1、.-, - - - ,.- -. -,-,1-Ta.qhirkfq tonOctol)cr 1.3-$(3.Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-. -!4-,9.*.,-w=NATIONAL ADVISORYACOkTTEE FOR AI!RONAUTICS- TECIH!JIGALNOTE NO. 778- - ,NGTES ON THE STALLIlrG OJ?VERTICAL TAIL SURFACESAND 01? FIN D
2、ESIGNBy 3. Lt Thompson and R. R. Gilruth,SUMMARYA discussion is given of the important aspects of thestalling of vertical tail surfaces. The type of instabil-ity encountere is described and the possibilities of in-advertent occurrence are noteii. The influence of direc-tional stability on the behavi
3、or of an airplane when thetail stall takes place is discussed. In this connection,fliqht tests of a twin-engine airplane in hi the Ver.ti.cal fin area was increased are cited. The reasons forinadequate directional stability in certain modern de-signs e,re accounted for and the properties and applica
4、-tion of dorsal ins are discussed. In addition, the chieffactors regulating the requirements for conventional finarea are given, in which connection a simplified criterionfor directional sta%ility is presented.It may be concluded that the stalling of vertical“tail surfaces is not in itself a dangero
5、us condition.Provided sufficient directional sta%ility exists at lareanqles of sideslip, the tail stall may occur with modernairplanes , as ?ith those of the past, without the knol-edge of or concern to the pilot.ItiTROIXJCTIONA deficiency in vertical fin area has been a rela-tively ctooil occurrenc
6、e in ,irplanes during the pa,stfewyears, and in many cases it has been necessary to increasethe vertical tail area of the”original design after pre-liminary fliqht tests. The difficulties experienced havebeen manifested in various ways, In sone cases it has .heeil an annoying tormovement must he car
7、tifully” coordi.n.ated with the ruldermovement to nrevent alvi.ng upon entry into e.sideslip andstalling on recovery from a sideslip. Another difficultyexperienced is that in some cases the uistalle mome:tshave exceeded the maximum capacity of the fin, so thatwhen sideslips have been produced intent
8、ionally or in-advertently due to moments produced %y the rudder, P.sym-metric power or gusts, stalling “of the vertical fin hasbeen produced and a reversal of rudder force experienced.This latter condition is the one given chief consideratior-in the present paper. The chief basis for this discussion
9、is data accumulated in the fliqht research laboratory ofthe National Advisory Committee for Aeronautics, in an ex-tensive flight investgation of the flyin; qualities ofvarious airplanes. This investiqat.ton has included testsof irpl,es of arfed size from smmll, light, two-placetCair.planes to the la
10、rgest multienine oom3tirs.1NSTABILIT% A,SSOCIATXD WITH TEE STALLINGUnder certain conditions, airplanes %ecome unstabledirectionally at large anqles of yaw as a result of thestalling of the vertica tail sura.ces. This directionalinstallility is-r,anifested in the form of a reversal of therudder hinge
11、 moment, as a result of which the pilot mustforce the, rudder hack tO neutral to return to unyawpdfliqht. This condition is of more concer for the largeairplanes than for the small ones ecause the rudderforces are so large in comparison ITi.ththe pi?!.otlsstrength.Airplanes otherwise possessing suff
12、icient directional sta-bility, may suffer this reversal of rudder force when tailstall occurs.In cases where the reversal of hinge moment is of amagnitude which exceeds the pilot strer.gth, equilibr-ium of yawinq moments occurs e.ta larc angle of yaw, asa result ot which the airplane ex-ocri,enc6s a
13、 rapid rota-tion in yaw acompanied ly a rolling motion, dcpeildingupon the dihedral effect. Conpar(able conditiorks may hesimulated in a normal airplane by holding the rudder OVC?I?rnanuallj-.-%.”.Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-BTACA
14、.Tech.nical Note No. 7+8 3-. w”.d“.48.In the relatively small airplanes, recovery from thiscondition is possible b:!returning the rudder against thes,ideslip. In large airplanes, as mentioned shove, therudder may be too heavy to return, and the proper use ofasymmetric throttle may %e the only recove
15、ry means avail-able to the pilot. It is irnportatit to note in this con-nection that the inherent pilot reaction, namely, gettingthe nose down, is undesirable following the rudder rever-sal, because the aerodynamic forces hich hold the rud,derover will be increased %y the increase in speed. It isals
16、o true, however, owing to the change in pitching momentwhich usually accompanies large side slip angles, that con-fusion may occur as to what t,he angle of a,ttack actuallyis. There 5.s, therefore, danger that the rotation in yawmay deenerate into that of a true spin.In cases where the sideslip is d
17、eliberate and #rad-ually entered as, for example, durinq tests, the imminenceof the tail stall has been j.ndica,ted by s,definite li%ht-eaing of the rudder force. This wrning is of littlevalue , however, where large sideslip angles are reached ase.result of atmospheric disturbances O? following sudd
18、enengine failure in n multiengined airplane.,. RIILA.TIONBETWEEN ,ANGLX OF 13AK ANDANGLE OF SID12SLIP.,The relation existinq betweenangle of hank and anqle0? s,ideslip in airplanes of modern type, throws anintgle of bank.can be readilydetermined. Thus it is usual to tiesort to the angle ofbank while
19、 sideslipping as an index of the angle of side-slip. The characteristic that actually determines whatthe anqleof oank can be for a given sideslip, is theamount of cross-wind force that the airplane can developat that angle of Sideslip. This cross-rind Sorce is madeUp chiefly of the lateral component
20、 Of propeller thrustand the side force on the fusel,age$ and these quantitiesare so variable that the angle of bank is totally unrelia-ble as an index of the angle of sideslip. During a per.Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-4 NACA T!ech
21、nica.1Note No. 778tion of the previously mentioned i.nvcsti%ation of flyingqualities of various airplanes, therefore, the NAM hasmade use of a sideslip indicator or recorder. !l?heinstru-ment consists simply Of a vane free to pivot about a ver-tical axis and aline itself with the relative :ind. Thea
22、nglo of the vane is either rocGrded, or ohservcd by some-on within the airplane. By thism cans, the mgle ofsideslip and corresponding angles of hank have been detcr-rt?inedfor a number of airplanes.An intcrcjsting feature of the results is that theside slip angles in many cases were surprisingly lar
23、qe,particularly in view of the relatively small nngles oflank experienced. The relation %etween angle of b:ml: andmngle Of sides15p for a typic,nl case for the power-on con-dition is shown in fi.qzre 1. It will be noted tkt theangle of bank for a qiven amount of sideslip varies withthe air speed, an
24、d that at low speed r.nangle of bank ofonly 4C corresponds to a sideslip anqlc of ln. !l!ha,tthisanqle of bank, which is the pil.otfs index of the maqii.tudeof the sideslip, rlay Ye very small is noterorthy in con-nection with the possj.?3ility of attaining exce”nsive sid.e-slip i.r.adertently. The
25、modern tendency seems to be to-ward characteristics that pcrnit thn ,e.irglnnes to sid.e-slip with so little bank that adequate directional sta.hil-ity at large anles of sideslip has become increasinglyimportant .In the e,%ove case, when the sidoslip anglo of 16 wasreached, the vertical tail stalled
26、, the rudder force re-versed, and strong rotational tendencies develoFed. Thisconaition o,ccurred with about one-third. full rudder de-flection and, as previously noted, the corresponding an-gle of oank at low speed was not more than 4. Characteri-stics of this general nature were experienced with s
27、ev-eral airglanes, but there were also sever,al czses tn whichequilibrium wp.s established at sideslip angles raningfrom 30 to 50 without the development of unstable tend-encies. These facts indicate that some fr.ctor in additionto the stalling of the vcrticnl tail surfaces nust regulatotho behavior
28、 of airplanes at lnrge a,n.qloscf sidoslip.They also indicnte that the wind tunnels must provide meansfor testing models at much larger .zngles of yr.-ivthan hag%een customary in the past.*.-*.h,Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-NAC,A T
29、echnical Note No. 778 5.,w“.DEP!zNDENOE OF BEHAVIOR ON DIRECTIONALSTA31LITY CEARACTE,RISTICSFlight tests of various airplanes have shown that thetail stnll is not in itself e.source of danger because, insome cases, angles of yaw as great a.s500have bsen record-ed in sideslims without the development
30、 of any unsta,lletendencies. +et , in other cases, fiirectional instabilityis ,devcloped at comparatively low angles of yaw. In thiscon,neotion$ detailed tests of a,particular twin-engine.,airplane wero very illuminating. On thi.sairplane it was: found that a vertical tail stall could %e produced in
31、 allcond.j.ti:onsof flight, hut that the behavior of the air-plane after the stall occurred was dependent on the rud-,. d,er.an+;le required to o%tain the stall. With flap up,.+- p,ower on, for exmlplc?,160 y?”v”,a rudder angle of only 9 produced,. Th,e resulting tail stail was accompanied by arever
32、sal.of rudder hine moment. lTith flap.”down, powerinefli;ht , the rudder forces far e.uj.libriuriyat zero yaw weronot affected. by the ade.iticnal fixed fin area. In therudder-free mode of operation, the additional directionalstability made possie y?.r,ingequiilm undr cndi-tins where the asmnetric m
33、mcnts had produced the tailstall and direction.1 instability in tlhc nriqinal airlane.REASONS FOR IWLIIEQUATE IIIRECTIONAL STABILITYThe reasons for a ;eleral tendency toward ins,dequabedirectional sta%ility at large angles of yaw, seem to4. lie in the effect of refnment in fuselage shape and a%efler
34、al increase of Vinq loading. AirBo.ne fuseaqes hbecome large in proportion to the wings and t the sametimo have %ecoma aerodynamically refined in shape andtherefore increasingly unste,tle. Desi%ns havo approachedin appearance .nairship with stub wings rather thau theso-called iflying-wing type,l The
35、 unstable moment thatthe fin must overcome in order to make the airplane diroc-tionally stable is cGntyibutsd chiefly b:?the fuselaqo,the contri”oution of the wings,I:rons,except with deflected e.i-13eing generally small. This fact indicats thatthe fin should be proportioned according to the size of
36、 ,the fuselage rather than in .cordance with the wing area.In ether words, it is hardly ijobe ex:pected that the finarea which was able to overcome satisfactorily the unsta-ble fuselage omnts on an airplane having. a win% loadingof 15 Pounds per square foot, can Ie reduced to one-halthat area when t
37、ha win.; loaiu.g is increased tc !zOpoundsyer squIt.+.+.-I8Rightangle of si3eslip for a typical- -I-t-i1+l.-,. -“m-1=12 16Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-TNACA Technical Note No. 778 Fig, 2.*/?-A., . . . 1 /-:150-“- - . - . - - . J-Ul
38、II 1-:-.-.,_.-.,.-,.-.-,._,_.t.-.-.-,.Iviodif” ed120 -”-” - 1 c) r,.- - . -. .-._-.-J ._.-_- ,_-. . .9i-l(i” d- - _- ,”.-_ .,_-.- ,- - _.+-0h$ Jf-l 80-.,.- ,-. ,-, .- -0 0 Tdg“h-. ._, . .-._.-_.,”_ _ .-“-”D = ogll . ,0L,!k”- 16” 1“-”-”. 40*311 ,. -4-,.=-8 _ .i IllI II _.-+&+_.-_ .,.-i 4 bii3+2elage alonel4- .-. _ -4 J_ .- “-J- I -r.-Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-
