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Wing-Strake Juncture . c. 1 NASA Technical Paper 1803 .. .. I. - .~ ,. Experimental " and Analytical .Study. .. of the Longitudin al. Aero-dynamic . ~ , .- Characteristics of Analyticallyand -, - Configurations at.. -Subcritical Speeds .. John E. Lamar and ,Neal .T..Frink .~ TECH LIBRARY WFB, NY NASA TechnicalPaper 1803 Experimentaland Analytical Study of theLongitudinal Aerodynamic Characteristics of Analytically and Empirically Designed Strake- Wing Configurationsat Subcritical Speeds John E. Lamar and Neal T. Frink Latzgley ResearchCenter Humpton, Virgillia National Aeronautics and Space Administration Scientific and Technical Information Branch 1981 SUMMARY Sixteen analytically and empirically designed strakes havebeen tested experimentally on a wing-body at three subcritical speeds in such a way as toisolate thestrake-forebody loads from the wing-afterbody loads.Analyti- cal estimates for these longitudinal results havebeen made using thesuction analogy and the augmented vortex lift concepts. The comparisons show thatthe pitch data, both total and components, arebracketed well by the high- andlow- angle-of-attack modelings of thevortex lift theories. The lift dataare gener- ally better estimated by thehigh-angle-of-attack vortex lift theory and then only until maximum lift orstrake-vortex breakdown occurs over the wing. The compressibilityeffects noted in thedata for the strake-forebody lift are explained theoretically by a reduction in the wing upwash associated with increasing Machnumber which leadsto smaller potential and vortex lifts on the forward lifting surfaces. Aerodynamic synergism was investigatedexperimentally; as expected, there wasan additional lift benefit for all configurations as a result of the interaction. Furthermore, there was a delay in pitch-upassociated with the synergism. Machnumber has a small effect on the"additional lifting surface effi- ciencyfactor" whereas changes in thestrake geometry have largereffects. Geometry changes such asincreasing area orslenderness ratio generally pro- duce a more efficientstrake. However, it is possibleto obtain the larger values of this factor with approximatelyhalf the area of the original, also the largest,gothic strake by using a suitableanalytical design for the gothic leading edge. These resultscorrelate well with strake-vortex-breakdown observations in the water tunnel. Strake geometry is also important in determiningthe maximum lift that a configuration will develop, with gothic leading-edgeshaping being preferred for ratios of strakearea to wing referencearea of less than 0.25 based on the strakesconsidered herein. INTRODUCTION Strake-wing aerodynamics are becomingof increasinginterest due tothe mutual benefitsderived from the combination.(See ref. 1.) For the wing, thesebenefits include: (1) minimal interferenceat or below thecruise1 angle of attack, (2) upper-surface boundary-layer control at moderate to high angle lIn particular, at cruise it is possiblethat the small impact of the strake may only be attainable by the use ofcamber or dihedral so as to "unload" thestrake under this condition.Neither oneof these is addressed in this paper,as only planar strakes are considered. of attack due to the strake vortex, (3) load redistribution due to effective useof the upper surface, and (4) reduced area required for maneuver loads. For the strake, thesebenefits are: (1) strake vortexstrengthened by upwash from the mainwing and (2) the need foronly a small area - hence, wetted area andcomparatively lightweight structure - to generate its significant contri- bution to the total lift becausethe strake provideslarge amounts of vortex lift. Inview of these strake benefits, it is appropriate to consider how best to maximizethem by propershaping of the strake. One way would be to use an empiricalapproach based on previousknowledge, a second would be cut-and-try, a third would be analytical, and a fourth would be a combination of thepreced- ingthree. At the time ofdevelopment of the lightweightfighters F-16 and YF-17, onlythe first two procedures were available.After these airplanes were developed,reports were written,references 2 and 3, whichsummarized the wind-tunnel test results of about 100 different strakes foreach airplane, along with an analysis to helpguide future strake-wing integrations. However, thesereports still do notgive the aerodynamicist an analytical method for shapingthe strake leadingedge. One possible approachwould be to isolate some critical parameter,such as leading-edgesuction, and then design the strake inthe presence of the wing while monitoring this parameter. As a step in this direction, a simplerapproach with the emphasis on delayingstrake-vortex breakdown has been developed and reported inrefer- ence 4. There the shape of the isolated strake is determineduniquely in a flow which is simpler but related to thethree-dimensional potential by specifyingprimarily the leading-edgesuction distribution. Reference 4 reports the firstdesign application of this method in which the resulting shape was area scaled until the three-dimensional suction distribution over boththe strake and the wing was considered to be acceptable. Thewind- tunnel test of the strake-wingcombination showed it to perform well. How- ever, to determineif this method could be used to develop better strakes, it was applied to the developmentof over 200 configurations. Only 24 were consideredsuitable, or interesting enough, forfurther evaluation. These, alongwith 19 empiricallydesigned strakes mountedon the same wing-body, were tested, in a cooperativeprogram with the authors, in the Northrop 16- by 24-Inch Diagnostic Water Tunnel. From the results reported inreferences 5 and 6, only 16 strake-wingconfigurations, 7 analyticallydesigned and 9 empir- icallydesigned, were considered of sufficient interest to be tested on a similar wing-body in a wind tunnel.These tests, like those in water, were to be done at zerosideslip because of the large test matrixinvolved. It is recognizedthat the effects of sideslip and leading-and trailing-edge flaps are important with regard to vortex breakdownand the resulting amount of usefullift attainable; however,these effects are beyond the scope ofthe presentstudy. This report documents the wind-tunnel results and presentsthe analytical estimates forboth the complete configurations and thecomponents using the method described inreferences 1, 4, and 7. Use of trade names or names ofmanufacturers in this report does not constitute an officialendorsement of such products or manufacturers,either expressed or implied, by the NationalAeronautics andSpace Administration. SYMBOLS AND ABBREVIATIONS Dimensional quantities are given in both SI Units and U.S. CustomaryUnits. Measurementsand calculations were made in U.S. CustomaryUnits. AD analyticallydesigned b span (b, = 50.8 cm (20 in.)) C constantpressure specification in strake design Drag CD coefficient,drag - qSref cD,O experimentalvalue of drag coefficient at CL = 0 Lift CL liftcoefficient, - Gref CL,maxmaximum valueof CL,tot Cm pitching-momentcoefficient about 56.99 percent body lengthstation, Pitch ingmomen t qgrefcref kP lifting pressure coefficient CS leading-edgesuction-force coefficient, Kv, sin2le Leading-edgethrust CT leading-edgethrust-force coefficient, %$ref C chord, cm (in. ) - - C characteristiclength used determinationin of KVISe, cm (in.) Crefreference chord, 23.33 cm (9.185 in.) Section suction force CS sectionsuction-force coefficient, LC dFS differentialleading-edge suction force (see sketch D) dZ differentialleading-edge length ED empiricallydesigned 3 f additional lifting surface efficiency factor, a (Normal force/q,Sref) po tentia l lift factor,potential lift (KP in table IV) KP a(sin a cos a) KV vortex lift factor (KV in table IV) Kv, le leading-edgevortex lift factor, 1 a(IS.F-I le,left + Is-F-I Ie,right)~ " (KV LE in table IV) qmSre f a sin2 a Kv ,se side-edgevortex lift factor, (KV SE in table IV) KV,Z augmented vortex lift factor, (Kv, le/2) c (see appendix A) 1 distance along leading edgefrom apex, cm (in. ) M free-stream Machnumber P polynomial pressure specification in strakedesign qco free-stream dynamic pressure, N/m2 (lb/ft2) Ra ratio of exposed strakearea to wing referencearea, Ss/Sref Rb exposed semispan ratio, [(b/2)s/(b/2)w]exp RS strakeslenderness ratio, (Length/Semispan)e,p r radius of curvature, cm (in.) S area Sref reference wing area, 0.1032 m2 (1.1109 ft2) S.F. potential-flowsuction force CSC S - a2(b/2) U free-streamvelocity, m/sec (ft/sec) 4 ~ .. ._..... sum ofinduced downwash and Ua at a = 1 rad, m/sec (ft/sec) averagevalue of wnet, m/sec (f t/sec) local coordinatesdefining strake planform, cm (in.) (see 'table 111) locationof centroid of particular loading, cm (in.) locationof reference point from nose of model, 54.832 cm (21.587 in.) (X SUB REF in table IV) - = xref - xc,i, cm (in.) (i standsfor subscripts p, le, se, and se) angle of attack, deg (ALPHA intable IV) equivalentcirculation associated with leading-edge suction, m2/sec (ft2/sec) averagevalue of r (11, m2/sec (ft2/sec) fractionof exposed strake semispan leading-edgesweep angle, deg densityof fluid, kg/m3 (slugs/ft3) three-dimensional Subscripts : BD-TE strake vortex breakdown at wing trailing edge in water tunnel e XP exposed inb'd inboard le leadingedge max maximum outb' d outboard P potential r root S strake se sideedge - se augmented sideedge 5 swb strake-wing-bodyconfiguration tot total configuration vz e vortexeffect due to leadingedge vsevortex effect due to side edge - vsevortex effect due to augmented
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