Summary of the Effects of Engine Throttle Response on Airplane Formation-Flying lities

30 Pages·01.74 MB·English

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NASA Technical Memorandum 4465
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Summary of the Effects of Engine
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Formation-Flying lities
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NASA Technical Memorandum 4465
Summary of the Effects of Engine
Throttle Response on Airplane
Formation-Flying Qualities
Kevin R. Walsh
Dryden Flight Research Facility
Edwards, California
National Aeronautics and
Space Administration
Office of Management
Scientific and Technical
Information Program
1993

SUMMARY OF THE EFFECTS OF ENGINE THROTTLE
RESPONSE ON AIRPLANE FORMATION-FLYING QUALITIES
Kevin R. Walsh*
NASA Dryden Flight Research Facility
P.O. Box 273
Edwards, California 93523-0273
Abstract
validated. No criteria for defining good or bad engine
throttle response characteristics currently exist.
A flight evaluation was conducted to determine the
effect of engine throttle response characteristics on pre- In the past 10 years, engine throttle response prob-
cision formation-flying qualities. A variable electronic lems have been encountered in several airplanes, for ex-
throttle control system was developed and flight-tested ample, the F-15 (McDonnell Douglas Corporation, St.
on a TF-104G airplane with a J79-11B engine at the Louis, Missouri) with the developmental F100 engine
NASA Dryden Flight Research Facility. This airplane model derivative (Pratt & Whitney, West Palm Beach,
was chosen because of its known, very favorable thrust Florida), _ AV-8B (McDonnell Douglas Corporation,
response characteristics. Ten research flights were St. Louis, Missouri), 3 and F-18 (McDonnell Douglas
flown to evaluate the effects of throttle gain, time delay, Corporation, St. Louis, Missouri, and Northrop Cor-
and fuel control rate limiting on engine handling quali- poration, Hawthorne, California). Problems in these
ties during a demanding precision wing formation task. examples ranged from excessive initial time delay or
Handling quality effects of lag filters and lead compen- lag in the F-15 and AV-SB airplanes to high throttle
sation time delays were also evaluated. The Cooper sensitivity in the initial F-18 installation. Such exam-
and Harper Pilot Rating Scale was used to assign lev- ples illustrate the need for handling quality guidelines
els of handling quality. Data from pilot ratings and or design specifications for advanced engine control sys-
comments indicate that throttle control system time tems. These engine design specifications are analogous
delays and rate limits cause significant degradations in to those developed over the last two decades for ad-
handling qualities. Threshold values for satisfactory vanced flight control systems. As a result, data are
(level 1) and adequate (level 2) handling qualities of required to develop handling qualities criteria to quan-
these key variables are presented. These results may tify the effects of thrust response dynamics on a pilot's
provide engine manufacturers with guidelines to assure ability to complete precision flight control tasks.
satisfactory handling qualities in future engine designs.
A brief flight research program was conducted at
Introduction the NASA Dryden Flight Research Facility (DFRF)
to investigate the effects of varying engine throttle re-
The ability to conduct such precise flying tasks as sponse on airplane handling qualities. An electronic
close-formation flight or aerial refueling is strongly af- variable-throttle response system was developed and
fected by the engine throttle response, that is, thrust installed on a two-seat TF-104G airplane (Lockheed
response caused by throttle changes. With the ad- Corporation, Burbank, California). This airplane was
vent of digital engine control systems, control soft- an ideal choice because its J79-11B engine (General
ware has become commonly used to modify engine re- Electric, Lynn, Massachusetts) responds to throttle
sponse characteristics) The engine throttle response changes extremely quickly. The variable-response elec-
may be degraded if such modifications are improperly tronic throttle enabled the pilot to evaluate throttle
system degradations and resulting effects on engine
handling qualities.
"Aerospace Engineer.
Copyright (_)1992 by the American Institute of Aeronautics This program provides initial data for developing
and Astronautics, Inc. No copyright isasserted in the United handling qualities criteria and design guidelines for at-
States under Title 17, U.S. Code. The U.S. Government has
taining satisfactory (level 1) throttle response of high-
a royalty-free license to exercise all rights under the copy-
performance airplane engines. Data were obtained at
right claimed herein for Governmental purposes. All other
rights are reserved by the copyright owner. one flight condition: the airspeed was 300 kn, and the
altitudewas15,00f0t. Thisflightconditionrepresents Figure 1 shows the TF-104G airplane. Notable fea-
aformation-flyintgask. tures include the extremely thin ttight surfaces, the
short and straight wings with 10° anhedral, and a con-
Thispapersummarizethseresultsoftheflightre-
trollable horizontal stabilizer mounted at the top of
searchprogramanddescribetshevariable-throttlree-
the vertical stabilizer. The wings have leading- and
sponsesystemasinstalledin theTF-104Gairplane.
trailing-edge flaps and a boundary-layer control sys-
Timehistorydatafortherepresentativpeilotevalua-
tem used with the trailing-edge flaps to reduce landing
tionsandacompilatioonfpilotcommenwtsithrespect
speeds.
to thetimehistorydataarepresentedT. hesedata
showthe differencebsetweensatisfactorya,dequate,
andinadequate(level1,2,and3)handlingqualities
ontheCooperandHarperPilotRatingScale4. Data
showingtimedelayandratelimit thresholdfsorthe
differenltevelsofhandlingqualitiesarealsopresented.
Additionaldatashowtheeffectsofa first-orderlag
filterandofalead-lagfilterincombinatiownithaddi-
tionaltimedelayonhandlingqualities.
TheworkofGeorgEe.CoopearndRoberPt .ltarper,
Jr., isgratefullyacknowledg4ed.Thisworkwasin-
strumentainl developintghisenginehandlingqualities
flighttestprogram.
Nomenclature
AX aircraft longitudinal acceleration, 9
9 acceleration of gravity, ft/sec 2
EC 80-12366
HQR handling quality rating
Fig. 1. The TF-104G airplane.
L/L lead-lag
LVDT linear variable-differential transformer The J79-11B is an axial-flow, high-pressure-ratio tur-
MTE mission task element bojet engine with variable-inlet guide vanes, variable-
stator vanes, and single-rotor compressor. This engine
N1 compressor speed, rpm
has a cannular combustor, a three-stage turbine, and a
PCM pulse code modulation fully modulating afterburner with a variable-area con-
PIO pilot-induced oscillation verging and diverging exhaust nozzle.
PLA power lever angle, deg
Variable-Response Throttle System
PLACMD power lever angle command, deg
Description
PLAFH power lever angle position feedback, deg
The electronic variable-response throttle control sys-
rpm revolutions per minute
tem was developed by Calspan (Buffalo, New York)
RVDT rotary variable-differential transtbrmer
specifically for the DFRF TF-104G airplane. Neal and
TCU throttle control unit
Sengupta described the implementation and operation
AT' change in time delay, msee of the throttle control system. 5 The throttle in the
6x throttle position limit commanded by forward cockpit was modified to command the exper-
throttle control unit, deg imental system. Figure 2 shows the main components
of the system. These components consist of an elec-
re L/L denominator time constant
tronic throttle control unit (TCU), integrated servomo-
rn L/L numerator time constant tor and clutch assembly, and position sensors. A cable
w,_ second-order lag natural frequency linkage connected the servomotor with the engine fuel
controller. The throttle in the aft cockpit remained in
¢ second-order lag damping ratio
the production configuration and served as the safety
Airplane and Engine Description backup system.
Figure 3 shows a simplified block diagram of the
The test airplane was a TF-104G: a high-
modified propulsion control system. Additional throt-
performance, two-place, trainer-fighter-interceptor air-
tle system dynamics were generated by the TCU and
plane with a maximum Maeh number of 2.0.
ORIGINAL PAGE
BLACK AND WHITE PHOTe(3,RAPI-I
Throttle
control
Servomotor
Rotary variable-
Linear variable-_diff'erential transforme_'_ '_: ..........
differential transformer
920645
Fig. 2. Electronic throttle system hardware.
Figure ,1 shows a functional block diagram of the
Pilot_ electronic throttle control system. The descriptors out-
,) side the polygon represent, the mechanical system, and
the e.lcctronic system is shown inside the polygon.
PLACMD
Mechanical System
i r
E
In the production TF-104C, airphme, a conventional
cable and pulley system connects the forward arm aft
throttles to the engine fuel control. The two throttle
handles are linked so that, whe.n one throttle handh'.
TCU is moved, the other tracks its position. As a re,suit,
variables the position of the forward and aft throttles match at
all time.s.
To incorporate the electronic throttle system, the
forward or evaluation throttle was disconnected by re-
Fig. 3. Modified propulsion control system.
moving the throttle linkage to the fuel control cables.
the servomotor. For example,, the power lever angle A rotary variable-differential trarisf(_rmer (I{VI)T) was
commanded (t-'LACMD) by the pilot is modified by conrw.cted to the throttle handle through gears and in-
the TCU to command the servomotor. The resulting stalled in the throttle housing to sense forward throb
change in the power lew.'r angle (f)LA) caused by the tie position, l)isconnecling the forward throttle from
servomotor is the input to the fuel controller. This an- the cable system eliminated the inherent friction on the
gle provides PLA position feedback (PLAFI3) to the throttle handle; therefore, an ad,justabh.' friction device
electronic control system. Measurements of compres- was installed in the housing to maintain throttle feel.
sor speed (N1) and aircraft longitudinal acceleration This device, a small phenolic block and bracket, created
(AX) are also represented. Throttle gain or sensitiv- drag against the throttle-handle axle. The device was
ity, 9/deg; transport time delay, sec; and rate limiting, intended to be adjustable to any friction level; however,
deg/sec were the primary variables for the experiment. the throttle stick force was adjustable to a maximum
The secondary variables included first-order lag filter of 2 lb or approximately one'-half of the normal stick
time constants and lead-lag time constants. force for the production throtth:.
EILACK AND WHITE PH_OT_9(_PH
ThrottleControlUnit(locatedinaftcockpit)
RVDT Commandsignalfiltem
/--- position (lead-lag,second-orderlag,--
sensor _I Vadeble- timedelay,orcascade
-I commandgain +'-+,_: combinations)
Forwardcockpit
throttlecommand
__ StepInput_
(electrical) Linearmode
_--" ( ,coCmmUentdest (nofilters) o
o
Verlable.lrequency
sinewave
generator Slnueoldal
input
Aftcockpit
throttlecommand
(mechanical) L cVoamrimabalned- --_ cVoamriambalen-d _R Servomotor L__ Servomotor! (Cablelinkage)
I position rote .-b /- Double
iI limit limit ddveamplifiersI TaascsheommbeltyerI! / drivepulley
atefeedback i tJF- --'I'0ovanlv*erol 'l
Positionfeedback LVDT
position
sensor
Originalcablelinkage
1 920191
Fig. 4. The throttle control system.
A servomotor, clutch, and cable assembly was in- system were optimized by adjusting the rate feedback
stalled in the engine bay to position the filel controller. gain.
A linear variable-differential transformer (LVDT) was
Figure 5 shows the TCU panel which the aft pi-
used to sense PLA at the fuel controller.
lot used to activate the system, select the operating
The TCU was installed in the left-hand console of mode, set the system variables, and enter test signals.
the aft cockpit and provided the desired throttle re- The TCU inserted time delays, rate limits, lead-lag
sponse variations for the experiment. The backseat time constants, first- and second-order lags, and po-
pilot could disengage or override the electronic throt- sition limits into the command path. These variables
tle control system at any time. The mechanical con- acted directly on the pilot throttle commands. A ro-
trol system was always functional from the aft cock- tary switch on the TCU panel was used to select the
pit. Since the aft cockpit throttle was mechanically desired test variables. Thumbwheel switches on the
linked to the fuel control valve, the rear throttle po- control panel were used to set the value of the test vari-
sition tracked the throttle commands. The front seat ables. The variables could be tested individually with-
pilot had no way to control the engine if the TCU were out altering other variables. They could also be cas-
disengaged. caded, such as testing a time delay followed by a lead-
lag filter. Positive and negative rate limits could be ad-
Electronic System
justed independently to simulate an engine that would
The block diagram inside the shaded polygon of increase revolutions per minute (rpm) at a different
Fig. 4 shows the electronics of the throttle control sys- rate than it would decrease rpm. The output signal of
tem. The RVDT generated an electrical signal which the circuit selected was then rate and position limited.
was amplified by a gain factor in the TCU. The TCU
Circuits in the TCU could selectively generate two
checked the amplified signal against position (ampli-
test inputs: a step signal and a sinusoidal signal. The
tude) and rate limits. This signal was compared with
step input had either positive or negative polarity, and
the actual throttle position at the fuel control valve
the sinusoidal input could vary from 0.0 to 2.9 Hz. The
which was measured by the INDT. The difference be-
test inputs were used during the flight program; how-
tween the RVDT command and the LVDT feedback
ever, the data are not presented in this paper. Re-
signals was an error signal used to advance or retard
fer to Ref. 5 for additional information concerning the
the servomotor position. A tachometer on the servo-
implementation and operation of the variable-response
motor assembly provided a rate feedback to the TCU.
electronic throttle system in the TF-104G airplane.
The damping characteristics of the electronic control
_n _d Test frequency
Thumbwheel switches
Lead-lag
for setting control system
NN
variable parameters
_ mn Test
Second-order lag
ND
AT 5x max Command gain I
delay position I
Red LED Indicates limit ,I /- Red LED indicates
servoclutch has
[_ [_ V power to servomotor
been powered --_ Ra_'e limit (Sx max) CON/] _ Yellow LED indicates
0 r----Tellt--_ AT -_ //.*L/L-ZIT OSTBY .if power to TCU
Engage ISin Poa[ 2nd \ //2nd-,,IT ON J electronics
_- Sequence select switch for
choosing mode of operation 92O192
Fig. 5. The throttle control unit panel.
Instrumentation in a TF-104G airplane. According to U.S. Air Force
military specification MIL-F-8785C, the TF-104G air-
Information from the airplane, engine, and TCU
plane, being a highly maneuverable fighter and inter-
was obtained on a pulse code modulation (PCM)
ceptor, is considered a class IV aircraft. 7
data acquisition system. The serial PCM data were
telemetered to the ground, decoded and formatted for Formation flying was the mission of interest for the
real-time display on cathode ray tubes and strip charts, evaluations. The specification classifies formation fly-
and recorded for postflight analysis. Instrumented pa- ing as a category A flight phase. 7 This flight phase
rameters were measured at 200 samples/sec. requires rapid maneuvering, precision tracking, or pre-
cise flightpath control.
Measured values of pressure and temperature were
used to generate computed values of airspeed, altitude, The primary variable for obtaining handling qualities
Mach number, static temperature, and standard day data was thrust control. Other typical handling quality
temperature at altitude. These calculated parameters variables, such as airframe stability and control char-
were also formatted for real-time display. Tables 1 and acteristics as well as cockpit interface elements, were
not considered.
2 in Ref. 5show the airplane, engine, and TCU param-
eters that were measured for this experiment.
The five pilots who participated in the program were
experienced in evaluating handling qualities of class IV
Development of Handling Qualities
airplanes. Their role was important in determining
Experiment
pilot and vehicle performance during the task. Fig-
ure 6 shows a modified Cooper and Harper Handling
This discussion describes the development of the
Qualities Rating (HQR) Scale. This scale defines sat-
electronic throttle handling qualities experiment and
isfactory, adequate, and inadequate (level 1, 2, and 3)
is based on the work of Cooper and Harper. 4 To
perform ance.
achieve reliable data and comparable ratings among
the pilots, care was taken in developing the experiment Mission Description
objectives, mission description, mission task elements
The required operation for the formation-flying task
(MTE) 6, rating criteria, pilot assessments, and flight
was to attain and maintain fore and aft position rel-
test procedures.
ative to the wing of the lead airplane by using visual
Objectives references. This high gain, precise, closed-loop track-
ing task requires the pilot to devote full-attention to
In general, the experiment obtained highly definitive
airplane control and to use the throttle as the primary
handling qualities data for a modified J79-11B engine
control input.
5
Demandsonthepilotin Pilot ] Handling'_
Adoerqrueaqcuyifroerdsoepleercatetidontask ) CAircraftcharacteristics " selectedtaskorrequiredoperation' ratingI quality,,)
• Excellent, • Pilotcompensationnotafactor 1
highlydesirable fordesiredperformance
Good, Pilotcompensationnotafactor LevelI,
negligibledeliciencies • fordesiredperformance 2 satisfactor
Fair,somamildly • Minimalpilotcompensationrequired 3
_unpleasantdeficiencies fordesiredpedormance
Yea
• Minorbutannoying Desiredpedormancerequires
deficiencies • moderatepilotcompensation
Deficiencies
without warrant Moderatelyobjectionable• Adequateperformancerequires Level2,
deficiencies considerablepilotcompensation adequate
improvement
Veryobjectionablebut • Adequateperformancerequires
_tolerabledeficiencies extensivepilotcompensation
Yes
Adequateperformancenotattainablewithmaximum 7
Majordeficiencies • tolerablepilotcompensation;controllabilitynotinquestion
Deliclencies'_
attainablewith Majordeficiencies • Considerablepilotcompensation Level3,
isrequiredforcontrol 8 inadequate
Intensepilotcompensation
Majordeficiencies • 9
_.. isrequiredtoretainconlrol
Yes
Controlwillbelostduringsome
itcontrollable_ Majordeliciencies •
portionofrequiredoperation
Yea
Pilotdecisions
Fig. 6. Cooper and Harper Pilot Rating Scale.
The lead airplane for the formation task was either keeping. This subphase was intended to be represen-
a T-38 (Northrop Corporation, tiawthorne, Calilbrnia) tative of the precision required in similar operational
or an F-18. Nominal flight conditions used for the test tasks, such as in air-to-air refueling or close-formation
program were an altitude of 15,000 ft above mean sea flight under adverse instrument conditions. Without
level and an indicated airspeed of 350 kn. This flight the introduction of suitable task perturbations to chal-
condition was chosen so that throttle motion was al- lenge the pilot and the throttle response system, the
ways in partial power. Only in the worst case TCU task would not provide the desired degree of handling
configurations did the throttle hit the military or idle qualities discrimination. Close-formation flight with a
power detents. smooth leader does not, in itself, provide the necessary
discrimination; the evaluation pilot cannot separate the
Mission Task Elements
optimum from the marginally acceptable cases. Note,
Developing a well-defined MTE with handling qual- for example, that even large relatively ponderous ships
ity performance criteria reduces the uncertainties and can fly accurate close formations during refueling tasks
extrapolation to the real-world equivalent required by at sea.
the pilot, ttaving the pilots consider the real-world
Since the emergence of full-authority electronic en-
equivalent during the evaluation lends itself to some
gine control allows the designer some flexibility, the
pilot extrapolation. Such extrapolation differs for each
task must allow definition of the satisfactory (level 1)
pilot because of variations in training, knowledge, ex-
throttle response characteristics. The MTE selected,
perience, and ability to assess beyond the specific task.
therefore, involved precision wing station keeping
The flight subphase chosen after preliminary evalua- during small and unannounced step throttle changes
tions of several formation tasks was close-wing station by the formation leader. F_rther, the evaluation pilot
6