was spent operating
in the speed range at which this mode
was prevalent.
Analysis and practical testing was
able to show that control of this flutter
mode was highly dependent on the
tribology of the snubber faces. During
operation, the snubbers can become
locked due to centrifugal and
aerodynamic loads on the blade.
Excitation of the stall flutter mode can
reverse the loading on the snubbers,
leading to a situation whereby higher-
order modes become excited due to
stick-slip motion as the snubber faces
move relative to each other.
The snubber faces are coated
with tungsten carbide and a dry film
lubricant, and will lock when the
critical coefficient of friction,
determined by the angle of the snubber
face is exceeded. For the existing
design, having a snubber angle ofCOALITION DEVELOPMENT
The US Marine Corps (USMC) has flown the first BRU-
70/A digital improved triple ejector rack (DITER)-
equipped AV-8B Harrier aircraft in support of the
US-led coalition campaign against the Islamic State
(IS) militants in Iraq and Syria. Deliveries of the DITER
production units were originally scheduled for June
this year, but were fast-tracked after USMC requested
for the systems to maximise current mission capability
in December 2014.20,280
The thrust in pounds generated
by the Pegasus engine340
The number of Harrier variant
AV-8Bs manufactured20°, this value is 0.36. It was indicated
that loss or lack of lubrication between
the two faces can impact the behavior
of the mode.
In order to understand this
phenomenon, a friction wear test was
initiated at the Rolls-Royce sponsored
University Technology Centre (UTC) at
Oxford University. This test was set up
to simulate the relative motion between
two snubbers in service and was
performed at a frequency in line with
what would be seen in service. The rig
used is shown in the image on the left.
The results from the rig enabled
an understanding of the bedding-in
process and indicated that the level
of friction between the two faces
increases as the dry film lubricant
wears away and debris builds up. TheLEFT: Friction wear
part test rig