\mthi-ii ^ KB No. L5F25a NATIONAL ADVISORY COMMITTEE FOR AERONAUTICS WARTIME REPORT ORIGlNALLy ISSUED August 19**-5 aa Eestrlcted Bulletin L5F25a A COMPARISON OF DATA OBTAIHED BY TWO FLIGHT TECHNIQUES FOR ItETERMENIRG THE SHiESLIP CHARACTERISTICS OF A FIGHTER AIRPLANE By Harold I. Johnson Langley Meanorial Aeronautical Laboratory Langley Field, Va. .--•* ^,"rT^T^,■. >- , NACA WASHINGTON : NACA WARTIME REPORTS are reprints of papers originally issued to provide rapid distribution 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- ' iiically edited. All have been reproduced without change in order to expedite general distribution. L - 87 DOCUMENTS DEPARTMENT Digitized by the Internet Arclnive in 2011 witln funding from University of Florida, George A. Smathers Libraries with support from LYRASIS and the Sloan Foundation http://www.archive.org/details/comparisonofdataOOIang NAG A- HB No. 15^^25 a NATIONAL ADVISORY COLMITTiE FOR AiiRONAUTICS RESTRICTED BULLETIN A COMPARISON OP DATA OBTAINED BY TWO PLIGHT TECHNIOL^ES FOR DETERMINING THE SIDESLIP CHARACTERISTICS OF A FIGHTER AIRPLAl'S 3y Harold I, Johns on STO'J'i^ARY In order to deterir.ine the validity of sideslip data obtained in flight by the continuous -re cord Tiethod, side- slip data obtained frcn a figh-cer airplene in slo'«?ly increasing sideslips have been compared ".vith sideslip data obtained frorr. che sarie airplane in sideslips in vvhich all the flight conditions were stabilized. The results of the comparisor showed no essential difference in the sideslio characteristics obtained by the tv/o flight techniques even though the continuous sideslips were purnosely executed with a yawing velocity about double the usual rate of 1° per second in order to accent iste the effects of the small yavjing and rolling velocities inherent in the method. Approximate theoretical calcu- lations confirned the experimental results but indicated that rates of yawing or rolling lower than 1"-^ per second are desirable if the continuous-sideslip technique is eniL/loyed in testing airplanes much larger than current fighters. The method of measuring sideslip characteristics under steady conditions is preferred to the method of measuring sideslip characteristics in slowly increasing sideslips when the airplane is directionally unstable, when it has a large pitching moment due to sideslip, or when a poor rei-^tionship exists among the rudder, aileron, and elevator control forces in sideslips. INTRODUCTION From time to time questions have arisen concerning the validity of data on sideslip characteristics obtained by the continuous -record method as differentiated from the usual steady-record method. The usual flight tech ini que KACA R3 No. L5F25a for riepsuring sideslip characteristics consists in taking records of control forces e.nd angles under steady condi- tions of sirspeed, hesding, angle of bank, and angle of sideslip, at various attitudes of the airplane within the sideslip range to be covered. Although tliis method of testing obviously yields the desired results, it is uneconomical v/ith regard to flight and instrument time cons^'Jitied because only a relatively liinited number of runs can be obtained in any one flight. In order to speed up the test program ma-cerially, sideslip char?cterlstics have often been determined by the continuous-record method. This method consists in switching the recording instruments on at laterally level flight trim and then slowly moving the three controls together in such a way as to maintain constant airspeed and heading v;hile ceusing the angle of sideslip or bank to increase at a slov/ rate (not e:cceeding 1° per sec) until one of the controls is fully deflected, or high control forces or other limitations are reached - at which time the Instrioi'^ients are switched off. The resulting data ere handled under the ass'jjirption that the variables measured at any particular instsnt during the maneuver represent the steady sideslip values. The error of this assu-TTiption, of course, lies in the neglect of the small incrementel control forces and deflections required to maintain the small angular velocities of the airplane during the sideslipping maneuver. DESCRIPTION OF TESTS In order to investigate the magnitude of the error involved in the continuous-record technique for determining sideslip characteristics, flight tests were made with a current fightsr-type airplane (fig. 1) and sideslip data were recorded during the tests by both the steady-record and the continuous-record techniques. Sideslip records Vifere taken st indicated airspeeds of I52 - 5 miles per hour with norm^al rated power at about 5OCO feet altitude. The slowly Increasing sideslips were purposely executed with a yawing velocity about double the usual rate in order to accentuate; the differences between the results obtained by the steady-record and continuous -re cord techniques. All control positions were measured at the control surfaces and the control forces ivere measured at the stick and rudder pedals. NACA RB No. L5F25a RESULTS AND DISCUSSION Results of tne tests are shown in figure 2. These results indie £-te thst very little error is incurred in sideslip-char ^jcteristics data beca-.ise of the use of the continuous -record metnod. As would be exrected, the test points obtained fror.. the continuous records, v.'hen compsred with the test points from the steady records, generally indicate th?t slightly greater rudder- and aileron-angle and control-force differences froiri the values corresponding to laterally level flight tririi are required for a given sideslip angle. The slight discontinuity that appears in the curves v/here the direction of sideslipping is reversed is partly the result of the control deflections ond forces ■required to accelerate the airplane from steady flight to the condition of small uniform yawing and rolling veloc- ities. Control friction also concributes to the discon- tinuity in the control-force curves. Inasmuch as the continuous records of figure 2. were obtained for a yawinp^ velocity aporoximately twice as great as the recommended maximum rate for fighter-type airplanes (1° sideslip per sec at low airspeeds), it becomes evident that, for the size .-snd flight speed of the airplane tested, the dis- crepaxicy to be expected between sideslip data obtained by the two methods is within the scatter to be expected from successively repeated tests made by either of the methods . Approximate theoretical calculstions have been made of the variation of rudder-angle and total-aileron-angle errors vifith service indicated airspeed for the same fighter airplane in slowly increasing sideslips. Only the two most important yawing or rolling derivatives were considered in calculating the I'udder or aileron error, respectively. For the rudder-angle error, these deriv- atives are the rate of change of airplane y awing-moment coefficient v/ith yawing velocity due to the ver'cical tail and the rate of change of yawing -moment coefficient with rolling velocity due to the v/ing. For the total-aileron- angle error, these derivatives include the variation of wing rolling-moment coefficient '-vith rolling velocity anl the variation of wing rolling-moment coefficient with yawing velocity. Values for the wing derivatives were determxincd from bhe charts of reference 1. Vertical-tail effectiveness was estimated from, reference 2. Aileron effectiveness was obtained from orevious flight -tests on the same airplane end from the theoretical charts of k NACA RB No. L5F25a rafsrence J" The side-force cliprecter-lstics of the fighter airrlsne usoa were as3uiaed to be ciKllsr to those measured for another fighter airplane having nearly identical geometric and weight characteristics. This ass'-inption wss necessary because no data were available for the airplane used in the tests from which to determine the ratio of rolling velocity to yawing velocity as a function of speed in properly executed sideslips. The errors wei-e calculated both for ? constant rolling velocity of 1° per second at all speeds and for a constant yawing velocity of 1° per second at all speeds. In practice, the rate of executing concinuous sideslips is generally set by limiting the rolling velocity to roughly 1° per second at high speeds and by limiting the yawing velocity to about 1° per second at low speeds. Results of the cslculr.tions for errors in ruddez' £o:jd aileron angles ctaised by use of the continuous-record method with the fighter airplane Cegted are ^resented in the curves of figures 3 and li, respectively. The errors are given as functions of service indicsted airspeed. A positive error indicates that more control deflection from the initial trim condition is required in a slovi'ly increasing sideslip tiian in a steedy sideslip. As noted previously, the rrte of executing continuous sideslips is controlled by limiting the rolling velocity at high speeds end by limJ.ting the yawing velocity at lov^ speeds. An inspection of the calcul-?ted cuj^ves indicates thf^t, if neither the rolling nor the y??wing velocity were ellov;ed to exceed 1^ per second, the error in rudder-angle mieaisurements should not exceed 0.2° et 100 miles per hour service indicated airspeed and should nearly vanish at high speeds; similsrly, the error in total-aileron- wngle measurements should not exceed 0.6° at 100 miles per hour service indicated airspeed and should become much less at high speeds. If the rates of yav/ing and rolling and the airplane s-oeed are held constrnt and the cix'-plane size is increased, the errors in rudder'- angle and aileron-angle mef.surem.ents in continuous sideslips increase. Errors, relating to the wing ya"«ing or rolling derivatives increase in direct proportion to the increase in wing span, whereas bhe rudder-angle ei^ror di:e to vertical-tail damping increases in direct proportion to the increase in distance between the center of gravity and the vertical tc.il. Hence, if the continuouo-record m.ethod for deterxr.ining sideslip characteristics is used in testing airplanes m.uch larger NACA RB No. L3F25a 5 than current fighter types (span, approximately L}.0 ft; vertical-tail length, approximately 20 ft), the maximum allovvable rates of yawing or rolling probsbly should be reduced belovv the 1*^ Der second limit recommended herein. It is desirsble to use the stesdy method for measuring sideslip characteristics for any flight con- dition if the relationship among the various control forces is poor, if the airplane is dlrectionally unstable, or if there is a Isrge pitching moment due to sideslip present. Wnen none of these objectionable conditions are present and v/hen continuous -recording instruraents are available, the continuous-record method for determining sideslip characteristics can be used advantageously to save time in flight testing. CONCLUDING REIvlARKS Trie continuous-record nethol for measuring sideslip characteristics In flig.ht yields accurate results for cui''i''ent fighter airplanes if the rate of ya'A'ing or rolling is not allowed to exceed 1° per second; lov;-3r rates of yawing or rolling are desirable if the continuous-sideslip technique is employed in testing lerger airpl^.nes in sideslips. when the relationship among the various con- trol forces in sideslips is poor, when the airplane is dlrectionally unstable, or when the pitching moment due to sideslip is unusually large, the steady-record method for determining sideslip characteristics is preferable to the cent inuc us -re cord method. Langley Memorial Aeronautical Laborator^f National Advisory Committee for Aeronautics Langley Field, Va. NAG A RB No. L5F25a REFERENCES 1, Pearson, Heiiry A., and Jones, Robert T.: Theoretical Stebilicy and Control Characteristics of V;/lngs ivith Various Amounts of Taoer end Tvy-lst. NAG A Rep. No. 655, 1938. 2. Pass, H. R. : Analysis of 'kind-Tunnel Data on Directional Stabili-cy and Control. MCA TN Nc. 775, 19^0. 5. Gilruth, R. R., and Turner, W. ¥., '. Lateral Control Required for Satisfsctcry Flying Qualities Based on Flight Tests of Numerous Airiolejies . NAG A Rep. No. 715, ^9hi. NACA RE No. L5F25a Fig. 1 Figure 1.- Three-view drawing of fighter airplane use n the tests. Fig. 2 NACA RB No. L5F?5a w E;\ '~' rr-T F — >*=■ fO ■ '-J 1 " ~ ~ r- . -,-- ^:jtj;: 'i jn. '^ — T ytrr "~I "^ -— ■-^-z ?) r^ ■^--i — - ^ P-. r ^ IT 1 — r"" .*'' ; <5 l!l. ~ ^ 5^ "^ sr -":- --.■; fjA' g iC of iH *?- Q) -T J u •> Q 33 ^ a & >-. "?1 10 s° u :- "" /.t r: -;- ,- • ;-_ ■.| ifej r ~r- . -y': -' -^ •- :: ^ -•:,- !t'^^ ■^ B,< 4. if l<1 J IT 4 ' lU ^ ;t ^ r f =>Ff 19 Ir |0 1 ft T — ,> .r^ ,: i "i r) .f 'J w : ; S'l K ^ § U o b' ^ ^ ' a P . r ■~-J {fl -£ p -- - ^- :- . -- ir\ ■^ t>._ o . ■-^ - - ? '•J ■-, -- « - :■ -i &i fc 1-*^ -O;"^ 10 ■i ^-T > — — • — -' fe-- 'fi Y "s: ? n - J. ll: >J ^ ■- f? '^. n n 1 - -' 1 Ui to (p f\ qS - H f ft^ 7® £: -- n \J t \f> - -, ' . ^^ ot^ § V O ^ ; ; .-.' , . ■B -^■3 1,^1 ft ■9? a* ■^ iS ~ -fl ■k: \' D" V: r " s ni c -'• - ^ ^^; l!| . =»- !,° • U jr i?i a n " 1 ) B J o < _-j pi ' XT ■ w - -a -^Aa — _-; ; Br- S at f\j - ' ;.- — ^ ^ 5 — n ■-. o B 5 -J •- 1 - "b .^ r-.i K: f • [- n 5 >^ - _,_ 1 - U ': -^ ft ; : -t " ' ? --- f^-* S- 3 c& - ■1-t ^-" - fjpi ' ■ 5j' 1= It; t ■1/ ">o ?g i; ^ J( nj — ^ : 3' - - — — - her n ^ da / 3 ai. ^ N '^4 } — — — e 5 3 n^- i- -■ [^ ^ Q — — — - — — ^ ( M _.- — — — .■2 — — i> — o c >e 5 n. ccr< ^5 K I ; — le — ~-- % > s £ ?fW€ eco S- V o 5 'eoc y t h fil hii) 'Nv ■~j-' ■ t [-^ — — — 1 y- IN 1 1 t"^ ? ■ '■ l.r 0- 1 I i^: i — , ■ °Q ^- ^'R - - ni nn 1 Ui f* ■^1 fofia i 'B INA g- . -; ^ -■r. \q\in fO m NATIC L ADVISORY J 1 ~- T 1 — - ^ — — --U " _ I r-H» j « MMITTCE m »E«OIIAUTICS jT) T ' -< >. r» 1 ■t !5- ^ • -^ i?TV 1 ■. ^ , ' » , i &- ft ■f- — - -- ~i ^i 7 - I — P ?a al a gc _'?? St. 51 °/= r ^ 1 Is ?i^ > r. >.- / ^ ^ ¥'' f ■■■ — ^ !^ ?-^< ^ ( fT R^ 9' if -- — — ¥ '■:] iil; ttil h: .. :;:: Flejva 2.- Comparlion of data obtained In continuous and ■tead7 aldaallpa. Current flgbter-type airplane; normal rated power; Indicated airspeed, 132 t J miles pmr boxir; altitude, approximately 3OOO feet. NACA RB No. L5F25a Fig. 3a, b « « s-a •H ^ ■d © n . » n C o o o ^ r! « o (3 vi 3 o O iH at» on -d o -d "•PC « (D O ■d 3 0) •do© fl 54* •H 0<« .rt © o a (< o © ^■d ^ © -H eg a 2. 4J bOCQ a xHOD Salc\J © Oi c « •d t«« © rH M ■P » c • ^ • a a « rH O o « a "H p.a O 4J T) a I « iH fliH boo « 9tS '^5 to I d - o « h o • • a H n •HA" « -H 4> iH a O 0) • «s •p-d O -H «> ■pah