An innovative inlet total-pressure distortion measurement rake has been designed and developed for the F/A-18 A/B/C/D aircraft inlet. The design was conceived by NASA and General Electric Aircraft Engines (Evendale, Ohio). This rake has been flight qualified and flown in the F-18 High Alpha Research Vehicle (HARV) at NASA Dryden Flight Research Center. The rake's eight-legged, one-piece wagon wheel design was developed at a reduced cost and offers reduced installation time compared with traditional designs. The rake features 40 dual measurement ports for both low- and high-frequency pressure measurements with the high-frequency transducer mounted at the port. The high-frequency transducer offers direct absolute pressure measurements from low frequency to the highest frequency of interest, thereby allowing the rake to be used during highly dynamic aircraft maneuvers. Outstanding structural characteristics are inherent to the design through its construction and use of lightweight materials

Burley, Richard R.

Lechtenberg, Leon

Ray, Ronald J.

Steenken, William G.

Thornton, Don

Yuhas, Andrew J.

English

NASA Technical Reports Server

/ /Xslo -7 N95- 14241
Design and Development of an F/A-18 Inlet Distortion Rake:
A Cost and Time Saving Solution
An innovative inlet total-pressure distortion measurement rake has been
designed and developed for the F/A-18 NB/C/D aircraft inlet. The design was conceived
by NASA and General Electric Aircraft Engines (Evendale, Ohio). This rake has been
flight qualified and flown in the F-18 High Alpha Research Vehicle (HARV) at NASA
Dryden Flight Research Center. The rake's eight-legged, one-piece wagon wheel design
was developed at a reduced cost and offers reduced installation time compared with
traditional designs. The rake features 40 dual measurement ports for both low- and
high-frequency pressure measurements with the high-frequency transducer mounted at
the port. The high-frequency transducer offers direct absolute pressure measurements
from low frequency to the highest frequency of interest, thereby allowing the rake to be
used during highly dynamic aircraft maneuvers. Outstanding structural characteristics
are inherent to the design through its construction and use of lightweight materials.
Design and Development of
an F/A-18 Inlet Distortion Rake:
a Cost and Time Saving Solution
Andrew J. Yuhas and Ronald J. Ray
NASA Dryden Flight Research Center ....
Edwards, California
Richard R. Burley
NASA Lewis Research Center
Cleveland, Ohio
William G. Steenken,
Leon Lechtenberg,
and Don Thornton
General Electric Aircraft Engines
_ ,dllr
Evendale, Ohio .,. -
4th NASA High Alpha Conference
Propulsion and Inlet Dynamics and Thrust Vectoring Section
NASA Dryden Flight Research Center
July 12-14, 1994 114-1/'2
https://ntrs.nasa.gov/search.jsp?R=19950007827 2020-06-16T10:16:43+00:00Z
Outline
This presentation overviews the objective, description, installation, and
qualification testing of the HARV inlet rake system. Comparisons of cost and installation
time between this design and a previous design are made. The report describes the
pressure transducer selection, along with all stages of flight qualification testing, from
laboratory testing to flight test. The presentation ends with a summary of concluding
remarks.
Outline
• Objective
• Rake description
• Rake Installation
• Pressure transducer selection
• Flight qualification testing
• Concluding remarks
114-173
Objective
The design, development, and installation of an inlet total-pressure distortion rake
can be expensive and time consuming. The goal of the F-18 HARV inlet research
program is to evaluate the inlet characteristics of high-performance aircraft during both
stabilized and highly dynamic maneuvers at high angles of attack. The F-18 HARV inlet
research program required a low-cost, quickly installed, low-maintenance solution for a
total-pressure inlet distortion measurement system.
Design requirements were established for the rake system. The most important
requirement was to provide as much commonality as practical with the planned HARV
inlet wind-tunnel test at the NASA Lewis Research Center and with previous F/A-18 inlet
testing. Commonality considerations with past and present testing include, but are not
limited to, instrumentation setup, rake positioning, and probe configuration. In addition, it
was desirable to follow established industry guidelines wherever possible, especially
those established by the Society of Automotive Engineers. Meeting aerodynamic and
structural requirements was also an important consideration.
Objective
Design and develop an inlet total-pressure distortion
measurement rake
• Investigate high angle-of-attack inlet flow
characteristics during dynamic aircraft maneuvers
• Offer reduced cost and Installation time
• Maintain commonality
• Adhere to technical guidelines
• Meet aerodynamic and structural considerations
94-174
F-18 High Alpha Research Vehicle
The F-18 HARV is a one-of-a-kind research aircraft located at NASA Dryden
Flight Research Center. This preproduction, single-seat fighter-attack aircraft has been
uniquely modified to perform extensive flight testing in the high-angle-of-attack region.
These modifications include a thrust-vectoring control system that uses three paddles
per engine nozzle to deflect the jet exhaust. The aircraft has two General Electric F404-
GE-400 turbofan engines. The inlet rake was installed in the right inlet (aft looking
forward).
The aerodynamic design flight envelope coincides with the normal operating
envelope of the HARV aircraft and was chosen to allow unrestricted flight with the inlet
rake installed. Inlet research test points are primarily focused at the low-speed portion of
the envelope between Mach numbers (M) 0.3 to 0.4. The worst-case dynamic pressure
condition is M-- 0.7 at sea level conditions in which the free-stream total pressure is
20.4 psia and the hot day total temperature is 618 °R.
F/A-18 High Alpha Research Vehicle (HARV)
Original F/A-18 Cantilevered Inlet Rake
An evaluation of the rake used in the original F/A-18 inlet compatibility program,
flown in the mid-1970's on the second preproduction F/A-18A, indicated current costs
more than $1.5 million and installation time of a year or more. Driving both these factors
was the complexity of using eight individual cantilever rakes. Those designs had to be
developed, tested, and installed independently. Installation in the HARV would require
the aft portion of the inlet duct to be extensively reinforced. To meet the complex inlet
rake structural requirements, the bulkhead on aircraft #2 was specifically designed to
accommodate the inlet rake mounting requirements. It was quickly apparent that this
was not a viable approach for the HARV project. During an early design conception
meeting NASA and General Electric personnel conceived an alternative approach in
which all eight rake legs would be joined at the center of the inlet with a hub similar to
that of a wagon wheel to simplify design.
Original F/A-18 Cantilevered Inlet Rake
¢
HARV Rake Front and Side Views
The HARV rake is much like a wagon wheel, with the streamlined centerbody
acting as the hub, the eight aerodynamic rake bodies as the spokes, and the inlet duct
as the rim. The load-bearing structure is a welded steel unit that joins the rake bodies
and the central hub into a single piece, which is supported by integral foot pads and
bolted to the aircraft inlet duct flange. Each of the eight rake legs contains five probes
located on the centroids of five equal areas of the flow area. The body comprises a steel
frame with a bonded elastomer. The elastomer, which acts as an excellent damping
material, allowed the overall weight to be reduced and the rake struts to be
aerodynamically shaped easily (in comparison with an all-metal body). The weight of the
entire rake assembly is approximately 15 lb. No physical contact is made between the
engine and the rake system. A detailed description of the rake hub, rake body cross-
section, measurement port, footpad, and mounting installation follows.
HARV Rake Front and Side Views
i: Mounting Insta_
I T Measurement p°rt "_'_ I ":';'" -
// ._ 4 II fR,ke body
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per rin .---'II
l \/", _n,,°e_u,,=°o,eJ
/_ _._ =:^^, Side vlew of rake
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l i Front view of rake
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Rake Hub
All of the rake bodies are gathered in a central hub and welded to an inner ring.
The hub also contains an isolated metal damper ring potted in the polyurethane
centerbody. This allows the damping material to dissipate vibration energy more
effectively than an all-metal body would. The same polyurethane material forms the
streamlining of centerbody.
Rake Body Cross-Section
The rake bodies (or spokes) are made by forming sheet metal into the leading
edge and sides of the airfoil shape. The sheet metal is left open at the trailing edge. This _
allows the installation of the sensor and lead-out tubes. The rake bodies are filled the
elastomer, and the trailing edges of the rake bodies are aerodynamically formed with
the elastomer. The rake body is 2.5-in. long with maximum thickness of about 0.39 in.
Rake Body Cross-Section
i F........................................................................................................................................................................................... :
:t
Elastomer filler
I_tU
Rake Measurement Port
The rake sensors are shielded total-pressure measuring sensors consisting of a
high-frequency response pressure transducer and a 1/16-in. diameter low-frequency
response pressure tube. The stagnation shield configuration was tested to show its
ability to measure the true total input pressure at flow angles of +25 ° in yaw and +15 °
and -25 ° in pitch (positive angle is toward engine centerline). The probes are aligned
within 2 ° of the anticipated steady-flow streamlines. The innermost probe is the only one
that had to be angled (5.5 °) with respect to the rake body. The high-response
transducers are installed in carder tubes. The tubes have a counterbore to receive the
transducers. Installation is accomplished by feeding the electrical leads through the
carrier tube from the sensor end, coating the back of the transducer with an adhesive,
and inserting the transducer into the counterbore. Next, the transducer is covered with
heat-shrinkable tubing. This arrangement gives a secure mounting for the transducers
but allows replacement while the rake is still in the aircraft. Transducer replacement was
demonstrated with the replacement of seven transducers requiring less than 2 days
once the engine was removed.
Rake Measurement Port
Stagnation
High-frequency
response
Low-frequency response
pneumatic tubing
Rake Footpad
The rake bodies are welded to the foot pads. The footpads allow the duct flange
to support the rake without inducing any bending load in the sheet metal wall of the
duct. The lead-out tubes are carried between the toes of the footpad.
Rake Footpad
Lead-out
Bolt holes
Rake Mounting Installation
Installation of the rake was accomplished after the right engine and K-seal had
been removed. The minimal modifications that were required on the airframe consisted
of 16 bolt holes drilled in the inlet duct flange aft of the rear bulkhead. Backup washers
were placed at each hole on the outside of the duct and epoxied in place to provide a
solid and flat surface for seating the nuts of the rake mounting bolts. The rake was
placed in the duct, and an even fit for each rake strut was achieved by placing shims
between the metal footpad and the inlet duct wall. The rake was then installed with an
adhesive injected between the foot pads and the shims. The foot pads were bolted in
place and the adhesive cured giving the assembly a firm, elastomer-damped mounting.
The K-seal was contoured to allow the rake tubes to pass freely under it. The seal was
then bolted in place on the aft duct flange, and the electrical leads and the pneumatic
tubes were routed to their respective connector locations.
Rake Mounting Installation
\ /
Installed HARV Inlet Rake
The NASA/General Electric design greatly simplifies installation and aircraft
modifications required for an inlet rake system. General Electric has designed,
developed, and built one prototype and two flight-worthy rakes for less than $500,000.
One flight-worthy rake had an entire set of high-response transducers included in the
cost. The rework to the airframe, installation of the rake assembly, modification to the K-
seal, and installation of the seal ready for lead routing was accomplished in two and a
half 8-hour shifts. This installation time shows a significant reduction in aircraft down
time compared with more traditional designs.
Installed HARV Inlet Rake
g4-11_
Pressure Transducer Selection
The pressure transducer requirement was to develop an instrumentation setup
that would allow for accurately measuring the pressure level and the time-dependent
component of the pressure during highly dynamic maneuvers. Minimization of two
known uncertainties that affect the ability to measure an accurate pressure level during
a dynamic maneuver were addressed: (1) pneumatic lag and (2) thermal zero shift. The
pneumatic lag describes the condition in which the pressure signal is delayed, in
reference to time, to the transducer at the end of the tubing and, therefore, affects low-
response accuracy. The thermal zero shift affects the ability of the transducer to
accurately measure the pressure level at varying inlet temperature conditions. Thermal
zero shift describes the calibration shift of the zero voltage condition experienced as a
pressure transducer sensing element varies with temperature.
The high-response probe used a temperature-compensated pressure transducer
with an absolute pressure range of 0 to 20 psia. The transducer was selected because
of its ability to minimize thermal zero drift through passive temperature compensation.
To further increase the accuracy of the transducer measurement, a series of pressure
calibrations were performed over the entire required pressure and temperature range,
up to 20 psia and at -65, -30, 0, 75, and 150 °F. These calibrations, along with the
measured engine inlet temperature, would allow for any remaining zero thermal drift to
be removed during postflight data processing. The low-response measurements will be
used to verify the pressure levels of the high-response measurements at stabilized
conditions. This high-response transducer setup will allow for the accurate
measurement of high-frequency pressure levels during highly dynamic maneuvers and
also meets the system accuracy requirements.
The low-frequency response probe uses a differential transducer with a reference
pressure. This transducer unit was thermally stabilized to increase accuracy by
minimizing thermal zero drift. This was accomplished by wrapping the transducer unit in
temperature-controlled thermal blanket. Another transducer feature that was used to
increase accuracy was its ability to perform in-flight calibrations. This allows for any
calibration bias error to be removed during postflight data processing. The in-flight
calibration is accomplished by applying the reference pressure to both sides of the
differential transducer.
!Pressure Transducer Selection
Challenge: Measure inlet pressures during highly
dynamic aircraft maneuvers by minimizing pneumatic
lag and thermal zero shift
• High-frequency response Instrumentation
- Absolute pressure range of 0-20 psia
- Passive temperature compensation
- Laboratory calibrations at temperatures from -65
to 150 °F
• Low-frequency response instrumentation
- Differential transducers with reference pressure
- Thermal blanked transducers
- In-flight calibration capability
N-116
Fllght Quallficatlon--Test Phases
The flight qualification testing was broken into three phases: (1) laboratory, (2)
ground, and (3) flight. The laboratory phase determines the baseline structural and
vibrational characteristics of the rake. This phase consists of NASTRAN computer
modeling of the rake structure, along with ping testing and vibrational shake table
testing of a prototype rake. The baseline results from the laboratory tests were used for
comparison with the results from the remaining phases: installed ground and flight
testing. Ground testing consisted of a ping test being performed on the inlet rake
installed in the HARV. Then, the installed rake was ground tested on the HARV with the
aircraft tied down. The right engine was operated through its full range with a slow
acceleration from idle power to full maximum afterburning and a slow acceleration back
to idle power. This procedure allows predominant frequencies to be identified over the
entire fan rotor speed range. Flight testing for the rake consisted of flight maneuvers to
give maximum unsteady loads (o_= 60 ° at 20,000 ft), maximum temperature, and
pressure (M = 0.7 on the deck), and maximum combination of temperature, pressure,
and unsteady loads (M= 0.9 at 18,000 ft) within the HARV flight envelope. The latter
two points were at the limits of the HARV flight envelope. The first point was flown to a
high-angle-of-attack condition, while the latter two obtained maximum #-limit loading.
Flight Qualification -Test Phases
• Laboratory testing
- NASTRAN modeling
- Ping testing
- Vibrational shake table testing
° Ground testing
- Ping testing
- Engine operation over entire fan rotor speed range
• Flight testing
- Flight maneuvers with combinations of maximum
unsteady loads, temperature, and pressure
_M-IH
Flight Qualification--Results
The rake structure was demonstrated to be highly damped and was successfully
flight qualified for the entire HARV flight envelope. The spectra of the rake vibrations
experienced during laboratory testing remained consistent with the ground and flight test
results. The maximum stress levels observed were less the 30 percent of limits (30,000
Ibf/in 2, peak to peak). The stress levels observed during high-(z flight were 26 percent of
limits and less than 10 percent of limits during maximum-g flight. The highest stress
limits observed were 30 percent of limits during aircraft takeoff. Based on the laboratory,
ground, and flight test results, the rake is now fully cleared for conducting flight research
within the entire HARV flight envelope with no restrictions. The inlet rake system has
flown over 60 successful research flights.
Flight Qualification - Results
• Highly damped structure
• Successfully cleared for entire design envelope
• Flight and ground testing verified expected frequency
response predicted by laboratory testing
• All stress levels observed were less than 30 percent
of limits
• Flown in over 60 research flights
IM1-1110
Concluding Remarks
An improved cost- and installation-time-saving inlet distortion pressure rake was
successfully designed, built, and validated for flight testing on the F/A-18 HARV
research aircraft. The cost for one prototype and two flight-worthy rakes along with
required development testing was under $500,000. The innovative design consists of a
one-piece, wagon wheel approach that resulted in ease of installation with minimal
aircraft modifications. The demonstrated installation time was under 3 days. Design
features include lightweight, high-strength, low structural resonance, low flow blockage,
and easy transducer removal and replacement. Instrumentation selection allowed for
direct pressure measurement during highly dynamic aircraft maneuvers. A prototype of
the new rake was environmentally tested in a laboratory where it passed all vibration
structural requirements. Ground test verified the expected frequency response predicted
from laboratory test. Flight qualification was completed and the rake is now cleared for
flight testing on the HARV aircraft with over 60 flights performed. All stress levels
observed during ground and flight qualification were less then 30 percent of limits.
Concluding Remarks
• NASA/GEAE Inlet rake design demonstrated improved
cost and installation-time savings
• Cost for entire design and development of two flight-
worthy rakes was under $500K
• Rake installation in under 3 days
• Direct pressure measurements during dynamic aircraft
maneuvers
• Flight qualification successfully completed (60+ flights)
• Structural stress levels were less than 30 percent of
limits
° Ease of transducer replacement demonstrated
g4-tgl



Design and development of an F/A-18 inlet distortion rake: A cost and time saving solution

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