Is a Horizontal Tail Necessary?

IO A LJ/'M-MTr/'MkJT A I All I I AMU Kl ADX/O Beginning digesta elementary ofthe basicand functions oftaila whether it is positioned at the front or rear or even hiding somewhere disguisedas something else. Parti FigurEquilibriu— 1 e1- m series, they may not be the only ones wing lift would change and the GeorgeBy CollingeB. aroun plain t thesi dbu e n e ar Englis h exactly counterpoised vectors would (EAA 67, Lifetime) and will mesh readily with observa- be disarranged. 5037 MarlinWay tions of other related aerodynamic e anglIth f f attaco e k alterse th , Oxnard, 93030CA phenomena without revamping the- cente f lifo r t (cp) begin movo st e for- ories midstream. ward or backward (more on this Illustrations by the Author The business of a tail is in large later) o unlesS . s somehow con- part, concerned with longitudinal sta- strained a runawa, p coulc y d com- bility which conventionally is ex- pletely toppl e airplaneth e reaA . r amine itselfn di , separate from both tail prevents this from happening and IT IS ANTICIPATED that even the lateral and directional stability. n extremela n doei t i s y simpld an e most knowledgeable will give assent To start, airplane'n a f i s four princi- elegant fashion, which might hel- pex to a review of some fundamental as- pal forces were hypothetically bal s beeo -s ha nr t i witplaifo y s hu nwh pects of why there are tails, how they anced through a single point at a long. wor don'd kan t work, especiallo s s ya steady speed, there would be no need An airplane is conventionally made many different shapes of airplanes for a tailplane (Figure 1-1). Of course, stable by the use of "aerodynamic de- are now coming onto the aeronautical this condition evef i , r achieved, could calage" (Ref. 1) or "Longitudinal di- scene. not remain long. For if the speed hedral" (Ref show) 2 . n exaggeraten di As with all explanations in this should change jus a littlt e bite th , Figure 1-2 certain O . n machinee sth Figure 1-2 — Most Airplanes SPORT AVIATION 27 The airplane is again balanced, but in a glide. To climb throttle ,th openes ei d dan the slipstream and downwash increase the stab down-force to cause a nose-up condition. Thus, longitudinal dihedral makes an airplane seek to initialln a flt a y y programmed angle of attack. If opening the throttle cause abrup muco o to sto r h o noseta - up movement, a possible alleviating remed sligha s yi t down-thrust built into the motor mount (Figure 1-3). distressins i tha t w I tfe thia o sgt conventional airplane has to carry an PietenpoFigur— 3 1- e r Campe Ai l r induced tail-down-load, however 1932 small. Yet, a look at the whole picture shows that the commonly used "un- stable-type" airfoil generates quite high values of lift and can very easily sacrific tinea y percentag taie th l o et return i thir nfo s most conveniend tan highly practical system of stabiliza- tion. Innutshella e degreth , pitcf eo h stability is governed by the CG loca- tion stae ,th b area angle ,th whict ea h sets ii t s aspecit , dise t th rati - d oan tance from the CG. Tails also function in other ways. The tailplane along d ruddean n rfi wit serve th h e like feathers on an arrow, to quickly point airplane th directioe th n eactui s i t ni - ally going, after a large disturbance such as a lomcevak or tail slide (Fig- FigurArro— 4 we1- Stability Goino gT ure 1-4). The tail's chord is often Work large, to benefit from as high a Reynolds number as possible. Addi- tionally, low-aspect ratio helps the tai resiso lt t separation (stallingd )an to remain effective at high angles, especially after the main wing is stalle r malpositioneddo . Tail cross- section symmetrif o e sar c proportions if they are to operate equally well, negatively positivelyr o . On most airplanes, more up than down elevator is provided, to be able to rais nose eth e adequately duringa flar landinr t a fo e s i gG wheC e nth the full-forward limit and the speed is low. Elevator down-travel is based on Figure 1-5 — A Pitts it ain't. safe stall recovery when at full-aft CG limit "normalf I . " airplanes were made to do aerobatics and inverted flight, their elevators migh havt no t e incidence setting tailplane ofth e will s (toer r some reason e speeth )- in d enough powe hol o t re nos dth e "up" be positive, wit o geometrin h c lon- creases. This create strongesa r down- (Figure 1-5). This lac f controo k l gitudinal dihedral. However, there e stabilizerth forc n o e , which con- woul particularle db y evidene th f i t should still be effective dihedral be- sequently bring e nosth s e bac, up k wing airfoil was highly cambered, as caus taie operatins eth i l downn gi - reducin e presee speeth th g o t td this type becomes inordinately stable was thid causn han sca e actua eth l value nose th ef I .shoul displacee db d when inverted. It doesn't lift well angle of attack to be less than the upward, the airplane slows and the either, upside down, necessitating a angle of incidence. stab down-force lessens sufficiently to high angle of attack to support the A rear tail function followe th n i s- allow the CG to lower the nose. e weighairplaneth f o t . Therefore, ing manner a selecte t A . d speeda , e enginIth f e stops e airplanth , e greater elevato rneede e poweb y drma deliberate nose-down couple, com- slows and the stab down-force di- just when there might not be any weighd an p c t prise(CGe th f s )i do minishe befores sa t thisBu . time eth more. equalize smala y db l downloae th n do nose lowers and stays down. Speed On the other hand, inverted flight stabilizer. This arrangement per- increases until countere orige th -y db woulprobleo n e airplane th db mf i e forms automatically nose th ef I . low- inally established stab down-force. was expressly designed, with a sym- APRI8 2 L 1984 reduction correspondingly demands a pull force. And this elevator displace- men again ca t n resule parn th i t f o t tail pushing dow pard nan t pushing up. For example: during an approach, speee th decreases di applyiny db ga gradually stronger pull, deflecting e elevatoth . - Initiallyup up r e th , elevator combines wit longitudie hth - nal dihedral to intensify the dwin- dling tail-dow nspeedsw forcelo t A ,. Figure 1-6 — Much better. the aft end is down so far that the normal stab down-load diminisheo st zero fact n sta e I .star n th ,b ca t lifting, tryin movo gt nose eth e downwardo ,t dutifully recover the original angle of attac airspeedd kan opposinn I . e gth stab with up-elevator the pilot once more sets up contrary reactions over the tail (Figure 1-8). A high-positioned tailplane cannot benefit greatly from downwash. Therefore, a one-piece design may be appreciably more efficien n thii t s case, presentin a gsingle , uninter- rupted surfac singla d ean reaction. Next month . more on pitch stability. References: . Aerodynami1 c Decalage, Aerodynamics of the Airplane by Clark B. Milikan, TaiFigur— l7 plan1- e t crosa e s pur- John Wiley & Sons, Inc., New York, divea posen I . s.. 1941, page 145. 2. Longitudinal Dihedral, Mechanics of . KermodeC . FlighA r y Isaab Si ,t c Pitman & Sons, Ltd., London, 1942, page 152. ABOU AUTHOE TTH R George Collinge eon s (EAi ) A67 of the earliest EAA members . early enough that he was the de- signer of the EAA logo. A native of Canada enlistee h ,e RCA th n di F in 1940, learned to fly in the system, then became an instructor approachn a n O Figur— .8 e1- eventualld an ye attaineth f o e don top military flight instructional (a- 1) certificates. Most type f airo s - metrical main-wing airfoi witd an lh Accordingly, if a pilot wishes to dive e inventorcrafth n i t y were flown elevators of ample travel, both up and muse h t provid oppositn ea e forcd ean regularly, from Tiger Motho t s down. Wing and tail would be set at pus hcontinud intan t oi e pushino gt Lancasters. Also during World zero degrees incidence. Longitudinal hold it in (Figure 1-7). Stick force War II, he lectured on aerody- dihedral would pertai o matten n r should naturally increase with speed. namics, engine handling and downwaso t e du , hup s whicwa s hwa If he releases the pressure, the nose range/endurance at CFS Trenton from the front wing flowing over the immediately rises. and ECFS Hullavington. tail (Figure 1-6)therd An .e shoule db Normal use of trimming devices in From 1947 to 1951 George was a little trim change f i any, , when altery n inherenoe wa sth t stabilizing t fighteje r pilot wit Canadiae hth n switching from upright to inverted mechanism. However, its employ- 400 Squadron, after which he and vicd an e versa.

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