The performance seeking control algorithm optimizes total propulsion system performance. This adaptive, model-based optimization algorithm has been successfully flight demonstrated on two engines with differing levels of degradation. Models of the engine, nozzle, and inlet produce reliable, accurate estimates of engine performance. But, because of an observability problem, component levels of degradation cannot be accurately determined. Depending on engine-specific operating characteristics PSC achieves various levels performance improvement. For example, engines with more deterioration typically operate at higher turbine temperatures than less deteriorated engines. Thus when the PSC maximum thrust mode is applied, for example, there will be less temperature margin available to be traded for increasing thrust

Orme, John S.

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N95- 33020
NASA Dryden Flight Research Center
"Summary"
Table of Contents
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CONTENTS:
• Adaptive Features (p. 1)
• Performance Improvements (p. 2)
• Algorithm Flexibility (p. 3)
• The Future (p. 4)
• Final Thoughts (p. 5)
Author." John S. Orme
Affiliation: NASA Dryden Flight Research Center
Phone: (805)258-3683
Fax: (805)258-3744
Address: P.O. Box 273, MS D-2033, Edwards, CA 93523
e-mail: orme@alien.dfrf nasa.gov
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"Summary", page 1
Adaptive Features
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The Performance Seeking Control algorithm optimizes total propulsion system performance. This
adaptive, model-based optimization algorithm has been successfully flight demonstrated on two
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engines with differing levels of degradation. Models of the engine, nozzle and inlet produce reliable,
accurate estimates of engine performance. But, because of an observability problem, component levels
of degradation cannot be accurately determined.
Depending on engine-specific operating characteristics PSC achieves various levels performance
improvement. For example, engines with more deterioration typically operate at higher turbine
temperatures than less deteriorated engines. Thus when the PSC maximum thrust mode is applied, for
example, there will be less temperature margin available to be traded for increasing thrust.
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"Summary", page 2
Performance Improvements
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Flight results show substantial benefits from the F-15 PSC algorithm. The PSC system benefits in
general accrue from more accurate, real-time knowledge of various safety margins - that is, where the
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system currently is and where it can safely go. The PSC system takes advantage of this difference to
maximize benefits. To its credit, the system operated in an exceedingly safe manner. No unrecoverable
stalls, engine over-temps, or ingested shocks occurred over the 72 PSC test flights. In one instance,
however, because of unsteady conditions, just after engaging the system, PSC caused a self-clearing
pop-stall of the fan and immediately the system automatically disengaged. Also, during optimizations in
which the fan turbine inlet temperature was driven to its maximum limit, the limit was exceeded
transiently, but never more than by 10 deg.F. The pilots who flew with the PSC system characterize its
operation as exceptionally reliable and were most impressed with its acceleration and deceleration
performance.
In the Maximum Thrust mode, increases of up to 15 percent at subsonic and 10 percent at supersonic
flight conditions were identified. Thrust increases were achieved essentially by trading available fan
stall margin and operating at higher turbine temperatures. The Maximum Thrust mode reduced the time
to accelerate by 15 percent at military power and between 4 and 7 percent at maximum afterburner.
Performance improvements of this magnitude could be useful in a combat situation.
The Minimum Fan Turbine Inlet Temperature mode demonstrated temperature reductions exceeding 100
deg.F at high altitudes. If temperature were the only factor affecting engine life, these reductions would
more than double engine life. In addition, lower operating temperatures could mean less required engine
maintenance. The primary means of accomplishing the decreases in temperature were by reducing trim
drag and lessening the thrust required for cruise.
Savings in fuel consumption of up to 2 percent in the subsonic regime and almost 10 percent
supersonically were observed in the Minimum Fuel mode. Fuel consumption improvements like these
could offer significant cost savings and/or range improvements to commercial airlines or the military. A
large portion of the fuel savings are attained by down trimming the afterburner and also by reducing
trim drag. Thrust was maintained in both the Minimum Fan Turbine Inlet Temperature mode and the
Minimum Fuel mode as evidenced by the constant flight condition.
Supersonic decelerations with the PSC Rapid Deceleration mode produced dramatic results. At 45,000
feet, time to decelerate from Mach 2 to 1.1 was reduced by 50 percent. At 30,000 feet, time to decelerate
was cut by approximately 30 percent. For in-flight emergencies, the benefits of this mode include
increased controllability and safety. For military aircraft flying supersonic intercept missions, rapid
deceleration gives the pilot increased control when engaging the adversary. Reducing infrared signature
by lowering engine exhaust temperature may also be desired.
Overall, the PSC system can provide significant benefits for economy and performance. As a design
tool, PSC could be used to reduce aircraft weight. PSC offers advantages to existing commercial
subsonic and high performance military aircraft, as well as any future aircraft including the High Speed
Civil Transport aircraft. For existing aircraft, PSC performance could be gained without any weight
penalty. PSC could be used as a low cost and low weight retrofit to an entire class of aircraft. If PSC were
incorporated in the design stage, the resulting configuration would reflect PSC's contribution by
reductions in weight, maintenance costs, and performance.
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"Summary", page 3
Algorithm Flexibility
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The flexibility of the PSC algorithm, its architecture and implementation contributed greatly to a
successful test program. The ability to rapidly change software configuration, greatly facilitated the
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debuggingand trouble-shooting of the system.Two new modeswhich weren't even consideredin the
initial PSCdesignwere addedwith very little difficulty, the Rapid Decelerationand PSCExcitation
modes.The performanceobjective of thePSCalgorithm canbechangedvery easily aswas thecasein
the Rapid Decelerationmode wherethe performanceindex wasjust theoppositesign of the Maximum
Thrust mode. In addition, theability for the pilot selectthe algorithm configuration via theNavigation
Control Unit (NCI) allowed for numerousparametricstudiesto be conducted.Changing the numberof
control effectors andthe measuringthe effect of biases,for example,would have beenextremely
cumbersomeif a new OFPhad to be releasedeachtime configuration changed.
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"Summary", page 4
The Future
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The F-15 PSC program developed a technical approach and methodology that can enhance the
performance of high-performance and transport aircraft. The PSC algorithm as it was implemented on
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the F-15, however, requiresaccuratemodels that predict actual flight hardwareperformanceoperation.
In addition, the adaptiveestimationtechniquedependson accuratemeasurementsof the inputs and
outputsof the systembeing optimized. Becauseof the model-based open-loop approachusedby the
F-15 PSC,errors in modeling andmeasurementsproduceestimatedoptimal trim commandsrather than
measurement-based,true optimal trim commands.To improve systemperformance,several
improvementscould be made.By increasingthe number of measurementsand adding feedback,the
systemwould rely lesson the modelsandwould simultaneously improve modeling accuracy.
Alternative estimatorsto the Kalman filter shouldalsobe explored; the dynamic Kalman filter employed
for PSCwasunnecessarilycomplicatedand hadanobservability problem. At somepoint in the future, it
mat be desiredto expandthe valid aircraft maneuveringenvelopefor PSCbeyondjust quasi
steady-stateto moredynamic conditions. It would alsobe of interestto expandthe integratedcontrols
methodologyto include more direct aerodynamiccontrol effectors in the PSCoptimization suchas
stabilator and ailerons.
Someof the areasfor further researchmentionedabovearecurrently being addressedin related
programs.A joint NASA, USAF, MDA, and P&W programcalled Adaptive Aircraft Performance
Technology (ADAPT) is a follow-on PSCproject. ADAPT will continue to advancethe optimal
performancetechnology basewith a performanceoptimization algorithm that is measurement-based
and includes feedbacks.The modified F-15 ShortTake-off andLanding/Maneuvering Technology
Demonstrator(S/MTD) aircraft will beusedto demonstratethis technology. Initial planning is directed
at quasi-steady optimization modessuchasminimum fuel consumption at constantthrust or maximum
thrust for a fixed fuel flow. The ADAPT optimization approachusesmeasurementfeedbackof
performancemetrics to ensureoptimality. The ADAPT algorithm primarily optimizes with aerodynamic
effectors to achieveits results,but also will control an axi-symmetric pitch/yaw vectoring nozzle.
Two other plannedprogramsare relatedto the PSCresearch.The High Stability Engine Control
(HISTEC) will investigatea multi-variable controller for direct operating-line engine control to provide
distortion tolerant control for thepurposesof increasingperformance. IntegratedMethodology for
Propulsion and Airframe Control Technology (IMPACT) is a programfor developing a global control
designmethodology. The idea of IMPACT is to capitalize on the inherent coupling betweenthe engine
andairframe.
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"Summary", page 5
Final Thoughts
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The Performance Seeking Control experience is an excellent example of how flight test research
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benefitsemerging technologies.The close-knit relation betweenworking partnersin governmentand
industry hasbeenmutually beneficial. From the PSCflight testprogram a high risk technology was
demonstratedand maturedto the extent that industry is alreadycommercializing it. The typical
developmentcycle time for anew high risk technology suchasPSCis anywherefrom 7 to 10years.
Evenbefore the testprogram endedin 1993,portions of the PSCtechnology were being incorporated as
a standardpart of new military aircraft engines.This demonstratesthe value of flight test research.The
governmentgainedexperiencewith a new technology and fulfilled its mission of technology transfer.
Without NASA's aid, MDA andP&W probably would not havedevelopedthe PSCtechnology to the
point wherecommercial productsresult becausethe costsand risks arejust too high.
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PCA Session Information
As a result of several accidents in which all or major parts of the flight control
system was lost, NASA Dryden investigated the capability for a "Propulsion
Controlled Aircraft" (PCA) system, one which used only engine thrust for
flight control.
Initial flight studies with the pilot manually controlling the throttles and all
flight controls locked in the NASA F-15 showed that it was possible to
maintain gross control. For instance, a climb could be initiated by adding an
equal amount of power to both engines. Bank control could be achieved by
adding power to one engine and reducing power to the opposite engine.
Using these techniques, altitude could be maintained within a few hundred
feet and heading to within a few degrees. These same flights showed that it
was extremely difficult to land on a runway. This was due to the small control
forces and moments of engine thrust, difficulty in controlling the phugoid
oscillations, and difficulty in compensating for the slow engine response.
Studies in flight simulators at Dryden and at McDonnell Douglas were able to
duplicate the flight results. These simulators also established the feasibility of
a PCA mode, shown below, using feedback of parameters such as flight path
angle and bank angle to augment the throttle control capability and to
stabilize the airplane.
The NASA F-15 was an ideal testbed airplane for this research. The HIDEC
digital engine controls, digital flight controls, general-purpose computer and
data bus architecture minimized the equipment that had to be added for PCA.
The only equipment added to the airplane was a control panel containing 2
thumbwheels, one for flightpath command, and the other for bank angle
command. These papers will describe the design, development, and flight test
results.
Agenda
Frank W. Burcham Jr., "Background and Principles of Throttles-Only Flight
Control"
Edward A. Wells, James M. Urnes, Sr., "Propulsion Controlled Aircraft
Design and Development"
Frank W. Burcham Jr., Trindel A. Maine, "Flight Test of a Propulsion
Controlled Aircraft System on the NASA F-15 Airplane"
Stephen Corda, Mark T. Stephenson, Frank W. Burcham Jr., "Dynamic
Ground Effects Flight Test of the NASA F-15 Airplane"
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PCA Session Information (Concluded)
Agenda (Concluded)
Trindel A. Maine, Frank W. Burcham Jr., Peter Schaefer, John Burken,
"Design Challenges Encountered in the F-15 PCA Flight Test Program"
Frank W. Burcham Jr., "F-15 PCA Conclusions and Lessons Learned"
Session Chair
Chair: Bill Burcham
Affiliation: NASA Dryden Flight Research Center
Phone: 805-258-3126
Fax: 805-258-3744
Address: M. S. D2033, P. O. Box 273, Edwards, CA 93523-0273
e-mail: BilI_Burcham@QMGATE.DFRF.NASA.GOV
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Performance seeking control (PSC) for the F-15 highly integrated digital electronic control (HIDEC) aircraft

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