The minimum fuel mode of the NASA F-15 research aircraft is designed to minimize fuel flow while maintaining constant net propulsive force (FNP), effectively reducing thrust specific fuel consumption (TSFC), during cruise flight conditions. The test maneuvers were at stabilized flight conditions. The aircraft test engine was allowed to stabilize at the cruise conditions before data collection initiated; data were then recorded with performance seeking control (PSC) not-engaged, then data were recorded with the PSC system engaged. The maneuvers were flown back-to-back to allow for direct comparisons by minimizing the effects of variations in the test day conditions. The minimum fuel mode was evaluated at subsonic and supersonic Mach numbers and focused on three altitudes: 15,000; 30,000; and 45,000 feet. Flight data were collected for part, military, partial, and maximum afterburning power conditions. The TSFC savings at supersonic Mach numbers, ranging from approximately 4% to nearly 10%, are in general much larger than at subsonic Mach numbers because of PSC trims to the afterburner

Nobbs, Steven G.

Orme, John S.

English

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N95- 33015
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NASA Dryden Flight Research Center
"Minimum Fuel Mode Evaluation"
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CONTENTS:
• Minimum Fuel Mode Predictions (p. 1)
• Minimum Fuel Mode Subsonic Analysis (D, 2)
• Minimum Fuel Mode Supersonic Analysis (p. 3)
• Minimum Fuel Mode Suoersonic Analysis (contd.) (p. 4)
• Summary 9f Results for the Minimum Fuel Mode (o. 5)
• Summary of Results for the Minimum Fuel Mode (contd.) (p. 6/
Author." John S. Orme
f_oliation: NASA D_den Flight Research Center
he: (805)258-3683
Fat: (805)258-3744
Address: P.O. Box 273, MS D-2033, Edwards, CA 93523
e-mail: orme@alien.dfrfnasa.gov
Author: Steven G. Nobbs
Affiliation: McDonnell Douglas Corporation
Phone: (314)232-2717
Fax: (314)232-4141
Address: MC 1069020, P.O. Box 516, St. Louis, MO 63166
e-mail: m236054%etd, decnet@mdcgwy.mdc, com
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"Minimum Fuel Mode Evaluation", page 1
Minimum Fuel Mode Predictions
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In the Minimum Fuel Mode, fuel flow is reduced while baseline engine thrust is maintained. Thrust Specific Fuel
Consumption (TSFC) reductions for the Minimum Fuel Mode are predicted to range from 0.5% to 3%at cruise power
settings subsonically and 6% to 10% at maximum power supersomcally. These results were generated using the Dynamic
Propulsion System Simulation. At part power settings, core fuel flow is reduced while baseline engine thrust is
maintained. Much greater TSFC reductions are obtained at maximum power because fuel flow in me inefficient
augmentor is reduced and thrust in the engine core is increased.
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Author: John S. Orme
e-mail: orme@alien.dfrf.nasa.gov
Author: Steven G. Nobbs
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"Minimum Fuel Mode Evaluation", page 2
Minimum Fuel Mode Subsonic Analysis
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The Minimum Fuel mode is designed to minimize fuel flow while maintaining constant FNP (effectively
reducing TSFC) during cruise flight conditions. The test maneuvers were at stabilized flight conditions.
The aircraft test engine was allowed to stabilize at the cruise conditions before data collection
initiated; data were then recorded with PSC not-engaged, then data were recorded with the PSC system
engaged. The maneuvers were flown back-to-back to allow for direct comparisons by minimizing the effects
of variations in the test day conditions. The Minimum Fuel mode was evaluated at subsonic and supersonic
Mach numbers and focused on three altitudes: 15,000, 30,000, and 45,000 feet. Flight data were collected
for part, military, partial and maximum afterburning power conditions.
Analysis for a typical Minimum Fuel mode demonstration during the single-engine subsonic test phase is
shown. The cruise flight condition was Mach, 0.88, and 45,000 feet. When necessary, the pilot maintained
flight cond!tion by commanding the non-test engine throttle and stick. This was done for all single
engine testing.
Time histories are presented for performance parameters (M, P-TIT, and TSFC), and engine operating
92
arameters(EPR,DEECcalculatedfanairflow,andfanstallmargin).ThePSCsystemwasnotengagedfrom
toapproximately30sec.ThesteadystatevalueofTSFCwiththePSCsystemdisengagedwas
approximately0.99.ThePSCsystemwasengagedfrom30secondsthrought eendoftherun.ThePSC
algorithmheldFNPtowithin+/-2%oftheinitialvalueafterthePSCsystemwasengaged.Thesteady
stateTSFCwiththePSCsystemengagedwasapproximately0.97,anearly2%improvementonfuel
consumption.ThefuelreductionwasachievedbydecreasingEPRandincreasing1anairflowaswellas
repositioningthecompressorandfanvariableguidevanes.Thefanstallmarginwasdrivenlowerbythe
changeinengineoperatingcondition.ThisflightconditionisneartheoptimalminimumTSFCconditionforthebaselineaircraft.
Itisofinteresttonotethereductioninturbinet mperatureofnearly40deg.R.SinceFTITwasnot
includedaspartoftheperformanceindexofthePSCoptimization,thetemperaturedecreasewas
coincidental.Aswillbeshownlater,theMinimumFuelmodedoesnotalwaysproduceaF-TITreduction.
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Author." Steven G. Nobbs
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"Minimum Fuel Mode Evaluation", page 3
Minimum Fuel Mode Supersonic Analysis
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The figure shown presents time histories for a typical test of the Minimum Fuel mode demonstration
during the dual-engine supersonic test phase. The cruise flight condition was Mach !.50 at an altitude
of 30,000 feet. At supersonic conditions, PSC controls the inlet ramps and afterburner fuel flow in
addition to all the other engine controls. Because this was a two engme test, the pilot made no
throttle inputs and Mach number was controlled indirectly by the PSC system maintaining a constant Net
Propulsive Force (FNP). Any model errors in FNP will show up in a change in Mach number. During the test
Mach number was unaffected by engaging PSC, lending confidence in the modeled FNP being maintained well
within 2% of initial FNP.
Time histories are presented for flight condition (M and altitude), performance parameters (FNP, FTIT,
and TSFC), engine operating parameters (EPR, airflow, total fuel flow, and variable vane angle) and
inlet parameters (inlet ramp angles and shock displacement ratio). The PSC system was not engaged from 0.
to approximately 25 sec. The steady state value of TSFC with the PSC system disengaged was approximately
1.95. The PSC system was engagedfrom 25 seconds through the end of the run. The steady state TSFC with
the PSC system engaged was approximately 1.77, over a 9% improvement on fuel consumption.
94
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"Minimum Fuel Mode Evaluation", page 4
Minimum Fuel Mode Supersonic Analysis (contd.)
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A fuel reduction of about 2500 pph was achieved by increasing EPR and fan airflow, while reducing
afterburner fuel flow. In effect, thrust produced by the less efficient afterburner was traded for
thrust produced from the engine core. The result as evident from the increase in turbine temperature,
reflecting more thrust and fuel flow in the core. The shock displacement ratio, a measure of the
distance the oblique shock wave stands from the inlet cowl, was driven to its lower limit of
approximately 10% by the change in airflow and inlet cowl position.
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"Minimum Fuel Mode Evaluation", page 5
Summary of Results for the Minimum Fuel Mode
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A comparison of measured and predicted TSFC savings which resulted from the PSC system during the
single-engine and deteriorated engine test phase is presented above as a function of test engine power
setting and flight condition. Data were collected at Mach 0.9 at an altitude of 15,000 and 30,000 feet,
and at Mach 0.88 at an altitude 45,000 feet for both the refurbished and degraded engines. The TSFC
savings are in general relatively small. The calculation of TSFC is especially sensitive to the
parameters that define it (TSFC = total fuel flow/net propulsive force) and the relativel_¢ short run of
data collected. In spite of the scatter, the TSFC savings are clearly established witla savings ranging
from a few tenths of a percent at the lowest power settings to one and one-half percent savings at the
MIL power setting. The flight data are in good agreement with the predictions at the high PLAs but are
noticeably lower than predictions at 50deg. PLA. In general, the best improvements appear to be at
45,000 feet altitude. Based on the general similarity of the data, it is clear that the PSC algorithm
has the ability to adapt to the specific health state ot the engine.
Although not large, the TSFC reductions could significantly reduce takeoff gross weight or increase
range when considering long-range cruise segments, as might be encountered for a second-generation
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supersonic transport.
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Author: Steven G. Nobbs
e-mail: m236054%etd.decnet@mdcgwy.nwlc.com
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"Minimum Fuel Mode Evaluation", page 6
Summary of Results for the Minimum Fuel Mode (contd.)
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Measured TSFC savings which resulted from the PSC system during the dual-engine test phase are presented
above as a function of test engine net propulsive force (FNP) and flight condition. Data were collected
at altitudes of 30,000 and 45,000 feet. The TSFC savings at supersonic Mach numbers are in general much
larger than at subsonic Mach numbers because of PSC trims to the afterburner (A/B). Supersonically, TSFC
savings range from approximately 4% to nearly 10 %. The magnitude of these savings is phenomenal.
Reductions m TSFC of this order usually come about only through significant and costly hardware
reconfigurations. PSC has achieved very substantial results with computer software alone.
The results indicate more TSFC savings at higher FNP levels. At higher FNP levels the afterburner is
consuming more fuel, allowing for larger afterburner fuel flow down trims. It is clear from the data
that the PSC algorithm produces similar results independent of the specific engine it is applied to.
The TSFC reductions could significantly reduce takeoff gross weight or increase range when considering
long-range cruise segments, as might be encountered for a second-generation supersonic transport.
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