A_T_I_2015_04_

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APRIL 2015

AEROSPACETESTINGINTERNATIONAL.COM

company proposed a number of design

changes to the blade to mitigate this

issue: the angle of the snubber face

was increased to 25° to make the

adjacent faces slide more easily and

an improved under-platform damper

was introduced to control the dynamic

response of the blade.

VALIDATION PROGRAM

Having identified the design solution,

the company’s efforts turned to

proving the change. It was established

that the design changes would be

validated by both rig testing and

engine running. The validation process

commenced with a frequency survey

of the revised blade standard and the

results compared with the data

previously available from the existing

standard. With the results of the

frequency survey in line with pre-test

FE predictions, the next step was a

fatigue test of the new blade. Six

sample blades were tested to establish

each of their failing fatigue strengths.

Each blade was mounted on a

vibration table and restrained at the

inner dovetail fixing and the snubber

to accurately represent engine

conditions. The rig used is shown

in the figure below right.

From the testing, it was

demonstrated that the basic strength

of the new blade was within the

scatter of the legacy blades and that

no new potential failure modes had

been introduced.

ENGINE TRIALS

Rolls-Royce followed a structured

process to establish the verification

and validation requirements for the

revised blade. It was determined by

the engineers that a number of engine

tests would be necessary to gather all

the evidence required to clear the new

blade for operational duty.

The last production engine was

shipped from the Rolls-Royce Filton

factory in 2005 and the testbed was

mothballed in 2010. Therefore

establishing an engine test capability

was the first major challenge. The first

of the engine tests, to calibrate the

testbed, was completed in May 2013.

With the testbed deemed serviceable,

the program to validate the design change

could begin in earnest. The first test of the

structured validation program was a build

of the engine, incorporating its existing

baseline configuration, against which the

intended design improvements could be

assessed. The build of the engine

incorporated two systems

to measure the dynamic response

of the blade.

between adjacent snubber faces is also

a major contributor to blade lock up.

In order to promote flutter during the

test, the fan assembly was built with

dry film lubricants removed and

snubber gaps at the bottom limit of

tolerance. Fan blade flutter can be

influenced by a number of factors,

the two principal being engine inlet

distortion and blockage downstream

of the fan. During the test program

the blockage in the front (cold) nozzles

of the engine was varied to further

promote flutter.

The first of these tests was completed

in September 2013 and featured the

existing blade fitted with the revised

damper. The purpose of the test was to

establish the individual benefit of the

damper, with the testing completed in

the flutter promoting configuration.

The next test, again in the flutter

promoting configuration, featured both

the revised blade and the damper. The

final test, completed in August 2014,

featured the revised blade and damper

in the production standard

configuration with all surface

lubrication reapplied. The results of

the test program clearly showed that

the redesign of the blade and the

incorporation of the new damper

led to a great reduction in flutter

amplitude of the blade. In the final

production standard configuration,

flutter has been all but eliminated.

Despite the twin technical and

logistical challenges, the Rolls-Royce

team compiled a comprehensive

understanding of the issue and devised

a substantial design improvement,

which is currently being evaluated by

the customer. z

Tim Williams is chief engineer – Pegasus,

transport and helicopter engines, with Rolls-

Royce plc, based in Filton, UK

900

The maximum range in nautical

miles of the USMC Harrier variant

1,347

The number of Pegasus

engines produced

9.25

The wingspan in meters

of the USMC Harrier

The first system is the blade tip

timing, which uses optical probes to

detect the arrival time of the tip of the

blade at a number of points around the

casing. The system measures the speed

of the engine spool and differences in

the time at which the tip of the blade

passes the probe compared with the

expected timing, and can be used to

determine the amplitude and

frequency of any blade resonance.

The second system is the FM grid,

and is similarly used to measure blade

dynamic response with a magnet

attached to the blade tip and a

conductive wire grid around the

engine casing. The motion of the

blade tip causes a signal to be induced

in the grid, which is proportional to

the frequency and displacement of the

blade tip and from which the blade

dynamic characteristics can be

deduced. Subsequent engine builds

and testing were aimed at

understanding the impact of the

individual improvements in the

proposed design changes.

As discussed earlier, the propensity

for flutter is dependent on the

lubrication of the snubber and other

contact faces on the blade. The gap

BELOW: (Below)

Vibration rig used

for blade fatigue

strength evaluation

z Harrier engine upgrade

ABOVE: Southern

Helmand province,

Afghanistan after

conducting an

aerial refuel