7/30/2019 A Technique to Determine Lift and Drag Polars in Flight

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VO L. 20, NO. 7 , JUL Y 1983 J. A I R C R A F T 587

A Technique to Determine Lift and Drag Polars in FlightA . Knaus*Messerschmitt-Bolkow-Blohm GmbH, Munich, Federal Republic of Germany

Performance trials of the European combat aircraft Tornado have concentrated on the systematicmeasurement of lift and drag polars. Such polars have been successfully measured by means of well-adapted testinstrumentation, a data reduction system, and a high calibration standard of the aircraft and engines. The use ofa certain com bination of steady-state and dynamic test m aneuvers has resulted in a drastic reduction in theamount of flight time required to obtain sufficient data for the determination of the zero lift drag, induced dragcharacteristics, and drag increments due to aircraft configuration changes. Flight test results are presented whichdemonstrate the advantage of the utilized test techn ique and the high data quality achieved.

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Nomenclature= nozzle exit area calculated= nozzle exit area m easured= acceleration along flight path= acceleration n orm al to flight path= acceleration corrected = coefficient of drag an d lift ,respectively= gross thrust coefficient= nozzle discharge coefficient= center of gravity= drag= ram drag= excess thrus t= gross thrust= gross thrust ideal (calculated)= net thrust= gravitational constant= angle-of-attack calibrationcoefficients= lift=main fuel mass flow= reheat fuel mass flow= Mach = t rue Mach num ber= air mass flow at engine entry= gas mass flow at nozzle entry= gas mass flow at nozzle exit='bleed mass flow= high-pressure spool speed=load factor= ambient pressure= mixed total pressure

(fromPT4+PT4LB)= measured total pressure at nozzleexit= total pressure at engine entry= total pressure at low-pressureturbine exit= total pressure at bypass exit= power o f f t a k e= roll rate= pitch rate= angle-of-attack sensorx axis distance to e.g.= angle of attack sensory axis distance to e.g.= sea levelPresented as Paper 81-2420 at the AIAA/SETP/SFTE/SAE/ITEA/IEEE 1st Flight Testing Con ference, Las Vegas, Nev., N ov. 11-13, 1981; submit ted N ov. 16 , 1981; revision received Oct. 22 , 1982.Copyright American Inst itute of Aeronautics an d Astronautics,Inc., 1981. All righ ts reserved.*Head o f Flight Test Analysis, Flight Test Departm ent.

7T,TT 0.7/0.905 1.42Fig. 10 Lift/drag polars, comparison flight test/prediction.

Airframe/OptimizationA s already mentioned at the beginning, th e test method , asoutlined in this paper, w as used for a number of performanceinvestigations in order to measure th e effect of aerodynamicchanges on lift an d drag. An example is given in Fig. 9.This figure show s the airplane zero lift drag variation withMach number for aircraft A /C 1, (unfavorable rear end) andfo r aircraft A/C 6 (improved afterbody shape and ex-crescences). The zero lift drag at a given Mach number wasdetermined from measured CD vs C 2L curves, as shown in Fig.7. In all cases the airplane induced drag characteristics w ereobtained from roller-coaster and wind-up turn maneuvers .The quality and quantity of these drag measurements giveevidence to the overall drag change due to air framemodif icat ions . It was possible, as s h o w n in Fig. 9, to reducethe drag level of A/C 6 by about 8% , which was later con-f irmed by drag measurements on A/C 11 as well as A/C 13.

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A/C 11. O FLIGHT 236/COMBAT ACCEL/TEST ALT.30000 FT

A FLIGHT 241 /COMBAT, M/R ACCEL/TEST ALT. =30000 FTx FLIGHT 242/COMBAT ACCEL/TEST ALT. =36000 FT FLIGHT 224/STEADY LEVELA/C 13H FLIGHT 22-37/STEADY LEVEL

Fig. 11 Zero lift drag variation with Mach number.

DRAG POLARHEAVY STORE CONFIG.

CLEAN AIRCRAFT

HEAVY STORE CONFIG. A/C 11 STEADY LEVEL+ A/C 11 ROLLER COASTERCLEAN AIRCRAFT A/C 11 STEADY LEVELX A/C 11 ROLLER COASTER* A/C 13STEADY LEVEL

HEAVY STORE CONFIG.

AA.0A = 25STEADY LEVELA = 45STEADY LEVELA = 67STEADY LEVELA = 25MAX DRY ACCEL.A =67MAX DRY DECEL

MACHFig. 12 External store drag.

Comparison with PredictionsSome representative comparisons between flight data an dpredictions follow. The predictions ar e based on wind-tunnelresults w hich were adjusted from model-test to full-scaleconditions.Figure 10 shows subsonic an d supersonic l ift/drag polars atM a = 0.7 for 25-deg wing sweep and at Ma = 0.9 and 1.42 fo r67-deg wing sweep. Such polar measurements have beencarried out for four dif ferent wing sweeps in 0.05 or 0.1 Machincrements within the entire envelope. This allows the ac-curate determination of drag variation w ith Ma c h n u mb e r .Again the large quantity and high quality (low data scatter) oftest data obtained from dynamic maneuvers permit an ac-curate assessment of the polar w hich would not be possibleusing steady-state m aneuvers only.In general, comparison between flight test data an dpredictions have s h o w n good agreement for all wing sweeps.Minor discrepancies w ere found in the lift and induced dragof the 25-deg w ing .Figure 11 presents th e zero l if t drag variation wi th Machn u mb e r . Th e C D() figures, at a given Mach n u m b e r , weredetermined f rom th e basic CD vs C 2L curves, as given, forexample in Fig. 11. The comparison shows excellentagreement in the subsonic and higher supersonic range. Flightdata show a somew hat steeper drag rise in the transonic area,but lower drag in the low supersonic Mach range.Figure 12 shows typical results of store drag measuremen tsfo r a heavy store configuration. The measured total aircraf tdrag as compared to the basic drag polar of the clean aircraf t

is given, for the most interesting angle of attack range, on thelef t-hand side of Fig. 12. After both polars have beencorrected to the same reference condi t ions, th e differencebetween the two polars represents the drag increment due toexternal stores/Resu lts f rom measuremen ts at dif ferent Machn u m b e r s are plotted on the r ight-hand s ide of Fig. 12 and arecompared wi th predictions.

7/30/2019 A Technique to Determine Lift and Drag Polars in Flight

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JULY 1983 LIFT AND DRAG POLARS 593Concluding Remarks

Experience gathered in the course of an extensive per-formance test program has show n tha t , using test methodscurrently in existence, it is possible to meas ure lift an d dragpolars accurately in-flight.The use of dynamic maneuvers allows a drastic reduction inflying t ime as well as the measuremen t of the induced dragan d zero lift drag characteristics of an aircraf t . This , in tu rn ,makes it feasible to verify in-flight, over the entire angle-of-attack range, th e effects of aerodynamic modif icat ions on liftand drag.To achieve the indicated accuracies, high demands areplaced on th e f l ight test instrumentation, which must bespecifically tailored to the task, as well as on the calibrationstandard of the engines involved in the thrust in-fl ightdeterminat ion.It is self-evident that demands of this type necessitatecertain expenditures. This aspect is therefore currently the

subject of an intensive effor t to fur ther improve th e costeffectiveness of the flight test of the future .

AcknowledgmentsTh e tasks of the Tornado Overall Flight Test Program ar eshared among Aeritalia at Caselle /Turin, British Aerospace

at War ton , an d Messerschmitt-Bolkow-Blohm at the FlightTest Center at Manch ing . The author wishes to t hank hiscolleagues at AIT, BAe, and MB B for their close cooperationduring the various segments of the Tornado performance testphases.

ReferenceV ., "Performance Assessment of an Advanced ReheatTurbo Fa n Engine," AIAA Paper 81-2447, Nov. 1981.

F rom the AIAA Progress in Astronautics and Aeronautics SeriesALTERNATIVE HYDRO CARBON FUELS:

COMBUSTION AND CHEMICAL KINETICSv. 62A Project SQUID W o r k s h o p

Edited by Craig T. Bowman, Stanford Universityan d J < j > r g e n Birkeland, Department of EnergyTh e c u r re n t generat ion of internal com bus t i on engines is the result of an extended period of s imultaneous evolut ion ofengines an d fuels. During t h i s period, the engine designer was relatively free to specify fuel proper t ies to meet engine per-f o rma n c e r equ i r em ents , and the petroleum in d u s t ry responded by pr oduc i ng fuels wi th th e desired spec if ica t ions. H o w e v e r ,t oday ' s r is ing cost of pe t ro leum, coupled wi th the realization t h a t pet roleum supplies wil l not be able to meet the long-termd e m a n d , h as s t imu la t e d an in te rest in a l te rna t ive l iquid fuels, part icular ly those t h a t ca n be derived f rom coal . A w i d evar i e t y of l iquid fuels can be produced f rom coal , and f rom o t her hyd rocarbon and carbohydrate sources as well , r ang i ngf rom m e t h a n o l to high molecular w ei gh t , lo w volati li ty oils. This volume is based on a set of original papers delivered at aspecial w o r k s h o p cal led by the Depar tment of Energy and the Depar tment of Defense fo r th e purpose of discussing th epr ob lem s of swi t ch ing to fuels producible f rom such nonpetroleum sources for use in au tom ot i ve eng i nes , a irc ra f t gast u r b i n e s , and s t a t io n a ry p o w e r plan t s . The au t ho r s were asked also to indicate how research in the areas of combus t ion , fuelc h e m i s t r y , an d chemica l kine t ics can be di rected toward achieving a t ime l y t r a n s i t io n to such fuels, should i t becomenecessary. Research scientists in those f ields, as well as development engineers concerned wi th engines and power plants , wil lf ind th i s v o lu m e a usefu l up- to- da te analysis of the c ha n g in g fuels picture .

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