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 in- crease 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 sta- bility.

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 IsaaSi b ,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 sys- tem, 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. ment is avoided in this review in his family emigrated to the U.S. Because a pilot must change an air- order to hold explanations to a where he has subsequently worked craft's attitud n ordei e o climbt r , minimum t maythai e B s . a t , whena e computefoth r d aircrafan r t zoom, descen diver o t wille h , , under- trimme operateds ri sense th , e should equipment industrie Southern si n standabl t thesya e times, override eth primare same bth e th s ea y controls; a Charte Californias wa re H . basic stabilizing function of the tail. that is, forward to relieve a stick membe Fernandn Sa botf o re hth o For example, in a dive, the speed and push-force backward an , easo t d a e and Santa ChaptersPaulA EA a . longitudinal dihedra increaso t t ac l e pull-force. Throughout EAA's existence, e taith l down-load forcin e reath g r As already described, to dive or to Georg s beeha e a nfrequen t con- end downward bringing the nose back increase speed should necessiate a tributor of both articles and up and the airplane out of the dive. push on the stick. A climb or speed artwor SPORr kfo T AVIATION.

SPORT AVIATION 29 IO A LJ/MTH SMITAI All I I ^Mi- M OO ADX/O

Stability Continued

Part 2

By George B. Collinge (EAA 67, Lifetime) 5037 Marlin Way Oxnard, CA 93030

Illustrations by the Author

1JONGITUDINAL STABILITY Fig. 2-1 What will it do? CONCERNS the action of an airplane, where, after a pitch distur- it can produce, hence the concept of employed on blades and tailless bance and without pilot interference increased efficiency in an all-wing airplanes. it either returns to or moves farther airplane by eliminating the tail, is As the CG is positioned farther for- away from the original state (see Fig- flawed by the very nature of its wing's ward, an airplane becomes more and ure 2-1). It is usually studied in two low lifting-power. more stable, balancing out at a higher main categories - Static and Dynamic. The terms "stable" and "unstable" speed. Stick forces become heavier Static stability refers only to the as applied to airfoils, while undoub- because there is a greater effective beginning phase; that is, whether or tedly fixtures as regards accepted weight to maneuver. A loss of "nose- not an airplane initially begins to re- nomenclature, may not be altogether up" elevator power would be evident turn. If it simply starts, but doesn't appropriate, especially when ascer- in the low-speed range, requiring a complete the movement, it is still taining the stability characteristics tail of greater influence. Therefore, classed as "statically stable". of the total airplane. Airplanes with the forward CG limit is determined Dynamic stability pertains to the sub- unstable airfoils are, of course, easily chiefly by control rather than stabil- sequent and remaining motion, what- made stable and some aircraft with ity. ever that might turn out to be. stable airfoils can at times be any- A stability standard, the com- Most airplanes have "unstable" air- thing but stable. Although academic, pliance of which has resulted in the foils. These have positively-cambered beyond the stall the cp of all airfoils general good-handling of modern median lines and generate high lift- migrates toward a mid-chord position. airplanes, mandates that a stable coefficients. They push a lot of air At a 90 degree angle of attack, all will airplane requires a large stick-move- downward. Unstable airfoils are sad- exhibit the flat-plate predisposition ment and an increase of pressure to dled with a cp that moves forward of a 50 percent chord cp location. start a speed change. Also it will re- when the angle of attack is increased, The airfoils in Figure 2-2 show a quire a progressively stronger stick- tending to further increase the angle. range of types, the most cambered force to increase the rate of change. With low angles (high speed) the cp median-line (top example) indicative In other words, to pull faster means moves rearward causing a nose-down of the highest lifting power (CD of to pull harder at the same time. reaction. Instability of this kind is the group. For comparison, the vari- As the CG of an airplane is made to one of the chief reasons for a tail in ous median lines are all wrapped with move aft, a point will be reached the first place. the same streamline-function of about where stability is neutral. It will tend In contrast, a "stable" airfoil is one 15 percent thickness. to stay in whatever attitude it is put. where the median line is flattened or Airfoil No. 1 is very unstable, has Past this point and the airplane be- perhaps reflexed. The cp is stationary, high lift and is for lower-speed air- gins to change pitch too easily, espe- or almost. Some of these airfoils incor- craft. No. 2 is less unstable, provides cially in rough air. It will be work for porate a cp that even moves aft with moderate lift for a wide speed-range. the pilot to keep it from getting worse. angle increase and forward with a No. 3 is stable and has lower lift. It If disturbed, the airplane will tend to decrease, so helping to restore the requires minimal trim alterations diverge. Example: if the airplane's original angle of attack and airspeed. with variations of speed, also used on nose goes up due to a disturbance, the "Tailless" aircraft generally have to rotary-wing blades and tailless wing will lift into an ever-tighter favor these airfoils. Unfortunately, airplanes. No. 4 has a reflex to loop. Even forward stick might not the more stable an airfoil, the less lift stabilize its cambered entry. It too is stop it.

SPORT AVIATION 49 To recap - as it is moved aft, the ability of the CG to lower the nose is reduced. Stability thereby decreases. Stick forces become lighter as the lever-arm length between the CG and the tail gets shorter, although control may seem adequate if only because there is less nose weight to overcome. With continued aft CG travel, the airplane will eventually run out of elevator nose-down power so that stall and spin recovery will become more difficult if not impossible. In a dive, this reduced leverage has the same effect as too small a tail or too short a fuselage, the airplane would continue to accelerate though the stick would be fully back! It might seem a paradox, but moving the CG forward (making the nose heavier) allows an easier dive recovery. A more forward CG increases the power Fig. 2-2 Airfoil types. (leverage) of the tail. Normal allowable CG travel for an average airfoil on an average airplane -^•^ /«g is seldom more than 20 percent of the mean aerodynamic chord (see Figure 2-3). The rear CG position can be roughly determined as that which al- lows hands-off flying and the forward position that for good control on land- ing. An aft CG condition can tighten up Fig. 2-3 CG limits. a turn or pull out, such as occurred in the early Spitfires with full fuel load. A push force was needed to prevent too much G. Instability can show up as any or all of the following partial list of symptoms: to recover from a dive, instead of releasing a forward push, the airplane needs a strong pull, if an airplane will increase speed eas- ily with only a small push but while continuing to dive the stick comes DOWH back to its original position, if an airplane demands a push force just before a three-point touchdown. Standardization of terminology was (/N/T//9L) (PHUCTO/&) early decided. Delineation of the mo- tions that decide the pitch-stability classifications is shown in Figure 2-4. Conditions 1, 2, 3 and 6 are statically Z stable because they tend to return to 3 level flight, even though 3 and 6 never achieve it. An average airplane can take 20 to 60 seconds between each oscillation, the usual damping factor results in two or three phugoids before ending. It is possible that an Fig. 2-4 The Gamut. airplane is so stable and so sluggish in response that after an upset only one cycle or perhaps a half cycle is performed, though up to 60 seconds in length. Another aircraft is classed less stable because it requires five cycles notwithstanding that it com- pletes them all in less time. And it is possible that oscillations in an unsta- ble sense can be so slow that it might not be considered dangerous or dif- ficult to control. Fig. 2-5 Weight lifter.

50 MAY 1984 cruise. But once immersed in the highly-angled downwash flowing off large-area flaps it generates the necessary increased authority. Some- times inverted slats are essential to prevent tails from stalling, particu- larly near the roots where the airstream is degraded by the fuselage. Aircraft across the full spectrum of size utilize inverted tails, from the Beechcraft Musketeer to the McDon- nell F-4 to the multi-engined Breguet 941. These examples should not be con- fused with what was purely an early application of longitudinal dihedral, exhibited by the WW I Pfalz III (see Figure 2-6). It was superseded on the Pfalz Ilia by a symmetrical section with greater chord and area (Refer- ence 1, 2, 3). A number of high-wing monoplanes Fig. 2-6 Pfalz III 1917. can tolerate a more aft CG and still be quite stable because during a pitch-up for instance, the CG effec- tively moves forward of even an unst- able cp, helping to lower the nose (see Figure 2-7). Contrariwise, with its CG above the low wing, an airplane can become progressively more unstable as the nose goes higher (see Figure 2-8). Low-wing monoplanes as a con- sequences are happier with a more forward CG and/or a larger tail. A more stable airfoil also helps. While not considered in these elementary notes, the effect of the fuselage, propeller(s) and engine nacelles are all additional factors which, with the wing cp, combine to form an overall airplane cp. Their Fig. 2-7 Pendulum effect. sometimes strong influence can help explain abnormal deviations of air- craft behavior from those prognosti- cated by basic airfoil action alone. As the CG is so important, it is possible on large transports to adjust its location during flight. The French Airbus 310-300 was planned to have the capability of pumping fuel into its horizontal tail while cruising, to de- crease the negative load. The aft shift in CG unloads the main wing, reduc- ing its wing loading and induced drag (Reference 4). Next month . . . Lifting tails and servos. References - Fig. 2-8 Upsetting. 1. Cross & Cockade, Vol. 1, No. 1, Winter, Santa Ana USA, 1960. Pages The use of camber-increasing flaps are mandatory in conjunction with 36, 37 and 47. to obtain extremely high CL from thin really big flaps because so much more 2. Jane's All the World's Aircraft, highspeed airfoils is eminently possi- air at extraordinary velocity goes over Sampson Low Marston UK, 1919. ble as long as there is a tail out back. the nose which would otherwise invite Pages 339a through 343a. While lowered flaps always cause the separation. 3. Pfalz Dili, Profile Publications cp to move aft normally creating a In some cases the tail is modified to Ltd., Hills & Lacy Ltd., No. 43, Lon- nose-down attitude, on certain air- produce an extra-large down-load to don UK, 1965. Page 5. craft the increased downwash over handle the stability decrease with 4. Airbus A310-300 Definition the tail is so strong as to initially flaps (see Figure 2-5). An inverted Completed by Jeffrey M. Lenorovitz, cause a nose-up tendency. tail is set to produce only enough Aviation Week, Aug. 29, 1982. Page Leading-edge slats or Kreuger flaps down-load for normal stability during 31.

SPORT AVIATION 51 IO A LJ/tBI Al TAN 10 I **L. I OO ADX/O iOO*M1 T S' Lifting Tails, One-Piece Tails and Servos

Parts

By George B. Collinge (EAA 67, Lifetime) 5037 Marlin Way Oxnard, CA 93030

Illustrations by the Author

D,ISTINCs T FROM SYMMETRIC- airplane would stiffen on the top of a ensures little trim drag throughout SECTION tails that are set at a small loop and attempts to roll off or execute the flight envelope and only smallish positive angle of incidence but in real- an Immelman (lots of rudder on this tails are needed (see Fig. 3-2). ity are operating at a negative angle aircraft due to low-response ailerons) In contrast, one of the most difficult of attack in downwash and in direct frequently resulted in a flat but this aircraft to give reasonable longitudi- contrast to negatively-arched stabiliz- time inverted spin, a condition as- nal stability and handling is the high ers, there have been such things as sisted by the now-inverted tail and by thrust-line boat. A typical early "lifting" tails. Rarely employed, ex- the addition of a large squarish example, a British Bradford-built cept in the very early days, their use winter-canopy. Recovery was some- twin, incorporated a tail with an in- has been for purposes other than basic times difficult, especially with a pair verted RAF 15 section, mounted at a longitudinal stability. Example: the of green student pilots aboard doing moderate angle to provide nominal prototype of a popular biplane of the "mutual practice" and suffering acute "engines-off longitudinal dihedral 30's exhibited a tendency to go flat disorientation. A number of these but which was greatly augmented by during a spin (Ref. 1). Rather than airplanes spun into the ground, keel the increased tail down load in the redesign and adjust the CG to a more upwards. "engines-on" slipstream. The large forward position and/or enlarge the For a designer to arrange the four nose-down couple created by power existing tail area, the stabilizer was forces (lift, weight, thrust and drag) application was thereby neatly coun- given the quick fix of a positively- through a single point is an ideal terbalanced (see Fig. 3-3). cambered airfoil section. A degree only, but the airplane type that seems Some early designers were obvi- of longitudinal dihedral remained to most closely approach that state of ously concerned about the simultane- which gave acceptable stability, but perfection in numbers has to be the ous up and down loads that occurred spin behavior was improved. The so-called midget racer. In this cate- with the two-piece tailplane. On the cambered stabilizer helped in lifting gory, mid-wings predominate in com- other hand, the one-piece, all-moving the aft end and forcing the nose down. pany with minimum cp-travel air- tail was always smooth and flat and Spin recovery was further encouraged foils. Fuel location close to the CG required only a small deflection to be by the restriction — "Solo from front seat only". When this airplane was produced in quantity early in WW-II and put into elementary military-training, it was naturally subjected to an ex- tremely wide-range of aerodynamic situations not normally experienced in civilian use. As the main wings employed a highly-cambered airfoil section, they became super stable when inverted (see Fig. 3-1). The Fig. 3-2 Look alikes.

38 JUNE 1984 effective. The French Morane was a pioneer in the utilization of this kind of tail, as were its many copies, including Fokker and Pfalz (see Fig. 3-4). However, unless the control column was held firmly by the pilot (or by other means) longitudinal stability was nil. This because a plain free- floating surface tends to trail, thus providing no resistance to pitch changes. Steady flight would have re- quired constant pilot attention (Ref. 2). Early airplanes with an all-moving Fig. 3-3 P-5 Cork 1 1918. tailplane generally had the highly- cambered main-wing airfoil. A nose- down disturbance would move the cp well aft. Unless checked immediately, an ever-increasing angle of dive would result because of the aforemen- tioned absence of an automatically imposed down-load on the tail (see Fig. 3-5). During a nose-up displace- ment, the cp forward-travel would pull the nose up farther, requiring a forward push-force to stop it (see Fig. 3-6). This pitch instability, accompanied by the heavy feel of wing warping, no fixed fin plus the handling quirks of Fig. 3-4 M5K Fokker 1914. a rotary engine produced airplanes difficult to fly, particularly as gun platforms, a requirement that sud- denly became very important in those years. As speeds approached 100 mph this type of tail became increasingly difficult to handle, falling into disuse. In the latter part of WW-I and into the post-war years, airplanes went faster and controls became much heavier to move. Some got very heavy. This situation would probably have remained for a lot longer than it did except for the fact that many air- craft were multi-engined and non- feathering propellers exacerbated the difficulty of engine-out flying. Pilots Fig. 3-5 Pull against dive. just did not have the leg strength to push on and hold on adequate rudder to fully compensate for the asymmet- rical thrust of the live engine com- bined with the drag of a windmilling propeller. Hence, servos first became popular on rudders. Anton Flettner had invented the servo in Germany during WW-I. Sub- sequent royalty fees financed his later extensive helicopter work (Ref. 3). The servo was for years referred to as the "Flettner tab" and has been used in various roles on a great many of the world's airplanes. Its original basic function was to provide a light-to-operate control which in turn aerodynamically moved

Fig. 3-6 Push against dive.

SPORT AVIATION 39 a large main-surface. Early adaptions were sometimes given the added leverage of an extended boom (Fig. 3-7). A variation, the spring-centered servo, gained wide usage during and after WW-II although it could result in sluggish or soggy response on the initial portion of take-offs and landing runs. With other servo types, the pilot directly commanded the main surface which incorporated a geared servo. In this case the servo always moved a fixed percentage of the angular travel of the boosted main-surface. In concert with these developments, Fig. 3-7 Bolton-Paul 1934. servos doubled as trimmers on all main control-surfaces; in the pitching plane on many aircraft, it displaced the adjustable stabilizer. Servos or boosters employed on ailerons lighten the feel and in effect, increase the rate of roll (Fig. 3-8). Servos are used extensively, right up to the realm of irreversible, hydraulically-actuated surfaces with artificial feel. The elevens of an English (general aircraft) experimental tailless-glider of 1940 each utilized a tab that functioned as a servo to lighten the aileron action. With elevator move- ment the same tab operated only as a trim tab. This ingenious adaption kept the control feel properly har- Fig. 3-8 Harvard mkll 1940. monized, that of the elevators heavy and that of the ailerons light (Ref. 4). Widespread activity with tabs was bound to lead to their application as "anti" servos, to revitalize the long disused one-piece or slab tail. While the slab's undesirable floating or trailing characteristics could be minimized by centering it with a bungee, springs per se tend to veil true feel. A better system is a geared servo, reversed in its action so as to oppose the main surface angular travel. Pow- erful aerodynamic centering action results, but with a high degree of feel, the magnitude of which can be readily tailored to suit a specific airplane-de- sign (Fig. 3-9). It can double at the Fig. 3-9 Glider anti-servo tab. same time as a bias (trimmer). This concept, applied for in July 1945, was granted a U.S. patent in August 1951 to John W. Thorp, assignor to Lock- heed Aircraft Corporation. The "anti-servo tab", as it came to References: be called, has moreover been designed 1. Aerobatic and Amateur Built stab/elevator" (later changed). It was into elevators of two-piece tails to re- Aircraft by Robert Whittier, SPORT tiring because of "oscillating flight". duce trail and/or restore feel when AVIATION, April 1972, pages 13-15. 3. German Rotocraft Pioneer Comes masked by springs or weights in the 2. Soaring, August 1982, page 2. Back by David A. Anderton, Aviation system. Harland Ross describes flight in his Week, Nov. 29, 1954, pages 26-28. Next month — Pitch stability homemade 1937 RS-1 Zanonia 4. Towed Tailless, Flight, Sept. 26, further examined. sailplane which had a plain "flying 1946, pages 328-329. ,

40 JUNE 1984 IO A LJS%BI SMUT A I All IO I I MIL. ADX/O T S

Pitch Stability Further Examined Part 4

By George B. Collinge (EAA 67, Lifetime) 5037 Marlin Way Oxnard, CA 93030

Illustrations by the Author

PILOT NEEDS to feel stick There is yet a third category. When 1), while the aircraft's movements are forces to help judge speed and how an airplane is purposely disturbed or watched. The test is repeated except much load to apply to the airframe. If given a gross displacement for obser- that the stick is pulled backward, the control forces, from a trimmed vational intent and allowed to fly by then centralized. state, increase with any attitude itself, a difference is noted if the stick Usually, in each case, the nose change, then the airplane is, in all is then held firmly or if it is left free. should come back to the beginning likelihood, stable. However, there is a Stick Fixed - For this mode, at a position, go past (due to inertia) then little more to add. So far in this re- trimmed steady speed, the stick is reverse and so damp out in a few cy- view, longitudinal stability has been pushed about one half of its forward cles. If it does not subside, the CG is broadly covered under a two-part travel until the nose is well down. too far aft and/or the stabilizer is too classification. Terms, static and The stick is then immediately re- small or at the wrong angle. The dis- dynamic were used to sequence the turned to its original position and tance between the stabilizer and the reaction to an upset. held or clamped immobile (Figure 4- CG could be too short as well.

Fig. 4-1 Fixed.

SPORT AVIATION 39 Fig. 4-2 Free.

As the CG position is moved aft due Stick Free - For this test, again the and CG position and is generally per- to design or loading (passengers, same stick movements as for "fixed" manent once the airplane has been cargo or fuel) the degree of stick only it is not brought back to the ref- built, the degree of stick-free stability movement necessary to cause a speed erence position but completely re- can be easily enhanced, with only change will get less and less until, leased (Figure 4-2). When stick free, small modifications to the control sys- just past the neutral point, the airplanes take longer to decay after tem. airplane will go either way, up or an upset, depending on how readily In other words, it is possible that an down, by itself (mentioned in Part 2 the elevator floats and at what angles. airplane with only neutral stick-fixed of this series). Only the fixed part, the stabilizer, stability can be persuaded to accept a It just might be satisfactory for an has any real effect and it is of consid- measure of stick-free stability. This airplane to have neutral stick-fixed erably less area than the entire tail. is because one or more automatic con- stability provided that the controls An airplane will be stable in the trivances actually move the elevator feel normal and it still requires a push stick-free mode as long as the correct to cause a nosedown recovery. By so to go faster and a pull to go slower, as stick movements are still necessary doing, the airplane can then perhaps pressure is more important to the to change the airspeed from a tolerate an even more aft position of pilot than actual movement or travel trimmed condition. Whereas stick- the neutral point. This increase in of the stick. fixed stability is governed by tail size stick-free stability in the low-speed

Fig. 4-3 Fix.

40 JULY 1984 Fig. 4-4 More fixes. mode and provision of a nose-down than the stick-fixed mode. Springs are Some designers of high-perfor- message through the stick to the pilot not effected by acceleration (g) but mance gliders settle for reduced sta- is done with a spring tab (Figure 4-3). weights are. Therefore, weights are bility in exchange for reduced drag, At low speeds, air pressure di- superior because they give an increas- by following the idea of rather small- minishes so that the spring pulls the ing force with acceleration. From this area tailplanes. By choosing the two- tab up. The tab in turn moves the it is obvious that a tail-trimming sys- piece design and intentionally omit- elevator down, making the airplane tem based on springs can alter stabil- ting weighty mass-balance, some nose down to restore speed. At high ity characteristics over the speed stick-free stability is retained. For speeds the tab has no effect as air range. example, due to inertia, an up-gust pressure is too high for the spring A fourth item, the geared anti-servo causes the elevator to lag. This down- strength to overcome. tab, is more effective with speed at elevator helps lower the nose to re- Two other devices to give the pilot increasing stick-free stability and has cover airspeed. better feel and to increase stick-free a further advantage of working both One of the trade-offs is that the stability are either a spring or weight up and down (Figure 4-5). One-piece elevator inertia, feeding back through bearing directly on the stick or some- tails with anti-servos can also be re- the stick will tend to increase the nor- where in the control run (Figure 4-4). duced in area, but not too much, be- mal pull-force that is required in a One or all of these three fixes can cause then stick-fixed stability will turn or a pull-out. improve stick-free stability better begin to suffer. Next month . . . Tailless.

Fig. 4-5 Anti-servo.

SPORT AVIATION 41 IO A All 10 I *ML M ADX/O

TAILLESS Parts

By George B. Collinge (EAA 67, Lifetime) 5037 Marlin Way Oxnard, CA 93030 i Illustrations by the Author

"TAILLESS" AIRPLANES Unfortunately, wings of this kind its then terrible propensities was em- really tailless? plainly create less CL than "unstable" ployed by Alexander Lippisch in his By utilizing fixed wings for lift, wings. To carry the same load more passion to see his brainchild accepted manually-controlled stable flight is wing area is needed. This and the im- as what else, a superior airplane. But most difficult if not impossible with- possibility of effectively using high- that was not to be, although a num- out a tail (ref. 1). Nevertheless the lift trailing-edge flaps (because of the ber of Komets did become operational. primary inclination of designers of aft cp shift) defeats much of the orig- It had the usual features of a single- all-wing aircraft has been to increase inal rationale of the all-wing air- wing, moderate aspect-ratio aircraft. efficiency by eradicating the fuselage plane. It quickly becomes apparent The 23.3 degrees of sweep was a con- and tail as non-essentail drag and that any claimed advantage of the cession, not to boost the critical mach weight-producing appendages. The type is sharply compromised. number, rather to allow a rearward "all-wing" purists favor the concept All-wing aircraft have at times displacement of the ailerons where that the wing itself should contain been fitted with trailing-edge flaps they could do double duty providing everything the airplane has to carry. that go down but at the same time control in pitch. Although eighty years" 'and millions have an equal area go up. This is to A large amount of twist was neces- of airplanes has resulted in the con- average out the median line and sary to provide sufficient longitudinal ventional tail-at-the-rear formula, maintain balance. About the only re- dihedral (Fig. 5-1). The landing flaps, aviation history records many efforts sult is drag, though quite a useful and referred to in German literature as to do it differently. But however it is on occasion a most desirable force. brakes or dive flaps, were positioned disguised or camouflaged, the tail or Within their small operating en- in a mid-chord location, to minimize rather the tail-substitute, is always velopes, all-wing aircraft can add to the resultant aft cp movement. there someplace, in some form or their already necessarily stable Other all-wing aircraft have used shape. characteristics by incorporating the same flap location, and also just A positively-cambered airfoil when springs and/or weights in the elevator for drag. The Mel63 flaps could only used inverted, or a sufficiently re- control-system. As with a conven- have been intended for drag because flexed (bent up) airfoil tend to inhibit tional airplane, by limiting control- ' they gave a CL increase of barely 0.1 cp travel or even reverse its move- surface travel, an all-wing airplane and this is not subtracting the CL loss ment so that its effect is to resist any can also be made to resist stalling and due to up-travel trimmers. change in angle of attack thus provid- spinning at low speeds, even when the It may be worth noting that the ef- ing a degree of stability. The rear por- stick is fully aft, which capability fect in pitch of Komet's flaps was one- tions of these sections act as nega- seems to be a penchant of a fair per- third down-nose slightly up. From tively-angled tails, the forward parts centage of designers (ref. 2). two-thirds down-nose lowered, requir- as conventional wings. There will always be dreamers with ing "tail-heavy trim". Tailless wings usually have a large a favorite or pet formula in spite of With the positions of the cp and CG amount of twist in order that the the realization that their ideals may pretty well fixed, only leading-edge outer areas (routinely swept at least harbor innate flaws. It can be surpris- slats were permissible without prob- slightly) can act as a tail although ing to note the tremendous human en- lems. They increase the usable angle like the reflexed airfoils, they func- deavour that has been expended, in of attack but do not cause significant tion with minimal leverage. And be- cases, in efforts to bring such ideas to cp movement. cause of their limited power, the successful fruition. Enabled by wartime German deci- range of CG travel that they control Case in point, the Mel63 (ref. 3 to sion-system which was essentially must also be severely restricted, if not 10). Beset by inherent tailless prob- controlled by political people, the immovable. lems, the liquid-rocket motor, with all Mel63 "proved" the concept, sort of. A

38 AUGUST 1984 total of 360 were built in Germany (ref. 10) and 7 in Japan (ref. 11). Other than this one decreed adap- tion, there could be no real useful work for the tailless. One good thing about the Mel63 was what it spawned. It is obvious that Lippisch was finally convinced of the all-wing dead end and in May 1943 he left Germany to do new design work in Vienna. When the wings of a tailless design are swept sufficiently to provide re- ally useful longitudinal dihedral plus a means of ensuring an adequate lever-arm for the elevators, closing in of the space between the arms of the wing was natural and inevitable and resulted in the delta as is known today. Besides structural advantages and a large increase of internal vol- ume, it is an unchallenged fact that the narrow-angle delta planform has ideal aerodynamic characteristics for Fig. 5-1 Me 163 1942. certain high-speed aircraft while at the same time is capable of great lift at low speeds, without the use of flaps keeps it momentarily going straight had invented the swept and twisted which it, like the all-wing cannot use ahead. Thus the huge and instantane- wing, to be flown without a separate anyway. The large factor of induced ous increase in CL, coupled with a tre- tailplane. He had begun his experi- drag at the controllable high-angles mendous ground effect, put the delta ments in 1904, through monoplane to of attack well substitutes for flaps in on gently, the aerodynamic drag slow- triplane, his biplanes being the best the desirable steepening of the ap- ing it automatically until the nose known. proach, in the lowering of landing wheel is lowered onto the runway. Dunne, along with Samuel speeds and practically eliminating Upon examining history, it be- Franklin Cody in 1906, were employ- any semblance of unwanted float. comes clear that the revolutionary ees of the Royal Aircraft Establish- When the elevens of the delta are Mel63 was not so revolutionary after ment, Balloon Factory, at Farn- raised for the flare, the entire all. Because, back in the early days, borough. Both these men (among airplane rotates though its inertia a man named John William Dunne others) were trying to be the first to

Fig. 5-2 Dunne d-5 1910.

SPORT AVIATION 39 Fig. 5-3 Swallow 1946. fly in the UK. Cody succeeded in sus- cause, so (for three more years) they Gotha (ref. 19). Not surprisingly, lit- tained flight on Oct. 16, 1908; concentrated on kites, balloons and tle was accomplished. Dunne's No. 5 machine flew in the airships (ref. 17). Dunne and Cody In 1945, the British Ministry of spring of 1910. were fired, though both continued ex- Supply sponsored a program of tail- The Wrights were still using a for- periments on their own. less research (ref. 20) which resulted ward elevator and their airplanes This seemingly congenital urge of in a number of experiments. On were not stable. In fact, at this time English politicians to terminate the March 15, 1946 the first flight took they stated that inherent stability airplane by fiat made one of its ap- place of a single-seat DeHavilland was undesirable. Their system was pearances in 1965 with the scuttling 108 Swallow (Fig. 5-3), one of three to for "hand-controlled equilibrium". of a number of projects, notably the be built (ref. 21). A few months later, Dunne had made his own calculations TsR2. An all-missile or remotely-con- DeHavilland announced the Swallow based on the Zanonia seed well before trolled air force was to be the wave of as an "... experimental basis for later stability and control had been the future (ref. 18) and this, sadly, types" (ref. 22) and the upcoming "clarified" by such as Lanchester (ref. from a country that has contributed a Brabazon IV transport (Comet) in 12). good share of the world's finest flying particular. A 1946 full-page ad in The Wright type front-elevator de- machines. Flight showed an artist's rendering of sign greatly influenced beginning Back to WW-II. A group headed by a proposed all-wing airliner (ref. 23). aviation, including Cody. Not so the Horten brothers, Reimar and Wal- On Sept. 27, the number two DH108 Dunne. He felt that airplanes should ter, convinced the German Air Minis- disintegrated in the air with no es- be definitely stable. Not that they try that they, too, should be supported cape for the test pilot Geoffrey Raoul should be able to fly without a pilot, for their own particular brand of tail- DeHavilland, (ref. 24). There were no but instead not require hectic, con- less. As the Hortens could not provide more Swallows built and no all-wing stant attention to pitch control as did sufficient production facilities, the airliner. the Wright types. He tried forward Ministry -later gave their design to John Knudsen Northrop was an control-surfaces on one of his early gliders, for more nose-up control dur- Fig. 5-4 IA-38 1960. ing landings. It was discarded, larger rear elevators worked better (ref. 13). A typical Dunne airplane, his first really successful flyer, the D.5 (Fig. 5-2) was a single-engined, twin-pro- peller biplane with a 52 degree sweep and a pronounced twist. This powerful longitudinal dihedral gave the pilot a firm and steady airplane. His "hori- zontal rudders", as they were called then, were flaps at the wing tips which on his D.4 were actuated by a "modern" wheel on a column, the wheel for roll and push/pull for pitch, the original elevons! (Ref. 14) Dunne aircraft had fixed fins but no rud- ders. At least one of his machines was built under franchise by Nieuport in France (ref. 15) and several by W. Starling Burgess (ref. 16) in the U.S.A., who was trying to avoid the Wright patent on ailerons. In 1909 the British Committee of Imperial Defence, in all their wisdom, decided that the Aeroplane was a lost

40 AUGUST 1984 References: 1. Aerodynamics of the Airplane by Clark B. Millikan, General Publishing Co. Limited, Toronto 1941, page 142. 2. Homebuilt Aircraft, Werner and Werner Corporation, Santa Monica, 198, page 8. 3. Rocket Fighter by John T. Dodson, Flying, Jan. 1950. 4. Developing a Rocket Fighter by Rudolph Opitz and Robert Randell Air In- ternational, UK, Vol. 21, 1965. 5. Wings of the Luftwaffe by Capt. Eric Brown, Doubleday & Co., Inc., USA 1978, pages 167 to 176. 6. The Komet by William Green, RAF Flying Review, UK, Vol. 18, No. 8, April 1963. 7. Raketjager Mel63 by Mano Ziegler, Fig. 5-5 Fauvel AV-45 1960. Motor Press Verlag, Stuttgart, WG, 1961. American "tailless" zealot and was re- found necessary to carry anything the 8. Ein Dreick Fliegt, The Delta Wing least bulky, again calls attention to by Alexander Lippisch, Motorbuch Verlag, sponsible for a bevy of aircraft of this Stuttgart, WG, 1976. kind (ref. 25) including the B-35 and the obvious futility of trying to make 9. Das Buch der Duetschen Luftfart- YB-49. He, too, could never make his the all-wing concept a practical car- technik by Bruno Lage, Verlag Dieter wings thick enough to get everything rier (ref. 29). Not helping in his re- Hoffman, Mainz, WG, 1970, pages inside, there were bumps, pods, blis- gard was a massive retractible nose- 552,553. ters and engines all over. Up to the wheel, pivoted under the pilot's 10. The Aeroplane Spotter, March 6, last, the problem of trying to devise cockpit. It took up much of the fuse- 1948, page 58. suitable high-lift devices eluded lage interior space. The IA-38 made 11. The Aeroplane Spotter, April 3, Northrop although he always claimed a flight on Dec. 9, 1960 (Fig.5-4). 1948, page 82. 12. Early Aviation (at Farnborough) by they were under study (ref. 26). For the last few decades, no large Percy Walker, MacDonald & Co., Ltd., His Flying-Wing bomber prototype aircraft concern has been really seri- London 1974, pages 169, 171. was being flown over the Air Force ous about the classic all-wing aircraft. 13. Early Aviation (at Farnborough) by flight test center at Muroc, California However, individuals have produced Percy Walker, MacDonald & Co., Ltd. Lon- on June 5, 1948. During a stall series some examples (Fig. 5-5). These have don 1974, page 231. the airplane began such violent som- been and are mostly single placers 14. Earl Aviation (at Farnborough) by ersaulting that the resultant strain with restricted activity and little tol- Perry Walker, MacDonald & Co., Ltd., separated some of the structure and erance for variation of CG position. London 1974, page 185. it crashed and exploded with the en- All have some accommodation to ac- 15. Aviation Magazine, Paris, June cepted flying techniques. 1959, page 28. tire crew onboard. The contract for 16. Flight, Jan. 3,1930, page 41 and Dec. thirty YRB-49 aircraft was im- The goal of putting everything in- 11, 1953, page 755; Contact! by Henry S. mediately cancelled and the remain- side a wing has proved illusive, nul- Villard, Crown Publishers, Inc., NY 1968, ing Flying Wings were scrapped. The lifying the touted advantage of the pages 166, 167, 238. Air Force went with the B-36. Muroc tailless concept. In fact, modern trans- 17. Early Aviation (at Farnborough) by was renamed in honor of the dead ports evidence just the opposite end, Percey Walker, MacDonald & Co., Ltd., pilot, Captain Glen Walter Edwards where relatively tiny and thin wings London 1974, page 327. (ref. 27). support ever-larger fuselages. The 18. Project Cancelled by Dereck Wood, An Institute Aerotechnico type 38 sketch of the DC-9 (Fig. 5-6) has been The Bobbs-Merrill Company, Ind., In- cargo transport was designed by Dr. traced exactly from one of a number dianapolis 1965. 19. Nazi Jet-Bats Which Never Took Reimar Horten and constructed in of well-publicized photographs and so Wing by Erwin J. Bulban, Aviation USA, Argentina (ref. 28). It suffered from contains no biased distortions which Oct 1945. cooling problems with its four semi- might otherwise serve to enhance this 20. Aviation Week, Sept. 15, 1952, page buried, extended-shaft air-cooled 450 premise. 21. hp El Gaucho radial engines. Its rela- Next month — Pitch and roll retro- 21. Inter-Avia, Vol. 4, Oct. 1949, page tively high-drag fuselage, which was spect. 610. 22. Flight, June 6, 1946, page 562. Fig. 5-6 DC9 Super 80 1982. 23. Flight, Nov. 7, 1946, page v. 24. Flight, Oct. 3, 1946, pages 364, 365; Aeroplane, Oct. 4, 1946, pages 380, 395. 25. Northrop Activities, Flight, May 9, 1946, pages 469, 470. 26. The Northrop "All-Wing" Airplane by John K. Northrop, Aviation USA, Dec. 1941; All-Wing Aircraft by John K. North- rop, Flight, June 55 and 12, 1947. 27. Model Airplane News, June 1963. 28. Air Pictorial, Air League of the British Empire, London, July 1961, page 193; Jane's Encyclopedia of Aviation, Vol. 3, Grolier Educational Corporation, Dan- bury USA 1980, page 478. 29. Tailless Problems by G. H. Lee, RAes paper, Nov. 14, 1946, Flight, Nov. 28, 1946; Stalling Phenomena and the Tail- less Aeroplane by A. R. Weyl, series, Flight 1947.______SPORT AVIATION 41 IO A A I All IO I Kl OO ADX/O

PITCH AND ROLL RETROSPECT Part 6 By George B. Collinge (EAA 67, Lifetime) , 5037 Marlin Way Oxnard, CA 93030

Illustrations by the Author J.N THE PAST, if an airplane had braced rear tails that incorporated A giant brouhaha was to develop two wings placed one behind the other longitudinal dihedral. While Mont- and it got going about the same time and they were of nearly equal area, gomery and Langley favored the tan- that changes were taking place which the term "Tandem" would apply. If dem arrangement, they nevertheless would help to standardize the method the front surface was of considerably added Penaud tails for pitch stability of pitch control also. So as the two smaller area, the airplane was a (see Figure 6-1). axes of control were intertwined after "Canard". It is fair to assume that The Wrights, of course, did not use a fashion, a touch of history on the both these types are basically differ- a rear tail in the beginning. They aileron is included here. ent configurations of the same for- were successful with a front elevator. The Wrights quite simply regarded mula. Therefore, in the interests of A lot of would-be aeronauts around direct lateral-control as their prop- conformity and with deference to the world decided to copy. And no one erty, which others could use. But not what is already accepted nomencla- appeared upset, least of all the for making money. When profit be- ture, this review regards the entire Wrights, about these obvious imita- came a factor, which it almost always general class as tandems. The forward tions of the Wright-style front did, the Wrights wanted in. wing is the canard regardless of its elevator. What did bother the Wrights There were some legitimate techni- size. no end was what they considered im- cal conflicts naturally, a few of more The aircraft designs of Penaud, itations of their lateral control! Over substance than others, between the Caley, Lilienthal and Chanute all em- this they had legal exclusivism. Wright's coupled rudder/warp system and that used, for example, by Dunne. He had just two differential-elevators, no rudder surface at all! The original Wright patent applica- tion of March 23,1903 (Ref. 1), though denied, resulted in a second request which was ultimately issued as No. 821,393 on May 22, 1906. It carried 16 claims for "improvements in flying machines" but included that most sig- nificant award, the one that was to cause all sorts of consternation in the aeronautical world, the one covering manipulation of wing tips in any way Fig. 6-1 Penaud-tailed Langley 1903 or manner in order to achieve lateral control (Ref. 2). Many would try but, in most countries involved, there seemed no way around this legality. Apparently discounted or ignored by the U. S. Patent Office was early activity regarding lateral handling because at least two existing U. S. patents clearly described such work. One was held by Montgomery, another by Mouillard. Montgomery's manned glider flight in 1885 was made with a glider equipped with aileron control (Ref. 3). The Mouillard glider, which had been built under the aegis of Chanute, incorporated "annularies", sections of each trailing edge that could be independently low- Fig. 6-2 Voisin Goupy No. 1 1908 ered by the pilot. While Chanute was 24 SEPTEMBER 1984 \ financing his patent (No. 582,757, a low seventy-five thousand. France available to all since disclosure in May 18, 1897), Mouillard wrote to contested buying but their courts up- 1906, and in addition were printed in him ". . . this device is indispensable held the Wrights. Germany ignored detail in the 1906 French publication ... it is this which permits going to any fee. They said Chanute, in an of L'Aerophile (Ref. 16). left and right" (Ref. 4). Additionally, early lecture, had talked about wing Because of this, a great many early it is on record that Edson Gallaudet warping and in Germany that was airplanes around the world, if not ac- had developed a differential wing-lift sufficient to invalidate the entire pa- tually Wrights, certainly looked like technique a year before the Wrights tent (Ref. 13). Canada obviously had Wrights. The front elevator arrange- (Ref. 5) and that Mathew Boulton of also not recognized the legitimacy of ment was very popular on pushers England had patented a small mova- the Wright patents (Ref. 14). although as 1910 arrived, different ble wing-tip as far back as 1868 (Ref Between 1909 and 1913, the airplane configurations had been 6). Wrights had sold licenses in seven tried by many individual designers. When Chanute first learned of the countries, including a syndicate in Among the best known of the tractors proposed Wright coverage, he natur- France and companies in England, were Bleriot, Esnault-Peltrie, Bre- ally thought that prior patents would Germany and Italy (Ref 15). The guet, Antoinette, Nieuport, Voisin certainly invalidate their claims (Ref Wright patent drawings had been (Goupy) and A. V. Roe. 7). But as the world knows, the Wright patent was granted (Ref. 8). Almost no one could fly for money unless a licensing fee of over one thousand dollars a day was paid to the Wrights (Ref 9). That was a lot of cash in those days, so it is not too surprising that after the initial euphoria over the Wright demonstration flights in the USA and abroad, there gradually developed re- sentment and noncompliance of the patent. Even the granting of another patent, covering midwing "lateral balancing rudders" to Dr. Alexander Graham Bell of the Aerial Experi- ment Association only served to muddy the legal waters. This patent Fig. 6-3 Voisin 1909 was issued on Dec. 5, 1911 after a three-year wait. (Ref. 10). Well, as it turned out, the Wrights sued a lot of people and litigations made news up to the full 17 year pa- tent life (Ref 11). Many designers sought to thwart the document that they thought was improperly broad and inclusive. For instance, Burgess and a number of constructors, U. S. and European, used ailerons that only moved downward. A few others, wranked over the controversy, dis- pensed with lateral control altogether and using side curtains, performed skidding turns with rudder alone (Fig. 6-2). In fact, journalists of the day called attention between flat Fig. 6-4 Curtiss D111 1910 turns and banked turns, the latter rating a higher accolade. By 1915, Curtiss was making airplanes in Buffalo and shipping them to England without ailerons. Ailerons were manufactured in To- ronto and sent abroad separately (Ref. 12). Because it was causing so much havoc, in 1917 with the U. S. almost at war and needing airplanes, Con- gress appropriated one million dollars to acquire the Wright basic patent by condemnation. Thereafter, a cross- licensing or pool of patents within the new Manufacturers Aircraft Associa- tion solved most problems. Down from an original two hundred thousand dollar price, beleaguered England was granted rights to the airplane for Fig. 6-5 Wright with auxiliary elevator 1910 SPORT AVIATION 25 It is well known that the early Wright airplanes were unstable. Wil- bur said, "We would arrange the machine so that it would not tend to right itself (Ref. 7). This statement was made well after the fact. And the fact was that their airplanes could not have been much else except un- stable, what with highly-cambered main-wings and free-floating, non- loaded front elevators! The skill necessary to aviate satisfactorily must have been of high order. Fig. 6-6 Wright model B 1911 Accordingly, to stabilize airplanes with forward elevators, various con- structors employed a long-levered, everyone else had been doing (Fig. fixed rear-tail, but still retained the 6-5). Eventually, the entire front ubiquitous front elevator for pitch References for Part 6: control. Curtiss, Farman, Voisin, et elevator was removed and in 1910, al, were originally of this configura- for a more complete reversal of their 1. Solving the Control Riddle by Robert tion (Feb. 6-3). Later, about when the original policy and to complete the Burkhart, Air Line Pilot, Dec. 1976. flying fraternity had grudgingly re- metamorphosis (Ref. 22), the Wrights 2. American Science and Invention by produced a factory airplane without a Mitchell Wilson, Bonanza Books, NY solved to go ahead and use warp or 1960, pages 346-351. ailerons and to pay the toll, elevator forward elevator at all and finally a standard wheeled landing-gear (Ref. 3. Design For Flying by David B. action was incorporated into the rear 23). This was the "headless Wright" Thurston, McGraw-Hill Book Company, tail (perhaps due to tractor influence) NY 1978, page 2, 3. but initially only in conjunction with or Model B (Fig. 6-6). 4. and 5. A Dream of Wings by Tom D. the existing elevator in front (Fig. It is possible that crashes, in which Crouch, W. W. Norton & Company, NY 6-4). After this, it was only a rela- the heavy mass of the pusher engine 1981, page 70, 71; page 307. tively short interim before even the let go and thumped the pilot and/or 6. Dreams and Realities of the Conquest diehards gave up their forward passengers, influenced greatly the as- of the Skies by Beril Becker, Atheneum, cendancy of the tractor. Its crash NY, 1976, page 149. elevators. Detailed in the literature 7. Glenn Curtiss, Pioneer of Flight by are a number of interesting stories of worthiness was recognized as superi- or. Tractor airplanes could also be C. R. Roseberry, Doubleday and Company, how, for example, the Curtiss front made smaller, cleaner, faster and Inc., Garden City, NY, 1972, page 191. elevators were eventually discarded. 8. One Day At Kitty Hawk by John E. with greater regard for pilot comfort. One source describes them as being Walsh, Thomas Y. Crowell Company, NY, knocked off running into a fence by Tractor propellers were, in the long 1975, pages 20, 175, 216. Lincoln Beachey while barnstorming. run, more efficient and less hazardous 9. Curtiss by Louis S. Casey, Crown to the pilot in the air. The existing The airplane was then hurriedly Publishers, Inc., NY, 1981, pages xi, xii. disenchantment with the forward 10. Homebuilt Aircraft, Werner & flown without and what a difference Werner Corporation, CA, Sept. 1981, it made! (Ref. 18) elevator also served to hasten the al- most exclusive use of the tractor al- pages 24, 25. Al J. Engel suggested to Curtiss though there were a few German 11. Aerial Age, Aerial Age Company, that the front elevator not be installed Inc., NY, March 1923, page 139. pushers pressed into service at the 12. Glenn Curtiss, Pioneer of Flight by on new machines at the factory and beginning of World War 1, with the be removed from all their existing C. R. Roseberry, Doubleday and Company, occasional experimental model pro- Inc., Garden City, NY, 1972, page 398. machines in the field. They became duced by the factories as the war con- 13. The Story of Flying by Archibold "headless" (Ref. 19). tinued. However, it was the English Black, McGraw-Hill Book Company, Inc., In a 1909 London Sphere illustra- who gave the pusher a renewed lease NY, 1940, pages 92 to 98. tion, Latham is shown in his tractor on life, if only a brief one. The problem 14. Glenn Curtiss, Pioneer of Flight by monoplane over the channel. Among was non-aerodynamic - that of shoot- C. R. Roseberry, Doubleday and Company, the notations, its fixed horizontal ing bullets through a rotating propel- Inc., Garden City, NY, 1972, page 477. tailplane is labelled "stability fin" ler. It may have occurred to contempo- 15. Contact! by Henry S. Villard, (Ref. 20) indicative of its required pur- Bonanza Books, NY, 1968, page 24. rary tractor designers to place the 16. Aviation by Christopher Chant, pose and the trend of the day. guns outboard of the propeller arc and Chartwell Books, Inc., NJ, 1978, page 28. The Wrights had been concerned so bypass the complication and weight 17. The Flying Machine by Alien An- enough over the difficulty of flying of synchronizing gears, but the relia- drews, G. P. Putnam & Sons, NY, 1977, their airplanes to do work on various bility of available machine guns and page 92. devices to assist the pilot. At least one ammunition required much hand 18. The First To Fly by Sherwood Har- patent was granted to them (1909) for clearing of jams, necessitating that ris, Simon and Schuster, NY, 1970, page an automatic pitch-stabilizer. Ac- the breeches be close to the pilot. 226. tuated by a vane that controlled a 19. The Magnificent Old Man and His Until the Allied syncro-system was Flying Machine by William J. Alien, Air compressed-air supply, it was in turn firmed up, the pusher helped fill in. Line Pilot, Jan. 1976, page 18. connected to the elevator (Ref. 21). As years passed, at least up to re- 20. and 21. See Them Flying by Hous- This invention was soon abandoned cently, tandems have been built only ton Peterson, Richard W. Baron, NY, 1969, as unnecessary because after the occasionally, made pitch-stable page 393; page 44. crash in 1908 in which Selfridge was largely by a forward CG and the utili- 22. Early Aviation (At Farnborough) by killed and Orville seriously hurt, zation of a form of longitudinal dihed- Percy B. Walker, MacDonald & Co. Ltd., there was a transition period wherein ral, although not without some prob- London, 1974, page 173. the Wright airplane also flew with an lems intrinsically associated with the 23. The Wright Brothers by C. H. Gibbs-Smith, Science Museum, H. M. added, fixed tailplane. This was soon type. Stationery Office, London, 1963, page 26. modified to act as an elevator, to work Next and concluding part . . . Tan- together with the front one, much like dems. 26 SEPTEMBER 1984 A LJ/M^I-T^NMT A I t+ nv/ni I f*l_ I All M ADX/O I l~ MORE ON TANDEMS

Conclusion By George B. Collinge (EAA 67, Lifetime) 5037 Marin Way Oxnard, CA 93030

Illustrations by the Author u'.P TO 1910, the Wrights and are other arguments put forward for if the aft wing of a tandem pair is many others flew airplanes with for- double wings. But it is important to mounted at a higher incidence as on ward elevators. These aircraft cannot note that when a second wing, of any the biplane, a very unstable airplane be properly classified as tandems. size, is brought into position near a would be created (Fig. 7-4). On this Their primary element consisted of a first wing, either above it or below as configuration the airplane is balanced cambered, biplane wing-cell on which a biplane or behind it as a tandem, at a point equidistant between the cp the location of the center of gravity there will obviously be an interfer- of each wing. At a specific speed, the was based. To this unit was attached ence with the pressure distribution wings could be lifting equally, at the a free-floating "horizontal rudder" ac- around each (Fig. 7-2). same angle of attack, even though tuated by the pilot via a cable or strut. Generally, any kind of second-wing their incidence angles vary. This surface generated no sustained placement is going to rob the first of With a speed increase, the pilot lift and was not intended to share a significant amount of downthrust would normally lower the nose and support of the total weight (see Fig. and will at the same time and for the reduce CL to stay level. The angle of 7-1). It is difficult to imagine a flyable same reason reduce its own LD ratio downwash from each wing would then airplane more unstable or more for- (Ref. 1). lessen and the cp of each would move midable to control in the pitching Acknowledged worldwide is the as- aft causing a nose-down effect. To add plane. sumption that a single wing is the to this, the rear wing at its higher The Wrights originally utilized a most efficient mechanism for obtain- angle of incidence and now operating double or biplane design for purposes ing the greatest sub-sonic lift for the in less downwash, is lifting a greater of bracing. Later it became part of the least drag. This could nicely explain proportion of the total weight. There- concept of twisting or warping in why one never sees a biplane 707. fore, the overall airplane cp moves order to alter lift in step with rudder Normal decalage and positive stag- considerably rearward. An unstable movement. When dual wings are used ger (Fig. 7-3) have been routinely em- nose-down or "tuck" results. today, in many designs, it may still ployed to reduce performance loss in The pilot would have to pull on the be for the purpose of bracing. There the biplane (Ref. 2,3). Unfortunately, stick to prevent catastrophic diving,

Fig. 7-1 Wright type A 1908.

46 NOVEMBER 1984 Fig. 7-2 Potential perturbation.

Fig. 7-3 Lower wing suffers most. which action is definitely counter to duced angle, has a much lower cruise or very close to the center line, one- the requirements of a stable airplane. CL. engine-out control would be ex- At low speeds, just the opposite: the But the longitudinal dihedral now tremely difficult if not impractical ex- cp of both wings at high angles would provides stability for the tandem. If it cept at high speeds. move forward, pulling the nose up. goes faster than the selected cruise Lowering a flap increases the CL of The forward wing would lift more due speed, the canard lifts more, raising an airfoil but will not appreciably to flying in a greater upwash (higher the nose to slow the airplane. Less lift change the normal stalling angle angle of attack). Instead of a pilot at lower speeds depresses the nose, (Ref. 4, 5). This characteristic is, of continuing to pull at the lower speeds, speeding up the airplane. As with a course, what makes a canard elevator he would have to increasingly push to rear-tailed design, the restoring force possible (Fig. 7-6) for it is necessary keep from stalling. Again, most un- can be made powerful and pitch sta- to be able to depress elevator at low stable. bility made quite firm. Negative G, speed, say on the approach, and not Consequently, it is unusual on tan- as in real inverted flight, would com- be confronted with a sudden stall. dems to mount the aft wing at a smal- pletely upset the balance of a tandem. A total or abrupt canard stall must ler incidence even though it worsens The wider the interval between the be guarded against at all costs other- the total L/D ratio (Fig. 7-5). The re- two wings of a tandem, the greater wise the plain (flapless) rear-wing, sult of this causes the canard to lift the physical travel of CG that is possi- still lifting after the canard stalls, too great a proportion with speed. To ble, while still holding the limits to would put the airplane into a dive or offset this problem and still achieve the same percentage of the separa- somersault, from which it would be an acceptable equalibriurn at a given tion. This feature has appealed to awkward to recover and from which a speed (usually cruise) the CG is some designers of cargo aircraft. Di- large loss of height would occur. positioned more forward, which rectional stability becomes more of a So the canard is made resistant to necessarily increases the canard problem though, and large fin area is the low-speed stall. A wide range of wing-loading. The aft wing, at its re- needed. If multiple engines are not on modern airfoils offers a choice of lift

SPORT AVIATION 47 curves that do not peak sharply. In it might be needed most, for instance, a conventional airplane. This is be- yesteryears, the CL profile of conven- during a slow flare for landing. cause the conventional has its CG rel- tional sections was flattened by other On such an occasion, the only "up" atively close to its cp, which relation- means. The Focke-Wulf and Rohr pitch control left to the tandem pilot ship remains reasonably constant. canard surfaces (Fig. 7-7) both used would be the addition of engine The tandem's CG, however, is sup- similar forward-swept elevators, as- power. On a conventional airplane, ported between two widely-spaced lift sisted by low-aspect ratio (Ref. 6, 7, the increased slipstream might serve centers. Any diminution of lift greater 8,9). to hold down the tail and so help to on one wing over the other not only As with any flapped wing, there is reduce the landing speed. Adding causes a height decrease but at the a practical limit to down-travel. At power to a canard-controlled airplane, same time destroys the delicately- low speeds, if the canard elevator is under the same circumstances would tuned balance resulting in a situation depressed to the point of maximum only increase speed and/or cause it to that is not always fully controllable. lift, any additional travel (stick back) gain height. Consequently, the delegation of would only decrease the lift. This ef- Hihg-speed stalling, however, is some or all pitch management to the fective reduction of elevator power to usually possible despite slow-speed rear wing (Fig. 7-8) or the addition of raise the nose, restricts the pilot's restrictions and can result in some a horizontal tail (Fig. 7-9) is and has ability to create a full stall. It is simi- pretty wild rides. If sales records of been a practical method of increasing lar in result to the limited elevator- the past are to be given credence, tandem stability and controllability. travel occasionally encountered on "safe" limited-control airplanes are During the early thirties, the Flea conventionally-tailed airplanes and for the most part apparently disliked was a tandem employing a most in- which is also done to achieve stall/ by trained pilots. congruous scheme, that of a fixed rear spin resistance. If there is a deterioration of wing wing set at high incidence in company Thus, a few airplanes, conventional air-flow causing loss of lift, due to with a loaded but semi-floating and tandem alike, can be flown slowly bugs, grass, rain, frost or ice, it could canard! An aft elevator or trimmer (after a fashion) with the stick fully be felt primarily by the canard, due was found desirable by Henri Mignet, back, with little risk of an inadvertent to its higher wing-loading. Yet, a con- its designer, for his own personal Flea loss of lift. siderably larger aft wing may loose (HM18). With the angular travel of Regretably, it is impossible to de- sufficiently to equalize the effect or the forward wing limited to narrow sign for a "soft stall" or "no stall" even to cause a nose-up change. The limits, its envelope of manageability feature and at the same time retain reduction in lift can be much more still remained small. enough elevator authority to prevent noticeable on a tandem as opposed to Adding a rear elevator negates a running out of pitch control just when the same amount barely detected on lot of the design features and com-

Fig. 7-4 Unstable.

Fig. 7-5 Stable.

48 NOVEMBER 1984 promises originally part of the "ap- peal" of a tandem. But such a change could be seen as a logical and obvious development much like what took place in 1910, when pitch control moved from the front end to the aft end, on pushers and tractors alike. There are other items with which the designer of tandems has to cope. One is a tendency of a canard elevator to ride up in flight, because of the pressure differential, which causes the nose to lower. To hold the elevator down, the pilot would have to exert a pull force with increased speed while in a dive, which action once again is reversed to normal practice. To im- prove stability and generate a more acceptable feel, a spring-loaded trim- mer, or better, a simple locked servo (Fig. 10) is attached to the elevator. It has little effect at low speeds. Another item is flaps. Conventional Fig. 7-6 Flap increases CL. trailing-edge camber-increasing flaps would greatly improve airport perfor- mance of tandems. Long, flat ap- proaches could be advantageously steepened, prolonged floating reduced and touch-on speeds lowered. But this high-lift device causes an extreme nose-down trim change. Without a rear tail to automatically assume an increased, compensating down-load, the flapped tandem would be required to generate a greatly increased up- load on its canard. Trouble is, at approach speeds with front elevator deflected, the ordinary canard is already very close to its finely adjusted CL max. Therefore, if the aft wing is to even moderately lower or extend flap, then a different kind of canard must be used, one that can increase its CL to a high enough value to counter the CL increase of the aft wing and maintain a balanced airplane. Canard flaps of this kind obviously don't double well as elevator controls, that function then residing in the aft wing. Black-box coupling of front and Hg. 7-7 FW19a 1931 & Rohr 1947 rear flaps ensures a constantly bal- anced airplane (Fig. 7-11). Very low aspect ratio suppresses stalling, is suited to the delta configuration and is a much simpler expedient than slats or Kreuger flaps. A different approach is one where the canard itself sweeps forward, its increased leverage providing a bal- ancing up-load for a limited amount of trailing-edge flap extension. Com- plication on the march. While the pure delta layout already provides a steep approach and a slow landing speed, it cannot normally use trailing edge landing flaps either. Machines of this type have been briefly tested with retractable canards or "moustaches" on the Fig. 7-8 Despretz mk II 1965. theory that at high angles, not as

SPORT AVIATION 49 the canard has computer sensing and control inputs up to 40 times per sec- ond! (Ref. 10)

EPILOG The question, posed by the title of this series can possibly be answered "there is no free lunch." If the tail is discarded, then a substitute must take its place. It may be a reflexed trailing edge, sweep and twist, or a canard that reaches CL max before its companion aft wing. It would seem evident all the same, that since 1910 a simple rear-tail, separate or inte- gral (delta style) has, through usage, been the dominant, almost exclusive form of pitch control for manually Fig. 7-9 Luton 1936. controlled, fixed-wing airplanes. Whether it remains so depends on a replacement not just different but truly superior. References: 1. Tandem Arrangements, Air- plane Design by Edward P. Warner, McGraw-Hill Book Company, Inc., NY 1936. Pages 275, 277 and 278. "The performance of a tandem combi- nation is always poorer than that of the individual wings, as might be foreseen from the slightest considera- tion of induced drag theory. Since, if Fig. 7-10 Fix by fixed servo. the forward wing is lifting, its down- wash rotates the lift and drag axes of the aft wing, effectively increasing the value of the drag component of its lift vector. Unfortunately, the combi- nation of angles of attack offering the greatest aerodynamic efficiency has serious disadvantages on the score of stability." 2. Mechanics of Flight by A. C. Kermode, Sir Issac Pitman and Sons, Ltd., London 1942, page 66. 3. Aerodynamics of the Airplane by Clark B. Millikan, John Wiley and Sons, Inc., London 1941, page 77. 4. Tail First, Aeronautics Oct. 1939, page 36. 5. Report 824, Summary of Airfoil Data, NACA 1945. Fig. 7-11 The Viggen interconnection. 6. Midnight Oiler, Skyways, Nov. 1946. much up-elevon would be needed, dead-air region. 7. Lightweight Canard, Flight, leaving more lift on the wing, giving It has already been detailed that Jan. 16, 1947, pages 55, 56. even lower landing speeds. the Wrights originally flew a de- 8. The Ugly Dickling by Rex King, The practical result of these tests cidedly unstable arrangement that Aeroplane Monthly, Oct. 1973, pages has manifested in the now common demanded a high degree of pilot con- 275 through 278. use, on low aspect ratio, high perfor- centration. A modern, fly-by-wire, ex- 9. Das Buch der Deutschen Luf- mance airplanes, of small fixed for- perimental, aft-tailed fighter has fahrttechnik by Bruno Lange, Verlag ward surfaces. Called canards, a more been deliberately made likewise un- Dieter Hoffman, Mainz WB 1970, accurate term would be "king-size stable, by moving the CG aft of the pages 272, 273, 274. vortex-generators" which are em- neutral point. The theory this time is 10. Aviation Week, Dec. 6, 1982, ployed, as is the forward wing of the quicker maneuvering although (and page 67. Grumman Aerospace Corpo- Vigen, to prolong the main-wing, the secret of success) angular devia- ration ad ". . . without computers upper-surface attached-flow to a very tion requires close electronic monitor- sensing and moving these canards 40 steep angle and to improve control by ing to prevent lightning-quick, catas- times each second, the craft would energizing the stream across the fin(s) trophic divergence. Similarly, Grum- not be flyable because the pilot and rudder(s) when they would other- man is working on a tandem design couldn't adjust positions fast enough wise be immersed in an extensive where, to create a flyable airplane,

50 NOVEMBER 1984