NASA Technical Reports Server (NTRS) 19960012200: Overview of the preparation and use of an OV-10 aircraft for wake vortex hazards flight experiments

23 Pages·8.5 MB·English

by  NASA Technical Reports Server (NTRS)

AM A A
NASA-TM-111259
AIAA 95-3935
Overview of the Preparation and Use
of an OV-10 Aircraft for Wake Vortex
Hazards Flight Experiments
Robert A. Stuever, Eric C. Stewart,
and Robert A. Rivers
NASA Langley Research Center
Hampton, VA
(NASA-TM-111259) OVERVIEW OF THE N96-18437
PREPARATION AND USE OF AN OV-10
AIRCRAFT FOR WAKE VORTEX HAZARDS
FLIGHT EXPERIMENTS (NASA. Langley Unclas
Research Center) 23 p
G3/03 0099795
1st AIAA Aircraft Engineering, Technology,
and Operations Congress
September 19-21,1995/Los Angeles, CA
For permission to copy or republish, contact the American Institute of Aeronautics and Astronautics
370 L'Enfant Promenade, S.W., Washington, D.C. 20024
OVERVIEW OF THE PREPARATION AND USE OF AN OV-10
AIRCRAFT FOR WAKE VORTEX HAZARDS FLIGHT EXPERIMENTS
Robert A. Stuever*, Eric C. Stewart*, and Robert A. Rivers1
NASA Langley Research Center
Hampton, VA 23681-0001
ABSTRACT INTRODUCTION
An overview is presented of the development, As part of The National Aeronautics and
use, and current flight-test status of a highly- Space Administration's (NASA) Reduced Spacing
instrumented North American Rockwell OV-10A Operations (RSO) element of the Terminal Area
Bronco as a wake-vortex-hazards research aircraft. A Productivity (TAP) Program, means are being
description of the operational requirements and investigated to safely reduce separations between
measurements criteria, the resulting instrumentation aircraft landings and departures at major airports. A
systems and aircraft modifications, system-calibration key area being addressed by the Langley Research
and research flights completed to date, and current Center (LaRC) is the specification of aircraft
flight status are included. These experiments are being longitudinal separation to avoid encounters with
conducted by the National Aeronautics and Space hazardous wakes from preceding aircraft.1 Wake
Administration as part of an effort to provide the hazards (Figure 1), along with runway occupancy
technology to safely improve the capacity of the times, are currently the primary constraints on
nation's air transportation system and specifically to minimum safe longitudinal separation for takeoff
provide key data in understanding and predicting wake- and landing operations and thus seriously affect
vortex decay, transport characteristics, and the airport capacity.
dynamics of encountering wake turbulence. The OV- The product of this effort will be the delivery
10A performs several roles including meteorological- of an Aircraft Vortex Spacing System (AVOSS)2-3
measurements platform, wake-decay quantifier, and near the turn of the century. This system will
trajectory-quantifier for wake encounters. Extensive provide the capability to safely space arriving and
research instrumentation systems include multiple air- departing traffic based on 20- to 30-minute
data sensors, video cameras with cockpit displays, predictions of wake hazards along the terminal-area
aircraft state and control-position measurements, traffic corridors. The enabling technology of the
inertial aircraft-position measurements, meteorological AVOSS concept is the ground-based prediction and
measurements, and an on-board personal computer for possible real-time measurement of wake strength
real-time processing and cockpit display of research and location in the airport environment. This
data. To date, several of the preliminary system check information will allow capacity increases in most
flights and two meteorological-measurements conditions while preventing the rare encounter of a
deployments have been completed. Several wake- wake of sufficient strength to cause either an unsafe
encounter and wake-decay-measurements flights are approach, landing, or departure, or undesired
planned for the fall of 1995. maneuvering (e.g., a go-around). A full-up,
optimized system would employ real-time sensor
feedback to adjust the predicted separation
'Research Engineer, Flight Dynamics and Control estimates, and may allow approach/departure traffic
Division, Member, AIAA to proceed even though nonhazardous wakes exist in
'Senior Research Engineer, Flight Dynamics and the flight corridors.
Control Division, Member, AIAA To meet the AVOSS delivery goal, a
'Research Pilot, Flight Operations and Support multidisciplinary team within NASA is studying and
Division, Senior Member, AIAA
Copyright © 1995 by the American Institute of Aeronautics and Astronautics, Inc. No copyright is asserted in the United States
under Title 17, U.S. Code. The Government has a royalty-free license to exercise all rights under the copyright claimed herein
for Government purposes. All other rights are reserved by the copyright owner.
The use of trademarks or names of manufacturers in this report is for accurate reporting and does not constitute an official
endorsement, either expressed or implied, of such products or manufacturers by the National Aeronautics and Space
Administration.
developing techniques for predicting and measuring aircraft modifications, the system-calibration and
aircraft wake-vortex characteristics and predicting research flights completed to date, and the current
the subsequent hazards posed to aircraft flight status.
encountering these wakes. The Flight Dynamics
and Control Division, Research and Technology
Group (FDCD/RTG) at LaRC has been tasked, in RESEARCH AIRCRAFT
part, to lead an effort to provide flight data to
supplement the ground-based research required for The OV-10A (NASA 524, Serial No. 67-
delivery of the AVOSS. 14687) is a two-seat, twin-turbo-prop, high-wing,
There are four specific objectives of the flight high-tail, twin-tail-boom, unpressurized aircraft
efforts. The first objective is to provide data for the originally designed and operated as a forward-air-
development and validation of computer models to control and observation platform with munitions-
represent the flowfield, decay, and transport of a delivery capabilities (Figure 2). Basic characteristics
wake-vortex system in the atmosphere near the of the nominal OV-10A and NASA 524 in its
ground. The validation of these models requires research configuration are presented in Table 1, and a
accurate datasets describing aircraft wake vortices photograph of NASA 524 is shown in Figure 3.
and the meteorological conditions which affect their Instrumentation and research modifications are
transport and decay. The second flight objective is discussed in a later section. The OV-10A was
to provide key data for the development of wake- selected as a research platform due to its performance
encounter hazard models which will be used to envelope, ruggedness, relatively large cargo and aft-
define sensor-observable wake-strength thresholds cockpit capacity for housing research equipment, and
for satisfactory landing/departure operations in the relatively low cost to operate and maintain.
event a wake encounter occurs. Adequate
representation of vortex-induced aerodynamic forces
and moments on an aircraft and the subsequent OPERATIONAL REQUIREMENTS AND
dynamics of the encounter are key requirements. MEASUREMENT CRITERIA
The third flight objective is the evaluation of
airborne techniques/sensors for detecting nearby As previously stated, flight data will support
wakes. The optimum full-up AVOSS would employ the objectives of wake-vortex characterization,
ground-based detection of wakes along the approach vortex-encounter hazard definition, vortex-detection
corridor, but this does not exclude the evaluation of algorithm development, and AVOSS demonstration.
candidate airborne, in-situ vortex-detection methods This section presents an overview of the ensuing
which could be considered as potential back-up, operational requirements and measurements criteria.
early-warning systems for pilots. Finally, the fourth
objective is a flight demonstration of the operational Vortex Characterization
utility of the AVOSS in the airport environment.
A significant part of the AVOSS development
To facilitate completion of these objectives, a
effort is the characterization and prediction of wake-
multiple-aircraft flight-test program has been
vortex structure and dynamics. Measurements of
initiated at LaRC. A key test vehicle for this
vortex strength and position made by sensors, when
program is a highly-instrumented North American
coupled with simultaneous meteorological
Rockwell OV-10A Bronco. This vehicle became
information, provide data for the development and
operational as a research aircraft in November 1994,
validation of computer models to predict and
and plans exist to perform several types of missions
quantify vortex behavior. Flight data could be used
including (1) flying controlled trajectories near and
to augment both the strength and position
through the wake produced by another aircraft, (2)
measurements and the meteorological
tracking the trajectories of the wake of another
measurements made from the ground.
airplane, (3) performing as a meteorological-
measurements platform, (4) assisting in quantifying
Meteorological-Measurements Platform
the trajectory of a typical transport aircraft relative
One sensor, a lidar developed by The
to a wake it encounters, and (5) possibly
Massachusetts Institute of Technology Lincoln
demonstrating the safety of the new AVOSS
Laboratories (MIT/LL), is being used to gather
reduced-spacing criteria in the terminal area.
wake strength and transport data at selected airport
This paper presents an overview of the status
sites by operating under or near the approach
and development of the OV-10A flight-test program
corridor and measuring the wakes of passing
and includes a description of the operational commercial aircraft.4 The current meteorological-
requirements and measurements criteria, the
sensor array, which provides data to correlate with
resulting instrumentation systems and necessary
the lidar wake measurements, is limited to either
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very low altitudes and/or to specific times and Facility, Goddard Space Flight Center
spatial positions with respect to the airport. (WFF/GSFC)) are also required.
Meteorological flight data has been identified as A typical research scenario could involve the
potentially valuable in extending the range of the OV-10A first making atmospheric profiles to select
current sensor array up the actual airport-area the best test altitude of the day (nominally 5,000 ft
approach/departure corridors and reducing the but potentially much lower). Once at the altitude
extent to which other data sources must be selected, the wake-generating aircraft would fly at
extrapolated. constant altitude and airspeed in a racetrack pattern
The OV-10A has subsequently been typically oriented either up/downwind or cross-
designated as a suitable measurements platform for wind. The OV-10A would then begin flying behind
deployment to these sites and has been tasked with the wake-generating aircraft at various separation
sampling vertical and horizontal profiles of winds, distances, executing approaches near and through
turbulence level, temperature, dew-point, static the wake nominally from above, and simultaneously
pressure, and other parameters summarized in Table recording both the meteorological conditions and
2. Because data needs to be collected along both the the position of the smoke-marked vortex trails. In-
full approach/departure corridor and across and situ measurements of the flowfield made during
within the perimeter of the airfield, an array of vortex-core penetrations can provide an independent
navigational and air-data measurements is required measure of vortex strength for correlation with the
on the aircraft. strengths derived from the descent rate of the
vortex.
Wake-Decay Prediction Validation
Flowfield measurements made while flying Vortex Encounter Hazard Definition
approaches near and through the wakes of other Perhaps the most challenging aspect of
aircraft at altitude have been identified as a means optimizing the AVOSS is defining the threshold of
for validating out-of-ground-effect wake-decay satisfactory-operations for encounters with wakes
models. These models could then be implemented persisting in the traffic corridors. This entails
in near-term AVOSS prototypes. Specifically, one knowledge of the wake characteristics, an
prediction methodology5 could be validated which understanding of how the wake affects the aircraft as
requires not only the documentation of ambient it passes through, the expected pilot control inputs
meteorological conditions but also the quantification or automatic control system inputs, a measure of the
of the trajectory (descent/drift) of the aircraft wake. level of acceptable "upset," and the ultimate
These requirements indicate the need for application to the aircraft fleet. Simulations of
inertial, meteorological, and air-data instruments on vortex encounters will have to be utilized to address
the measurement platform (initially identified as the this,8'9 and those simulations will have to be
OV-10A) as well as measurements of its state and validated using a variety of techniques being
control positions to enable differentiation between addressed in a collection of several experimental,
vortex-induced and control-induced aircraft motions. numerical, and analytical studies.'
Additionally, a method to track the position of the One process for developing validated models
smoke-marked wake of the vortex-generating for simulating vortex encounters involves
aircraft is required. For these experiments, the • developing a simulator math model of a vortex-
location of the wake will be determined utilizing a encounter aircraft
downward-looking, stereo-photographic imaging • using measured and/or predicted vortex
system installed on the OV-10A (similar to that used strengths and vortex-induced aerodynamic
in a previous test6) coupled with the position forces and moments in a flight simulation to
measurements of the OV-10A. This method will predict airplane motions
inherently include measurement of the position of • modifying the simulator math model, as
the OV-10A with respect to the wake for necessary, to get correct responses
documentation of exact wake-encounter geometries. • extending application of the validated math
The photographic-ranging scheme entails models to other aircraft for definition of fleet-
measurements of the wake from above, which adds wide satisfactory-operation thresholds.
the additional potential for measuring the separation Flight data from vortex encounters flown at safe
of the vortex pair, in part, to characterize the Crow- altitudes and approach speeds could facilitate this
linking process.7 To determine the rate of vortex process. Specifically, measurement of the vortex
decay and transport, the position and basic flight strength, location, flowfield properties, encounter-
conditions of the wake-generating aircraft (currently airplane states and controls, and relative wake-to-
designated as the NASA C-130 from Wallops Flight aircraft positions can be used to validate ground-
based results. Although wake-encounter flight tests
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have been previously conducted,'g-10> "•12 most of the cockpit) and in post-flight analysis in computing
the flight work in the past has been aimed at vortex-pair characteristics as well as the position of
quantifying upset hazards rather than validating an aircraft with respect to that vortex pair.
encounter models for the ground-based The unique measurements required to
quantification of hazards. determine these quantities for a near-parallel
The NASA Boeing 737-100 (NASA 515) has approach to a vortex include the magnitudes and
currently been designated as the primary gradients of the vortex-induced angles of attack (a)
configuration for validation of wake-encounter and sideslip (P). This technique requires two
simulation techniques, and as such may fly independent measurements of a and p across the
approaches near and through the wake of another span of the airplane. Similarly, longitudinally-
aircraft. It represents a typical jet-transport spaced flow sensors are required for non-parallel
configuration, and a large portion of the ground- encounters. Data collected during flights to quantify
based simulation, wind-tunnel, and wake-encounter vortex characteristics in the atmosphere could, with
modeling effort at LaRC is centered around it. The the inclusion of the flowfield-gradient and
B-737 was selected for validation of a wake- magnitude measurements, also be used post-flight in
encounter simulation since an existing baseline the assessment of vortex-detection, location, and
simulation model was already available along with a strength algorithms.
wind-tunnel database.
For any flight experiments of this type, the AVOSS Demonstration
OV-10A will be tasked with simultaneously A flight verification of the safety of predicted
gathering atmospheric data in the vicinity of the aircraft spacings will be required during
wake of interest, photographically recording the demonstration of the AVOSS near the turn of the
trajectory of the B-737 from a nominal standoff century. This demonstration will probably involve
distance to quantify its position with respect to the one or more test aircraft safely flying approaches in
lead-aircraft's vortex pair, and penetrating these the airport environment using the predicted AVOSS
vortices in order to obtain quantitative vortex spacings.
flowfield data. Measurements from the OV-10A
similar to those required for wake characterization
are therefore needed. OV-10A MODIFICATIONS AND
A typical flight will proceed in a very similar INSTRUMENTATION
fashion to that of the previously-described wake-
decay-validation flight, except that selection of the Significant modifications to the OV-10A were
test-altitude would be more restrictive for safety required to convert it from a stock aircraft into a
purposes. In general, the B-737 would make several research platform. The modifications began in the
wake penetrations along the leg of the racetrack summer of 1992 and were completed in the fall of
pattern, then temporarily move out of the vicinity to 1994. This section presents a description of those
allow the OV-10A to sample (nearly) the same modifications and the resulting systems.
wake and atmospheric conditions as encountered by
the B-737. Longitudinal separation distances would
General System
be set to avoid substantial vortex-induced upsets on
The custom-built data acquisition system
the vortex-encountering aircraft, since the purpose
(DAS) acquires analog signals from many sources
of flight data is to validate simulation models and
and converts them into digital form for display,
not study upset hazard boundaries. The B-737
transmission, and recording. Basic requirements of
would be equipped with a nominal instrumentation
the system include a capability to record
complement to measure navigation, state, and
approximately two hours of data on one mission and
control parameters. Wake penetrations would be
a data rate sufficiently high to characterize high-
flown from several approach angles to the vortex
frequency atmospheric turbulence at values at least as
pair, at typical (and safe) approach speeds and flap
great as the lowest structural frequency of the
settings of interest.
airframe and instrumentation booms. The major
components of the DAS are listed in Table 3 and the
Vortex Detection
basic sensors for each of these systems are listed in
An in-situ method for predicting vortex
Table 4. Figure 4 also shows the system locations
encounters has been suggested6'n as a candidate
relative to the airframe, while Figure 5 depicts the
early-warning system for pilots. Such a system,
general paths of signals through the DAS. The initial
relying on measurements of the flow field in the
OV-10A data system configuration provides more
vicinity of the wake coupled with a predictive
than 150 flight measurements. The DAS contains six
algorithm, could be very useful in both real-time (in
major stages, including sensors, signal conditioning,
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pulse code modulation (PCM) data-encoding, tape Hz power for cockpit displays, navigation
recording, telemetry, and data display. The sensor instruments, and on-board personal computers.
outputs are signal conditioned, multiplexed and Several of the major DAS system components
digitized by a commercial 12-bit PCM subsystem that are described in detail below.
features functional flexibility, programmability, and
the ability to multiplex analog and digital signals into Navigation Systems
a serial format. Major components of the sensor suite Two navigation systems have been added to
include an IRIG-B time-code generator to provide an the OV-10A. The first is a multi-component system
accurate time base for all flight measurements, a ring- that supports the research measurements and includes
laser-gyro inertial navigation unit (INU) integrated a Litton LN-93 ring-laser gyro INU integrated with a
with a global positioning system (GPS) receiver and Honeywell 3A GPS receiver and a CPU-140/A
a standard central air-data computer (SCADC), a Standard Central Air-Data computer (SCADC), all
separate package of fast-response rate and attitude communicating with the PCM system via a MIL-
gyros, two pilot-static and flow-angle-measurement standard 1553 data bus through an 80486-
systems, a 5-hole probe, temperature probes, a dew- processor/33-MHz personal-computer bus controller.
point sensor, video/audio cameras/recorders, and The INU updates the GPS in the event of satellite
several control position transducers (CPTs). signal loss, but the GPS does not update the INU to
Except for sensors, displays, and control prevent drift errors in the INU measurements. The
panels, most of the equipment for the DAS system is INU also is capable of providing aircraft
installed in the OV-10A cargo bay. This space, accelerations, velocities, and attitudes, but due to
encompassing approximately 71 cubic feet of usable apparent aliasing due to dithering, the linear
volume on 22 square feet of flooring, was modified accelerations and angular rates from the INU must be
by first removing the floor and side walls, the oxygen supplemented by a separate package of fast-response
bottles, several pieces of avionics equipment plus rate gyros and accelerometers. The components of
their mounting shelf, and relocating the VHP radio the research navigation system reside in the cargo
and the VOR/ILS receivers to the right-hand tail bay on fixed plates and are not part of the removable
boom. A removable main instrumentation pallet, instrument pallet. The second navigation system
designed to fit in the aft portion of the cargo bay and entails a Garmin GPS 100 mounted in the forward
roll out onto a cart for system maintenance, was cockpit, and enables the pilot to precisely navigate
fabricated and populated with equipment (Figure 6). and/or set up research maneuvers.
Subsequent major modifications also included Although not a pure navigation system, a
installation of a removable metal floor with guide unique instrument developed at LaRC and installed
rails to support the main instrumentation pallet in the for precise trajectory control during vortex-
aft portion of the cargo bay, removable metal plates measurement maneuvers is the Electronic Vortex
to hold selected instrumentation and instrumentation- Optical Tracking System (EVOTS). This system,
power distribution equipment in the mid- and composed of a low-power light-emitting diode (LED)
forward-portion of the cargo bay floor, and shelves in coupled with a reflective boresight and mounted on a
the forward part of the cargo bay to support aircraft parallelogram swing-arm mechanism on either side
power distribution equipment and selected of the forward cockpit, is equivalent to a side-
instrumentation (Figure 7). Cargo-bay ventilation mounted gunsight. When properly calibrated in
was augmented by holes cut in the wing-root fairing flight, this instrument gives the pilot some means of
and in the honeycomb/fiberglass cargo-bay door. quantifying and validating vortex-approach angles.
Exhaust fans, operated by the pilot, were installed A photograph of the EVOTS instrument is shown in
over the holes in the cargo door. Figure 8.
The instrumentation system is operated with
28-volt DC power. The nominal OV-10A electrical Air-Data Systems
system consists of two DC generators and two AC There are two air-data systems on the OV-
inverters. Each DC generator is capable of providing 10A. One system is the SCADC previously
a maximum of 300-amperes/28-volts (combined 600- described, which is coupled to the INU/GPS system.
amps/28-volts) to the DC buses, while the inverters The second system is the three-component pilot,
each provide 115 volts AC. Single-generator static, and flow-direction (ex/P) measuremenl system
operation is capable of supplying sufficient power for described here and used to obtain the Iwo flowfield
all basic aircraft electrical loads, while the inverters gradienls described in ihe section on vorlex-
provide 400 Hz power for the aircraft avionics. Two delection measurement requirements.
additional instrumentation inverters have been Air data measuremenls at both winglips and
installed to provide 115-volt/400-Hz power for the nose are facilitated by sensors mounted on Ihe
several instrumentation components and 115-volt/60 lips of graphite-epoxy research booms. These
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booms fit into aluminum tubes and support brackets, for the pilot-static instrumenls was installed inside
which allow relatively quick boom installation and the aft end of the nose boom mounting lube.
removal. As is customary, the booms were designed A commercial, healed Rosemounl 5-hole
to place sensors as far ahead of the predicted aircraft probe (Model 858 AJ) was mounted at the end of the
upwash influence as possible, yet have enough nose boom. The Rosemounl probe provides Ihe
lightness and stiffness (high natural frequency) to required pressure porls for determining a, p,
prevent unwanted vibration influence on the sensor airspeed, and altimelry measurements al Ihe nose, and
measurements. Thermally-controlled Setra pressure also provides a rugged inslrumenl housing. The tip
transducers, located approximately at the base of of Ihe mounted probe is located approximately 6 feel
each respective research boom, provide for in front of the aircraft nose and approximately 2 feet
enhanced accuracy in pressure measurements. right of the longiludinal cenlerline (Figure 10). The
Accelerometers mounted near the tips of these probe was connecled lo Ihe Selra pressure teansducers
booms provide information on boom vibration. by approximately 17 feel of 3/16-inch inside-
Sensors at the wingtips provide a measurement diameter flexible tubing. A one-inch-long reslriclor
of the gradient in angle of attack across the span. At wilh a 0.067-inch hole (#51 drill) was placed inside
each wingtip, the boom support bracket was Ihe lubing near Ihe probe lo reduce resonant
attached to the strengthened outer rib, and the overshoots in the response. Ground-based tests at
fiberglass wingtip fairings were modified to allow sea-level indicated the lolal system had a nalural
each boom to protrude through the leading edge. frequency of approximately 20 Hz wilh good
Thermal-control boxes for the pressure transducers damping.
were installed inside a cavity near the tip rib. A A pair of Rosemounl Model 102 non-deiced
standard NACA pilot-static probe with balsa a/P lolal lemperalure probes (one for Ihe SCADC and
vanes was mounted at the end of each wingtip boom one for the research system) and a General Eastern
approximately one chord length in front of the wing 1011 Aircraft Dew-Poinl Sensor were also integrated
leading edge. The balsa vanes have a high natural inlo Ihe air-data system. The temperature probes are
frequency of about 0.18 Hz per knot of indicated located on the underside of the wing approximately
airspeed in incompressible flow,14 so that at a midway between Ihe engine and wingtip on each side
nominal OV-10A airspeed of 120 knots, the natural (Figure 11), and Ihe dew-poinl sensor is mounted on
frequency of the vanes is approximately 22 Hz. Ihe left side of Ihe forward fuselage.
Figure 9 shows a boom/probe combination mounted
for flight. Airborne Video/Audio Systems
A flow-angle measurement at the nose The research instrumentation includes a
combined with one of the wingtip measurements video/audio system consisting of three miniature
provide a longitudinal gradient in the flow angles. lipstick-size Elmo video cameras and Ihree recorders.
The longitudinal distance between the nose-mounted One camera is located al Ihe tip of Ihe left vertical
sensor and those at the wingtips is approximately 12 tail, pointing in the general direction of flighl, and
feet. At the nose, the fiberglass shell and nearby includes Ihe lop of the wing and fuselage in ils field
support structure were modified to accept of view. This camera provides a good, qualitative
installation of the research boom on the right side of means of reviewing Ihe highlighls of a flighl in
the aircraft fuselage. (The nose itself was not preparation for data reduction and analysis. The lop,
sufficiently stiff to mount the boom directly at the left vertical fin cap fairing was modified to include
tip of the nose, and the need to locate this camera plus a small, clear-plastic window
instrumentation in the nose wheel well left no room (Figure 12). The other Iwo cameras are mounted
for additional support structure.) A fairing to cover near the tip of each wing and are pointed vertically
the nose-boom mounting structure was fabricated downward through "peepholes" bored oul of the
and installed. Four thermal enclosure boxes for the wingtip fairings (Figure 11). Algorithms are being
pressure transducers were installed inside the nose- developed to use Ihe stereo effeci of Ihe synchronized
gear wheel well, while aluminum debris shields frames from Ihe Iwo downward-looking views lo
were fabricated to protect these boxes. A small calculate the relative distance between vortex smoke
fiberglass fairing was fabricated for the upper-left trails and the OV-10A, and the relative distance
area of the nose cone to externally protect one box belween Ihe smoke Irails and Ihe B-737. Audio
that protruded through the nose. A pressure channels on Ihe video/audio system enable the pilot
manifold was installed at the base of the nose boom and FTE to record verbal data during maneuvers.
to efficiently route pressure tubing from the pressure The sponsons are small pods attached lo both
ports in the boom to the transducers in the wheel Ihe left and right sides of the fuselage near the waist
well. Finally, a small box housing power converters position (front view of Figure 2), and were originally
used to carry part of the OV-10A weapons system.
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Both sponsons on this aircraft were modified The flat-panel computer display is driven by a second
extensively to house equipment associated with the 80486-processor/33-MHz personal computer (located in
research video/audio system (e.g., Figure 13). the cargo bay directly behind the FTE) dedicated for
Additionally, aerodynamic/cosmetic fairings were real-time calculation and presentation of selected
. fabricated and installed on their exterior to cover the research parameters to the FTE. The FTE has the
voids created by the removal of the machine guns, ability to request any one of several display pages for
the ammunition-discharge system, and the bomb inspection, but cannot modify these pages nor control
racks. the calculations in flight. A depiction of the aft-cockpit
configuration is shown in Figure 14 and the
L-Band Telemetry system components are described in Table 5.
The telemetry system provides L-Band
transmitters for data transmission and C-Band Pilot's Station
Beacon transmitters for aircraft radar tracking, and The forward cockpit was modified to allow
includes associated miniature antennas which are partial instrumentation control and video display.
mounted externally to the aircraft structure. Modification included removal of
• HF radio control
Flight-Test Engineer's (FTE) Station • one VHF radio control
Although the nominal OV-10A can be safely • armament-control panel
operated by a single crew member, most flight • map case
experiments for this project require two crew • missile control panel
members. The pilot occupies the front cockpit and and installation of
has responsibility for the overall safe conduct of the • instrumentation and video/audio system control
flight, which includes guiding the aircraft to and panels
through the test maneuvers and overriding research • a video display directly in front of the pilot on top
instrumentation systems in the event of an of the control panel
emergency. The flight-test engineer (FTE) occupies • the Garmin GPS navigation unit to the left of the
the aft cockpit and operates instrumentation, monitors video display
research measurements, directs and evaluates test • o/p indicators to the right of the video display
maneuvers, and maintains audio contact with ground- • a battery-temperature monitoring system
based researchers. • a master power-interrupt switch for all video and
The FTE research station is, therefore, a major experimental equipment
part of the DAS system and encompasses the entire • the EVOTS instruments.
aft-cockpit area. Modification of the nominal aircraft The new panel presents the pilot with the capability of
in this area required removal of controlling data-recording, but does not allow the
• the control stick, rudder pedals, power levers, flexibility to selectively shut down certain systems
landing-gear lever, and map case (e.g., telemetry) nor evaluate data (other than video) in
• the observer's instrument panel, including the real time. However, the limited control is useful for
attitude indicator, the landing-gear warning light, certain flights (e.g., flutter clearance) where the FTE
the fuel-low and fuel-feed lights, the radio cannot occupy the back seat. The new forward-cockpit
magnetic indicator (RMI), and the microphone configuration is shown in Figure 15 and the
select switch components are summarized in Table 6.
• the oxygen regulator panel
and subsequent installation of
• side consoles on either side of the seat to house FLIGHT TEST PROCEDURES
research equipment, including the hygrometer
control/indicator unit, the video/audio system Scope
control panel, a data-computer-display keypad, Flight experiments with the OV-10A entail the
and the DAS system control panel proposed research missions discussed in the
• an intercom/microphone switch on the throttle introduction plus a substantial preliminary test program
console to calibrate the research systems. The preliminary test
• a new instrument panel, housing two video program has included clearing the modified OV-10A
displays; a flat-panel, 640-by-480-pixel flight envelope for flutter, calibrating the pilot-static
monochrome VGA computer display; and the and flow-angle measurement systems, validating the
nominal ship's airspeed indicator, altimeter, meteorological-measurement algorithms, calibrating
torque indicators, fire handles, and a fire- the stereo video system, and practicing photographic
extinguisher switch tracking techniques for three-ship wake-encounter
• extensive instrumentation wiring. flights. With the exception of two completed
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deployments, the preliminary flights have entailed most by the limitations described above, except for the "wet"
of the program to date and, therefore, are summarized cloud/rainshower avoidance. High-altitude operations
in this section along with an overview of the requiring the oxygen system are not necessary.
deployments. Specific technical results from the
aircraft calibrations may be presented in a later report.
Aircraft Test Limitations Flight Operations - Preliminary.
Research modifications resulted in the Calibration & Check Flights and Deployments
establishment of four artificial limits to the nominal
OV-10A performance envelope. First, NASA 524 is no Flutter clearance
longer capable of all-weather performance, since flight An available flutter analysis for an OV-10D
into "wet" clouds or in rain could expose the balsa o/P indicated a sensitivity of the horizontal tail to vibration
vanes to moisture and possible warpage. Planned modes. Because no flutter data were available for the
future replacement of these vanes with heated 5-hole OV-10A, and because the effect of the significant
probes, upon further satisfactory experience using the modifications to the nose and wingtips of NASA 524
nose-mounted probe, could ease this constraint. on the flutter modes were unknown, a flutter and
Second, even though the nominal OV-10A was envelope expansion flight test program15 was completed
designed for landings on unprepared strips, a landing in the fall of 1994. A summary of the operational
sink rate of less than 4 ft/sec has been recommended for procedures used in that flutter flight-test program is
NASA 524 to avoid excessive vertical accelerations and presented here.
subsequent stresses on the wingtip booms and mounting As the extent of the required nose and wingtip
hardware. Accelerometers have been placed on the modifications to the OV-10A became apparent,
booms to monitor these accelerations on landing, and aeroelastic specialists at LaRC were consulted to
required inspections are conducted following any firm determine the scope of a flutter flight-clearance
landings. Third, avoidance of roll rates exceeding 140 requirement. A comprehensive plan was developed
deg/sec during loaded maneuvers exceeding 4g has which included a series of ground vibration tests and
been recommended for the same purpose (although this envelope expansion flights. The ground vibration tests
is an unlikely condition for research experiments). were accomplished for both the baseline aircraft and the
Finally, the aircraft is limited to an altitude of 12,500 ft fully modified aircraft for purposes of comparison.
MSL due to the removal of the oxygen system. Analysis of these data did not indicate a strong
In terms of natural performance limits, there is possibility of flutter within the full aircraft envelope,
an approximately 25-30 KIAS (about 15%) reduction in prompting the decision to proceed to a full envelope
the maximum attainable level-flight airspeed at 10,000 expansion flight series.
ft MSL. The additional drag caused by the three NASA 524 was fully instrumented with the
research booms/fairings is the suspected cause. No DAS and telemetry system previously described, except
additional significant handling-quality and performance for the addition of accelerometers attached to the
differences between the unmodified and modified forward and aft left-wingtip spars. A high-speed taxi
versions of NASA 524 have been noted. Neither roll test was completed prior to the first flight. This test
performance nor pitch-rate capability were perceptibly showed no undamped vibratory modes, even though the
affected. The center of gravity was shifted from a near wingtip booms exhibited large-amplitude oscillations
aft limit with the instrumentation pallet (only) installed, due to runway roughness and the natural frequency of
to a near forward limit with the addition of the nose the aircraft during taxiing. Airborne testing was
boom and associated equipment in the nose wheel well. completed in two flights in November 1994. The test
This forward shift in the center of gravity results in a maneuvers included incremental airspeed increases
slight tendency for the nose to rotate sluggishly on from a straight-and-level condition, aileron raps on
takeoff. The OV-lOA's docile stall characteristics were condition to act as a frequency-sweep method, stick-
not affected. free rudder doublets to examine coupled
Wake turbulence tends to persist and be most lateral/directional excitations on the nose boom, and
hazardous during relatively "benign" weather dives to maximum attainable airspeeds (330 KEAS)
conditions, when winds, convective activity, and accompanied by aileron raps on condition. After these
precipitation are relatively light. Hence, with a few flights, the envelope was cleared for all flight
exceptions, most of the operations associated with operations with no evidence of flutter modes having
collection of wake-decay data, encounter upset been detected.
dynamics, and meteorological profiles occur during During flutter testing, data were being
fair-weather conditions coupled with typical pattern telemetered to the LaRC Flight Control Center (FCC)
approach speeds and nominal "g" loading. Required and analyzed in near-real-time by aeroelastlc
flight conditions are, therefore, not generally affected specialists. An FTE was not allowed to fly in the OV-
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10A because of the hazardous nature of the flutter tests Since the flights occurred during the winter months,
and because an FTE could not assess vibration data obtaining the proper weather conditions became
from the aft cockpit. However, a crew, which included problematic and resulted in several aborts. Both flights
a trained safety observer, chased the OV-10A in a for this procedure occurred on the same day. The
NASA T-34C turbo-prop aircraft. Progression through second flight was conducted during a period of
the test card was controlled by the flight-test director in changing wind conditions at the established limit of 10
the FCC with required inputs from the analysts. knots.
Air-Data System Calibrations Meteorological-Measurements Validation
Prior to delivery of recorded research data, a Several flights were completed in February
variation of the tower flyby method was executed in 1995, to gather data for validation of the meteorological
February 1995, for position-error calibration of the measurements and algorithms needed for further
three installed research booms and the Rosemount research flights, particularly turbulence algorithms and
temperature sensor using the unique capabilities of the computed winds.
Wallops Flight Facility. At WFF the basket of a 100-ft- Flight test maneuvers included constant-
tall "cherry picker" was instrumented with a spare OV- speed/altitude/angle-of-bank 450-degree turns;
10A pressure transducer and a radiosonde constant-heading/speed/altitude passes combined with
instrumentation pod measuring pressure and .elevator, aileron, rudder, and throttle doublets;
temperature. Both the basket and the OV-10A were constant-speed/altitude box patterns; and level-flight
also each outfitted with a laser-reflector cube, enabling acceleration/decelerations. Constant-speed spiral
precise position tracking of either by a laser tracker ascents and descents were used to validate temperature
mounted to one of the WFF radar installations. and dew-point measurements against weather-balloon
The flight profiles included multiple passes at measurements. Additionally, an airspeed calibration
incrementally increasing airspeeds flown abeam the using GPS-derived data was attempted using constant-
cherry-picker basket at 100 feet above ground level airspeedAaltitude and constant angle-of-bank 450-
(AGL), oriented along the centerline of WFF runway degree turns and constant-airspeed/altitude/heading
04. Laser-tracking acquisition of the OV-10A occurred passes on reciprocal headings.
approximately 5,000 feet from the cherry picker. These flights demonstrated some of the
Several combinations of landing gear position shortcomings of the aircraft, including the inability to
(up/down) and flap setting (up/intermediate/down) were hold a constant airspeed with a standard deviation of
flown. At set intervals during these passes, as the OV- less than one knot, and the continuous small lateral-
10A was in the pattern, the basket was lowered and directional oscillations of the aircraft. The former was
raised to obtain atmospheric profile data while being a result of the weak speed stability of the aircraft, while
simultaneously tracked by the laser tracker. At the start the latter was principally a result of, the constant
and conclusion of the pilot-static calibration flights, the variation of engine torque of ±25-50 ft-lb characteristic
OV-10A was taxied to the cherry-picker location on the of the installed Garret T-76 turboprop engines.
airport and 30 seconds of on-board data were recorded Additionally, the aircraft is characterized in both the
next to the lowered cherry-picker basket so that gear/flaps up and down configurations by a divergent
calibrations of the respective pressure transducers could spiral mode, a lightly damped Dutch-roll mode, and
be made at the same altitude. high positive dihedral effect. The combination of all of
During operations, data from all three research these effects results in "sloppy" lateral-directional
booms were recorded and video-tracking signals, characteristics in the context of precise tracking
pressure, and temperature data from the cherry picker required for these maneuvers. Maintaining zero
were time-stamped, recorded, and telemetered along sideslip requires constant attention, and non-zero
with the OV-10A data to the LaRC FCC via satellite. sideslip conditions excite the nuisance lateral-
The LaRC FCC was configured to receive the multiple directional modes.
data streams and accomplish partial real-time Due to these aircraft characteristics, the
processing and display of selected parameters for calibration maneuvers were not trivial. Even the
evaluation. Laser-tracking data was post processed slightest amount of atmospheric turbulence caused
along with on-board recorded data to give the final airspeed, heading, and angle-of-bank deviations.
position-error corrections for the research instruments. Modifications to the original test plan included
The onboard FTE was responsible for controlling constant-throttle passes to mitigate the speed instability
progress through the test card, although a flight-test and nuisance lateral-directional modes.
director was assigned to the LaRC FCC to suggest
changes based on real-time analysis of data. Video System Calibrations
The flights were scheduled for early morning The on-board stereo-video system, entailing
in order to obtain the calmest atmospheric conditions. the pair of downward-looking wingtip cameras,
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