>> AIRCRAFT ENGINE NOISE
IN AND AROUND AIRPORTS ARE
AN UNDESIRABLE PART
OF AIRPORT OPERATIONS,
AND ADVERSELY IMPACTS
THE QUALITY OF LIFE
IN THE SURROUNDING COMMUNITIES.
PART OF ITS MISSION TO IMPROVE
THE QUALITY OF LIFE,
THE NASA GLENN RESEARCH CENTER
IN LEWIS FIELD PLAYS
AN IMPORTANT ROLE IN
THE DEVELOPMENT OF TECHNOLOGIES
TO REDUCE AIRCRAFT ENGINE NOISE.
THE DEVELOPMENT OF SUCCESSFUL
NOISE REDUCTION STRATEGIES
RELIES HEAVILY ON
UNDERSTANDING THE MECHANISMS
AND CHARACTERISTICS
OF ENGINE NOISE.
A PRINCIPAL SOURCE OF NOISE
IN MODERN COMMERCIAL AIRCRAFT
ENGINES IS THE IMPINGEMENT
OF FAN WAKES ON THE DOWNSTREAM
EXIT GUIDE VANES.
THIS NOISE, OFTEN REFERRED TO
AS ROTOR-STATOR INTERACTION
NOISE, IS PRODUCED
IN DISCRETE FREQUENCIES
CALLED TONES AS WELL AS
BROADBAND FREQUENCIES.
TONES ARE GENERATED
AT REGULAR FREQUENCY INTERVALS,
DETERMINED BY THE BLADE
PASSING FREQUENCY, OR BPF.
TO BETTER UNDERSTAND
THE ROTOR-STATOR INTERACTION
NOISE CHARACTERISTICS,
IT IS NECESSARY TO MEASURE
THE LEVELS OF INTERACTION TONES.
THE ROTATING RAKE SYSTEM,
DEVELOPED AT NASA GLENN
RESEARCH CENTER, IS A UNIQUE
TOOL FOR MEASURING
THE ROTOR-STATOR INTERACTION
TONE LEVELS.
THE ROTATING RAKE SYSTEM WAS
DEVELOPED IN THE ADVANCED
NOISE CONTROL FAN FACILITY.
THE ANCF IS A FOUR-FOOT DIAMETER
LOW SPEED FAN THAT REPLICATES
THE RELEVANT FEATURES
OF TURBO FAN ENGINE TONE NOISE
IN A CONTROLLED ENVIRONMENT.
THE COMBINATION OF
THE EASILY-MODIFIED DUCT
ASSEMBLY WITH THE ROTATING RAKE
SYSTEM HAS ENABLED RIGOROUS
EVALUATION OF THE UTILITY
OF THE ROTATING RAKE SYSTEM.
THE ROTATING RAKE IS SHOWN HERE
IN THE EXHAUST PLANE
OF THE ANCF IN ORDER TO MEASURE
THE NOISE THAT PROPAGATES
TO THE FAR FIELD.
THE USE OF THE ROTATING RAKE
SYSTEM IN THE ANCF RIG
REPRESENTS A LOW TECHNOLOGY
READINESS LEVEL, OR TRL,
APPLICATION OF THE SYSTEM.
THE ROTATING RAKE CAN BE
DEPLOYED AT VARIOUS LOCATIONS
IN THE FAN DUCT INLET
AND EXHAUST IN ORDER TO MEASURE
THE INTERNAL PROPAGATION
OF THE NOISE FIELD.
THIS CAN BE
PARTICULARLY VALUABLE IN
EVALUATING AND VERIFYING
ACOUSTIC PREDICTION CODES.
THE ROTATING RAKE SYSTEM
WORKS AS FOLLOWS.
THE ROTATIONAL SPEED OF THE RAKE
IS SET AT AN EXACT FRACTION--
USUALLY 1/100
OF THE FAN'S SPEED.
IN THE CASE OF ANCF, THAT IS,
15 TO 20 RPM DEPENDING ON
THE SPEED OF THE FAN.
THIS SYNCHRONIZATION
OF ROTATIONAL SPEEDS IS
THE ENABLING PRINCIPLE
OF THE ROTATING RAKE SYSTEM.
THE ROTATION IMPARTS
A UNIQUE DOPPLER SHIFT
ON THE FREQUENCY OF
THE MEASURED ACOUSTIC FIELDS,
SEPARATING THE ACOUSTIC FIELD
COMPONENTS INTO
SLIGHTLY DIFFERENT FREQUENCIES.
THIS DISPLAY OF AN ONLINE
MEASUREMENT FROM
TWO OF THE RAKE MICROPHONES
ILLUSTRATES THE SEPARATION
OF THE ACOUSTIC COMPONENTS.
EACH VERTICAL LINE REPRESENTS
A DIFFERENT COMPONENT THAT
CAN BE ANALYZED INDEPENDENTLY.
IN THE STATIONARY FRAME,
THESE COMPONENTS WOULD BE
THE SAME FREQUENCY,
AND THEREFORE INDISTINGUISHABLE
TO THE OBSERVER.
THE ROTATING RAKE HAS BEEN
EXTENSIVELY USED
IN THE NASA GLENN 9 BY 15
WIND TUNNEL FOR FAN MODELS
THAT ARE MORE REPRESENTATIVE
OF ACTUAL TURBO FAN ENGINES.
THESE FANS ROTATE AT
MUCH HIGHER SPEEDS SO THAT
THE ROTATION SPEED OF THE RAKE
IS USUALLY SET AT
1/200 OF THE FAN'S SPEED,
OR UP TO 60 RPM.
THE USE OF THE ROTATING RAKE
SYSTEM IN THE WIND TUNNEL
REPRESENTS A MEDIUM
TECHNOLOGY READINESS LEVEL,
OR TRL, APPLICATION
OF THE SYSTEM.
HAVING BEEN PROVEN IN
THE LABORATORY AND WIND TUNNEL
SETTINGS, THE ROTATING RAKE
HAS BEEN USED TO MEASURE
THE ACOUSTIC CHARACTERISTICS
OF A PRODUCTION TURBO FAN
ENGINE, THE TFE731.
THIS SUCCESSFUL ENGINE TEST
WAS A DEMONSTRATION
OF THE UTILITY OF
THE ROTATING RAKE SYSTEM
AT A HIGH TRL LEVEL.
USING DIAGNOSTIC TECHNOLOGY
SUCH AS THE ROTATING RAKE
WILL ENABLE DESIGNERS TO CERATE
QUIETER ENGINES, HELPING TO MAKE
THE NOISE GOAL SET FOR
10 AND 25 YEARS IN THE FUTURE.
ULTIMATELY, OBJECTIONABLE
AIRCRAFT NOISE WILL BE CONTAINED
WITHIN THE AIRPORT BOUNDARIES.