(\CP,L -?47 iX NATIONAL ADVISORY COMMITTEE FOR AERONAUTICS WARTIME REPORT ORIGINALLY ISSUED October 19'<-1 as Advance Confidential Eeport FLIGHT BTVESTIGATICN OF WING-Gm FAIRINGS m A FIGHTER TYPE AIRPLANE By J. M. Nissen and M. D. White Langley Memorial Aeronautical Laboratory Langley Field, Va. UNIVERSITY OF FLORIDA DOCUMENTS DEPARTMENT 120 MARSTON SCIENCE LIBRARY P.O. BOX 11 7011 GAINESVILLE, FL 32611-7011 USA NACA > WASHINGTON NACA WARTIME REPORTS are reprints of papers originally issued to provide rapid distribuUon of advance research results to an authorized group requiring them for the war effort. They were pre- viously held under a security status but are now unclassified. Some of these reports were not tech- nically edited. All have been reproduced without change in order to expedite general distribution. L - 2U7 \ \ Digitized by tlie Internet Arcliive in 2011 witli funding from University of Florida, George A. Smathers Libraries with support from LYRASIS and the Sloan Foundation http://www.archive.org/details/flightinvestOOIang \7 ' ^ ^ FLIC-HT Ii;VE3TI&ATI01T OF ■.YliTa-GUlT FAIRIITGS OH A FIC-HTEH TYPE AIHPLAiTE By J. M. I'Tissen and J.I. D , White sui.:mahy Flijjht tssts were conducted on a Ilavy fighter airplane to deterLaine methods for fa.iring the wing-gun installation so ar; to retain the maxiiaua lift of the clean vring insofar as possihle. The unfaired-gun installation increased the stalling speed over that of the clean wing "by approximately 5 knots T7ith flaps down, po'ver off a.nd Dy approximately 3 knots with fliips dovrn, poorer on. Two arran,?eiiient s of fairings were developed that re- stored the lift of the Tving, One arrangement consisted of engine corrl-t^'-pe fairings for hoth projecting and suh- merged guns. This arran.r;en;ent provided an annular open- ing between the gun oarrel and the fairing lip for cool- ing the guns. The flush arrangement consisted of the en- gine cowl-type fairings for the pro jecti ng .guns and faired wing openings for the submerged guns. Successful opera- tion of this latter t;/pe of fairing, however, required that no air he admitted around the submerged guns. All arrangements of fairings as well as the unfaired guns im- proved the stalling characteristics of the airplane as compared with the clean-wing condition. It also appeared that the gun-fairing arranremsnts eliminated the- ground- looping tendencies of the airplane that were attributed to wing stalling. This was evidenced Qj a series of land- ings made with the wing guns faired and the small tail wheel installed in which no ground-looping tendencies were n c t e i. , On the basis of data from the full-scale wind tunnel, it appears that no reduction in top speed need be antici- pated with the four projecting fairings ventilated for cooling as coapf.red with't'ne unfaired gun condition. With the combination of projecting fairings and faired wing openings with no air admitted, the top speed may actually be increased 3 miles per hour as compared with the unfaired gun conditio n , \ IKTHODUCTIOH At the request of tae Eureav. of Aeronautics, flight tests have been conducted on a fighter type airplane "by the NACA at Langley ?ieli. The purpose of these tests was to determi::ie tho nodif icat ions required to corroat certain undcsirahlc characteristics of tho airplane. The investigation started on April 15, 1941 '^as suspended only for an interval from liay 28 to Juno 16, during which chock tests and necessary structural cha.ngos were made on the airplane hy the ITavy r.t Anacostia and iTorfolk, The present report covers the flight tests of gun fairings designed to correct the detrimenta.l effects of the projecting and subrcerged wing guns on the airplane. These effects, a G-lcnot increase in stalling speed as compared V7ith the clean-77ing condition, and a more pro- nounced tendency of the airplane to ground-loop in land- ings, wore Dclicvcd to he duo to early and unsyinractrical wing stalling produced by tho -Ting-contour irregularities of the gun installation. The aelief that rving stalling influenced the ground-loop:'.ng tendencies Ik based on flight tests of other airplanes that showed that violent ground-looping tendencies were caused by unsyraaotrical wing stalling in a three-point attitude. TK3: AIHPLArE AlTji li-TSTEUivISrT IlISTALLATIOl-T The airplane on v^hich the investigation was carried out is a G-rumman ?45'-5 single-place laid-ring monoplane fighter (fig. 1). Airplane ilo. 2533, rhich was delivered to the 1TA3A for the tost, ras a standard service model except for the following nod.if icat ions , The tail 'vheel of the tost airplane was cqiiipped with a pneumatic tiro that raised the tail approximately 5 inches as compared with the hard-rubber tail wheel used on service models, 'By this substitution the airplane ground angle was reduced about 2^, This rr.odif icat i on , which apparently is not suited to deck operation, was adopted during early tests to prevent ground-looping until specific investigation of that Tjroblem was undertaken, TTheel brakes of greater capacity than those in service models were also substitut- ed on the test airplane to provide additional ground con- trol. Du.ring the tests the airplane center-of-gravity posi- tion was maintained at approzinately <;8 . 5 percent IvI.A.C,, the location at r-hinh it was generally flown in service. The starting weight for each flight varied frora 5425 to 5735 pounds, the greatest part of the variation. (275 pounds) oeing due to the removal or replacement of the four guns, Tnis weight variation corresponds to a differ- ence in stalling speed of approximately 1,5 knots; for simplicity in analysis all the stalling speeds reported have "been corrected to a gross weight of 5725 pounds. The instruments used in all the tests rvere a record- ing air-speed meter instvalled on the airplane air-speed line and a thrse-element control-position recorder record- ing the novements oi the oievator, rudder, and ailerons. Tufts v/ere installed or. the upper surfaces of tlie wings and in some cases in the immediate vicinity of the gun fairings to s.id in the study of the behavior of the air- piano at the stall. The locations of the two 0, SG-caliber machine guns in each wing are illustrated in figure 2. Figures 3 through 7 are phot o ;;raphs of the various types of gun fair- ings tested. In figure 3(a) are shown the submerged gun in its unfaired condition and the projecting gun fitted with the fairing submitted by the Grumman Gcmpany. The G-rumman fairing resembles ar. engine cowl in appearance except that the space bct-.70on the gun barrel and the fair- ing was sealed with rubber grommet. Pigure 3(b) shows the projecting gun in its unfaired condition with the sub- merged gun removed. The Grumman fairing which was the only fairing used on the projecting guns was at first tested as submitted. In later tests the grommet was re- moved and the edge of the opening was bent in so as to provide an annular ^ypit-ce a,bout 1/8 inch in width around the gun barrel for the entry of cooling air. Several fairings for tlie submerged gun, designated for brevity iro . 1, l.'o . 2, I'o. 3, and ''faired opening," are illustrated, respectively, in figures -, 5, 5, and 7, Pairing IJo . 1 is a modified version of the Grumman fairing, peing somewhat more oval in cross section as compared with the flat sides of the latter. Pairings I'Tos, 2 and 3, v;hich are shorter versions of the i7o . 1 fairing, differ from each other only in width, ITo . 3 being the narro'fer. The faired opening sho^n in figure 7 is faired into a tube that en- circles the gun barrel for a distance back from the gun m'azzle of about 6 inches, and an annular space about I/B incli ??iie is provided tetTreen this tube and the gun "barrel to periiit the passage of cooling air. Sinilar annular spaces are provided "between "blast tu"be and fairing for the other fairings, TBS2S, HSSULT3, AITD DISCUSSIOIT Stall Character is tics The result:; of the stall tests with various arrange- ments of .:;ur. fairings are presented in ta"ble I for the two flii^ht conditions investigated. These flight conditions were the landin.p; condition, power off, f lapis down, and gear down, and the carrier-approach condition 23,5 inches of ;aercury manifold pressure and 2350 rpm, flaps down and gear down. In these flii^ht conditions continuous records were o'Dtaincd of stalls approached gradually in e. lateral- ly level attitude, the pilot noting the violence of the stall, tho rcsponro of the airplane to the ailerons and to power cipplicat ion in tho stall, and the tuft "behavior in tho stall approach. The results ta"bulatod in ta"ble I may be "briefly sura- narized as follows: 1, In the power-off, flap-down flight condition eaclx set of unfaired guns, projecting or su"bnierged, alone effected a 3-hnot incres,se in stalling speed over the clean-wing condition while in conbination. the increase was 5 knots. With power on, flaps down all arrangeinents of unfaired guns increasCvd tho stalling speeds 'cj a'oout 3 knots (tests 1, 2, 3, and 5) . 2. A fairing arra.ngc.ncnt con-iisting of the Gruraraan fairing on the projectixig guz: and the 2To, 1 fairing on the su'baerged gun (fig. 4 and test iTo . 7) effected an improYement over the unfaired-gun condition in the foliowin.-^ respects: {a) The stalling speed in the carrier- approach condition was reduced "oy 1 or 2 knots as cor-parcd with the unfaircd-gun condition and the landing-condition stalling speed was reduced to the clean-wing values. (b) Tho .stallir.g charac proved over the cloan-vying c catcd "by tho laildcr roll r.t iiicrcasod rcspoiisivonoss of aileron n',ovc?ncnt or powor ap stall. Conclusions regardin ity in the stall as listed i on tests in -^hich, irniediate ailerons were applied agains was applied and the stick mo enough to X-'^svont a sharp ri toristics vrorc im- ondition as indi- thc stall and tho the airplane to plication at the g the contr ollabil- n tahle. I are Isased ly after the stall, t the roll or po-.7or ved forward only so of the no so. 3, Another gun-fairing arrangoracnt consi of the &rum:nan fairing ori the projecting gun tho faired ving op-^ning for tho subr.orgod gu 7 and tests ITos. 14 and 15) gave rosv.ltf; sirn to those listed ur.d-jr paragraph (2) , under r cd conditicns; that is, tho effective functi of this o.rranger.iont required that no air flo o fairings, (Cctiparo te it is nocossarj- that ai p o rn: i 1 1 e d t hr du gh ITos . 11 .and 14. ) nit ted to cool the der to utilize this arraugeaent in service, sion vrould have to be laale for opening a.nd c an air seal around the gun in flight. during firing, then sting and n (fig. ilar estrict- oning 17 bo sts r be ad- in or- provi- losing the 4, Jlor.e of the other arrangements listed : table I was considered satisfactory. It is o: teront to note, hoTOver, that at least one of other fairings for the subiuerged guns was effective when tested alone with the projecting gun reaovcd, but v;as entirely ineffective in corabination with a fairing on the projecting gun (tests l"os. 8 and 9), Apparently dotriiv.ontal interference effects recult froK tho close prcxinity of tho two guns to each other, especially with power on. In additio'- to t'r-.o results .tabulated in table I, in- formation was obtained fron tuft studies that is consid- ered of interest. Unc tuft observations indicated that even in tho clean-Viring condition the initial breakdown of flow occurs in the vicinity of the gun locations. Jhis fact explains ': o sonc extent why tho gtxn-f airing design was critical. The tufts showed too thrt, in general, the character of the stall corro'-ponded with the rate and ex- tent of spauwise progress of t-'io flow breakdown, A sharp break and •oil in tho stall, for example, occurred \ Tfhen tiiG floTT "brealcdoTTii spread rapidly to the -^ing tip as in the cl-san-wing condition; on the other hand, a raild roll resulted uhen the flovr pro^jre'ssed only to a station ^o.nenhat irboT.rd of the ailerons as in the unfaired-gun^ conditions and as with the recomEended fairings. From the a.Dove, it is evident that the troubles ex- perienced following the installation of unfaired guns on the clean v,dng rrere due not to their harmful effects on stalling characteristics hut only to the increased stall- ing speeds that they produced. The mild rolls that fol- lorred the early advent of the stall '.vould cause disturbing moaonts on the grour.d r;hich, conbined rrith the inherently unstable landing-gear arrar.ge.jent , resulted in violent ground loops. Tests v.-ith tuf in the iamediate vicinity of the fairings showed Eainly t?iut it ras the fairing for the subaorged gun tact suffered froa interference, ihilQ the flor/ about the other fairing appeared to be maintained satisfactorily, Chech tests raride '.vith and rrithout the tufts near the fairings indicated the effects of the tufts to be negligible. Pollov/ing firing tests conducted on the recommended fairing arrangements by a squadron at ilorfclk, it was re- ported that a l/S-inch-wi de annular space betTjeen gun or blast tube and fairing gave adequate gun cooling. No fir- ing tests were conducted '-^ith openings sealed. Drag Estimate On the basis of fu.ll-scale win d-ti:.nnel tests conduct- ed on another airjjlane, it is estimated that tliere ■n'ill be no reduction in top speod due to the projecting fairing as com.parod vrith the x^nfaired rans, The use of the faired- ving opening i,7ith the flo^j sealed off ^Tould actually in- crease the top speed by about 3 aides per hour as compared v;ith the unfaired funs. Ground-Looping Tests Previous tects on other a.irT-)lanes have shoT/n that frequently objectionable ground-looping tendencies are as- sociated Tith an unsyrame t rical , early stalling of the v;ing in the groy.nd run. As rras stated earlier, it was with the idea of reiucin,^ the ground angle of the airplane "below the decreased stalling angle of the unfaired-gun arrangement that a pneumatic tail wht;el wcis installed on the airplaneo This tail wheel reduced the groiind angle by ahout 2°. It is calculated that in the power-off condition tho lift recovered "by the recomiii ended gun fairings corresponds to an increase in stalling angle as coapared with the unfaired- gun condition of 3 . With the gun fairings on, therefore, the ground angle could "be increased by as much as 3° with- out exceeding the stalling angle; hence, the pneumatic tail wheel no longer seemed necessary. To verify this conclusion, a series of landings was made with the original hard-rubber tail v/heel installed and the gnns faired with the G-rumraan and the ITo. 1 fairing. l\o grofind-looping tendency was noted in any of the land- ings, From those landings it is evident that the aerody- namic sourncs of groiind-lo oping tendencies wore eliminated by the gun fairings. It should be noted, however, that this modification in no way o,ffoctod the natural tenden- cies of the airplane to ground-loop; in fact, the landing- gear arrangement of this airplane appears less satisfac- tory from this standpoint than do many others. (JEITERAL RSMAHKS The problems associated with the installation of wing guns in the subject airplaz^ie appear to be of a rather gen- eral iia,turc. For example, the difficulties that necessi- tated the present investigation were due largely to tho introduction' of discoi'^tinui t ios in what is known to bo the most critical portion of tho v/ing chord, that is, tho up- per surface of the wing in the immediate vicinity of the loading edge. Corrective measures that might logically be employed in future designs would be: (1) to lower the gun within the wing possibly by turning the gun on its side so that it wo 111 d project belov/ the stagnation, point, or (2) to provide a faired opening with an air seal that can be opened and closed in flight if it is considered necessary to admit cooling air to tho guns. Prom the standpoint of simplicity of design and installation, the former alterna- \ tive reconruends Itself, For ready adaptation, however, the installation should "be incorporated in the origintil desi^^n since structural lii::itat ions will generally prevent relocation of the gux:s once the airplane has "been con- structed, as in the present instanco, x'ho second altornativo has tho di sadvan ta^^o that it might require the added complication of a novable air seal. This disadvantage would oe coapcnsatcd for, to a considcra'blQ extent, however, "by tho roduccd drag of this installation as compared with the lirr.t and "by the protec- tion from adverse \70athor condi'-.ions that it affords the gi^n , Regardless of the fairing installation employed, pro- vision must he ;;iade for bore-sighting the guns. This could be accomplished nost readily by first bore-sighting the gi.ns and then in'::taliing the fairings so that the guns are centered in tho openings. Another problem that merits at- tention in connection with \'-i:i.g-f;un installations is that of minimizing tho si2C of Icading-cdgo opening required to cover different s3ttin-;;s of the gun. In this' connec- tion, consideration might logically be given the possi- bility of changing the angle of the gun a'jout the muzzle instead of about the front support, "hatever the means employed, however, it appears desirable that some steps be taken in this direction, C02TCLUSI0ITS As a result of the fli>-'-ht investigation of wing-gun fairings on a fighter type airplane, the following con- clusions may be stated: 1, The installation of unf aired ;r;uns on the other- wise clean viing resulted in a premature stall that in- creased the stalling speed in the carrier-approach and landing conditions of flight, 2, 3y suitably fairing the guns it was possible to reduce the stalling speeds to very nearly the values cor- responding to the clean wing and at the same time elimi- nate tho obj cct ioi;o.blc stalling characteristics associ- ated with the cloan-wing condition. 3, For irnEediate adoption on airplanes now in serv- icBf a gun-fairing arraiiiPienient consisting of the G-n'oninan fairing on the projecting; gun. and a modification of this fairing for the suhi^erged gun recoiairiGnds itself largely ■bccavLSQ of its siiaplicity, 4, An alterntitive and equally,' effective arrangement consisting of the C-ruianan fairing for the projecting gun and a fairod T7ing opening for the suoaerged gun dopcndod for its effectiveness on the scaling off of cooling air arovi.nd the gun., so that in service raoans might have to oe provided for opening and closing an air seal in flight, 5, In a poricr of landings made r^/ith the original hard-ruobcr tail •jhcol installed and the guns faired, no ground-looping tendency was noted. The landing-gear arrangement on this airplane, however, a-ppears less satis- factory fro;j a ground-looping standpoint than do many others. Langley Memorial Aeronautical Laooratory, iJational Advicory ComLiiitee for Acronautfcs, Langley Field, Va, NACA < tj o «: !: ^ ig S -J '^ (V V) ■^ < ^ a o t o ^ ^ Q s ^ ^ V ^ ^ Hi 5; r Oi "t vi ^ «) "0 t 3 Table 1 ^2 "7 ^ ^ 5 ^ sj ii * k U) we* s a o ^ 1. « .° 1 1^ ^0 1^ k 50* Si ^ k (^ k I}; 10 m 5' I * '5 ■^ ^ Ml •^ M) ^ s * * * 5 « N Vi •s) 5 ft; k 5 Vj 2 t t * « - o Q o k) ^ Jl I" X NACA l; It ^>' *- u I ^ I b V V;- i^_j:r lu' 1^ k 4 s? 5 5: >0 y.-y% 'l ^ ^ It ^ 6 ^ Q k "3 14. -J kjl ^ -J ^ >k "0 ^ '^ ^ u, -I ■J $ (D I o a> +> o t:) tM r-l W sj .a 43 a ^H (0 \ NACA I 1q. _ .^1 \ MCA Pig. 3 Figure 3a.- View of submerged gun in tinfaired condition and projecting gun with Grumman fairing. Ruliber grommets installed aroxind edges of fairing and wing opening. Figure 3b,- View of projecting gun in imf aired condition with submergea gun removed. MCA Fig. 4 Figure 4.- Views of Ho, 1 fairing on submerged gun and Grumman fairing on projecting gun. Both fairings provide annular space about 1/8" wide around gun barrel or blast tube for cooling air. mcA Fig. 5 Figure 5.- Views of No. 2 (wide) fairing on sulMnerged gun and Grumman fairing on projecting gun. Both fairings provide annular space about 1/8" wide around gun barrel or blast tube for cooling air. \ NACA Pig. 6 Figure 6.- Views of No. 3 (narrow) fairing on submerged gun emd Grumman fairing on projecting gun. Both fairings provide anmilar space about 1/8** wide around gun barrel or blast tube for cooling air. "x MCA Fig. 7 Figure 7.- Views of faired wing opening for submerged gun and Grunnneua fairing on projecting gua. Both fairings provide annular space about 1/8" wide around gun barrels for cooling air. \ UNIVERSITY OF FLORIDA 1262 08106 492 4 UNIVERSITY OF FLORIDA DOCUMENTS DEPARTMENT 1 20 MARSTON SCIENCE LIBRARY RO. BOX 117011 GAINESVILLE, FL 32611-70m^A \